prompt_front: Answer the question based on the given passages. Only give me the answer and do not output any other words.

Question: Who is the mayor presiding now where Merrill Elam was born?

The following are given passages.
Context 1: Passage 1:
2017 Cincinnati mayoral election
The 2017 Cincinnati mayoral election took place on November 7, 2017, to elect the Mayor of Cincinnati, Ohio  The election was officially nonpartisan, with the top two candidates from the May 2 primary election advancing to the general election, regardless of party  Incumbent Democratic Mayor John Cranley won re-election to a second term 
While the election was nonpartisan, all the candidates were known Democrats 

Primary election
Candidates
Declared
John Cranley (D), incumbent Mayor of Cincinnati
Rob Richardson, Jr  (D), former University of Cincinnati board chairman
Yvette Simpson (D), Cincinnati City Council member

Declined
Greg Hartmann (R), former Hamilton County Commissioner
Charlie Winburn (R), Cincinnati City Council member

Endorsements
Polling
Results
General election
Candidates
John Cranley (D), incumbent Mayor of Cincinnati
Yvette Simpson (D), Cincinnati City Council member, President Pro-Tempore

Endorsements
Polling
Results
Passage 2:
2017 Atlanta mayoral election
The 2017 Atlanta mayoral election occurred on November 7, 2017, with a runoff election held on December 5, 2017   Incumbent mayor Kasim Reed, a member of the Democratic Party who had been in office since 2010, was ineligible to run for reelection due to term limits 
A total of 14 candidates qualified for the November 7 non-partisan election   However, no candidate revived a majority, so a runoff was held between Keisha Lance Bottoms and Mary Norwood, the top two finishers, on December 5   On election night, unofficial results suggested that Keisha Lance Bottoms had narrowly won, but Norwood called for a recount The election was first certified on December 11, 2017, and had Bottoms winning by a margin of 823 votes   This margin was less than the 1 percent threshold needed to avoid a mandatory recount   The recount, which occurred on December 14, resulted in Norwood gaining five votes and Bottoms losing six in Fulton County, while the vote totals in DeKalb County remained the same   The election was re-certified on December 17 after Fulton County accepted the new results   Norwood officially conceded the race on December 21, and Lance Bottoms was inaugurated as the 60th mayor of Atlanta on January 2, 2018 

Candidates
Declared
Peter Aman, former Chief Operating Officer of Atlanta
Rohit Ammanamanchi, Georgia Institute of Technology graduate
Keisha Lance Bottoms, Atlanta City Councilmember (District 11)
John Eaves, Fulton County Chairman, District 7, At-Large
Vincent Fort, Georgia State Senator 
Kwanza Hall, Atlanta City Councilmember (District 2)
Laban King, co-founder of the real estate investment firm, Millennial Global Investments
Ceasar Mitchell, Atlanta City Council President
Mary Norwood, Atlanta City Councilmember (Post 2 At Large) and candidate for Mayor in 2009
Cathy Woolard, former Atlanta City Council President
Glenn Wrightson

Failed to qualify
Alex Barrella, cartoonist

Dropped out
Al Bartell, Certified Mediator  Dropped out August 22, 2017 
Margaret Kaiser, Georgia State Representative, District 59, 2007–2016  Dropped out October 25, 2016 
Michael Sterling, former Executive Director of the City of Atlanta Workforce Development Agency  Dropped out October 24, 2017

Endorsements
Polling
Results
General election
Runoff
See also
Mayor of Atlanta
2017 United States elections
Passage 3:
Merrill Elam
Merrill Elam is an American architect and educator based in Atlanta, Georgia  She is a principal with Mack Scogin in Mack Scogin Merrill Elam Architects where their work spans between buildings, interiors, planning, graphics and exhibition design, and research 

Biography and education
She received a Bachelor of Architecture from the Georgia Institute of Technology in 1971 and a Master of Business Administration at the J  Mack Robinson College of Business Georgia State University in 1982 

Career
She worked as an architect and senior associate at Heery & Heery Architects & Engineers for 12 years prior to establishing Scogin Elam and Bray Architects in 1984  In 2000, Mack Scogin and Merrill Elam Architects was established  She has been a licensed architect in 13 states, a member of the American Institute of Architects and is NCARB certified 
In addition to an active practice, she  has been a visiting architecture professor in architecture programs across the country including Harvard University, Yale University, University of Toronto, University of Virginia, Ohio State University, Syracuse University, and University of Texas at Austin  She also formerly acted as president of the Georgia State Board of Architects  In addition she aided in founding the Architecture Society of Atlanta and is the former organization's former president 

Selected projects
The Round House, Connecticut, 2020 
One Midtown Plaza Lobby, Atlanta, Georgia, 2007
Health Services Center, Yale University, New Haven, Connecticut, 2006
Ernie Davis Hall, Syracuse University, Syracuse, New York, 2005
Gates Center for Computer Science and Hillman Center for Future Generation Technologies, Carnegie Mellon University, Pittsburgh, Pennsylvania, 2005
Zhongkai Sheshan Villas, Shanghai, China, 2004
Carroll A  Campbell Jr  Graduate Engineering Center, Clemson University, Clemson, South Carolina, 2004
U S  Federal Courthouse, General Services Administration, Austin, Texas, 2003
Lulu Chow Wang Campus Center and Davis Garage, Wellesley College, Wellesley, Massachusetts, 2001
Carol Cobb Turner Branch Library, Marrow, Georgia, 1991
John J  Ross - William C  Blakley Law Library, Arizona State University, Tempe, Arizona, 1993
Austin E  Knowlton School of Architecture, Ohio State University, Columbus, Ohio, 1998
Jean Gray Hargrove Music Library, University of California, Berkeley, Berkeley, California, 1998
Mountain Tree House, Dillard, Georgia, 1998

Honors and awards
2014 Design Leader award, Women in Architecture Awards, Architectural Record, 2014
Shutze Medal, Georgia Institute of Technology with Mack Scogin, 2013
Cooper-Hewitt National Design Award for Architecture with Mack Scogin, 2012 
Arnold W  Brunner Memorial Prize in Architecture, American Academy of Arts and Letters with Mack Scogin, 2011
Royal Institute of British Architects, International Fellow, 2008
Chrysler Award for Innovation in Design with Mack Scogin, 1996
Arts and Letters Award in Architecture, American Academy of Arts and Letters with Mack Scogin, 1995
Passage 4:
Joseph Rakowski
Joseph Rakowski is an American Democratic Party politician who served as Acting Mayor of Jersey City, New Jersey  He became acting mayor when he was elected to serve as City Council President of Jersey City  Rakowski was the third person to hold the office in the same year 

Biography
During his four-month term of office, Rakowski worked with New York City Mayor David Dinkins and New Jersey Attorney General Robert J  Del Tufo to address a problem where trucks full of garbage were being driven from New York City to Jersey City, New Jersey and their full trailers abandoned  Rakowski did not seek to complete the unexpired term as mayor in the special election that was held during his tenure  He was succeeded by Bret Schundler 
Passage 5:
Joan McBride
Joan Leslie McBride (born 1951 or 1952) is an American politician  A Democrat, she was the mayor of Kirkland, Washington, from 2010 to 2013, after which she was elected to the Washington House of Representatives as a member from District 48  She also served on the Kirkland City Council since 1998, sitting as deputy mayor from 2002 until her election as mayor in 2010 McBride originally announced a bid to succeed state Senator Rodney Tom  However, in April 2014, she and state Representative Cyrus Habib announced they would switch races, with Habib running for Tom's seat and McBride running to replace Habib in the House  McBride was elected in November and sworn on January 12, 2015  She was named assistant Democratic whip McBride announced her retirement from the State legislature on March 29, 2018 

Personal life
McBride is married with children  She has a background in nonprofit work and community relations  She resides in Kirkland 
Passage 6:
Rusty Paul
Russell K  Paul (born June 23, 1952) is an American politician serving as the mayor of Sandy Springs, Georgia since 2014 

Early life and education
Paul was born in 1952, and grew up in the Birmingham, Alabama area  He received a Bachelor’s degree at Samford University and attended graduate school at Georgia State University 

Career
Paul was the Stone Mountain city council member from 1977 to 1983  From1989 to 1993, he served as Assistant Secretary for Congressional and Intergovernmental Relations for the Housing and Urban Development Department under Secretary Jack Kemp  He chaired the Georgia Republican Party from 1995 to 1999, and became a State Senator representing northern Fulton County from 2001 to 2003  In 2005, he was elected to Sandy Springs city council  Paul was elected to succeed Eva Galambos in November 2013 

Personal life
Paul is married to Jan Paul and has 5 children  He is an Episcopalian 

Controversies
Paul has courted controversy for his lobbying work in relation to the Development Authority of Fulton County while serving as a publicly elected official 
Passage 7:
Kraków
Kraków (Polish: [ˈkrakuf] (listen)) is the second-largest and one of the oldest cities in Poland  Situated on the Vistula River in Lesser Poland Voivodeship, the city dates back to the seventh century  Kraków was the official capital of Poland until 1596 and has traditionally been one of the leading centres of Polish academic, economic, cultural and artistic life  Cited as one of Europe's most beautiful cities, its Old Town with Wawel Royal Castle was declared a UNESCO World Heritage Site in 1978, one of the world's first sites granted the status 
The city has grown from a Stone Age settlement to Poland's second-most-important city  It began as a hamlet on Wawel Hill and was reported by Ibrahim Ibn Yakoub, a merchant from Córdoba, as a busy trading centre of Central Europe in 985  With the establishment of new universities and cultural venues at the emergence of the Second Polish Republic in 1918 and throughout the 20th century, Kraków reaffirmed its role as a major national academic and artistic centre  As of 2022, the city has a population of 800,653, with approximately 8 million additional people living within a 100 km (62 mi) radius of its main square After the invasion of Poland by Nazi Germany at the start of World War II, the newly defined Distrikt Krakau (Kraków District) became the capital of Germany's General Government  The Jewish population of the city was forced into a walled zone known as the Kraków Ghetto, from where they were sent to Nazi extermination camps such as the nearby Auschwitz, and Nazi concentration camps like Płaszów  However, the city was spared from destruction and major bombing 
In 1978, Karol Wojtyła, archbishop of Kraków, was elevated to the papacy as Pope John Paul II—the first non-Italian pope in 455 years  Also that year, UNESCO approved Kraków's entire Old Town and historic centre and the nearby Wieliczka Salt Mine as Poland's first World Heritage Sites  Kraków is classified as a global city with the ranking of "high sufficiency" by the Globalization and World Cities Research Network  Its extensive cultural heritage across the epochs of Gothic, Renaissance and Baroque architecture includes Wawel Cathedral and Wawel Royal Castle on the banks of the Vistula, St  Mary's Basilica, Saints Peter and Paul Church and the largest medieval market square in Europe, Rynek Główny  Kraków is home to Jagiellonian University, one of the oldest universities in the world and traditionally Poland's most reputable institution of higher learning  The city also hosts a number of institutions of national significance such as the National Museum, Kraków Opera, Juliusz Słowacki Theatre, National Stary Theatre and the Jagiellonian Library  The city is served by John Paul II International Airport, the country's second busiest airport and the most important international airport for the inhabitants of south-eastern Poland 
In 2000, Kraków was named European Capital of Culture  In 2013, Kraków was officially approved as a UNESCO City of Literature  The city hosted World Youth Day in July 2016 

Etymology
The name of Kraków is traditionally derived from Krakus (Krak, Grakch), the legendary founder of Kraków and a ruler of the tribe of Vistulans  In Polish, Kraków is an archaic possessive form of Krak and essentially means "Krak's (town)"  The true origin of the name is highly disputed among historians, with many theories in existence and no unanimous consensus  The first recorded mention of Prince Krakus (then written as Grakch) dates back to 1190, although the town existed as early as the seventh century, when it was inhabited by the tribe of Vistulans  It is possible that the name of the city is derived from the word kruk, meaning 'crow' or 'raven' The city's full official name is Stołeczne Królewskie Miasto Kraków, which can be translated as "Royal Capital City of Kraków"  In English, a person born or living in Kraków is a Cracovian (Polish: krakowianin or krakus)  Until the 1990s the English version of the name was often written as Cracow, but now the most widespread modern English version is Krakow 

History
Kraków's early history begins with evidence of a Stone Age settlement on the present site of the Wawel Hill  A legend attributes Kraków's founding to the mythical ruler Krakus, who built it above a cave occupied by a dragon, Smok Wawelski  The first written record of the city's name dates back to 965, when Kraków was described as a notable commercial centre controlled first by Moravia (876–879), but captured by a Bohemian duke Boleslaus I in 955  The first acclaimed ruler of Poland, Mieszko I, took Kraków from the Bohemians and incorporated it into the holdings of the Piast dynasty towards the end of his reign 
In 1038, Kraków became the seat of the Polish government  By the end of the tenth century, the city was a leading centre of trade  Brick buildings were constructed, including the Royal Wawel Castle with St  Felix and Adaukt Rotunda, Romanesque churches such as St  Andrew's Church, a cathedral, and a basilica  The city was sacked and burned during the Mongol invasion of 1241  It was rebuilt practically identically, based on new location act and incorporated in 1257 by the high duke Bolesław V the Chaste who following the example of Wrocław, introduced city rights modelled on the Magdeburg law allowing for tax benefits and new trade privileges for the citizens  In 1259, the city was again ravaged by the Mongols  A third attack in 1287 was repelled thanks in part to the newly built fortifications 

In 1335, King Casimir III the Great (Polish: Kazimierz) declared the two western suburbs to be a new city named after him, Kazimierz (Latin: Casimiria)  The defensive walls were erected around the central section of Kazimierz in 1362, and a plot was set aside for the Augustinian order next to Skałka  The city rose to prominence in 1364, when Casimir founded the University of Kraków, the second oldest university in central Europe after the Charles University in Prague  Casimir also began work on a campus for the academy in Kazimierz, but he died in 1370 and the campus was never completed 
The city continued to grow under the joint Lithuanian-Polish Jagiellon dynasty  As the capital of the Kingdom of Poland and a member of the Hanseatic League, the city attracted many craftsmen from abroad, businesses, and guilds as science and the arts began to flourish  The royal chancery and the university ensured a first flourishing of Polish literary culture in the city 

Kraków's "Golden Age"
The 15th and 16th centuries were known as Poland's Złoty Wiek or Golden Age  Many works of Polish Renaissance art and architecture were created, including ancient synagogues in Kraków's Jewish quarter located in the north-eastern part of Kazimierz, such as the Old Synagogue  During the reign of Casimir IV, various artists came to work and live in Kraków, and Johann Haller established a printing press in the city after Kasper Straube had printed the Calendarium Cracoviense, the first work printed in Poland, in 1473 In 1520, the most famous church bell in Poland, named Zygmunt after Sigismund I of Poland, was cast by Hans Behem  At that time, Hans Dürer, a younger brother of artist and thinker Albrecht Dürer, was Sigismund's court painter  Hans von Kulmbach made altarpieces for several churches  In 1553, the Kazimierz district council gave the Jewish Qahal (council of a Jewish self-governing community) a licence for the right to build their own interior walls across the western section of the already existing defensive walls  The walls were expanded again in 1608 due to the growth of the community and influx of Jews from Bohemia  In 1572, King Sigismund II, the last of the Jagiellons, died childless  The Polish throne passed to Henry III of France and then to other foreign-based rulers in rapid succession, causing a decline in the city's importance  Furthermore, in 1596, Sigismund III of the House of Vasa moved the administrative capital of the Polish–Lithuanian Commonwealth from Kraków to Warsaw  The city was destabilised by pillaging in the 1650s during the Swedish invasion, especially during the 1655 siege  Later in 1707, the city underwent an outbreak of bubonic plague that left 20,000 of the city's residents dead 

19th century
Already weakened during the 18th century, by the mid-1790s the Polish–Lithuanian Commonwealth had twice been partitioned by its neighbors: Russia, the Habsburg empire and Prussia  In 1791, the Austrian and Holy Roman Emperor Joseph II changed the status of Kazimierz as a separate city and made it into a district of Kraków  The richer Jewish families began to move out  However, because of the injunction against travel on the Sabbath, most Jewish families stayed relatively close to the historic synagogues  In 1794, Tadeusz Kościuszko initiated an unsuccessful insurrection in the town's Main Square which, in spite of his victorious Battle of Racławice against a numerically superior Russian army, resulted in the third and final partition of Poland In 1802, German became the town's official language  Of the members appointed by the Habsburgs to the municipal council only half were Polish  From 1796 to 1809, the population of the city rose from 22 000 to 26 000 with an increasing percentage of nobles and officials  In 1809, Napoleon Bonaparte captured former Polish territories from Austria and made the town part of the Duchy of Warsaw  During the time of the Duchy of Warsaw, requirements to upkeep the Polish army followed by tours of Austrian, Polish and Russian troops, plus Russian occupation and a flood in the year 1813 all added up to the adverse development of the city with a high debt burden on public finances and many workshops and trading houses needing to close their activities  Following Napoleon's defeat, the 1815 Congress of Vienna restored the pre-war boundaries but also created the partially independent Free City of Kraków  An insurrection in 1846 failed, resulting in the city being annexed by Austria under the name the Grand Duchy of Kraków (Polish: Wielkie Księstwo Krakowskie, German: Großherzogtum Krakau) 

This Republic of Kraków (1815–1846) included the towns of Chrzanow, Trzebinia and Nowa Gora and 224 villages  Outside the city, mining and metallurgy started developing  The population of Cracow itself grew in this time from 23 000 to 43 000, the one in the Republic from 88 000 to 103 000  The population of the city had an increasing number of catholic clergy, officials and intelligentsia with which the rich townspeople sympathised  They were opposed to the conservative landed aristocracy who also were drawn more and more to the city real estates even though their income still mainly came from their agricultural possessions in the Republic, the Kingdom of Poland and Galicia  The percentage of the Jewish population in the city also increased in this time from 20 8% to 30 4%  A free trade zone led to positive economic development  But because of the unstable political situation and insecurity about the future, not much of the accumulated wealth was invested  Through the increase of taxes, customs and regulations, prices soared and the city fell into a recession  From 1844 to 1850 the population was diminished by over 4 000 inhabitants  
In 1866, Austria granted a degree of autonomy to Galicia after its own defeat in the Austro-Prussian War  Politically freer Kraków became a Polish national symbol and a centre of culture and art, known frequently as the "Polish Athens" (Polskie Ateny)  Many leading Polish artists of the period resided in Kraków, among them the seminal painter Jan Matejko, laid to rest at Rakowicki Cemetery, and the founder of modern Polish drama, Stanisław Wyspiański  Fin de siècle Kraków evolved into a modern metropolis; running water and electric streetcars were introduced in 1901, and between 1910 and 1915, Kraków and its surrounding suburban communities were gradually combined into a single administrative unit called Greater Kraków (Wielki Kraków) At the outbreak of World War I on 3 August 1914, Józef Piłsudski formed a small cadre military unit, the First Cadre Company—the predecessor of the Polish Legions—which set out from Kraków to fight for the liberation of Poland  The city was briefly besieged by Russian troops in November 1914  Austrian rule in Kraków ended in 1918 when the Polish Liquidation Committee assumed power 

20th century to the present
Following the emergence of the Second Polish Republic in 1918, Kraków resumed its role as a major Polish academic and cultural centre, with the establishment of new universities such as the AGH University of Science and Technology and the Jan Matejko Academy of Fine Arts, including a number of new and essential vocational schools  The city became an important cultural centre for Polish Jews, including both Zionist and Bundist groups  Kraków was also an influential centre of Jewish spiritual life, with all its manifestations of religious observance - from Orthodox to Hasidic and Reform Judaism - flourishing side by side 
Following the invasion of Poland by Nazi Germany in September 1939, the city of Kraków became part of the General Government, a separate administrative region of the Third Reich  On 26 October 1939, the Nazi régime set up Distrikt Krakau, one of a total of four districts within the General Government  On the same day, the city of Kraków also became the capital of the administration  The General Government was ruled by Governor-General Hans Frank, who was based in the city's Wawel Castle  The Nazis envisioned turning Kraków into a completely Germanised city; after removal of all the Jews and Poles, renaming of locations and streets into the German language, and sponsorship of propaganda trying to portray it as a historically German city  On 28 November 1939 Hans Frank set up Judenräte ('Jewish Councils') to be run by Jewish citizens for the purpose of carrying out orders for the Nazis  These orders included the registration of all Jewish people living in each area, the collection of taxes, and the formation of forced-labour groups  The Polish Home Army maintained a parallel underground administrative system On the eve of World War II some 56,000 Jews resided in Krakow, almost one-quarter of a total population of about 250,000  By November 1939, the Jewish population of Krakow had grown to approximately 70,000  According to German statistics from 1940, over 200,000 Jews lived within the entire Kraków District, comprising more than 5 percent of the total population in the district  These statistics, however, probably underestimate the situation In November 1939, during an operation known as "Sonderaktion Krakau", the Germans arrested more than 180 university professors and academics and sent them to the Sachsenhausen and Dachau concentration camps, though the survivors were later released on the request of prominent Italians 

Before the formation of ghettos, which began in the Distrikt in December 1939, Jews were encouraged to flee the city  For those who remained the German authorities decided in March 1941 to allocate a then suburban neighborhood, Podgórze District, to become Kraków's ghetto - there many Jews would die of illness or starvation  Initially, most ghettos were open and Jews were allowed to enter and exit freely  However, with time ghettos were generally closed and security became tighter  From autumn 1941, the SS developed the policy of Extermination through labour, which further worsened the already bleak conditions for Jews  The ghetto inhabitants were later murdered or sent to German Extermination camps, including Bełżec and Auschwitz, and to Kraków-Płaszów concentration camp  The largest deportations within the Distrikt occurred from June to September 1942  More specifically, the Kraków ghetto deportation occurred in the first week of June 1942, and in March 1943 the ghetto was definitely liquidated Roman Polanski, the film director, survived the Kraków ghetto  Oskar Schindler selected employees from the ghetto to work in his enamelware factory Deutsche Emailwaren Fabrik (Emalia for short), saving them from the camps  Similarly, many men capable of physical labor were saved from the deportations to extermination camps and instead sent to labor camps across the General Government  By September 1943, the last of the Jews from the Kraków ghetto had been deported  Although looted by occupational authorities, Kraków remained relatively undamaged at the end of World War II, with most of the city's historical and architectural legacy spared  Soviet forces under the command of Marshal Ivan Konev entered the city on 18 January 1945, and began arresting Poles loyal to the Polish government-in-exile or those who had served in the Home Army 

After the war, under the Polish People's Republic (officially declared in 1952), the intellectual and academic community of Kraków came under complete political control  The universities were soon deprived of printing rights and autonomy  The Stalinist government of Poland ordered the construction of the country's largest steel mill in the newly created suburb of Nowa Huta  The creation of the giant Lenin Steelworks (now Sendzimir Steelworks owned by Mittal) sealed Kraków's transformation from a university city into an industrial centre  The new working-class population, drawn by the industrialization of Kraków, contributed to rapid growth 
In an effort that spanned two decades, Karol Wojtyła, cardinal archbishop of Kraków from 1964 to 1978, successfully lobbied for permission to build the first churches in the newly-industrial suburbs  In 1978 the Catholic Church elevated Wojtyła to the papacy as John Paul II, the first non-Italian pope in 455 years  In the same year, UNESCO, following the application of local authorities, placed Kraków Old Town on the first-ever list of World Heritage Sites 

Geography
Kraków lies in the southern part of Poland, on the Vistula River, in a valley at the foot of the Carpathian Mountains, 219 m (719 ft) above sea level; halfway between the Jurassic Rock Upland (Polish: Jura Krakowsko-Częstochowska) to the north, and the Tatra Mountains 100 km (62 mi) to the south, constituting the natural border with Slovakia and the Czech Republic; 230 km (143 mi) west from the border with Ukraine 
There are five nature reserves in Kraków, with a combined area of ca  48 6 hectares (120 acres)  Due to their ecological value, these areas are legally protected  The western part of the city, along its northern and north-western side, borders an area of international significance known as the Jurassic Bielany-Tyniec refuge  The main motives for the protection of this area include plant and animal wildlife and the area's geomorphological features and landscape  Another part of the city is located within the ecological 'corridor' of the Vistula River valley  This corridor is also assessed as being of international significance as part of the Pan-European ecological network  The city centre is situated on the left (northern) bank of the river 

Climate
Officially, Kraków has a temperate oceanic climate, denoted by Köppen classification as Cfb, best defined as a semicontinental climate  In older reference periods it was classified as a warm summer continental climate (Dfb)  By classification of Wincenty Okołowicz, it has a warm temperate climate in the centre of continental Europe with the "fusion" of different features Due to its geographic location, the city may be under marine influence, sometimes Arctic influence, but without direct influence, giving the city variable meteorological conditions over short spaces of time Being towards Eastern Europe and a relatively considerable distance from the sea, Krakow has significant temperature differences according to the progress of different air masses, having four defined seasons of the year  Average temperatures in summer range from 18 6 to 20 4 °C (65 to 69 °F) and in winter from −0 6 to 0 8 °C (31 to 33 °F)  The average annual temperature is 10 0 °C (50 °F)  In summer temperatures often exceed 25 °C (77 °F), even reaching 30 °C (86 °F), while in winter temperatures drop to −5 °C (23 °F) at night and about 0 °C (32 °F) during the day  During very cold nights the temperature can drop to −15 °C (5 °F)  The city lies near the Tatra Mountains, there are often occurrences of halny blowing (a foehn wind), causing temperatures to rise rapidly, and even in winter reach up to 20 °C (68 °F) In relation to Warsaw, temperatures are very similar for most of the year, except that in the colder months southern Poland has a larger daily temperature range, more moderate winds, generally more rainy days and with greater chances of clear skies on average, especially in winter  The higher sun angle also allows for a longer growing season  In addition, for older data there was less sun than the capital of the country, about 30 minutes daily per year, but both have small differences in relative humidity and the direction of the winds is northeast The climate table below presents weather data from 2000 to 2012, although the official Köppen reference period was from 1981 to 2010 (therefore not being technically a climatological normal)  According to ongoing measurements, the temperature has increased during these years as compared with the last series  This increase averages about 0 6 °C (1 1 °F) over all months  Warming is most pronounced during the winter months, with an increase of more than 1 0 °C (1 8 °F) in January 

Cityscape
Developed over many centuries, Kraków provides a showcase setting for many historic styles of architecture  As the city expanded, so too did the architectural achievements of its builders  It is for this reason that the variations in style and urban planning are so easily recognisable Built from its earliest nucleus outward, and having escaped much of the destruction endured by Poland during the 20th-century wars, Kraków's many architectural monuments can typically be seen in historical order by walking from the city centre out, towards its later districts  Kraków is one of the few medieval towns in Poland that does not have a historic Ratusz town hall in its Main Square, because it has not survived the Partitions of Poland Kraków's historic centre, which includes the Old Town, Kazimierz and the Wawel Castle, was included as the first of its kind on the list of UNESCO World Heritage Sites in 1978  The Stare Miasto is the most prominent example of an old town in the country  For many centuries Kraków was the royal capital of Poland, until Sigismund III Vasa relocated the court to Warsaw in 1596  The whole district is bisected by the Royal Road, the coronation route traversed by the Kings of Poland  The Route begins at St  Florian's Church outside the northern flank of the old city-walls in the medieval suburb of Kleparz; passes the Barbican of Kraków (Barbakan) built in 1499, and enters Stare Miasto through the Florian Gate  It leads down Floriańska Street through the Main Square, and up Grodzka to Wawel, the former seat of Polish royalty, overlooking the Vistula river  Old Town attracts visitors from all over the World  Kraków historic centre is one of the 13 places in Poland that are included in the UNESCO World Heritage Sites  The architectural design of the district had survived all cataclysms of the past and retained its original form coming from the medieval times 

In addition to the old town, the city's district of Kazimierz is particularly notable for its many renaissance buildings and picturesque streets, as well as the historic Jewish quarter located in the north-eastern part of Kazimierz  Kazimierz was founded in the 14th century to the south-east of the city centre and soon became a wealthy, well-populated area where construction of imposing properties became commonplace  Perhaps the most important feature of medieval Kazimierz was the only major, permanent bridge (Pons Regalis) across the northern arm of the Vistula  This natural barrier used to separate Kazimierz from the Old Town for several centuries, while the bridge connected Kraków to the Wieliczka Salt Mine and the lucrative Hungarian trade route  The last structure at this location (at the end of modern Stradom Street) was dismantled in 1880 when the northern arm of the river was filled in with earth and rock, and subsequently built over 

By the 1930s, Kraków had 120 officially registered synagogues and prayer houses that spanned across the old city  Much of Jewish intellectual life had moved to new centres like Podgórze  This in turn, led to the redevelopment and renovation of much of Kazimierz and the development of new districts in Kraków  Most historic buildings in central Kazimierz today are preserved in their original form  Some old buildings, however, were not repaired after the devastation brought by the Second World War, and have remained empty  Most recent efforts at restoring the historic neighborhoods gained new impetus around 1993  Kazimierz is now a well-visited area, seeing a booming growth in Jewish-themed restaurants, bars, bookstores and souvenir shops 

As the city of Kraków began to expand further under the rule of the Austro-Hungarian Empire, the new architectural styles also developed  Key buildings from the 19th and early 20th centuries in Kraków include the Jan Matejko Academy of Fine Arts, the directorate of the Polish State Railways as well as the original complex of Kraków Główny railway station and the city's Academy of Economics  It was also at around that time that Kraków's first radial boulevards began to appear, with the city undergoing a large-scale program aimed at transforming the ancient Polish capital into a sophisticated regional centre of the Austro-Hungarian Empire  New representative government buildings and multi-story tenement houses were built at around that time  Much of the urban-planning beyond the walls of the Old Town was done by Polish architects and engineers trained in Vienna  Some major projects of the era include the development of the Jagiellonian University's new premises and the building of the Collegium Novum just west of the Old Town  The imperial style planning of the city's further development continued until the return of Poland's independence, following the First World War  Early modernist style in Kraków is represented by such masterpieces as the Palace of Art by Franciszek Mączyński and the 'House under the Globe'  Secession style architecture, which had arrived in Kraków from Vienna, became popular towards the end of the Partitions 

With Poland's regained independence came the major change in the fortunes of Kraków—now the second most important city of a sovereign nation  The state began to make new plans for the city development and commissioned a number of representative buildings  The predominant style for new projects was modernism with various interpretations of the art-deco style  Important buildings constructed in the style of Polish modernism include the Feniks 'LOT' building on Basztowa Street, the Feniks department store on the Main Square and the Municipal Savings Bank on Szczepański Square  The Józef Piłsudski house is also of note as a particularly good example of interwar architecture in the city After the Second World War, new government turned toward Soviet influence and the Stalinist monumentalism  The doctrine of Socialist realism in Poland, as in other countries of the People's Republics, was enforced from 1949 to 1956  It involved all domains of art, but its most spectacular achievements were made in the field of urban design  The guidelines for this new trend were spelled-out in a 1949 resolution of the National Council of Party Architects  Architecture was to become a weapon in establishing the new social order by the communists  The ideological impact of urban design was valued more than aesthetics  It aimed at expressing persistence and power  This form of architecture was implemented in the new industrial district of Nowa Huta with apartment blocks constructed according to a Stalinist blueprint, with repetitious courtyards and wide, tree-lined avenues 

Since the style of the Renaissance was generally regarded as the most revered in old Polish architecture, it was also used for augmenting Poland's Socialist national format  However, in the course of incorporating the principles of Socialist realism, there were quite a few deviations introduced by the communists  One of these was to more closely reflect Soviet architecture, which resulted in the majority of works blending into one another  From 1953, critical opinions in the Party were increasingly frequent, and the doctrine was given up in 1956 marking the end of Stalinism  The soc-realist centre of Nowa Huta is considered to be a meritorious monument of the times  This period in postwar architecture was followed by the mass-construction of large Panel System apartment blocks, most of which were built outside the city centre and thus do not encroach upon the beauty of the old or new towns  Some examples of the new style (e g , Hotel Cracovia) recently listed as heritage monuments were built during the latter half of the 20th century in Kraków After the Revolutions of 1989 and the birth of the Third Republic in the latter half of the 20th century, a number of new architectural projects were completed, including the construction of large business parks and commercial facilities such as the Galeria Krakowska, or infrastructure investments like the Kraków Fast Tram  A good example of this would be the Manggha Museum of Japanese Art and Technology designed by Arata Isozaki, the 2007-built Pawilon Wyspiański 2000, which is used as a multi-purpose information and exhibition space, or the Małopolski Garden of Arts (Małopolski Ogród Sztuki), a multi-purpose exhibition and theatre complex located in the historic Old Town 

Parks and gardens
There are about 40 parks in Kraków including dozens of gardens and forests  Several, like the Planty Park, Botanical Garden, Zoological Garden, Royal Garden, Park Krakowski, Jordan Park and Błonia Park are located in the centre of the city; with Zakrzówek, Lasek Wolski forest, Strzelecki Park and Park Lotników (among others) in the surrounding districts  Parks cover about 318 5 hectares (787 acres), 1 2 sq mi) of the city 
The Planty Park is the best-known park in Kraków  It was established between 1822 and 1830 in place of the old city walls, forming a green belt around the Old Town  It consists of a chain of smaller gardens designed in various styles and adorned with monuments  The park has an area of 21 hectares (52 acres) and a length of 4 kilometres (2 5 mi), forming a scenic walkway popular with Cracovians The Jordan Park founded in 1889 by Dr Henryk Jordan, was the first public park of its kind in Europe  The park built on the banks of the Rudawa river was equipped with running and exercise tracks, playgrounds, the swimming pool, amphitheatre, pavilions, and a pond for boat rowing and water bicycles  It is located on the grounds of a larger Kraków's Błonia Park  The less prominent Park Krakowski was founded in 1885 by Stanisław Rehman but has since been greatly reduced in size because of rapid real estate development  It was a popular destination point with many Cracovians at the end of the 19th century 

Environment
There are five nature reserves in Kraków with a total area of 48 6 hectares (120 acres)  Smaller green zones constitute parts of the Kraków-Częstochowa Upland Jurassic Landscape Parks' Board, which deals with the protection areas of the Polish Jura  Under its jurisdiction are: the Bielany-Tyniec Landscape Park (Park Bielańsko-Tyniecki), Tenczynek Landscape Park (Park Tencziński) and Kraków Valleys Landscape Park (Park Krajobrazowy Dolinki Krakowskie), with their watersheds  All natural reserves of the Polish Jura Chain are part of the CORINE biotopes programme due to their unique flora, fauna, geomorphology and landscape  The western part of Kraków constitutes the so-called Obszar Krakowski ecological network, including the ecological corridor of the Vistula river  The southern slopes of limestone hills provide conditions for the development of thermophilous vegetation, grasslands and scrubs 
The city is spaced along an extended latitudinal transect of the Vistula River Valley with a network of tributaries including its right tributary Wilga, and left: Rudawa, Białucha, Dłubnia and Sanka  The rivers and their valleys along with bodies of water are some of the most interesting natural wonders of Kraków 
Kraków and its environment, surrounded by mountains, suffer from Europe's dirtiest air pollution because of smog, caused by burning coal for heating, especially in winter 

Governance
The Kraków City Council has 43 elected members, one of whom is the mayor, or President of Kraków, elected every four years  The election of the City Council and of the local head of government, which takes place at the same time, is based on legislation introduced on 20 June 2002  The President of Kraków, re-elected for his fourth term in 2014, is Jacek Majchrowski  Several members of the Polish national Parliament (Sejm) are elected from the Kraków constituency  The city's official symbols include a coat of arms, a flag, a seal, and a banner 

The responsibilities of Kraków's president include drafting and implementing resolutions, enacting city bylaws, managing the city budget, employing city administrators, and preparing against floods and natural disasters  The president fulfills his duties with the help of the City Council, city managers and city inspectors  In the 1990s, the city government was reorganised to better differentiate between its political agenda and administrative functions  As a result, the Office of Public Information was created to handle inquiries and foster communication between city departments and citizens at large In 2000, the city government introduced a new long-term program called "Safer City" in cooperation with the Police, Traffic, Social Services, Fire, Public Safety, and the Youth Departments  Subsequently, the number of criminal offences dropped by 3 percent between 2000 and 2001, and the rate of detection increased by 1 4 percent to a total of 30 2 percent in the same period  The city is receiving help in carrying out the program from all educational institutions and the local media, including TV, radio and the press 

Districts
Kraków is divided into 18 administrative districts (dzielnica) or boroughs, each with a degree of autonomy within its own municipal government  Prior to March 1991, the city had been divided into four quarters which still give a sense of identity to Kraków – the towns of Podgórze, Nowa Huta, and Krowodrza which were amalgamated into the city of Kraków as it expanded, and the ancient town centre of Kraków itself 

The oldest neighborhoods of Kraków were incorporated into the city before the late-18th century  They include the Old Town (Stare Miasto), once contained within the city defensive walls and now encircled by the Planty park; the Wawel District, which is the site of the Royal Castle and the cathedral; Stradom and Kazimierz with its historic Jewish quarter, the latter originally divided into Christian and Jewish quarters; as well as the ancient town of Kleparz 
Major districts added in the 19th and 20th centuries include Podgórze, which until 1915, was a separate town on the southern bank of the Vistula, and Nowa Huta, east of the city centre, built after World War II 

Among the most notable historic districts of the city are: Wawel Hill, home to Wawel Castle and Wawel Cathedral, where many historic Polish kings are buried; the medieval Old Town, with its Main Market Square (200 metres (660 ft) square); dozens of old churches and museums; the 14th-century buildings of the Jagiellonian University; and Kazimierz, the historical centre of Kraków's Jewish social and religious life The Old Town district of Kraków is home to about 6,000 historic sites and more than 2,000,000 works of art  Its rich variety of heritage architecture includes Romanesque (e g , St  Andrew's Church, Kraków), Renaissance, Baroque and Gothic buildings  Kraków's palaces, churches, theatres and mansions display a great variety of color, architectural details, stained glass, paintings, sculptures, and furnishings In the Market Square stands the Gothic St  Mary's Basilica (Kościół Mariacki)  It was rebuilt in the 14th-century and features the famous wooden altar (Altarpiece of Veit Stoss), the largest Gothic altarpiece in the world, carved by Veit Stoss  From the church's main tower a trumpet call (hejnał mariacki), is sounded every hour  The melody, which used to announce the opening and closing of city gates, ends unexpectedly in midstream  According to legend, the tune was played during the 13th-century Tatar invasion by a guard warning citizens against the attack  He was shot by an archer of the invading Tatar forces whilst playing, the bugle call breaking off at the moment he died  The story was recounted in a book published in 1928 called The Trumpeter of Krakow, by Eric P  Kelly, which won a Newbery Award 
The current divisions were introduced by the Kraków City Hall on 19 April 1995  Districts were assigned Roman numerals as well as the name: Stare Miasto (I), Grzegórzki (II), Prądnik Czerwony (III), Prądnik Biały (IV), Łobzów (V), Bronowice (VI), Zwierzyniec (VII), Dębniki (VIII), Łagiewniki-Borek Fałęcki (IX), Swoszowice (X), Podgórze Duchackie (XI), Bieżanów-Prokocim (XII), Podgórze (XIII), Czyżyny (XIV), Mistrzejowice (XV), Bieńczyce (XVI), Wzgórza Krzesławickie (XVII), and Nowa Huta (XVIII) 

Map of districts of the City of Kraków

Economy
Kraków is one of Poland's most important economic centres and the economic hub of the Lesser Poland (Małopolska) region  Since the fall of communism, the private sector has been growing steadily  There are about 50 large multinational companies in the city, including Google, Uber, IBM, Shell, UBS, HSBC, Motorola, Aptiv, MAN, General Electric, ABB, Aon, Akamai, Cisco, Hitachi, Altria, Capgemini, and Sabre Holdings, along with other British, German and Scandinavian-based firms  The city is also the global headquarters for Comarch, an enterprise software house  Kraków is the second most-visited city in Poland (after Warsaw)  According to the World Investment Report 2011 by the UN Conference for Trade and Development (UNCTAD), Kraków is also the most emergent city location for investment in global BPO projects (Business Process Outsourcing) in the world 

In 2011, the city budget, which is presented by the Mayor of Kraków on 15 November annually, has a projected revenue of 3,500,000,000 złoty  The primary sources of revenue were as follows: 14% from the municipal taxation on real estate properties and the use of amenities, 30% in transfers from the national budget, and 34% in state subsidies  Projected expenditures, totaling 3,520,000,000 złoty, included 21% in city development costs and 79% in city maintenance costs  Of the maintenance costs, as much as 39% were spent on education and childcare  The City of Kraków's development costs included; 41% toward construction of roads, transport, and communication (combined), and 25% for the city's infrastructure and environment  The city has a high bond credit rating, and some 60% of the population is under the age of 45 Unity Tower was completed in 2020 after almost 30 years, creating a new business and residential centre  It is the tallest building in the city 

Entrepreneurship
Krakow has a long history of entrepreneurship, perhaps best reflected in the fact the most important square in the city is called the Main Market Square (Rynek Główny) 

Startup community
Since the early 2000s a startup community has emerged in Krakow, In the early days the Krakow: Europe's Silicon Valley web page was the on line hub of the community  Most important now is the OMGKRK foundation and its Facebook group which has over 5000 members and acts as a community notice board for the startup community 

Entrepreneurs
Jan Thurzo, a Hungarian entrepreneur and mining engineer who was from 1477 an Alderman and later Mayor of Kraków  He established the Fugger–Thurzo company with Jakob Fugger  Fugger monopolised copper mining and trade in the Holy Roman Empire around 1500 and has been described as the richest man who has ever lived Michal Hornstein, born in Krakow, and graduate of a Krakow Business School, escaped from a Nazi death camp transport  He moved to Montreal in 1951 where he founded Federal Construction Ltd , a real estate company focussing on apartments and shopping centres  He was recognised as a major philanthropist in Montreal and supported the arts, education and medicine, for example with this Gift of Old Masters to the Montreal Museum of Fine Arts 
Helena Rubinstein, born in Kraków, established the Helena Rubenstein inc  cosmetics company which was sold to Colgate Palmolive in 1973 for $142 3 million in stock and cash, and was said to be one of the world's richest women 
Janusz Filipiak established the successful IT company Comarch in 1993 which in 2018 employed 5500 people, and sponsors the Cracovia football team 
Piotr Wilam established the Pascal Publishing House, the internet portal Onet pl and seed capital fund Innovation Nest 
Rafal Brzoska Rafal Brzoska is the founder and CEO of InPost, which went public in January 2021 raising $3 billion 

Knowledge and innovation community
Kraków is one of the co-location centres of Knowledge and Innovation Community (Sustainable Energy) of The European Institute of Innovation and Technology (EIT) InnoEnergy is an integrated alliance of reputable organisations from the education, research and industry sectors  It was created based on long standing links of cooperation as well as the principles of excellence  The partners have jointly developed a strategy to tackle the weaknesses of the European innovation landscape in the field of sustainable energy 

Transport
Public transport is based on a fairly dense network of tram and bus routes operated by a municipal company, supplemented by a number of private minibus operators  Local trains connect some of the suburbs  The bulk of the city's historic area has been turned into a pedestrian zone with rickshaws and horse-drawn carriages; however, the trams run within a three-block radius  The historic means of transportation in the city can be examined at the Museum of Municipal Engineering in the Kazimierz district, with many old trams, cars and buses 

Railway connections are available to most Polish cities, e g  Katowice, Częstochowa, Szczecin, Gdynia and Warsaw  International destinations include Bratislava, Budapest, Vienna, Prague, Berlin, Hamburg, Lviv, Kyiv, and Odesa (June–September)  The main railway station is located just outside the Old Town District and is well-served by public transport 
Kraków's airport, officially named Kraków John Paul II International Airport (IATA: KRK), is located 11 km (7 mi) west of the city  Direct trains cover the route between Kraków Główny train station and the airport in 20 minutes  Kraków Airport served around 5,800,000 passengers in 2017  Also, the Katowice International Airport is located 80 kilometres (50 miles) or about 75 minutes from Kraków In Autumn 2016 Poland's oldest Bicycle-sharing system was modernized and now offers 1,500 bikes at 150 stations under the name of Wavelo (pl), which is owned by BikeU of the French multinational company Egis 

Demographics
Kraków had a recorded population of 774,839 in 2019  Selected demographic indicators are presented in a table (below), compiled on the basis of only the population living in Kraków permanently  The larger metropolitan area of the city encompasses a territory in which (in 2010) 1,393,893 inhabitants live Already in the Middle Ages, the population of Kraków consisting of numerous ethnic groups, began to grow rapidly  It doubled between 1100 and 1300 from 5,000 to 10,000, and in 1400 counted 14,000 inhabitants  By 1550, the population of metropolitan Kraków was 18,000; although it decreased to 15,000 in the next fifty years due to calamity  By the early 17th century the Kraków population had reached 28,000 inhabitants In the historical 1931 census preceding World War II, 78 1% of Cracovians declared Polish as their primary language, with Yiddish or Hebrew at 20 9%, Ukrainian 0 4%, German 0 3%, and Russian 0 1%  The ravages of history have greatly reduced the percentage of ethnic minorities living in Kraków 
In the last two decades, Kraków has seen a large growth of immigrant population  In the 2002 census, only 0 25% of respondents living in the city declared a non-Polish nationality primarily Ukrainian and Russian  As of 2019, it was estimated that foreigners accounted for as much as 10% of the city's population, with Ukrainians being the most numerous group (between 11,000 and 50,000) 
Population growth in Kraków since 1791

Religion
The metropolitan city of Kraków is known as the city of churches  The abundance of landmark, historic temples along with the plenitude of monasteries and convents earned the city a countrywide reputation as the "Northern Rome" in the past  The churches of Kraków comprise over 120 places of worship (2007) of which over 65 were built in the 20th century  More are still being added  In addition to Roman Catholicism, other denominations present include Jehovah's Witnesses, Mariavite Church, Polish Catholic Church, Polish Orthodox Church, Protestantism and Latter-Day Saints As of 2017, weekly Mass attendance in the Archdiocese of Krakow was 49 9 percent, above the national Polish average of 38 3 percent 

Kraków contains also an outstanding collection of monuments of Jewish sacred architecture unmatched anywhere in Poland  Kraków was an influential centre of Jewish spiritual life before the outbreak of World War II, with all its manifestations of religious observance from Orthodox to Hasidic and Reform flourishing side by side  There were at least 90 synagogues in Kraków active before the Nazi German invasion of Poland, serving its burgeoning Jewish community of 60,000–80,000 (out of the city's total population of 237,000), established since the early 12th century Most synagogues of Kraków were ruined during World War II by the Nazis who despoiled them of all ceremonial objects, and used them as storehouses for ammunition, firefighting equipment, as general storage facilities and stables  The post-Holocaust Jewish population of the city had dwindled to about 5,900 before the end of the 1940s  Poland was the only Eastern Bloc country to allow free Jewish aliyah (emigration to Israel) without visas or exit permits upon the conclusion of World War II  By contrast, Stalin forcibly kept Russian Jews in the Soviet Union, as agreed to in the Yalta Conference  In recent time, thanks to efforts of the local Jewish and Polish organisations including foreign financial aid from the American Jewish Joint Distribution Committee, many synagogues underwent major restorations and serve religious and tourist purposes 

Education
Kraków is a major centre of education  Twenty-four institutions of higher education offer courses in the city, with more than 200,000 students  Jagiellonian University, the oldest university in Poland and ranked by the Times Higher Education Supplement as the second-best university in the country, was founded in 1364 as Studium Generale and renamed in 1817 to commemorate the royal Jagiellonian dynasty of Poland and Lithuania  Its principal academic asset is the Jagiellonian Library, with more than 4 million volumes, including a large collection of medieval manuscripts like Copernicus' De Revolutionibus and the Balthasar Behem Codex  With 42,325 students (2005) and 3,605 academic staff, the Jagiellonian University is also one of the leading research centres in Poland  Famous historical figures connected with the university include Saint John Cantius, Jan Długosz, Nicolaus Copernicus, Andrzej Frycz Modrzewski, Jan Kochanowski, King John III Sobieski, Pope John Paul II and Nobel laureates Ivo Andrić and Wisława Szymborska AGH University of Science and Technology, established in 1919, is the largest technical university in Poland, with more than 15 faculties and student enrollment exceeding 30,000  It was ranked by the Polish edition of Newsweek as the best technical university in the country in 2004  During its 80-year history, more than 73,000 students graduated from AGH with master's or bachelor's degrees  Some 3,600 persons were granted the degree of Doctor of Science, and about 900 obtained the qualification of Habilitated Doctor 

Other institutions of higher learning include Academy of Music in Kraków first conceived as conservatory in 1888, one of the oldest and most prestigious conservatories in Central Europe and a major concert venue; Kraków University of Economics, established in 1925; Pedagogical University, in operation since 1946; Agricultural University of Kraków, offering courses since 1890 (initially as a part of Jagiellonian University); Academy of Fine Arts, the oldest Fine Arts Academy in Poland, founded by the Polish painter Jan Matejko; Ludwik Solski Academy for the Dramatic Arts; The Pontifical Academy of Theology; AGH University of Science and Technology and Krakow University of Technology, which has more than 37,000 graduates 
Scientific societies and their branches in Kraków conduct scientific and educational work in local and countrywide scale  The Academy of Learning, Krakow Scientific Society, Association of Law Students' Library of the Jagiellonian University, Polish Copernicus Society of Naturalists, Polish Geological Society, Polish Theological Society in Kraków, Polish Section of Institute of Electrical and Electronics Engineers and Polish Society for Synchrotron Radiation all have their main seats in Kraków 

Culture
Kraków was named the official European Capital of Culture for the year 2000 by the European Union  It is a major attraction for both local and international tourists, attracting nearly 13 million visitors a year  Major landmarks include the Main Market Square with St  Mary's Basilica and the Sukiennice Cloth Hall, the Wawel Castle, the National Art Museum, the Zygmunt Bell at the Wawel Cathedral, and the medieval St  Florian's Gate with the Barbican along the Royal Coronation Route  Kraków has 28 museums and public art galleries  Among them is the Czartoryski Museum featuring works by Leonardo da Vinci and Rembrandt as well as the EUROPEUM - European Culture Centre and the Archaeological Museum of Kraków whose collection highlights include the Zbruch Idol and the Bronocice Pot 

Museums and national art galleries
Kraków's 28 museums are separated into the national and municipal museums; the city also has a number of art collections and public art galleries  The National Museum, established in 1879, as well as the National Art Collection on Wawel Hill, are all accessible to the general public 
The National Art Collection is located at the Wawel, the former residence of three dynasties of Polish monarchs  Royal Chambers feature art, period furniture, Polish and European paintings, collectibles, and a major collection of 16th-century monumental Flemish tapestries  Wawel Treasury and Armoury features Polish royal memorabilia, jewels, applied art, and 15th- to 18th-century arms  The Wawel Eastern Collection features Turkish tents and military accessories  The National Museum holds the largest body of artworks in the country with collections consisting of several hundred thousand items kept in big part in the Main Building at Ul  3 Maja, although there are eleven other separate divisions of the museum in the city, one of the most popular being The Gallery of the 19th Century Polish Art in Sukiennice with the collection of some of the best known paintings and sculptures of the Young Poland movement  The latest division called Europeum with Brueghel among a hundred Western European paintings was inaugurated in 2013 

Other notable museums in Kraków include the Manggha Museum of Japanese Art and Technology (at M  Konopnickiej 26), Stanisław Wyspiański Museum (at 11 Szczepanska St), Jan Matejko Manor House in Krzesławice, – a museum devoted to the master painter and his life, Emeryk Hutten Czapski Museum, and Józef Mehoffer Manor The Rynek Underground museum, under the main square, showcases Kraków's over 1,000-year history though its streets, activities and artifacts  The construction of the museum was preceded by extensive excavations which started in 2005 and, as more and more was found, continued eventually until 2010 
Krakil - Museum of illusions is a space where illusions meet scientific inventions and the arts: physics and optics are displayed together with artworks and classical riddles 
The Polish Aviation Museum, considered the world's eighth best aviation museum by CNN, features over 200 aircraft including a Sopwith Camel among other First World War biplanes; a comprehensive display of aero engines; and a complete collection of airplane types developed by Poland after 1945  Activities of smaller museums around Kraków and in the Lesser Poland region are promoted and supported by the Małopolska Institute of Culture; the Institute organises annual Małopolska Heritage Days  The Lenin Museum was open from 1954 to 1989 

Performing arts
The city has several famous theatres, including the Narodowy Stary Teatr (the National Old Theatre), the Juliusz Słowacki Theatre, the Bagatela Theatre, the Ludowy Theatre, and the Groteska Theatre of Puppetry, as well as the Opera Krakowska and Kraków Operetta  The city's principal concert hall and the home of the Kraków Philharmonic Orchestra is the Kraków Philharmonic (Filharmonia Krakowska) built in 1931 Kraków hosts many annual and biannual artistic events, some of international significance such as the Misteria Paschalia (Baroque music), Sacrum-Profanum (contemporary music), the Krakow Screen Festival (popular music), the Festival of Polish Music (classical music), Dedications (theatre), the Kraków Film Festival (one of Europe's oldest short films events), Etiuda&Anima International Film Festival (the oldest international art-film event in Poland), Biennial of Graphic Arts, and the Jewish Culture Festival  Kraków was the residence of two Polish Nobel laureates in literature, Wisława Szymborska and Czesław Miłosz; a third Nobel laureate, the Yugoslav writer Ivo Andric, lived and studied in Kraków  Other former longtime residents include internationally renowned Polish film directors Andrzej Wajda and Roman Polanski, both of whom are Academy Award winners 

Music
Opera Krakowska one of the leading national opera companies, stages 200 performances each year including ballet, operettas and musicals  It has, in its main repertoire, the greatest world and Polish opera classics  The Opera moved into its first permanent House in the autumn of 2008  It is in charge also of the Summer Festival of Opera and Operetta 
Kraków is home to two major Polish festivals of early music presenting forgotten Baroque oratorios and operas: Opera Rara, and Misteria Paschalia  Meanwhile, Capella Cracoviensis runs the Music in Old Krakow International Festival 
Academy of Music in Kraków, founded in 1888, is known worldwide as the alma mater of the contemporary Polish composer Krzysztof Penderecki and it is also the only one in Poland to have two winners of the International Chopin Competition in Warsaw among its alumni  The academy organises concerts of its students and guests throughout the whole year Music organisations and venues include: Kraków Philharmonic, Sinfonietta Cracovia (a k a  the Orchestra of the Royal City of Kraków), the Polish Radio Choir of Kraków, Organum Academic Choir, the Mixed Mariański Choir (Mieszany Chór Mariański), Kraków Academic Choir of the Jagiellonian University, the Kraków Chamber Choir, Amar Corde String Quartet, Consortium Iagellonicum Baroque Orchestra of the Jagiellonian University, Brass Band of T  Sendzimir Steelworks, and Camerata Chamber Orchestra of Radio Kraków 

Tourism
According to official statistics, in 2019 Kraków was visited by over 14 million tourists including 3 3 million foreign travellers  The visitors spent over 7 5 billion zlotys (ca  €1 7 billion) in the city (without travel costs and pre-booked accommodation)  Most foreign tourists came from Germany (14 2%), United Kingdom (13 9%), Italy (11 5%), France (11 2%), Spain (10 4%) and Ukraine (5 4%)  The Kraków tour-guide from the Lesser Poland Visitors Bureau indicated that not all statistics are recorded due to the considerable number of those who come, staying in readily available private rooms paid for by cash, especially from Eastern Europe The main reasons for visiting the city are: its historical monuments, recreation as well as relatives and friends (placing third in the ranking), religion and business  There are 120 quality hotels in Kraków (usually about half full) offering 15,485 overnight accommodations  The average stay last for about 4 to 7 nights  The survey conducted among the travelers showed that they enjoyed the city's friendliness most, with 90% of Polish tourists and 87% foreigners stating that they would recommend visiting it  Notable points of interest outside the city include the Wieliczka salt mine, the Tatra Mountains 100 km (62 mi) to the south, the historic city of Częstochowa (north-west), the well-preserved former Nazi concentration camp at Auschwitz, and Ojcowski National Park, which includes the Renaissance Castle at Pieskowa Skała  Kraków has been awarded a number of top international rankings such as the 1st place in the Top city-break destinations 2014 survey conducted by the British consumer association Which? 

Sports
Kraków was the host city of the 2014 FIVB Men's Volleyball World Championship and 2016 European Men's Handball Championship  It has also been selected as the European City of Sport for 2014 Football is one of the most popular sports in the city  The two teams with the largest following are thirteen-time Polish champion Wisła Kraków, and five-time champion Cracovia, both founded in 1906 as the oldest still existing in Poland  They have been involved in the most intense rivalry in the country and one of the most intense in all of Europe, known as the Holy War (Święta Wojna)  Other football clubs include Hutnik Kraków, Wawel Kraków, Wieczysta Kraków and one-time Polish champion Garbarnia Kraków  There is also the first-league rugby club Juvenia Kraków  Kraków has a number of additional, equally valued sports teams including twelve-time Polish ice hockey champions Cracovia and the twenty-time women's basketball champions Wisła Kraków 

The Cracovia Marathon, with thousands of participants from two dozen countries annually, has been held in the city since 2002  Poland's first F1 racing driver Robert Kubica was born and brought up in Kraków, as was former WWE tag team champion Ivan Putski, and Top 10 ranked women's tennis player Agnieszka Radwańska 
The construction of a new Tauron Arena Kraków began in May 2010; for concerts, indoor athletics, hockey, basketball, futsal and other events  The facility area has 61,434 m2, with maximum area of the arena court of 4 546 m2  The average capacity is 18,000 for concerts, and 15,000 for sport events, with maximum number of spectators being 22,000  The Arena boasts Poland's largest LED media façade, with a total surface of 5,200 m2 of LED strip lighting, wrapping around the stadium, and one of Europe's largest LED screens, measuring over 540 m2 Kraków was bidding to host the 2022 Winter Olympics with Jasná but the bid was rejected by a majority (69 72%) of the vote in a referendum on 16 May 2014  The referendum was organised after a wave of criticism from citizens who believed that the Olympics would not promote the city  The organizing committee of "Krakow 2022" spent almost $40,000 to pay for a citizen-approved logo, but many citizens considered this a waste of public money  The committee was rumoured to have fraudulently used several million zlotys for unknown expenses 
In May 2019, the Polish Olympic Committee announced Kraków as host of the Polish bid for the 2023 European Games, On 22 June 2019, The European Olympic Committees at the General Assembly in Minsk, Belarus announced that Kraków will host the 2023 edition 

Notable people
International relations
Contemporary foreign names for the city
Kraków is referred to by various names in different languages  An old English name for the city is Cracow; though it has become less common in recent decades, some sources still use it  The city is known in Czech, Slovak and Serbian as Krakov, in Hungarian as Krakkó, in Lithuanian as Krokuva, in Finnish as Krakova, in German and Dutch as Krakau, in Latin, Spanish and Italian as Cracovia, in French as Cracovie, in Portuguese as Cracóvia and in Russian as Краков  Ukrainian and Yiddish languages refer to it as Krakiv (Краків) and Kroke (קראָקע) respectively 

Twin towns and sister cities
Kraków is twinned, or maintains close relations, with 36 cities around the world:

See also
Cracow Circle Thomism
Tourism in Poland
Context 2: Passage 1:
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60  It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals  It is a hard, slightly malleable, silvery metal that quickly tarnishes in air and moisture  When oxidized, neodymium reacts quickly producing pink, purple/blue and yellow compounds in the +2, +3 and +4 oxidation states  It is generally regarded as having one of the most complex spectra of the elements  Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach, who also discovered praseodymium  It is present in significant quantities in the minerals monazite and bastnäsite  Neodymium is not found naturally in metallic form or unmixed with other lanthanides, and it is usually refined for general use  Neodymium is fairly common—about as common as cobalt, nickel, or copper—and is widely distributed in the Earth's crust  Most of the world's commercial neodymium is mined in China, as is the case with many other rare-earth metals 
Neodymium compounds were first commercially used as glass dyes in 1927 and remain a popular additive  The color of neodymium compounds comes from the Nd3+ ion and is often a reddish-purple  However, this changes with the type of lighting because of the interaction of the sharp light absorption bands of neodymium with ambient light enriched with the sharp visible emission bands of mercury, trivalent europium or terbium  Neodymium-doped glasses are used in lasers that emit infrared with wavelengths between 1047 and 1062 nanometers  These lasers have been used in extremely high-power applications, such as experiments in inertial confinement fusion  Neodymium is also used with various other substrate crystals, such as yttrium aluminium garnet in the Nd:YAG laser 
Neodymium alloys are used to make high-strength neodymium magnets—a type of powerful permanent magnet  These magnets are widely used in products like microphones, professional loudspeakers, in-ear headphones, high-performance hobby DC electric motors, and computer hard disks, where low magnet mass (or volume) or strong magnetic fields are required  Larger neodymium magnets are used in electric motors with a high power-to-weight ratio (e g , in hybrid cars) and generators (e g , aircraft and wind turbine electric generators) 

Characteristics
Physical properties
Metallic neodymium has a bright, silvery metallic luster  Neodymium commonly exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C  Neodymium, like most of the lanthanides, is paramagnetic at room temperature and becomes an antiferromagnet upon cooling to 20 K (−253 2 °C)  Neodymium is a rare-earth metal that was present in the classical mischmetal at a concentration of about 18%  To make neodymium magnets it is alloyed with iron, which is a ferromagnet 

Electron configuration
In the periodic table, it appears between the lanthanides praseodymium to its left and the radioactive element promethium to its right, and above the actinide uranium  Its 60 electrons are arranged in the configuration [Xe]4f46s2, of which the six 4f and 6s electrons are valence  Like most other metals in the lanthanide series, neodymium usually only uses three electrons as valence electrons, as afterwards the remaining 4f electrons are strongly bound: this is because the 4f orbitals penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this increases with higher ionic charge  Neodymium can still lose a fourth electron because it comes early in the lanthanides, where the nuclear charge is still low enough and the 4f subshell energy high enough to allow the removal of further valence electrons 

Chemical properties
Neodymium is the fourth member of the lanthanide series  It has a melting point of 1,024 °C (1,875 °F) and a boiling point of 3,074 °C (5,565 °F)  Neodymium, like other lanthanides, usually has the oxidation state +3, but it can also form in the +2 and +4 oxidation states, and even, in very rare conditions, +0  Neodymium metal quickly oxidizes at ambient conditions, forming an oxide layer like iron rust that spalls off and exposes the metal to further oxidation; a centimeter-sized sample of neodymium corrodes completely in about a year  Nd3+ is generally soluble in water  Like its neighbor praseodymium, it readily burns at about 150 °C to form neodymium(III) oxide; the oxide peels off, exposing the bulk metal to the further oxidation:
4Nd + 3O2 → 2Nd2O3Neodymium is a quite electropositive element, and it reacts slowly with cold water, or quickly with hot water, to form neodymium(III) hydroxide:

2Nd (s) + 6H2O (l) → 2Nd(OH)3 (aq) + 3H2 (g)Neodymium metal reacts vigorously with all the stable halogens:
2Nd (s) + 3F2 (g) → 2NdF3 (s) [a violet substance]
2Nd (s) + 3Cl2 (g) → 2NdCl3 (s) [a mauve substance]
2Nd (s) + 3Br2 (g) → 2NdBr3 (s) [a violet substance]
2Nd (s) + 3I2 (g) → 2NdI3 (s) [a green substance]Neodymium dissolves readily in dilute sulfuric acid to form solutions that contain the lilac Nd(III) ion  These exist as a [Nd(OH2)9]3+ complexes:
2Nd (s) + 3H2SO4 (aq) → 2Nd3+ (aq) + 3SO2−4 (aq) + 3H2 (g)

Compounds
Some of the most important neodymium compounds include:

halides: NdF3; NdCl2; NdCl3; NdBr3; NdI2; NdI3
oxides: Nd2O3
hydroxide: Nd(OH)3
carbonate: Nd2(CO3)3
sulfate: Nd2(SO4)3
acetate: Nd(CH3COO)3
neodymium magnets (Nd2Fe14B)Some neodymium compounds have colors that vary based on the type of lighting 

Organoneodymium compounds
Organoneodymium compounds are compounds that have a neodymium–carbon bond  These compounds are similar to those of the other lanthanides, characterized by an inability to undergo π backbonding  They are thus mostly restricted to the mostly ionic cyclopentadienides (isostructural with those of lanthanum) and the σ-bonded simple alkyls and aryls, some of which may be polymeric 

Isotopes
Naturally occurring neodymium (60Nd) is composed of five stable isotopes—142Nd, 143Nd, 145Nd, 146Nd and 148Nd, with 142Nd being the most abundant (27 2% of the natural abundance)—and two radioisotopes with extremely long half-lives, 144Nd (alpha decay with a half-life (t1/2) of 2 29×1015 years) and 150Nd (double beta decay, t1/2 ≈ 7×1018 years)  In all, 33 radioisotopes of neodymium have been detected as of 2022, with the most stable radioisotopes being the naturally occurring ones: 144Nd and 150Nd  All of the remaining radioactive isotopes have half-lives that are shorter than twelve days, and the majority of these have half-lives that are shorter than 70 seconds; the most stable artificial isotope is 147Nd with a half-life of 10 98 days 
Neodymium also has 13 known metastable isotopes, with the most stable one being 139mNd (t1/2 = 5 5 hours), 135mNd (t1/2 = 5 5 minutes) and 133m1Nd (t1/2 ~70 seconds)  The primary decay modes before the most abundant stable isotope, 142Nd, are electron capture and positron decay, and the primary mode after is beta minus decay  The primary decay products before 142Nd are element Pr (praseodymium) isotopes and the primary products after are element Pm (promethium) isotopes  Four of the five stable isotopes have been predicted to decay to isotopes of cerium or samarium and are only observationally stable  Additionally, some observationally stable isotopes of samarium are predicted to decay to isotopes of neodymium Neodymium isotopes are used in various scientific applications  142Nd has been used for the production of short-lived Tm and Yb isotopes  146Nd has been suggested for the production of 147Pm, which is a source of radioactive power  Several neodymium isotopes have been used for the production of other promethium isotopes  The decay from 147Sm (t1/2 = 1 06×1011 y) to the stable 143Nd allows samarium–neodymium dating  150Nd has also been used to study double beta decay 

History
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite  Thirty years later, fifteen-year-old Wilhelm Hisinger, a member of the family owning the mine, sent a sample to Carl Scheele, who did not find any new elements within  In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide, which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier  Ceria was simultaneously and independently isolated in Germany by Martin Heinrich Klaproth  Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius; he separated out two other oxides, which he named lanthana and didymia  He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid  The metals that formed these oxides were thus named lanthanum and didymium, officially discovered in Vienna in 1885 by Carl Gustaf Mosander  Von Welsbach confirmed the separation by spectroscopic analysis, but the products were of relatively low purity  Didymium was discovered by Carl Gustaf Mosander in 1841, and pure neodymium was isolated from it in 1925  The name neodymium is derived from the Greek words neos (νέος), new, and didymos (διδύμος), twin Double nitrate crystallization was the means of commercial neodymium purification until the 1950s  Lindsay Chemical Division was the first to commercialize large-scale ion-exchange purification of neodymium  Starting in the 1950s, high purity (>99%) neodymium was primarily obtained through an ion exchange process from monazite, a mineral rich in rare-earth elements  The metal is obtained through electrolysis of its halide salts  Currently, most neodymium is extracted from bastnäsite and purified by solvent extraction  Ion-exchange purification is used for the highest purities (typically >99 99%)  The evolving technology, and improved purity of commercially available neodymium oxide, was reflected in the appearance of neodymium glasses in collections today  Early neodymium glasses made in the 1930s have a more reddish or orange tinge than modern versions, which are more cleanly purple, because of the difficulties in removing traces of praseodymium using early technology, namely fractional crystallization Because of its role in permanent magnets used for direct-drive wind turbines, it has been argued that neodymium will be one of the main objects of geopolitical competition in a world running on renewable energy  This perspective has been criticised for failing to recognise that most wind turbines do not use permanent magnets, and for underestimating the power of economic incentives for expanded production 

Occurrence and production
Occurrence
Neodymium is rarely found in nature as a free element, instead occurring as ores, such as monazite and bastnäsite (these are mineral group names rather than single mineral names) that contain small amounts of all rare-earth metals  In these minerals neodymium is rarely dominant; some exceptions include monazite-(Nd) and kozoite-(Nd)  The main mining areas are in China, United States, Brazil, India, Sri Lanka, and Australia  World reserves of neodymium are estimated at eight million tonnes The Nd3+ ion is similar in size to the early lanthanides of the cerium group (those from lanthanum up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (MIIIPO4) and bastnäsite (MIIICO3F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y, with somewhat less Pr and Nd)  Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved  The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride  The product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution 

In space
Neodymium's per-particle abundance in the Solar System is 0 083 ppb (parts per billion)  This figure is about two thirds of that of platinum, but two and a half times more than mercury, and nearly five times more than gold  The lanthanides are not usually found in space, and are much more abundant in the Earth's crust 

In the Earth's crust
Neodymium is classified as a lithophile under the Goldschmidt classification, meaning that it is generally found combined with oxygen  Although it belongs to the rare-earth metals, neodymium is not rare at all  Its abundance in the Earth's crust is about 38 mg/kg, making it the 27th most common element  It is similar in abundance to lanthanum  Cerium is the most common rare-earth metal, followed by neodymium, and then lanthanum 

Production
The world's production of neodymium was about 7,000 tons in 2004  The bulk of current production is from China  Historically, the Chinese government imposed strategic material controls on the element, causing large fluctuations in prices  The uncertainty of pricing and availability have caused companies (particularly Japanese ones) to create permanent magnets and associated electric motors with fewer rare-earth metals; however, so far they have been unable to eliminate the need for neodymium  According to the US Geological Survey, Greenland holds the largest reserves of undeveloped rare-earth deposits, particularly neodymium  Mining interests clash with native populations at those sites, due to the release of radioactive substances during the mining process 

Neodymium is typically 10–18% of the rare-earth content of commercial deposits of the light rare-earth-element minerals bastnäsite and monazite  With neodymium compounds being the most strongly colored for the trivalent lanthanides, it can occasionally dominate the coloration of rare-earth minerals when competing chromophores are absent  It usually gives a pink coloration  Outstanding examples of this include monazite crystals from the tin deposits in Llallagua, Bolivia; ancylite from Mont Saint-Hilaire, Quebec, Canada; or lanthanite from the Saucon Valley, Pennsylvania, United States  As with neodymium glasses, such minerals change their colors under the differing lighting conditions  The absorption bands of neodymium interact with the visible emission spectrum of mercury vapor, with the unfiltered shortwave UV light causing neodymium-containing minerals to reflect a distinctive green color  This can be observed with monazite-containing sands or bastnäsite-containing ore The demand for mineral resources, such as rare-earth elements (including neodymium) and other critical materials, has been rapidly increasing owing to the growing human population and industrial development  Recently, the requirement for a low-carbon society has led to a significant demand for energy-saving technologies such as batteries, high-efficiency motors, renewable energy sources, and fuel cells  Among these technologies, permanent magnets are often used to fabricate high-efficiency motors, with neodymium-iron-boron magnets (Nd2Fe14B sintered and bonded magnets; hereinafter referred to as NdFeB magnets) being the main type of permanent magnet in the market since their invention  NdFeB magnets are used in hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs), and fuel cell vehicles (FCVs) (hereinafter referred to as xEVs), wind turbines, home appliances, computers, and many small consumer electronic devices  Furthermore, they are indispensable for energy savings  Toward achieving the objectives of the Paris Agreement, the demand for NdFeB magnets is expected to increase significantly in the future 

Applications
Neodymium has an unusually large specific heat capacity at liquid-helium temperatures, so is useful in cryocoolers 
Neodymium acetate can be a substitute for the radioactive and toxic uranyl acetate (used as a standard contrasting agent in electron microscopy) 
Probably because of similarities to Ca2+, Nd3+ has been reported to promote plant growth  Rare-earth element compounds are frequently used in China as fertilizer 
Samarium–neodymium dating is useful for determining the age relationships of rocks  and meteorites
Neodymium isotopes recorded in marine sediments are used to reconstruct changes in past ocean circulation 

Magnets
Neodymium magnets (an alloy, Nd2Fe14B) are the strongest permanent magnets known  A neodymium magnet of a few tens of grams can lift a thousand times its own weight, and can snap together with enough force to break bones  These magnets are cheaper, lighter, and stronger than samarium–cobalt magnets  However, they are not superior in every aspect, as neodymium-based magnets lose their magnetism at lower temperatures and tend to corrode, while samarium–cobalt magnets do not Neodymium magnets appear in products such as microphones, professional loudspeakers, headphones, guitar and bass guitar pick-ups, and computer hard disks where low mass, small volume, or strong magnetic fields are required  Neodymium is used in the electric motors of hybrid and electric automobiles and in the electricity generators of some designs of commercial wind turbines (only wind turbines with "permanent magnet" generators use neodymium)  For example, drive electric motors of each Toyota Prius require one kilogram (2 2 pounds) of neodymium per vehicle In 2020, physics researchers at Radboud University and Uppsala University announced they had observed a behavior known as "self-induced spin glass" in the atomic structure of neodymium  One of the researchers explained, "…we are specialists in scanning tunneling microscopy  It allows us to see the structure of individual atoms, and we can resolve the north and south poles of the atoms  With this advancement in high-precision imaging, we were able to discover the behavior in neodymium, because we could resolve the incredibly small changes in the magnetic structure "  Neodymium behaves in a complex magnetic way that had not been seen before in a periodic table element 

Glass
Neodymium glass (Nd:glass) is produced by the inclusion of neodymium oxide (Nd2O3) in the glass melt  Usually in daylight or incandescent light neodymium glass appears lavender, but it appears pale blue under fluorescent lighting  Neodymium may be used to color glass in delicate shades ranging from pure violet through wine-red and warm gray The first commercial use of purified neodymium was in glass coloration, starting with experiments by Leo Moser in November 1927  The resulting "Alexandrite" glass remains a signature color of the Moser glassworks to this day  Neodymium glass was widely emulated in the early 1930s by American glasshouses, most notably Heisey, Fostoria ("wisteria"), Cambridge ("heatherbloom"), and Steuben ("wisteria"), and elsewhere (e g  Lalique, in France, or Murano)  Tiffin's "twilight" remained in production from about 1950 to 1980  Current sources include glassmakers in the Czech Republic, the United States, and China The sharp absorption bands of neodymium cause the glass color to change under different lighting conditions, being reddish-purple under daylight or yellow incandescent light, but blue under white fluorescent lighting, or greenish under trichromatic lighting  This color-change phenomenon is highly prized by collectors  In combination with gold or selenium, red colors are produced  Since neodymium coloration depends upon "forbidden" f-f transitions deep within the atom, there is relatively little influence on the color from the chemical environment, so the color is impervious to the thermal history of the glass  However, for the best color, iron-containing impurities need to be minimized in the silica used to make the glass  The same forbidden nature of the f-f transitions makes rare-earth colorants less intense than those provided by most d-transition elements, so more has to be used in a glass to achieve the desired color intensity  The original Moser recipe used about 5% of neodymium oxide in the glass melt, a sufficient quantity such that Moser referred to these as being "rare-earth doped" glasses  Being a strong base, that level of neodymium would have affected the melting properties of the glass, and the lime content of the glass might have had to be adjusted accordingly Light transmitted through neodymium glasses shows unusually sharp absorption bands; the glass is used in astronomical work to produce sharp bands by which spectral lines may be calibrated  Another application is the creation of selective astronomical filters to reduce the effect of light pollution from sodium and fluorescent lighting while passing other colours, especially dark red hydrogen-alpha emission from nebulae  Neodymium is also used to remove the green color caused by iron contaminants from glass 

Neodymium is a component of "didymium" (referring to mixture of salts of neodymium and praseodymium) used for coloring glass to make welder's and glass-blower's goggles; the sharp absorption bands obliterate the strong sodium emission at 589 nm  The similar absorption of the yellow mercury emission line at 578 nm is the principal cause of the blue color observed for neodymium glass under traditional white-fluorescent lighting  Neodymium and didymium glass are used in color-enhancing filters in indoor photography, particularly in filtering out the yellow hues from incandescent lighting  Similarly, neodymium glass is becoming widely used more directly in incandescent light bulbs  These lamps contain neodymium in the glass to filter out yellow light, resulting in a whiter light which is more like sunlight  During World War I, didymium mirrors were reportedly used to transmit Morse Code across battlefields  Similar to its use in glasses, neodymium salts are used as a colorant for enamels 

Lasers
Certain transparent materials with a small concentration of neodymium ions can be used in lasers as gain media for infrared wavelengths (1054–1064 nm), e g  Nd:YAG (yttrium aluminium garnet), Nd:YAP (yttrium aluminium perovskite), Nd:YLF (yttrium lithium fluoride), Nd:YVO4 (yttrium orthovanadate), and Nd:glass  Neodymium-doped crystals (typically Nd:YVO4) generate high-powered infrared laser beams which are converted to green laser light in commercial DPSS hand-held lasers and laser pointers 

Trivalent neodymium ion Nd3+ was the first lanthanide from rare-earth elements used for the generation of laser radiation  The Nd:CaWO4 laser was developed in 1961  Historically, it was the third laser which was put into operation (the first was ruby, the second the U3+:CaF laser)  Over the years the neodymium laser became one of the most used lasers for application purposes  The success of the Nd3+ ion lies in the structure of its energy levels and in the spectroscopic properties suitable for the generation of laser radiation  In 1964 Geusic et al  demonstrated the operation of neodymium ion in YAG matrix Y3Al5O12  It is a four-level laser with lower threshold and with excellent mechanical and temperature properties  For optical pumping of this material it is possible to use non-coherent flashlamp radiation or a coherent diode beam 

The current laser at the UK Atomic Weapons Establishment (AWE), the HELEN (High Energy Laser Embodying Neodymium) 1-terawatt neodymium-glass laser, can access the midpoints of pressure and temperature regions and is used to acquire data for modeling on how density, temperature, and pressure interact inside warheads  HELEN can create plasmas of around 106 K, from which opacity and transmission of radiation are measured Neodymium glass solid-state lasers are used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion  Nd:glass lasers are usually frequency tripled to the third harmonic at 351 nm in laser fusion devices 

Substitute for uranyl acetate
Uranyl acetate has been the standard contrasting agent in transmission electron microscopy (TEM) for decades  However, its use is increasingly hampered by regulations by governments due to its radioactive properties as well as its high toxicity  Therefore, alternatives are being searched for, including lanthanide acetates or platinum blue  as well as the use of less defined substances such as oolong tea extract  Despite these published alternatives, uranyl acetate (UAc) is still the standard for EM contrasting In the periodic table the vertical ordering of elements in groups is based on the presence of the same number of electrons in their outermost shell, which determines their chemical and physical properties  Because neodymium (Nd) is right above uranium (U) the chemical properties of UAc and NdAc would be very similar in binding to tissue in ultrathin sections thus leading to a similar amount of contrast 

Biological role and precautions
The early lanthanides have been found to be essential to some methanotrophic bacteria living in volcanic mudpots, such as Methylacidiphilum fumariolicum: lanthanum, cerium, praseodymium, and neodymium are about equally effective  Neodymium is otherwise not known to have a biological role in any other organisms Neodymium metal dust is combustible and therefore an explosion hazard  Neodymium compounds, as with all rare-earth metals, are of low to moderate toxicity; however, its toxicity has not been thoroughly investigated  Neodymium salts are regarded as more toxic if they are soluble than if they are insoluble if they are ingested  Neodymium dust and salts are very irritating to the eyes and mucous membranes, and moderately irritating to skin  Breathing the dust can cause lung embolisms, and accumulated exposure damages the liver  Neodymium also acts as an anticoagulant, especially when given intravenously Neodymium magnets have been tested for medical uses such as magnetic braces and bone repair, but biocompatibility issues have prevented widespread application  Commercially available magnets made from neodymium are exceptionally strong and can attract each other from large distances  If not handled carefully, they come together very quickly and forcefully, causing injuries  For example, there is at least one documented case of a person losing a fingertip when two magnets he was using snapped together from 50 cm away Another risk of these powerful magnets is that if more than one magnet is ingested, they can pinch soft tissues in the gastrointestinal tract  This has led to an estimated 1,700 emergency room visits and necessitated the recall of the Buckyballs line of toys, which were construction sets of small neodymium magnets 

See also
Neodymium compounds
Category:Neodymium compounds
Lanthanides
Period 6 elements
Rare earth metals

Notes
Passage 2:
James Chadwick
Sir James Chadwick,  (20 October 1891 – 24 July 1974) was a British physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932  In 1941, he wrote the final draft of the MAUD Report, which inspired the U S  government to begin serious atom bomb research efforts  He was the head of the British team that worked on the Manhattan Project during World War II  He was knighted in Britain in 1945 for his achievements in physics 
Chadwick graduated from the Victoria University of Manchester in 1911, where he studied under Ernest Rutherford (known as the "father of nuclear physics")  At Manchester, he continued to study under Rutherford until he was awarded his MSc in 1913  The same year, Chadwick was awarded an 1851 Research Fellowship from the Royal Commission for the Exhibition of 1851  He elected to study beta radiation under Hans Geiger in Berlin  Using Geiger's recently developed Geiger counter, Chadwick was able to demonstrate that beta radiation produced a continuous spectrum, and not discrete lines as had been thought  Still in Germany when World War I broke out in Europe, he spent the next four years in the Ruhleben internment camp 
After the war, Chadwick followed Rutherford to the Cavendish Laboratory at the University of Cambridge, where Chadwick earned his Doctor of Philosophy degree under Rutherford's supervision from Gonville and Caius College, Cambridge, in June 1921  He was Rutherford's assistant director of research at the Cavendish Laboratory for over a decade at a time when it was one of the world's foremost centres for the study of physics, attracting students like John Cockcroft, Norman Feather, and Mark Oliphant  Chadwick followed his discovery of the neutron by measuring its mass  He anticipated that neutrons would become a major weapon in the fight against cancer  Chadwick left the Cavendish Laboratory in 1935 to become a professor of physics at the University of Liverpool, where he overhauled an antiquated laboratory and, by installing a cyclotron, made it an important centre for the study of nuclear physics 
During the Second World War, Chadwick carried out research as part of the Tube Alloys project to build an atom bomb, while his Manchester lab and environs were harassed by Luftwaffe bombing  When the Quebec Agreement merged his project with the American Manhattan Project, he became part of the British Mission, and worked at the Los Alamos Laboratory and in Washington, D C  He surprised everyone by earning the almost-complete trust of project director Leslie R  Groves, Jr  For his efforts, Chadwick received a knighthood in the New Year Honours on 1 January 1945  In July 1945, he viewed the Trinity nuclear test  After this, he served as the British scientific advisor to the United Nations Atomic Energy Commission  Uncomfortable with the trend toward Big Science, he became the Master of Gonville and Caius College in 1948  He retired in 1959 

Education and early life
James Chadwick was born in Bollington, Cheshire, on 20 October 1891, the first child of John Joseph Chadwick, a cotton spinner, and Anne Mary Knowles, a domestic servant  He was named James after his paternal grandfather  In 1895, his parents moved to Manchester, leaving him in the care of his maternal grandparents  He went to Bollington Cross Primary School, and was offered a scholarship to Manchester Grammar School, which his family had to turn down as they could not afford the small fees that still had to be paid  Instead he attended the Central Grammar School for Boys in Manchester, rejoining his parents there  He now had two younger brothers, Harry and Hubert; a sister had died in infancy  At the age of 16, he sat two examinations for university scholarships, and won both of them Chadwick chose to attend Victoria University of Manchester, which he entered in 1908  He meant to study mathematics, but enrolled in physics by mistake  Like most students, he lived at home, walking the 4 miles (6 4 km) to the university and back each day  At the end of his first year, he was awarded a Heginbottom Scholarship to study physics  The physics department was headed by Ernest Rutherford, who assigned research projects to final-year students, and he instructed Chadwick to devise a means of comparing the amount of radioactive energy of two different sources  The idea was that they could be measured in terms of the activity of 1 gram (0 035 oz) of radium, a unit of measurement which would become known as the curie  Rutherford's suggested approach was unworkable—something Chadwick knew but was afraid to tell Rutherford—so Chadwick pressed on, and eventually devised the required method  The results became Chadwick's first paper, which, co-authored with Rutherford, was published in 1912  He graduated with first class honours in 1911 Having devised a means of measuring gamma radiation, Chadwick proceeded to measure the absorption of gamma rays by various gases and liquids  This time the resulting paper was published under his name alone  He was awarded his Master of Science (MSc) degree in 1912, and was appointed a Beyer Fellow  The following year he was awarded an 1851 Exhibition Scholarship, which allowed him to study and research at a university in continental Europe  He elected to go to the Physikalisch-Technische Reichsanstalt in Berlin in 1913, to study beta radiation under Hans Geiger  Using Geiger's recently developed Geiger counter, which provided more accuracy than the earlier photographic techniques, he was able to demonstrate that beta radiation did not produce discrete lines, as has been previously thought, but rather a continuous spectrum with peaks in certain regions  On a visit to Geiger's laboratory, Albert Einstein told Chadwick that: "I can explain either of these things, but I can't explain them both at the same time " The continuous spectrum would remain an unexplained phenomenon for many years Chadwick was still in Germany at the start of the First World War, and was interned in the Ruhleben internment camp near Berlin, where he was allowed to set up a laboratory in the stables and conduct scientific experiments using improvised materials such as radioactive toothpaste  With the help of Charles Drummond Ellis, he worked on the ionisation of phosphorus, and the photochemical reaction of carbon monoxide and chlorine  He was released after the Armistice with Germany came into effect in November 1918, and returned to his parents' home in Manchester, where he wrote up his findings over the previous four years for the 1851 Exhibition commissioners Rutherford gave Chadwick a part-time teaching position at Manchester, allowing him to continue research  He looked at the nuclear charge of platinum, silver, and copper, and experimentally found that this was the same as the atomic number within an error of less than 1 5 per cent  In April 1919, Rutherford became director of the Cavendish Laboratory at the University of Cambridge, and Chadwick joined him there a few months later  Chadwick was awarded a Clerk-Maxwell studentship in 1920, and enrolled as a Doctor of Philosophy (PhD) student at Gonville and Caius College, Cambridge  The first half of his thesis was his work with atomic numbers  In the second, he looked at the forces inside the nucleus  His degree was awarded in June 1921  In November, he became a Fellow of Gonville and Caius College 

Researcher
Cambridge
Chadwick's Clerk-Maxwell studentship expired in 1923, and he was succeeded by the Russian physicist Pyotr Kapitza  The Chairman of the Advisory Council of the Department of Scientific and Industrial Research, Sir William McCormick arranged for Chadwick to become Rutherford's assistant director of research  In this role, Chadwick helped Rutherford select PhD students  Over the next few years these would include John Cockcroft, Norman Feather and Mark Oliphant, who would become firm friends with Chadwick  As many students had no idea what they wanted to research, Rutherford and Chadwick would suggest topics  Chadwick edited all the papers produced by the laboratory 

In 1925, Chadwick met Aileen Stewart-Brown, the daughter of a Liverpool stockbroker  The two were married in August 1925, with Kapitza as Best Man  The couple had twin daughters, Joanna and Judith, who were born in February 1927 In his research, Chadwick continued to probe the nucleus  In 1925, the concept of spin had allowed physicists to explain the Zeeman effect, but it also created unexplained anomalies  At the time it was believed that the nucleus consisted of protons and electrons, so nitrogen's nucleus, for example, with a mass number of 14, was assumed to contain 14 protons and 7 electrons  This gave it the right mass and charge, but the wrong spin



ground_truth: Lance Bottoms

prompt_front: Answer the question based on the given passages. Only give me the answer and do not output any other words.

Question: What undertaking included the person who discovered that the number of protons in each element's atoms is unique?

The following are given passages.
Context 2: At a conference at Cambridge on beta particles and gamma rays in 1928, Chadwick met Geiger again  Geiger had brought with him a new model of his Geiger counter, which had been improved by his post-doctoral student Walther Müller  Chadwick had not used one since the war, and the new Geiger–Müller counter was potentially a major improvement over the scintillation techniques then in use at Cambridge, which relied on the human eye for observation  The major drawback with it was that it detected alpha, beta and gamma radiation, and radium, which the Cavendish laboratory normally used in its experiments, emitted all three, and was therefore unsuitable for what Chadwick had in mind  However, polonium is an alpha emitter, and Lise Meitner sent Chadwick about 2 millicuries (about 0 5 μg) from Germany In Germany, Walther Bothe and his student Herbert Becker had used polonium to bombard beryllium with alpha particles, producing an unusual form of radiation  Chadwick had his Australian 1851 Exhibition scholar, Hugh Webster, duplicate their results  To Chadwick, this was evidence of something that he and Rutherford had been hypothesising for years: the neutron, a theoretical nuclear particle with no electric charge  Then in January 1932, Feather drew Chadwick's attention to another surprising result  Frédéric and Irène Joliot-Curie had succeeded in knocking protons from paraffin wax using polonium and beryllium as a source for what they thought was gamma radiation  Rutherford and Chadwick disagreed; protons were too heavy for that  But neutrons would need only a small amount of energy to achieve the same effect  In Rome, Ettore Majorana came to the same conclusion: the Joliot-Curies had discovered the neutron but did not know it 

Chadwick dropped all his other responsibilities to concentrate on proving the existence of the neutron, assisted by Feather and frequently working late at night  He devised a simple apparatus that consisted of a cylinder containing a polonium source and beryllium target  The resulting radiation could then be directed at a material such as paraffin wax; the displaced particles, which were protons, would go into a small ionisation chamber where they could be detected with an oscilloscope 
In February 1932, after only about two weeks of experimentation with neutrons, Chadwick sent a letter to Nature titled "Possible Existence of a Neutron"  He communicated his findings in detail in an article sent to Proceedings of the Royal Society A titled "The Existence of a Neutron" in May  His discovery of the neutron was a milestone in understanding the nucleus  Reading Chadwick's paper, Robert Bacher and Edward Condon realised that anomalies in the then-current theory, like the spin of nitrogen, would be resolved if the neutron has a spin of 1/2 and that a nitrogen nucleus consisted of seven protons and seven neutrons The theoretical physicists Niels Bohr and Werner Heisenberg considered whether the neutron could be a fundamental nuclear particle like the proton and electron, rather than a proton–electron pair   Heisenberg showed that the neutron was best described as a new nuclear particle, but its exact nature remained unclear   In his 1933 Bakerian Lecture, Chadwick estimated that a neutron had a mass of about 1 0067 u  Since a proton and an electron had a combined mass of 1 0078 u, this implied the neutron as a proton–electron composite had a binding energy of about 2 MeV, which sounded reasonable, although it was hard to understand how a particle with so little binding energy could be stable   Estimating such a small mass difference required challenging precise measurements, however, and several conflicting results were obtained in 1933–4   By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained a large value for the mass of a neutron, but Ernest Lawrence's team at the University of California produced a small one   Then Maurice Goldhaber, a refugee from Nazi Germany and a graduate student at the Cavendish Laboratory, suggested to Chadwick that deuterons could be photodisintegrated by the 2 6 MeV gamma rays of 208Tl (then known as thorium C"):

An accurate value for the mass of the neutron could be determined from this process  Chadwick and Goldhaber tried this and found that it worked   They measured the kinetic energy of the proton produced as 1 05 MeV, leaving the mass of the neutron as the unknown in the equation   Chadwick and Goldhaber calculated that it was either 1 0084 or 1 0090 atomic units, depending on the values used for the masses of the proton and deuteron   (The modern accepted value for the mass of the neutron is 1 00866 u ) The mass of the neutron was too large to be a proton–electron pair For his discovery of the neutron, Chadwick was awarded the Hughes Medal by the Royal Society in 1932, the Nobel Prize in Physics in 1935, the Copley Medal in 1950 and the Franklin Medal in 1951  His discovery of the neutron made it possible to produce elements heavier than uranium in the laboratory by the capture of slow neutrons followed by beta decay  Unlike the positively charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier, and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium  This inspired Enrico Fermi to investigate the nuclear reactions brought about by collisions of nuclei with slow neutrons, work for which Fermi would receive the Nobel Prize in 1938 Wolfgang Pauli proposed another kind of particle on 4 December 1930 in order to explain the continuous spectrum of beta radiation that Chadwick had reported in 1914  Since not all of the energy of beta radiation could be accounted for, the law of conservation of energy appeared to be violated, but Pauli argued that this could be redressed if another, undiscovered, particle was involved  Pauli also called this particle a neutron, but it was clearly not the same particle as Chadwick's neutron  Fermi renamed it the neutrino, Italian for "little neutron"  In 1934, Fermi proposed his theory of beta decay which explained that the electrons emitted from the nucleus were created by the decay of a neutron into a proton, an electron, and a neutrino   The neutrino could account for the missing energy, but a particle with little mass and no electric charge was difficult to observe  Rudolf Peierls and Hans Bethe calculated that neutrinos could easily pass through the Earth, so the chances of detecting them were slim  Frederick Reines and Clyde Cowan would confirm the neutrino on 14 June 1956 by placing a detector within a large antineutrino flux from a nearby nuclear reactor 

Liverpool
With the onset of the Great Depression in the United Kingdom, the government became more parsimonious with funding for science  At the same time, Lawrence's recent invention, the cyclotron, promised to revolutionise experimental nuclear physics, and Chadwick felt that the Cavendish laboratory would fall behind unless it also acquired one  He therefore chafed under Rutherford, who clung to the belief that good nuclear physics could still be done without large, expensive equipment, and turned down the request for a cyclotron 

Chadwick was himself a critic of Big Science in general, and Lawrence in particular, whose approach he considered careless and focused on technology at the expense of science  When Lawrence postulated the existence of a new and hitherto unknown particle that he claimed was a possible source of limitless energy at the Solvay Conference in 1933, Chadwick responded that the results were more likely attributable to contamination of the equipment  While Lawrence rechecked his results at Berkeley only to find that Chadwick was correct, Rutherford and Oliphant conducted an investigation at the Cavendish that found that deuterium fuses to form helium-3, thereby causing the effect that Lawrence had observed  This was another major discovery, but the Oliphant-Rutherford particle accelerator was an expensive state-of-the-art piece of equipment In March 1935, Chadwick received an offer of the Lyon Jones Chair of physics at the University of Liverpool, in his wife's home town, to succeed Lionel Wilberforce  The laboratory was so antiquated that it still ran on direct current electricity, but Chadwick seized the opportunity, assuming the chair on 1 October 1935  The university's prestige was soon bolstered by Chadwick's Nobel Prize, which was announced in November 1935  His medal was sold at auction in 2014 for $329,000 Chadwick set about acquiring a cyclotron for Liverpool  He started by spending £700 to refurbish the antiquated laboratories at Liverpool, so some components could be made in-house  He was able to persuade the university to provide £2,000 and obtained a grant for another £2,000 from the Royal Society  To build his cyclotron, Chadwick brought in two young experts, Bernard Kinsey and Harold Walke, who had worked with Lawrence at the University of California  A local cable manufacturer donated the copper conductor for the coils  The cyclotron's 50-ton magnet was manufactured in Trafford Park by Metropolitan-Vickers, which also made the vacuum chamber  The cyclotron was completely installed and running in July 1939  The total cost of £5,184 was more than Chadwick had received from the University and the Royal Society, so Chadwick paid the rest from his 159,917 kr (£8,243) Nobel Prize money At Liverpool the Medicine and Science faculties worked together closely  Chadwick was automatically a committee member of both faculties, and in 1938 he was appointed to a commission headed by Lord Derby to investigate the arrangements for cancer treatment in Liverpool  Chadwick anticipated that neutrons and radioactive isotopes produced with the 37-inch cyclotron could be used to study biochemical processes, and might become a weapon in the fight against cancer 

Second World War
Tube Alloys and the MAUD Report
In Germany, Otto Hahn and Fritz Strassmann bombarded uranium with neutrons, and noted that barium, a lighter element, was among the products produced  Hitherto, only the same or heavier elements had been produced by the process  In January 1939, Meitner and her nephew Otto Frisch astounded the physics community with a paper that explained this result  They theorised that uranium atoms bombarded with neutrons can break into two roughly equal fragments, a process they called fission  They calculated that this would result in the release of about 200 MeV, implying an energy release orders of magnitude greater than chemical reactions, and Frisch confirmed their theory experimentally  It was soon noted by Hahn that if neutrons were released during fission, then a chain reaction was possible  French scientists, Pierre Joliot, Hans von Halban and Lew Kowarski, soon verified that more than one neutron was indeed emitted per fission  In a paper co-authored with the American physicist John Wheeler, Bohr theorised that fission was more likely to occur in the uranium-235 isotope, which made up only 0 7 percent of natural uranium 

Chadwick did not believe that there was any likelihood of another war with Germany in 1939, and took his family for a holiday on a remote lake in northern Sweden  The news of the outbreak of the Second World War therefore came as a shock  Determined not to spend another war in an internment camp, Chadwick made his way to Stockholm as fast as he could, but when he arrived there with his family, he found that all air traffic between Stockholm and London had been suspended  They made their way back to England on a tramp steamer  When he reached Liverpool, Chadwick found Joseph Rotblat, a Polish post-doctoral fellow who had come to work with the cyclotron, was now destitute, as he was cut off from funds from Poland  Chadwick promptly hired Rotblat as a lecturer, despite his poor grasp of English In October 1939, Chadwick received a letter from Sir Edward Appleton, the Secretary of the Department of Scientific and Industrial Research, asking for his opinion on the feasibility of an atom bomb  Chadwick responded cautiously  He did not dismiss the possibility, but carefully went over the many theoretical and practical difficulties involved  Chadwick decided to investigate the properties of uranium oxide further with Rotblat  In March 1940, Otto Frisch and Rudolf Peierls at the University of Birmingham re-examined the theoretical issues involved in a paper that became known as the Frisch–Peierls memorandum  Instead of looking at unenriched uranium oxide, they considered what would happen to a sphere of pure uranium-235, and found that not only could a chain reaction occur, but that it might require as little as 1 kilogram (2 2 lb) of uranium-235, and unleash the energy of tons of dynamite 

A special subcommittee of the Committee for the Scientific Survey of Air Warfare (CSSAW), known as the MAUD Committee, was created to investigate the matter further  It was chaired by Sir George Thomson and its original membership included Chadwick, along with Mark Oliphant, John Cockcroft and Philip Moon  While other teams investigated uranium enrichment techniques, Chadwick's team at Liverpool concentrated on determining the nuclear cross section of uranium-235  By April 1941, it had been experimentally confirmed that the critical mass of uranium-235 might be 8 kilograms (18 lb) or less  His research into such matters was complicated by all-but-incessant Luftwaffe bombings of the environs of his Liverpool lab; the windows were blown out so often that they were replaced by cardboard In July 1941, Chadwick was chosen to write the final draft of the MAUD Report, which, when presented by Vannevar Bush to President Franklin D  Roosevelt in October 1941, inspired the U S  government to pour millions of dollars into the pursuit of an atom bomb  When George B  Pegram and Harold Urey visited Britain to see how the project, now known as Tube Alloys, was going, Chadwick was able to tell them: "I wish I could tell you that the bomb is not going to work, but I am 90 per cent sure that it will "In a recent book about the Bomb project, Graham Farmelo wrote that "Chadwick did more than any other scientist to give Churchill the Bomb      Chadwick was tested almost to the breaking point " So worried that he could not sleep, Chadwick resorted to sleeping pills, which he continued to take for most of his remaining years  Chadwick later said that he realised that "a nuclear bomb was not only possible—it was inevitable  Sooner or later these ideas could not be peculiar to us  Everybody would think about them before long, and some country would put them into action"  Sir Hermann Bondi suggested that it was fortunate that Chadwick, not Rutherford, was the doyen of UK physics at the time, as the latter's prestige might otherwise have overpowered Chadwick's interest in "looking forward" to the Bomb's prospects 

Manhattan Project
Owing to the danger from aerial bombardment, the Chadwicks sent their twins to Canada as part of a government evacuation scheme  Chadwick was reluctant to move Tube Alloys there, believing that the United Kingdom was a better location for the isotope separation plant  The enormous scope of the effort became more apparent in 1942: even a pilot separation plant would cost over £1 million and strain Britain's resources, to say nothing of a full-scale plant, which was estimated to cost somewhere in the vicinity of £25 million  It would have to be built in America  At the same time that the British became convinced that a joint project was necessary, the progress of the American Manhattan Project was such that British cooperation seemed less essential, although the Americans were still eager to utilise Chadwick's talents The matter of cooperation had to be taken up at the highest level  In September 1943, the Prime Minister, Winston Churchill, and President Roosevelt negotiated the Quebec Agreement, which reinstated cooperation between Britain, the United States and Canada  Chadwick, Oliphant, Peierls and Simon were summoned to the United States by the director of Tube Alloys, Sir Wallace Akers, to work with the Manhattan Project  The Quebec Agreement established a new Combined Policy Committee to direct the joint project  The Americans disliked Akers, so Chadwick was appointed technical advisor to the Combined Policy Committee, and the head of the British Mission Leaving Rotblat in charge in Liverpool, Chadwick began a tour of the Manhattan Project facilities in November 1943, except for the Hanford Site where plutonium was produced, which he was not allowed to see  He became the only man apart from Groves and his second in command to have access to all the American research and production facilities for the uranium bomb  Observing the work on the K-25 gaseous diffusion facility at Oak Ridge, Tennessee, Chadwick realised how wrong he had been about building the plant in wartime Britain  The enormous structure could never have been concealed from the Luftwaffe  In early 1944, he moved to Los Alamos, New Mexico, with his wife and their twins, who now spoke with Canadian accents  For security reasons, he was given the cover name of James Chaffee 

Chadwick accepted that the Americans did not need British help, but that it could still be useful in bringing the project to an early and successful conclusion  Working closely with the director of the Manhattan Project, Major General Leslie R  Groves, Jr , he attempted to do everything he could to support the effort  He also endeavoured to place British scientists in as many parts of the project as possible in order to facilitate a post-war British nuclear weapons project to which Chadwick was committed  Requests from Groves via Chadwick for particular scientists tended to be met with an immediate rejection by the company, ministry or university currently employing them, only to be overcome by the overriding priority accorded to Tube Alloys  As a result, the British team was critical to the Project's success Although he had more knowledge of the project than anyone else from Britain, Chadwick had no access to the Hanford site  Lord Portal was offered a tour of Hanford in 1946  "This was the only plant to which Chadwick had been denied access in wartime, and now he asked Groves if he could accompany Portal  Groves replied that he could, but if he did then 'Portal will not see very much' " For his efforts, Chadwick received a knighthood in the New Year Honours on 1 January 1945  He considered this to be a recognition of the work of the whole Tube Alloys project By early 1945, Chadwick was spending most of his time in Washington, D C , and his family relocated from Los Alamos to a house on Washington's Dupont Circle in April 1945  He was present at the meeting of the Combined Policy Committee on 4 July when Field Marshal Sir Henry Maitland Wilson gave Britain's agreement to use the atom bomb against Japan, and at the Trinity nuclear test on 16 July, when the first atom bomb was detonated  Inside its pit was a polonium-beryllium modulated neutron initiator, a development of the technique that Chadwick had used to discover the neutron over a decade before  William L  Laurence, The New York Times reporter attached to the Manhattan Project, wrote that "never before in history had any man lived to see his own discovery materialize itself with such telling effect on the destiny of man "

Later life
Shortly after the war ended, Chadwick was appointed to the Advisory Committee on Atomic Energy (ACAE)  He was also appointed as the British scientific advisor to the United Nations Atomic Energy Commission  He clashed with fellow ACAE member Patrick Blackett, who disagreed with Chadwick's conviction that Britain needed to acquire its own nuclear weapons; but it was Chadwick's position that was ultimately adopted  He returned to Britain in 1946, to find a country still beset by wartime rationing and shortages At this time, Sir James Mountford, the Vice Chancellor of the University of Liverpool, wrote in his diary "he had never seen a man 'so physically, mentally and spiritually tired" as Chadwick, for he "had plumbed such depths of moral decision as more fortunate men are never called upon even to peer into     [and suffered]     almost insupportable agonies of responsibility arising from his scientific work' "In 1948, Chadwick accepted an offer to become the Master of Gonville and Caius College  The job was prestigious but ill-defined; the Master was the titular head of the College, but authority actually resided in a council of 13 fellows, of whom one was the Master  As Master, Chadwick strove to improve the academic reputation of the college  He increased the number of research fellowships from 31 to 49, and sought to bring talent into the college  This involved controversial decisions, such as hiring in 1951 the Chinese biochemist Tien-chin Tsao and the Hungarian-born economist Peter Bauer  In what became known as the Peasants' Revolt, fellows led by Patrick Hadley voted an old friend of Chadwick's off the council and replaced him with Bauer  More friends of Chadwick's were removed over the following years, and he retired in November 1958  It was during his mastership that Francis Crick, a PhD student at Gonville and Caius College, and James Watson discovered the structure of DNA Over the years, Chadwick received many honours, including the Medal for Merit from the United States, and the Pour le Mérite from Germany  He was elected a Fellow of the Royal Society in 1927  He became a foreign member of the Royal Netherlands Academy of Arts and Sciences in 1946 and an International member of the American Philosophical Society in 1948  He was made a Companion of Honour in the New Year Honours on 1 January 1970 for "services to science", and went to Buckingham Palace for the investiture ceremony  He became more frail, and seldom left his flat, although he travelled to Liverpool for celebrations of his eightieth birthday   A lifelong atheist, he saw no reason to adopt religious faith in later life  He died in his sleep on 24 July 1974 His papers are held at the Churchill Archives Centre in Cambridge, and are accessible to the public  The Chadwick Laboratory at the University of Liverpool is named after him, as is its Sir James Chadwick Chair of Experimental Physics, which was named after him in 1991 as part of celebrations of the centenary of his birth  A crater on the moon is also named after him  The James Chadwick Building, which houses part of the School of Chemical Engineering and Analytical Sciences, University of Manchester is named in his honour  He was described by the United Kingdom Atomic Energy Authority official historian Lorna Arnold as "a physicist, a scientist-diplomat, and a good, wise, and humane man "

Notes
Passage 3:
Synthetic element
A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; thus, they are called "synthetic", "artificial", or "man-made"  The synthetic elements are those with atomic numbers 95–118, as shown in purple on the accompanying periodic table: these 24 elements were first created between 1944 and 2010  The mechanism for the creation of a synthetic element is to force additional protons into the nucleus of an element with an atomic number lower than 95  All known (see: Island of stability) synthetic elements are unstable, but they decay at widely varying rates: the half-lives of their longest-lived isotopes range from microseconds to millions of years  
Five more elements that were first created artificially are strictly speaking not synthetic because they were later found in nature in trace quantities: 43Tc, 61Pm, 85At, 93Np, and 94Pu, though are sometimes classified as synthetic alongside exclusively artificial elements  The first, technetium, was created in 1937  Plutonium (Pu, atomic number 94), first synthesized in 1940, is another such element  It is the element with the largest number of protons (atomic number) to occur in nature, but it does so in such tiny quantities that it is far more practical to synthesize it  Plutonium is known mainly for its use in atomic bombs and nuclear reactors No elements with atomic numbers greater than 99 have any uses outside of scientific research, since they have extremely short half-lives, and thus have never been produced in large quantities 

Properties
All elements with atomic number greater than 94 decay quickly enough into lighter elements such that any atoms of these that may have existed when the Earth formed (about 4 6 billion years ago) have long since decayed  Synthetic elements now present on Earth are the product of atomic bombs or experiments that involve nuclear reactors or particle accelerators, via nuclear fusion or neutron absorption Atomic mass for natural elements is based on weighted average abundance of natural isotopes in Earth's crust and atmosphere  For synthetic elements, there is no "natural isotope abundance"  Therefore, for synthetic elements the total nucleon count (protons plus neutrons) of the most stable isotope, i e , the isotope with the longest half-life—is listed in brackets as the atomic mass 

History
Technetium
The first element to be synthesized, rather than discovered in nature, was technetium in 1937  This discovery filled a gap in the periodic table, and the fact that technetium has no stable isotopes explains its natural absence on Earth (and the gap)  With the longest-lived isotope of technetium, 97Tc, having a 4 21-million-year half-life, no technetium remains from the formation of the Earth  Only minute traces of technetium occur naturally in Earth's crust—as a product of spontaneous fission of 238U, or from neutron capture in molybdenum—but technetium is present naturally in red giant stars 

Curium
The first entirely synthetic element to be made was curium, synthesized in 1944 by Glenn T  Seaborg, Ralph A  James, and Albert Ghiorso by bombarding plutonium with alpha particles 

Eight others
Synthesis of americium, berkelium, and californium followed soon  Einsteinium and fermium were discovered by a team of scientists led by Albert Ghiorso in 1952 while studying the composition of radioactive debris from the detonation of the first hydrogen bomb  The isotopes synthesized were einsteinium-253, with a half-life of 20 5 days, and fermium-255, with a half-life of about 20 hours  The creation of mendelevium, nobelium, and lawrencium followed 

Rutherfordium and dubnium
During the height of the Cold War, teams from the Soviet Union and the United States independently created rutherfordium and dubnium  The naming and credit for synthesis of these elements remained unresolved for many years, but eventually shared credit was recognized by IUPAC/IUPAP in 1992  In 1997, IUPAC decided to give dubnium its current name honoring the city of Dubna where the Russian team worked since American-chosen names had already been used for many existing synthetic elements, while the name rutherfordium (chosen by the American team) was accepted for element 104 

The last thirteen
Meanwhile, the American team had created seaborgium, and the next six elements had been created by a German team: bohrium, hassium, meitnerium, darmstadtium, roentgenium, and copernicium  Element 113, nihonium, was created by a Japanese team; the last five known elements, flerovium, moscovium, livermorium, tennessine, and oganesson, were created by Russian–American collaborations and complete the seventh row of the periodic table 

List of synthetic elements
The following elements do not occur naturally on Earth  All are transuranium elements and have atomic numbers of 95 and higher 

Other elements usually produced through synthesis
All elements with atomic numbers 1 through 94 occur naturally at least in trace quantities, but the following elements are often produced through synthesis: technetium, promethium, astatine, neptunium, and plutonium, which were discovered through synthesis before being found in nature 
Passage 4:
Atomic number
The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus  For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element  The atomic number can be used to uniquely identify ordinary chemical elements  In an ordinary uncharged atom, the atomic number is also equal to the number of electrons 
For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A  Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A 
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes  A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight  Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century 
The conventional symbol Z comes from the German word Zahl 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights  Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context 
The rules above do not always apply to exotic atoms which contain short-lived elementary particles other than protons, neutrons and electrons 

History
The periodic table and a natural number for each element
Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and so they can be numbered in order 
Dmitri Mendeleev claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of atomic weight ("Atomgewicht")  However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed tellurium (atomic weight 127 6) ahead of iodine (atomic weight 126 9)  This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time 

A simple numbering based on periodic table position was never entirely satisfactory, however  Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties  However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time) 

The Rutherford-Bohr model and van den Broek
In 1911, Ernest Rutherford gave a model of the atom in which a central nucleus held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms  This central charge would thus be approximately half the atomic weight (though it was almost 25% different from the atomic number of gold (Z = 79, A = 197), the single element from which Rutherford made his guess)  Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also known as element number, atomic number, and symbolized Z)  This proved eventually to be the case 

Moseley's 1913 experiment
The experimental position improved dramatically after research by Henry Moseley in 1913  Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of Z 
To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminium (Z = 13) to gold (Z = 79) used as a series of movable anodic targets inside an x-ray tube  The square root of the frequency of these photons (x-rays) increased from one target to the next in an arithmetic progression  This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i e  the element number Z  Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time 

Missing elements
After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen to uranium (Z = 92) were examined by his method  There were seven elements (with Z < 92) which were not found and therefore identified as still undiscovered, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91  From 1918 to 1947, all seven of these missing elements were discovered  By this time, the first four transuranium elements had also been discovered, so that the periodic table was complete with no gaps as far as curium (Z = 96) 

The proton and the idea of nuclear electrons
In 1915, the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood  An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one  However, as early as 1907, Rutherford and Thomas Royds had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times  If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms 
In 1917, Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas, and believed he had proven Prout's law  He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles)  It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei  A helium nucleus was presumed to be composed of four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two of the charges  At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of +79, consistent with its atomic number 

The discovery of the neutron makes Z the proton number
All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932  An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive nuclear charge now was realized to come entirely from a content of 79 protons  Since Moseley had previously shown that the atomic number Z of an element equals this positive charge, it was now clear that Z is identical to the number of protons of its nuclei 

Chemical properties
Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z (the atomic number)  The configuration of these electrons follows from the principles of quantum mechanics  The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior  Hence, it is the atomic number alone that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of any mixture of atoms with a given atomic number 

New elements
The quest for new elements is usually described using atomic numbers  As of 2023, all elements with atomic numbers 1 to 118 have been observed  Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created  In general, the half-life of a nuclide becomes shorter as atomic number increases, though undiscovered nuclides with certain "magic" numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability  
A hypothetical element composed only of neutrons has also been proposed and would have atomic number 0 

See also



ground_truth: Manhattan Project

prompt_front: Answer the question based on the given passages. Only give me the answer and do not output any other words.

Question: Who is the employer of the person Mach's principle was named after?

The following are given passages.
Context 3: Passage 1:
Vinko Dvořák
Vinko Dvořák (January 21, 1848 – May 6, 1922) was a Czech-Croatian physicist, professor and academician He studied mathematics and physics at the Charles University in Prague, and after graduating he became an assistant to professor Ernst Mach  After obtaining his doctorate in Prague in 1873/1874 he came to Zagreb (at the time also part of Austria-Hungary) and founded the Physics Cabinet at the Faculty of Philosophy in 1875 
Dvořák made many important discoveries in the field of experimental acoustics and optics, which are known as the Dvořák-Rayleigh current, the Dvořák acoustic repulsion, and the Dvořák circuit  His work on acoustic radiometers coincided with that of Lord Rayleigh He was the dean of the Faculty of Philosophy in 1881/82 and again in 1891/92 and the rector of the University of Zagreb in 1893/94 
Professor Dvorak made constant advancements in physics experimentation at the Faculty—in 1896 he obtained a Röntgen radiation device just six months after it was discovered He became a member of the Academy of Sciences and Arts in 1883 (associate) and 1887 (full member)  He was also an associate member of the Czech Academy of Franz Joseph I, a member of the Société francaise de physique (French Physics Society) and the Paris Société internationale des électriciens, and a member of the Royal Czech Society of Sciences in Prague 
Dvořák retired in 1911 
Passage 2:
Boeing VC-25
The Boeing VC-25 is a military version of the Boeing 747 airliner, modified for presidential transport and commonly operated by the United States Air Force (USAF) as Air Force One, the call sign of any U S  Air Force aircraft carrying the president of the United States 
Only two examples of this aircraft type are in service; they are highly modified Boeing 747-200Bs, designated VC-25A and having tail numbers 28000 and 29000  Although technically the Air Force One designation applies to the aircraft only while the president is on board, the term is commonly used to refer to the VC-25 in general  The two aircraft often operate in conjunction with Marine One helicopters, which ferry the president to airports whenever a vehicle motorcade would be inappropriate  Two new aircraft, based on the Boeing 747-8I and designated VC-25B, have been ordered by the USAF to replace the aging VC-25As 

Development
By 1985, the pair of Boeing 707-based VC-137s used as the presidential aircraft had been in service for 23 and 13 years respectively, and the USAF began searching for an eventual replacement  The Request for Proposal issued stated that the aircraft to be selected should have at least three engines and an unrefueled range of at least 6,000 miles (9,700 km)  Both Boeing with its 747 and McDonnell Douglas with the DC-10 were in competition to be selected, with the Boeing entry the eventual winner   The fabrication of the current 747s began during the presidency of Ronald Reagan (1981–1989) 
The VC-25s were completed in 1986 and first flew in 1987   The interior designs were created by First Lady Nancy Reagan, who used designs reminiscent of the American Southwest  Problems with interior wiring for communication systems delayed delivery of the two aircraft until 1990, during the administration of George H  W  Bush 
The Air Force reported that the operating cost for each VC-25A in 2014 was $210,877 per hour 

Design and configuration
The VC-25 is capable of flying 7,800 miles (12,600 km)—roughly one-third the distance around the world—without refueling  The VC-25A can accommodate more than 70 passengers  Each VC-25A cost approximately $325 million  While the VC-25 has two main decks and a cargo area, like a regular Boeing 747, its 4,000 square feet (370 m2) of floor space has been reconfigured for presidential duties  Its lowest level is mostly cargo space, carrying luggage and the onboard food supply 
The main passenger area is on the second floor or main deck  There are three entrances on board, two on the main and one on the lower deck  Typically the president boards and disembarks from the front, main deck entrance via an airstair, while journalists and other passengers enter at the rear door of the main deck  Facilities for the press and other passengers are configured like an ordinary airliner's first-class cabin 

The "White House"
The front section of the aircraft is informally called the "White House", a reference to the president's official residence in Washington, D C  The president's executive suite includes sleeping quarters with two couches that can be converted into beds, lavatory and shower, vanity, double sink, and a private office, or the president's "Oval Office aboard Air Force One"  If necessary, the president can address the nation from the office  This capability was added after the September 11 attacks, during which the aircraft had to land at Barksdale Air Force Base for President George W  Bush to address the nation  These offices, including the president's suite, are mostly located on the starboard (right) side, and a long corridor runs along the port (left) side  The aircraft also contains a conference room, originally designed as a situation room, but now used for meeting with staff while traveling  This room includes a 50-inch plasma screen television which can be used for teleconferencing  The aircraft has fully equipped office areas with telecommunication systems (including 87 telephones and 19 televisions) On board the VC-25 is a medical annex, which includes a fold-out operating table, emergency medical supplies, and a well-stocked pharmacy  George W  Bush had a treadmill added to Air Force One during his term in office  Every flight is staffed by a doctor and nurse  The aircraft is self-sufficient, such as carrying all the food it will need  Meals are prepared in two galleys, which together are equipped to feed up to 100 people at a time  The president gets a personal menu  An area where guests sit is near the center of the aircraft, outside the "White House" There are separate quarters for guests, senior staff, Secret Service and security personnel, and the news media located in the aft area of the main deck  Protocol states that one may wander aft of one's assigned seat, but not forward of it  Communications equipment and the cockpit are on the upper deck  There are also secure and non-secure voice, fax and data communications facilities  While the aircraft's luggage capacity is adequate to carry the belongings of the passengers, the logistics train of the president means that the aircraft must fly preceded by an aerial convoy of several cargo transports, which carry the helicopters, motorcade vehicles, and other equipment required by the presidential entourage 

Operational history
VC-25A
The VC-25A replaced the VC-137C (a military version of the Boeing 707) as the mainstay of the Air Force One fleet  On some occasions, the VC-25s serve as transport for the US vice president, for which service they use the Air Force Two call sign  The VC-25A aircraft are maintained and operated as military operations by the Presidential Airlift Group, part of Air Mobility Command's 89th Airlift Wing, based at Joint Base Andrews in Camp Springs, Maryland 
The aircraft can also be operated as a military command center in the event of an incident such as a nuclear attack  Operational modifications include aerial refueling capability and countermeasures against anti-aircraft missiles  The electronics on board are connected with approximately 238 miles (383 km) of wiring, twice that of a regular 747  All wiring is covered with heavy shielding for protection from a nuclear electromagnetic pulse in the event of a nuclear attack  The aircraft also has electronic countermeasures (ECMs) to jam enemy radar, flares to avoid heat-seeking missiles, and chaff to avoid radar-guided missiles  All small arms and ammunition stores not in the physical possession of the Secret Service on board the VC-25s are stowed and secured by the Secret Service in separate locked compartments, each with a different locking mechanism for added security   Many of the VC-25's other capabilities are classified for security reasons 
After a presidential inauguration resulting in a change in office, the outgoing president is provided transport on a VC-25 aircraft to their home destination   The aircraft for this flight does not use the Air Force One call sign because it is not carrying the president in office   For both Presidents Bill Clinton and George W  Bush, the flight was known as Special Air Mission 28000, where the number represents the aircraft's tail number 

The VC-25As have also been used to transport deceased former presidents, as the guest area aft of "the White House" has chairs and tables that can be removed and the casket laid in their place  The bodies of Ronald Reagan, Gerald Ford, and George H W  Bush were transported to Washington for their state funerals, and then on to their final resting places  Colonel Mark Tillman, pilot for President George W  Bush, said, "We'll take care of the president from basically when he's in office to when he lays in state " For the funeral of President Ronald Reagan in 2004, Tillman said that the crew converted the front of the aircraft to look the way it would have appeared when Reagan was president; President and Nancy Reagan's Air Force One jackets were placed on the chairs to "make them feel at home"  A specially designed hydraulic lifter (similar to the type used by airline catering) with the presidential seal affixed to the sides lifts the casket up to the portside aft door to enter the VC-25A  The tradition of placing the caskets in the passenger cabin dates back to the assassination of John F  Kennedy, when the crew did not want the president's body placed in the cargo hold, and again during the state funeral of Lyndon B  Johnson The two VC-25As are slated for retirement by the end of 2025 

VC-25B
The VC-25A aircraft are aging and have become less cost-effective to operate  The USAF Air Mobility Command investigated possible replacements, with early press coverage suggesting that the USAF would consider the Boeing 747-8 and the Airbus A380  On 7 January 2009, Air Force Materiel Command issued a new Sources Sought notice for a replacement aircraft to enter service by 2017 with an additional two aircraft to follow in 2019 and 2021  On 28 January 2009, EADS North America representing EADS and its Airbus division confirmed it would not respond to the US Air Force notice, as assembling only three airplanes in the US would not make financial sense  This made Boeing the only aircraft manufacturer interested in supplying the replacement aircraft, and was reported to be exploring a 787 option also  On 28 January 2015, the Air Force announced the selection of the Boeing 747-8 to replace the aging VC-25A for presidential transport On 10 May 2016, the Air Force posted online an amendment to its Air Force One contract authorizing Boeing to begin preliminary design activities  This version of the contract synopsis confirmed that the government will buy two modified 747-8 aircraft  Boeing was awarded a contract in January 2016 to identify cost reduction opportunities in areas including maintenance, aerial refueling and communications  On 15 July 2016, Boeing received another contract for pre-engineering and manufacturing development (EMD) risk-reduction to address "system specification, the environmental control system, the aircraft interior, the electrical and power system and sustainment and maintenance approaches" to reduce development risks and life-cycle costs On 1 August 2017, Defense One reported that in an effort to pay less for the replacement program, the USAF entered into a contract to purchase two 747-8 Intercontinentals from Boeing, which had originally been ordered by Transaero, a Russian airline, in 2011  Before they could be delivered, the company filed for bankruptcy and was closed down; the two aircraft were stored at Southern California Logistics Airport in the Mojave desert to prevent corrosion  On 27 February 2018, the White House announced a US$3 9 billion agreement with Boeing to modify the two unsold 747-8s to replace the current VC-25As  The new aircraft will be designated VC-25B  These aircraft are to be retrofitted with telecommunications and security equipment to bring them to the required security level for the presidential aircraft, and are due to be delivered by 2024  Once the new aircraft are delivered, the VC-25As will be retired and placed in museums In June 2019, President Trump announced his plans to revamp the VC-25's livery from the traditional white and ultramarine shades to one of red, white, and blue  This would have been the first deviation from the Raymond Loewy livery scheme since it was introduced in 1962 on the VC-137C which was first used as Air Force One during the Kennedy administration  The Biden administration abandoned the new design, citing "additional engineering, as well as increased time and cost " A modified version of the traditional scheme was announced in March 2023 The 747s begun undergoing modification work at the Boeing's San Antonio facility in 2020  According to The Wall Street Journal, the development process has been hit by multiple "production mishaps", including the discovery of empty tequila mini-bottles on one of the aircraft, and the use of jacks that were not rated to support the weight of the aircraft  While the jacking did not result in damage to the planes, "the Pentagon's contractor-management agency formally requested Boeing improve its operations " While the initial delivery date was set to 2024, the Pentagon expects the jets to be two to three years late In April 2022, Boeing's CEO Dave Calhoun revealed that he expects the company to have a loss of $660 million on the VC-25B program, after the contract was renegotiated by President Trump 

Variants
VC-25A
based on Boeing 747-200B
VC-25B
based on Boeing 747-8 Intercontinental, to replace the VC-25As

Operators
United StatesUnited States Air Force
89th Airlift Wing, Presidential Airlift Group (PAG) - Andrews AFB, Maryland

Specifications (VC-25A)
Data from Boeing BDSGeneral characteristics
Crew: 26: 2 pilots, flight engineer, navigator, + cabin crew + mission crew
Capacity: 76 passengers
Length: 231 ft 10 in (70 66 m)
Wingspan: 196 ft 8 in (59 94 m)
Height: 63 ft 5 in (19 33 m)
Max takeoff weight: 833,000 lb (377,842 kg)
Powerplant: 4 × General Electric CF6-80C2B1 turbofan engines, 56,700 lbf (252 kN) thrust eachPerformance

Maximum speed: 547 5 kn (630 1 mph, 1,014 0 km/h) at 35,000 ft (10,668 m)
Maximum speed: Mach 0 92
Cruise speed: 500 kn (580 mph, 930 km/h) / M0 84 at 35,000 ft (10,668 m)
Range: 6,800 nmi (7,800 mi, 12,600 km)
Service ceiling: 45,100 ft (13,700 m)

Notable appearances in media
The VC-25 "Air Force One" is a prominent symbol of the U S  presidency and its powers; with the White House and presidential seal, it is among the most recognized presidential symbols  Air Force One has often appeared in popular culture and fiction, including the setting of the 1997 action movie Air Force One where the aircraft had an escape pod and a parachute ramp, unlike the actual presidential aircraft 

See also
Air Force One photo op incident
Air Force Two
Air transports of heads of state and governmentRelated development

Boeing 747
Boeing E-4B NightwatchAircraft of comparable role, configuration, and era

VC-137C SAM 26000
VC-137C SAM 27000
Boeing C-32
Boeing C-40 Clipper
Airbus A330 MRTT Vespina
Passage 3:
SOLID
In software engineering, SOLID is a mnemonic acronym for five design principles intended to make object-oriented designs more understandable, flexible, and maintainable  The principles are a subset of many principles promoted by American software engineer and instructor Robert C  Martin, first introduced in his 2000 paper Design Principles and Design Patterns discussing software rot : 2–3 The SOLID ideas are

The Single-responsibility principle: "There should never be more than one reason for a class to change " In other words, every class should have only one responsibility 
The Open–closed principle: "Software entities     should be open for extension, but closed for modification "
The Liskov substitution principle: "Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it " See also design by contract 
The Interface segregation principle: "Clients should not be forced to depend upon interfaces that they do not use "
The Dependency inversion principle: "Depend upon abstractions, [not] concretions "The SOLID acronym was introduced later, around 2004, by Michael Feathers Although the SOLID principles apply to any object-oriented design, they can also form a core philosophy for methodologies such as agile development or adaptive software development 

See also
Code reuse
GRASP (object-oriented design)
Inheritance (object-oriented programming)
List of software development philosophies
Passage 4:
Form I-9
Form I-9, officially the Employment Eligibility Verification, is a United States Citizenship and Immigration Services form  Mandated by the Immigration Reform and Control Act of 1986, it is used to verify the identity and legal authorization to work of all paid employees in the United States  All U S  employers must ensure proper completion of Form I-9 for each individual they hire for employment in the United States 

Requirements
The Immigration Reform and Control Act of 1986 (IRCA) required employers to verify that all newly hired employees presented facially valid documentation verifying the employee's identity and legal authorization to accept employment in the United States  The I-9 form, or more properly the Employment Eligibility Verification Form, is provided by the federal government for that purpose Every employee hired after November 6, 1986 must complete an I-9 form at the time of hire  Employees must complete Section 1 of the form upon commencing employment  The employer must complete Section 2 within three days of the employee's starting date at work  The employer is responsible for ensuring that the forms are completed properly and in a timely manner  
The I-9 is not required for unpaid volunteers or for contractors  However, a company could still find itself liable if it contracts work to a contractor it knows either is or employs unauthorized workers Multiple versions of Form I-9, Employment Eligibility Verification, have been issued since the form was first introduced in 1987; not all versions are valid for use  Currently, only forms showing the following revision date are valid: "Rev  07/17/2017 N*" If an employee cannot read or cannot write in English, a translator or preparer may complete the form and sign it on behalf of the employee  The form also requires the employee's own signature 
In October 2004, new legislation made it possible to complete the I-9 electronically 

Prerequisites for employees
In completing form I-9, prospective employees attest, under penalty of perjury, that they are in one of the following categories:

A U S  citizen,
A U S  non-citizen national,
A lawful permanent resident or
An alien authorized to work 

Documentation for proof of identity or employment authorization
With the form, prospective employees must provide documents that prove their eligibility to work  A variety of documents are acceptable  The prospective employee must provide either:

One document that establishes both identity and employment eligibility (on List A on the I-9)
One document that establishes identity (on List B) together with another document that establishes employment eligibility (on List C) All documents must be unexpired

List A
Documents that may be used under "List A" of the I-9 form to establish both identity and employment eligibility include:

An unexpired U S  Passport,
A U S  Passport Card,
A Permanent Resident Card (often called a "green card") or Alien Registration Receipt Card with photograph,
An unexpired Temporary Resident Card,
An unexpired foreign passport with an I-551 stamp, or with Form I-94 (For the certain alien who is authorized to work with restrictions  The person should also attach the documents which indicate an unexpired employment authorization ),
An unexpired Employment Authorization Document issued by the United States Department of Homeland Security that includes a photograph (Form I-766) or
An unexpired Employment Authorization Card 

List B
Documents that may be used under "List B" of the I-9 to establish identity include:

Driver's license or identification card issued by a U S  state or outlying possession of the U S  provided it contains a photograph or identifying information such as name, date of birth, sex, height, eye color and address;
Federal or state identification card provided it contains a photograph or identifying information such as name, date of birth, sex, height, eye color and address;
School identification card with photograph;
U S  Armed Services identification card or draft record;
Voter Registration Card;
U S  Coast Guard Merchant Mariner Card;
Native American tribal document;
Driver's license issued by a Canadian government authority or
Trusted traveler documentation (Global Entry, NEXUS, SENTRI) For individuals under the age of eighteen, the following documents may be used to establish identity:

School record or report card;
Clinic, doctor or hospital record or
Daycare or nursery school record Employees who supply an item from List B (to establish identity) must also supply an item from List C (to establish employment eligibility) 

List C
Documents that may be used under "List C" of the I-9 to establish employment eligibility include:

A U S  Social Security card issued by the Social Security Administration unless it indicates one of the following:
NOT VALID FOR EMPLOYMENT (generally issued to non-immigrant aliens unauthorized to work)
VALID FOR WORK ONLY WITH INS AUTHORIZATION
VALID FOR WORK ONLY WITH DHS AUTHORIZATION,
A birth certificate issued by the U S  State Department (Form FS-545 or Form DS-1350),
Original or certified copy of a birth certificate from the U S  or an outlying possession of the U S , bearing an official seal,
A Certificate of U S  Citizenship (Form N-560 or N-561),
A Certificate of Naturalization (Form N-550 or N-570),
Native American tribal document,
U S  Citizen ID Card (Form I-197),
An ID Card for the use of a Resident Citizen in the United States (Form I-179),
An unexpired employment authorization card issued by the Dept  of Homeland Security (other than those included on List A) or
Consular Report of Birth Abroad (Form FS-240) U S  citizens who have lost their social security card can apply for a duplicate at the Social Security Administration  
Employees who supply an item from List C (to establish employment eligibility) must also supply an item from List B (to establish identity) 

Reverification
Employers must update or reverify certain ID documents at or prior to their expiration date  This does not apply to already presented and accepted non-expired U S  Passports or Permanent Resident Cards when they reach their expiration date, nor to any List B documents (e g , state driver's licenses and state IDs)  The USCIS website, in the Employer section, Employer Bulletins, lists the limited requirements and allowed instances for reverification 
For U S  citizens, I-9s are valid continuously unless a break of more than a year of employment occurs  International employees on F-1 (student), H-1B (specialty occupation), or J-1 (exchange visitor) visas must have their I-9 reverified each time their visa has expired with a new work authorization permit (renewed visa with work authorization, EAD, Permanent Residence Card, etc ) 

Retention
Employers must retain a Form I-9 for all current employees  Employers must also retain a Form I-9 for three years after the date of hire, or one year after the date employment ends, whichever is later  Employers must show their employees' I-9 form any time the immigration or labor authority requests it 

Anti-discrimination provisions
The Immigration Reform and Control Act which introduced the requirement leading to the promulgation of the I-9 form also included anti-discrimination provisions  Under the Act, most U S  citizens, permanent residents, temporary residents, asylees or refugees who are legally allowed to work in the United States cannot be discriminated against on the basis of national origin or citizenship status  This provision applies to employers of three or more workers and covers both hiring and termination decisions  In addition, an employer must accept any valid document or combination of documents specified in the I-9 form as long as the documents appear genuine For example, an employer could not refuse to hire a candidate because his I-9 revealed that he was a non-citizen (such as a permanent resident or a refugee) rather than a U S  citizen  For this reason some immigration lawyers advise companies to avoid requiring an I-9 until a candidate is hired rather than risk a lawsuit  As another example, a company could not insist that an employee provide a passport rather than, say, a driver's license and social security card  Another anti-discrimination provision requires that employers must enforce I-9 compliance in a uniform manner  For example, an employer must not require some employees to complete an I-9 before being hired, but allow others to complete the form after starting employment Employers must not assume that the employee is unauthorized to work just because the individual either could not bring the proof of employment authorization or has brought the unaccepted documents until the start date of the employment  Instead, employers should encourage that employee to bring the acceptable documents which are under the List A, B and C  Employers may terminate the employment only if the employee cannot attest the person's work authorization by bringing the proof after the start date 
The Office of Special Counsel for Immigration-Related Unfair Employment Practices ("OSC") is a section within the Department of Justice's Civil Rights Division that enforces the anti-discrimination provision of the Immigration and Nationality Act ("INA")  The OSC can help workers by calling employers and explaining proper verification practices and, when necessary, by providing victims of discrimination with charge forms  Upon receipt of a charge of discrimination, OSC investigations typically take no longer than seven months  Victims may obtain various types of relief including job relief and back pay 
OSC also has an extensive outreach program  It provides staff to speak at outreach events throughout the country, and has free informational brochures, posters and tapes for distribution 

Types of discrimination
The OSC investigates the following types of discriminatory conduct under the anti-discrimination provision of the Immigration and Nationality Act (INA), 8 U S C  § 1324b:

Citizenship or immigration status discrimination
With respect to hiring, firing, recruitment or referral for a fee by employers with four or more employees, employers may not treat individuals differently because they are or are not U S  citizens or work-authorized individuals  U S  citizens, recent permanent residents, temporary residents, asylees and refugees are protected from citizenship status discrimination  However, permanent residents who do not apply for naturalization within six months of eligibility are not protected from citizenship status discrimination  Citizenship status discrimination which is otherwise required to comply with law, regulation, executive order or government contract is permissible by law 

National origin discrimination
With respect to hiring, firing, recruitment or referral for a fee by employers with more than three and fewer than fifteen employees, employers may not treat individuals differently because of their place of birth, country of origin, ancestry, native language, accent or because they are perceived as looking or sounding foreign  All U S  citizens, lawful permanent residents and work authorized individuals are protected from national origin discrimination  The Equal Employment Opportunity Commission has jurisdiction over employers with fifteen or more employees 

Unfair documentary practices
Relating to verifying the employment eligibility of employees, employers may not request more or different documents than are required to verify employment eligibility, reject reasonably genuine-looking documents or specify certain documents over others with the purpose or intent of discriminating on the basis of citizenship status or national origin  U S  citizens and all work authorized individuals are protected from document abuse 

Retaliation
Individuals who file charges with OSC; who cooperate with an OSC investigation; who contest action that may constitute unfair documentary practices or discrimination based upon citizenship, immigration status, or national origin; or who assert their rights under the INA's anti-discrimination provision are protected from retaliation 

Penalties
The IRCA includes penalties for I-9 noncompliance  Federal law provides for imprisonment or fines for making false statements or using false documents in connection with the completion of the I-9  An employer who hires an unauthorized worker can be fined between $250 and $5,500 per worker  In addition, such an employer can be barred from federal government contracts for a year  An employee who knowingly accepts fraudulent documentation can also be criminally prosecuted under other immigration laws An employer who fails to keep proper records that I-9s are properly filed can be fined $110 per missing item for each form, up to $1100 per form, even if the employee is legally authorized to work in the United States  Since 2009, Immigration & Customs Enforcement (ICE) has conducted over 7,500 audits and imposed over $80 million in fines  In 2011 alone, ICE conducted 2,740 audits and assessed over $7 million in fines An individual who knowingly commits or participates in document fraud may be fined between $375 and $3,200 per document for the first offense and between $3,200 and $6,500 per document for subsequent offenses 

See also
Immigration to the United States
E-Verify
Passage 5:
Mach bands
Mach bands is an optical illusion named after the physicist Ernst Mach  It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system 

Explanation
The Mach bands effect is due to the spatial high-boost filtering performed by the human visual system on the luminance channel of the image captured by the retina  Mach reported the effect in 1865, conjecturing that filtering is performed in the retina itself, by lateral inhibition among its neurons   This conjecture is supported by observations on other (non-visual) senses, as pointed out by von Békésy   The visual pattern is often found on curved surfaces subject to a particular, naturally-occurring illumination, so the occurrence of filtering can be explained as the result of learnt image statistics   The effect of filtering can be modeled as a convolution between a trapezoidal function that describes the illumination and one or more bandpass filters   A tight approximation is obtained by a model employing 9 even-symmetric filters scaled at octave intervals The effect is independent of the orientation of the boundary 

In radiology
This visual phenomenon is important to keep in mind when evaluating dental radiographs for evidence of decay, in which grayscale images of teeth and bone are analyzed for abnormal variances of density  A false-positive radiological diagnosis of dental caries can easily arise if the practitioner does not take into account the likelihood of this illusion  Mach bands manifest adjacent to metal restorations or appliances and the boundary between enamel and dentin  Mach bands may also result in the misdiagnosis of horizontal root fractures because of the differing radiographic intensities of tooth and bone Mach effect can also lead to an erroneous diagnosis of pneumothorax by creating a dark line at the lung periphery (whereas a true pneumothorax will have a white pleural line) 

In computer graphics
Mach bands can also appear when there is a discontinuity in the derivative of a gradient, a visual effect common when intensities are linearly interpolated such as in Gouraud shading 
Computer image processing systems use edge-detection in a way analogous to the brain, using unsharp masking to clarify edges in photos for example 

See also
Acutance
Cornsweet illusion
Hermann grid illusion
Lateral inhibition
Optical illusions
Watercolour illusion
Gibbs phenomenon
Passage 6:
Sound barrier
The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound  When aircraft first approached the speed of sound, these effects were seen as constituting a barrier, making faster speeds very difficult or impossible  The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime  Flying faster than sound produces a sonic boom 
In dry air at 20 °C (68 °F), the speed of sound is 343 metres per second (about 767 mph, 1234 km/h or 1,125 ft/s)  The term came into use during World War II when pilots of high-speed fighter aircraft experienced the effects of compressibility, a number of adverse aerodynamic effects that deterred further acceleration, seemingly impeding flight at speeds close to the speed of sound  These difficulties represented a barrier to flying at faster speeds  In 1947, American test pilot Chuck Yeager demonstrated that safe flight at the speed of sound was achievable in purpose-designed aircraft, thereby breaking the barrier  By the 1950s, new designs of fighter aircraft routinely reached the speed of sound, and faster 

History
Some common whips such as the bullwhip or stockwhip are able to move faster than sound: the tip of the whip exceeds this speed and causes a sharp crack—literally a sonic boom  Firearms made after the 19th century generally have a supersonic muzzle velocity The sound barrier may have been first breached by living beings about 150 million years ago  Some paleobiologists report that computer models of their biomechanical capabilities suggest that certain long-tailed dinosaurs such as Brontosaurus, Apatosaurus, and Diplodocus could flick their tails at supersonic speeds, creating a cracking sound  This finding is theoretical and disputed by others in the field  Meteorites in the Earth's upper atmosphere usually travel at higher than Earth's escape velocity, which is much faster than sound 

Early problems
The tip speed of a propeller blade depends on the propeller speed and the forward speed of the aircraft  When the aircraft speed is high enough, the tips reach supersonic speeds  Shock waves form at the blade tips and sap the shaft power driving the propeller  To maintain thrust, the engine power must replace this loss, and must also match the aircraft drag as it increases with speed  The required power is so great that the size and weight of the engine becomes prohibitive  This speed limitation led to research into jet engines, notably by Frank Whittle in England and Hans von Ohain in Germany  The jet engine is suitable for two reasons  It produces the required power, in terms of thrust, from a relatively small size compared to the piston engine it replaced  The whirling blades in the front of the jet engine are not adversely affected by high aircraft speeds in the same way as the propeller  
Nevertheless, propeller aircraft were able to approach their critical Mach number, different for each aircraft, in a dive  Unfortunately, doing so led to numerous crashes for a variety of reasons  Flying the Mitsubishi Zero, pilots sometimes flew at full power into terrain because the rapidly increasing forces acting on the control surfaces of their aircraft overpowered them  In this case, several attempts to fix it only made the problem worse  Likewise, the flexing caused by the low torsional stiffness of the Supermarine Spitfire's wings caused them, in turn, to counteract aileron control inputs, leading to a condition known as control reversal  This was solved in later models with changes to the wing  Worse still, a particularly dangerous interaction of the airflow between the wings and tail surfaces of diving Lockheed P-38 Lightnings made "pulling out" of dives difficult; however, the problem was later solved by the addition of a "dive flap" that upset the airflow under these circumstances  Flutter due to the formation of shock waves on curved surfaces was another major problem, which led most famously to the breakup of a de Havilland Swallow and death of its pilot Geoffrey de Havilland, Jr  on 27 September 1946  A similar problem is thought to have been the cause of the 1943 crash of the BI-1 rocket aircraft in the Soviet Union 
All of these effects, although unrelated in most ways, led to the concept of a "barrier" making it difficult for an aircraft to exceed the speed of sound  Erroneous news reports caused most people to envision the sound barrier as a physical "wall", which supersonic aircraft needed to "break" with a sharp needle nose on the front of the fuselage  Rocketry and artillery experts' products routinely exceeded Mach 1, but aircraft designers and aerodynamicists during and after World War II discussed Mach 0 7 as a limit dangerous to exceed 

Early claims
During WWII and immediately thereafter, a number of claims were made that the sound barrier had been broken in a dive  The majority of these purported events can be dismissed as instrumentation errors  The typical airspeed indicator (ASI) uses air pressure differences between two or more points on the aircraft, typically near the nose and at the side of the fuselage, to produce a speed figure  At high speed, the various compression effects that lead to the sound barrier also cause the ASI to go non-linear and produce inaccurately high or low readings, depending on the specifics of the installation  This effect became known as "Mach jump"  Before the introduction of Mach meters, accurate measurements of supersonic speeds could only be made remotely, normally using ground-based instruments  Many claims of supersonic speeds were found to be far below this speed when measured in this fashion 
In 1942, Republic Aviation issued a press release stating that Lts  Harold E  Comstock and Roger Dyar had exceeded the speed of sound during test dives in a Republic P-47 Thunderbolt  It is widely agreed that this was due to inaccurate ASI readings  In similar tests, the North American P-51 Mustang demonstrated limits at Mach 0 85, with every flight over Mach 0 84 causing the aircraft to be damaged by vibration 

One of the highest recorded instrumented Mach numbers attained for a propeller aircraft is the Mach 0 891 for a Spitfire PR XI, flown during dive tests at the Royal Aircraft Establishment, Farnborough in April 1944  The Spitfire, a photo-reconnaissance variant, the Mark XI, fitted with an extended "rake type" multiple pitot system, was flown by Squadron Leader J  R  Tobin to this speed, corresponding to a corrected true airspeed (TAS) of 606 mph  In a subsequent flight, Squadron Leader Anthony Martindale achieved Mach 0 92, but it ended in a forced landing after over-revving damaged the engine Hans Guido Mutke claimed to have broken the sound barrier on 9 April 1945 in the Messerschmitt Me 262 jet aircraft  He states that his ASI pegged itself at 1,100 kilometres per hour (680 mph)  Mutke reported not just transonic buffeting, but the resumption of normal control once a certain speed was exceeded, then a resumption of severe buffeting once the Me 262 slowed again  He also reported engine flame-out This claim is widely disputed, even by pilots in his unit  All of the effects he reported are known to occur on the Me 262 at much lower speeds, and the ASI reading is simply not reliable in the transonic  Further, a series of tests made by Karl Doetsch at the behest of Willy Messerschmitt found that the plane became uncontrollable above Mach 0 86, and at Mach 0 9 would nose over into a dive that could not be recovered from  Post-war tests by the RAF confirmed these results, with the slight modification that the maximum speed using new instruments was found to be Mach 0 84, rather than Mach 0 86 In 1999, Mutke enlisted the help of Professor Otto Wagner of the Munich Technical University to run computational tests to determine whether the aircraft could break the sound barrier  These tests do not rule out the possibility, but are lacking accurate data on the coefficient of drag that would be needed to make accurate simulations  Wagner stated: "I don't want to exclude the possibility, but I can imagine he may also have been just below the speed of sound and felt the buffeting, but did not go above Mach-1 "One bit of evidence presented by Mutke is on page 13 of the "Me 262 A-1 Pilot's Handbook" issued by Headquarters Air Materiel Command, Wright Field, Dayton, Ohio as Report No  F-SU-1111-ND on January 10, 1946:

Speeds of 950 km/h (590 mph) are reported to have been attained in a shallow dive 20° to 30° from the horizontal  No vertical dives were made  At speeds of 950 to 1,000 km/h (590 to 620 mph) the air flow around the aircraft reaches the speed of sound, and it is reported that the control surfaces no longer affect the direction of flight  The results vary with different airplanes: some wing over and dive while others dive gradually  It is also reported that once the speed of sound is exceeded, this condition disappears and normal control is restored 
The comments about restoration of flight control and cessation of buffeting above Mach 1 are very significant in a 1946 document  However, it is not clear where these terms came from, as it does not appear the US pilots carried out such tests In his 1990 book Me-163, former Messerschmitt Me 163 "Komet" pilot Mano Ziegler claims that his friend, test pilot Heini Dittmar, broke the sound barrier while diving the rocket plane, and that several people on the ground heard the sonic booms  He claims that on 6 July 1944, Dittmar, flying Me 163B V18, bearing the Stammkennzeichen alphabetic code VA+SP, was measured traveling at a speed of 1,130 km/h (702 mph)  However, no evidence of such a flight exists in any of the materials from that period, which were captured by Allied forces and extensively studied  Dittmar had been officially recorded at 1,004 5 km/h (623 8 mph) in level flight on 2 October 1941 in the prototype Me 163A V4  He reached this speed at less than full throttle, as he was concerned by the transonic buffeting  Dittmar himself does not make a claim that he broke the sound barrier on that flight and notes that the speed was recorded only on the AIS  He does, however, take credit for being the first pilot to "knock on the sound barrier" The Luftwaffe test pilot Lothar Sieber (7 April 1922 – 1 March 1945) may have inadvertently become the first human to break the sound barrier on 1 March 1945  This occurred while he was piloting a Bachem Ba 349 "Natter" for the first crewed vertical takeoff of a rocket in history  In 55 seconds, he traveled a total of 14 km (8 7 miles)  The aircraft crashed, and he perished violently in this endeavor There are a number of unmanned vehicles that flew at supersonic speeds during this period  In 1933, Soviet designers working on ramjet concepts fired phosphorus-powered engines out of artillery guns to get them to operational speeds  It is possible that this produced supersonic performance as high as Mach 2, but this was not due solely to the engine itself  In contrast, the German V-2 ballistic missile routinely broke the sound barrier in flight, for the first time on 3 October 1942  By September 1944, V-2s routinely achieved Mach 4 (1,200 m/s, or 3044 mph) during terminal descent 

Breaking the sound barrier
In 1942, the United Kingdom's Ministry of Aviation began a top-secret project with Miles Aircraft to develop the world's first aircraft capable of breaking the sound barrier  The project resulted in the development of the prototype Miles M 52 turbojet-powered aircraft, which was designed to reach 1,000 mph (417 m/s; 1,600 km/h) (over twice the existing speed record) in level flight, and to climb to an altitude of 36,000 ft (11 km) in 1 minute 30 seconds 
A huge number of advanced features were incorporated into the resulting M 52 design, many of which hint at a detailed knowledge of supersonic aerodynamics  In particular, the design featured a conical nose and sharp wing leading edges, as it was known that round-nosed projectiles could not be stabilised at supersonic speeds  The design used very thin wings of biconvex section proposed by Jakob Ackeret for low drag  The wing tips were "clipped" to keep them clear of the conical shock wave generated by the nose of the aircraft  The fuselage had the minimum cross-section allowable around the centrifugal engine with fuel tanks in a saddle over the top 

Another critical addition was the use of a power-operated stabilator, also known as the all-moving tail or flying tail, a key to supersonic flight control, which contrasted with traditional hinged tailplanes (horizontal stabilizers) connected mechanically to the pilots control column  Conventional control surfaces became ineffective at the high subsonic speeds then being achieved by fighters in dives, due to the aerodynamic forces caused by the formation of shockwaves at the hinge and the rearward movement of the centre of pressure, which together could override the control forces that could be applied mechanically by the pilot, hindering recovery from the dive  A major impediment to early transonic flight was control reversal, the phenomenon which caused flight inputs (stick, rudder) to switch direction at high speed; it was the cause of many accidents and near-accidents  An all-flying tail is considered to be a minimum condition of enabling aircraft to break the transonic barrier safely, without losing pilot control  The Miles M 52 was the first instance of this solution, which has since been universally applied 
Initially, the aircraft was to use Frank Whittle's latest engine, the Power Jets W 2/700, which would only reach supersonic speed in a shallow dive  To develop a fully supersonic version of the aircraft, an innovation incorporated was a reheat jetpipe – also known as an afterburner  Extra fuel was to be burned in the tailpipe to avoid overheating the turbine blades, making use of unused oxygen in the exhaust  Finally, the design included another critical element –  the use of a shock cone in the nose to slow the incoming air to the subsonic speeds needed by the engine 
Although the project was eventually cancelled, the research was used to construct an unmanned missile that went on to achieve a speed of Mach 1 38 in a successful, controlled transonic and supersonic level test flight; this was a unique achievement at that time, which validated the aerodynamics of the M 52 
Meanwhile, test pilots achieved high velocities in the tailless, swept-wing de Havilland DH 108  One of them was Geoffrey de Havilland, Jr , who was killed on 27 September 1946 when his DH 108 broke up at about Mach 0 9  John Derry has been called "Britain's first supersonic pilot" because of a dive he made in a DH 108 on 6 September 1948 

The first aircraft to officially break the sound barrier
The British Air Ministry signed an agreement with the United States to exchange all its high-speed research, data and designs and Bell Aircraft company was given access to the drawings and research on the M 52, but the U S  reneged on the agreement, and no data was forthcoming in return  Bell's supersonic design was still using a conventional tail, and they were battling the problem of control 

They utilized the information to initiate work on the Bell X-1  The final version of the Bell X-1 was very similar in design to the original Miles M 52 version  Also featuring the all-moving tail, the XS-1 was later known as the X-1  It was in the X-1 that Chuck Yeager was credited with being the first person to break the sound barrier in level flight on 14 October 1947, flying at an altitude of 45,000 ft (13 7 km)  George Welch made a plausible but officially unverified claim to have broken the sound barrier on 1 October 1947, while flying an XP-86 Sabre  He also claimed to have repeated his supersonic flight on 14 October 1947, 30 minutes before Yeager broke the sound barrier in the Bell X-1  Although evidence from witnesses and instruments strongly imply that Welch achieved supersonic speed, the flights were not properly monitored and are not officially recognized  The XP-86 officially achieved supersonic speed on 26 April 1948 On 14 October 1947, just under a month after the United States Air Force had been created as a separate service, the tests culminated in the first crewed supersonic flight, piloted by Air Force Captain Charles "Chuck" Yeager in aircraft #46-062, which he had christened Glamorous Glennis  The rocket-powered aircraft was launched from the bomb bay of a specially modified B-29 and glided to a landing on a runway  XS-1 flight number 50 is the first one where the X-1 recorded supersonic flight, with a maximum speed of Mach 1 06 (361 m/s, 1,299 km/h, 807 2 mph) 
As a result of the X-1's initial supersonic flight, the National Aeronautics Association voted its 1947 Collier Trophy to be shared by the three main participants in the program  Honored at the White House by President Harry S  Truman were Larry Bell for Bell Aircraft, Captain Yeager for piloting the flights, and John Stack for the NACA contributions Jackie Cochran was the first woman to break the sound barrier, which she did on 18 May 1953, piloting a plane borrowed from the Royal Canadian Air Force, with Yeager accompanying her On December 3, 1957, Margaret Chase Smith became the first woman in Congress to break the sound barrier, which she did as a passenger in an F-100 Super Sabre piloted by Air Force Major Clyde Good In the late 1950s, Allen Rowley, a British journalist, was able to fly in a Super Sabre at 1000mph one of the few non-American civilians to exceed the speed of sound and one of the few civilians anywhere to make such a trip On 21 August 1961, a Douglas DC-8-43 (registration N9604Z) unofficially exceeded Mach 1 in a controlled dive during a test flight at Edwards Air Force Base, as observed and reported by the flight crew; the crew were William Magruder (pilot), Paul Patten (co-pilot), Joseph Tomich (flight engineer), and Richard H  Edwards (flight test engineer)  This was the first supersonic flight by a civilian airliner, achieved before the Concorde or the Tu-144 flew 

The sound barrier understood
As the science of high-speed flight became more widely understood, a number of changes led to the eventual understanding that the "sound barrier" is easily penetrated, with the right conditions  Among these changes were the introduction of thin swept wings, the area rule, and engines of ever-increasing performance  By the 1950s, many combat aircraft could routinely break the sound barrier in level flight, although they often suffered from control problems when doing so, such as Mach tuck  Modern aircraft can transit the "barrier" without control problems By the late 1950s, the issue was so well understood that many companies started investing in the development of supersonic airliners, or SSTs, believing that to be the next "natural" step in airliner evolution  However, this has not yet happened  Although the Concorde and the Tupolev Tu-144 entered service in the 1970s, both were later retired without being replaced by similar designs  The last flight of a Concorde in service was in 2003 
Although Concorde and the Tu-144 were the first aircraft to carry commercial passengers at supersonic speeds, they were not the first or only commercial airliners to break the sound barrier  On 21 August 1961, a Douglas DC-8 broke the sound barrier at Mach 1 012, or 1,240 km/h (776 2 mph), while in a controlled dive through 41,088 feet (12,510 m)  The purpose of the flight was to collect data on a new design of leading edge for the wing 

Breaking the sound barrier in a land vehicle
On 12 January 1948, a Northrop unmanned rocket sled became the first land vehicle to break the sound barrier  At a military test facility at Muroc Air Force Base (now Edwards AFB), California, it reached a peak speed of 1,019 mph (1,640 km/h) before jumping the rails On 15 October 1997, in a vehicle designed and built by a team led by Richard Noble, Royal Air Force pilot Andy Green became the first person to break the sound barrier in a land vehicle in compliance with Fédération Internationale de l'Automobile rules  The vehicle, called the ThrustSSC ("Super Sonic Car"), captured the record 50 years and one day after Yeager's first supersonic flight 

Breaking the sound barrier as a human projectile
Felix Baumgartner
In October 2012 Felix Baumgartner, with a team of scientists and sponsor Red Bull, attempted the highest sky-dive on record  The project would see Baumgartner attempt to jump 120,000 ft (36,580 m) from a helium balloon and become the first parachutist to break the sound barrier  The launch was scheduled for 9 October 2012, but was aborted due to adverse weather; subsequently the capsule was launched instead on 14 October  Baumgartner's feat also marked the 65th anniversary of U S  test pilot Chuck Yeager's successful attempt to break the sound barrier in an aircraft Baumgartner landed in eastern New Mexico after jumping from a world record 128,100 feet (39,045 m), or 24 26 miles, and broke the sound barrier as he traveled at speeds up to 833 9 mph (1342 km/h, or Mach 1 26)  In the press conference after his jump, it was announced that he was in freefall for 4 minutes 18 seconds, the second longest freefall after the 1960 jump of Joseph Kittinger for 4 minutes 36 seconds 

Alan Eustace
In October 2014, Alan Eustace, a senior vice president at Google, broke Baumgartner's record for highest sky-dive and also broke the sound barrier in the process  However, because Eustace's jump involved a drogue parachute, while Baumgartner's did not, their vertical speed and free-fall distance records remain in different categories 

Legacy
David Lean directed The Sound Barrier, a fictionalized retelling of the de Havilland DH 108 test flights 

See also
Sonic boom
Vapor cone
Passage 7:
Global Alliance in Management Education
CEMS - The Global Alliance in Management Education or CEMS (formerly the Community of European Management Schools and International Companies) is a cooperation of leading business schools and universities with multinational companies and NGOs  The CEMS Global Alliance includes 34 academic institutions on every continent, nearly 70 corporate partners and eight social partners (NGOs) from around the globe  CEMS administers the delivery of the CEMS MIM degree in its member schools, supports the CEMS Alumni Association (CAA) and facilitates general cooperation among its members 

CEMS MIM
CEMS Master in International Management (CEMS MIM) is a one-year degree program for a select group of students at the member institutions,  taught jointly by CEMS business schools and universities  Created in 1988 by founding members from the University of Cologne, HEC Paris, ESADE and the Bocconi University, it was the first supra-national MSc  The aim of CEMS is to develop knowledge and provide education that is essential in the multilingual, multicultural and interconnected business world The MIM Programme consists of three terms: two academic terms and one internship term  The two academic terms must be consecutive, while the internship term can take place at any time during the graduate period of studies  Students must spend at least two out of the three terms abroad  In addition to completing one's home degree, graduation from CEMS also requires completion of a business project, skill seminars, an international internship, and two foreign language exams 
Each CEMS academic member has a limited number of places available  In many cases, schools have prerequisites to be admitted into the selection process, including a high-grade average and proof of language skills  The selection process typically requires the student to already be enrolled or selected for a Master of Business degree with a member university before applying for the CEMS MIM  CEMS graduates receive a degree from their home institution as well as from CEMS 

Global ranking
The CEMS MIM has consistently ranked among the top 10 in the Masters in Management Ranking since 2005 by the Financial Times 

Academic members
Schools offering the CEMS Master's in International Management (CEMS MIM):

Corporate partners
Nearly 70 corporate partners contribute financially on an annual basis and provide the programme with human resources and input into the curriculum itself in each country they are based  They chart skills courses, give lectures or invite CEMS students to company activities where students learn about corporate practices or solve cases  These connections are very useful to promote the company to the students, while the students gain insight into real-world problems and solution approaches 
Students are advised to be responsible and arrange by themselves an internship partner which will accept the student intern for at least ten consecutive weeks  There are student visa requirements that each student takes responsibility to abide by, and the regulation varies by each local government 

Social partners
The first CEMS social partners joined the organisation in December 2010  These are the first in a series of non-profits and NGOs that contribute to the alliance in a way identical to corporate partners (selection and admission of students, governance, curriculum delivery, proposal of internships and employment opportunities)  This new initiative is part of a major sustainability drive from within CEMS  In the same vein, CEMS has also signed the PRME (Principles for Responsible Management Education) declaration 

Alumni association
The CEMS Alumni Association (CAA), founded in 1993 by CEMS graduates, is an international network of CEMS graduates throughout the world  To date, there are nearly 16,000 CEMS alumni  The Graduation Ceremony takes place each year during the CEMS Annual Events (usually at the end of November) which is hosted by one of the CEMS member schools The CAA is led by an alumni board and is present in many countries through local committees of CEMS alumni  The local committees are responsible for keeping in contact with CEMS alumni and organizing professional and social activities  They meet regularly to discuss the activities and development of the association  The alumni board comprises the CAA President, the CEMS Executive Director, a representative of the CEMS Student Board, a representative of CEMS member schools, three local committee representatives, two senior alumni and two junior alumni  It proposes and develops initiatives to foster career and personal development opportunities for its alumni members and represents alumni interests on the CEMS Executive Board 
While students stay at partner universities, there are support groups called CEMS clubs through which CEMS students share their identities  The extended network of CEMS students spans schools across the world 

Footnotes
Notes
Passage 8:
Confederation of Finnish Industries
The Confederation of Finnish Industries (EK, Finnish: Elinkeinoelämän Keskusliitto, Swedish: Finlands Näringsliv) is the largest employers' association in Finland  It was formed at the beginning of 2005 when the two employers' associations Palvelutyönantajat (Employers of the Service Sector) and Teollisuuden ja Työnantajain Keskusliitto (Union of Industries and Employers) merged  EK's member companies collectively contribute over 70% of Finland's GDP, and over 95% of Finland's exports  It has considerable negotiating power, since Finland has universal validity of collective labour agreements, and often a national income policy agreement is reached 
EK focuses its activity on the following goals:

A business environment which stimulates growth and success for companies
Securing the competitiveness of Finnish work
Ways to benefit from the opportunities offered by globalisation
Economic policies promoting competitiveness
Efficient member servicesThe organisation consists of:

43 branch associations
About 15,000 member companies, 96% of which are small and medium enterprises (SMEs)
About 900,000 employees

External links
The Confederation of Finnish Industries  Official site
Passage 9:
Mach's principle
In theoretical physics, particularly in discussions of gravitation theories,  Mach's principle (or Mach's conjecture) is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach  The hypothesis attempted to explain how rotating objects, such as gyroscopes and spinning celestial bodies, maintain a frame of reference  
The proposition is that the existence of absolute rotation (the distinction of local inertial frames vs  rotating reference frames) is determined by the large-scale distribution of matter, as exemplified by this anecdote:

You are standing in a field looking at the stars  Your arms are resting freely at your side, and you see that the distant stars are not moving  Now start spinning  The stars are whirling around you and your arms are pulled away from your body  Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?

Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame  If you see all the stars whirling around you, Mach suggests that there is some physical law which would make it so you would feel a centrifugal force  There are a number of rival formulations of the principle, often stated in vague ways like "mass out there influences inertia here"  A very general statement of Mach's principle is "local physical laws are determined by the large-scale structure of the universe" Mach's concept was a guiding factor in Einstein's development of the general theory of relativity  Einstein realized that the overall distribution of matter would determine the metric tensor which indicates which frame is stationary with respect to rotation  Frame-dragging and conservation of gravitational angular momentum makes this into a true statement in the general theory in certain solutions  But because the principle is so vague, many distinct statements have been made which would qualify as a Mach principle, and some of which are false  The Gödel rotating universe is a solution of the field equations that is designed to disobey Mach's principle in the worst possible way  In this example, the distant stars seem to be revolving faster and faster as one moves further away  This example does not completely settle the question of the physical relevance of the principle because it has closed timelike curves 

History
Mach put forth the idea in his book The Science of Mechanics (1883 in German, 1893 in English)  Before Mach's time, the basic idea also appears in the writings of George Berkeley  After Mach, the book Absolute or Relative Motion? (1896) by Benedict Friedlaender and his brother Immanuel contained ideas similar to Mach's principle 

Einstein's use of the principle
There is a fundamental issue in relativity theory: if all motion is relative, how can we measure the inertia of a body? We must measure the inertia with respect to something else  But what if we imagine a particle completely on its own in the universe? We might hope to still have some notion of its state of motion  Mach's principle is sometimes interpreted as the statement that such a particle's state of motion has no meaning in that case 
In Mach's words, the principle is embodied as follows:
[The] investigator must feel the need of    knowledge of the immediate connections, say, of the masses of the universe  There will hover before him as an ideal insight into the principles of the whole matter, from which accelerated and inertial motions will result in the same way 
Albert Einstein seemed to view Mach's principle as something along the lines of:
   inertia originates in a kind of interaction between bodies   
In this sense, at least some of Mach's principles are related to philosophical holism  Mach's suggestion can be taken as the injunction that gravitation theories should be relational theories  Einstein brought the principle into mainstream physics while working on general relativity  Indeed, it was Einstein who first coined the phrase Mach's principle  There is much debate as to whether Mach really intended to suggest a new physical law since he never states it explicitly 
The writing in which Einstein found inspiration was Mach's book The Science of Mechanics (1883, tr  1893), where the philosopher criticized Newton's idea of absolute space, in particular the argument that Newton gave sustaining the existence of an advantaged reference system: what is commonly called "Newton's bucket argument" 
In his Philosophiae Naturalis Principia Mathematica, Newton tried to demonstrate that one can always decide if one is rotating with respect to the absolute space, measuring the apparent forces that arise only when an absolute rotation is performed  If a bucket is filled with water, and made to rotate, initially the water remains still, but then, gradually, the walls of the vessel communicate their motion to the water, making it curve and climb up the borders of the bucket, because of the centrifugal forces produced by the rotation  This experiment demonstrates that the centrifugal forces arise only when the water is in rotation with respect to the absolute space (represented here by the earth's reference frame, or better, the distant stars) instead, when the bucket was rotating with respect to the water no centrifugal forces were produced, this indicating that the latter was still with respect to the absolute space 
Mach, in his book, says that the bucket experiment only demonstrates that when the water is in rotation with respect to the bucket no centrifugal forces are produced, and that we cannot know how the water would behave if in the experiment the bucket's walls were increased in depth and width until they became leagues big  In Mach's idea this concept of absolute motion should be substituted with a total relativism in which every motion, uniform or accelerated, has sense only in reference to other bodies (i e , one cannot simply say that the water is rotating, but must specify if it's rotating with respect to the vessel or to the earth)  In this view, the apparent forces that seem to permit discrimination between relative and "absolute" motions should only be considered as an effect of the particular asymmetry that there is in our reference system between the bodies which we consider in motion, that are small (like buckets), and the bodies that we believe are still (the earth and distant stars), that are overwhelmingly bigger and heavier than the former 
This same thought had been expressed by the philosopher George Berkeley in his De Motu  It is then not clear, in the passages from Mach just mentioned, if the philosopher intended to formulate a new kind of physical action between heavy bodies  This physical mechanism should determine the inertia of bodies, in a way that the heavy and distant bodies of our universe should contribute the most to the inertial forces  More likely, Mach only suggested a mere "redescription of motion in space as experiences that do not invoke the term space"   What is certain is that Einstein interpreted Mach's passage in the former way, originating a long-lasting debate 
Most physicists believe Mach's principle was never developed into a quantitative physical theory that would explain a mechanism by which the stars can have such an effect  Mach himself never made his principle exactly clear : 9–57  Although Einstein was intrigued and inspired by Mach's principle, Einstein's formulation of the principle is not a fundamental assumption of general relativity, although the principle of equivalence of gravitational and inertial mass is most certainly fundamental 

Mach's principle in general relativity
Because intuitive notions of distance and time no longer apply, what exactly is meant by "Mach's principle" in general relativity is even less clear than in Newtonian physics and at least 21 formulations of Mach's principle are possible, some being considered more strongly Machian than others : 530   A relatively weak formulation is the assertion that the motion of matter in one place should affect which frames are inertial in another 
Einstein, before completing his development of the general theory of relativity, found an effect which he interpreted as being evidence of Mach's principle   We assume a fixed background for conceptual simplicity, construct a large spherical shell of mass, and set it spinning in that background   The reference frame in the interior of this shell will precess with respect to the fixed background   This effect is known as the Lense–Thirring effect   Einstein was so satisfied with this manifestation of Mach's principle that he wrote a letter to Mach expressing this:

it    turns out that inertia originates in a kind of interaction between bodies, quite in the sense of your considerations on Newton's pail experiment     If one rotates [a heavy shell of matter] relative to the fixed stars about an axis going through its center, a Coriolis force arises in the interior of the shell; that is, the plane of a Foucault pendulum is dragged around (with a practically unmeasurably small angular velocity) 
The Lense–Thirring effect certainly satisfies the very basic and broad notion that "matter there influences inertia here"   The plane of the pendulum would not be dragged around if the shell of matter were not present, or if it were not spinning   As for the statement that "inertia originates in a kind of interaction between bodies", this, too, could be interpreted as true in the context of the effect 
More fundamental to the problem, however, is the very existence of a fixed background, which Einstein describes as "the fixed stars"   Modern relativists see the imprints of Mach's principle in the initial-value problem   Essentially, we humans seem to wish to separate spacetime into slices of constant time   When we do this, Einstein's equations can be decomposed into one set of equations, which must be satisfied on each slice, and another set, which describe how to move between slices   The equations for an individual slice are elliptic partial differential equations   In general, this means that only part of the geometry of the slice can be given by the scientist, while the geometry everywhere else will then be dictated by Einstein's equations on the slice In the context of an asymptotically flat spacetime, the boundary conditions are given at infinity   Heuristically, the boundary conditions for an asymptotically flat universe define a frame with respect to which inertia has meaning   By performing a Lorentz transformation on the distant universe, of course, this inertia can also be transformed 
A stronger form of Mach's principle applies in Wheeler–Mach–Einstein spacetimes, which require spacetime to be spatially compact and globally hyperbolic   In such universes Mach's principle can be stated as the distribution of matter and field energy-momentum (and possibly other information) at a particular moment in the universe determines the inertial frame at each point in the universe (where "a particular moment in the universe" refers to a chosen Cauchy surface) : 188–207 There have been other attempts to formulate a theory that is more fully Machian, such as the Brans–Dicke theory and the Hoyle–Narlikar theory of gravity, but most physicists argue that none have been fully successful  At an exit poll of experts, held in Tübingen in 1993, when asked the question "Is general relativity perfectly Machian?", 3 respondents replied "yes", and 22 replied "no"   To the question "Is general relativity with appropriate boundary conditions of closure of some kind very Machian?" the result was 14 "yes" and 7 "no" : 106 However, Einstein was convinced that a valid theory of gravity would necessarily have to include the relativity of inertia:

So strongly did Einstein believe at that time in the relativity of inertia that in 1918 he stated as being on an equal footing three principles on which a satisfactory theory of gravitation should rest:
The principle of relativity as expressed by general covariance 
The principle of equivalence 
Mach's principle (the first time this term entered the literature): … that the gµν are completely determined by the mass of bodies, more generally by Tµν In 1922, Einstein noted that others were satisfied to proceed without this [third] criterion and added, 
"This contentedness will appear incomprehensible to a later generation however "

It must be said that, as far as I can see, to this day, Mach's principle has not brought physics decisively farther  It must also be said that the origin of inertia is and remains the most obscure subject in the theory of particles and fields  Mach's principle may therefore have a future – but not without the quantum theory 

Inertial induction
In 1953, in order to express Mach's Principle in quantitative terms, the Cambridge University physicist Dennis W  Sciama proposed the addition of an acceleration dependent term to the Newtonian gravitation equation  Sciama's acceleration dependent term was 
  
    
      
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    {\textstyle F=G{\frac {m_{A}m_{B}{\bf {a}}}{rc^{2}}}\ }
   where r is the distance between the particles, G is the gravitational constant, a is the relative acceleration and c represents the speed of light in vacuum  Sciama referred to the effect of the acceleration dependent term as Inertial Induction 

Variations in the statement of the principle
The broad notion that "mass there influences inertia here" has been expressed in several forms 
Hermann Bondi and Joseph Samuel have listed eleven distinct statements that can be called Mach principles, labelled Mach0 through Mach10   Though their list is not necessarily exhaustive, it does give a flavor for the variety possible 

Mach0: The universe, as represented by the average motion of distant galaxies, does not appear to rotate relative to local inertial frames 
Mach1: Newton's gravitational constant G is a dynamical field 
Mach2: An isolated body in otherwise empty space has no inertia 
Mach3: Local inertial frames are affected by the cosmic motion and distribution of matter 
Mach4: The universe is spatially closed 
Mach5: The total energy, angular and linear momentum of the universe are zero 
Mach6: Inertial mass is affected by the global distribution of matter 
Mach7: If you take away all matter, there is no more space 
Mach8: 
  
    
      
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Mach9: The theory contains no absolute elements 
Mach10: Overall rigid rotations and translations of a system are unobservable 

See also



ground_truth: Charles University