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Conducted in 1887, the Michelson-Morley experiment has been called "the most successful failed experiment in science." It has also been called "the kicking-off 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) ether, much in vogue among physicists at the time.
Consider how both physical and audio waves travel through mediums; a liquid or gas such as water or air. Since James Clerk Maxwell's work in 1861, it was known that light was an electromagnetic wave. Physicists began to postulate that this wave moved through a medium, made of particles much smaller than those within air, and much less dense. They called this substance the aether. Maxwell himself helped popularize the notion of the aether, and soon it became taken for granted that the aether existed.
The idea of the aether had numerous problems, as some physicists of the time noted. To serve as a medium for electromagnetic energy while accounting for observed data, the aether had to be fluid — to fill space, a million times more rigid than steel — to support the high frequencies of light waves, massless and without viscosity — otherwise it would slow the planets in their orbits, transparent — or more distant stars would have apparent magnitudes dropping off quicker than the square of the distance, non-dispersive, incompressible, and continuous on very small scales. This was a lot to ask of any substance, and aether served 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 the 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 be moving with respect to it due to its rotation about its axis, orbit around the Sun, and orbit of the Sun around the galaxy. Even if the aether itself moved, it would be unlikely to be moving perfectly in lockstep with the Earth, which varies both its direction of motion and velocity over time.
The expected effect was an "aether wind" that would cause slight variations in the speed of light depending on which way the light was traveling. Because the velocity of the Earth's orbit around the Sun is only about one hundredth of one percent of the speed of light, the effect was thought to be slight. Various experimental setups to detect the aether wind had been set up in the mid-1800s, but the instruments at the time were simply not precise enough.
The Michelson-Morley experiment was designed to measure tiny variations in light speed by bouncing two beams at right angles using half-slivered and fully reflective mirrors, then recombining them and observing the interference pattern. If there was even a tiny difference in the speed of the two light beams, it would be evident in the pattern of constructive and destructive interference on the detecting element. To eliminate disruptions of the experiment, the whole thing took place in the basement of a stone building, and the apparatus was put on a large marble tablet floating in a mercury pool. This allowed the Michelson-Morley experiment to be rotated, and variations in aether speed depending on direction would cause a measurable effect.
In the end, the "aether effect" was found to be so low as to be almost undetectable — more than 10 times less than the expected effect. It was such a slight effect that, taking margin of error into account, it could have been zero.
Subsequent, progressively more precise experiments confirmed what nobody wanted to hear: the aether was imaginary. Light somehow propagated as a wave through vacuum, and that was it. The Michelson-Morley experiment was just the first to discover this. These findings set the stage 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.