Light and waves
1887
The experiment that found nothing
Albert Michelson and Edward Morley
How do you detect a wind that nobody can feel?
If light is a wave, something must be waving: physicists called it the ether. As the Earth moves through it, light should travel faster in some directions. Michelson and Morley built an extremely precise instrument to see the difference and found none.
How they figured it out
The Earth moves around the Sun at about 30 km/s. If light travels through an “ether”, light going with that motion and light going across it should take very slightly different times: a difference of about one part in a hundred million.
Albert Michelson split a light beam into two arms at right angles and recombined them, so any timing difference would shift the pattern of light and dark fringes. In 1887, with Edward Morley, he floated the instrument on a pool of mercury so it could turn smoothly. They expected a shift of about 0.4 fringe. They saw almost nothing. Michelson considered it a failure.
The equation
Light’s speed came out the same whichever way the instrument pointed.
See it in real life
The colours of a soap bubble
The swirling colours on a soap bubble are interference: light reflected from the front and back of the film adding or cancelling. Noise-cancelling headphones use the same idea with sound. And LIGO is a Michelson interferometer with arms 4 km long.
Where engineers use it
Interferometers
The same layout measures tiny distances in factories, labs and LIGO.
Null results
Experiments designed to see nothing are powerful tests; engineers use null measurements in bridges and balances.
Where it came from
- Light is electromagnetism 1865
Led to
If light is a wave, what is waving? Michelson and Morley tested for the ether and found nothing.
What it led to
- Special relativity 1905
Led to
Relativity made the missing ether drift natural. Historians still debate how much the experiment itself guided Einstein.