Significance of the failure to detect absolute motion.

The idea

The Michelson–Morley experiment used an interferometer to compare the speed of light along two perpendicular arms as Earth moved through the hypothesised aether. A shift in interference fringes was expected if Earth travelled through a stationary aether, because the light travel times along the arms would differ. No fringe shift was observed, so no motion relative to an absolute frame was detected. This null result was significant: it removed support for the aether as a medium for light, showed that absolute motion cannot be detected by such experiments, and helped motivate Einstein's special relativity, in which the speed of light in vacuum is invariant and there is no privileged absolute rest frame.

What earns the marks

• 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.

Common mistakes and how to put them right

• Thinking the null result proved that light has no speed.

  Put it right: 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.

  Put it right: No fringe shift was observed, so no absolute motion was detected.

• Confusing the aether with air.

  Put it right: The aether was a hypothetical medium filling space, not the atmosphere.

• Assuming the result immediately proved special relativity.

  Put it right: It removed support for the aether and motivated relativity, but relativity was developed later.

Exam tips

• 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.