Special relativity

Michelson Morley Experiment

A key component of classical physics was the existence of the all pervading ether. According to this concept, the speed of light should have changed with the rotation of the earth in ether - light moving in the direction of motion of earth should have moved slower with respect to the earth than light moving in the opposite direction, as everything had an absolute speed relative to ether. Classical physicists believed this, but nobody had been able to prove it.

In 1887, Albert Michelson and Edward Morley designed the most famous experiment ever to prove this and put it beyond doubt.

As the legend goes, they failed and created history.

The Principle of Relativity

This is the most fundamental theorem of physics. According to it, there is nothing called absolute uniform motion. All motion is relative. We all know that (if we consider the motion of a train to be uniform) the landscape seems to be moving by someone in the train, and the train seems to be moving by to someone outside. The Principle of Relativity says it is impossible to ascertain that one of them is correct while the other is wrong.

The speed of light postulate

The speed of light is the same for all people - regardless of their relative uniform velocities. This was something that was proved by Michelson and Morley, but can also be gathered from a simple thought experiment.

Suppose you are sitting in a vessel that is moving at the speed of light. If the speed of light is not constant, it should be zero relative to you. Then if you look back, the part behind you should be dark, as it is impossible for the light from there to reach you. So you can ascertain that you are moving, which is a violation of the Principle of Relativity.