# Explain why the Michelson–Morley experiment is inconsistent with the Galilean model of relativity and how Einstein's two postulates resolve this inconsistency.

> IB DP Physics Higher Level (2023 syllabus) — A.5 Galilean and special relativity (HL only) · Explain · 4 marks

> The Michelson–Morley experiment used an interferometer to compare the speed of light measured along two perpendicular arms, one aligned with Earth's orbital motion and one perpendicular to it. No fringe shift was observed between the two arms, regardless of the orientation of the apparatus.

## Mark scheme (4 marks)

1. The Galilean model predicts that the speed of light should depend on the motion of the observer/source relative to the luminiferous aether, so the two arms of the interferometer should record different light travel times, producing a fringe shift.
2. The null result (no fringe shift observed) contradicts the Galilean prediction, showing the speed of light is the same in both arms and independent of Earth's motion.
3. Einstein's first postulate (the principle of relativity) states that the laws of physics, including Maxwell's equations, are the same in all inertial frames, so there is no preferred frame such as the aether frame.
4. Einstein's second postulate states that the speed of light in a vacuum is the same constant c for all inertial observers regardless of the motion of the source or observer, directly accounting for the null result.

## Key terms

- [Michelson–Morley experiment](https://www.gradenine.co.uk/glossary/michelson-morley-experiment)
- [postulate](https://www.gradenine.co.uk/glossary/postulate)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-michelson-morley-experiment-is-14199a6c) · Published by Druglandscape Ltd.