In 1887 Michelson and Morley built an interferometer which is shown below. In this interferometer, you shoot a beam of light at a beam splitter. The beamsplitter splits the light into two parts, which each bounce off of their own mirror. The light is then recombined by the beam splitter and goes to a detector. Light is a wave, so when the path lengths are equal, the waves add to produce a bright spot at the detector. When the path length changes by only half a wavelength (which for visible light comes out to be about 250nm) the waves cancel each other to produce a dark spot. For more information about this, see Interference. Even though this instrument can be a few meters big, it can detect changes in distance of hundreds of nanometers.

The idea was to use this interferometer to measure the movement of the Earth through the Eather. As we'll see, if the concept of the aether was correct, light would take different amounts of time to go through the arm facing in the direction of motion as it would through the arm perpendicular to it.

Here is what they thought the interferometer would look like if it was standing still in the eather. Think of the sphere as a photon or a wavefront traveling at the speed of light when you first turn on the light source. Each arrow is an equal distance from the previous one and since the speed of light is constant, one arrow turns on for every clock tick.

	Movie: AVI
Michelson Morley Interferometer, stationary in the Aether

If the interferometer is moving through the aether, the light that is going in the direction of motion takes longer to get to the detector. They thought that by rotating the whole interferometer, one arm would go from being in the direction of motion to being perpendicular to it and the light traveling down this arm would at first have a longer distance to travel and then a shorter distance. They should have been able to detect this change by seing bright and dark fringes pass by their detector.

	Movie: AVI
Michelson Morley Interferometer, moving Through the Aether

Wouldn't the two paths also be the same length of the top arm gets bigger and the side arm stays the same? What about some combination?

If space gets expanded perpendiclar to the direction of motion to compensate by making the top arm longer, it would violate the Principle of Relativity -- you could tell if you were the one moving or standing still. For the example shown below, the peg perfectly fits into the hole when both are at rest. If the peg was "at rest" and the hole moved toward it and got expanced, it would be bigger than the peg. On the other hand if the hole was "at rest" and the peg was zooming toward it, it would be expanded and be too big to fit through the hole. The Principle of Relativity is violated since you would know who was moving faster even though all constant velocity motion should be indistinguishable.

So isn't be squished something that you could measure too? It turns out that when you combine Lorentz Contraction with other Special Relativity effect known as and , there is no way to tell that you are squashed. In fact, since things are equally good from your perspective, everything you're wizzing by looks squashed too.