Albert Einstein developed the theory of relativity. His special theory of relativity grew out of an attempt to reconcile the laws of mechanics with the laws of the electromagnetic field. Michelson and Morley sent light beams along and across Earth's motion through space. They could find no difference. At age 16, Einstein imagined chasing after a beam of light. This thought experiment played a role in his later work. In 1905, Einstein published four groundbreaking science articles while he worked at the patent office. One of them was "On the Electrodynamics of Moving Bodies," and it proposed special relativity.

Special relativity rests on two ideas. The speed of light is the same for any observer. The laws of physics are the same in every frame of reference that is not speeding up or slowing down. Special relativity revealed that space and time are united in a fabric known as space-time. According to special relativity, the faster you move, the slower time passes for you. According to relativity, a twin who travels near the speed of light and returns would be younger than the twin who stayed on Earth.

A fourth 1905 paper by Einstein was about the relationship between mass and energy, which had been viewed as completely separate. Einstein is famous for the equation E=mc2. It says that energy equals mass times the speed of light squared. Einstein's ideas changed how scientists think about time, space and the universe. Albert Einstein became famous around the world.

Einstein later published his general theory of relativity in 1915. General relativity says that gravity is how mass warps space and time. Einstein recognized that if you move away from a clock near the speed of light, the clock you see will be slow. Newton's equations are still used to calculate all but the most extreme motions, even though Einstein's theory superseded them.