The Michelson-Morley experiment was one of the most important and famous experiments in the history of physics. It is not only a testament to dedication and ingenuity, but it also demonstrates how one can work hard for years only to discover that all their ideas were wrong. In 1887, Albert Abraham Michelson and Edward Morley conducted an experiment to measure the speed of the Earth relative to the ether, and ultimately proved that the ether did not exist. Their results would become the experimental foundation for Einstein's Special Theory of Relativity.
In the late nineteenth century, the extraordinary physicist James Clerk Maxwell (1831-1879) had proposed that light was a form of wave, similar to sound but with a much higher frequency. Any wave, from a wave in the sea to a vibrating string, needs a more or less elastic support to serve as a medium for its propagation. Sound, for example, propagates by "pushing" and "pulling" molecules of air (or water, or any other material), but it is incapable of propagating in a vacuum.
Maxwell's theory needed a medium capable of transporting light waves even through the vacuum of space, since it was hard not to notice that the light from distant stars somehow managed to reach Earth. At that time it was inconceivable that a wave—of any kind—could propagate in a vacuum without any material medium to support it, so the existence of a hypothetical material substance on which light propagated was postulated. This material—which had to have extremely strange characteristics and occupy every corner of the universe—was named the ether.
For reasons that only a physicist can properly explain, the speed of light depends on the density of the medium it traverses. Indeed, the speed of light differs in vacuum, air, or water, being slower the denser the medium. The most immediate evidence of this is the "deformation" that any object placed in a glass of water experiences, such as a spoon or a knife. This is due to the change in direction of the light ray when passing from one medium to another, caused by the change in the speed of light, and is described by Snell's law of refraction.
For the ether to conform to the observed behavior of light in a vacuum, it had to have an infinitely small density and a high coefficient of elasticity, and it also had to be able to pass through any material. This was necessary because, somehow, it appeared even inside a container that had been evacuated in a laboratory. This explanation—far from seeming far-fetched or at least unlikely—was endorsed by Maxwell himself, Lord Kelvin, and Nikola Tesla, among other prominent minds of the era. For scientists a century ago, the concept of the ether was as accepted as electromagnetic fields are today.
To make everything fit, it became essential to prove the existence of this fluid. An experiment had to be designed that would clearly establish the characteristics of the ether, but it was clear from the start that it would not be easy. The ether, to fulfill its role of carrying light, had to be very (but very) tenuous, which would undoubtedly make its detection difficult. Albert Abraham Michelson (1852-1931) and Edward Morley (1838-1923) had an idea. They would create an apparatus that could measure the speed of light in two perpendicular directions, thereby not only demonstrating the existence of this fluid but also finding the speed at which the Earth moved relative to it. The experience of these two physicists would later be known as "The Michelson-Morley experiment".
The experiment
Each year, the Earth completes a revolution around the Sun, traveling at a speed of 30 km/s (or about 100,000 km/h). In the late 19th century, it was believed that the direction of the "ether wind" relative to the position of the Solar System should vary as the Earth moved in one direction or another, just as a boat receives a different push from the water of a river depending on whether it moves with, against, or across the current.
Michelson and Morley assumed that the Earth was the boat and the river was the ether. For success, the experiment needed to be carried out at various times during the year. In this way, light, arriving at Earth from different positions relative to the ether, would do so at different speeds. The problem was that the speed of light is 300,000 km/s, while the Earth's speed is "only" 30 km/s, so the difference in speeds to be measured was very small. However, Michelson, who was highly skilled in measuring the speed of light, devised a way to measure this minimal difference.
In a building located almost at sea level, Michelson and Morley constructed what is now known as a "Michelson interferometer". The device, relatively simple, uses a half-silvered lens (or half-mirror) to split monochromatic light into two beams that travel at right angles to each other. This allows two identical light rays—since they come from the same source—to be sent simultaneously in perpendicular directions, made to travel equal distances, and received at a point where an "interference pattern" is created. The "drawing" of this pattern depends on the speed of light in each of the interferometer's arms. Any difference between these speeds, caused by the different direction of motion of light relative to the motion of the ether, could be detected. Unfortunately, the apparatus never found any difference.
Indeed, after several months of preparation and several more of testing, the experiment was declared a failure. Or a success, if we analyze it from the right perspective. In every instance, the interferometer behaved as if there were no "ether wind," and although many explanations were attempted, such as the Earth dragging the ether along in some way, none proved correct. Michelson and Morley, instead of demonstrating the properties of the ether, demonstrated its nonexistence. Ernst Mach was one of the first physicists to consider the result of the experiment a success and suggested a new theory. As a corollary to the investigations initiated by the experiment, an alternative theory was developed, the Lorentz contraction, which did not require the ether for the transmission of light. The development of this theory led to Einstein's Special Relativity. Not bad for a failure, right?
Strangely, some theoretical works by HongSheng Zhao, from the University of St. Andrews, which attempt to incorporate dark matter and dark energy into the same theoretical framework, postulate the existence of something similar to the ether that kept Michelson and Morley up at night a century ago. Zhao has found that a dark energy similar to a fluid can behave like dark matter if it reaches a sufficiently high density.
This idea would eliminate the need for the existence of the massive weakly interacting particle (WIMP), affecting, among other things, the speed at which galaxies can rotate. Zhao's work fits perfectly with the experimental data obtained to date. Who knows, perhaps right now, somewhere in the world, a team of scientists is fine-tuning the "version 2.0" of the experiment that made these two physicists famous many years ago.
