Speed of light - Aetherwisp

The speed of light $c$ is the speed of a ray of light in the vacuum. In SI units it is exactly equal to

c=299\ 792\ 458\text{ m/s}

by definition. What a "ray of light" constitutes depends on the branch of physics, but in general it is the speed at which electromagnetic interaction is communicated between different objects. In classical physics, it is the propagation speed of an electromagnetic wave. In quantum physics, it is the velocity of a Photon. It is related to the vacuum permittivity $\varepsilon_{0}$ and the vacuum permeability $\mu_{0}$ by

c=\frac{1}{\sqrt{ \varepsilon_{0}\mu_{0} }}

In nuclear and atomic physics, it is sometimes useful to express the square of speed of light in terms of electronvolts and atomic mass units:

c^{2}=931.494\text{ MeV/u}

This is useful when converting from atomic masses to atomic rest energies with $E=mc^{2}$ .