Great firewall debate about black holes, and he's come to the conclusion that black holes well, basically the event horizon, which essentially defines a black hole they don't exist.

He was actually one of the 1st 2 demonstrate possible problems in terms of what it's known as the information paradox.

But, Sir, there's been work recently over the last two years by people mainly in this West coast of America who have been suggesting that the black hole itself may not be a straightforward is was initially thought that maybe this circles firewall we know that that black holes, you know, they're not as black as they seem, that they give off this radiation powders hawking radiation.

This is the kind of radiation that you get from black holes.

How How does this appear?

Well, it appears when you apply quantum mechanics to a black hole, so you can imagine near the event horizon of the black hole, which is like sort of the point of no return, if you like.

Near this event horizon, you get two particles created like a particle anti particle pair, one just inside on one just outside.

Now these air we call entangled with one another.

Now the guy outside escapes the guy inside falls into the black hole singularity.

This is where everything just crunches up.

By the time that the black hole is evaporated completely, you've completely lost knowledge of the guy that fell in.

That information is gone.

So you end up with an end state, which is basically you go in, you begin at the beginning of its life.

The star that's collapsing to form this black hole is described by what we call a pure state of pure quantum state.

But by the end of it, by the time the black hole is evaporated, you're you have what's called a mixed state, this information being lost.

It's not pure anymore on that.

Information's essentially lost to the singularity, is locked in the singularity but is an outside observer.

You just see information is gone on.

This violates one of the really sort of key principles of quantum mechanics that called unitary ity, that essentially what goes in must come out.

People were thinking about this and it leads of ideas like black hole complementarity that actually you don't really lose information that it's essentially get a duplicate copy of it, but no observer can actually see both copies.

You know, you know, no observer can actually access both copies.

And so there's no violation of what's called a no cloning theory and these sorts of things that you can't make copies of quantum information like that.

And then along came the firewall, okay, and this is the notion.

The black hole event horizon isn't just a place in space where nothing much happens.

But actually, if you cross an event horizon, you'll burn up.

Andi.

It just seems strange, it seems, to a stranger many levels, but one of them, it seems, to break things like the equivalence principle of relativity.

I mean, we discussed in another video, so no point.

Going over again, right?

As always, is a leak there.

If the black hole is old enough, it has to be an old enough black hole.

That horizon will turn into a firewall.

Where does this fire will come from?

And hawking, I think, has just decided to try and understand what might be going on, especially said it's not there.

A lot of people don't like firewalls, and to be honest having some time passed since our first video.

Brady.

I'm sort of sharing this slight dislike for firewalls.

I think at the end of the day, you've got a situation where you really have a very weak gravitational field at the event horizon.

It shouldn't be a special place.

And yet you having this very dramatic thing happened that you would just bear not without knowing about it.

It doesn't really fit.

The claim is this that you would you would approach the black portal event horizon, not experiencing and anything much, and then suddenly, bang, You're bent up and it just doesn't sit well.

But the prices paid for saying it's not there.

Is that its coverage of the event horizon as well?

So he's got rid of the underlying definition of a black hole, and it's led to press releases everywhere.

Saying Hawking says black holes don't exist.

It's fair to say that original authors of the fire wall paper have sparked something can have sparked research and that's fantastic.

Hawking's way then, and he's very anti fire will.

Hey gives this paper is very short.

It gives a bunch of reasons why he doesn't like the fireball and then come to the conclusion that his resolution of it is that event horizon don't actually form.

So I mean, I can go through a couple of his arguments against the firewall.

One of them says that basically, he looks.

It's a black hole in and what's called anti to sister space.

It's a bit like this sort of familiar space down, but it has this this negative sort of energy pervading through what's called a negative cosmological constant.

If you take anti the sitter space and you put some energy in it, depending on how much energy you kick it by, you can either have some radiation.

Or you can form a black hole because anticipates a bit like a box so that radiation will eventually it can, in principle, gather to form a black hole.

So what he says is, he says, what's the what's the configuration you would expect?

Have I kick this this this situation?

What's what's what's the configuration?

I would expect what I'd expected, in essence, as sort of superposition of both sonar is the radiation in the black hole in principle.

But what he also argues is that the situation should be symmetric under something called Cpt.

And through this he argues that it should be there should be the collapse of the back whole should be the time reverse of the evaporation of the black hole.

Okay, so if the collapse of the black hole in the beginning, if you think about the initial stages of collapse, there's no event horizon.

Okay?

It's not some points of phones, but originally there isn't one.

And so if the evaporation is the reverse of that, in some sense, then there shouldn't be an event horizon at the end.

And I think what he means is, I don't know, I asked him, but I think what he means is they don't You've got to redefine what you say is a is a black hole Is not, for example, suggesting it find if everyone turns their major telescopes to the center of our galaxy, we're not going to find an object which is behaving Justus.

If it's a supermassive black hole, it's still there.

It hasn't disappeared.

Sit talking, right?

It's paper.

But whether or not it's got an event horizon is ah is a matter of debate.

Is he's replacing this event horizon with this, where he's called on a parent's horizon.

So what is an event horizon?

An event horizon is to find in the following way.

It's a region of space and time from which you cannot reach somewhere called future Noel Infinity Fusion.

All infinity.

Intuitively, it's really just infinitely far in the future, infinitely far away Earth kind of what it is.

Maybe we should make a distinction between an event horizon and what Hawking says it's replaced by which is something called an apparent horizon.

To all intents and purposes, look pretty close to an event arise, and there are still regions within which light sort of goes into what you would call the black hole.

But the slight difference is it.

Light can escape from this over over a long, long period of time.

It's It's a kind of meta stable configuration, and so it is different from the event horizon.

In that sense now, the key difference is the following.

Basically, an apparent horizon could disappear.

Okay, so you would, when you look at a parent highs and the thing about an apparent horizon is, it's it's Ah, it's dependent on an observer on a set of coordinates.

Okay, so you take her slicing a snapshot in time and you look at your your space on you realize that there's a place where a ll the light rays are bunching up, so there's kind of a place where light can't get across.

Okay, that's your apartment.

Arise and okay, that's not the same as an event horizon.

An event Horizon says that I look at the space time global.

I look at it overall time and identify those regions of space and time from which I cannot reach this future.

Nall Infinity place.

Okay, And that's not the same thing is the apparent horizon.

Funny thing about an event horizon is that that it requires you to know about the whole space time?

It's kind of weird.

It's sort of you need to know about the future.

It that way.

It's more might worry about causality, but actually there's no issue, really.

But it's a kind of strange property is defined by some boundary condition in the future.

We're taking a snapshot in time.

I'm saying this what looks like an event horizon, but it's not really it's an apparent arising where the light rays are bunching up.

I see you cross it.

I think he has no what?

You're coming back.

But I hang on and I hung on and I hang on and hang on and eventually it's a partner.

Rise and disappears and you escape.

That's it for me.

The most exciting thing about this current to bed that's going on of which have you can tell I'm not an expert is that people are really using these environments of strong gravity to actually test out theories of gravity and quantum mechanics.

And so that's the crucial element here that has led to all of this that they're trying to properly incorporate quantum theory quantum mechanics on dhe.

They're suggesting things get modified either through the creation of these firewalls, which seemed to be emerging.

Or if you don't want the firewalls, you you have you getting rid of the event horizon so that the true classical solution isn't necessarily the solution in a In a theory, including quantum theory.

Well, actually, that's probably no massive surprise problem in the paper.

This is no calculations, Okay, so it's all very nice, but to say, Oh, everything's chaotic inside the apparent horizon and this prevents me from predicting what's going to come out bloody, bloody, bloody or very nice.

All very I mean, he has great physical insight is no denying that.

There's no calculation in here, though, and it's sort of like, What's he really saying?

I mean, you need you need to see the mass before you can.

You can see that whether or not Stevens managed to really determine what the true outcome is of this situation, it remains to be seen.

It's a very short paper with ideas with the bulk of the paper is just reasons why you shouldn't believe the firewall ideas on then, right at the end, he has a little section on where he says he has a resolution of his own in which the ends up getting rid of the event horizon, replacing it with this apparent arrives.

And so this thing still can look like a compact object, which is very massive.

In other words, like a black hole before King was right and we don't have event horizon, we have apparent horizons.

What does this mean for the way we think of black holes is as beasts?

Well, I think you would even call them black holes anymore.

I think that's the bottom line.

If he's right, then yeah, there's no such thing as a black hole.

I want to find a black hole by the by.

The existence of an event horizon okay is formally, as in light, Cannot not even like nothing could escape from this from a particular region of space time to this future Nall Infinity place.

There is no such place now.

There is no such a region of space time, which which can't reach future nor infinity.

Now that's just doesn't exist anymore.

So therefore, there's no black hole.

There is a lot Lot could get out one day.

Yeah, it just has to wait.

Okay, well, I mean, I think this is a pretty radical notion he's saying here, but there are no equations in this paper.

It's full of ideas on dhe.

I think until their equations there that we can we can look at and solve and try and understand how we're incorporating the quantum mechanics with gravity.

Then it still remains a bit hoping Stephen Hawking made his reputation on black holes.

He did.

He did, but well, yeah, yeah, yeah, he is, but, you know, I mean kind of it.

Is this not part of the story, right?

The that he's developed such a great understanding of black hole physics that he is, that it, you know, he can make the seat, make these insights.

He refers to a number of papers to justify Why not to believe the firewall on Does do have calculations in.

But this particular idea, where he gets rid of the event, arrives and replaces it with an apparently rising I presume in the back of his head.

He's got some equations that he's thinking will do it for him, but he probably just doesn't manage to, you know, get them down here.

You know, as you know, it's difficult for him to write down anything, and so he's probably thought through it in his own way.

I think Steven's a very intuitive kind of physicist.

He's he probably knows the answer before he's even bother solving the equations.

That's the way probably thinks his way through it.

Firewalls have sort of friend of a puzzle, and people called radical, really radical suggestions that these these entangled states are connected by air by wormholes stretching right through through through a space.

It's yes, but this is what this is.

The beauty.

The file is really got.

People imagine it imaginative on, and Hawking has gone up and said, no black holes would be best to say that Black Hole's temporary, that their temporary well hawking, of course, showed their temporary.

He showed that they evaporate, which is one of the for me.

That's his magic.

Major contribution is just such a staggering result has led to all of this that they were always thought black holes suck right there.

Just everything collapses in.

And he then demonstrated when you include quantum theory, actually, no.

They evaporate.

They lose everything s O that we've always known.

They're temporary, but with their temporary long time scales, he's he's saying effectively again.

This is another way in which their temporary they got a parent horizon and light can escape.

Eventually through that what we previously thought.

So now you've got this little bit of radiation just outside the stretched horizon.

There's nothing special about the event horizon.