Science › Classical physics
Michelson–Morley: no "ether wind"
1887· Discovery / experiment
ProvenTested many times; part of working physics.
An 1887 experiment that looked for Earth's motion through the supposed 'ether' carrying light, and found none.
See it on the Science map →How it works
Physicists assumed light waves needed a medium, the ether, filling space. Earth moves through space at about 30 km per second, so light should travel at slightly different speeds in different directions, like a swimmer going with or against a current. In Cleveland, Ohio, Albert Michelson and Edward Morley split a light beam, sent the halves at right angles, and recombined them. The expected shift never appeared.
Why it matters
The most famous null result in science. It undermined the ether and set the stage for Einstein's 1905 conclusion that light's speed is the same for everyone.
How we know
Their instrument could detect a shift far smaller than the one expected. The test has been repeated many times since with lasers and optical cavities, at vastly higher precision, always with the same answer.
Still a mystery
Historians debate how much this experiment directly influenced Einstein. The physics result itself is settled.
Connections (3)
▸Light is an electromagnetic wave sent physicists hunting for the ether in Michelson–Morley: no "ether wind"Proven
The relationship
Sound travels through air and ocean waves through water, so 19th-century physicists assumed light waves also travel through a medium, an invisible ether. If Earth moves through it, light's measured speed should vary with direction. Michelson and Morley built a sensitive instrument to detect that variation, a direct test of the wave-in-a-medium picture.
How the link was found
Albert Michelson first tried in Berlin and Potsdam in 1881, then refined the test with Edward Morley in Cleveland, Ohio, in 1887.
Why it's proven
Their null result has been repeated with ever greater precision. Einstein's 1905 relativity explained it: light needs no medium, and its speed is the same for every observer.
Open questions and mysteries
George FitzGerald and Hendrik Lorentz first explained the result with shrinking lengths, an idea relativity later reinterpreted. How much it directly influenced Einstein is debated.
▸Michelson–Morley: no "ether wind" showed the same speed in every direction for The speed of light is constantProven
The relationship
Physicists expected light to move through an invisible "ether," so Earth's motion should create an ether wind changing light's speed by direction. Michelson and Morley split a light beam into two perpendicular paths and compared them. No difference appeared. That null result is exactly what follows if light's speed is the same for every observer.
How the link was found
Albert Michelson and Edward Morley, Cleveland, Ohio, 1887, using a rotating interferometer floating on mercury.
Why it's proven
Repeated many times with lasers and, since the 2000s, rotating optical cavities that rule out any direction effect to about one part in a hundred million billion.
Open questions and mysteries
Historians debate how much the experiment directly influenced Einstein, who said it played a smaller role in his thinking than many assume.
▸Michelson–Morley: no "ether wind" was first explained with Length contractionProven
The relationship
To rescue the ether after the 1887 null result, FitzGerald and Lorentz proposed that anything moving through the ether shrinks along its motion by exactly enough to hide the ether wind. It was a patch with no deeper reason. Einstein later derived the same shrinking from relativity's two rules, with no ether needed.
How the link was found
George FitzGerald suggested it in 1889; Hendrik Lorentz independently in 1892. Einstein gave the modern explanation in 1905.
Why it's proven
Contraction alone could not explain every variant; the 1932 Kennedy-Thorndike experiment also required time dilation, which relativity supplies. Relativity's full package matches all such tests.
Open questions and mysteries
Settled. It is a classic example of the right formula appearing before the right explanation.