Someone not long ago asked me to explain the Michelson-Morley experiment, and why it was so important, so here it is.
When you make a sound, the sound travels away from you in all directions at about 340 m/s (770 mph). We can measure that speed pretty easily: just make a sound and time how long it takes to travel a certain distance.
Sound is a wave, and like most waves, it needs a medium through which to travel, in this case air. When we say the speed of sound is 340 m/s, that means that sound travels through air at that speed. Sound speed is not an innate property of sound but depends on the medium through which it travels. The speed of sound in the atmosphere of Mars, for example, is only 240 m/s.
Since the speed of sound is the speed in air, if you were to move through the air, the speed you observe would be different. Suppose you were on the platform of a train moving at 10 m/s (20 mph). If you measured the speed of sound in the direction you are moving, you would get a speed of 330 m/s. That’s because the sound is traveling through the air at 340 m/s, but you are traveling through the air in the same direction at 10 m/s, so the sound is moving 330 m/s relative to you. In the same way, if you measured the speed of sound in the opposite direction, you would get 350 m/s because of your motion. This is a key feature of waves traveling through a medium: they can be different in different directions because of your motion through the medium.
So now let’s look at light. Light can act like a wave, so it was thought that light must simply be a wave, similar to sound. Of course if that is the case, then light must have some kind of medium to travel through. They called this medium the luminiferous aether. The problem was that no one had ever observed this aether. It also had some strange properties like being massless and invisible.
Enter Michelson and Morley. They devised an experiment that would measure the speed of light in different directions. The idea was that since the Earth orbits the Sun, it must be moving relative to this aether, just as we can move through the air. That would mean the speed of light measured in the direction of Earth’s orbit would be a bit slower than expected. In the opposite direction it would be a bit faster. Measuring the speed of light is hard, so Michelson and Morley used a clever trick that just measured the shift in light speed, but the end result is the same.
So these guys expected to measure this difference in the speed of light, which would prove that the aether existed. What they found was that the speed of light was the same in every direction. No matter what direction they measured the speed of light, they always got the same answer. They couldn’t observe any effect of an aether.
After the Michelson-Morley experiment, several theorists tried to explain the results by supposing that Earth dragged the aether along with it, or imposed other conditions to explain our inability to observe the aether. But then Einstein proposed special relativity, which eliminated the need for aether altogether.
Of course, to get rid of the aether, Einstein had to rewrite physics, but that’s another story.