The Michelson-Morley Experiment

Or, the demise of the aether and the complete reconsideration of physics

Physics teachers like this experiment. They like to discuss in great detail the absolute surety that Albert Michelson and Edward Morley approached their experiment with, that they were convinced they would prove the existence of the aether, that theorized ubiquitous medium that had to exist for light waves to move. They also delight in having students remember every step of the experiment -- how the interferometer was set up, exactly what the expected lag time of light impeded by the aether should be. After having made sure that every step of their reasoning is understood conceptually and mathematically, physics teachers then like to declare that the experiment detected no such lag and that everything just said was wrong.

This says as much about physics teachers as it does about physicists. The experiment's still worth knowing, though.

Diagram of the interferometer With the theorized aether wind of velocity v blowing towards the left, the light traveling straight into it (toward mirror C) would have a velocity (according to Newton) of c - v; coming back, its velocity would be c + v. The light traveling perpendicular to the aether (toward mirror B) would cover the same distance but with a velocity equal (according to Pythagoras) to (c^2-v^2)^(1/2).
Thus the time the light travels would be the distance (L) over the rate, or L/(c-v) + L/(c+v) for the light that travels to mirror C, and 2L/((c²-v²)^(1/2)) for the light that travels to mirror B.
The two beams of light should arrive at different times, according to this reasoning; by some mathematical manipulation that we will not show here (because we hate having to write equations this way; you can figure it out yourself) we arrive at an approximate (because of the use of the binomial expansion theorem, unshown) time difference of 2Lv²/c³. The point is, they were expecting the light to arrive out of phase.
It didn't.
There was no time difference at all -- and this fact has been repeatedly shown many times since.

Confronted with this data, some physicists realized that they would have to reonsider their basic conception of the universe. Einstein later proposed a theory that contredicts common sense -- time and space are relative but the speed of light is absolute -- but which actually describes the universe.