The Michelson experiment

At the end of the nineteenth century, it was a common belief that light needs and has a medium in which it propagates: light is a wave in a medium called ether, as sound is a wave in air. This belief was shattered when Michelson (1881) tried to measure the velocity of the Earth on its way around the Sun. He used a sensitive interferometer, with one arm in the direction of the Earth's motion, and the other perpendicular to it. When rotating the instrument through an angle of 90°, a shift of the fringes of interference should take place: light propagates in the ether, and the velocity of the Earth had to be added that of the light in the direction of the respective arms. The result was zero: there was no velocity of the Earth with respect to the ether.

This negative result can be phrased differently. Since the system of the ether is an inertial system, and that of the Earth is moving with a (approximately) constant velocity, the Earth's system is an inertial system too. So the Michelson experiment (together with other experiments) tells us that the velocity of light is the same for all inertial systems which are moving with constant velocity with respect to each other (principle of the invariance of the velocity of light). The speed of light in empty space is the same for all inertial systems, independent of the motion of the light source and of the observer.