Albert Einstein worked as a postal clerk when he came up with many of his theories, including the Theory of Relativity. The theory postulates that the speed at which time happens is mutable. This theory came from a dream Einstein had where he was sledding down a mountain until he reached the speed of light, and the stars looked different.
The basic rules are that the speed of light is constant for all observers and that people moving at constant speeds are subject to the same physical laws. His equation for this, E=mc2, is probably the most famous equation in the world and was debated by scientists for years.