# Speed of Light - Fundamental Role in Physics

Fundamental Role in Physics

See also: Introduction to special relativity, Special relativity, and One-way speed of light

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. 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 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) 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. 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. 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. 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 = mc2), length contraction (moving objects shorten), 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. 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. 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.

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. No conclusive evidence for such changes has been found, but they remain the subject of ongoing research.

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 (see Hughes–Drever experiment), and of rotating optical resonators (see Resonator experiments) have put stringent limits on the possible two-way anisotropy.

### Other articles related to "role, fundamental role in physics":

848 Naval Air Squadron - Current Role
... The Squadron took the role from 707 NAS which was responsible for the training of aircrew and maintainers in the Commando role ...
Xenia Tchoumitcheva - Career - Actress
... In June she gets a support role in a movie and shoots a small role in the 2011 film Without Men ... January 2011 Xenia finishes shooting a role as a rockstar in the French cinema production Bob et Les Sex Pistaches starring the French actor Jules Sitruk ...
845 Naval Air Squadron - Role
... The primary role of the squadron is in supporting Royal Marine Commando troops in amphibious assaults ...
Dannii Minogue - Filmography
... Film Year Film Role Notes 1992 Secrets Didi 2004 The Porter Bunny Stigler Short film 2007 White Diamond A Personal Portrait of Kylie Minogue Herself Documentary film Television Year ...
Speed Of Light - Fundamental Role in Physics - Upper Limit On Speeds
... More generally, it is normally impossible for information 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 ...

### Famous quotes containing the words physics, fundamental and/or role:

We must be physicists in order ... to be creative since so far codes of values and ideals have been constructed in ignorance of physics or even in contradiction to physics.
Friedrich Nietzsche (1844–1900)

The fundamental job of a toddler is to rule the universe.
Lawrence Kutner (20th century)

The addition of a helpless, needy infant to a couple’s life limits freedom of movement, changes role expectancies, places physical demands on parents, and restricts spontaneity.
Jerrold Lee Shapiro (20th century)