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[♪ INTRO]

In nature, most of our basic forces both attract and repel.

In fact, gravity is the only exception.

As far as we can tell, anything gravity acts on just goes one way: down.

As long as there's no other forces involved, anyway.

But there is actually an exception to the exception.

On a teeny-tiny scale, we're exposed to negative gravity every day.

Because, according to theorists, sound waves, of all things, have negative gravity.

And what's even more amazing is that you don't need fancy science

like quantum mechanics or general relativity to understand why.

You can understand real-life negative gravity using classical physics;

the stuff you probably learned in high school.

Now, I know the idea that gravity acts on sound at all might seem a little sketchy,

because sound waves aren't some special kind of matter.

They're just vibrations:

the scrunching up and stretching out of molecules like those in the air.

Still, scientists argue that you can think of them as particles.

Kind of like you can think of light as particles, or photons.

In sound, though, they're called phonons.

Phonons aren't like electrons or molecules or any other particles you're familiar with.

They're essentially packets of energy moving at a similar speed.

As a sound wave passes through the air,

molecules speed up as they squish together and slow down as they spread out.

And a phonon is a tiny packet of those vibrations.

So it's not exactly a single physical particle.

It's more like a flock of birds.

A flock is just a bunch of individual birds, but you can still identify it as its own unit.

And that's the idea with phonons as well.

It's like a “flock” of vibrations that emerge from a sound wave.

The weird thing about these phonons is, according to theory,

they move upward in a gravitational field.

It might seem like that just shouldn't happen; that's not how gravity works.

But the reason is actually pretty straightforward.

Picture a sound wave moving through the air.

Air pressure is slightly greater at the bottom of that wave than at the top,

because the lower air is denser.

It's just like how water pressure is greater near the ocean floor than it is near the surface.

Since sound vibrations travel faster through denser fluids,

the bottom of the sound wave, where the air is under a lot more pressure, travels faster.

And that makes the whole thing bend up. Just ever-so-slightly.

That means the sound wave, and the phonons that make it up, are going against gravity.

In other words, phonons have negative mass.

Because anything with a positive mass would get pulled down.

This is an actual, physical effect, not just an illusion.

It's not like an airplane taking off, or a bird flying, either.

Gravity is still pulling down on those things, even as they move up.

But sound waves are actually falling up.

And that comes with some pretty wild consequences.

The first thing is, over large enough distances, all sound waves should curve upward.

This probably wouldn't have any real-world impacts,

like, it's not going to change how we communicate.

The amount of predicted curving is so small

that we don't even have instruments sensitive enough to detect it.

But scientists think this effect could be more relevant in objects like neutron stars,

where sound waves travel through super-dense fluids.

There, phonons could significantly affect the star's behavior.

But that's not the end of the story, either.

Because on top of defying gravity themselves,

sound waves should also push away anything with mass.

See, anything with mass has gravity.

And not just big things, like, you have your own gravitational field.

And so do phonons.

Except, since phonons have negative mass, they also have negative gravity.

Meaning they should repel anything with mass.

Again, we don't have the technology to measure this kind of effect yet,

since gravity is pretty weak on a microscopic level.

But this is still fascinating.

And this weird behavior is all based on pretty simple, old-school physics;

stuff that's been sitting right in front of us for literally hundreds of years.

So, what do you know? It turns out, now and then, old physics can do new tricks.

If you're the kind of person who likes using what you know to figure out

new things about the world, you might also like courses offered by Brilliant.

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The courses are hands-on, with interactive quizzes and guided problems with explanations.

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To find out more, head over to Brilliant.org/SciShow.

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you'll get 20 percent off an annual premium subscription!

And as always, thanks for watching SciShow.

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