The Michelson-Morley experiment disproved the existence of the luminiferous aether, a substance thought to be the medium through which light waves traveled. The experiment measured tiny changes in the speed of light and found the expected “ether effect” to be almost imperceptible, leading to the conclusion that the ether was imaginary. This discovery laid the foundations for 20th-century theoretical physics, including general relativity and quantum theory.
Conducted in 1887, the Michelson-Morley experiment has been called “the most successful failed experiment in science.” It has also been called “the starting point for the theoretical aspects of the Second Scientific Revolution”. The Michelson-Morley experiment provided strong evidence against the idea of a luminiferous (light-bearing) aether, which was very popular among physicists of the time.
Consider how physical and audio waves travel through mediums; a liquid or gas such as water or air. From the work of James Clerk Maxwell in 1861, it was known that light was an electromagnetic wave. Physicists began postulating that this wave moved through a medium, made of particles much smaller than those within the air, and much less dense. They called this substance the ether. Maxwell himself helped popularize the notion of the aether, and it was soon assumed that the aether existed.
The aether idea had several problems, as some physicists of the time noted. To serve as a medium for electromagnetic energy given the observed data, the aether had to be fluid – space filling, a million times stiffer than steel – to support the high frequencies of light waves, massless and without viscosity – otherwise the planets would be slow in their orbits, transparent – or more distant stars would have apparent magnitudes falling more rapidly than the square of the distance, non-dispersive, incompressible and continuous on very small scales. That was a lot to ask of any substance, and the ether played a role more of a theoretical crutch than anything else.
The Michelson-Morley experiment, conducted by Albert Michelson and Edward Morley at what is now Case Western Reserve University in Ohio, was the beginning of the end for aether theories. Physicists realized that if the aether existed, the Earth would move relative to it due to its rotation around its axis, its orbit around the Sun, and its orbit around the galaxy. Even if the aether itself were moving, it would be unlikely to move perfectly in step with the Earth, which varies both its direction of motion and its speed over time.
The intended effect was an “aether wind” that would cause slight variations in the speed of light depending on the direction in which light was travelling. Since the speed of Earth’s orbit around the Sun is only about one-hundredth of one percent the speed of light, the effect was thought to be small. Various experimental setups for detecting aether wind had been set up in the mid-1800s, but instruments at the time simply weren’t accurate enough.
The Michelson-Morley experiment was designed to measure tiny changes in the speed of light by bouncing two beams at right angles using half-sliver, fully reflecting mirrors, then recombining them and observing the interference pattern. If there were even the slightest difference in the speed of the two light beams, it would be evident in the pattern of constructive and destructive interference on the sensing element. To eliminate interruptions to the experiment, the whole thing took place in the basement of a stone building and the apparatus was placed on a large marble tablet floating in a pool of mercury. This allowed the Michelson-Morley experiment to be rotated and changes in the speed of the aether depending on the direction would have caused a measurable effect.
In the end, the “ether effect” was found to be so low as to be almost imperceptible, more than 10 times less than the expected effect. It was such a small effect that, allowing for the margin of error, it could have been zero.
Subsequent experiments, increasingly precise, confirmed what no one wanted to hear: the ether was imaginary. The light somehow spread like a wave through the vacuum, and that was it. The Michelson-Morley experiment was only the first to discover this. These results laid the foundations for 20th-century theoretical physics, including general relativity and quantum theory.
For his work in physics, Albert Michelson was awarded the Nobel Prize in 1907.
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