Ocean waves travel across the water and sound waves travel through air. In fact, those waves don’t work without a medium — something to pass them along from place to place. So it makes sense that light waves need something to transmit them across the vacuum of space.

For centuries, scientists called that medium the ether. They couldn’t explain what it was. But they knew that you couldn’t see it, feel it, or weigh it. In fact, it wouldn’t interact with “normal” matter in any way. But it was infinite — it filled the entire universe.

By the mid-1800s, though, many scientists had some doubts about it. And an experiment in 1887 all but ruled it out.

Albert Michelson and Edward Morley figured that as Earth orbited the Sun, its speed relative to the ether was changing. And since light waves were carried by the ether, the scientists expected to be able to see a change in the speed at which light travels. So they measured the speed of light in Earth’s direction of motion, and at a right angle to that direction. But they saw no change — the speed of light was always the same.

Later, Albert Einstein’s Theory of Special Relativity explained that the speed of light in a vacuum is constant — it never changes. That ruled out the need for an ether to transmit light waves through the universe.

That doesn’t mean that the vacuum of space is a complete void, though — or that we know everything about it. More about that tomorrow.
 

Script by Damond Benningfield

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