Explain why the Michelson–Morley experiment is inconsistent with the Galilean model of relativity and how Einstein's two postulates resolve this inconsistency.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
The Galilean model assumes that velocities add vectorially, so the speed of light would be c±v relative to the aether. In the Michelson–Morley set‑up the arm aligned with Earth's orbital motion would have a longer light‑travel time than the perpendicular arm, giving a measurable fringe shift.
The experiment found no shift, showing that the light speed is the same in both arms and independent of Earth's motion. This null result contradicts the Galilean prediction.
Einstein’s first postulate removes the aether as a preferred frame: the laws of physics, including Maxwell’s equations, are identical in all inertial frames.
His second postulate declares that the speed of light in vacuum is the invariant constant c for all inertial observers, regardless of source or observer motion. This directly explains the null result.
The experiment found no shift, showing that the light speed is the same in both arms and independent of Earth's motion. This null result contradicts the Galilean prediction.
Einstein’s first postulate removes the aether as a preferred frame: the laws of physics, including Maxwell’s equations, are identical in all inertial frames.
His second postulate declares that the speed of light in vacuum is the invariant constant c for all inertial observers, regardless of source or observer motion. This directly explains the null result.
Examiner tips
- Show the velocity‑addition argument first, then the null result, then state both Einstein postulates in order.
- Use the exact wording ‘principle of relativity’ and ‘speed of light is the same constant c for all inertial observers’.
- Keep the answer to 4 points – one per mark.
Common mistakes
- Mixing up Galilean addition with relativistic addition; writing ‘c±v’ without explaining the vector nature.
- Failing to mention that the null result shows light speed is independent of Earth’s motion.
- Using vague phrases like ‘light behaves strangely’ instead of the precise postulates.
Mark scheme (4 marks)
- 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.
- 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.
- 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.
- 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 in this question
Michelson–Morley experiment · postulate
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Galilean and special relativity (HL only) questions
- Explain how the concept of simultaneity in special relativity leads to the concl…
- Explain how the Lorentz transformation for time differs from the Galilean transf…
- Explain why a muon created in the upper atmosphere by cosmic-ray collisions can …
- Explain why two observers in different inertial frames of reference will disagre…