How Fast Does Light Travel? | The Speed of Light How Fast Does Light Travel? | The Speed of Light By Nola Taylor Redd, Space.com Contributor | May 22, 2012 08:37pm ET MORE Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes. Credit: Iscatel | Shutterstock The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second. Early scientists, unable to perceive light’s motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise.

Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called "c" — is thought to be not acheivable by anything with mass, for reasons explained below. That doesn’t stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe. Speed of light: History of the theory The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed. In 1667, the Italian astronomer Galileo stood two people on a hill at a distance of less than a mile, each holding a shielded lantern. One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder (and factoring out reaction times), he thought he could calculate the speed of light.

Unfortunately, Galileo's distances were too small to see a difference, so he could only determine that light traveled at least ten times faster than sound. In the 1670s, Danish astronomer Ole Römer used eclipses of Jupiter 's moon, Io, as a chronometer for the speed of light. Over the course of several months, as Io passed behind the giant gas planet, Römer found that the eclipses came later than calculations anticipated, although over the course of several months, they drew closer to the predictions. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer. He concluded that light took ten to eleven minutes to travel from the sun to Earth, an overestimate since it in fact takes 8 minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s). In 1728, English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth's travels around the sun.

He put the speed of light at 185,000 miles per second (301,000 km/s), accurate to within about 1 percent. Two attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly-rotating toothed wheel, with a mirror set up 5 miles away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel. The two independent methods each came within about 1,000 miles per second of the speed of light measured today. Prussian-born Albert Michelson, who grew up in the United States, attempted to replicate Focault's method in 1879, but used a longer distance, as well as extremely high-quality mirrors and lenses.

His result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, when Michelson remeasured it. Einstein and special relativity In 1905, Albert Einstein wrote his first paper on special relativity . In it, he established that light travels at the same speed no matter how fast the observer moves. Even using the most precise measurements possible, the speed of light remains the same for an observer standing still on the face of the Earth as it does for one traveling in a supersonic jet above its surface. Similarly, even though the Earth is orbiting the sun, which is itself moving around the Milky Way, which is a galaxy traveling through space, the measured speed of light coming from our sun would be the same whether one stood inside or outside of the galaxy to calculate it. Similarly, Einstein calculated that the speed of light doesn't vary with time or place. What is a light-year? The speed of light can cause the universe to seem to host a time machine. The distance light travels in the course of a year is called a light-year.

A light-year is a measure of both time and distance. It is not as hard to understand as it seems. Think of it this way: Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight take about 8 minutes to reach our eyes, so the sun is about 8 light-minutes away. Light from the nearest star system, Alpha Centauri , is requires roughly 4.3 years to get here, so that star system is said to be 4.3 light-years away. Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. Thus, when astronomers study objects that lie a light-year away or more, they are seeing it as existed at the time that light left it, not as it would appear if they stood near its surface today. In this sense, everything we see in the distant universe is, literally, history . This principle allows astronomers to see how the universe as it looked after the Big Bang, which took place about 13.7 billion years ago.

Examining objects that are, say, 10 billion light-years away, we see them as they looked 10 billion years ago, relatively soon after the beginning of the universe, rather than how they appear today. Is the speed of light really constant? Light travels in waves, and, like sound, can be slowed depending on what it is traveling through. Nothing can outpace light in a vacuum. However, if a region contains any matter, even dust, light can bend when it comes in contact with the particles, which results in a decrease in speed. Light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, while light passing through a diamond is slowed to less than half that speed. Still, it travels through the gem at over 277 million mph (almost 124,000 km/s) — not a speed to scoff at. Can we travel faster than light? Science fiction loves to speculate about this, because “warp speed,” as faster-than-light travel is popularly known, would allow us to travel between stars in time frames otherwise impossibly long. And while it has not be proven to be impossible, the practicality of traveling faster than light renders the idea pretty farfetched.

According to Einstein's general theory of relativity , as an object moves faster, its mass increases, while its length contracts. At the speed of light, such an object has an infinite mass, while its length is 0 — an impossibility. Thus, no object can reach the speed of light, the theory goes. That doesn’t stop theorists from proposing creative and competing theories. The idea of warp speed is not impossible , some say, and perhaps in future generations people will hop between stars the way we travel between cities nowadays. One proposal would involve a spaceship that could fold a space-time bubble around itself in order to exceed the speed of light. Sounds great, in theory. Further Reading: How "Fast" is the Speed of Light? How "Fast" is the Speed of Light? Light travels at a constant, finite speed of 186,000 mi/sec. A traveler, moving at the speed of light, would circum-navigate the equator approximately 7.5 times in one second. By comparison, a traveler in a jet aircraft, moving at a ground speed of 500 mph, would cross the continental U.S. once in 4 hours.

Please send suggestions/corrections to: Molecular Expressions Microscopy Primer: Physics of Light and Color - Speed of Light Movie Gallery Speed of Light Somewhere in outer space, billions of light years from Earth, the original light associated with the Big Bang of the universe is blazing new ground as it continues moving outward. In stark contrast, another form of electromagnetic radiation originating on the Earth, radio waves from the inaugural live episode of The Lucy Show are broadcasting a premier somewhere in deep space, although greatly reduced in amplitude. The basic concept behind both events involves the speed of light (and all other forms of electromagnetic radiation), which scientists have thoroughly examined, and is now expressed as a constant value denoted in equations by the symbol c. Not truly a constant, but rather the maximum speed in a vacuum, the speed of light, which is almost 300,000 kilometers per second, can be manipulated by changing media or with quantum interference. Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or perturbed in some other manner.

This well-established scientific fact is not a product of the Atomic Age or even the Renaissance, but was originally promoted by the ancient Greek scholar, Euclid, somewhere around 350 BC in his landmark treatise Optica. However, the intensity of light (and other electromagnetic radiation) is inversely proportional to the square of the distance traveled. Thus, after light has traveled twice a given distance, the intensity drops by a factor of four. When light traveling through the air enters a different medium, such as glass or water, the speed and wavelength of light are reduced (see Figure 2), although the frequency remains unaltered. Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index of 1.3; see Figure 2) and 200,000 kilometers per second in glass (refractive index of 1.5).

In diamond, with a rather high refractive index of 2.4, the speed of light is reduced to a relative crawl (125,000 kilometers per second), being about 60 percent less than its maximum speed in a vacuum. Because of the enormous journeys that light travels in outer space between galaxies (see Figure 1) and within the Milky Way, the expanse between stars is measured not in kilometers, but rather light-years, the distance light would travel in a year. A light-year equals 9.5 trillion kilometers or about 5.9 trillion miles. The distance from Earth to the next nearest star beyond our sun, Proxima Centauri, is approximately 4.24 light-years. By comparison, the Milky Way galaxy is estimated to be about 150,000 light-years in diameter, and the distance to the Andromeda galaxy is approximately 2.21 million light-years. This means that light leaving the Andromeda galaxy 2.21 million years ago is just arriving at Earth, unless it was waylaid by reflecting celestial bodies or refracting debris.

When astronomers gaze into the night skies, they are observing a mixture of real time, the recent past, and ancient history. For example, during the period that pioneering Babylonians, Arab astrologers, and Greek astronomers described the stellar constellations, Scorpius (Scorpio to astrologers) still had the whiptail of a scorpion. The tail star and others in this constellation had appeared as novae in the skies between 500 and 1000 BC, but are no longer visible to today's stargazers. Although some of the stars that are observed in the night skies of Earth have long since perished, the light waves that carry their images are still reaching human eyes and telescopes. In effect, the light from their destruction (and the darkness of their absence) has not yet crossed the enormous distances of deep space because of insufficient time. Empedocles of Acragas, who lived around 450 BC, was one of the first recorded philosophers to speculate that light traveled with a finite velocity.

Almost a millennium later, around 525 AD, Roman scholar and mathematician Anicius Boethius attempted to document the speed of light, but after being accused of treason and sorcery, was decapitated for his scientific endeavors. Since the earliest application of black powder for fireworks and signals by the Chinese, man has wondered about the speed of light. With the flash of light and color preceding the explosive sound by several seconds, it did not require a serious calculation to realize that the speed of light obviously exceeded the speed of sound. Interactive Java Tutorial Speed of Light in Transparent Materials Discover how the speed of light is reduced in proportion to the refractive index of a material when the light enters a new transparent medium, such as air, water, or glass. The Chinese secrets behind explosives made their way to the West during the middle of the thirteenth century, and with them, came questions about the speed of light. Prior to this period, other investigators must have considered the flash of lightning followed later by the clap of thunder, typical of a thunderstorm, but offered no plausible scientific explanations about the nature of the delay.

The Arabic scholar Alhazen was the first serious optical scientist to suggest (around 1000 AD) that light had a finite speed, and by 1250 AD, British optics pioneer Roger Bacon wrote that the speed of light was finite, although very rapid. Still, the widely held opinion by a majority of scientists during this period was that the speed of light is infinite and could not be measured. In 1572, the famous Danish astronomer Tycho Brahe was the first to describe a supernova, which occurred in the constellation Cassiopeia. After watching a "new star" suddenly appear in the sky, slowly intensify in brightness, and then fade from view over an 18-month period, the astronomer was mystified, but intrigued. These novel celestial visions drove Brahe and his contemporaries to question the widely held notion of a perfect and unchanging universe having an infinite speed of light. The belief that light has infinite speed was hard to displace, although a few scientists were beginning to question the speed of light in the sixteenth century. As late as 1604, the German physicist Johannes Kepler speculated that the speed of light was instantaneous.

He added in his published notes that the vacuum of space did not slow the speed of light down, hampering, to a limited degree, the quest by his contemporaries for the ether that supposedly filled space and carried the light. Shortly after the invention and some relatively crude refinements to the telescope, Danish astronomer Ole Roemer (in 1676) was the first scientist to make a rigorous attempt to estimate the speed of light. By studying Jupiter's moon Io and its frequent eclipses, Roemer was able to predict the periodicity of an eclipse period for the moon (Figure 3). However, after several months, he noticed that his predictions were slowly becoming less accurate by progressively longer time intervals, reaching a maximum error of about 22 minutes (a rather large discrepancy, considering how far light travels in that time span). Then, just as oddly, his predictions again became more accurate over several months, with the cycle repeating itself. Working at the Paris Observatory, Roemer soon realized that the observed differences were caused by variations in the distance between the Earth and Jupiter, due to orbital pathways of the planets. As Jupiter moved away from the Earth, light had a longer distance to travel, taking additional time to reach the Earth.

Applying the relatively inaccurate calculations for the distances between Earth and Jupiter available during the period, Roemer was able to estimate the speed of light at about 137,000 miles (or 220,000 kilometers) per second. Figure 3 illustrates a reproduction of the original drawings by Roemer delineating his methodology utilized to determine the speed of light. Roemer's work stirred the scientific community, and many investigators began to reconsider their speculations about the infinite speed of light. Sir Isaac Newton, for example, wrote in his landmark 1687 treatise Philosophiae Naturalis Prinicipia Mathematica (Mathematical Principles of Natural Philosophy), "For it is now certain from the phenomena of Jupiter's satellites, confirmed by the observations of different astronomers, that light is propagated in succession and requires about seven or eight minutes to travel from the sun to the earth", which is actually a remarkably close estimate for the correct speed of light. Newton's respected opinion and widespread reputation was instrumental in jump-starting the Scientific Revolution, and helped launch new research by scientists who now endorsed light's speed as finite. The next in line to provide a useful estimate of the speed of light was the British physicist James Bradley.

In 1728, a year after Newton's death, Bradley estimated the speed of light in a vacuum to be approximately 301,000 kilometers per second, using stellar aberrations. These phenomena are manifested by an apparent variation in the position of stars due to the motion of the Earth around the sun. The degree of stellar aberration can be determined from the ratio of the Earth's orbital speed to the speed of light. By measuring the stellar aberration angle and applying that data to the orbital speed of the Earth, Bradley was able to arrive at a remarkably accurate estimate. In 1834, Sir Charles Wheatstone, inventor of the kaleidoscope and a pioneer in the science of sound, attempted to measure the speed of electricity. Wheatstone invented a device that utilized rotating mirrors and capacitative discharge through a Leyden jar to generate and clock the movement of sparks through almost eight miles of wire. Unfortunately, his calculations (and perhaps his instrumentation) were in error to such a degree that Wheatstone estimated the velocity of electricity at 288,000 miles per second, a mistake that led him to believe that electricity traveled faster than light.

Wheatstone's research was later expanded upon by French scientist Dominique Fran�ois Jean Arago. Although he failed to complete his work before his eyesight failed in 1850, Arago correctly postulated that light traveled slower in water than air. Meanwhile in France, rival scientists Armand Fizeau and Jean-Bernard-Leon Foucault independently attempted to measure the speed of light, without relying on celestial events, by taking advantage of Arago's discoveries and expanding on Wheatstone's rotating mirror instrument design. In 1849, Fizeau engineered a device that flashed a light beam through a toothed wheel (instead of a rotating mirror), and then onto a fixed mirror positioned at a distance of 5.5 miles away. By rotating the wheel at a rapid rate, he was able to steer the beam through a gap between two of the teeth on the outward journey and catch reflected rays in the neighboring gap on the way back. Armed with the wheel speed and distance traveled by the pulsed light, Fizeau was able to calculate the speed of light.

He also discovered that light travels faster in air than in water (confirming Arago's hypothesis), a fact that fellow countryman Foucault later confirmed through experimentation. Foucault employed a rapidly rotating mirror driven by a compressed air turbine to measure the speed of light. In his apparatus (see Figure 4), a narrow beam of light is passed through an aperture and then through a glass window (acting also as a beamsplitter) with a finely graduated scale before impacting on the rapidly spinning mirror. Light reflected from the spinning mirror is directed through a battery of stationary mirrors in a zigzag pattern designed to increase the path length of the instrument to about 20 meters without a corresponding increase in size. In the amount of time it took the light to reflect through the series of mirrors and return to the rotating mirror, a slight shift in the mirror position had occurred. Subsequently, light reflected from the shifted position of the spinning mirror follows a new pathway back to the source and into a microscope mounted on the instrument. The tiny shift in light could be seen through the microscope and recorded.

By analysis of the data collected from his experiment, Foucault was able to calculate the speed of light as 298,000 kilometers per second (approximately 185,000 miles per second). The light path in Foucault's device was short enough to be utilized in the measurement of light speeds through media other than air. He discovered that the speed of light in water or glass was only about two-thirds of the value in air, and he also concluded that the speed of light through a given medium is inversely proportional to the refractive index. This remarkable result is consistent with the predictions about light behavior developed hundreds of years earlier from the wave theory of light propagation. Following Foucault's lead, a Polish-born American physicist named Albert A. Michelson attempted to increase the accuracy of the method, and successfully measured the speed of light in 1878 with a more sophisticated version of the apparatus along a 2,000-foot wall lining the banks of Maryland's Severn River.

Investing in high quality lenses and mirrors to focus and reflect a beam of light over a much longer pathway than the one utilized by Foucault, Michelson calculated a final result of 186,355 miles per second (299,909 kilometers per second), allowing for a possible error of about 30 miles per second. Due to the increased sophistication of his experimental design, the accuracy of Michelson's measurement was over 20 times greater than Foucault's. During the late 1800s it was still believed by most scientists that light propagates through space utilizing a carrier medium termed the ether. Michelson teamed with scientist Edward Morley in 1887 to devise an experimental method for detecting the ether by observing relative changes in the speed of light as the Earth completed its orbit around the sun. In order to accomplish this goal, they designed an interferometer that splits a beam of light and re-directs the individual beams through two different pathways, each over 10 meters in length, using a complex array of mirrors.

Michelson and Morley reasoned that if the Earth is traveling through an ether medium, the beam reflecting back and forth perpendicular to the flow of ether would have to travel farther than the beam reflecting parallel to the ether. The result would be a delay in one of the light beams that could be detected when the beams were recombined through interference. The experimental apparatus built by Michelson and Morley was massive (see Figure 5). Mounted on a slowly rotating stone slab that was over five feet square and 14 inches thick, the instrument was further protected by an underlying pool of mercury that acted as a frictionless shock absorber to remove vibrations from the Earth. Once the slab was set into motion, achieving a top speed of 10 revolutions per hour, it took hours to reach a halt again. Light passing through a beamsplitter, and reflected by the mirror system, was examined with a microscope for interference fringes, but none were ever observed. However, Michelson utilized his interferometer to accurately determine the speed of light at 186,320 miles per second (299,853 kilometers per second), a value that stood as the standard for the next 25 years.

The failure to detect a change in the speed of light by the Michelson-Morley experiment set in motion the beginnings of an end to the ether controversy, which was finally laid to rest by the theories of Albert Einstein in the early twentieth century. In 1905, Einstein published his Special Theory of Relativity followed by the General Theory of Relativity in 1915. The first theory related to the movement of objects at constant velocity relative to one another, while the second focused on acceleration and its links with gravity. Because they challenged many long-standing hypotheses, such as Isaac Newton's law of motion, Einstein's theories were a revolutionary force in physics. The idea of relativity embodies the concept that the velocity of an object can be determined only relative to the position of the observer. As an example, a man walking inside an airliner appears to be traveling at about one mile per hour in the reference frame of the aircraft (which itself is moving at 600 miles per hour). However, to an observer on the ground, the man seems to be moving at 601 miles per hour. Einstein assumed in his calculations that the speed of light traveling between two frames of reference remains the same for observers in both locations.

Because an observer in one frame uses light to determine the position and velocity of objects in another frame, this changes the manner in which the observer can relate the position and velocity of the objects. Einstein employed this concept to derive several important formulas describing how objects in one frame of reference appear when viewed from another that is in uniform motion relative to the first. His results led to some unusual conclusions, although the effects only become noticeable when the relative velocity of an object approaches the speed of light. In summary, the major implications of Einstein's fundamental theories and his often-referenced relativity equation: E = mc2 can be summarized as follows: The length of an object decreases, relative to an observer, as the velocity of that object increases. When a frame of reference is moving, time intervals become shorter. In other words, a space traveler moving at or near the speed of light could leave the Earth for many years, and return having experienced a time lapse of only a few months. The mass of a moving object increases with its velocity, and as the velocity approaches the speed of light, the mass approaches infinity.

For this reason, it is widely believed that travel faster than the speed of light is impossible, because an infinite amount of energy would be required to accelerate an infinite mass. Although Einstein's theory affected the entire world of physics, it had particularly important implications for those scientists who were studying light. The theory explained why the Michelson-Morley experiment failed to produce the expected results, discouraging further serious scientific investigations into the nature of ether as a carrier medium. It also demonstrated that nothing can move faster than the speed of light in a vacuum, and that this speed is a constant and unchanging value. Meanwhile, experimental scientists continued to apply increasingly sophisticated instruments to zero in on a correct value for the speed of light and reduce the error in its measurement. Measurements of the Speed of Light Date 299,792.4588 Table 1 During the late nineteenth century, advances in radio and microwave technology provided novel approaches for measuring the speed of light. In 1888, more than 200 years after Roemer's pioneering celestial observations, German physicist Heinrich Rudolf Hertz measured the speed of radio waves.

Hertz arrived at a value near 300,000 kilometers per second, confirming James Clerk Maxwell's theory that radio waves and light were both forms of electromagnetic radiation. Additional proof was gathered during the 1940s and 1950s, when British physicists Keith Davy Froome and Louis Essen employed radio and microwaves, respectively, to more precisely measure the speed of electromagnetic radiation. Maxwell is also credited with defining the speed of light and other forms of electromagnetic radiation, not through measurement, but by mathematical deduction. During his research attempts to find a link between electricity and magnetism, Maxwell theorized that a changing electrical field produces a magnetic field, the reverse corollary of Faraday's law. He proposed that electromagnetic waves are composed of combined oscillating electric and magnetic waves, and calculated the velocity of these waves through space as: Velocity (V) = 1/(e � m)1/2 where e is the permitivity and m is the permeability of free space, two constants that can be measured with a relatively high degree of accuracy. The result is a value that closely approximates the measured speed of light.

In 1891, continuing his studies on the speed of light and astronomy, Michelson created a large-scale interferometer using the refracting telescope at the Lick Observatory in California. His observations were based on the delay in the arrival time of light when viewing distant objects, such as stars, which can be quantitatively analyzed to measure both the size of celestial bodies and the speed of light. Almost 30 years later, Michelson moved his experiments to the Mount Wilson Observatory, and applied the same techniques to the 100-inch telescope, the world's largest at the time. By incorporating an octagonal rotating mirror into his experimental design, Michelson arrived at a value of 299,845 kilometers per second for the speed of light. Although Michelson died before completing his experiments, his co-worker at Mount Wilson, Francis G. Pease, continued to employ the innovative technique to conduct research into the 1930s. Using a modified interferometer, Pease made numerous measurements over several years and finally determined that the correct value for the speed of light is 299,774 kilometers per second, the closest measurement achieved to that date.

Several years later, in 1941, the scientific community set a standard for the speed of light. This value, 299,773 kilometers per second, was based on a compilation from the most accurate measurements of the period. Figure 6 presents a graphical representation of light speed measurements over the past 200 years. By the late 1960s, lasers were becoming stable research tools with highly defined frequencies and wavelengths. It quickly became obvious that a simultaneous measurement of frequency and wavelength would yield a very accurate value for the speed of light, similar to an experimental approach carried out by Keith Davy Froome using microwaves in 1958. Several research groups in the United States and in other countries measured the frequency of the 633-nanometer line from an iodine-stabilized helium-neon laser and obtained highly accurate results. In 1972, the National Institute of Standards and Technology employed the laser technology to measure the speed at 299,792,458 meters per second (186,282 miles per second), which ultimately resulted in the redefinition of the meter through a highly accurate estimate for the speed of light.

Starting with Roemer's 1676 breakthrough endeavors, the speed of light has been measured at least 163 times utilizing a wide variety of different techniques by more than 100 investigators (see Table 1 for a compilation of methods, investigators, and dates). As scientific methods and devices were refined, the error limits of the estimates narrowed, although the speed of light has not significantly changed since Roemer's seventeenth century calculations. Finally in 1983, more than 300 years after the first serious measurement attempt, the speed of light was defined as being 299,792.458 kilometers per second by the Seventeenth General Congress on Weights and Measures. Thus, the meter is defined as the distance light travels through a vacuum during a time interval of 1/299,792,458 seconds. In general, however, (even in many scientific calculations) the speed of light is rounded to 300,000 kilometers (or 186,000 miles) per second.

Arriving at a standard value for the speed of light was important for establishing an international system of units that would enable scientists from around the world to compare their data and calculations. There is a mild controversy over whether evidence exists that the speed of light has been slowing since the time of the Big Bang, when it may have moved significantly faster, as suggested by some investigators. Although arguments presented and countered perpetuate this debate, most scientists still contend that the speed of light is a constant. Physicists point out that the actual speed of light as measured by Roemer and his followers has not significantly changed, but rather point to a series of refinements in scientific instrumentation associated with increases in precision of the measurements utilized to establish the speed of light. Today, the distance between Jupiter and the Earth is known with a high degree of accuracy, as are the diameter of the solar system and the orbital trajectories of the planets. When researchers apply this data to rework the calculations made over the past few centuries, they derive values for the speed of light comparable to those obtained with more modern and sophisticated instrumentation. Contributing Authors Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657. Thomas J. Fellers, Lawrence D.

Zuckerman, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310. Why The Speed of Light Matters Why The Speed of Light Matters By Wynne Parry | September 26, 2011 05:01pm ET MORE Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second), as a cosmic speed limit. Some researchers think they may have broken this limit, and the implications are mind bending. Credit: Willem Dijkstra , Shutterstock When physicists announced last week that they had detected subatomic particles, called neutrinos, that appeared to be traveling faster than the speed of light, it seemed to be an exception to a cosmic speed limit set by Albert Einstein's special theory of relativity. Einstein's theory, which he proposed in 1905, describes the relativity of motion, particularly the motion of anything moving at or close to the speed of light.

At the time, people believed that light waves, just as sound waves, ocean waves or shock waves, had to travel through a medium. But rather than air, water or ground, they believed light waves traveled through a substance called ether, less tangible than air, that pervaded the universe. Scientists assumed that the laws of physics would be different for an object at rest with respect to the ether, and with the proper experiments it would be possible to figure out what was truly at rest, according to Peter Galison, a professor of physics and the history of science at Harvard University. [ Twisted Physics: 7 Mind-Blowing Findings ] "Einstein got rid of that," Galison said. "There are no physical properties that go with the statement 'I am truly at rest.' That's really what special relativity is about." In other words, the properties of physics are the same for me whether I am riding my bicycle or sitting on a park bench. Special relativity, however, does not apply to acceleration. Einstein would tackle this later in his general theory of relativity.

Special relativity is also based on a second assumption that gives the speed of light — 186,000 miles per second (300 million meters per second) — in a vacuum a special status. Einstein postulated that light always travels at the same speed for every observer, regardless of that observer's speed, Galison explained. So, if you have a fast enough car, in theory, you could catch up to a bullet. But you could never catch up to, or even reduce the apparent speed of a pulse of light, regardless of whether you were driving toward it or away from it. Ultimate speed limit Under Einstein's theory, the speed of light becomes a sort of ultimate speed limit. In fact, objects with mass, be they cars or neutrinos, can't reach the speed of light because they would need infinite energy to do so, according to the theory. Some experiments have appeared to play with the speed of light, but these effects are illusory, according to Galison. Light traveling through different mediums, such as chilled sodium gas, does slow substantially, but this is because the light is being bounced between the atoms within the medium.

But between interactions with atoms, it is still traveling at 186,000 miles per second (300 million meters per second), he said. Claims that it's possible to push light beyond 186,000 miles per second (300 million meters per second), are equally illusory, Galison said. Galison uses a hypothetical to explain why. If you shine a laser pointer on the surface of the moon and flick your wrist to sweep across the surface, wouldn't that mean that the bright dot is crossing the surface of the moon faster than the speed of light? No, because nothing is actually crossing the surface of the moon — the dot isn't an actual object, it is just a series of photons in the laser beam hitting the surface. "For 100 years, people have used these and more sophisticated paradoxes to try to say, 'Well isn't there this way to exceed the speed of light?'" Galison said. "They usually turn out to involve accelerating motion, something that is not really an object" — like the bright spot of the laser pointer — "or infinite energy." In other words, cheats.

In the lab, researchers can create the impression of sending light faster than the speed limit by tweaking the speed at which the wave crests of light propagate through space. This, however, does not increase the speed at which the actual electromagnetic information travels — this is conveyed by the overall shape of the wave's amplitude. Iron clad theory? Since Einstein introduced special relativity, the theory and the special status it gives to the speed of light have appeared iron-clad. Until now, that is. Scientists working on the OPERA experiment at the CERN laboratory in Switzerland beamed neutrinos 454 miles (730 kilometers) underground to Italy, and calculated how fast they made the trip. Shockingly, the neutrinos appeared to beat light speed by 60 billionths of a second. The finding appears to fly in the face of the last 106 years of physics. "Our understanding hasn't evolved at all, we've been doing extremely precise tests of special relativity since the very first days," said Ben Monreal, an assistant professor of physics at University of California, Santa Barbara. "Special relativity has been passing tests with flying colors for over 100 years now.

That is why this result is so surprising and unexpected." If the finding of the OPERA experiment does pan out, the implications are much more mind-bending. Under special relativity, if something travels faster than the speed of light, it goes backwards in time. Such a proposition could interfere with the basic rule that cause precedes effect, called causality. "The reason a lot of physicists are very unmoved by these claims is that it could make causality itself very problematic," Galison said. In other words, it raises the prospect of time travel . There is another issue too. Einstein introduced the speed of light as a mathematical constant, c. If neutrinos can indeed exceed the speed of light, then c loses its special status, giving rise to a host of other problems elsewhere in physics, where c has been used in calculations, such as the famous formula E=mc^2. [ Warped Physics: 10 Effects of Faster-Than-Light Discovery ] "For all of these reasons, people are going to need extra evidence to conclude that it is going to hold up," Galison said. You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry .

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook . Editor's Recommendations Speed of Light | Velocity of Light | Travel | mph | kilmometer | km | meter | per hour | per second | mps | special relativity | albert einstein | e=mc2 Special Relativity Speed of Light | Velocity of Light To buy the Strauss book critical of Special Relativity click or Light Speed: Maximum Limit of the Universe According to theory, the speed of light is the upper limit for all motion. In other words, it is believed that traveling at this speed or faster than light speed travel or far beyond is impossible. One of the unexpected consequences of Einstein's Relativity Theory is that as any object approaches this speed, its mass increases dramatically. In this manner, an object's mass approaches infinity, an impossible situation that can not be remedied. Strictly speaking a 'Speed' is a scalar quantity whilst a 'Velocity' denotes direction as well. Both will be used interchangeably herein; for example, traveling at light speed or velocity of light will mean the same thing for the purposes of this discussion.

Here are some values for light speed that a reader might find useful: Speed of Light in Miles per Second: 186,000 miles per second Velocity of Light in MPH: 671 million miles per hour mph Light Speed in Kilometers per second: 300,000 km per second Light Velocity Kilometers per hour: 1,080 million km per hour Velocity of Light in Meters per Second: 299,792,458 m/s Velocity of Light | A Constant 'C' As discussed previously, the speed of light is the upper limit beyond which nothing can travel. Matter, energy, information and anything else in the universe are all prohibited from exceeding this value. Many experiments have been made to measure this value and it is known with extreme precision. As a result, it has become known as a constant 'c' for traveling at the speed of light in a vacuum. The notation 'c' was given to the velocity of light because this refers to the Latin word 'celeritas' that means swiftness.

Travel at Light Speed | Physics | Einstein | Special Relativity In 1905, Albert Einstein published his famous work that revolutionized physics; some of its most important principles require that light speed 'c' was independent of a light source's motion and also independent of the frame of reference of an observer of that light source. His motivation for proposing this many faceted theory was a direct result of the failure of the theory of the 'lumineferous aether' or ether to produce a convincing showing in experiments designed to prove the existence of the aforementioned ether. In fact, the data appeared to show that the aether or ether did not even exist! Pondering this evidence and that from other observations, motivated Einstein to propose a dramatic solution to the many existing mathematical puzzles that developed in the latter part of the 19th century. He created a marvelous spacetime fabric that produced the incredible effects of time dilation, length contraction, twin paradoxes, the mass-energy equivalence E=mc2 and a constant speed of light. This value 'c' plays an important role in many areas of modern physics including those that are not directly related to light itself. Therefore, a demonstration that light was not a constant would be a major blow.

Recent Neutrino Results News is circulating around the world that the European CERN laboratories have found a neutrino traveling at a speed of light that is inacceptable according to theory. This news alone should give relativists pause as to the certainty of their ideas. But people love to follow the crowd and refuse to even consider the possibility of errors in what they have regurgitated their entire life. Expect some public apologies from relevant CERN scientists that will provide proof that they were completely wrong. They won't want to lose their tenure and research grants. What is really sad about all of this is there is a simple way to see that E=mc2 and the postulate of the constancy of light speed are both incorrect; therefore, neither of these are representative in the original theory itself of an Einsteinian universe. He himself did not even use a constant light speed in his works of 1905..... Notice: All rights reserved. No part of the book may be reproduced in any form or by any means, electronic, mechanical or optical, including photocopying or recording, or by any information storage retrieval system without permission in writing from the publisher. Purchasing the book signifies acceptance of these terms by the buyer.

Question: What is the approximate speed of light in miles per second?
Answer: 186,000