The “theory of relativity” (or simply “relativity”) generally refers to two theories of Albert Einstein, his Special Theory of Relativity (or simply special relativity) of 1905, and his General Theory of Relativity (or general relativity) of 1916. Along with quantum theory, relativity is one of the two main planks on which almost the whole of modern physics is built.
The idea of relativity had been studied almost three centuries earlier by Galileo, when he stated the principle of relativity in 1632 (that the fundamental laws of physics are the same for all bodies in uniform motion). Later in the 17th Century, Sir Isaac Newton also took the principle of relativity for granted, asserting that if his famous laws of motion held in one inertial frame, then they also held in a reference frame moving at a constant velocity relative to the first frame.
As we will see, Einstein’s theories are somewhat more involved, even if his starting point was in many respects the same. His ground-breaking theories take into account the speed of light, the structure of space-time and the equivalence of acceleration and gravity. They have led to some remarkable consequences, including the dilation of time, the contraction of length, mass-energy equivalence and the bending of light, as well as the prediction of the existence of black holes, wormholes and the “birth” of the universe in a Big Bang.
Einstein's theories still hold up well today, after exhaustive experimentation and testing, and have been described as the single most important contribition by one man to science.