Speed of light

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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:

[math] E = mc^{2} [/math]


  • 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 [math] E = mc^{2} [/math] specifically please see the hyperphysics article on relativity.