Special relativity, introduced by Einstein in 1905, states that the speed of light in a vacuum is uniform, and that the laws of physics only appear the same to observers in uniform motion. These ideas have some startling consequences. For example, an astronaut zipping around in the Earth's orbit will age a bit (a very small bit) more slowly than an observer standing on the Earth.

General relativity was developed in the 9 years following Einstein's ideas on special relativity, as a hypothesis on gravitation. Einstein proposed that spacetime is curved, and that objects follow the shortest path in spacetime. General relativity also results in consequences such as gravitational time dilation, whereby time ticks slower in higher gravitational fields.

The theory of relativity reveled in imaginative thought. Einstein was considered a genius for his ability to place himself in a completely different perspective in designing these theories.