Introduction
The Michelson-Morley Experiment is arguably the single most significant “failed” experiment in all of physics. Not only did the experiment simplify extra complicated maths but paved the way for modern physic in the 20th and 21th centuries.
The Ether
Prior to the Michelson-Morley Experiment, physicist had theorized the existence of ”the ether”, a mysterious and undetected medium for light waves to propagate through. This theory was based off our understanding of sound waves which vibrate through molecules and away from their source. In fact, outer space is almost entirely silent since it lacks a similar medium!
Michelson theorized that the existence of an ether would cause a disturbance of the light passing through it.
Albert A. Michelson (1852 - 1931)
The most popular analogy for this phenomenon is of a boat attempting to sail directly across a river. As the boat tries to cross it is inevitably pushed off course by the waters current. Similarly, as light travels through the ether it would inevitably be redirected by the “ether’s wind”
Here is a depiction of the path a boat must propel itself in inorder to end directly across the river:
In the case of light waves, this ether wind could have been imagined to be like the following:
Michelson’s Solution
Michelson’s solution was to take advantage of the “ether wind”. He realized that light emitted from the earth would be influenced in varying directions as the earth rotated and completes its orbit around the sun. To detect this change, he designed the world's first interferometer.
An interferometer is a device used to measure the interference between two light beams after being emitted from the same source. By analyzing these results he could infer the direction the ether was flowing through the universe.
However, to his surprise they was no significant interference between the light beams! For the next decade, Michelson continued to repeat his experiment with Morley before having collected enough data to disprove the existence of the ether.
Some More About Interferometers
Here is a video I found depicting how an interferometer functions.
Today we still rely very heavily on interferometers and even used them for our discovery of gravitational waves! These interferometers are much larger and much more sensitive. For example, LIGO’s interferometer emits light out to a distance 4 kilometers away from its source. This is 360 times larger than Michelson and Morley’s interferometer which had only 11 meter arms.
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Work Cited:
http://www.kcvs.ca/martin/astro/kingsu/flash/michelsonMorley/resources/background.html
https://www.aps.org/programs/outreach/history/historicsites/michelson-morley.cfm