Einstein's theory of relativity is one of the cornerstones of modern physics but it requires a total rethink of our notions of time and space. Strange things happen in a relative universe: length and time are not fixed and depend upon your frame of reference. If you move at high speed, time slows down, space contracts and weight increases. So a period of time for someone on Earth that lasts for hundreds of years may only be a couple of hours for someone travelling in a rocket close to the speed of light. It's mind-boggling stuff, but the theory of relativity has a real impact on our world today.