The Michelson-Morley Experiment
If the Earth really does move through a luminiferous ether, that motion should show up as a tiny, measurable difference in the speed of light travelling in different directions. In 1887, Michelson and Morley built an instrument precise enough to try to catch it — a single elegant piece of apparatus, exploiting nothing more exotic than a half-silvered mirror and the interference of light with itself.
Michelson's interferometer
Albert Michelson and Edward Morley set out to detect the predicted ether wind directly, using an extremely sensitive instrument of Michelson's own design: the Michelson interferometer. Its power lies in comparing the travel times of two light beams sent along different paths, using the wave nature of light itself as the measuring tool.
A beam of light from a single source is directed at a half-silvered mirror, angled at 45° to the beam, which splits it into two beams travelling at right angles to one another. One beam continues on through the half-silvered mirror and travels down one arm of the apparatus; the other is reflected off at 90° and travels down a second arm, positioned perpendicular to the first. Crucially, the two arms are built to be of equal length.
At the far end of each arm sits a plane mirror, positioned exactly perpendicular to the beam travelling down that arm, so that each beam reflects straight back the way it came. Both reflected beams arrive back at the half-silvered mirror at the same moment, where they recombine into a single beam and are directed toward a detector (in the original experiment, the observer's eye; in principle, any light-sensitive detector).
Why recombining the beams reveals a speed difference
Because the two beams both originated from the same source, they are perfectly coherent with one another — their waves are locked in step at the point where they split. When they recombine at the half-silvered mirror, the two beams' waves overlap and interfere, producing a pattern of bright and dark bands, or fringes, at the detector: bright where the waves arrive in phase and reinforce each other, dark where they arrive out of phase and cancel.
The exact position of this fringe pattern depends sensitively on the relative time the two beams took to complete their round trips down their respective arms and back. If the ether wind affects the speed of light differently depending on direction — as the ether hypothesis predicts — then the beam travelling parallel to the Earth's motion through the ether should take a very slightly different time to complete its round trip than the beam travelling perpendicular to that motion, even though the two arms are physically the same length.
The decisive step was to mount the whole apparatus on a stone slab floating on a pool of mercury, so it could be rotated smoothly through 90°. Rotating the interferometer swaps which arm points along the direction of the Earth's motion and which points across it — so if the ether wind is really affecting the two beams differently, the fringe pattern observed at the detector should shift by a small, precisely calculable amount as the apparatus turns.
Purpose: to detect the Earth's motion through the hypothesised luminiferous ether, by looking for the effect of the predicted "ether wind" on the speed of light travelling in different directions. Method: split a single beam of light into two perpendicular paths of equal length using a half-silvered mirror; reflect each beam back along its own path using a plane mirror at the far end of each arm; recombine the two beams at the half-silvered mirror to produce an interference pattern, sensitive to any difference in the beams' travel times; then rotate the entire apparatus through 90°, swapping the orientation of the two arms relative to the Earth's motion, and watch for the predicted shift in the fringe pattern that this rotation should cause if the ether wind is real.
What the rotation actually revealed — and what that revelation meant for the ether hypothesis — is the subject of the next lesson.
Key takeaways
- The Michelson interferometer splits a single light beam into two perpendicular beams using a half-silvered mirror, sends each down an arm of equal length to a mirror, and recombines them at the same half-silvered mirror.
- The recombined beams are coherent and interfere, producing a fringe pattern of bright and dark bands whose exact position depends sensitively on the relative travel time of the two beams.
- If the ether wind affects light's speed differently depending on direction, the two perpendicular beams should take very slightly different round-trip times, shifting the fringe pattern.
- Rotating the whole apparatus through 90° swaps which arm points along the Earth's motion and which points across it, so the ether hypothesis predicts a specific, calculable fringe shift on rotation.
- The experiment's purpose was to detect this shift as direct evidence of the Earth's absolute motion through the ether.