Speed of light

The speed of light, in any medium, which is usually denoted by ${\displaystyle c}$, is a physical constant important in many areas of physics. It is denoted by 'c^0' especially in vacuum medium, although the symbol 'c' can be used to refer to that in any medium. It is exactly 299,792,458 metres per second (983,571,056 feet per second) by definition.^[1][2] A photon (particle of light) travels at this speed in a vacuum.

According to special relativity, ${\displaystyle c}$ is the maximum speed at which all energy, matter, and physical information in the universe can travel. It is the speed of all massless particles such as photons, and associated fields—including electromagnetic radiation such as light—in a vacuum.

It is predicted by the current theory to be the speed of gravity (that is, gravitational waves). Such particles and waves travel at ${\displaystyle c}$ regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, ${\displaystyle c}$ interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc².^[3]

The special theory of relativity is based on the prediction, so far upheld by observations, that the measured speed of light in a vacuum is the same whether or not the source of the light and the person doing the measuring are moving relative to each other. This is sometimes expressed as "the speed of light is independent of the reference frame."

1. ↑ SI Brochure: The International System of Units (SI) (PDF) (9 ed.), BIPM, 2019, p. 128, retrieved 2020-01-12
2. ↑ Cox, Brian; Forshaw, Jeff (2010). Why does E=mc²?: (and why should we care?). Da Capo. p. 2. ISBN 978-0-306-81911-7.
3. ↑ Uzan, J-P; Leclercq, B (2008). The natural laws of the universe: understanding fundamental constants. Springer. pp. 43–4. ISBN 978-0387734545.