Michelson-Morley Experiment

The purpose of the experiment

Theories in physics in the 19th century assumed that, just as sound and water waves require a medium to transmit their wave motions, light also needs a medium to transmit its wave motions, luminiferous aether. It was even thought that a vacuum also has to be filled with aether, since the light can travel through it.

The purpose of the Michelson-Morley experiment was to prove the existence of luminiferous aether, which was based on the following statement: "depending on your relative position in space, the light should pass through the aether at varying speeds." If the existence of aether had been confirmed, the goal then would have been to determine the Earth's velocity in relation to this hypothetical medium.

The process of the experiment

Before the experiment took place in 1887, in 1886 Michelson and Morley were successful in confirming Fresnel's drag coefficient. This coefficient was seen as proof of the stationary aether theory since it measured the effect a moving medium had on light.

It was seen as proof of the theory to say that the aether is dragged by a moving body or moves with the moving body. This outcome boosted their confidence in detecting the aether wind. For the procedure, a Michelson interferometer was used.

Michelson suggested that by comparing the speed of light in the direction of the Earth's movement and the speed of light when seen at a certain angle with respect to the Earth's rotational direction, they could detect a difference. This difference would prove that the speed of light is constant with respect to the proposed aether through which the Earth would be moving. But there were no differences to be found from the results of the experiment.

The outcome of the experiment

Since the outcome was negative, they disproved their theories and the aether theories which came before them. Instead, the results of the experiment proved that there is no difference between the speed of light while traveling in different directions.

In the figure above are the results of the experiment, where the top solid line represents the midday observations and the bottom solid line represents nighttime observations. The theoretical and actual curves are not presented at the same scale; in fact, the dotted curves represent just one-eighth of the theoretical displacements.

Why is it known as “the most famous failed experiment”?

The experiment is known as “the most famous failed experiment” because while the results of the experiment were negative and they couldn't prove what they had wanted to prove, it has allowed physicists to prove that the speed of light is constant in any situation, while also being a foundation for the theory of special relativity.

Impact on the theory of special relativity

Albert Einstein released a paper in 1905, over two decades after the Michelson-Morley experiment, explaining its perplexing findings. He claimed that there is no aether, since light is a specific kind of wave that does not need a medium to transmit it. There is no specific frame of reference showing different measurements of the speed of light without the concept of aether.