The
speed of light in
vacuum, commonly denoted
c, is a universal
physical constant important in many areas of
physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s).
[Note 3] It is exact because, by international agreement, a
metre is defined as the length of the path travelled by
light in vacuum during a time interval of 1⁄299792458
second.
[Note 4][3] According to
special relativity, c is the upper limit for the speed at which conventional
matter, energy or any
signal carrying information can travel through
space.
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 the observer.
[Note 5] This invariance of the speed of light was postulated by Einstein in 1905,
[6] after being motivated by
Maxwell's theory of electromagnetism and the lack of evidence for the
luminiferous aether;
[16] it has since been consistently confirmed by many experiments.
[Note 6]
The special theory of relativity 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.[19][20] 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.
[21] These include the
equivalence of mass and energy (
E =
mc2),
length contraction (moving objects shorten),
[Note 7] and
time dilation (moving clocks run more slowly). The factor
γ by which lengths contract and times dilate is known as the
Lorentz factor and is given by
γ = (1 −
v2/
c2)−1/2, 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, 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
v = 99.5%
c.
The results of special relativity can be summarized by treating space and time as a unified structure known as
spacetime (with
c relating 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.
[24] 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, 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.
[25][Note 8] In
non-inertial frames of reference (gravitationally curved spacetime or
accelerated reference frames), the
local speed of light is constant and equal to
c, but the
speed of light along a trajectory of finite length can differ from
c, depending on how distances and times are defined.
[27]
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 the
speed of light may have changed over time.
[28][29] No conclusive evidence for such changes has been found, but they remain the subject of ongoing research.
[30][31]