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81-559: The d in Modern English thunder (from earlier Old English þunor ) is epenthetic , and is now found as well in Modern Dutch donder (cf. Middle Dutch donre ; also Old Norse þorr , Old Frisian þuner , Old High German donar , all ultimately descended from Proto-Germanic * þunraz ). In Latin the term was tonare "to thunder". The name of the Nordic god Thor comes from

162-404: A "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium . The popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped", it had ceased to be light. This type of behaviour

243-495: A further 4–24 minutes for commands to travel from Earth to Mars. Receiving light and other signals from distant astronomical sources takes much longer. For example, it takes 13 billion (13 × 10 ) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra-Deep Field images. Those photographs, taken today, capture images of the galaxies as they appeared 13 billion years ago, when

324-461: A light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri , the closest star to Earth after the Sun, is around 4.2 light-years away. Radar systems measure the distance to a target by the time it takes a radio-wave pulse to return to the radar antenna after being reflected by the target: the distance to the target is half the round-trip transit time multiplied by

405-615: A material-dependent constant. The refractive index of air is approximately 1.0003. Denser media, such as water , glass , and diamond , have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than- c speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to

486-400: A purely Latin alphabet of 26 letters . Speed of light The speed of light in vacuum , commonly denoted c , is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour). According to the special theory of relativity , c is the upper limit for

567-584: A result, if something were travelling faster than  c relative to an inertial frame of reference, it would be travelling backwards in time relative to another frame, and causality would be violated. In such a frame of reference, an "effect" could be observed before its "cause". Such a violation of causality has never been recorded, and would lead to paradoxes such as the tachyonic antitelephone . There are situations in which it may seem that matter, energy, or information-carrying signal travels at speeds greater than  c , but they do not. For example, as

648-416: A standard for the metre. As a dimensional physical constant , the numerical value of c is different for different unit systems. For example, in imperial units , the speed of light is approximately 186 282 miles per second, or roughly 1 foot per nanosecond. In branches of physics in which c appears often, such as in relativity, it is common to use systems of natural units of measurement or

729-412: A time dilation factor of γ  = 2 occurs at a relative velocity of 86.6% of the speed of light ( v  = 0.866  c ). Similarly, a time dilation factor of γ  = 10 occurs at 99.5% the speed of light ( v  = 0.995  c ). The results of special relativity can be summarized by treating space and time as a unified structure known as spacetime (with  c relating

810-459: A time interval of 1 ⁄ 299 792 458 of a second", fixing the value of the speed of light at 299 792 458  m/s by definition, as described below . Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of c . Outer space is a convenient setting for measuring the speed of light because of its large scale and nearly perfect vacuum . Typically, one measures

891-430: Is a shock wave, similar in principle to the shock wave formed by an explosion , or at the front of a supersonic aircraft . Near the source, the sound pressure level of thunder is usually 165 to 180 dB , but can exceed 200 dB in some cases. Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of

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972-424: Is described as a type of electromagnetic wave . The classical behaviour of the electromagnetic field is described by Maxwell's equations , which predict that the speed  c with which electromagnetic waves (such as light) propagate in vacuum is related to the distributed capacitance and inductance of vacuum, otherwise respectively known as the electric constant ε 0 and the magnetic constant μ 0 , by

1053-421: Is discussed in the propagation of light in a medium section below, many wave velocities can exceed  c . The phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than  c , although the initial movement of

1134-427: Is frame-independent, because it is impossible 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 at the detector should be synchronized. By adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition. The special theory of relativity explores

1215-511: Is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes , it can take minutes to hours for signals to travel. In computing , the speed of light fixes the ultimate minimum communication delay . The speed of light can be used in time of flight measurements to measure large distances to extremely high precision. Ole Rømer first demonstrated in 1676 that light does not travel instantaneously by studying

1296-459: Is generally microscopically true of all transparent media which "slow" the speed of light. In transparent materials, the refractive index generally is greater than 1, meaning that the phase velocity is less than c . In other materials, it is possible for the refractive index to become smaller than   1 for some frequencies; in some exotic materials it is even possible for the index of refraction to become negative. The requirement that causality

1377-406: Is heard. This is a consequence of the speed of light being much greater than the speed of sound . The speed of sound in dry air is approximately 343 m/s (1,130 ft/s) or 1,236 km/h (768 mph) at 20 °C (68 °F; 293 K). This translates to approximately 3 s/km (4.8 s/mi); saying "one thousand and one... one thousand and two..." is a useful method of counting

1458-505: Is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a refractive index . The refractive index of a material is defined as the ratio of c to the phase velocity  v p in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization , or direction of propagation; in many cases, though, it can be treated as

1539-486: Is impossible for signals or energy to travel faster than  c . One argument for this follows from the counter-intuitive implication of special relativity known as the relativity of simultaneity . If the spatial distance between two events A and B is greater than the time interval between them multiplied by  c then there are frames of reference in which A precedes B, others in which B precedes A, and others in which they are simultaneous. As

1620-422: Is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity and, in doing so, showed that the parameter c had relevance outside of the context of light and electromagnetism. Massless particles and field perturbations, such as gravitational waves , also travel at speed c in vacuum. Such particles and waves travel at c regardless of

1701-495: Is known as the Lorentz factor and is given by γ = (1 − v / c ) , where v is the speed of the object. The difference of γ from   1 is negligible for speeds much slower than  c , such as most everyday speeds – in which case special relativity is closely approximated by Galilean relativity  – but it increases at relativistic speeds and diverges to infinity as v approaches c . For example,

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1782-491: Is not violated implies that the real and imaginary parts of the dielectric constant of any material, corresponding respectively to the index of refraction and to the attenuation coefficient , are linked by the Kramers–Kronig relations . In practical terms, this means that in a material with refractive index less than 1, the wave will be absorbed quickly. A pulse with different group and phase velocities (which occurs if

1863-554: Is observed, so information cannot be transmitted in this manner. Another quantum effect that predicts the occurrence of faster-than-light speeds is called the Hartman effect : under certain conditions the time needed for a virtual particle to tunnel through a barrier is constant, regardless of the thickness of the barrier. This could result in a virtual particle crossing a large gap faster than light. However, no information can be sent using this effect. So-called superluminal motion

1944-473: Is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than c ). When a charged particle does that in a dielectric material, the electromagnetic equivalent of a shock wave , known as Cherenkov radiation , is emitted. The speed of light is of relevance to telecommunications : the one-way and round-trip delay time are greater than zero. This applies from small to astronomical scales. On

2025-405: Is prevented from dispersing vertically as it would in non-inversion conditions, and is thus concentrated in the near-ground layer. Cloud-to-ground lightning (CG) typically consists of two or more return strokes, from ground to cloud. Later return strokes have greater acoustic energy than the first. The most noticeable aspect of lightning and thunder is that the lightning is seen before the thunder

2106-401: Is seen in certain astronomical objects, such as the relativistic jets of radio galaxies and quasars . 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 approaching Earth at a small angle to the line of sight: since the light which was emitted when the jet

2187-920: Is the form of the English language that has been spoken since the Great Vowel Shift in England , which began in the late 14th century and was completed by the 17th century . With some differences in vocabulary, texts which date from the early 17th century, such as the works of William Shakespeare and the King James Bible , are considered Modern English texts, or more specifically, they are referred to as texts which were written in Early Modern English or they are referred to as texts which were written in Elizabethan English. Through colonization , English

2268-454: Is used for more purposes than any other language". Its large number of speakers, plus its worldwide presence, have made English a common language (lingua franca) "of the airlines, of the sea and shipping, of computer technology, of science and indeed of (global) communication generally". Modern English evolved from Early Modern English which was used from the beginning of the Tudor period until

2349-565: The Ethnologue , there are almost one billion speakers of English as a first or second language. English is spoken as a first or a second language in many countries, with most native speakers being in the United States , the United Kingdom , Australia , Canada , New Zealand and Ireland . It "has more non-native speakers than any other language, is more widely dispersed around the world and

2430-547: The Deep Space Network determine distances to the Moon, planets and spacecraft, respectively, by measuring round-trip transit times. There are different ways to determine the value of c . One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and Earth-based setups. It is also possible to determine c from other physical laws where it appears, for example, by determining

2511-574: The Interregnum and Stuart Restoration in England. By the late 18th century, the British Empire had facilitated the spread of Modern English through its colonies and geopolitical dominance. Commerce, science and technology, diplomacy, art, and formal education all contributed to English becoming the first truly global language. Modern English also facilitated worldwide international communication. English

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2592-617: The Old Norse word for thunder. The shared Proto-Indo-European root is * tón-r̥ or * tar- , also found in Gaulish Taranis . The cause of thunder has been the subject of centuries of speculation and scientific inquiry . Early thinking was that it was made by deities, but the ancient Greek philosophers attributed it to natural causes, such as wind striking clouds ( Anaximander , Aristotle ) and movement of air within clouds ( Democritus ). The Roman philosopher Lucretius held it

2673-400: The geometrized unit system where c = 1 . Using these units, c does not appear explicitly because multiplication or division by   1 does not affect the result. Its unit of light-second per second is still relevant, even if omitted. The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of

2754-459: The local speed of light is constant and equal to  c , but the speed of light can differ from  c when measured from a remote frame of reference, depending on how measurements are extrapolated to the region. It is generally assumed that fundamental constants such as  c have the same value throughout spacetime, meaning that they do not depend on location and do not vary with time. However, it has been suggested in various theories that

2835-430: The printed circuit board refracts and slows down signals. Processors must therefore be placed close to each other, as well as memory chips, to minimize communication latencies, and care must be exercised when routing wires between them to ensure signal integrity . If clock frequencies continue to increase, the speed of light may eventually become a limiting factor for the internal design of single chips . Given that

2916-400: The quantum states of two particles that can be entangled . Until either of the particles is observed, they exist in a superposition of two quantum states. If the particles are separated and one particle's quantum state is observed, the other particle's quantum state is determined instantaneously. However, it is impossible to control which quantum state the first particle will take on when it

2997-414: The speed of light may have changed over time . No conclusive evidence for such changes has been found, but they remain the subject of ongoing research. It is generally assumed that the two-way speed of light is isotropic , meaning that it has the same value regardless of the direction in which it is measured. Observations of the emissions from nuclear energy levels as a function of the orientation of

3078-543: The Earth with speeds proportional to their distances. Beyond a boundary called the Hubble sphere , the rate at which their distance from Earth increases becomes greater than the speed of light. These recession rates, defined as the increase in proper distance per cosmological time , are not velocities in a relativistic sense. Faster-than-light cosmological recession speeds are only a coordinate artifact. In classical physics , light

3159-434: The advantage which radio waves travelling at near to the speed of light through air have over comparatively slower fibre optic signals. Similarly, communications between the Earth and spacecraft are not instantaneous. There is a brief delay from the source to the receiver, which becomes more noticeable as distances increase. This delay was significant for communications between ground control and Apollo 8 when it became

3240-413: The apparent motion of Jupiter 's moon Io . Progressively more accurate measurements of its speed came over the following centuries. In a paper published in 1865, James Clerk Maxwell proposed that light was an electromagnetic wave and, therefore, travelled at speed c . In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame of reference is a constant and

3321-418: The appearance of certain high-speed astronomical objects , and particular quantum effects ). The expansion of the universe is understood to exceed the speed of light beyond a certain boundary . The speed at which light propagates through transparent materials , such as glass or air, is less than c ; similarly, the speed of electromagnetic waves in wire cables is slower than c . The ratio between c and

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3402-401: The cloud and ground during a temperature inversion . The resulting thunder sounds have significantly greater acoustic energy than those produced from the same distance in non-inversion conditions. In a temperature inversion, the air near the ground is cooler than the air higher up. Inversions often happen when warm, moist air passes above a cold front. Within a temperature inversion, sound energy

3483-587: The consequences of this invariance of c with the assumption that the laws of physics are the same in all inertial frames of reference. One consequence is that c is the speed at which all massless particles and waves, including light, must travel in vacuum. Special relativity has many counterintuitive and experimentally verified implications. These include the equivalence of mass and energy ( E = mc ) , length contraction (moving objects shorten), and time dilation (moving clocks run more slowly). The factor  γ by which lengths contract and times dilate

3564-658: The course of the 20th century. Note, however, that these are generalizations, and some of these may not be true for specific dialects: Up until the American–British split (1600–1725), some major phonological changes in English included: After the American-British split , further changes to English phonology included: Changes in alphabet and spelling were heavily influenced by the advent of printing and continental printing practices. Consequently, Modern English came to use

3645-451: The distance between two objects in a frame of reference with respect to which both are moving (their closing speed ) may have a value in excess of  c . However, this does not represent the speed of any single object as measured in a single inertial frame. Certain quantum effects appear to be transmitted instantaneously and therefore faster than c , as in the EPR paradox . An example involves

3726-474: The emitting nuclei in a magnetic field (see Hughes–Drever experiment ), and of rotating optical resonators (see Resonator experiments ) have put stringent limits on the possible two-way anisotropy . According to special relativity, the energy of an object with rest mass m and speed v is given by γmc , where γ is the Lorentz factor defined above. When v is zero, γ is equal to one, giving rise to

3807-418: The equation In modern quantum physics , the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons . In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum. Extensions of QED in which

3888-512: The equatorial circumference of the Earth is about 40 075  km and that c is about 300 000  km/s , the theoretical shortest time for a piece of information to travel half the globe along the surface is about 67 milliseconds. When light is traveling in optical fibre (a transparent material ) the actual transit time is longer, in part because the speed of light is slower by about 35% in optical fibre, depending on its refractive index n . Straight lines are rare in global communications and

3969-495: The famous E = mc formula for mass–energy equivalence. The γ factor approaches infinity as v approaches  c , and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass, and individual photons cannot travel faster than the speed of light. This is experimentally established in many tests of relativistic energy and momentum . More generally, it

4050-556: The first crewed spacecraft to orbit the Moon : for every question, the ground control station had to wait at least three seconds for the answer to arrive. The communications delay between Earth and Mars can vary between five and twenty minutes depending upon the relative positions of the two planets. As a consequence of this, if a robot on the surface of Mars were to encounter a problem, its human controllers would not be aware of it until approximately 4–24 minutes later. It would then take

4131-416: The group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time. None of these options allow information to be transmitted faster than c . It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the front velocity). It can be shown that this is (under certain assumptions) always equal to c . It

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4212-429: The individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency ) propagate is called the phase velocity   v p . A physical signal with a finite extent (a pulse of light) travels at a different speed. The overall envelope of the pulse travels at the group velocity   v g , and its earliest part travels at the front velocity   v f . The phase velocity

4293-478: The lightning channel, measured by spectral analysis , varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000  K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about 20,400 K (20,100 °C; 36,300 °F). This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel. The resultant outward-moving pulse

4374-503: The lightning strike was very near. Close-in lightning has been described first as a clicking or cloth-tearing sound, then a cannon shot sound or loud crack/snap, followed by continuous rumbling. The early sounds are from the leader parts of lightning, then the near parts of the return stroke, then the distant parts of the return stroke. Modern English Modern English , sometimes called New English ( NE ) or present-day English ( PDE ) as opposed to Middle and Old English ,

4455-502: The massive photon is described by Proca theory , the experimental upper bound for its mass is about 10 grams ; if photon mass is generated by a Higgs mechanism , the experimental upper limit is less sharp, m ≤ 10   eV/ c   (roughly 2 × 10  g). Another reason for the speed of light to vary with its frequency would be the failure of special relativity to apply to arbitrarily small scales, as predicted by some proposed theories of quantum gravity . In 2009,

4536-484: The motion of the source or the inertial reference frame of the observer . Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. In the theory of relativity , c interrelates space and time and appears in the famous mass–energy equivalence , E = mc . In some cases, objects or waves may appear to travel faster than light (e.g., phase velocities of waves,

4617-495: The observation of gamma-ray burst GRB 090510 found no evidence for a dependence of photon speed on energy, supporting tight constraints in specific models of spacetime quantization on how this speed is affected by photon energy for energies approaching the Planck scale . In a medium, light usually does not propagate at a speed equal to c ; further, different types of light wave will travel at different speeds. The speed at which

4698-421: The observer. This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for motion against the luminiferous aether . It has since been consistently confirmed by many experiments. It is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again)

4779-408: The other hand, some techniques depend on the finite speed of light, for example in distance measurements. In computers , the speed of light imposes a limit on how quickly data can be sent between processors . If a processor operates at 1   gigahertz , a signal can travel only a maximum of about 30 centimetres (1 ft) in a single clock cycle – in practice, this distance is even shorter since

4860-420: The parameter  c is ubiquitous in modern physics, appearing in many contexts that are unrelated to light. For example, general relativity predicts that  c is also the speed of gravity and of gravitational waves , and observations of gravitational waves have been consistent with this prediction. In non-inertial frames of reference (gravitationally curved spacetime or accelerated reference frames ),

4941-403: The phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion . Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light . The opposite, group velocities exceeding c , was proposed theoretically in 1993 and achieved experimentally in 2000. It should even be possible for

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5022-412: The photon has a mass have been considered. In such a theory, its speed would depend on its frequency, and the invariant speed  c of special relativity would then be the upper limit of the speed of light in vacuum. No variation of the speed of light with frequency has been observed in rigorous testing, putting stringent limits on the mass of the photon. The limit obtained depends on the model used: if

5103-470: The process. Other causes have also been proposed, relying on electrodynamic effects of the enormous current acting on the plasma in the bolt of lightning. The shock wave in thunder is sufficient to cause property damage and injury, such as internal contusion , to individuals nearby. Thunder can rupture the eardrums of people nearby, leading to permanently impaired hearing . Even if not, it can lead to temporary deafness. Vavrek et al. (n.d.) reported that

5184-427: The seconds from the perception of a given lightning flash to the perception of its thunder (which can be used to gauge the proximity of lightning for the sake of safety). To estimate the distance from the lightning strike, divide the counted seconds by five for miles, or three for kilometers. A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a thundercrack , therefore indicates that

5265-431: The sounds of thunder fall into categories based on loudness , duration, and pitch . Claps are loud sounds lasting 0.2 to 2 seconds and containing higher pitches. Peals are sounds changing in loudness and pitch. Rolls are irregular mixtures of loudness and pitches. Rumbles are less loud, last for longer (up to more than 30 seconds), and are of low pitch. Inversion thunder results when lightning strikes occur between

5346-417: The speed v at which light travels in a material is called the refractive index n of the material ( n = ⁠ c / v ⁠ ). For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at ⁠ c / 1.5 ⁠ ≈ 200 000  km/s ( 124 000  mi/s) ; the refractive index of air for visible light is about 1.0003, so

5427-473: The speed at which conventional matter or energy (and thus any signal carrying information ) can travel through space . All forms of electromagnetic radiation , including visible light , travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. Much starlight viewed on Earth

5508-466: The speed of light in air is about 90 km/s (56 mi/s) slower than c . The speed of light in vacuum is usually denoted by a lowercase c , for "constant" or the Latin celeritas (meaning 'swiftness, celerity'). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √ 2 times the speed of light in vacuum. Historically,

5589-470: The speed of light in vacuum. Since 1983, the constant c has been defined in the International System of Units (SI) as exactly 299 792 458  m/s ; this relationship is used to define the metre as exactly the distance that light travels in vacuum in 1 ⁄ 299 792 458 of a second. By using the value of c , as well as an accurate measurement of the second, one can thus establish

5670-470: The speed of light. A Global Positioning System (GPS) receiver measures its distance to GPS satellites based on how long it takes for a radio signal to arrive from each satellite, and from these distances calculates the receiver's position. Because light travels about 300 000  kilometres ( 186 000  miles ) in one second, these measurements of small fractions of a second must be very precise. The Lunar Laser Ranging experiment , radar astronomy and

5751-439: The speed of waves in any material medium, and c 0 for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as related electromagnetic constants: namely, μ 0 for the vacuum permeability or magnetic constant, ε 0 for the vacuum permittivity or electric constant, and Z 0 for the impedance of free space . This article uses c exclusively for

5832-509: The spot is delayed because of the time it takes light to get to the distant object at the speed  c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed  c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than  c , after a delay in time. In neither case does any matter, energy, or information travel faster than light. The rate of change in

5913-407: The symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c , which by then had become the standard symbol for the speed of light. Sometimes c is used for

5994-471: The travel time increases when signals pass through electronic switches or signal regenerators. Although this distance is largely irrelevant for most applications, latency becomes important in fields such as high-frequency trading , where traders seek to gain minute advantages by delivering their trades to exchanges fractions of a second ahead of other traders. For example, traders have been switching to microwave communications between trading hubs, because of

6075-468: The units of space and time), and requiring that physical theories satisfy a special symmetry called Lorentz invariance , whose mathematical formulation contains the parameter  c . Lorentz invariance is an almost universal assumption for modern physical theories, such as quantum electrodynamics , quantum chromodynamics , the Standard Model of particle physics , and general relativity . As such,

6156-529: The universe was less than a billion years old. The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the evolution of stars , of galaxies , and of the universe itself. Astronomical distances are sometimes expressed in light-years , especially in popular science publications and media. A light-year is the distance light travels in one Julian year , around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs . In round figures,

6237-436: The values of the electromagnetic constants ε 0 and μ 0 and using their relation to c . Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling c . This is described in more detail in the "Interferometry" section below. In 1983 the metre was defined as "the length of the path travelled by light in vacuum during

6318-622: Was adopted in North America, India, parts of Africa, Australia, and many other regions. In the post-colonial period, some newly created nations that had multiple indigenous languages opted to continue using Modern English as the official language to avoid the political difficulties inherent in promoting one indigenous language above another. The following is an outline of the major changes in Modern English compared to its previous form (Middle English), and also some major changes in English over

6399-780: Was adopted in many regions of the world by the British Empire , such as Anglo-America , the Indian subcontinent , Africa , Australia and New Zealand . Modern English has many dialects spoken in many countries throughout the world, sometimes collectively referred to as the English-speaking world . These dialects include (but are not limited to) American , Australian , British (containing Anglo-English , Scottish English and Welsh English ), Canadian , New Zealand , Caribbean , Hiberno-English (including Ulster English ), Indian , Sri Lankan , Pakistani , Nigerian , Philippine , Singaporean , and South African English . According to

6480-498: Was farther away took longer to reach the Earth, the time between two successive observations corresponds to a longer time between the instants at which the light rays were emitted. A 2011 experiment where neutrinos were observed to travel faster than light turned out to be due to experimental error. In models of the expanding universe , the farther galaxies are from each other, the faster they drift apart. For example, galaxies far away from Earth are inferred to be moving away from

6561-413: Was from the sound of hail colliding within clouds. By the mid-19th century, the accepted theory was that lightning produced a vacuum and that the collapse of that vacuum produced what is known as thunder. Scientists have agreed since the 20th century that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. The temperature inside

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