Michelson–Morley · The ether
Is there really a sea for light to travel through?
That time, that place
If light is a wave, what is waving? Sound is air trembling; a ripple is water rising and falling. A wave needs a medium.
So it was assumed that the universe is filled with an invisible substance, the ether, and that light is a disturbance in it.
Why this question
Then it can be checked. The Earth circles the Sun at thirty kilometers a second, so it is swimming through the ether.
In a river, going against the current and going across it take different times. Likewise, light should measure a different speed along the Earth's motion than across it.
What was found
Michelson built an instrument to catch that minute difference: split a beam, send the halves along perpendicular paths, and recombine them. Any difference in arrival shows as fringes, and rotating the apparatus should shift them.
They floated the apparatus on mercury and turned it slowly, measuring in every orientation.
There was no difference. Repeating across seasons, still none.
It was long called a failed experiment, since it did not find what it sought. But the nothing it found was the answer.
The universe was filled with an ether to carry light
Interference fringes that would not shift in any orientation
Eighteen years later Einstein discarded the ether entirely and took the constancy of light's speed as a starting point