What is special relativity?

The theory is based on two key concepts.

• First, the natural world allows no “privileged” frames of reference. As long as an object is moving in a straight line at a constant speed (that is, with no acceleration), the laws of physics are the same for everyone. It’s a bit like when you look out a train window and see an adjacent train appear to move — but is it moving, or are you? It can be hard to tell. Einstein recognized that if the motion is perfectly uniform, it's literally impossible to tell — and identified this as a central principle of physics.