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Michelson-Morley Experiment

Key idea: Understand the Michelson–Morley null result, what it implies about the ether idea, and how it supports the special relativity postulates.

  • GCE A-Level H3 Physics 2027
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Learning objectives

  • discuss qualitatively the results of the Michelson–Morley interferometer experiment and its implications on the ether theory (knowledge of the details of the experiment is not required)
  • state the postulates of the special theory of relativity, that in all inertial frames, the laws of physics are the same and the speed of light in free space is the same regardless of the motion of the light source or observer
  • appreciate the failure of Galilean transformation equations when applied to a moving source of light
  • discuss the concept of simultaneity
  • show an understanding of the terms proper time and proper length
  • apply the Lorentz transformation equations to solve one-dimensional problems
  • Derive the time dilation formula and the length contraction formula, making use of the Lorentz factor.
  • apply the time dilation formula and the length contraction formula in related situations (e.g. the lifetime of fast-moving muons) or to solve problems
  • use the one-dimensional relativistic velocity addition formula to calculate velocities in different inertial frames or to solve problems
  • Apply the relativistic energy–momentum relation E² = (pc)² + (mc²)² to solve problems, including selecting its limiting form.
  • Show that E² = (pc)² + (mc²)² reduces to E = pc for massless particles and to E = mc² + ½mv² at low speeds.

The Michelson–Morley experiment matters in H3 because it provides a famous null result that is inconsistent with a simple “ether wind” model and is consistent with Einstein’s special relativity postulates (universal c in inertial frames).

Use this page as the evidence-first entry point, then move immediately into Postulates of Relativity for the formal model.

A source sends light to a beam splitter, which directs it along two perpendicular arms to mirrors. The returning beams recombine at a detector; rotating the apparatus tests for a change in the interference fringes.
A half-silvered mirror sends light along perpendicular paths. An ether-wind model predicted that rotating the apparatus would alter their relative travel times and shift the interference fringes.

1. Definitions (Must Know)

  • Ether (historical): a proposed medium that fills space and defines an absolute “rest frame” for light.
  • Ether wind: the apparent flow of ether past Earth if Earth moves through the ether.
  • Interferometer: an instrument that splits light into two paths and recombines them to produce interference fringes.
  • Fringe shift: a change in the interference pattern that would indicate a change in relative travel times along the two arms.
  • Null result: observing no measurable change where a non-zero change was expected.

2. Key Ideas (What Earns Marks)

  • If an ether rest frame existed, Earth’s motion could make the measured light travel time depend on direction (parallel vs perpendicular to the ether wind).
  • Rotating the apparatus should then change the interference pattern (a fringe shift).
  • Michelson–Morley saw no fringe shift of the expected size (null result), undermining the simplest ether-wind picture.

Expected vs observed (qualitative):

If an “ether wind” exists (simple model)Michelson–Morley observed
Light travel times differ by directionNo detectable difference (at expected sensitivity)
Rotating the apparatus changes fringesNo expected fringe shift

3. Detailed Explanations

A. What the experiment tries to do (one sentence)

Compare the light travel times along two perpendicular arms and look for a change when the instrument is rotated relative to Earth’s motion.

B. What the null result implies (H3-safe)

The null result means there is no evidence for an ether wind in the simple model where light speed would differ by direction due to motion through a medium.

In H3, you connect this to Einstein’s postulates:

4. Common Mistakes

  • Saying “Michelson–Morley proved special relativity” (it did not; it supports the postulates by ruling out a simple ether-wind model).
  • Forgetting to mention “null result” and what it rules out.
  • Writing about “signal delay” rather than the experiment’s aim (direction-dependent travel times).

5. Exam Tips

  • Use a 2-line mark-scheme-safe core:
    1. “They looked for an interference fringe shift when rotating the interferometer.”
    2. “No expected shift was observed (null result), contradicting a simple ether wind model and motivating Einstein’s postulates.”
  • If asked “implication”, say “no detectable ether rest frame / no evidence for ether wind”.

6. Worked Examples

Modelled example 1

State the result and implication (1–2 sentences)

Core

Problem

What was the key experimental result, and what does it imply about the ether-wind idea?
Study the worked solution

  1. State the observation

    Method

    No fringe shift of the expected size was detected when the apparatus was rotated.

    Reason

    This is the experiment’s null result.

    Working

    Δ N_observed ≈ 0 at the predicted sensitivity

  2. Draw the bounded inference

    Method

    The result gives no evidence for the direction-dependent travel times predicted by the simple ether-wind model.

    Reason

    A preferred ether rest frame should have made rotation change the relative travel times.

    Working

    null result ⇒ simple ether-wind prediction unsupported

Guided practice 2

What would a fringe shift have meant? (concept)

About 4 min

Problem

If a reproducible fringe shift had appeared when the apparatus was rotated, what would that have suggested within the ether model?

Try this before viewing the solution

Hints

Hint 1: trace the optical cause
A fringe shift means the relative phase, and hence the relative travel time along the two arms, changed.
Hint 2: connect to rotation
Ask why exchanging the arms’ directions relative to an ether wind would change those times.
View solution step by step

  1. Interpret the fringes

    Method

    A shift would show that the two beams’ relative travel time changed.

    Reason

    Fringe position depends on their phase difference when they recombine.

    Working

    Δ t changes ⇒ Δφ changes

  2. Interpret the rotation

    Method

    The change would suggest direction-dependent light travel times relative to Earth’s motion.

    Reason

    That is the qualitative prediction of a simple ether wind and preferred frame.

    Working

    t_∥ ≠ t_⊥

Common misconception 3

Is “no fringe shift” the same as “no motion”? (concept)

Find and correct the mistake

Learner claim

“No fringe shift was observed, so Earth must have been stationary.” Explain what is wrong with the claim.

Try this before viewing the solution

Best explanation

View solution step by step

  1. Identify what was varied

    Method

    The apparatus orientation changed while Earth continued its motion.

    Reason

    The test looked for direction-dependent optical travel times.

    Working

    rotation → predicted fringe change

  2. State what the null result limits

    Method

    The expected direction-dependent effect was not detected.

    Reason

    That challenges the simple ether-wind model, not the independent evidence that Earth moves.

    Working

    Δ N ≈ 0not ⇒ v_Earth = 0

Examiner practice 4

Link to the postulates (exam phrasing)

4 marks

Examination question

Explain how the Michelson–Morley result supports the special relativity postulates. [4 marks]

Try this before viewing the solution

View solution step by step

  1. State the prediction

    1 mark

    Method

    A simple ether wind predicts direction-dependent light travel times.

    Reason

    The apparatus moves relative to the proposed preferred ether frame.

    Working

    t_∥ ≠ t_⊥

  2. State the observation

    1 mark

    Method

    No expected fringe shift appeared on rotation.

    Reason

    No corresponding travel-time difference was detected.

    Working

    Δ N_observed ≈ 0

  3. Link to the postulates

    1 mark

    Method

    The result is consistent with invariant vacuum light speed and no detectable preferred inertial frame.

    Reason

    Those ideas remove the predicted ether-wind directional effect.

    Working

    c' = c

  4. Qualify the conclusion

    1 mark

    Method

    The result supports rather than proves the full theory.

    Reason

    One experiment cannot establish every consequence of special relativity.

    Working

    consistent evidence ≠ complete proof

Challenge 5

“Consistent with” vs “proves”

Minimal support

Independent transfer

A new experiment finds no effect predicted by model A, while model B predicts no effect. Write one scientifically careful sentence about the evidence, then apply that wording to Michelson–Morley.

Try this before viewing the solution

Hints

Hint 1: calibrate the verb
Use “challenges” for the failed prediction and “supports” or “is consistent with” for the surviving model.
View solution step by step

  1. State the general logic

    Method

    The null result challenges model A and is consistent with model B.

    Reason

    The result discriminates between these predictions without proving every part of B.

    Working

    ¬ P_A observed; P_B remains consistent

  2. Apply it

    Method

    Michelson–Morley challenges the simple ether-wind model and supports the special relativity postulates.

    Reason

    The expected ether fringe shift was absent, as invariant c and no preferred inertial frame are consistent with.

    Working

    supports ≠ proves

7. Mind Stretchers

Mind stretcher 1: What kind of result would refute “universal c”?Extension

Describe (qualitatively) what kind of measurement would directly contradict the claim “all inertial observers measure c in vacuum.”

Answer

Any reliable experiment in vacuum showing that different inertial observers (or the same observer after changing inertial motion) measure different values of the light speed for the same light beam (after proper corrections) would contradict the postulate.

Mind stretcher 2: Why the result is a “big deal”Extension

Why was the null result so surprising in the 19th-century “waves need a medium” mindset?

Answer

If light were like mechanical waves, you’d expect a medium that defines a rest frame and therefore an “ether wind” effect on measured speed. The absence of the expected effect undermined that assumption and pushed physics toward a new spacetime framework.

8. Optional/Enrichment

Next in the maintained sequence: Postulates of Relativity.

Continue with the next resource in this course.

Course and syllabus information
Course
GCE A-Level H3 Physics
Edition
GCE A-Level H3 Physics 2027