The aether hypothesis
Once light was understood as a wave, physicists expected it to require a medium, as familiar waves such as sound and water waves do. A hypothetical medium filling space was therefore proposed and called the luminiferous aether.
If this aether were stationary, it could provide a reference against which motion through space was measured.
Absolute motion is motion measured relative to a fixed universal reference, which physicists hoped the aether would provide.
Predicted effect of Earth's motion
Fizeau's work with light in moving water had shown that motion of a medium could affect measured light speeds. Michelson and Morley applied a similar idea to the proposed aether.
If Earth moved through stationary aether, an observer on Earth would experience an apparent aether wind. Light travelling parallel to Earth's motion was therefore predicted to have a different round-trip time from light travelling perpendicular to it.
| Light path | Prediction if stationary aether existed |
|---|---|
| Parallel to Earth's motion | The outward and return journeys would be affected by motion through the aether. |
| Perpendicular to Earth's motion | The round-trip time would differ from that of the parallel beam. |
Detecting the resulting difference between the two beams would provide a way of measuring the absolute motion of Earth relative to the aether.