Speed of light

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.

To learn more about the speed of light and its strange effects please see hyperphysics, for the equation specifically please see the hyperphysics article on relativity.

Authors and Editors

Allison Campbell, Jordan Hanania, Kailyn Stenhouse, Jason Donev