# 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:

$E = mc^{2}$

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 $E = mc^{2}$ specifically please see the hyperphysics article on relativity.

## Authors and Editors

Allison Campbell, Jordan Hanania, Kailyn Stenhouse, Jason Donev