The Michelson-Morley experiment (A-level only) — AQA A-Level Physics

The Michelson-Morley experiment, conducted in 1887 by Albert Michelson and Edward Morley, was designed to detect the relative motion of the Earth through the hypothesised luminiferous aether, a medium thought to permeate all space and carry light waves. Using an interferometer, they split a beam of light into two perpendicular paths and recombined them to observe interference fringes. Any difference in the speed of light along the two arms due to the Earth's motion through the aether would cause a shift in the fringe pattern when the apparatus was rotated.

The experiment yielded a null result: no fringe shift was observed, implying that the speed of light is constant regardless of the observer's motion. This surprising outcome challenged the aether theory and paved the way for Einstein's special theory of relativity. In the AQA A-Level Physics specification, this experiment is studied as a pivotal piece of evidence that led to a paradigm shift in physics, illustrating the importance of experimental evidence in shaping theoretical frameworks.

• →The luminiferous aether was a hypothetical medium believed to be necessary for the propagation of light waves, analogous to how sound waves require a medium.
• →The Michelson interferometer splits a light beam into two perpendicular paths, reflects them back, and recombines them to produce an interference pattern sensitive to differences in travel time.
• →If the Earth moved through the aether, the speed of light relative to the Earth would differ in the two arms, causing a time difference and thus a shift in the interference fringes when the apparatus was rotated by 90 degrees.
• →The null result (no fringe shift) indicated that the speed of light is invariant and independent of the Earth's motion, providing strong evidence against the aether.
• →This experiment was a crucial motivation for Einstein's special relativity, which postulates the constancy of the speed of light in all inertial frames and eliminates the need for an aether.

• A part-silvered mirror (beam splitter) divides one light beam into two coherent beams travelling along mutually perpendicular arms.
• Each beam reflects from a mirror at the end of its arm and returns to the part-silvered mirror, where the beams recombine.
• Recombination produces an interference pattern of bright and dark fringes because the two paths have different optical path lengths.
• The path difference determines the phase difference between the recombining waves and therefore the fringe positions.
• Rotating the interferometer by 90° exchanges the two arm lengths relative to any motion through the aether, so a change in travel time would shift the fringes.
• Michelson and Morley observed no measurable fringe shift, which is evidence that the speed of light is invariant and that the aether hypothesis is not supported.
• 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.
• The experiment compared light travel times along two perpendicular arms of an interferometer as Earth moved relative to the hypothesised aether.
• A change in orientation or Earth's motion should have produced a measurable shift in the interference pattern if an aether existed.
• The observed result was a null result: no fringe shift beyond experimental uncertainty was detected.
• The null result provided evidence against the aether and against a detectable absolute frame of rest.
• It showed that absolute motion cannot be detected by this method, supporting the principle that only relative motion is physically meaningful.
• It contributed to the development of special relativity and the idea that the speed of light in vacuum is invariant.
• The speed of light in a vacuum, $c$, is constant for all inertial observers.
• This speed is entirely independent of the relative motion of the light source and the observer.
• The invariance of $c$ is a core postulate of special relativity, supported by the Michelson-Morley experiment's null result.
• It contradicts classical Galilean relativity, where velocities simply add together as vectors.

• 💡Sketch the layout with source, part-silvered mirror, two arms, end mirrors and detector, and label the two light paths.
• 💡Explain the role of the 90° rotation explicitly, because this is what converts a possible aether effect into an observable fringe shift.
• 💡Link the absence of a fringe shift to the invariance of the speed of light rather than simply saying the experiment failed.
• 💡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.
• 💡State clearly that the result was a null result and explain what would have been observed if the aether existed.
• 💡Link the null result to the invariance of the speed of light and the absence of a privileged absolute frame.
• 💡Use precise terms such as interference fringes, perpendicular arms and relative motion rather than vague phrases like 'it didn't work'.
• 💡When asked for significance, go beyond describing the apparatus: explain the consequences for the aether model and for later relativity.
• 💡Always specify that the speed of light is constant 'in a vacuum' and 'for all inertial observers' to secure full marks in definition questions.
• 💡Link the invariance of $c$ directly to the null result of the Michelson-Morley experiment when discussing experimental evidence for special relativity.
• 💡When describing the experiment, always mention that the apparatus was rotated by 90 degrees to double the expected fringe shift and eliminate systematic errors.
• 💡Use precise terminology: 'null result' rather than 'no result', and 'interference fringes' rather than 'light patterns'.
• 💡In explanations, link the null result to the constancy of the speed of light and how this contradicts the aether hypothesis, but avoid overstating the experiment's role in proving relativity.

• Thinking the two beams are incoherent: the beams originate from the same source and are coherent, which is why stable fringes form.
• Believing the mirrors absorb the light: the end mirrors reflect the beams back, so each beam traverses its arm twice.
• Assuming the apparatus measures the speed of light directly: it compares travel times along two arms by observing fringe shifts, not by timing a pulse.
• Treating the aether as proven: 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: it compared light travel times and looked for a fringe shift, from which a speed difference could be inferred.
• Ignoring the rotation step: without rotating the apparatus, a constant path difference would not reveal the changing aether wind direction.
• Thinking the null result proved that light has no speed; the correction is that it showed the speed of light in vacuum is invariant and independent of the observer's motion.
• Claiming the experiment detected a small aether wind; the correction is that no fringe shift was observed, so no absolute motion was detected.
• Confusing the aether with air; the correction is that the aether was a hypothetical medium filling space, not the atmosphere.
• Assuming the result immediately proved special relativity; the correction is that it removed support for the aether and motivated relativity, but relativity was developed later.
• Adding the speed of a moving source to $c$ (e.g., $c + v$); the correction is that all inertial observers measure the vacuum speed of light as exactly $c$.
• Applying the invariance principle to non-inertial frames; the correction is that this specific postulate of special relativity applies strictly to inertial (non-accelerating) observers.
• Confusing the invariance of $c$ with the constancy of frequency or wavelength; the correction is that while the speed $c$ is invariant, the observed frequency and wavelength will shift due to the Doppler effect (though their product remains $c$).
• Students often think the experiment 'proved' the aether does not exist. In fact, it provided strong evidence against it, but did not definitively disprove it; later experiments and theoretical developments did.
• Students sometimes believe the experiment directly proved special relativity. It did not; it was a key motivation for Einstein, but relativity was a theoretical framework that explained the null result and made further predictions.
• Students may incorrectly state that the speed of light was measured to be constant in the experiment. The experiment did not measure the speed of light directly; it looked for a difference in the speed of light in different directions, which would manifest as a fringe shift.

1. 1Start by reviewing the historical context: what was the aether, why was it proposed, and what was the expected outcome of the experiment?
2. 2Learn the detailed setup of the Michelson interferometer, including the role of the beam splitter, mirrors, and the compensator plate. Draw a labelled diagram to aid memory.
3. 3Work through the mathematical derivation of the expected fringe shift, ensuring you can substitute values and calculate ΔN. Practice with different numbers.
4. 4Study the null result and its implications: why was it surprising, and how did it lead to special relativity? Make a list of the key conclusions.
5. 5Attempt past exam questions on this topic, focusing on explanations and calculations. Review mark schemes to understand what examiners expect.

• 📋Descriptive questions asking you to explain the purpose of the experiment, the function of the interferometer, and the expected outcome if the aether existed. Advice: Use clear, sequential language and include the rotation of the apparatus.
• 📋Calculation questions requiring you to compute the expected fringe shift using the formula ΔN = (2Lv^2)/(λc^2). Advice: Ensure units are consistent and give your answer to an appropriate number of significant figures.
• 📋Explanation questions on the significance of the null result and its relation to special relativity. Advice: Emphasise that the result provided evidence against the aether and motivated Einstein, but avoid claiming it proved relativity.
• 📋Questions asking you to compare the Michelson-Morley experiment with other experiments or to discuss how later experiments improved precision. Advice: Mention the use of lasers and optical resonators in modern versions, which have increased sensitivity.