What is the Theory of Relativity?
There is no “theory of relativity” — only two theories that both occasionally fall under the same designator — the theory of special relativity and the theory of general relativity. Both were devised by the famous physicist Albert Einstein between 1905 and 1915. Both these theories upended Newton’s classic mechanics, which had held for centuries prior. The special theory of relativity is a theory of the structure of spacetime, which asserts that 1) the laws of physics are the same for all observers in uniform motion relative to one another and 2) the speed of light in a vacuum is the same for all observers, regardless of their motion of the motion of the light source. This leads to bizarre consequences. For instance, imagine moving on a train going 100 mph next to another train moving in the same direction at 100 mph. From your perspective, the train next to you looks almost still. But imagine being on a train moving at the speed of light, while next to a light beam. Instead of the l
