At the turn of the century, there was a great mystery in physics: it was discovered that all observers measure the speed of light to be the same (about 300,000 km/s). This violates common sense; if one is travelling toward a light source, it should appear to be moving faster than if one is moving away from it. This is like walking on a moving train; if you walk towards the front of the train, the trees outside will be moving faster relative to you than if you walk towards the back. In the case of light, however, the speed does not change; it always moves the same relative to the observer. How can this be? Einstein solved this problem by stating that time itself is relative; time flows at different rates for different people. As an observer moves toward light, his time is slowed down, so the light appears to be travelling at the same speed, rather than being sped up. This means that simultaneity is not the same for all observers; if I see two events occur at the same time, you may see them hours apart.

One of the consequences of special relativity is that nothing can travel faster than the speed of light. Another is that matter and energy are equivalent, as expressed in Einstein's famous equation: E=mc^2