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Reid: There were lots of things scientists believed a century or two ago that might sound weird to us today. For one thing, they once thought atoms looked like plum pudding. But over time, as researchers have done more experiments and developed new theories, those ideas have evolved into the science we know today.

One of the most game-changing experiments ever was what’s known as the Michelson-Morley Experiment -- not because of what it did find, but because of what it didn’t. See, physicists used to think that all of space and time was filled with what they called the aether: an invisible material that wasn’t interacting with anything, but was giving light its speed. No one had ever detected this aether, but scientists figured it had to be there -- they knew that light acted like a wave, and waves seemed to need a material to travel through.

Throughout the 19th century, evidence was piling up that light was a wave -- it reflected like a wave, it interfered with itself like a wave, so it would make sense for light to get around like other waves, too -- by moving particles back and forth. In ocean waves, for example, the water molecules move up and down. And in sound waves, air particles move back and forth. So it would make sense for light to be moving around something, even if it was invisible -- and physicists called that something the aether.

In the spring of 1887, in Cleveland, Ohio, Albert Michelson and Edward Morley set up an experiment to learn more about the aether, by looking for the effects of something called the aether wind. You know how when you try to yell into a particularly strong gust of wind, your voice won’t get very far, but if you yell with the wind, it’ll go much farther? Well, scientists assumed that as Earth moved through this aether, it would create a similar kind of wind -- but one that would affect the speed of light from Earth’s perspective.

So Michelson and Morley set up beams of light in a way that should have given them different speeds, depending on whether they were traveling with, against, or perpendicular, to the aether wind. They then bounced the two beams off of mirrors so that they eventually reunited with each other, and predicted the beams would arrive at slightly different times -- since they’d been sped up or slowed down by the wind.

But the result? One of the greatest failed experiments of all time. There seemed to be no difference between the two beams. It was like Earth wasn’t moving through any kind of aether at all. Over the next few years, physicists tried to figure out how that could possibly make sense. Some said that the experiment just wasn’t accurate enough -- and tried repeating it with better and better equipment. But they still didn’t find anything. Eventually, they had to accept that it seemed like there was no such thing as an aether. Instead, light is just... different from other waves.

Turns out that the speed of light isn’t affected by an invisible aether, and light speed through a vacuum is always the same. Space and time will even bend themselves to fit -- which is where Einstein’s Special Theory of Relativity comes in. He helped put all these pieces together, showing that physics could work even without the aether. It just meant that completely mind-bending things had to be happening, like time slowing itself down. And unlike with the aether, experiments have shown that Einstein was right.

Meanwhile, more physicists were working on some of the other questions raised by the fact that the aether didn’t exist. Mainly: if light wasn’t like a regular wave, what was it? After a whole lot of experimenting, they realized that even though light did act like a wave sometimes, it also acted like a particle -- what’s known as the wave-particle duality. It was another brain-melting idea, and one that eventually led to another whole new field in physics -- quantum mechanics, the science of the very small. So, the Michelson-Morley Experiment might have banished the aether to the history books. But its failure also led to some of the most important ideas in science.

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