The Michelson-Morley Experiment

This experiment was designed to test the theory that the Earth was traveling through an aether; the substance that supposedly allowed light waves to propagate. The setup involved a light ray being split in half by a half-silvered mirror, heading off to a mirror and both rays returning to an observer. The observer would see a pattern where the light rays superposed to cause interference patterns.

The Earth was supposedly moving through the aether, so physicists thought there would be an ‘aether wind’ caused by this movement. This would mean that light would take different times to travel in different directions. For example, two boats that travel at a constant speed in a river travel 2 kilometers away from a point and then back. One travels with the river flow and the other travels across it. The boat that travels across the river is slower because it has to struggle against the current the entire way while the one that travels with the flow only has to struggle for half the journey and then can ride the current back.

The observer would view the interference patterns as black lines. These patterns would supposedly change if the apparatus was swung in a different direction (the aether wind would cause the light to slow/speed up). The fringes of the lines were expected to change by a certain amount (around 40%). This was to become a definitive experiment because the apparatus was known to be very accurate.

The experiment found that there was no change in the interference patterns so disproved the idea of an aether. People tried to explain this by coming up with other theories to fit the results. For example, the Earth contracts in the direction of movement or that the aether is dragged with the Earth. These theories were disproved by further testing and observation of the stars.

This experiment was important in the development of new theories and ideas. Albert Einstein used the results in his Theory of Special Relativity.