The Michelson-Morley Experiment

While studying Maxwell's equations¹ we read that electromagnetic waves propagate at a speed of . In general, waves travel at a speed relative to something. Sound, for instance, propagates at a speed relative to air. This is because waves propagate in a medium of some kind, air, water, you name it. Sound travels faster in water or other denser mediums, not that we recommended shouting to deter shark attacks. Swim away!

Physicists in the late 19^th century figured light waves were no exception to this propagation rule. When Maxwell's equations showed light was traveling at a speed c, the entire physics community shouted, "Relative to what?"

This is how the concept of the ether came to be: an invisible, slender substance that filled every crack of the universe so that light could propagate anywhere. The speed of light then had to be traveling with respect to the ether, an "absolute frame of reference" at rest with respect to the entire universe. Earth, while orbiting the Sun, would then perceive the ether as moving, just like if you drive past a barn, you'll see the barn moving in the other direction. Not only that, but the ether would create a drag on Earth as our planet tried to fight its way through this strange mysterious substance. This sort of drag was referred to as the "ether wind."

It was on this premise that two science guys, Michelson and Morley, designed an interferometer in 1887: an apparatus built to measure the velocity of Earth relative to the ether in order to prove it existed. They figured that if they measured the speed of light, the value of c should change depending on Earth's direction. If the Earth moved with velocity v in the same direction as the ether wind, the speed of light would be c − v. If the Earth moved with velocity v in the opposite direction as the ether wind, the speed of light would be c + v.

Michelson and Morley's interferometer consists of a light source hitting an inclined plate that splits the beam of light into two beams. The first split-beam travels through the plate and strikes a mirror after traveling a length L. That beam bounces off and goes back to the beam splitter, where it's redirected to a detector. The second split-beam travels perpendicular to the first split-beam (again, length L) to a second mirror, then travels back to the splitter, and finally to the detector too.

The detector measured the interference pattern of both waves added together. If the two waves traveled at the same speed, then they would constructively interfere and create a certain kind of pattern; if the two waves traveled at different speeds, they would destructively interfere and create another kind of interference pattern.

If v equaled zero, then the longitudinal (in the horizontal direction) light waves and transverse (in the vertical direction) light waves would arrive at the telescope in sync. However, because of the ether's suggested existence, Michelson and Morley expected an interference pattern that points towards destructive interference to form since the two light beams would take different times to travel the same length.

Yeah, that didn't happen.

People called this the most famous failed experiment of history. No interference pattern was seen, to the great shock of all. Michelson was forced to conclude that, "The result of the hypothesis of a stationary ether is thus shown to be incorrect." ²

Although another physicist, Hendrik Lorentz, tried to substantiate the existence of the ether by suggesting a phenomenon known as "length contraction," which we'll chat about in a bit, the Michelson-Morley experiment stands at the foundation of special relativity and explains why no interference pattern was seen in the Michelson-Morley experiment.

Albert Einstein is the one who figured it out in the end. Not only is the ether complete bogus, but the speed of light c is always constant. When Einstein was asked what electromagnetic waves were traveling relative to, he answered, "relative to whomever is measuring them."