The speed of light, almost always written as c (for celerity, an archaic term for swiftness of motion), is a constant which is a strange consequence of Einstein's theory of relativity. Photons (packets of light) always travel at the speed of light in a vacuum. Einstein's theories also led to the rather surprising consequence that mass and energy are related by the speed of light in what has become the most famous equation in physics:
- E is energy
- m is mass (often a mass difference, for example in nuclear reactions)
- c is the speed of light.