Concept preview

What you will be able to do

1. 01Explain what a frame of reference is and why motion is relative
2. 02State Einstein's two postulates of special relativity
3. 03Explain why the constancy of the speed of light is revolutionary

Sit on a train and the coffee on your table is at rest — but to someone on the platform it's hurtling by at 100 km/h. Who's right? Both are. There is no such thing as 'absolute' motion; you can only measure motion relative to a chosen viewpoint, called a frame of reference. Every measurement of position, velocity and even time is made from some frame.