The Michelson–Morley experiment — the end of the ether

What just happened?

The Michelson interferometer splits the beam into two perpendicular arms and recombines them on the screen, where interference fringes appear. We increased the hypothetical "ether wind" $v$ — and the fringes did not shift. The measurement gave zero.

What the ether predicted:

If light travelled through a stationary medium (the ether), its speed relative to the Earth would depend on direction, and the time difference between the two arms would give a fringe shift $\Delta$ growing with the speed $v$ of the "ether wind".

What was actually measured:

The result was zero at every orientation of the instrument. Conclusion: there is no "ether wind", and the speed of light is the same in all directions.

Invariant — the second postulate of SR:

The speed of light in a vacuum is constant for every observer, regardless of their motion and the direction of measurement. This is the foundation on which Einstein built special relativity.

Why this matters for your exam:

The Michelson–Morley experiment is the classic "null result" that overturned the ether hypothesis and justified the constancy of the speed of light. Trick: remember the chain — no fringe shift → no ether → c = const → theory of relativity.

Remember: the fringes did not budge despite the "ether wind" — the speed of light is always the same. There is no ether, and $c = \text{const}$ is the starting point of SR.