So I guess we have to start with the basic physics, which is the uncertainty principle.

It's this weird thing that you can't measure the position and the momentum of a particle at the same time.

And actually, for this video, the important one is you can't measure the time of something and the energy of something at exactly the same time.

So if you want you got a photo on, you can measure exactly when it arrives if you want.

But then you don't know what its energy is, or you could measure exactly what its energy is, but then you don't know what exactly what time it arrives.

And there's a fundamental uncertainty in that energy, which works even if the photon has zero energy.

So if the photo on fundamentally has zero energy, then there's an uncertainty that actually you can't tell whether it's zero energy.

It could actually have some energy.

So that means, and a pervert on zero energy isn't there at all, basically got nothing there, and that means that even when there's nothing there, you can actually have a photo on with suddenly acquiring an energy.

And that leads to this weird effect that even in the vacuum of space, you can have photons popping into existence for a brief period of time.

So basically, you can sort of cheat nature.

You can borrow energy as long as you put it back before anyone notices.

And that's this.

Trade off between the amount of energy you've got on the amount of time you've got on them or energy you borrow, the less time you can borrow it for.

And that means that the vacuum of space is actually deceiving massive particles and anti particles being created.

There's some rules so that you have to kind of create particles in pairs.

But beyond that, even in the vacuum of space, you can have particles popping into existence and disappearing again.

So this idea that vacuum is empty is kind of is actually not correct, right?

That even in vacuum, you have these virtual particles that appear and disappear.

So it applies to every particle, absolutely anything you know.

So I was talking initially about photons, but you could do it with electrons and positrons appearing.

You could have protons and antiprotons.

You can have anything you want, but obviously the more massive the up the particle is remember, from Einstein's famous equation E equals M.

C squared.

The more massive the particle is, the more energy your borrowing to create it, and therefore the quicker you have to put it back before anyone notices.

So the least massive particles could exist for the longest period of time couldn't create like a planet or a gold.

In principle, you could create anything.

But of course, by the time you get upto what on the cosmic scale of things that they have very large masses, then they disappear on such a short time scale that really that, you know, they effectively don't exist.

It sounds like it's made up, but actually there are experimental tests you can do to show that a vacuum is actually receiving massive particles.

It's not empty space.

There is this famous experiment, ah, phenomenon thing called the cashmere effect that was predicted way back in the 19 forties that says that if you do an experiment where you've got two plates very, very close together.

So about a micron apart, a 10 to the minus six meters apart.

Then actually, what happens?

Well, well, you've got this evening mass of photons being created out here by this uncertainty principle.

And you've got photos being created in the gap between two plates as well.

But the interesting thing is, because you've now got this pair of plates that actually restricts the kind of photons that you can create.

You can't, for example, create like with very long wavelengths because it wouldn't fit into that gap.

So just by having a pair of plates, Although this effect is still going on, you create fewer particles in the gap in between.

Then you do out here.

Now, the photons out here bouncing around than hitting the plate exerted pressure on it.

And the photons in here bouncing around that are being created by this virtual effect created pressure this side.

But of course, now we're creating less photons in this gap, which means a bigger pressure this side than there is on this side.

So there's actually a force, and this force is now be measured that you can actually detect this force.

You set up this experiment, you can actually detect this force and the plates actually get pushed together by this fact?

Call the Casimir Effect, where you're actually seeing this effect off the virtual photons being created deceiving massive virtual particles, but just more of them on one side of the plate than the other.

It explains things, but the main thing it does is create a huge problem.

Because if you are a theoretical physicist, you could do a calculation that says, Okay, so I'm creating all these virtual particles they've got mass associated with them.

How much mass?

Um, I actually creating in the universe that just from this stuff popping into existence.

And the answer is, unless you're very careful how you do the calculation, you end up with an infinite mess.

So it's actually one of these problems that theoretical physicists have that if you do the naive calculation of just as well, let's let's let this affect go.

What's gonna happen?

And the answer is, you end up with the universe that just doesn't work because you end up with infinite mass density everywhere and the whole thing.

You get infinities appearing in your mathematics so theoretical physicists have to duel this clever maths, using a process called re normalization, which basically is a way of kind of shoveling all those infinities under a rug or not worrying about that anymore.

I get the impression this must be a very faint phenomena because, like, for example, when a rocket ships going to the moon, that's not it's not running into a barrage of particles that keep popping into it.

Well, it is.

But actually, the year you're right, the impact it has on the rocket is tiny because the forces involved a very tiny and that's this effect of this again, if you just the one night of calculations that says How big should this affect B If you're not careful, you'll end up predicting that should be a huge effect.

And somehow it ends up being normalized down to this very small effect.

But it is your right, is it?

It's a pretty small effect in everyday life.

Okay, you promise me black.

Okay, so we need to do another thing before we get to black holes, which is that was the cashmere effect.

Now is a thing called the dynamical Kazimir effect, which says that Okay, so if I got this pair of plates, then this phenomenon goes on and we have these virtual particles being created out here and on either side.

But if you're in here and that's creates the force now the dynamical Kazemi effect says that actually, if you think about the virtual particles being created in here, if I accelerate one of the plates very quickly so instead of just having them sitting next to each other, I move it backwards and forwards very quickly.

Some of those virtual particles become riel.

So actually, the things which were popping in and out of existence, some of them actually become real particles.

And there's two ways to think about this either.

You can think about it of saying that, actually, when you rent them apart that these little pairs of particles will pop into existence and then re combine and disappear again.

They kind of lose track of each other because they kind of get pulled apart as well.

They lose their twin aural turns of you want a dollar explanation?

You can say that actually, the interaction of the kind of the electromagnetic field you're creating in this space and the metal plate when you accelerate it ends up creating photons from the surface, and the photons are then emitted from the service.

But either way, in some sense, the energy that was originally in those virtual photons in the Gap gets converted into real photons and you haven't cheated.

It seems like you're getting energy for nothing now that you know, you haven't paid back the debt.

But actually you have because in order to accelerate this plate, that's a force, remember?

And when you accelerate something against the force, you end up doing work.

So actually, just pulling the plate backwards and forwards puts energy into the system.

So you're not cheating nature.

And for a long time that this was thought to be kind of.

This was just the four experiment, and the trick is, the problem is from an experimental point of view that actually you have to move the plate of relativistic speeds.

So the plate that you're moving backwards and forwards has to move at some decent fraction of the speed of light, which you just can't do with real plates.

But actually, just a couple of years ago, someone did a very subtle experiment where they, instead of having a pair of plates and a gap in between.

They kind of created an electronic analog of this, which is they had a wave guide, which is just something that kind of forces photons to traveling in one dimension on.

At one end of it they put one of these things called a squid, which is a superconducting device where by changing the properties of the squid, you can effectively change the length of the wave guide at whatever speed you want.

So they could actually drive it at, like about 5% of the speed of light.

So get up to a speed where actually they were effectively changing the gap, things kind of cavity of those kinds of speeds.

When they did that, they found that this cavity started admitting microwave photons.

So actually, the effect has finally actually being detected in 2011 for the first time.

So that then gets us on two black holes, A black hole.

Obviously, you know, you haven't got accelerations in a black hole, but you've got gravity.

And there's this thing in general relativity called the equivalence principle that basically says that the way gravity behaves is indistinguishable from the way that acceleration behaves And actually, anyone who's ever stood in the lift is familiar with this and that when it starts going up when you start getting accelerated upwards, you feel heavier on.

Actually, you've no way of knowing whether the gravitational field, unless you could look out through the window.

You know where know whether the gravitational field of the earth has increased, which is way fuel heavy or where you're being accelerated, which is why you feel heavier.

So everything that you know, basically the physics of acceleration on the physics of gravity, is very similar.

So in a in a black hole, you don't have accelerating plates.

But you have gravity, plenty of gravity.

And exactly the same thing happens close to the event horizon of a black hole where you have these gravitational field that basically the virtual photons that are popping into existence and disappearing around the black hole lead to real photons being produced.

Now again, you can think about this two ways.

Either you can say that this interaction of the electromagnetic field with the event horizon of the black hole makes the event horizon emit photons.

Or you can have the rather more romantic picture that these pairs of photons are being produced and then disappearing.

But actually, once in a while, one of the pair falls into the black hole, and then the other photo on is kind of doomed to never find its friend ever again and has the wonder of the universe forever as a real particle.

But it is exactly the same physics that is, what's going on now here, then the question is, how do you repay the debt?

Because now you've created energy, and now you no longer have acceleration.

You no longer have things going.

Moving back and forth is actually only one place that the energy can come from, which is from the mess of the black hole, because the energy is mass times the speed of light squared, so the mass of the black hole is a source of energy.

So what has to happen bizarrely is you've got something falling into a black hole makes the mass of the black hole decrease over time.

And this is this process of a black hole, effectively admitting photons and in the process of omitting photons, losing mass is this thing called hawking radiation on why black holes eventually, if left to their own devices will evaporate.

I gather from that that all black holes are doomed to evaporate away because they can't help this thing happening at their event horizon.

But there's so that's certainly true.

But there are two things to bear in mind.

Firstly, it's an incredibly slow process, which means that actually, you know, black holes, which formed very early in the universe unless they really low mass, black holes won't reach that stage even today.

And the other thing is that actually, that's how black hole loses mass.

But, of course, the Black Open always gain mass.

Just buy stuff falling into it on almost certainly most black holes in the universe.

Is Maur stuff falling in that then?

Then they're actually losing by this evaporation process.

So, actually, even though the process is always there, probably most black holes are actually putting on weight rather than losing it.

I mean, it's never been detected directly, so in that sense it's just a theory.

But actually, it's a pretty solid theory in the sense that all the physics that goes into it is we reasonably comfortable.

There's something slightly uncomfortable about this whole area of physics because you're combining gravitation with quantum mechanics.

You got the gravity of the black hole.

You've got the quantum mechanics of these virtual particles.

And there is.

It's well known that there's a missing piece of physics that we don't really understand how to put quantum mechanics and gravity together.

But from the limited understanding we do have, it looks like this is a pretty solid result that actually black holes will indeed evaporate by this process.

You got a little detective here, So in simple terms, all you're doing is you're firing some light out there, bounces back, gets detected and then you got a little thing that says, OK, Whenever I detect some, I'll send another pulse up.