Speed of lightWord文档格式.docx

1、Exact valuesMetres per second299,792,458Planck units1Approximate valueskilometres per second300,000kilometres per hour1,080 millionmiles per second186,000miles per hour671 millionastronomical units per day173Approximate light signal travel timesDistanceTimeone foot1.0 nsone metre3.3 nsfrom geostatio

2、nary orbit to Earth119 msthe length of Earths equator134 msfrom Moon to Earth1.3 sfrom Sun to Earth （1 AU）8.3 minfrom nearest star to Sun （1.3 pc）4.24 yearsfrom the nearest galaxy （the Canis Major Dwarf Galaxy） to Earth25,000 yearsacross the Milky Way100,000 yearsfrom the Andromeda Galaxy to Earth2.

3、5 million yearsThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time.1 In im

4、perial units this speed is approximately 186,282 miles per second.According to special relativity, c is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed of all massless particles and associated fieldsincluding electromagnetic radiation such a

5、s lightin vacuum, and it is predicted by the current theory to be the speed of gravity （that is, gravitational waves）. Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, c interrelates space and

6、 time, and appears in the famous equation of massenergy equivalence E=mc2.2The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the materia

7、l （nc/v）. For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 200,000km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90km/s slower than c.In most practical case

8、s, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft or vice versa. The

9、 light we see from stars left them many years ago, allowing us to study the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. Finally, the spee

10、d of light can be used with time of flight measurements to measure large distances to high precision.Ole Rmer first demonstrated in 1676 that light travelled at a finite speed （as opposed to instantaneously） by studying the apparent motion of Jupiters moon Io. In 1865, James Clerk Maxwell proposed t

11、hat light was an electromagnetic wave, and therefore traveled at the speed c appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light with respect to any inertial frame is independent of the motion of the light source,3 and explored the consequences of

12、 that postulate by deriving the special theory of relativity and showing that the parameter c had relevance outside of the context of light and electromagnetism. After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458m/s with a measurement uncerta

13、inty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units （SI） as the distance travelled by light in vacuum in 1299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.4edit Numer

14、ical value, notation, and unitsThe speed of light in vacuum is usually denoted by c, for constant or the Latin celeritas （meaning swiftness）. Originally, the symbol V was used, introduced by James Clerk Maxwell in 1865. In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch used c for a constant later

15、shown to equal 2 times the speed of light in vacuum. 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.56Sometimes c is used for the

16、 speed of waves in any material medium, and c0 for the speed of light in vacuum.7 This subscripted notation, which is endorsed in official SI literature,4 has the same form as other related constants: namely, 0 for the vacuum permeability or magnetic constant, 0 for the vacuum permittivity or electr

17、ic constant, and Z0 for the impedance of free space. This article uses c exclusively for the speed of light in vacuum.In the International System of Units （SI）, the metre is defined as the distance light travels in vacuum in 1299,792,458 of a second. This definition fixes the speed of light in vacuu

18、m at exactly 299,792,458m/s.8910 As a dimensional physical constant, the numerical value of c is different for different unit systems.Note 1 In branches of physics in which c appears often, such as in relativity, it is common to use systems of natural units of measurement in which c = 1.1213 Using t

19、hese units, c does not appear explicitly because multiplication or division by 1 does not affect the result.edit Fundamental role in physicsSee also: Introduction to special relativity, Special relativity,and One-way speed of lightThe speed at which light waves propagate in vacuum is independent bot

20、h of the motion of the wave source and of the inertial frame of reference of the observer.Note 2 This invariance of the speed of light was postulated by Einstein in 1905,3 after being motivated by Maxwells theory of electromagnetism and the lack of evidence for the luminiferous aether;14 it has sinc

21、e 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） is frame-independent, because it is impossible to measure the one-way speed of light （for example, from a source to

22、a distant detector） without some convention as to how clocks at the source and at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition.1315 The special theory of relativ

23、ity explores the consequences of this invariance of c with the assumption that the laws of physics are the same in all inertial frames of reference.1617 One consequence is that c is the speed at which all massless particles and waves, including light, must travel in vacuum.The Lorentz factor as a fu

24、nction of velocity. It starts at 1 and approaches infinity as v approachesc.Special relativity has many counterintuitive and experimentally verified implications.18 These include the equivalence of mass and energy （E = mc2）, length contraction （moving objects shorten）,Note 3 and time dilation （movin

25、g clocks run slower）. The factor by which lengths contract and times dilate, is known as the Lorentz factor and is given by = （1 v2/c2）1/2, where v is the speed of the object. The difference of from 1 is negligible for speeds much slower thanc, such as most everyday speedsin which case special relat

26、ivity is closely approximated by Galilean relativitybut it increases at relativistic speeds and diverges to infinity as v approaches c.The results of special relativity can be summarized by treating space and time as a unified structure known as spacetime （withc relating the units of space and time）

27、, and requiring that physical theories satisfy a special symmetry called Lorentz invariance, whose mathematical formulation contains the parameterc.21 Lorentz invariance is an almost universal assumption for modern physical theories, such as quantum electrodynamics, quantum chromodynamics, the Stand

28、ard Model of particle physics, and general relativity. As such, the parameterc is ubiquitous in modern physics, appearing in many contexts that are unrelated to light. For example, general relativity predicts thatc is also the speed of gravity and of gravitational waves.2223 In non-inertial frames o

29、f reference （gravitationally curved space or accelerated reference frames）, the local speed of light is constant and equal toc, but the speed of light along a trajectory of finite length can differ fromc, depending on how distances and times are defined.24It is generally assumed that fundamental con

30、stants such asc 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 the speed of light may have changed over time.2526 No conclusive evidence for such changes has been found, but they

31、 remain the subject of ongoing research.2728It also is generally assumed that the 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 the emitting nuclei in a magnetic field （see HughesDrever experiment）, and of rotating optical resonators （see Resonat