# 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:

Where:

*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.