Everything about Speed Of Light totally explained
» "Lightspeed" redirects here. For other uses, see Lightspeed (disambiguation).:
For other uses of "speed of light", see speed of light (disambiguation)
The
speed of light in the
vacuum of free space is an important
physical constant usually denoted by the letter
c. It is the speed of all
electromagnetic radiation, including visible
light, in free space. It is the speed of anything having zero
rest mass.
In SI units, the speed of light in a vacuum is exactly
299,792,458 metres per second (1,079,252,849 km/h). 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 approximate value of 3 m/s is commonly used in rough estimates (the error is 0.07%). In
imperial units, the speed of light is about 670,616,629.4 miles per hour or 983,571,056.4
feet per second (roughly one
foot per
nanosecond), which is about 186,282.397
miles per second.
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. The ratio of the speed of light in the vacuum to the observed
phase velocity is called the
refractive index of the medium. See
dispersion (optics). 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. When measured
locally, light in a vacuum always passes an observer at
c.
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. According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3 metres per second, or about 30
centimetres (1
foot) per
nanosecond). See
metre.
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. 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's the constant speed
c, rather than light itself, that's fundamental to special relativity; thus if light is somehow manipulated to travel at less than
c, this manipulation won't 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. 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.
Use of the symbol 'c' for the speed of light
The symbol 'c' for 'constant' or the
Latin celeritas ("swiftness") is generally used for the speed of light.
NIST and
BIPM practice is to use
c0 for the speed of light in vacuum. 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's 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". Such a violation of causality has never been recorded.
To put it another way, information propagates to and from a point from regions defined by a
light cone. The
interval AB in the diagram to the right is "
time-like" (that is, there's 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's no frame of reference in which event A and event B occur simultaneously). Thus, it's 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's a frame of reference in which event A and event C occur simultaneously, separated only in space; see
simultaneity). 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 isn't possible for any matter (or information) to travel from A to C or from C to A. Thus there's 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 kilometres or about 5.88 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 s.
The actual transit time is longer, in part because the speed of light is slower by about 30% in an
optical fiber depending on its
refractive index 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. A typical time as of 2004 for a
U.S. to
Australia or
Japan computer-to-computer
ping is 0.18 s. The speed of light additionally affects
wireless communications design.
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.
This effect forms the basis of the
Global Positioning System (GPS) and similar
navigation systems. 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. 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. If a processor operates at 1
GHz, a signal can only travel a maximum of 300 mm in a single cycle. 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.
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.
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, don't 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:
»
These constants appear in
Maxwell's equations.
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. This speed was determined by the (
permittivity and
permeability) of the aether.
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. 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. 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. 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.
Ernst Mach was among the first physicists to suggest that the experiment amounted to a disproof of the aether theory. Developments in theoretical physics had already begun to provide an alternative theory,
Fitzgerald-Lorentz contraction, which explained the null result of the experiment.
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. The constant speed of light is one of the fundamental Postulates (together with
causality and the
equivalence of inertial frames) of special relativity.
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