Special Relativity was put forth by Albert Einstein and is possibly his most famous contribution to science. It is based on the assumptions that 1. The basic laws of physics are the same in all reference frames and 2. That the speed of light in a vacuum is always measured to be the same regardless of the velocity of the observer. The practical upshot of this is that time flows differently for observers moving at different velocities. The following definitions and equations should be applied to any situation when the speed of an observer or object is greater than one tenth of the speed of light. Source: [wikipedia: list of relativistic equations]