Michelson And Morley's "Failed" 1887 Experiment Changed The Course Of Physics, And Put The Aether To Bed

In the past few centuries, scientists have conducted countless experiments that have revolutionized our understanding of the universe. Very few have been as impactful as the Michelson-Morley experiment, which attempted to find evidence of the luminiferous aether, and found nothing.

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First, a little context. In the 19th Century, physicists revolutionized our understanding of light, in several other incredibly impactful experiments. In 1801, Thomas Young took a light and shone it through a double-slit onto a screen behind it. At the time, the corpuscular idea of light, proposed by Isaac Newton, was the prevailing hypothesis about what light is, suggesting that light was a massless particle dubbed a "corpuscle".

But when Young shone a light through the double-slits, the pattern it made on the screen behind it was not what you would expect if light were simply a particle. Rather than two blobs of light, it produced an interference pattern, as if two ripples of waves were emerging on the other side of the slit and interfering with each other.

Physicists began to take the wave-nature of light seriously (ignore photons for now, there's no need to complicate things further thankyouverymuch, Einstein). Scientists at the time thought (pretty naturally, let's not past-shame) that in order for light waves to exist, they must be waves in a medium. The luminiferous (light-bearing) aether was now proposed as a medium through which light waves can propagate. 

For many scientists at the time, including American physicist Albert A. Michelson, detecting the aether was a priority. Though it was apparently invisible to us but for the waves we can see in it, they attempted to figure out its properties. These properties would have to be pretty remarkable, being both light and difficult to compress in order to support light's speed, but also allowing matter to pass through it without causing noticeable drag. 

There are many different proposed versions of the aether, with the majority of these models being thought up before special relativity made such explanations unnecessary, explaining light and electromagnetic waves without the need for a medium. Many of these aether hypotheses suggest effects that it would have on matter, and ways in which it might be detected. One suggested effect was the "aether wind". 

"Michelson realized that, just as the speed of sound is relative to the air, so the speed of light must be relative to the aether," Michael Fowler, professor in the Physics department at the University of Virginia, explains in a University of Virginia blog post. "This must mean, if you could measure the speed of light accurately enough, you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and the difference of the two measurements should be twice the windspeed."

Unfortunately, measuring the one-way speed of light turns out to be quite tricky, and may actually be impossible. But Michelson and Edward W. Morley came up with a way it could be done. The idea was essentially to race light, sent out in different directions. Light would be sent toward a half-transparent mirror, so that half of the light continues in a straight line, whilst half is reflected by the mirror placed at an angle of 45 degrees. Both these packages of light were then reflected off distant mirrors, and returned to the observer.

If there is an aether wind that affects the speed of light passing through it, then the beams of light should arrive at the observer at slightly different times. Given how fast light is, looking for this difference was achieved by Michelson and Morley through use of the interference pattern created by light. In short, if the light traveling one path takes longer, it will fall out of step with light traveling the other path, and the result may be the waves canceling each other out, meaning nothing will be seen by the observer. 

The device used by Michelson and Morley in 1887 was able to rotate, allowing them to test for aether wind in plenty of directions. The pair performed the experiment with the hope of detecting evidence for the aether, but found none.

"The Experiments on relative motion of Earth and ether have been completed and the result is decidedly negative," Michelson wrote in a letter. "The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe – the maximum displacement was 0.02 and the average much less than 0.01 – and then not in the right place. As displacement is proportional to squares of the relative velocities it follows that if the ether does slip past the relative velocity is less than one sixth of the earth’s velocity."

Further experiments have taken place attempting to detect aether drag, including at the tops of mountains, but none have found evidence of an all-pervading luminiferous aether. The Michelson-Morley experiment is sometimes referred to as the most famous "failed" experiment in history. But though their hypothesis was incorrect, the experiment was as thorough as it was ingenious, and they published their null results for all to see and learn from, providing key evidence about how the universe operates.

"If the Michelson–Morley experiment had not brought us into serious embarrassment," Einstein later said of it, "no one would have regarded the relativity theory as a (halfway) redemption."