The special theory of relativity is based on looking at nature in a different way than people looked at it before.

• We think of space and movement of time as being constants^*.
• But special relativity says that the speed of light is constant.
• Everything else (including spacetime) changes.

Special relativity describes situations in which mass^* moves at a constant velocity^* relative^* to another mass.

• First it says that you cannot tell the difference between not moving and moving at a constant velocity.
• The second part is harder to understand. It says that light is different from any thing we experience. The observed speed of light is the same no matter how you are moving when you observe it.

One of the consequences of this theory is the equivalence of mass and energy^*.