Outline of the experiment as a means of detecting absolute motion.

The idea

The experiment tested whether the Earth's motion through the aether could be detected as a change in the speed of light along different directions. If the aether existed, light travelling along the Earth's motion would have a different speed relative to the apparatus than light travelling perpendicular to it. Michelson and Morley split a light beam into two perpendicular arms and recombined the beams to form fringes. They then rotated the whole apparatus through 90° so that the arms exchanged their orientation relative to the supposed aether wind. Any change in relative light travel time would shift the fringe pattern. The expected shift was small but detectable with their interferometer. They found no shift beyond experimental uncertainty, so no absolute motion was detected.

What earns the marks

• The experiment assumes an aether filling space, providing a preferred frame against which absolute motion could be measured.
• If the Earth moves through the aether, the speed of light relative to the apparatus should differ along directions parallel and perpendicular to the motion.
• The interferometer compares light travel times in two perpendicular arms by recombining the beams and observing interference fringes.
• Rotating the apparatus through 90° should interchange the parallel and perpendicular roles of the arms and therefore shift the fringes if an aether wind existed.
• Michelson and Morley observed no significant fringe shift, so the expected aether effect was not detected.
• The null result is interpreted as evidence that the speed of light is constant for all observers, which contributed to the development of special relativity.

Common mistakes and how to put them right

• Treating the aether as proven.

  Put it right: The experiment was designed to detect it, and the null result removed the need for it.

• Saying the experiment measured the Earth's speed directly.

  Put it right: It compared light travel times and looked for a fringe shift, from which a speed difference could be inferred.

• Ignoring the rotation step.

  Put it right: Without rotating the apparatus, a constant path difference would not reveal the changing aether wind direction.

Exam tips

• State the prediction first, then describe the measurement, then state the null result and its significance.
• Use the phrase 'no fringe shift within experimental uncertainty' rather than claiming the shift was exactly zero.
• Explain that the result supports the invariance of the speed of light and the rejection of a preferred aether frame.