"Lightspeed" redirects here. For other uses, see Lightspeed (disambiguation).

For other uses of "speed of light", see speed of light (disambiguation)

A line showing the speed of light on a scale model of Earth and the Moon, taking about 1⅓ seconds to traverse that distance. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001

Light traveling through a medium such as air (for example, this laser) travels slower than light through a vacuum. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission.

The speed of light in the vacuum of free space is an important physical constant usually denoted by the letter c. In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes A physical Constant is a Physical quantity that is generally believed to be both universal in nature and constant in time ^[1] It is the speed of all electromagnetic radiation, including visible light, in free space. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 It is the speed of anything having zero rest mass. ^[2] The SI metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h). The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International To help compare different Orders of magnitude, the following list describes various Speed levels between 1 ^[3] The speed of light can be assigned a definite numerical value because the fundamental SI unit of length, the metre, has been defined since October 21, 1983, as the distance light travels in a vacuum in 1/299,792,458 of a second; in other words, any increase in the measurement precision of the speed of light would refine the definition of the metre, but not alter the numerical value of c. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg. Year 1983 ( MCMLXXXIII) was a Common year starting on Saturday (link displays the 1983 Gregorian calendar) The approximate value of 3×10⁸ m/s is commonly used in rough estimates (the error is 0. 07%). In imperial units, the speed of light is about 670,616,629. Imperial units or the Imperial system is a collection of units first defined in the British Weights and Measures Act of 1824 4 miles per hour or 983,571,056. 4 feet per second (roughly one foot per nanosecond), which is about 186,282. The foot per second (plural feet per second) is a unit of both Speed (scalar and Velocity (vector quantity which includes direction A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. 397 miles per second. A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum. In Optics, transparency (also called pellucidity) is the Material property of allowing The ratio of the speed of light in the vacuum to the observed phase velocity is called the refractive index of the medium. The phase velocity (or phase speed) of a Wave is the rate at which the phase of the wave propagates in space The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium See dispersion (optics). In Optics, dispersion is the phenomenon in which the Phase velocity of a wave depends on its frequency In general relativity c remains an important constant of spacetime, however the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Gravitation is a natural Phenomenon by which objects with Mass attract one another When measured locally, light in a vacuum always passes an observer at c. In Mathematics, a phenomenon is sometimes said to occur locally if roughly speaking it occurs on sufficiently small or arbitrarily small Neighborhoods

Overview

The speed of light in vacuum is now viewed as a fundamental physical constant. This postulate, together with the principle of relativity that all inertial frames are equivalent, forms the basis of Einstein's theory of special relativity. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3×10⁸ metres per second, or about 30 centimetres (1 foot) per nanosecond). A centimetre ( American spelling: centimeter, symbol cm) is a unit of Length in the Metric system, equal to one hundredth A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. See metre. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International

Experimental evidence has shown that the speed of light is independent of the motion of the source. It has also been confirmed experimentally that the two-way speed of light (for example from a source, to a mirror, and back again) is constant. It is not, however, possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and receiver should be synchronized. ^[4] Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way.

It is worth noting that it is the constant speed c, rather than light itself, that is fundamental to special relativity; thus if light is somehow manipulated to travel at less than c, this manipulation will not directly affect the theory of special relativity.

Observers traveling at large velocities will find that distances and times are distorted in accordance with the Lorentz transforms; however, the transformations distort times and distances in such a way that the speed of light remains constant. In Physics, the Lorentz transformation converts between two different observers' measurements of space and time where one observer is in constant motion with respect to An observer moving with respect to a collection of light sources would find that light from the sources ahead would be shifted toward the violet end of the spectrum while light from those behind was redshifted. Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by

Use of the symbol 'c' for the speed of light

The symbol 'c' for 'constant' or the Latin celeritas ("swiftness")^[5] is generally used for the speed of light. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. Celeritas is a Latin word translated as "swiftness" or "speed" NIST and BIPM practice is to use c₀ for the speed of light in vacuum. The International Bureau of Weights and Measures ( Bureau international des poids et mesures, in French) is an international Standards organization, one Occasionally, some writers use c for the speed of light in media other than vacuum. Throughout this article c is used exclusively to denote the speed of light in a vacuum.

In branches of physics in which the speed of light plays an important part, for example relativity, it is common to use a system of units in which c is 1, thus no symbol for the speed of light is required.

Causality and information transfer

If information could travel faster than c in one reference frame, causality would be violated: in some other reference frames, the information would be received before it had been sent, so the "effect" could be observed before the "cause". Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in Such a violation of causality has never been recorded. ^[6]

A light cone defines locations that are in causal contact and those that are not. In Special relativity, a light cone (or null cone) is the pattern describing the temporal evolution of a flash of Light in Minkowski spacetime Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in

To put it another way, information propagates to and from a point from regions defined by a light cone. In Special relativity, a light cone (or null cone) is the pattern describing the temporal evolution of a flash of Light in Minkowski spacetime The interval AB in the diagram to the right is "time-like" (that is, there is a frame of reference in which event A and event B occur at the same location in space, separated only by their occurring at different times, and if A precedes B in that frame then A precedes B in all frames: there is no frame of reference in which event A and event B occur simultaneously). SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS Thus, it is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the "cause" and B the "effect").

On the other hand, the interval AC in the diagram to the right is "space-like" (that is, there is a frame of reference in which event A and event C occur simultaneously, separated only in space; see simultaneity). SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS However, there are also frames in which A precedes C (as shown) or in which C precedes A. Barring some way of traveling faster than light, it is not possible for any matter (or information) to travel from A to C or from C to A. Thus there is no causal connection between A and C.

Light years

Astronomical distances are sometimes measured in light years (the distance that light would travel in one Earth year, roughly 9.46×10¹² kilometres or about 5. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by To help compare different Distances this page lists lengths starting at 1015 m (1 Pm or 1000000 million km or 6700 Astronomical units 88×10¹² miles). Because light travels at a large but finite speed, it takes time for light to cover large distances. Thus, the light we observe from distant objects in the universe was emitted from them long ago: in effect, we see their distant past. Even in terms of our own star we see into the past as well. Light from the sun takes around eight and one-third minutes to reach the earth.

Communications and GPS

The speed of light is of relevance to communications. For example, given the equatorial circumference of the Earth is about 40,075 km and c about 300,000 km/s, the theoretical shortest amount of time for a piece of information to travel half the globe along the surface is 0. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 0668 s.

The actual transit time is longer, in part because the speed of light is slower by about 35% in an optical fiber^[7] depending on its refractive index n, v = c / n and straight lines rarely occur in global communications situations, but also because delays are created when the signal passes through an electronic switch or signal regenerator. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium A typical time as of 2004 for a U.S. to Australia or Japan computer-to-computer ping is 0. The United States of America —commonly referred to as the For a topic outline on this subject see List of basic Australia topics. For a topic outline on this subject see List of basic Japan topics. Ping is a Computer network tool used to test whether a particular host is reachable across an IP network it is also used to self test the network interface 18 s. The speed of light additionally affects wireless communications design. Wireless communication is the transfer of information over a distance without the use of electrical conductors or " Wires quot

Another consequence of the finite speed of light is that communications with spacecraft are not instantaneous, and the gap becomes more noticeable as distances increase. This delay was significant for communications between Houston ground control and Apollo 8 when it became the first spacecraft to orbit the Moon: for every question, Houston had to wait nearly 3 seconds for the answer to arrive, even when the astronauts replied immediately. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units

This effect forms the basis of the Global Positioning System (GPS) and similar navigation systems. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth Navigation is the process of reading and controlling the movement of a craft or vehicle from one place to another One's position can be determined by means of the delays in radio signals received from a number of satellites, each carrying a very accurate atomic clock, and very carefully synchronized. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element It is remarkable that, to work properly, this method requires that (among many other effects) the relative motion of satellite and receiver be taken into effect, which was how (on an interplanetary scale) the finite speed of light was originally discovered (see the following section).

Similarly, instantaneous remote control of interplanetary spacecraft is impossible because it takes time for the Earth-based controllers to receive information from the craft, and an equal time for instructions to be received by the craft. It can take hours for controllers to become aware of a problem, respond with instructions, and have the spacecraft receive the instructions.

The speed of light can also be of concern on very short distances. In supercomputers, the speed of light imposes a limit on how quickly data can be sent between processors. A supercomputer is a Computer that is at the frontline of processing capacity particularly speed of calculation (at the time of its introduction If a processor operates at 1 GHz, a signal can only travel a maximum of 300 mm in a single cycle. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. Processors must therefore be placed close to each other to minimize communication latencies. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single chips. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside

Physics

Constant velocity from all inertial reference frames

Most individuals are accustomed to the addition rule of velocities: if two cars approach each other from opposite directions, each traveling at a speed of 50 km/h, relative to the road surface, one expects that each car will measure the other as approaching at a combined speed of 50 + 50 = 100 km/h to a very high degree of accuracy. (For the South African airport with IATA code "KMH" see Johan Pienaar Airport.

However, as speeds increase this rule becomes less accurate. Two spaceships approaching each other, each traveling at 90% the speed of light relative to some third observer between them, do not measure each other as approaching at 90% + 90% = 180% the speed of light; instead they each measure the other as approaching at slightly less than 99. 5% the speed of light. This last result is given by the Einstein velocity addition formula:^[8]

where v and w are the (positive) velocities of the spaceships as measured by the third observer, and u is the measured velocity of either space ship as observed by the other. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical The velocity-addition formula is one of two physics equations that relates the velocities of a moving object in different reference frames Galilean Addition Of Velocities ^[9] This reduces to u = v + w for sufficiently small values of v and w (such as those typically encountered in common daily experiences), as the term vw / c² approaches zero, reducing the denominator to 1.

If one of the velocities for the above formula (or both) are c, the final result is c, as is expected if the speed of light is the same in all reference frames. Another important result is that this formula always returns a value which is less than c whenever v and w are less than c: this shows that no acceleration in any frame of reference can cause one to exceed the speed of light with respect to another observer. Thus c acts as a speed limit for all objects with respect to all other objects in special relativity.

Luminiferous aether (discredited)

Interference pattern produced with a Michelson interferometer

Before the advent of special relativity, it was believed that light travels through a medium called the luminiferous aether. In the late 19th century " luminiferous aether " (or " ether " meaning light-bearing aether, was the term used to describe a medium for the propagation Maxwell’s equations predict a given speed of light, in much the same way as is the speed of sound in air. Sound is a vibration that travels through an elastic medium as a Wave. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The speed of sound in air is relative to the movement of the air itself, and the speed of sound in air with respect to an observer may be changed if the observer is moving with respect to the air (or vice versa). Wind is the flow of Air or other Gases that compose an Atmosphere (including but not limited to the Earth's) The speed of light was believed to be relative to a medium of transmission for light that acted as air does for the transmission of sound—the luminiferous aether.

The Michelson–Morley experiment, arguably the most famous and useful failed experiment in the history of physics, was designed to detect the motion of the Earth through the luminiferous aether. The Michelson–Morley experiment, one of the most important and famous experiments in the History of physics, was performed in 1887 by Albert Michelson and It could not find any trace of this kind of motion, suggesting, as a result, that it is impossible to detect one's presumed absolute motion, that is, motion with respect to the hypothesized luminiferous aether. The Michelson–Morley experiment said little about the speed of light relative to the light’s source and observer’s velocity, as both the source and observer in this experiment were traveling at the same velocity together in space.

Interaction with transparent materials

The refractive index of a material indicates how much slower the speed of light is in that medium than in a vacuum. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium The slower speed of light in materials can cause refraction, as demonstrated by this prism (in the case of a prism splitting white light into a spectrum of colours, the refraction is known as dispersion). Refraction is the change in direction of a Wave due to a change in its Speed. In Optics, a dispersive prism is a type of optical prism, normally having the shape of a geometrical triangular prism. In Optics, dispersion is the phenomenon in which the Phase velocity of a wave depends on its frequency

In passing through materials, the observed speed of light can differ from c. The ratio of c to the phase velocity of light in the material is called the refractive index. The phase velocity (or phase speed) of a Wave is the rate at which the phase of the wave propagates in space The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium The speed of light in air is only slightly less than c. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Denser media, such as water and glass, can slow light much more, to fractions such as and of c. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many Through diamond, light is much slower—only about 124,000 kilometres per second, less than of c. ^[10] This reduction in speed is also responsible for bending of light at an interface between two materials with different indices, a phenomenon known as refraction. Refraction is the change in direction of a Wave due to a change in its Speed.

Since the speed of light in a material depends on the refractive index, and the refractive index may depend on the frequency of the light, light at different frequencies can travel at different speeds through the same material. This effect is called dispersion.

Classically, considering electromagnetic radiation to be a wave, the charges of each atom (primarily the electrons) interact with the electric and magnetic fields of the radiation, slowing its progress. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J

A more complete description of the passage of light through a medium is given by quantum electrodynamics. Quantum electrodynamics ( QED) is a relativistic Quantum field theory of Electrodynamics.

Accelerated frames of reference and general relativity

Since the early part of the 20th century lightspeed in vacuum has been considered a property of our spacetime metric, i. This article discusses metrics in General relativity, for a discussion of metrics in general see Metric tensor. e. an exchange rate between seconds and meters and thus an effective "limit speed for energy" in general. ^[11]

Although it is constant in inertial frames of reference in special relativity, the speed of light can vary based on its position for accelerated frames of reference in special relativity and in general relativity. Before heading into this discussion, it must first be noted that in all cases the speed of light locally remains c in these cases. So when an observer measures the speed of light at his own position, the constancy of its speed holds. The issue arises at positions distant from the observer in these situations.

The cause of this change is gravitational time dilation. Gravitational time dilation is the effect of time passing at different rates in regions of different Gravitational potential; the higher the local distortion of Spacetime As clocks at lower gravitational potentials tick slower, a beam of light will take longer to move along a rod at a lower gravitational potential than it would take to move along an identical rod at ones own potential. This light is considered to be moving more slowly at lower potentials. This slowdown becomes extreme as the light approaches the event horizon of a black hole, where both time and light will appear to stop. In General relativity, an event horizon is a boundary in Spacetime, an area surrounding a Black hole or a Wormhole, inside which events cannot A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e Similarly, light will appear to go faster at higher gravitational potentials.

In general relativity, the curvature of spacetime can also affect the number of rods between certain positions. This will add another factor to magnitude of the apparent speed change.

Faster-than-light observations and experiments

Main article: Faster-than-light

The blue glow in this "swimming pool" nuclear reactor is Čerenkov radiation, emitted as a result of electrons traveling faster than the speed of light in water. This article is a subarticle of Nuclear power. A nuclear reactor is a device in which Nuclear chain reactions are initiated controlled Čerenkov radiation (also spelled Cerenkov or Cherenkov) is Electromagnetic radiation emitted when a charged particle (such as an The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J

It is generally considered that it is impossible for any information or matter to travel faster than c. Information as a concept has a diversity of meanings from everyday usage to technical settings Matter is commonly defined as being anything that has mass and that takes up space. The equations of relativity show that, for an object travelling faster than c, some physical quantities would be not represented by real numbers. In Mathematics, the real numbers may be described informally in several different ways However, there are many physical situations in which speeds greater than c are encountered.

Things which can travel faster than c

Wave velocities and synchronized events

It has long been known theoretically that it is possible for the "group velocity" of light to exceed c. The group velocity of a Wave is the Velocity with which the variations in the shape of the wave's amplitude (known as the modulation or envelope ^[12] One recent experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 In 2002, at the Université de Moncton, physicist Alain Haché made history by sending pulses at a group velocity of three times light speed over a long distance for the first time, transmitted through a 120-metre cable made from a coaxial photonic crystal. The University of Moncton ( Fr Université de Moncton is a French language university in Moncton New Brunswick serving the Acadian community of Atlantic Alain Haché (born December 14, 1970, in Tracadie, New Brunswick) is an experimental physicist and associate professor at the University Photonic crystals are periodic Optical (nanostructures that are designed to affect the motion of Photons in a similar way that periodicity of a Semiconductor ^[13] However, it is not possible to use this technique to transfer information faster than c: the velocity of information transfer depends on the front velocity (the speed at which the first rise of a pulse above zero moves forward) and the product of the group velocity and the front velocity is equal to the square of the normal speed of light in the material. Information as a concept has a diversity of meanings from everyday usage to technical settings In Physics, Front velocity is the speed at which the first rise of a pulse above zero moves forward

Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people distantly spaced in a line, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the previous person shouting. Another example can be seen when watching ocean waves washing up on shore. With a narrow enough angle between the wave and the shoreline, the breakers travel along the waves' length much faster than the waves' movement inland.

Light spots and shadows

If a laser is swept across a distant object, the spot of light can easily be made to move at a speed greater than c. ^[14] Similarly, a shadow projected onto a distant object can be made to move faster than c. In neither case does any matter or information travel faster than light.

Quantum mechanics

The speed of light may also appear to be exceeded in some phenomena involving evanescent waves, such as tunnelling. An evanescent wave is a nearfield standing Wave exhibiting Exponential decay with distance In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a Experiments indicate that the phase velocity and the group velocity of evanescent waves may exceed c; however, it would appear that the front velocity does not exceed c, so, again, it is not possible for information to be transmitted faster than c. The phase velocity (or phase speed) of a Wave is the rate at which the phase of the wave propagates in space

In quantum mechanics, certain quantum effects may be transmitted at speeds greater than c (indeed, action at a distance has long been perceived by some as a problem with quantum mechanics: see EPR paradox, interpretations of quantum mechanics). Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Physics, action at a distance is the Interaction of two objects which are separated in Space with no known mediator of the interaction In Quantum mechanics, the EPR paradox is a Thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities An interpretation of quantum mechanics is a statement which attempts to explain how Quantum mechanics informs our Understanding of Nature. For example, the quantum states of two particles can be entangled, so the state of one particle fixes the state of the other particle (say, one must have spin +½ and the other must have spin −½). In Quantum physics, a quantum state is a mathematical object that fully describes a quantum system. Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin Until the particles are observed, they exist in a superposition of two quantum states, (+½, −½) and (−½, +½). Quantum superposition is the fundamental law of Quantum mechanics. If the particles are separated and one of them is observed to determine its quantum state then the quantum state of the second particle is determined automatically. If, as in some interpretations of quantum mechanics, one presumes that the information about the quantum state is local to one particle, then one must conclude that second particle takes up its quantum state instantaneously, as soon as the first observation is carried out. However, it is impossible to control which quantum state the first particle will take on when it is observed, so no information can be transmitted in this manner. The laws of physics also appear to prevent information from being transferred through more clever ways and this has led to the formulation of rules such as the no-cloning theorem and the no-communication theorem. The no cloning theorem is a result of Quantum mechanics which forbids the creation of identical copies of an arbitrary unknown quantum state In Quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two observers is impossible

Closing speeds

If two objects are travelling towards one another, each at 0. 8c as measured in a particular inertial frame of reference, then they are getting closer together at 1. 6c as measured in that frame. ^[15] This is called a closing speed. Note that a closing speed does not represent the speed of any object in an inertial frame.

Proper Speeds

If a spaceship travels to a planet one light year (as measured in the Earth's rest frame) away from Earth at high speed, the time taken to reach that planet could be less than one year as measured by the traveller's clock (although it will always be more than one year as measured by a clock on Earth). The value obtained by dividing the distance travelled, as determined in the Earth's frame, by the time taken, measured by the traveller's clock, is known as a proper speed or a proper velocity. Proper-velocity, the distance traveled per unit time elapsed on the clocks of a traveling object equals coordinate velocity at low speeds There is no limit on the value of a proper speed as a proper speed does not represent a speed measured in a single inertial frame. Note that a light signal that left the Earth at the same time as the traveller would always get to the destination before the traveller.

Things which only appear to travel faster than c

So-called superluminal motion is seen in certain astronomical objects, such as the jets of radio galaxies and quasars. In Astronomy, superluminal motion is the apparently Faster-than-light motion seen in some radio galaxies, Quasars and recently also in some galactic The lower-energy non-relativistic version of this phenomenon is described at Polar jet. Radio galaxies and their relatives radio-loud Quasars and Blazars, are types of Active galaxy that are very luminous at radio wavelengths (up A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. However, these jets are not moving at speeds in excess of the speed of light: the apparent superluminal motion is a projection effect caused by objects moving near the speed of light and at a small angle to the line of sight. In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called

Travel faster than the speed of light in a medium

Although it may sound paradoxical, it is possible for shock waves to be formed with electromagnetic radiation. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called As a charged particle travels through an insulating medium, it disrupts the local electromagnetic field in the medium. An insulator, also called a Dielectric, is a material that resists the flow of Electric current. Electrons in the atoms of the medium will be displaced and polarised by the passing field of the charged particle, and photons are emitted as the electrons in the medium restore themselves to equilibrium after the disruption has passed. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations (In a conductor, the equilibrium can be restored without emitting a photon. In Science and engineering, a conductor is a material which contains movable Electric charges. ) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, if the disruption travels faster than the photons themselves travel, as when a charged particle exceeds the speed of light in that medium, the photons constructively interfere and intensify the observed radiation. The result (analogous to a sonic boom) is known as Čerenkov radiation. The term sonic boom is commonly used to refer to the shocks caused by the Supersonic flight of an aircraft Čerenkov radiation (also spelled Cerenkov or Cherenkov) is Electromagnetic radiation emitted when a charged particle (such as an

General relativity

Some topics (such as the expansion of the universe, and wormholes) require the application of general relativity and are covered in the main faster than light article. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916

Other theories concerning the speed of light

Particles that travel faster than light, dubbed tachyons, have been proposed by particle physicists but have yet to be observed, and would potentially violate causality if they were. A tachyon (from the Greek, takhyónion, from, takhýs, ie swift fast is any hypothetical particle that travels at Faster-than-light Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them Causality (but not causation) denotes a necessary relationship between one event (called cause and another event (called effect) which is the direct consequence

Some physicists, notably João Magueijo and John Moffat, have proposed that in the past light traveled much faster than the current speed of light. João Magueijo (born Évora, 1967 is a Portuguese cosmologist and professor in Theoretical Physics at Imperial College London John Moffat may refer to John Moffat (physicist (born 1932 professor at the University of Toronto known for his work suggesting the speed of light has varied This theory is called variable speed of light (VSL) and its supporters claim that it has the ability to explain many cosmological puzzles better than its rival, the inflation model of the universe. The variable speed of light (VSL concept states that the Speed of light in a vacuum usually denoted by c, may not be Constant in some cases Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy However, it has not gained wide acceptance.

Slow light

Main article: Slow light

Refractive phenomena, such as this rainbow, are due to the slower speed of light in a medium (water, in this case). Slow light is the literal slowing of the Speed of light. It is the propagation of an optical pulse or other modulation of an optical carrier at a very low Group velocity Refraction is the change in direction of a Wave due to a change in its Speed. A rainbow is an optical and meteorological phenomenon that causes a spectrum of Light to appear in the Sky when the Sun

Light traveling through a medium other than a vacuum travels below c as a result of the time lag between the polarization response of the medium and the incident light. In Classical electromagnetism, the polarization density (or electric polarization, or simply polarization) is the Vector field that expresses However, certain materials have an exceptionally high group index and a correspondingly low group velocity. The group velocity of a Wave is the Velocity with which the variations in the shape of the wave's amplitude (known as the modulation or envelope In 1999, a team of scientists led by Lene Hau were able to slow the speed of a light pulse to about 17 metres per second;^[16] in 2001, they were able to momentarily stop a beam. Lene Vestergaard Hau (born in Vejle Denmark, on November 13, 1959) is a Danish Physicist. ^[17]

In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a Bose–Einstein condensate of the element rubidium, the atoms of which, in Lukin's words, behaved "like tiny mirrors" due to an interference pattern in two "control" beams. The Lebedev Physics Institute of the Russian Academy of Sciences, situated in Moscow, is one of the leading Russian research institutes specializing in Physics Moscow (Москва́ romanised: Moskvá, IPA: see also other names) is the Capital and the largest city of A Bose–Einstein condensate (BEC is a State of matter of Bosons confined in an external Potential and cooled to Temperatures very near to Rubidium (ruːˈbɪdiəm /rəˈbɪdiəm/ is a Chemical element with the symbol Rb and Atomic number 37 ^[18]

History

Until relatively recent times, the speed of light was largely a matter of conjecture. Empedocles maintained that light was something in motion, and therefore there had to be some time elapsed in travelling. Empedocles ( Greek:, ca 490–430 BC was a Greek Pre-Socratic Philosopher and a citizen of Agrigentum, a Greek colony in Aristotle said that, on the contrary, "light is due to the presence of something, but it is not a movement". Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. Furthermore, if light had a finite speed, it would have to be very great; Aristotle asserted "the strain upon our powers of belief is too great" to believe this.

One of the ancient theories of vision was that light was emitted from the eye, instead of entering the eye from another source. Using this theory, Heron of Alexandria advanced the argument that the speed of light must be infinite, since distant objects such as stars appear immediately upon opening the eyes. Hero (or Heron) of Alexandria ( Ήρων ο Αλεξανδρεύς) (c Infinity (symbolically represented with ∞) comes from the Latin infinitas or "unboundedness

Medieval and early modern theories

Early Muslim philosophers initially agreed with Aristotle's view that the speed of light was infinite. Early Islamic philosophy or classical Islamic philosophy is a period of intense philosophical development beginning in the 2nd century AH of the Islamic calendar Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. In the 11th century, however, the Iraqi scientist Ibn al-Haytham (Alhacen), using an early experimental scientific method in his Book of Optics, discovered that light has a finite speed. For a topic outline on this subject see List of basic Iraq topics. TemplateInfobox Muslim scholars --> ( Arabic: ابو علی، حسن بن حسن بن هيثم Latinized In scientific inquiry an experiment ( Latin: Ex- periri, "to try out" is a method of investigating particular types of research questions or Scientific method refers to bodies of Techniques for investigating phenomena The Book of Optics ( Arabic: Kitab al-Manazir, Latin: De Aspectibus or Opticae Thesaurus Alhazeni Some of his contemporaries, notably the Persian scientists Avicenna and al-Biruni, also agreed with Alhacen that light has a finite speed. layout and formatting it should ensure no clashes with the top of the infobox TemplateInfobox Muslim scholars --> ( Persian /ابو علی الحسین ابن عبدالله ابن سینا (born Avicenna "observed that if the perception of light is due to the emission of some sort of particles by a luminous source, the speed of light must be finite". ^[19] Al-Biruni further discovered that the speed of light is much faster than the speed of sound. Sound is a vibration that travels through an elastic medium as a Wave. ^[20]

Commenting on a verse in the Rigveda ("Swift and all beautiful art thou, O Surya, maker of the light; illuminating all the radiant realm. The Rigveda ( Sanskrit sa ऋग्वेद ṛgveda, a compound of ṛc "praise verse" and veda "knowledge" "),^[21] the 14th century Indian scholar Sayana wrote "Thus it is remembered: [O Sun] you who traverse 2202 yojanas [1 yojana is between 4 and 9 miles] [ca. India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country Sāyaṇa ( सायण, with honorific Sāyaṇācārya;died 1387 was an important commentator on the Vedas He flourished under King Bukka I A yojana (Hindi: योजन) is a Vedic measure of distance used in ancient India. 14,000 to 32,000 km] in half a nimesa [a nimesa corresponds to ca. Hinduism’s understanding of time is as grandiose as time itself 0. 5 s]",^[22] corresponding to between 56,000 and 128,000 km/s for low or high values of yojana, a definition that roughly approaches the dimension of the actual speed of light.

Johannes Kepler believed that the speed of light is infinite since empty space presents no obstacle to it. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer Francis Bacon argued that the speed of light is not necessarily infinite, since something can travel too fast to be perceived. Francis Bacon 1st Viscount St Alban KC QC (22 January 1561 – 9 April 1626 was an English Philosopher, Statesman, and author René Descartes argued that if the speed of light were finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 A lunar eclipse occurs whenever the Moon passes through some portion of the Earth's shadow Since such misalignment had not been observed, Descartes concluded the speed of light is infinite. Descartes speculated that if the speed of light was found to be finite, his whole system of philosophy might be demolished. ^[23]

Measurement of the speed of light

Early attempts

Isaac Beeckman proposed an experiment (1629) in which a person would observe the flash of a cannon reflecting off a mirror about one mile away. Isaac Beeckman ( December 10, 1588 - May 19, 1637) was a Dutch Philosopher and Scientist. | NOTE Throughout this article "cannon" is used as BOTH the || singular and plural A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. Galileo proposed an experiment (1638), with an apparent claim to having performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher A lantern is a Portable Lighting device used to illuminate broad areas This experiment was carried out by the Accademia del Cimento of Florence in 1667, with the lanterns separated by about one mile. The Accademia del Cimento (Academy of Experiment an early scientific society, was founded in Florence 1657 by students of Galileo, Evangelista Florence ( Italian: Firenze Florentia and Fiorenza) is the Capital City of the Italian region of Tuscany No delay was observed. Robert Hooke explained the negative results as Galileo had by pointing out that such observations did not establish the infinite speed of light, but only that the speed must be very great. Robert Hooke, FRS (18 July 1635 – 3 March 1703 was an English Natural philosopher and Polymath who played an important role in the

Rømer's observations of the occultations of Io from Earth.

Astronomical techniques

The first quantitative estimate of the speed of light was made in 1676 by Ole Christensen Rømer, who was studying the motions of Jupiter's moon, Io, with a telescope. Ole Christensen Rømer (o(ːlə ˈʁœːˀmɐ in Danish 25 September 1644, Århus – 19 September 1710, Copenhagen) In Physics, motion means a constant change in the location of a body TemplateInfobox Planet.--> Io (ˈaɪoʊ, or as Greek A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. It is possible to time the orbital revolution of Io because it enters and exits Jupiter's shadow at regular intervals (at C or D). In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star With regards to Time, an interval or period is the Duration between two events or occurrences of similar events Rømer observed that Io revolved around Jupiter once every 42. 5 hours when Earth was closest to Jupiter (at H). The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 He also observed that, as Earth and Jupiter moved apart (as from L to K), Io's exit from the shadow would begin progressively later than predicted. It was clear that these exit "signals" took longer to reach Earth, as Earth and Jupiter moved further apart. This was as a result of the extra time it took for light to cross the extra distance between the planets, time which had accumulated in the interval between one signal and the next. The opposite is the case when they are approaching (as from F to G). On the basis of his observations, Rømer estimated that it would take light 22 minutes to cross the diameter of the orbit of the Earth (that is, twice the astronomical unit); the modern estimate is about 16 minutes and 40 seconds. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the

Around the same time, the astronomical unit was estimated to be about 140 million kilometres. The astronomical unit and Rømer's time estimate were combined by Christiaan Huygens, who estimated the speed of light to be 1,000 Earth diameters per minute. Christiaan Huygens (ˈhaɪgənz in English ˈhœyɣəns in Dutch) ( April 14, 1629 &ndash July 8, 1695) was a Dutch This is about 220,000 kilometres per second (136,000 miles per second), 26% lower than the currently accepted value, but still very much faster than any physical phenomenon then known.

Isaac Newton also accepted the finite speed. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements In his 1704 book Opticks he reports the value of 16. Opticks is a book written by English physicist Isaac Newton that was released to the public in 1704. 6 Earth diameters per second (210,000 kilometres per second, 30% less than the actual value), which it seems he inferred for himself (whether from Rømer's data, or otherwise, is not known). The same effect was subsequently observed by Rømer for a "spot" rotating with the surface of Jupiter. And later observations also showed the effect with the three other Galilean moons, where it was more difficult to observe, thus laying to rest some further objections that had been raised.

Even if, by these observations, the finite speed of light may not have been established to everyone's satisfaction (notably Jean-Dominique Cassini's), after the observations of James Bradley (1728), the hypothesis of infinite speed was considered discredited. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer This article describes the English astronomer for other people sharing the name see James Bradley (disambiguation James Bradley (March 1693 Bradley deduced that starlight falling on the Earth should appear to come from a slight angle, which could be calculated by comparing the speed of the Earth in its orbit to the speed of light. This "aberration of light", as it is called, was observed to be about 1/200 of a degree. The aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an Apparent motion Bradley calculated the speed of light as about 298,000 kilometres per second (185,000 miles per second). This is only slightly less than the currently accepted value (less than one percent). The aberration effect has been studied extensively over the succeeding centuries, notably by Friedrich Georg Wilhelm Struve and de:Magnus Nyrén. Friedrich Georg Wilhelm von Struve (Vasily Yakovlevich Struve ( April 15, 1793 &ndash November 23, 1864 ( Julian calendar

Diagram of the Fizeau–Foucault apparatus. The Fizeau–Foucault apparatus ( 1850) was designed by the French physicists Hippolyte Fizeau and Léon Foucault for measuring the

Earth-bound techniques

The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Armand Hippolyte Louis Fizeau (1819-1896 French physicist, was born in Paris. (This measures the speed of light in air, which is slower than the speed of light in vacuum by a factor of the refractive index of air, about 1. 0003. ) Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror several thousand metres away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second. Fizeau's method was later refined by Marie Alfred Cornu (1872) and Joseph Perrotin (1900). Marie Alfred Cornu ( March 6, 1841 &mdash April 12, 1902) was a French Physicist. Henri Joseph Anastase Perrotin ( December 19 1845 &ndash February 29 1904) was a French Astronomer.

Leon Foucault improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Jean Bernard Léon Foucault (ʒɑ̃ bɛʁnaʁ leɔ̃ fu'ko ( 18 September 1819 &ndash 11 February 1868) was a French physicist Foucault's estimate, published in 1862, was 298,000 kilometres per second. Foucault's method was also used by Simon Newcomb and Albert A. Michelson. Simon Newcomb ( March 12 1835 &ndash July 11 1909) was a Canadian American Astronomer and Mathematician Albert Abraham Michelson ( December 19, 1852 &ndash May 9, 1931) was a Polish - American Physicist known Michelson began his lengthy career by replicating and improving on Foucault's method.

In 1926, Michelson used a rotating prism to measure the time it took light to make a round trip from Mount Wilson to Mount San Antonio in California, a distance of about 22 miles (36 km). In Optics, a prism is a transparent optical element with flat polished surfaces that refract Light. Mount Wilson is one of the more prominent peaks in the San Gabriel Mountains, part of the Angeles National Forest in Los Angeles County California, USA Mount San Antonio, known locally as Old Baldy or Mount Baldy, is the highest peak in the San Gabriel Mountains, and the highest California ( is a US state on the West Coast of the United States, along the Pacific Ocean. The precise measurements yielded a speed of 186,285 miles per second (299,796 kilometres per second).

Laboratory-based methods

During World War II, the development of the cavity resonance wavemeter for use in radar, together with precision timing methods, opened the way to laboratory-based measurements of the speed of light. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including A resonator is a device or system that exhibits Resonance or resonant behavior that is it naturally oscillates at some frequencies, called its resonance Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships In 1946, Louis Essen in collaboration with A. Louis Essen ( September 6, 1908 &ndash August 24, 1997) was an English Physicist whose most notable achievements were in C. Gordon-Smith used a microwave cavity of precisely known dimensions to establish the frequency for a variety of normal modes of microwaves—which, in common with all electromagnetic radiation, travels at the speed of light in vacuum. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Frequency is a measure of the number of occurrences of a repeating event per unit Time. A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric Their result, 299,792±3 km/s, was substantially greater than those found by optical techniques, and prompted much controversy. However, by 1950 repeated measurements by Essen established a result of 299,792. Essen (ˈɛsən is a City in the center of the Ruhr Area in North Rhine-Westphalia, Germany. 5±1 km/s; this became the value adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. The International Union of Radio Science (in French, l'Union Radio-Scientifique Internationale - URSI) is one of 26 international scientific unions affiliated Most subsequent measurements have been consistent with this value.

With modern electronics (and most particularly the availability of oscilloscopes with time resolutions in the sub-nanosecond regime) the speed of light can now be directly measured by timing the delay of a light pulse from a laser or a LED in reflecting from a mirror, and this kind of experiment is now routine in undergraduate physics laboratories. An oscilloscope (commonly abbreviated to scope or O-scope) is a type of Electronic test equipment that allows signal Voltages to be viewed ^[24][25][26]

Speed of light set by definition

In 1983, the 17th Conférence Générale des Poids et Mesures defined the metre in terms of the distance traveled by light in a given amount of time, which amounts to adopting a standard value for the speed of light in vacuum:^[27]

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International ^[3]

Here, the term vacuum is meant in the technical sense of free space. In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes This definition of the metre relies on the definition of the second, which is:

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units ^[28][29]

The consequence of this definition is that further refinements in the current experimental value of the speed of light would only adjust the length of a metre. ^[30] This point is made explicit by nondimensionalization in the article on Maxwell's equations. Nondimensionalization is the partial or full removal of units from a Mathematical equation by a suitable substitution of Variables. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric The value of c, or c₀,^[31] namely:

combined with the definition of magnetic constant μ₀, also defines the electric constant ε₀ in SI units. The vacuum permeability, referred to by international standards organizations as the magnetic constant, and denoted by the symbol μ 0 (also Vacuum permittivity, referred to by international standards organizations as the electric constant, and denoted by the symbol ε0 is a fundamental Physical The magnetic constant μ₀ is not dependent on c and as a result of the definition of the ampere, has a standard value in SI units of:^[32]

. The vacuum permeability, referred to by international standards organizations as the magnetic constant, and denoted by the symbol μ 0 (also The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second

The electric constant has then the exact value^[33]

These constants appear in Maxwell's equations. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric

Special relativity

After the work of James Clerk Maxwell, it was believed that light travelled at a constant speed relative to the "luminiferous aether", the medium that was then thought to be necessary for the transmission of light. James Clerk Maxwell (13 June 1831 &ndash 5 November 1879 was a Scottish mathematician and theoretical physicist. In the late 19th century " luminiferous aether " (or " ether " meaning light-bearing aether, was the term used to describe a medium for the propagation This speed was determined by the (permittivity and permeability) of the aether. Permittivity is a Physical quantity that describes how an Electric field affects and is affected by a Dielectric medium and is determined by the ability In Electromagnetism, permeability is the degree of Magnetization of a material that responds linearly to an applied Magnetic field.

A schematic representation of a Michelson interferometer, as used for the Michelson-Morley experiment. Albert Abraham Michelson ( December 19, 1852 &ndash May 9, 1931) was a Polish - American Physicist known Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of

In 1887, the physicists Albert Michelson and Edward Morley performed the influential Michelson-Morley experiment to measure the speed of light relative to the motion of the earth, the goal being to measure the velocity of the Earth through the aether. Albert Abraham Michelson ( December 19, 1852 &ndash May 9, 1931) was a Polish - American Physicist known Edward Williams Morley ( January 29, 1838 - February 24 1923) was an American Scientist famous for the Michelson-Morley The Michelson–Morley experiment, one of the most important and famous experiments in the History of physics, was performed in 1887 by Albert Michelson and EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 As shown in the diagram of a Michelson interferometer, a half-silvered mirror was used to split a beam of monochromatic light into two beams traveling at right angles to one another. Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of A beam splitter is an optical device that splits a beam of Light in two Monochrome comes from the Greek μονόχρωμος ( monochromos) meaning “of one color” which is a combination In Geometry and Trigonometry, a right angle is an angle of 90 degrees corresponding to a quarter turn (that is a quarter of a full circle After leaving the splitter, each beam was reflected back and forth between mirrors several times (the same number for each beam to give a long but equal path length; the actual Michelson-Morley experiment used more mirrors than shown) then recombined to produce a pattern of constructive and destructive interference. A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern Any slight change in speed of light along each arm of the interferometer (because the apparatus was moving with the Earth through the proposed "aether") would change the amount of time that the beam spent in transit, which would then be observed as a change in the pattern of interference. In the event, the experiment gave a null result. Generally a null result is a result which is Null (nothing that is the proposed result is absent

Ernst Mach was among the first physicists to suggest that the experiment amounted to a disproof of the aether theory. Ernst Mach (max ( February 18, 1838 &ndash February 19, 1916) was an Austrian Physicist and Philosopher and Developments in theoretical physics had already begun to provide an alternative theory, Fitzgerald-Lorentz contraction, which explained the null result of the experiment. Length contraction, according to Hendrik Lorentz, is the physical phenomenon of a decrease in Length detected by an observer in objects that travel at any non-zero

It is uncertain whether Albert Einstein knew the results of the Michelson-Morley experiment, but the null result of the experiment greatly assisted the acceptance of his theory of relativity. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. The constant speed of light is one of the fundamental Postulates (together with causality and the equivalence of inertial frames) of special relativity. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest

See also

• Čerenkov radiation
• Electromagnetic wave equation
• Faster-than-light
• Free space
• Fundamental Speed
• Mathematical descriptions of the electromagnetic field
• Maxwell's equations
• Meter
• SI units
• Sinusoidal plane-wave solutions of the electromagnetic wave equation
• Speed of sound
• Vacuum

References

Footnotes

Historical references

Modern references

External links

• Speed of light in vacuum (at NIST)
• Definition of the metre (BIPM)
• Data Gallery: Michelson Speed of Light (Univariate Location Estimation) (download data gathered by A.A. Michelson)
• Switching light on and off (news article on stopping light)
• Beam smashes light barrier (news article on group velocity experiment)
• Subluminal (Java applet demonstrating group velocity information limits)
• De Mora Luminis at MathPages
• Light discussion on adding velocities
• Speed of Light (University of Colorado Department of Physics)
• How would we know if the speed of light was varying with time?
• How is the speed of light measured?