Michelson-Morley experiment

The Michelson-Morley experiment is the name given out to an experiment first performed in 1887, intended to test whether or not light travels through a medium, the same way that sound needs to travel through solids, liquids or gases.

The Experiment
The theory supposed was that if there was a medium, named Luminiferous Aether, then due to the speed at which the Earth moves and rotates, it should be expected to drag behind the Earth to some degree. Any object moving through a medium will find that the medium appears to move with respect to that object; this is why you feel the wind on your face when you run. The aether should have a similar 'wind' that would affect how light moves.

The experiment tested to see how long it took light to travel in different directions. If it took longer to move one way, this would indicate the medium was distorted.

The conclusion of the experiment was that the speed of light is constant in all directions, and so there is no luminiferous aether, and light does not require a medium to travel through.

Criticism
This experiment relies upon the assumption of a moving Earth. If the Earth is non-rotating and stationary then the predicted end results with or without luminiferous aether are indistinguishable.