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The Michelson-Morley experiment is an important physics experiment conducted in the late 19th century.

It was already known in earlier times that light propagates in waves and compared it to water waves, which require a medium, namely water. Michelson and Morley thought that light must also have a medium, namely the aether,and they wanted to prove this through an experiment.

What is the aether?

The ether is the hypothetical medium through which light travels through the universe. This idea can be easily illustrated by everyday examples such as water waves or sound waves, which also require a medium to propagate. It was thought at the time that the aether medium existed throughout the universe to allow light to propagate. It also need not be dense, since planets are not affected by it.

The behavior of the aether

It was also assumed that the aether is at rest and does not move with the earth, much like the air around a car. When an object is moved through the aether, a kind of aether wind is created, similar to the wind created around a moving object.

As the earth moves through the aether, an aether wind should also be created. The experiment of Michelson and Morley attempted to confirm the theory of the aether by means of the aether wind. Anything moving with the ether wind would have to move faster than against the ether wind, similar to the airplane.

If were to fly against the wind, it would be slower than with a tailwind. If you have a crosswind, i.e. the wind comes from the left or right side, this does not change the speed either.

It was precisely this behavior that Michelson and Morley wanted to test with their experiment.

If the aether were real, light would have to behave exactly like an airplane, flying faster when it has a tailwind and slower when it is flying against the wind.

The experiment

Michelson and Morley’s experiment was designed to detect the aether wind. To do this, they used an interferometer, which consisted of a semi-transparent mirror, two perpendicular arms and a receiver.

A laser beam was aimed at the semi-transparent mirror, which split the beam into two halves, each passing through one arm of the interferometer before hitting a mirror at the end of the arm and being reflected. The reflected beam returned to the semi-transparent mirror where it met with the other Beam overlapped and created interference patterns that could be observed on the receiver.

The expectations

The interesting thing about this experiment was that if the interferometer had moved with the aether wind, there should have been a shift in the interference pattern. This would arise due to the different speeds of the light rays, each traveling against and with the aether wind.

If there were no aether, both light beams would travel at the same speed, regardless of the direction of the interferometer, and no difference in the interference pattern could be observed.

Surprising results

However, the result of the experiment surprised physicists because it showed no shift in the interference pattern. No matter which direction the interferometer was pointed, there was no measurable change. This result was a great surprise because it contradicted the assumption at the time that there must be an etheric medium.

The Michelson-Morley experiment was a significant step in the history of physics because it questioned the belief in the existence of the ether and thus in a fundamental property of the universe.

The experiment showed that there is no aether wind and that light moves at the same speed in all directions, regardless of the observer’s movement. This result was the starting point for Einstein’s theory of special relativity, which revolutionized the understanding of spacetime and continues to play a central role in modern physics today.