Throughout history, one thing has never changed - time. It is something we rely on to plan our lives, and it is consistent, regular and ceaseless. But is it? High in the Alps, Michio encounters a mystery - tiny particles called muons which shouldn't exist. They don't last long enough to be detected on Earth - and yet here they are. The answer to this mystery lies in one of the greatest discoveries of all time - Einstein's theory of relativity. The faster you travel, the slower time ticks. So time is not fixed at all.