Looking Back at the Michelson–Morley Experiment
It is common knowledge that space is an empty vacuum. Before this was common knowledge, Aristotle and other philosophers thought that the space between planets had to be filled with something; he, alongside others, named this concept “the aether.” Today “the aether” sounds preposterous; however, with limited knowledge of space at the time other than telescopes and looking up at the night sky, the idea of a “void” was incomprehensible. Thus, they based new ideas and theories around “the aether,” and at the time “the aether” was well accepted.
A significant reason why the aethers were accepted was because of light. The classical understanding of light was that it was only a wave, given its wave-like properties. However, Thomas Young’s famous Double-Slit experiment in 1801 confirmed that light is a wave by showing constructive and destructive interference (exclusively wave-like properties). A wave is a disturbance that passes through a medium from one part to another, such as sound, which passes through the air as its medium. Because light is a wave, classical physicists believed it had to pass through some medium as it unravels from the sun to earth. They believed this medium was the aether.
Additionally, they understood that the denser a medium is, the faster a wave will move through it–at least as long as the molecules are small. They assumed that because the speed of light is so high, it must propagate through highly dense material. Physicists believed that the extreme density made the aether nearly impossible to compress, which is why it filled up all of space. Lastly, they believed the aether was in the form of a fluid with no viscosity; otherwise, the planets would slow down and eventually stop.
After Young's Double-Slit experiment, the aether was commonly accepted in the scientific community. However, although it was accepted, no experiment concretely proved the existence of the aether. Thus, Albert Michelson set off to prove the aether once and for all. The device Michelson planned to use was an Interferometer. This device took a laser and pointed it at a partially transparent and particle-reflective mirror, sending two beams of light in perpendicular directions. Both beams of light reflect off of mirrors at equal distances to the original mirror, and these mirrors, alongside the original mirror, reflect the beams of light to the same point. This is similar to how in the famous ‘Swimmer Problem,’ two swimmers swim in a river: one swims back and forth perpendicular to the river’s flow, and one swims to and against the river’s flow. In this experiment, the aether is the river, and the two light beams are the swimmers.
Michelson’s classical understanding of physics led him to predict that the beam moving sideways to the motion of the earth would reach the final point first as he believed that the earth and, subsequently, the beam of light, is moving relative to the aether just like the river in the swimmer problem. So he believed that the beam of light traveling perpendicular to the aether would travel the shortest distance as the light would essentially form a triangle and go the equivalent distance of two hypotenuses, rather than the total distance of the two other lengths of the triangle two times, like the particle moving relative to the direction of motion of the earth and aether.
However, Michelson’s prediction was wrong, as although he set out to prove the existence of the aether, the experiment proved that there was no aether, as light is not relative to anything. An Einsteinian or modern-day physicist would see one crucial flaw in Michelson’s prediction: there is no river. Or rather, the light is not moving to or against the direction of the aether. For one, there is no aether, and secondly, light is not relative to anything. Modern-day physicists understand that light is the same speed for all observers, regardless of their relative speeds. Thus, both beams of light will end at the same time.
Michelson's experiment helped Einstein understand and prove relativity, which is the principle that individuals moving at a constant velocity are subject to the same laws of physics as those at rest. This is why light is the same speed whether it is moving in the direction the earth is moving or when it is moving perpendicular to the direction the earth is moving.
Today the effects of Michelson's experiment are still being felt, so much so that it is often considered the most influential experiment in the field of physics, let alone all science. The experiment disproved the aether and helped Einstein develop his theory of “General Relativity” and “Special Relativity,” and although the latter theory is incomplete, it is still crucial. Additionally, the Michelson Interferometer is still being used today for all sorts of applications: measuring the diameters of stars, detecting the direction of gravitational waves, and detecting atmospheric temperatures and winds, as well as many other unique and essential applications.
Michelson is said to have lived the rest of his life believing that the experiment was wrong and that the aether was absolute. Michelson never realized the full impact that his experiment had in shaping the modern-day understanding of physics and the universe as a whole. It may not be possible to dive into space, as it is not as fluid as previously believed. Still, it is possible to dive into the burgeoning and constantly expanding understanding of the universe today.
References
Britannica. (n.d.). Young's double-slit experiment. Britannica. Retrieved October 17, 2022, from https://www.britannica.com/science/light/Youngs-double-slit-experiment
CalTech. (n.d.). What is an Interferometer? ligo.caltech.edu. Retrieved October 17, 2022, from https://www.ligo.caltech.edu/page/what-is-interferometer
Goodstien, D. L., Dr. (n.d.). Episode 41: The Michelson Morley Experiment - The Mechanical Universe [Speech transcript]. Youtube. Retrieved October 17, 2022, from https://www.youtube.com/watch?v=Ip_jdcA8fcw
The University of Virginia. (n.d.). The Michelson-Morley Experiment. phys.virginia.edu. Retrieved October 17, 2022, from https://galileoandeinstein.phys.virginia.edu/lectures/michelson.html#Detecting%20the%20Aether%20Wind:%20the%20Michelson-Morley%20Experiment
Video physical science (Producer). (2015). Learning Physics_Light and Relativity: Breakdown of Classical Relativity with Light Example [Video]. https://www.youtube.com/watch?v=_EES1amZqyM
