The Michelson and Morley Experiment
Relativity
Relativity begins with Galileo. He believed motion must be relative which means it involved the displacement of objects relative to a reference system. Most scientists, at the time, rejected the idea of the Earth rotating on its axis and around the sun because if you drop a stone from a tall building the stone should fly away as the Earth rotates. Galileo explained this by saying that the Earth and stone shared the same motion and because of them sharing horizontal motion the stone would fall straight down. In 1642 Galileo made an experiment up that could prove his theory, an object was dropped from a ship with a tall mast while the ship was in motion, and of course the object fell straight down. Galileo stated that this theory was to be known as Galilean Relativity.

The Ether
Ancient Greeks initially thought that Earth was surrounded by water and the water was surrounded by air and the air by ether. Beyond the ether was said to be nothing.

There were many contributors of the ideas and theories of the ether but the Frenchman Augustin Fresnel is known and credited as the real founder of the wave theory of light, he found that it was necessary for ether to be the medium in which light travelled. The ether was supposed to be a liquid or a very elastic solid that occupied all of the space in which matter consisted. It was believed that light was transmitted in a series of waves through the ether as the ether remained motionless. Scientists compared light through ether as waves through the ocean because as a wave travels through water the particles don’t move it is just the motion of the wave that moves. If scientists could show that the ether had no motion of its own and that as matter passed through it without changing the ether motion as they passed through space then scientists would have a reliable starting point for a general analysis of motion.

Michelson and Morley
Albert Michelson

Albert A. Michelson and Edward W. Morley were two American scientists, in 1887 they attempted to measure the motion of the earth relative to the ether by using wave interference. This is simply demonstrated with water waves. If you put two fingers in a bowl of water so they are just touching the surface and are about 20cm apart. Then vibrate them at the same rate and you will get two expanding circular ripples, as the ripples overlap a complicated pattern of waves occurs which is called interference.

Edward Morley

As the Earth was supposed to be moving through stationary ether, there should have been an ether resistance or ether wind. The speed of light was supposed to be constant in the ether, therefore the ether wind should slow light down as the light headed into it, relative to us. Michelson realised this and compared it to sound travelling through air and how sound is relative to the air. This means if you can measure the speed of light accurately enough then you could measure the speed of light travelling upwind and compare it to the speed of light downwind, the difference of the two measurements would be twice the ether wind speed. Michelson thought up a very clever idea to solve this problem and it was based on:

“Suppose we have a river of width w (say, 100 feet), and two swimmers who both swim at the same speed v feet per second (say, 5 feet per second). The river is flowing at a steady rate, say 3 feet per second. The swimmers race in the following way: they both start at the same point on one bank. One swims directly across the river to the closest point on the opposite bank, then turns around and swims back. The other stays on one side of the river, swimming upstream a distance (measured along the bank) exactly equal to the width of the river, then swims back to the start. Who wins?”

-quoted from - http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html

The answer to this question is the across stream swimmer will win no matter what speed they are travelling (as long as they are going faster then the current). Michelson’s idea was to race light like in the swimming