Physics

{{bronze}}
[[File:Magnet 4.jpg|thumb|right|165px|A levitating magnet. The Meissner effect, or the will of [[God]]?]]
{{Physics}}
{{hatnote|If you happen to be lysdexic, you might be looking for [[psychic]]s instead.}}
{{cquote|The simplicity of nature is not to be measured by that of our conceptions. Infinitely varied in its effects, nature is simple only in its causes, and its economy consists in producing a great number of phenomena, often very complicated, by means of a small number of general laws.|||Pierre-Simon de Laplace, ''Explosition du système du monde''.<ref name="Gribbin"/>}}
'''Physics''' is the study of space, time, matter, energy and their interactions with one another. Its scope ranges in size from the smallest [[subatomic particles]] to [[Cosmology|the entire Universe]]. Physics is therefore the most basic of all natural sciences.<ref>[http://www.merriam-webster.com/dictionary/physics Physics]. Merriam-Webster Online Dictionary.</ref><ref>Giancoli D. ''Physics: Principles with Applications''. 6th ed. Upper Saddle River, NJ: Pearson Education; 2005. {{isbn|0130606200}}</ref>

Laws, [[hypotheses]] and [[theories]] of physics are most often expressed in the language of [[mathematics]]. In many cases, breakthroughs in physics are made possible by using already existing mathematical techniques. For example, the pioneers of quantum mechanics took advantage of the mathematical methods of linear algebra, complex variables, and partial differential equations developed in the eighteenth and nineteenth centuries. In other cases, however, the development of physics motivates that of mathematics. A classic example is the development of calculus by Sir Isaac Newton who needed it in his work on classical mechanics and gravitation.<ref>Kakalios J. ''The Physics of Superheroes''. New York: Gotham Books; 2005. "Chapter 21: Not A Dream! Not A Hoax! Not An Imaginary Tale!{{---}}Quantum Mechanics". {{isbn|0715635492}}</ref>

==Early physics==
[[Aristotle]] believed that everything was made up of one of the five elements: earth, fire, air, water, and "quintessence", or the "fifth essence". He also thought the heavenly bodies were perfect and unchanging. His ideas were simply accepted. It is interesting to note that in Eastern philosophy, the Universe is considered to comprise of the five elements: metals, wood, water, fire and earth. There is not much empirical evidence to support either claim. At this stage, research in the natural sciences in general and physics in particular were conducted by philosophers, who could not be bothered to verify their claims with careful observations or experiments.<ref name="Gribbin"/> Aristotle in particular made the most casual of observations then drew the most general of conclusions from them. He taught, for example, that an apple falls to the Earth because it has gravity but smoke rises because it has levity.<ref name="BBC">[[BBC]] Documentary: Isaac Newton, the Last Magician. ([http://www.dailymotion.com/video/x1wjy2r_isaac-newton-the-last-magician-s01e01-480p-hdtv-x264-msd_creation Daily Motion link])</ref>

Things improved dramatically with the arrival of Archimedes of Syracuse. Starting from empirical observations and experiments, he discovered the principle of buoyancy in hydrostatics, the law of the lever and introduced the concept of the center of mass of a body.<ref>Archimedes, Heath T.L (''trans.'' and ''ed.''). ''The Works of Archimedes, Edited in Modern Notation with Introductory Chapters''. Mineola, NY: Dover Publications, Inc.; 1897. {{isbn|0486420841}}</ref> Archimedes also built a planetarium, which operated on the basis of a [[heliocentric theory]].<ref>Geymonat M. ''The Great Archimedes''. Waco, Tex.: Baylor University Press; 2010. {{isbn|9781602583115}}</ref> Observe that the ancient Greeks did in fact entertain the possibility of the Earth and other planets orbiting the Sun. Ultimately, [[geocentric]] models proved to be more popular. This was likely due to their fewer observational discrepancies as seen with the naked eye: chiefly, the lack of stellar parallax (it is impossible to state with certainty exactly why heliocentrism was rejected, as no works by heliocentric astronomers or their critics have survived; Aristarchus' heliocentrism is known only because of the reference in Archimedes' ''The Sand Reckoner''). Geocentrism would be held almost unanimously by astronomers until the later 17th century, when Kepler's heliocentric model proved instrumentally superior to the geocentric models (direct observational evidence of Earth's movement, completely shattering the idea that geocentric models were reality, was first done by James Bradley in the early 18th century).

The ancient Greeks made a number of important advances in the study of optics. Euclid of Alexandria, best known for his comprehensive treatise on geometry, published a book on geometric optics. He recognized that light propagated in straight lines, and enunciated the law of reflection, the angle of incidence equals the angle of reflection. He also treated a variety of different mirrors. Hero of Alexandria attempted to explain the behavior of light by assuming that light propagated in such a way that time taken to travel between two points is minimized. During the middle ages, some investigations by scholars working in Baghdad kept the subject alive. But by and large the progress in physics languished.<ref name="Hecht"> Hecht, Eugene. “Chapter 1 - A Brief History.” ''Optics''. Hecht, 5th ed., Addison-Wesley, 2017. {{isbn|9780133977226}}</ref>

==Seventeenth century physics: Warming up==
Willebrord Snell experimentally discovered the law of refraction, which now bears his name.<ref name="Hecht"/> [[Galileo Galilei]] argued that all bodies fall at the same rate regardless of their masses, if air resistance is negligible. He proved that if air resistance can be ignored, bodies thrown at an angle will fall along a parabolic trajectory. While telescopes were not unheard of at this time, Galileo built superior ones, using which he made observations disproving the Ptolemaic system, discovered the (Galilean) moons of Jupiter, sunspots, that the Moon is full of craters, among other imperfections, all of which contradict Aristotle's teachings. He formulated the law of the pendulum, but tried, and failed, to design a clock that was usable at sea. Galileo was among the very first scientists as we recognize them today. He fully understood the necessity of experiments and observations and made use of them whenever possible in his investigations. [[René Descartes|Réne Descartes]] correctly stated that inertia is the tendency of a massive body to travel in a straight line at constant speed.<ref name="Gribbin"/> This later became what we now call Newton's first law of motion. But perhaps his most important contribution was the discovery of coordinate (or Cartesian) geometry, bringing together the powers of symbolic algebra and geometry. The value of this new geometry in both mathematics and physics can hardly be overestimated; it enabled for much more convenient methods of proof and discovery. Furthermore, it paved the way for calculus, which underlies much of physics.<ref name="Ball">Ball, W. W. Rouse. ''A Short Account of the History of Mathematics''. 4th ed., Dover Publications, Inc., 1908. {{isbn|0486206300}}</ref>

{{wpl|Ole Rømer}} observed that from Earth, the moons of Jupiter appear to be regularly eclipsed. However, the times between these eclipse are not really constant; in fact they vary according to the position of Earth in its orbit about the Sun. He concluded that the speed of light must therefore be finite. Using the best estimate of the radius of the Earth's orbit available at the time, he calculated the speed of light to be 225,000 km/s. Although this value is not very accurate by modern standards, it was the first time that the speed of light was definitively shown to be finite.<ref name="Gribbin"/>

==Eighteenth century physics: Age of Newton==
[[File:Dispersive_Prism_Illustration.jpg|thumb|right|200px|A prism decomposes white light into its constituent colors. This indicates that the refractive index is a function of wavelength.]]

===Mechanics and Optics===
Sir [[Isaac Newton]] dominated this era. In his masterpiece the ''Philosophiae Naturalis Principia Mathematica'', or ''Principia'' for short, he laid down the basics of calculus, the laws of motion and gravitation. Newton gave a theoretical derivation of [[Kepler]]'s laws of orbital motion, previously obtained from astronomical data provided by Tycho Brahe, and an explanation of tides.<ref name="Gribbin"/> While physics has its roots in astronomy and a number of important results have been published before Newton, e.g. Archimedes' principle, it was Newton who constructed physics as a mathematical formalism. It was he who demonstrated that natural phenomena, for all their apparent complexities, arise from a small number of fundamental laws, in a very effective manner. In ''Opticks'', Newton described his famous prism experiment, discussed the laws of optics and articulated his corpuscular theory of light.<ref name="Gribbin"/> Christian Huygens and a few others, however, favored a wave theory of light. He also studied the behavior of pendulums, formulating a sophisticated theory of oscillations and inventing a pendulum clock in the process.<ref name="Gribbin"/> He suggested that clocks be made using a cycloidal pendulum, which he showed to be isochronous, meaning the period is unaffected by the amplitude of motion. He studied uniform circular motion and obtained the famous formula for the centripetal acceleration.<ref name="Ball"/>

After Newton's death in 1727, Leonard Euler, Daniel Bernoulli, Jean le Rond d'Alembert, Joseph-Louis Lagrange, Pierre-Simon de Laplace, among others, continued the development of calculus and brought classical mechanics to great new heights, as can easily be noted by opening up an advanced textbook on the subject.<ref name="Hand">Hand L, Finch J. ''Analytical Mechanics''. Cambridge: Cambridge University Press; 1998. {{isbn|0521575729}} paperback.</ref> D'Alembert introduced the principle of virtual work, essentially a reformulation of Newton's laws but one that employs energy, a scalar, rather than forces, which are vectors, or quantities with both magnitudes and direction. This simplifies the analysis of mechanical problems. Euler and Lagrange showed that, equivalently, from the principle of least action, one could, using the machinery of the calculus of variations, which they developed, obtain the equations of motion exactly identical to those derived by a direct application of Newton's laws. Furthermore, the Euler-Lagrange equations retain the same form regardless of coordinate systems whereas Newton's second law in the familiar form is only valid in Cartesian coordinates. Euler gave a systematic treatment of the dynamics of rigid bodies. Lagrange invented the notion of potentials. Euler and Lagrange studied the three-body problem, such as the Sun-Earth-Moon system, which they solved approximately.<ref group="note">This led him to what we now call the five Lagrange points, L<sub>1</sub> to L<sub>5</sub>, the first three unstable while the last two stable. In nature, debris tend to collect in the vicinity of Lagrange points and many artificial satellites have been made to orbit these locations.</ref> Lagrange managed to explain lunar libration (apparent oscillations seen from Earth) and why the Moon always shows the same face to the Earth. Lagrange discussed all these amazing developments in mechanics in his masterpiece, ''Méchanique Analytique''.<ref name="Ball"/>

Near the end of the century in 1791, Giovanni Guglielmini observed the Coriolis Effect by dropping balls down the inside the Tower of Bologna, providing a direct observational detection of the Earth's rotation (the need for such an effect in a rotating Earth was noted by Giovanni Riccioli in the 1650s, and Robert Hooke had previously attempted the experiment, but was not confident in his results). In 1797-98, Cavendish publicized his determination of Newton's gravitational constant <math>G</math> (which appears in his law of universal gravitation) to a high degree of accuracy and his discovery that the Earth's core must consist of a very dense material, using a torsion balance.<ref name="Gribbin"/> This is because the mean density of the Earth is five and a half times that of water, which covers much of its surface.<ref name="Ball"/> 

It was also this period when astronomers completed the shift from geocentrism to heliocentrism (the Vatican didn't acknowledge this change for almost another century, in 1820). Most notably, the Astronomer James Bradley discovered stellar aberration in γ Draconis, which provided a direct demonstration of the motion of the Earth through space, inexplicable by all geocentric systems.

===Electricity===
[[File:Lightning simulator questacon.jpg|thumb|right|275px|Now, that's some Palpatine shit.]]
Charles Augustin de Coulomb announced his eponymous inverse square law of electrostatics.<ref name="Brian"/> It is interesting to note that Sir Henry Cavendish had himself discovered the same thing using similar experimental apparatus, a torsion balance, but did not publish his findings; they were unearthed by Maxwell in the next century. [[Benjamin Franklin]] performed his kite experiment, which convincingly showed that lightning is an electrical phenomenon. Franklin then invented the lightning rod, which was considered to be [[Heresy|heretic]] by the Vatican, who argued that it interfered with the will of 
their imaginary friend
 God. Franklin also put forth the convention of positive and negative charges.<ref name="Brian">Baigrie B. ''Electricity and Magnetism: A Historical Perspective''. Westport, Conn. [u.a.]: Greenwood Press; 2007. {{isbn|0313333580}}</ref>

===Heat===
Sadi Carnot explored the ability of heat to do work. His rather slim book on a theoretical heat engine, named after him, founded the study of [[thermodynamics]]. In fact, Carnot's theorem &mdash; there exists no heat engine more efficient than the corresponding Carnot engine &mdash; is an early form of the second law of thermodynamics.<ref name="Lewis"/>

==Nineteenth century physics: Heat, electricity and magnetism, and chaos==
===Mechanics===
Laplace demonstrated the stability of the Solar System, which he modeled as a collection of rigid bodies moving in a vacuum, and expounded upon the [[nebula]] hypothesis for its origin.<ref name="Ball"/> Drawing from his work in analytical probability theory, Laplace showed that almost certainly all bodies the Solar System formed from the same cloud of gas. He also reintroduced the concept of a black hole, previously put forth by John Mitchell, based on the escape velocity formula in Newton's theory of gravity.<ref name="Gribbin">Gribbin J. ''Science, A History, 1543-2001''. London: Allen Lane; 2002. {{isbn|0713995033}}</ref> William Rowan Hamilton, Simeon Denis Poisson, and Carl Gustav Jacobi introduced new mathematical methods for classical mechanics and generalized the subject.<ref name="Ball"/><ref name="Hand"/>

In the final decade of this century, Henri Poincare investigated the three-body problem and came to a surprising conclusion: in general, it has no solutions.<ref group="note">Special cases have been solved (approximately) by Euler, Lagrange and others.</ref> He also recognized the existence of dynamical systems with extreme sensitivity to small changes to the initial conditions. Any changes to the initial conditions, no matter how small, will in time lead to entirely different behavior. Such systems are called chaotic. Unfortunately, while the discovery that Newton's laws predicted [[Chaos theory|chaos]] all along is quite remarkable, it was overshadowed by the fanfare associated with what happened right at the start of the next century.<ref name="Hand"/>

===Optics===
Thomas Young considered both the wave and corpuscular models of light and found himself supporting the former.<ref name="Gribbin"/> Augustin Jean Fresnel built upon the work on the wave model of light by Huygens and worked out a mathematical description of diffraction. In addition, he successfully accounted for the rectilinear propagation of light in a homogeneous medium, thus dispelling a major obstacle to the wave model.<ref name="Hecht"/> Young performed the double-slit experiment, which definitively vindicated the wave model. He used Newton's experimental data to calculate the wavelengths of red and violet light to a high degree of accuracy, even by modern standards.<ref group="note">This tells us how skilled an experimentalist Newton was and how insightful a theorist Young was.</ref> He also gave a rough estimate of the size of atoms, which remained hypothetical at this time.<ref name="Gribbin"/>

===Thermodynamics and Statistical Mechanics===
Jean-Baptiste Fourier formulated his analytical theory of heat diffusion using experiments and observations. The mathematical technique he discovered during his investigation, Fourier analysis, is of great value in theoretical physics.<ref name="Gribbin"/> Fourier predicted the phenomenon of [[global warming]].<ref name="Lewis"/>

Benjamin Thomson (Count Rumford) and James Joule demonstrated by experiment that heat is a form of motion. William Thomson (Lord Kelvin), Rudolf Clausius, Hermann von Helmholtz, and others furthered the work of Carnot and established the laws of thermodynamics. Using these, Lord Kelvin estimated the [[age of the Earth]] to between 50 to 500 million years.<ref name="Gribbin"/> Their work marked the beginning of the kinetic theory of gases. Now that thermodynamics was established, engineers began designing ever more efficient heat engines and refrigerators.<ref name="Lewis"/>

Maxwell and Ludwig Boltzmann derived the first ever statistical law of physics, the Boltzmann-Maxwell distribution of molecular speeds in an ideal gas, from Newton's laws of motion.<ref>Gribbin J. ''In Search of Schrödinger's Cat: Quantum Physics and Reality''. Toronto: Bantam Books; 1984.</ref> Boltzmann then spent much of life life developing the kinetic theory of gases and statistical mechanics.<ref name="Gribbin"/> Perhaps Boltzmann's greatest contribution is the statistical interpretation of the second law of thermodynamics. A system tends to be in a state of maximal entropy because such a state is the most likely.<ref name="Schroeder">Schroeder D. ''An Introduction to Thermal Physics''. San Francisco, CA: Addison Wesley; 2000. {{isbn|0201380277}}</ref> He also showed that the entropy of a given system is directly proportional to the natural logarithm of its thermodynamic probability; the constant of proportionality is known as Boltzmann's constant in his honor, and is ubiquitous in statistical mechanics.<ref name="Schroeder"/>

Gustav Kirchoff, perhaps best known for his laws of circuitry, used the laws of optics and thermodynamics to show that the radiation emitted by a black body is a function only of the temperature of that black body and its wavelength.<ref>Planck M, Masius M (''trans''). ''The Theory of Heat Radiation''. New York: Dover Publications, Inc.; 1959</ref> He then challenged his colleagues to determine such a function. Kirchoff is also noted for his investigations of diffraction. It is interesting to note that even though he employed the elastic-solid model for light propagation, working before the advent of Maxwell's electromagnetic theory, his results are still correct, in the sense that they agree well with experiment.<ref name="Hecht"/> (In about the same year, [[Charles Darwin]] published his masterpiece, ''The Origin of Species by Means of Natural Selection''.) William Strutt (Lord Rayleigh) and Sir James Jeans deduced the Rayleigh-Jeans law in response. But it only works for short wavelengths and disturbingly fails for longer ones, predicting that even a mildly heated body is a source an infinite amount radiation. This became known as the ultraviolet catastrophe.<ref name="Lewis">Lewis J. ''Heat and Thermodynamics: A Historical Perspective''. Westport, CT: Greenwood Press; 2007. {{isbn|9780313333323}}</ref>

===Electricity and Magnetism===
{{cquote|When we turn our attention to the general case of electrodynamics… our first impression is surprise at the enormous complexity of the problems to be solved.|||Max Planck.<ref>[https://aapt.scitation.org/doi/10.1119/1.4913414 ''Modern Electrodynamics'' by Andrew Zangwill]. {{isbn|9780521896979}} Reviewed by James S. Russ. ''American Journal of Physics''. June 22, 2015.</ref>}}
[[File:EM_Spectrum_Properties_edit.svg|thumb|left|500px|A diagram of the electromagnetic spectrum comparing different wavelengths to other physical objects. Note that names are merely labels for electromagnetic radiation at different wavelength orders of magnitude.]]
Hans Christian Oersted discovered by accident that an electric current induces a magnetic field. Felix Savart and Jean-Baptiste Biot established via a series of experiment that such a magnetic field obeys an inverse square law. Lord Kelvin, von Helmholtz, Denis Poisson, and others conducted further mathematical investigations of electricity and magnetism. Drawing upon Oersted's fundamental discovery, Andre-Marie Ampere initiated the study of electrodynamics. Michael Faraday showed that a changing magnetic flux induces an electric field and built the first electric motor. Its technological value can hardly be overestimated. Just six decades after Faraday's discovery of electromagnetic induction, electric trains entered service in the United States, Great Britain and Germany.<ref name="Brian"/><ref group="note">The London Underground opened in 1863 and became electrified in 1890, making it the first metro system that uses electric-propulsion.</ref> James Clerk Maxwell did for [[electromagnetism]] what Newton had done for gravitation by building it into a coherent theory, now called Maxwell's equations. Maxwell's equations not only summarize everything there is to know about classical electromagnetism but also predict the existence of electromagnetic waves, propagating at precisely the speed of light <math>c</math>. He could scarcely avoid the conclusion that light is itself an example of an electromagnetic wave. Maxwell's equations imply that the velocity of light does not depend on the velocity of the source or the observer, provided they are not accelerating. This seems to contradict Newtonian mechanics, in which the speed measured depends on the frame of reference. This puzzle would not be resolved till the next century.<ref name="Gribbin"/> Shortly after Maxwell's death, Heinrich Hertz conducted a series of experiments in which he generated a low-frequency electromagnetic wave, now called radio waves. Like light, it can be reflected, refracted, diffracted and polarized. In doing so, he constructed a primitive radio antenna as well as forerunners of satellite dishes. But more importantly, his experiments verified Maxwell's electromagnetic theory of light.<ref name="Brian"/>

===New Physics===
Henri Becquerel discovered a mind-boggling natural phenomenon which apparently violated the conservation of energy, later dubbed [[radioactivity]] by Marie Curie in an experiment involving a salt of uranium. Energy was given off for no apparent reasons. What is bizarre about this phenomenon is that it is affected neither by temperature nor chemical reactions. Curie and her husband went on to isolate and identify two new elements, polonium and radium, both of which are radioactive.<ref name="Gribbin"/>

At this point, the basics of classical physics were all established. Using Newton's laws of motion and gravitation, Maxwell's equations of electromagnetism and the laws of thermodynamics, one could explain pretty much everything in the known world. Despite its encouraging success, nineteenth-century physics encountered a number of major problems that it was unable to explain: (1) the fact that the speed of light <math>c</math> is independent of the reference frame, (2) the ultraviolet catastrophe, as mentioned above, (3) whether atoms are real or merely theoretical constructs, (4) radioactivity and (5) the photoelectric effect. Solutions to these problems took physicists to places where no one had gone before.

==Twentieth century physics: Mainly relativity and quantum mechanics==
===Quantum Mechanics===
[[File:Double-slit_experiment_results_Tanamura_2.jpg|thumb|left|150px|The double-slit experiment for electrons.]]
Max Planck discovered that if he treated light as if it was made up of discrete particles, then he could resolve the ultraviolet catastrophe. During the process, he formulated Planck's law of blackbody radiation.<ref name="Gribbin"/> [[Albert Einstein]] then entered the stage in the most spectacular manner possible. For his doctoral thesis, Einstein gave a mathematical description of Brownian motion. Brownian motion is the random movements of tiny particles, such as pollen, immersed in a fluid, such as water. In doing so, Einstein gave a convincing argument why atoms are real entities rather than theoretical constructs. Using Planck's quantum hypothesis, he explained the photoelectric effect in full detail.

Ernest Rutherford discovered the law of radioactive decay: the rate of decay is directly proportional to the amount present. He identified two kinds of radiation emitted by a radioactive substance, which he dubbed alpha and beta rays. Both of these were found to be deflected by a magnetic field and as such were electrically charged. Further experiments revealed that alpha rays were the nuclei of a helium-4 atom and beta rays were electrons (or positrons, not yet known at this time). A third kind of radiation, gamma rays, were later unearthed. Unlike the previous two, gamma rays were unaffected by magnetism and turned out to be electromagnetic waves of very short wavelengths. (See figure above.)

Rutherford realized that radioactive decay enabled him to shoot a stream of particles at a target, observe how they scatter, and study its structure.<ref group="note">In order for you to see an object, your eyes must be able to pick up photons emitted by, say, a candle, bouncing off that object. The same principle is at play here.</ref> His gold foil experiment revealed that most of the atom is actually empty space and that the electrons are moving about the nucleus much like the way the Earth and other planets orbit the Sun. Unfortunately, his planetary model seems fatally flawed; Maxwell's equations predict that accelerating charged particles emit electromagnetic radiation. If this was true, electrons will continuously lose energy as they spiral towards the nucleus; atoms would collapse in tens of nanoseconds. Niels Bohr saw a way out. His atomic model took into account the quantization of energy. Electrons can only absorb or emit discrete amounts of energy; those at the ground state have no energy to emit and will not collide with the nucleus. Bohr's model successfully reproduces the spectrum of hydrogen and hydrogen-like atoms, those with just one electron, but falters for more complex ones. The cavalry soon arrived. In the 1920s, a group of physicists, perhaps some of the most brilliant in history, developed the modern theory of [[quantum mechanics]]. Louis de Broglie and Albert Einstein pointed out the importance of wave-particle duality.<ref group="note">Richard Feynman stated very simply that the wave-particle duality was '''the''' mystery of quantum mechanics and nothing could be said about it other than it had to be taken by faith. Of course, it has been verified experimentally for a gazillion times but we don't have anything by way of explanation. If a reactionary Christian reads this, (s)he will probably salivate: "See, the physicists base something on faith!" But it is a very different kind of faith, especially regarding the numerous experiments.</ref> Werner Heisenberg enunciated the crucial [[Quantum_mechanics#Uncertainty|uncertainty principle]], which differentiates the quantum world from what we are used to. It states that certain pairs of dynamical variables, such as position and momentum, are such that the more precisely one is measured, the less precisely the other one is known. There is no way around it; uncertainty is built into the fabric of nature. [[Paul Dirac]] succeeded in unifying special relativity and quantum mechanics for the first time with his relativistic wave equation for the electron.<ref name="Gribbin"/> The prediction of antimatter by Dirac and [[J. Robert Oppenheimer]] soon followed. Due to its inherent counter-intuitiveness, quantum mechanics has been a [[quantum woo|favorite target]] of [[pseudoscience|pseudoscientists]]. Most professional scientists were simply too happy that this amazing new theory works, and went on to use it in their research. Einstein, however, was deeply troubled about the probabilistic nature of quantum mechanics, which he thought to be incomplete. He tried multiple times to devise a thought experiment that would be able to show the flaw of quantum mechanics. But Bohr managed to defeat him.<ref group="note">{{wpa|Bohr-Einstein debates}}</ref> Quantum mechanics survives largely unscathed.

Interrupted by [[World War II]], the development of quantum field theory resumed at full pace in the postwar years. The first major result was quantum electrodynamics, independently developed by Sin-Itiro Tomogana, Julian Schwinger and [[Richard Feynman|Richard Feynman]]. As one of the most accurate theories of physics ever, it is considered to be a crown jewel of science.<ref name="Greene"/><ref name="Gribbin2">Gribbin, John R., and Mary Gribbin. ''Richard Feynman: a Life in Science''. New York, NY, Penguin, 1997. {{isbn|052594124X}}</ref> Building upon this encouraging success, physicists then developed quantum chromodynamics<ref group="note">It has nothing to do with color in the everyday sense of the word.</ref> to describe the strong nuclear force, and the electroweak theory, which addresses the unification between the electromagnetic and weak nuclear forces, and finally the [[Standard Model]] of particle physics, which encompasses all three non-gravitational interactions.<ref name="Greene"/> Superfluidity and superconductivity attracted quite a bit of attention.<ref name="Gribbin2"/>

===Special and General Relativity===
Postulating that the speed of light is the same for all observers and that the laws of physics remain the same in all inertial reference frames, Einstein developed the special theory of relativity, which reduces to classical mechanics if the speeds involved are no more than 10% that of light. Fundamental special relativistic effects are time dilation for moving bodies, length contraction in the direction of motion, and the loss of simultaneity of clocks moving at different speeds. As an afterthought, he derived what is probably the most famous equation in science, <math>E = mc^2 </math>, which gives the energy equivalent of a massive body at rest. This impressive creative outburst took place in 1905,<ref name="Gribbin"/> a great year for Einstein as well as physics.

As is often the case in science, his success in formulating the special theory of relativity pointed Einstein to the next big problem. While special relativity proclaims that no causal influence can travel faster than the speed of light, Newton's theory of gravity implicitly assumes that gravitational interactions are instantaneous. Einstein realized that (if air resistance is non-existent or negligible) falling bodies can consider themselves to be at rest and the ground is accelerating towards them; in other words, gravitation and acceleration are equivalent. Einstein considered this to be the happiest moment of his life. His opinion is quite reasonable, since the principle of equivalence lies at the core of his theory of gravity, general relativity, nowadays thought to be his ''magnum opus''. Moreover, it is a crucial insight into the nature of the Universe. Special relativity stipulates that all inertial frames are equivalent, as far as the laws of physics are concerned. With the principle of equivalence, the basic laws of physics hold in ''all'' frames of reference, inertial or not. Einstein quickly recognized that in his new framework, spacetime itself is curved, which means the conventional Euclidean geometry cannot be used. Fortunately, the necessary mathematics, differential geometry, had already been introduced six decades before by Bernhard Riemann. After ten years of hard work, Einstein finally published his field equations for gravity in 1916. John Wheeler eloquently summarized them thus, "Mass [and energy] tells space [strictly, spacetime] how to curve. Space tells mass how to move."<ref name="Greene">Greene B. ''The elegant universe : superstrings, hidden dimensions, and the quest for the ultimate theory''. New York, NY: W. W. Norton and Company, Inc.; 1999. ISBN 0375708111.</ref>According to general relativity, the influence of gravity travels at exactly the speed of light. Indeed, a jiggling massive body emits gravitational waves, or radiation. Unfortunately, since gravity is so feeble, detecting gravitational waves is a Herculean task.<ref>Gribbin J, Gribbin M. ''The Universe: A Biography''. London: Penguin books; 2006.</ref> Just a few months later, Karl Schwarzschild obtained the first non-trivial exact solution<ref group="note">Of course, a vacuum is an obvious solution; spacetime is perfectly flat.</ref> to Einstein's field equations.<ref name="McEvoy">McEvoy, J. P, et al. Stephen Hawking for Beginners. Icon, 1995. {{isbn|1874166250}}</ref> [[black hole|Schwarzschild's solution]] describes a spherically symmetric body whose gravitational pull is so strong that not even light on its surface can escape. Wheeler later gave this object the name "black hole". Einstein showed that general relativity correctly accounts for the precession of the perihelion of Mercury, something Newton's theory of gravity cannot explain. Since spacetime is curved, the path of light near a massive body must be also curved. Sir Arthur Eddington verified this prediction by astronomical observations in 1919.<ref name="Greene"/> Subrahmanyan Chandrasekhar, J. Robert Oppenheimer and others set the stage for future developments in relativistic astrophysics. Unfortunately, most pure research came to a halt when Germany invaded Poland in 1939.<ref name="Thorne">''Thorne K. Black Holes and Time Warps: Einstein's Outrageous Legacy''. W. W. Norton and Company; 1994. {{isbn|0393035050}}</ref> Interest in general relativity returned in the postwar era and important results concerning cosmic expansions and black holes were built upon. Indeed, the period from the 1960s to the mid-1970s was the golden age of general relativity.<ref name="Thorne"/>

===Classical Mechanics===
In the 1960s, physicists rediscovered Poincare's results in chaos theory via computer simulations. Since then, this latest branch of classical mechanics has received considerable attention, not least because of its relevance to weather forecasting. Since the behavior of a weather system depends sensitively on its initial condition, which cannot be measured with absolute accuracy, long-term forecasting is all but impossible.<ref name="Hand"/>

===New Physics===
As they currently stand, the Standard Model and general relativity contain almost everything physicists know for certain (within the limits of experimental uncertainty, pun intended) about how the Universe works. However, contemporary physics has very little to say about the behavior and characteristics of neutrinos, dark matter, and dark energy; cosmological observations suggest the latter two comprise the overwhelming majority of our Universe.<ref name="Greene"/> So expect some ground-breaking results in the twenty-first century.

===Condensed-matter Physics===
Perhaps due to the influence of popular books, people outside of the physics community tend to think that all current research in physics consists of just the fields mentioned above, namely astrophysics and cosmology on one hand and particle physics on the other. However, the largest and most active field of physics is actually condensed matter physics.<ref>[http://www.aps.org/units/dcmp/history.cfm History of Condensed Matter Physics]. American Physical Society.</ref> As a matter of fact, about one in four physicists wrote their doctoral dissertation on a topic in this branch of physics. Meanwhile, specialists in (general) relativity are on what Chandrasekhar would call the "lonely byways of science".<ref>Wali, Kameshwar C. ''Chandra: A Biography of. S. Chandrasekhar''. The University of Chicago Press, 1992. {{isbn|0226870545}}</ref><ref name="physPhD">[https://www.aps.org/careers/statistics/upload/trends-phd0214.pdf Trends in Physics PhDs]. American Physical Society.</ref><ref group="note">To give this some context, Subrahmanyan Chandrasekhar made this morose remark some time during the late 1970s and the 1980s, after the Golden Age of General Relativity had ended, leaving him on his own personal and final quest to clarify what was known and to tie up some loose ends, which resulted in the epic monograph ''The Mathematical Theory of Black Holes'' (1983), and to study the collisions of gravitational plane waves.</ref> Condensed matter physics explores the properties of large numbers of interacting particles in the liquid or gaseous states. More specifically, a condensed-matter physicist studies the electric, magnetic, optical, thermal and tensile properties of a material. Therefore, insulators, conductors, semiconductors, and superconductors are all topics of this branch of physics. Of great interest is crystalline materials, whose regular atomic arrangement makes them amenable to quantum-mechanical treatment.<ref>[https://www.britannica.com/science/condensed-matter-physics Condensed-matter physics]. Encyclopedia Britannica. Accessed February 11, 2018.</ref>

==Twenty-first century physics: High-energy physics, gravitation, and condensed matter physics==
===High-energy Physics===
On the Fourth of July, 2012, the European Center for Nuclear Research ([[CERN]] in French) announced it had observed a particle whose behavior is consistent with what theorists call the 
God particle
 [[Higgs boson]] to a very high degree of statistical significance.<ref>[http://abcnews.go.com/Technology/higgs-boson-evidence-god-particle-reported-fermilab-physicists/story?id=16695742 Physicists See Best Proof Yet of 'The God Particle' (ABC News)]</ref><ref group="note">Note the ironic date. This could easily have been a U.S. discovery in light of the fact that the Superconducting Super Collider was designed to smash subatomic particles at 20 GeV while the Large Hadron Collider could only do so at 16 GeV after its most recent upgrade. Alas, the Super Collider was cancelled in the 1990s by "budget hawks".</ref> The Higgs boson, first predicted by Peter Higgs and others, is of fundamental importance in quantum field theory. In interacts with some of the other particles in such a way that it gives them [[mass]].

In May 2018, physicists at the Fermi Accelerator National Laboratory (Fermilab), the most powerful particle accelerator in the U.S., found some evidence for the hypothetical fourth flavor of neutrinos, the sterile neutrino, not part of the [[Standard Model]]. Sterile neutrinos do not interact via the weak nuclear force, unlike its cousins. However, this result contradicts other experiments and needs further research.<ref>[http://www.bbc.co.uk/news/science-environment-44370751 Has US physics lab found a new particle?]. [[BBC]] News. June 6<sup>th</sup>, 2018</ref>

===Gravitation===
[[File:Wavy.gif|thumb|200px|A computer animation showing gravitational waves being generated as two black holes or neutron stars spiral around each other before they coalesce.]]
In February 2016, about one hundred years after Einstein completed his general theory of relativity, physicists at the Laser Interferometer Gravitational Observatory (LIGO) revealed after painstaking data analysis they had indeed detected signatures of gravitational waves, predicted by Einstein himself.<ref>[http://news.mit.edu/2016/ligo-first-detection-gravitational-waves-0211 Scientists Make First Direct Detection of Gravitational Waves]. MIT News. February 11, 2016.</ref> This detection event actually took place on September 14, 2015, but the researchers wanted to be absolutely certain before publicizing this landmark discovery. The signal they detected came from two merging black holes about 30 times the mass of our Sun. This has opened up an entirely new possibility. Astronomers have traditionally observed celestial bodies using the electromagnetic radiation emitted at different wavelengths. Now, they can do so via gravitational waves.<ref group="note">Gravitational waves are a kind of radiation. In physics, radiation is a mechanism that transports energy and momentum. Thus, sound is another example of radiation. And so is wind, but unlike the previous examples, matter is also transported in this special case.</ref> In the foreseeable future, we can expect more detection of gravitational waves coming from not just other pairs of black holes but also pairs of neutron stars, a neutron star-black hole system, and even supernovae, if they are close enough. As a matter of fact, LIGO announced in June, 2016, their second detection, recorded on December 26, 2015. This time, the signal came from another pair of black holes, but with fourteen and eight solar masses. Calculations indicate that the chances of LIGO being fooled by a random vibration of the same appearance as a gravitational wave signal is negligible, one in twenty billion. When LIGO's sister project in Italy, VIRGO<ref group="note">Nothing superstitious here. Move along, folks!</ref> finally comes online, physicists and astronomers will be able to locate the source of the signals with greater confidence.<ref>PBS Space Time: The Future of Gravitational Waves ([https://www.youtube.com/watch?v=eJ2RNBAFLj0 YouTube link]).</ref> Indeed, in September, 2017, teams at LIGO and VIRGO announced the fourth detection of gravitational waves, this time of two colliding black holes of similar masses two billion light years away. Not only did they triangulate the source, they also measured its polarization, or the direction of vibration.<ref>[http://www.bbc.com/news/world-australia-41420188 Gravitational Wave Hunters Bag Fourth Detection]. BBC News. September 17, 2017.</ref> One month later, the LIGO-VIRGO collaboration announced the first detection of gravitational waves emitted by two neutron stars spiraling towards each other. Observation by telescopes operating at the entire electromagnetic spectrum followed, and a wealth of data was collected. This detection helps explain the origins of heavy elements of the periodic table, such as gold, silver and platinum, and lends some evidence to the hypothesis that collisions of neutron stars are responsible for at least some of the gamma-ray bursts we observe.<ref>[http://www.bbc.com/news/science-environment-41640256 Einstein's waves detected in star smash]. BBC News. October 16, 2017.</ref> Previously, it was thought that heavier elements were manufactured by nuclear reactions taking place in the interior of a large star in its final moments. Subsequent calculations suggested this was not enough. Debris ejected from a collision of neutron stars, observations show, provides the missing amounts.<ref>[https://news.nationalgeographic.com/2017/10/gravitational-waves-discovered-neutron-stars-pictures-science/ In a First, Gravitational Waves Linked to Neutron Star Crash]. ''[[National Geographic]]''. December 20, 2017.</ref>

In 2018, physicists announced that general relativity passed yet further stringent tests, on gravitational lensing and other strong field effects.<ref>[https://www.scientificamerican.com/article/milky-ways-black-hole-provides-long-sought-test-of-einsteins-general-relativity/ Milky Way's Black Hole Provides Long-Sought Test of Einstein's General Relativity]. The ''Scientific American''. July 26<sup>th</sup>, 2018.</ref><ref>Laura Otto, University Wisconsin-Milwaukee. [https://phys.org/news/2018-06-einstein-theory-gravity-extreme-conditions.html Einstein's theory of gravity holds – even in extreme conditions]. Phys.org. July 4<sup>th</sup>, 2018.</ref><ref group="note">[http://www.eso.org/public/usa/images/eso1825a/ Artist's impression of light bending around the black hole at the center of the Milky Way].</ref>

Cosmological measurements suggest that the overwhelming majority of the Universe consists of dark matter and dark energy, thus named because they do not interact electromagnetically. The properties of dark matter and dark energy remain largely unknown at this point.

Another active research topic is how gravity works at the quantum level. For decades, physicists' attempts to unify general relativity with quantum mechanics have been in vain, as these two immensely successful theories as we know them are fundamentally incompatible with each other. A calculation involving both yields nonsensical answers, such as infinities. Unfortunately, nobody has yet figured out how to circumvent this obstacle completely. A variety of approaches have been pursued, such as quantum field theory in curved spacetime, Hawking-Hartle gravity, twister theory, loop quantum gravity, among others. Besides theories, physicists have devised and conducted a number of experiments intended to probe the quantum-mechanical nature of gravity; they have yet to succeed.<ref>[https://www.scientificamerican.com/article/is-gravity-quantum/ Is Gravity Quantum?]. [[Scientific American]]. August 14, 2018.</ref>

Another potential way out that has captured much attention is [[String Theory|string theory.]] It essentially "tames" the infinities by spreading out interactions in spacetime. An appealing aspect of string theory is that it also accounts for the other three fundamental forces of nature. It is thus called a "theory of everything" because it says that all four of the known fundamental forces ([[gravity|gravitational]], weak nuclear, strong nuclear, and [[electromagnetic]]) are manifestations of one underlying mechanism. The problem with string theory is, however, that there is neither empirical evidence for nor against it. Therefore, it should be strictly speaking called the "string hypothesis" until it gives some empirically verifiable predictions and be able to explain phenomena on which other theories are either silent or give incorrect results. In the meantime, one has every right to remain skeptical. Recent research suggests that dark matter may be incompatible with string theory. This means either cosmology is very different from what we think, or that string theory is wrong.<ref>[https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180809/ Dark Energy May Be Incompatible With String Theory]. ''Quanta Magazine''. August 9, 2018.</ref> Other alternative theories have been proposed, such as {{wpl|loop quantum gravity}}.

===Condensed-matter Physics===
Condensed matter physics continues to make up the majority of physics research. In addition to the traditional study of "hard" condensed matter systems such as metals or crystals, in the twenty-first century new subfields of condensed matter physics are added to cover other solid- or liquid-like materials.

==Astrophysics==
'''Astrophysics''' is simply the physics of [[astronomy]]. Astrophysicists use [[spectroscopy]] on [[visible light|light]] to determine the composition of [[star]]s, nebulae, and other [[matter]] in the [[universe]]. They observe [[radiation]] such as [[microwave]]s emitted from pulsars, [[red shift]]ed light from the [[big bang]], and the [[energy]] emitted by [[black hole]]s. They develop and utilize [[Theory|theories]] of [[relativity]], [[String theory|string]] cosmology, [[dark matter]], and [[dark energy]]. This [[science|scientific]] field utilizes other fields of physics such as [[Laws of thermodynamics|thermodynamics]] and is related to [[cosmology]], which deals with properties of immense bodies.

The field of astrophysics was formed when [[Isaac Newton]] coupled [[Galileo Galilei]]'s [[hypothesis]] of the dynamics of [[Planet|planetary]] orbits, itself based on [[Nicolaus Copernicus]]' notion of [[heliocentrism]], with [[Johannes Kepler]]'s empirical laws of planetary motion, thereby demonstrating that [[gravity]] works the same on celestial bodies as it does on [[Earth]]. Newton's [[Law of Universal Gravitation]] is included in his work ''[[Philosophiæ Naturalis Principia Mathematica]]''.

==See also==
{{div col}}
*[[Quantum physics terms|Glossary of quantum physics terms]]
*[[Astrology]]
*[[Sean Carroll]]
*[[Stephen Hawking]]
*[[Carl Sagan]]
*[[Sally Ride]]
{{div col end}}

==External links==
*Kaiser, David. "[https://www.journals.uchicago.edu/doi/10.1086/664983 A Tale of Two Textbooks: Experiments in Genre]". ''Isis'', A Journal of the History of Science Society. The University of Chicago Press. Volume 103, Number 1. March 2012. This is the story of one of the most famous physics textbooks ever and one of the most well-known yet also controversial.
*[https://www.quantamagazine.org/what-makes-the-hardest-equations-in-physics-so-difficult-20180116/ What Makes The Hardest Equations in Physics So Difficult?] Quanta Magazine. Abstraction Blog. January 16, 2018. Surprisingly, the most notoriously hard-to-solve equations in physics actually model phenomena familiar to daily life, namely fluid flow.
*[https://www.youtube.com/watch?v=2EkHB_WtKRQ What You Never Learned about Mass]. Fermilab. December 19, 2017. This video explains a crucial insight that led Einstein to the general theory of relativity.

==Notes==
{{reflist|group="note"|2}}

==References==
{{reflist|2|90%}}

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