Fundamental role in physics

The speed at which light waves (or any wave for that matter) propagates in a vacuum (or otherwise) is independent both of the motion of the wave source and of the inertial frame of reference of the observer. ^{[Note 3]} 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 the luminiferous ether;^[14] it has since been consistently confirmed by many experiments.^{[Note 4][13][15]} The theory of special 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.^[16][17] One consequence is that c is the speed at which all massless particles and waves, including light, must travel in a vacuum.

The Lorentz factor γ as a function of velocity. It starts at 1 and approaches infinity as v approaches c.

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 5]} and time dilation (moving clocks run slower). 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.
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.^[21] 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.^[22][23] In non-inertial frames of reference (gravitationally curved space 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.^[24]
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.^[25][26] No conclusive evidence for such changes has been found, but they remain the subject of ongoing research.^[27][28]
It 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 and of rotating optical resonators have put stringent limits on the possible anisotropy.