What did the most famous failed experiment in physics find?
Two physicists went hunting for an invisible wind of aether in 1887. It wasn't there, and that changed physics.
▶ Start the storyIn 1887 two American physicists, Albert Michelson and Edward Morley, set out to measure how fast the Earth moves through the "luminiferous aether", the invisible medium that nineteenth-century physics assumed filled all of space and carried light waves. The logic was natural. Water waves need water and sound needs air, so light, which crosses even a vacuum, was thought to need a medium too, and even a vacuum was assumed to be filled with it.
If the Earth ploughs through the aether at about 30 kilometres per second on its orbit, light should travel at slightly different speeds along and across that "aether wind". The effect would be tiny, since 30 kilometres per second is only about one hundredth of one percent of the speed of light. So the pair built an interferometer: a device that splits light into two beams at right angles, sends them along two arms and recombines them, so that the pattern of interference depends on the relative time light takes along each arm. To keep it steady, they mounted it on a heavy sandstone block floating in a trough of mercury, so it could rotate almost without friction, sweeping the arms through every angle to the supposed wind.
Step 1: Light hits a half-silvered mirror
Step 2: It splits into two beams at right angles
Each goes down a long arm and is reflected back
Step 3: The beams recombine in the eyepiece
Their interference pattern depends on the relative travel times
Step 4: The whole device rotates on mercury
Any aether wind would shift the pattern as the arms turn
They found essentially nothing. The displacement they measured was at most one-fortieth of what the aether theory predicted, within the range of experimental error that would allow it to be zero. The experiment has been called the most famous failed experiment in history. Later physicists tried to rescue the aether: FitzGerald and Lorentz suggested that objects contract along their motion through it. In 1905 Einstein took another route. Starting from the constancy of the speed of light and the relativity principle, he derived length contraction and time dilation, and left the aether no role at all. He wrote that if the experiment had not brought such embarrassment, no one would have regarded relativity as a (halfway) redemption.
The test has been repeated with ever greater sensitivity; in 2009 optical resonator experiments confirmed no aether wind at the 10⁻¹⁷ level.
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Recap
Light showed no 'wind': its speed did not depend on how the Earth moved.
💡 A trick to remember it · They listened for an aether wind and heard dead calm in every direction: the 'failure' was the discovery.
Surprising fact · It has been called the most famous failed experiment in history, and in 2009 resonators confirmed no aether wind at the 10⁻¹⁷ level.
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