This is a box.

And now it's empty.

Well, not really.

We can't see it, but it's still full of dust particles, water vapor, air molecules, and on a smaller scale, photons, electrons and other elementary particles.

But if we could somehow remove every last bit of matter, could we make that space really, truly EMPTY?

Is space just an absence of things... or is space a thing itself?

[INTRO] Ancient Greeks gave us the word "aether," which has been used for centuries to describe space as a thing.

Whether space is a "something" or a "nothing" sounds like one of those questions your philosophy major friend likes to ask late at night.

But the very nature of reality hinges on this question.

It all has to do with relativity.

No, not his relativity.

Old-school relativity.

Way before Einstein, physicists knew velocity was relative.

To say an object is moving at a certain speed in a certain direction only has meaning relative to other objects.

If two astronauts pass each other in deep space, each can claim that they were at rest and the other was moving.

And they're both equally right.

But accelerated motion is different.

Changes in speed or direction, daily experience tells us these seem to have value even without other objects as reference points.

Isaac Newton illustrated this with a thought experiment: You have a bucket of water at the end of a rope.

As you twist the rope, the bucket starts to spin.

Eventually the water is pushed against the sides.

You're probably like "so what?"

but think about it.

The bucket and water are moving together.

If all motion were relative, for the water to be pushed against the bucket, it should be moving relative to some other thing.

That's fine, we have plenty of options: "the rope!"

or "the air!"

or "the earth!"

But now let's picture this experiment in the vacuum of space.

No Earth, no sun... nothing.

And instead of water, it's you strapped to the inside of the bucket.

As it starts to spin, you feel pressed against the side.

But why?

Relative to the bucket, you seem to be still.

What are you accelerating relative to?

And if the answer is NOTHING, then what's making you feeel thiiiiis waaaaaay?

Newton's solution to this riddle was that SPACE MUST BE A THING.

An invisible but very real structure that gives all motion in the cosmos a master point of reference.

He called this absolute space.

Nobody could agree what it was made of, but for the next couple hundred years, scientists accepted that space was something--until another famous physicist offered a new, crazy idea.

NOPE, NOT QUITE THERE YET.

In the mid-1800s, Austrian physicist Ernst Mach decided he didn't want to ride up Isaac's bucket.

He pointed out a flaw with Newton's reasoning: our universe is NOT actually empty.

We share the cosmos with billions of stars and other things, and even though they're really far away, they can still act as points of reference.

According to Mach, in a truly empty universe, with just you in it, acceleration really would have no meaning, there really would be no way to tell spinning from not spinning.

In a universe with half as much matter, you'd feel half as much.

It isn't space that gives meaning to acceleration, it's all the stuff in space.

SPACE IS NOT A THING.

Around the same time, another brilliant physicist--NO, NOT YET!

YOU'LL KNOW WHEN IT'S TIME!--James Clerk Maxwell was discovering some important things about light: It travels in a wave-like manner at exactly one speed.

Now, sound waves and ocean waves both need a medium to travel through, so scientists figured light traveled through a medium too, one that filled space itself.

They called it the luminiferous aether, and once again space was A THING.

But there was a problem with this idea: Two boats will measure an ocean wave's speed differently depending on their own velocities through the water.

But light waves, unlike anything else in the universe, ALWAYS have the same speed no matter who's measuring.

An object moving toward a light source at a million meters per second, an object moving away from a light source at a million meters per second and an object sitting still will all measure the light approaching them at the exact same speed.

HOW CAN THIS BE TRUE?

GOSH, IF ONLY THERE WERE SOME SORT OF FAMOUS GENIUS WHO COULD EXPLAIN THIS PARADOX.

ONE WITH, LIKE, CRAZY HAIR WHOSE FACE IS ON A LOT OF DORM ROOM POSTERS... Oh, forget it, it was Albert Einstein.

NOPE TOO LATE!

YOU MISSED YOUR CHANCE!

Einstein said "forget the aether."

He said that light, different from any other kind of wave, could travel through empty space.

But without a medium, what is light's speed relative to?

This is where Einstein blew our minds.

He explained light's bizarre behavior by reasoning that space and time are not fixed.

Distances and durations depend on the perspective of the observer.

But if space is not absolute, as Newton argued, it can't be used as an ultimate point of reference for motion, so MAYBE SPACE ISN'T REALLY A THING?

Not so fast.

According to Einstein, even though space isn't absolute, and time isn't absolute, what we get when we put them together, SPACETIME is absolute.

The faster an object moves through space, the slower it moves through time.

An object not moving through space is moving at top speed through time.

An object moving as fast as possible, at the speed of light, no longer moves through time.

None of these objects will agree about distances and durations, but there is one thing they will all agree on: That an accelerating object will always have a curved trajectory through this spacetime.

This is how spacetime provides a reference point for acceleration all by itself.

Space may not be a thing, but SPACETIME IS A THING.

You'll feel that bucket spinning even if there's no other matter in the universe.

Sorry, Mach.

I know this might make your head spin faster than Newton's bucket, it may even sound unbelievable, but Einstein's equations have survived decades of experimentation, and we've witnessed the malleability of space and time in actual experiments, like this.

The idea that we're all swimming through an omnipresent invisible something, a master framework of space and time, may sound a bit like Einstein invented a new "aether," but that's just like, your perspective man.