[APPLAUSE]
JENNIFER: Thank you so much. I am going to open by saying that, I am a huge, huge fan of physics history;
I am well-aquainted with this famous room, and I am completely intimidated and unworthy.
But we are all here for the same reason; because we love science, and I am especially honoured that
I get to give the first joint talk with my own husband, Sean Carroll.
This is the kind of thing we talk about at home, and it's the first time that we've actually both talked about this topic.
So, you are kind of in for a first.
Ultimately, this is a story about Alice and Bob. Alice and Bob, for those of you who don't know,
are kind of fixtures when it comes to thought experiments, particularly those involving information theory,
cartography, and especially quantum information in cartography.
It just seems a little more fun to play around with the ideas of these actual two characters, Bob and Alice,
and the different adventures that they get into.
The subject tonight, Black hole Firewalls, is another kind of thought experiment that physicists started to play around with,
around the questions of what happens when that daredevil Alice, who the the more risk taking of the two,
jumps into a black hole.
You know, Bob kind of stays behind.
Now, there's been a traditional way of thinking about this, sort of standard textbook, popular science, physics stuff for decades,
that we thought we knew pretty well what happened,
that she would be ripped to shreds by the power of gravity at the singularity,
that point of infinite density at the black hole centre.
But when the physicists started playing around, in the summer of 2012,
with some of the big ideas of one of the other thought experiments that deals with information laws,
they actually discovered a paradox. They discovered that it was possible that instead of being ripped to shreds by gravity,
Alice might be burnt up alive in this enormous wall of radiation just in the event horizon. She never even gets the chance to be ripped apart to shreds.
So this talk is about that. And we are going to start off easy and slow,
but it's going to get mind-bending towards the end.
The good news is, you have Sean Carroll doing the really mind-bending stuff and he is a tremendous communicator.
So, a paradox, for those of you who want a definition: "a statement that apparently contradicts itself and yet might be true. "
In physics, they are quite commonly used; this is a maybe not that specific definition. It is more that they are revealed by though experiments,
and they are used to eliminate certain counter-intuitive truths, to reveal flaws in the assumptions or in the logic behind some of the reasoning in physics.
So there is a long tradition in this, and it goes all the way back to the Zeno's paradoxes, including Archilles and the tortoise,
how the fleet Archilles can give the tortoise a head start, and although Archilles can run so much faster he never quite catches up.
It became a foundation of calculus, figuring out what the limit was in calculus.
Maxwell's daemon was used to illuminate thoughts on entropy and thermodynamics.
In special relativity, Einstein came up with a Twin paradox: what happens when one twin stays behind on earth,
and the other twin zips off into space, at speed of light. When that twin comes back, he will be much younger than the twin he left behind.
That is a paradox, and it illuminates certain factors of special relativity.
Tonight, we are going to focus on a lot of the quantum mechanical paradoxes. Sean will be telling you about the Schrodinger's cat paradox.
He will be mentioning Heisenberg uncertainty principle. Central to this entire thing, is the Black hole information paradox.
It is in fact, the thing out of which Black hole firewalls came.
But first, a little bit on the history of Black holes. I'd like to do this because, as I said, I love physics history.
And I also love this guy, John Michell.
He was basically an English rector in the 18th century. Common descriptions of him at that time basically stated that
he was "short swarthy and fat".
But he was known as a brilliant thinker, and in fact, he was far more brilliant than his contemporaries realised,
because he was literally 150 years ahead of his time.
He was trying to figure out, one day, how you could calculate the mass of a star.
And he came up with this method, you know, thinking that he could basically measure the reduction of the speed of light that was being emitted by it.
And his thinking was very Newtonian. Remember, at this point in time, the debate over whether light was a particle, a wave was still going on.
He very much described, in Newton's notion that, light was a particle, or a "core puzzle", as I think Newton called them.
So he thought that gravity would slow down the light, and he could basically measure how much the light was slowing down;
he could use that to calculate the mass, through a series of calculation.
He also had another insight: when he actually started doing the maths, he realised that at some some point,
gravity would become so strong that the escape velocity would exceed the speed of light.
The escape velocity is what happens ---- say when you fire a cannon, and it was a famous thought experiment by Newton to describe gravity - how objects orbit.
They are basically in free fall, if you escape the earth's atmosphere, you'd go into near-earth orbit.
An object, a space station, satellite are just constantly in free fall around the earth.
But you need to be able to get beyond that; you'd need to be able to escape that part.
So what happens if light can't escape? Then we wouldn't even be able to see the star.
So he called it a "dark star", and he also had another key insight:
he thought: well maybe you couldn't see it directly, but it might have a twin star near it that was bright,
and you could look at that star, and kind of figure out that there was a dark star next to it.
He was actually wrong about how to calculate the mass of a star, because as we now know, the speed of light is constant, that's thanks to Einstein.
so there is no way you could have used that to calculate the mass of a star.
But he was absolutely right about dark stars.
Unfortunately, he dies in complete obscurity; nobody took him seriously - they thought he was crazy.
This was so ahead of anything anyone had ever thought about, that they thought there's no way that he could possibly be right.
He was not the only one, however. Pierre Simon LaPlace actually came to a similar conclusion a few years later,
and he talked about it in his book: Exposition du Systeme du Monde, where he basically concluded that it is possible
that "the greatest luminous bodies in the universe are " on this account "invisible".
And again, he also was right.
So it would basically take another 100 years, or over 100 years before we finally had Albert Einstein
re-envisioning space-time.
Remember, in Newtonian days, they thought of space as three-dimensional, Euclidean geometry space,
and it was static; it didn't move.
Einstein kind of re-envisioned it.
There were three dimensions of space, and the fourth of time, and they combine to give you space-time.
It became dynamic.
It could twist and warp, and the things that did the warping and twisting were the massive objects in space.
And that is where gravity comes from. Gravity is less a force and more say,
objects and light following the curvature of space.
It was revolutionary; I don't think I need to tell any of you that.
It changed everything, and among the lives it changed was a physicist named Karl Schwarzschild.
At the age of forty, he enlisted to fight in World War One, even though he was much older than some of the men out there.
And it was, as you might imagine, a dreadful time in the trenches, dodging bullets, bombs, bayonets, and the whole bit.
And one of the ways that he escaped the horror of the war was to do physics.
He came upon Einstein's theory of general relativity, general theory of relativity.
And he became fascinated. In his spare time, he actually started working with some of the equations.
He was basically trying to do something similar to what Newton did.
Newton would measure the distance between the centre of mass of an object and another point in space.
and as you got closer and closer, eventually you would get down to a point.
Schwarzschild was trying to calculate the curvature of space. He hit a roadblock too, but it was well below zero.
He basically hit a point where the equation just blew up.
Nobody knew why. They didn't know what that point was.
Today I think we would call it the event horizon of a black hole.
What we think now is that there might be something beyond that point,
even though the equations stop at that point.
The good news is, John Wheeler dubbed these objects "Blacks holes" in 1968,
and shortly after that, we found compelling evidence that in fact, they do exist.
The first object that they discovered that could be a candidate black hole, was this X-ray binary star Cygnus X-1.
They found it, or they determined it in 1971, that there was no way it could be a neutron star.
It was far too dense, and it was probably going to be a Black hole.
And we know it was a binary star system, so Michell was absolutely right.
They could tell the Black hole was there based on its companion star and its behaviours.
This lovely animation here is based on actual data Sean found on the internet, a beautiful place, the internet.
This is the black hole, at the centre of our Milky Way  Galaxy.
What they had done is basically extrapolate well advancing through time, into the future,
to look at how the presence of the Black hole affects the movement of the bodies around it.
So Black holes, they are real, they are out there, they are probably at the centre of most galaxies.
So now, we get back to Alice.
Because really this is all about Alice.
Bob is kind of a smaller player on the side there.
Black holes have some really interesting properties.
We know that they have immense gravity, but what happens when she crosses the event horizon?
The event horizon is what Schwarzschild unknowingly discovered.
It's basically the point of no return; once you cross that point, you are done; you are not getting back.
No light can escape, no information or anything can get out of that.
Alice doesn't know it, because she isn't going to noticing anything unusual
as she crosses it, but she is pretty much doomed at that point.
She just might not know it for a bit.
Why is she doomed? Well, it's something that physicists like to call "no drama",
and it's based on Einstein's equivalence principle, this notion that gravity and acceleration are essentially the same thing.
What that means is, an object in free fall, and Alice is in free fall, does not feel the effects of gravity.
She's not going to notice any extreme effect. She is just going to be floating along, thinking that space is the same as it's always been.
Until...
But first, that's from her perspective.
Remember that we are taking about relativity, which means that frames of reference are important.
Bob watches her slightly form a distance jumping into a Black hole, but what he sees is a bit different.
He sees her getting slower and slower, because time slows down. Time dilates as you get closer to a black hole,
because of the extreme effects of gravity,
As she approaches the event horizon, she will seem to be moving slower and slower,
and right at the end, she is just going to seem to be frozen in space to him.
and that's where time ends for him, as far as Bob is concerned, and he doesn't really know what happens beyond that point.
For Alice, it's bad news,
because there is an evil singularity lurking there at the centre.
She drifts along, and she drifts along; she is in free fall, and after a while she notices an uncomfortable tugging.
that she's being stretched.
That is because she is falling feet-first towards this singularity.
At this point there is really only one way she could go, and it's towards her doom, towards the singularity.
it's just sort of like a...That's the only way she can travel.
The gravity on her feet becomes stronger and stronger, much much stronger than the gravity on her head.
So it keeps pulling and pulling, and she gets longer and longer
until she's basically...I think Kip Thorne called it being spaghettified.
Finally when she gets close enough to the singularity, she is just ripped to shreds,
and there is nothing left of poor Alice.
Again, poor Bob never knows the horror that awaited his partner.
Of course, this is not the end of the story.
This is a classical view of a Black hole according to Einstein's relativity.
Things get really complicated with quantum mechanics, and quantum mechanics actually is relevant here,
because there are two different rule books for different size scales when it comes to physics.
General relativity governs the macro-scale, the very large, the movement of the planet, things like that, you know, everyday physics.
Quantum mechanics governs the world of subatomic particles and the very small.
Those of you who are familiar with the discovery of Higgs boson will know that
that was the last piece in what's known as the standard model of particle physics.
That model is now complete.
Except for one bit: that model doesn't include gravity.
Gravity and quantum mechanics just don't play well together,
and physicists are yet to devise a quantum theory of gravity that works.
Blackholes, because they are basically where relativity and quantum mechanics meet,
that's why they are tremendously interesting to physicists.
So surprise. this is something we find out when we start to apply quantum mechanics to black holes.
Stephen Hawking, playing around with this in 1970s, came up with this idea of Hawking radiation.
Turns out that Black holes are not completely black.
That's partly because, empty space, when you are talking quantum mechanically, is not really empty.
Space-time is really...you've got these virtual particles that are popping in and out of existence,
antimatter and matter, Alice and Bob,
and what happens when they meet is that they annihilate into energy almost instantaneously,
so they don't violate the conservation of energy; they are not around long enough.
Nonetheless, space-time at the quantum level is this kind of bubbly frothy foam, so to speak.
So what happens when these virtual particle pairs, Alice and Bob,
pop up into existence right at the event horizon of a Black hole?
Alice is just a little bit closer to it, and she falls in, and Bob doesn't?
What happens? What happens is, she basically disappears.
It seems that the other particle Bob is light being emitted as radiation from the Black hole,
and the Black hole will lose mass as a result.
Black holes not only emit these very faint Hawking radiation, it evaporates over time.
A physicist named Don Page, who is a very charming man,
he's one of the people who just loves counting things.
One of the things he counted was essentially what amounts to the half-life of a black hole.
That point at which the black hole has radiated away exactly half its mass,  is called the Page time.
It's kind of like the Mid-life Crisis for a black hole.
It is relevant to the Firewalls debate.
Hawking radiation has some very troubling implications when it comes to information theory.
and specifically what happens to information that falls into a Black hole.
This is where we get to the information paradox. Sean will talk more about this in depth as it pertains to Firewalls.
I'm just going to give you a brief overview now.
It is very simple. We know that nothing can escape a Black hole.
So any information that falls into it, that Alice carries into it, should be lost.
There is a problem though, because there is a key precept in quantum mechanics,
that says that information must be conserved.
It cannot be lost.
Stephen Hawking decided that this was wrong,
and he put forward this very radical notion that information was lost.
I think his famous quote is: "Not only does God play dice, but sometimes he confuses us by throwing them when they can't be seen. "
Kip Thorne, one of his colleagues, agreed with him.
John Preskill, another physicist, did not, and they made a very very famous bet in 1990s,
where Hawking and Thorne basically said information is destroyed.
They said, OK technically you could say that it gets radiated out by Hawking radiation,
but it's so scrambled that when the Black hole finally evaporates completely,
there is just no way we could retrieve that information.
Basically, you are throwing a book into a bonfire, and it gets burnt up into complete ashes,
and then the ashes just integrate, and then there's nothing left.
What do you do? That information is gone.
Preskill just didn't like this, so he basically took the opposite side of the bed.
He said that information could be recovered in principle,
but he felt that physicists would need to devise a theory of quantum gravity
to really understand the mechanism by which this could occur.
We don't have a theory of quantum gravity yet, as I mentioned.
But Hawking did, in fact, concede the bet in July 2004.
This was actually based on some work by string theorists, again Sean will talk bout it more in depth.
It's called the holographic principle.
They basically found a cheat.
The cheat is, as long as Alice and Bob can see two different things or have their own little bits of information,
as long as they don't communicate with each other, it's OK.
Also, the information gets encoded not necessarily in the Black hole itself, but along the event horizon.
It's a question of volume, so 3D becomes 2D.
and that's a little trick they used mathematically to show that it was possible for information to be conserved.
What happened was Hawking presented Preskill with the eighth edition of Total Baseball: the ultimate baseball Encyclopaedia, "from which information can be retrieved at will".
And it was a big deal. Preskill was very happy.
But Kip Thorne refused to concede.
He basically said:  no there's more to the story. I don't think we have it.
It turns out that Thorne may have been correct not to concede,
because today we have a new paradox, and that is Black hole Firewalls.
This is something, again a paper that Joe Polchinski, a string theorist in Santa Barbara and Don Marolf
and a couple of their students came up with in the summer of 2012.
They were playing around with the information paradox,
and Joe decided that he was going to try a little fun thought experiment,
kind of run Hawking's thought experiments and a lot of thought processes backwards,
just as a way to double-check, and make sure his logic was good.
What he discovered was, there was an anomaly.
It was basically a logic problem that physicist Raphael Bousso calls it "the Menu from Hell".
What it basically says is that, there are three fundamental precepts
that physicists know and love and hold dear.
And what Polchinski at all found was that all of those three things cannot all be true.
They simply can't. You've got to pick one to give up.
One of those, which we already talked about, is "no drama". It's the equivalence principle,
the fact that Alice notices nothing unusual as she crosses the event horizon.
We would really like to hang onto that,
for reasons again which you will hear more of.
The second one is that we want information to be conserved.
We really want it to be conserved; it's a fundamental principle of quantum mechanics,
and if that's wrong, all hell breaks loose in quantum mechanics.
The third thing is Locality, which is a little harder to describe,
but essentially, if you picture dropping a pebble in a pond, and it starts to ripple and it ripples outwards.
The basic activity of dropping a pebble happens locally;
it doesn't happen everywhere at once, and that it spreads out.
That's locality.
Locality is actually one of the foundations of physics.
It's really really central. So central that it's hard to think of...certainly classical physics without it.
Which one has to go? Nobody wants to give away any of these.
I actually had the privilege of going to a Stanford workshop shortly after the Firewalls paper was published,
the first Firewalls paper,
and watching them fight it out.
It was fascinating to watch, because nobody knew the answer.
They still don't; they are still arguing about it.
But the most obvious, easiest solution to it is to get rid of "no drama",
and when you sacrifice "no drama", you get a Firewall,
a wall of radiation at the event horizon,
and poor Alice gets burnt alive.
I personally would like to save poor Alice; I don't know how we are going to do that.
I am hoping that my husband, Sean Carroll, can come up here, and take you through the next stage of the talk.
Hopefully he can find a way to save Alice.
[APPLAUSE]
SEAN: Yes, we'll see what we can do.
The answer is "no", we will not be able to save Alice.
This is not - sometimes I go around giving talks about the discovery of the Higgs boson,
and it's a wonderful privilege to do that, because there is a climax.
We discovered the Higgs boson.
It is a triumphant event.
There will be no climactic triumphal event at this talk.
This is going to say "hey look, there is this puzzle. I don't know what the answer is.
Maybe someone in the audience can help us out."
It all comes, as Jennifer said, from taking the idea of a Black hole, which is well established,
and which we understand in the context of classical mechanics, and applying quantum mechanics too.
Classical mechanics is what was handed down to us from Newton through Maxwell and Einstein.
It's the idea that you have a state of affairs in the universe,
and it evolves predictably through time.
Also, what you see is what there is.
In classical mechanics, there just IS something and you can observe it.
In quantum mechanics, that's not quite the case.
So here is your one minute introduction to quantum mechanics.
What you can observe is much less than what actually exists.
That's really the essence of quantum mechanics and why it's difficult to understand,
because nobody likes this. Nobody likes the fact that some fundamental deep facet of physics
is being expressed in terms of what human beings can observe.
That can't be right.
And this is called the ""measurement problem" of quantum mechanics.
Surely we don't understand quantum mechanics well enough if we are reduced to making statements like this.
And it turns out that we do understand quantum mechanics better than we used to.
We know a little bit more about what qualifies as an observer and a measurement.
We still don't know everything; we still don't have a complete picture that everyone agrees on,
on the basic quantum mechanical reality.
But we know how it works, in practice. How it works is, certain kinds of observations that you can make.
They return results that we can predict - the probability of getting a result.
But when you are not making those observations, when you are not looking at this system,
all of the different possibilities can simultaneously be there.
So here is a classical example - Jennifer mentioned:  The Schrodinger's cat.
One of a long line of experiments, that are among the most violent thought experiments that physicists like to talk about,
where we kill small helpless animals.
In this case, well we kill half of the animal.
We kill the animal at some branch of the wave function of the universe.
In this case, you have a cat; it's in a box,
and what you want to do, is that you want to take advantage of the fact that in quantum mechanics,
systems can be in superpositions of different classical possibilities.
So in the box, you hook up a Geiger counter to a radioactive source, a hammer that can fall,
and break a vile that is filled with poisoned gas.
It has to be a Geiger counter or something else that is sensitive to a quantum mechanical event,
like the decay of an atom.
So there's something fundamentally quantum mechanical; it happens with a certain probablity,
and you wait until that probability adds up to 50%.
Then according to the rules of quantum mechanics, Schrodinger points out that, in this box,
there is no such thing as an alive cat or a dead cat.
In this box, the reality of the situation is that there is a superposition of an alive cat and a dead cat.
We are used to, in classical mechanics, having boxes that we don't know what's in there.
You might say that there is a 50-50 chance that the cat is alive or dead.
But in that way of talking, there is a reality, you just don't know what it is.
it is a matter of our ignorance.
In quantum mechanics, it's not just a matter of our ignorance.
The truth of what's inside the box, is that it is both an alive cat and a dead cat,
a superposition of both possibilities.
And again, these days we know more about quantum mechanics than Schrodinger did,
so we know that, you don'y need to open the box to create either the alive cat or the dead cat.
There is an effective observation being made all the time by the air in the box, by the light in the box,
by the cat walking around and so forth.
Long before you open the box, the split has already happened.
But the classical explanation of this thought experiment is that, before you open the box,
there is a superposition of a live cat and a dead cat.
When you open it and observe, you only ever see either the cat alive, or the cat dead.
You never see a superposition of both possibilities, even though that's what quantum mechanics says is there
when you are not looking.
And that's the fundamental mystery of quantum mechanics.
The way that when you are not looking at something, it's in a superposition of all these different possibilities.
That leads to something that's very important for the Firewall story, something called entanglement.
This is not a picture of entanglement. This is just an excuse to put a cute cat on the slide.
Entanglement is the idea that in quantum mechanics, the way that we describe the world is not piece by piece.
it's all at once.
So in classical mechanics, let's say you have two boxes, and you can't see inside,
and you have a red ball and a blue ball.
You know that there is one of each, and each one is inside a box, but you don't know which one is in which box.
Then you would say that there are two possibilities: either that the blue ball is in box one,
and the red one is in box two;
OR the red one is in box one, and the blue one is in box two.
One of those possibilities is true in classical mechanics, whether or not you know it.
In quantum mechanics, inside the box you can have any superposition of maybe it's the red ball, maybe it's the blue ball.
So in quantum mechanics, there can be entanglement.
Not only that it can be true that in one box, a certain chance that the ball is blue,
but it can be the case that in the other box, whichever option is real depends on what's going on in the first box.
If you open just one box, it might be 50-50, the ball is red, or the ball is blue.
If you open the other box, likewise, 50-50, red versus blue.
But if you open the first box and the ball is red, then you know with 100% certainty the other box is blue.
That is quantum entanglement, and if you want to be a little more of a respectable particle physicist,
you can think of particles spinning or moving in certain directions,
rather than balls being red or blue.
But it's the same overall point,
that quantum mechanics does not say there is a state of this box, and a state of this box.
It says there is the state of the world.
In this case, the world is either, red over here and blue over there,
OR blue over here, red over there.
So this little picture is supposed to show not two balls in each box,
but the balls are in superposition of red and blue, that it is entangled between those two boxes.
This is another mystery of quantum mechanics. Forget about the measurement stuff.
The fact that things that are separated from each other can nevertheless be entangled with each other
bugs us also.
It certainly bugged Einstein.
He wrote a paper in 1935 with his friends Podolsky and Rosen.
He came up with one of the paradoxes that Jennifer mentioned.
The paradox, the EPR paradox - The Einstein-Podolsky-Rosen, says,
look, this entanglement between two different particles,
let's say a particle spinning clockwise, and a particle spinning counter-clockwise, or something like that,
or a ball being red or blue.
You see, you have one particle over here that's in one state;
another particle in another state, and they are entangled with each other.
That entanglement can persist no matter how far apart the particles are.
You can have one particle that is entangled with the other,
even though they can be separated by hundreds of thousands of light years.
They are on separate sides of a galaxy.
This is not a picture of our galaxy, by the way, we don't have any pictures of our galaxy.
This is the Andromeda galaxy.
But the principle is the same.
In fact, we have done experiments like this,
where two particles are entangled; they are separated by kilometres.
They are not separated by hundreds of thousands of light years.
But still kilometres are pretty good.
According to the rules of quantum mechanics, if you observe this one and then see the ball if blue,
you instantly know that the ball on the other side of the galaxy is red.
So this bugs us, because as Einstein pointed out,
nothing can travel faster than the speed of light.
But the reality of which ball is in the box seems to be changing instantaneously all over the universe.
That bothers us a great deal.
However, it turns out that this kind of instantaneous change is relatively harmless.
You can't use this to talk to anybody, on the other side of the galaxy.
If you think about it, your friend over here, what do they know before you open the box?
They know that if they open their box, the ball could be red or it could be blue.
So you open your box, and your find that the ball is red.
You know that when they open the box, it is going to be blue,
with 100% probability.
But they don't know that. They open the box and it is blue,
and they go:"Well, that was one of the chances"
No information has travelled.
You cannot send a telegraph using this kind of entanglement.
So quantum mechanics manages to be non-local in a very subtle way.
It bugged Einstein, but again, all he could do was say :"Look, this sounds weird"
and the rest of the community said: "Yes, that does sound weird, but it's true. "
You'll do the experiment and it's true. this is actually how nature works.
The reason why I am going to such a great length talking about this here, is because
the particles that are emitted by Black holes are entangled in a very very specific way.
Jennifer showed you some nice pictures of Black holes at one moment in time.
The Black holes sitting there in space, just a black sphere with fun radiation and people falling onto it.
I am the hard core quantitate of science, so I drew a little picture, in power point.
This is a space-time diagram of a Black hole.
The hole itself, the two-dimensional surface enclosing a three-dimensional volume
is here portrayed as a just a one-dimensional circle enclosing a two dimensional disc.
This circle is the black hole at one moment in time.
And the vertical axis is time evolving.
The black hole is just sitting there, so nothing is actually happening in this picture,
but we are going to make it a little bit more active in just a second.
And inside you see the singularity, and the event horizon defines the boundary of a Black hole.
For those of you that have been reading ahead, outside of class,
you will know that outside the event horizon time goes vertically, but inside, time heads into the singularity.
Once you are inside the event horizon,
moving towards the singularity is precisely the same as moving forward in time.
You can no more avoid heading towards the singularity than you can avoid heading tomorrow.
That is why the singularity is so inevitable.
Now what Stephen Hawking said is, out of this classical Black hole will come radiation,
and one way to think about that is, near the event horizon, according to the "no drama" principle,
the principle of equivalence that Jennifer talked about,
you are in, what physicists call, the vacuum state.
You are in empty space, there is nothing there.
You can pass through the event horizon with a little detector, looking for particles,
you don't see anything special; there's no sign posts, no "no escape after you meet this point"
or anything like that.
But it's an empty space that obeys the rules of quantum mechanics
and in quantum mechanics, even empty space is kind of interesting.
Empty space is actually a superposition of different ways that particles could be coming in and out of existence.
Now, here in this room, space is empty, and space-time is fairly flat,
because gravitational field is not that strong.
if you look at empty space, you truly don't see anything.
but in the vicinity of a Black Hole, what Hawking points out,
is that the gravitational field of a Black hole can tear apart these virtual particles that are sitting in the quantum vacuum.
So you can have one particle, Alice, falling into the Black hole, and another particle Bob escaping into infinity.
So what was two particles sitting there quietly in empty space
becomes two real actual particles.
That's very important; this one particle that goes out is what we call the Hawking radiation.
That's what we see evaporating away from the Black hole.
So it's important that whenever you make one Hawking particle, that escapes the Black hole,
secretly, although you don't see it from out here, there is another particle that you also made
that falls into the Black hole.
Given all the setup, you will not be surprised to hear that the two particles are entangled with each other.
That makes sense, because these two particles are created essentially out of nothing,
out of empty space.
Empty space does not have any electric charge, does not have any spin. It doesn't have any energy.
It's empty space,
But you've made particles out of it.
Therefore the two particles that you pulled out of empty space
need to be equal but also opposite.
If one of them has a positive electric charge, the other one has a negative electric charge.
One of them is spinning clockwise; the other one is spinning counter-clockwise.
If one of them has a positive amount of energy, the other one has a negative amount of energy.
And indeed, any particle that escapes to infinity, to the outside world has a positive amount of energy.
Therefore, as far as the outside observer is concerned,
Alice, the in-falling particle, has a negative amount of energy.
That's why the Black holes eventually shrink and lose their masses
because essentially, inside the Black hole,
you see a constant drizzle of negative energy particles decreasing the mass of a Black hole.
These two particles, Alice and Bob, that are created by Hawking radiation,
are entangled with each other.
if this one is spinning one way, this one is spinning the other way.
Empty space separated into sets of particles, will necessarily give you particles that are always entangled.
That's a consequence of the "no drama" principle.
however, we want more than that.
We want the information, having which we made the Black hole, to eventually escape
Once the whole Black hole has evaporated, we want to know exactly what it was made of.
We know that you have two particles entangled;
we know that all the particles coming out are supposed to tell us what went in in the first place.
But that's a puzzle, because Bob is entangled with Alice,
and what that means is that some of Bob's information is being left behind in the Black hole.
When Bob leaves, Bob is entangled, and we are not exactly sure how; therefore the information is not yet getting out.
What you'd say to yourself, if you are trying to make sense of this,
if you are a physicist who believes that Black holes evaporate,
but also believes that information is conserved, is that:
That's OK, because later there is another particle named Carrie, and Bob will just be entangled with Carrie,
in exactly the same way that he was entangled with Alice.
So Bob is spinning clockwise; both Alice and Carrie are spinning counter-clockwise,
and vice versa.
So maybe that's how the information gets out. This early radiation Bob, is entangled with the late radiation Carrie.
Then of course, Carrie herself, if she is a virtual particle that was torn out of the quantum vacuum,
she'll also be entangled with Dave, but Dave is just not going to be important to our story.
Sorry Dave.
Here is what we are trying to say.
We are trying to say that Bob started life entangled with Alice
and eventually is entangled with Carrie.
That's the only way that information can get out of Black holes, if there is "no drama" at the event horizon.
The problem is, that cannot happen.
Quantum entanglement does not work that way.
Quantum entanglement does not let you mess around.
Sorry
In fact, literally, the phrase this goes by is the "Monogamy of entanglement"
If you are a particle Bob, and you are maximumly entangled with another particle Alice,
(there is Alice and there is Bob),
you cannot possibly be entangled even a little bit with another particle called Carrie.
The rules of quantum entanglement are more strict than the rules of "free love" in the 60s,
when this was first discusses.
I got this slide, this picture, from John Preskill, who Jennifer already mentioned,
a colleague of mine in CalTech.
John got in trouble for showing this slide, because it says on the top: Monogamy is frustrating!
As if poor bob is frustrated because he is stuck in this relationship with Alice,
and he can't get together with Carrie.
So John started changing the labels and he made it Becky over here,
who is entangled with Adam,
and cannot be entangled with Charlie.
Then he got into trouble for that too, because people were saying:
"What are you saying about Beckie's morals anyway? Beckie is just sleeping around? Come on. "
So don't label your particles by people's names,
is the short lesson here that we should learn.
But this is the big puzzle. This is it! This IS the puzzle that we are faced with,
if you believe everything on the previous slides.
We have shown logically that the fact that the Hawking radiation comes from a region with "no drama",
flies through empty space, and eventually is going to bring all of its information back,
can't happen.
Not all of those things can all happen at the same time,
because entanglement is monogamous. Bob cannot be entangled with both Alice and Carrie
Something has to go. This is the AMPS argument.
AMPS are Almheiri, Marolf, Polchinski and Sully
the Santa Barbarians who wrote this paper in 2012.
And they went through the list. They went through the list that Jennifer mentioned,
from Raphael Bousso, what could be going wrong?
They are all kind of pretty dramatic.
One possibility is that information really is lost.
Maybe if you take encyclopaedia and throw them in the Black holes,
the information in those books is truly lost forever, not just in practice, but even in principle.
That would be a dramatic violation of everything we think is true about quantum mechanics.
In some sense that's OK. Everything that we think is true about quantum mechanics
is a dramatic violation to what we thought about classical mechanics,
and yet it turns out to be right.
It might be true that quantum mechanics is not the final theory of everything.
In fact , Stephen Hawking, who was the first to propose that this was true,
said very explicitly that he thought this was the right answer.
This is why he made that bet that he eventually conceded, and is now having second thoughts about conceding.
Despite the fact that it is a logical possibility that information is lost,
what we would really like to see is a well defined theory of how information is lost,
where it goes, under what circumstances it is destroyed and conserved, and so forth.
And nobody has been able to do that.
So no one has been able to come up with a consistent coherent framework
in which information really is lost.
That's not to say it's not true.
It's just that it's not one of those ideas that falls into your lap and everything suddenly makes sense.
It's an idea that is suggested; you think about it, and it just becomes more and more puzzling as time goes on.
Another possibility is locality. This is a somewhat subtle concept,
but it's basically the idea that things that happen in the world happen in different locations in the world.
So a particle doesn't just pop out of existence here, and suddenly reappear over there.
A particle propagates from here to there, obeying the laws of field theory,
of quantum field theory, in this case.
This photon, or electron, or whatever it is that we labelled Bob,
moves from the event horizon to very very far away, following the laws of particle physics,
which are local in space.
Maybe that's not what happens; maybe the information contained in Bob doesn't obey those laws.
That would be very dramatic and interesting.
Finally there is the possibility that, there is in fact "drama"at the event horizon.
This thought that we've had since the 1960s, that Alice could fall into the event horizon and never notice anything,
is just not right.
Instead, there is a tremendous amount of high energy radiation that Alice sees when she gets there.
If you think about it, this Hawking radiation, it comes to you;
it's very faint, it's very cold, it's very long-wavelength.
But when you leave a Black hole, a photon gets red-shifted; it loses energy.
It suffers what we call the gravitational red-shift.
So a photon out here, that is very low energy,
when we true it back in time, was very high energy, when it left.
Usually we say: Well, that's OK. It was high energy, but it was invisible because it was part of a quantum vacuum.
And this idea is saying actually no. It was just there as a super-duper high energy particle.
And when Alice tries to cross an event horizon, she would be incinerated by this wall of fire,
all of these high energy particles lurking near the event horizon.
And I fact, if this were true, there wouldn't even be anything called inside a Black Hole.
All the stuff that we think make up the Black hole would simply live on the event horizon.
Now be a very dramatic difference from how we think about Black holes were narrowing.
Sadly, neither Almheiri, Marold, Polchinski or Sully seemed to know that
the word Firewall is used in computer science,
to be a good thing, to stop evil people from getting in where you wanna go.
They used it as a bad thing, something that burns you to bits, as you fall into a black hole.
So the smart money, among people who are my personal friends,
is that locality is going to be the thing to go.
It  is a very dramatic thing, but it seems to be a little bit easier to live with than the other two, going.
So let me talk a little more about locality.
As Jennifer mentioned, you can visualise what locality is by thinking about
what happens when you drop a pebble into a pond,
or when you make a sound, or when you flash a light , or anything like that.
The effects of what you do
do not show up all throughout the universe instantaneously.
They move at a certain rate - the speed of light,
the speed of sound, or the speed of waves in the water
They move from one place to the next to next to the next.
That's a reflection of the fact that the way we talk about the world
is by talking about what's happening at every point in space.
So this is space, if I were to tell you what's happening in some region of space,
I tell you what's happening there, or what's happening there, and so forth.
If someone says: What's going on in this lecture hall?
I can tell them - the identity of every person and every seat in the lecture hall, and what they are doing.
Separate information in every point in space.
Now quantum mechanics complicates this a little bit,
because it says that there could be separate superpositions in classical realities.
But each on elf those classical realities is local in space.
There is a description of ... there are so many electrons over here, so many photons over there,
Everything is defined by where it is in space.
That is the principle of locality.
Entanglement makes the concept of locality a little bit more problematic,
but it doesn't break it; it just makes it more interesting.
The quantum state of these two particles that are very very far away from each other is not local.
However, the dynamics, how they actually change with time, how you can actually send a signal through space,
is all perfectly local, even with quantum mechanics and entanglement.
If you want something to happen, you just start it here
and you just travel through space to get anywhere else.
Despite the fact that locality is very precious to us,
it's probably not right.
In fact, other thought experiments involving Black holes have already given us reasons to believe that,
when it comes to quantum gravity, just what we are doing here, after all,
quantising gravity itself, the fabric of space-time.
When it comes to quantum gravity, locality is probably not an inviolable principle.
The question is, how much violation with locality can we get away with?
To what extent is the universe not really described by what's happening here,
plus what's happening there, plus what's happening there?
So we have two clues that are given to us, by thinking about Black holes.
One is called the holographic principle.
This actually goes back to Stephen Hawking once again, and his radiation from Black holes.
One of the implications of Hawking's discovery that Black holes radiate,
by the way, I should (because I'm in a venerable lecture hall), stop saying,
we have never seen a radiating Black hole.
This is not data that we are talking about here.
I'm sure that Black holes radiate; my confidence is very very high,
but I could be wrong about that.
It's cherished rules of physics that tell us that black holes radiate,
but it sure would be nice to have some experiments or some data that said the same thing.
Alright. Thought to the experimental is now complete,
now we can go back to crazy theorists' land.
Hawking showed that a Black hole actually has a lot of entropy.
Entropy is a way of talking about the disorderliness, the chaoticness of an arrangement of stuff in the universe.
When cream and coffee are all mixed together, they are high entropy;
when they are separated, they are low entropy.
So ordinarily, if you want to quantify actually the amount of entropy in the air in this room,
you just talk about the numbers of ways you can rearrange the molecules that make up the air in this room.
And the number of ways that you can rearrange the molecules just depends on the volume of space that we have in the room.
So it is an absolute rock-solid feature of entropy in the ordinary experimental macroscopic world,
that entropy scales like the volume of the system that you are looking at.
If you double the volume, you will double the available entropy.
What Hawking showed is that for a Black hole,
the entropy of a black hole scales as the area of its event horizon.
So a Black hole has a volume; there is a volume inside the event horizon.
There is a three-dimensional volume enclosed by the event horizon.
You might have guessed, that the entropy of a Black hole is proportional to that volume inside.
That is not right.
It is proportional to the area, some two-dimensional quantity,
the area of the event horizon, not the three-dimensional quantity of the volume inside.
People had known that for a long time;
It didn't really sink in seriously until the 1990s,
when , Huskin and others promoted this idea,
to not just a weird idosyncrasy of Black holes,
but a fundamental feature of nature.
They proposed the holographic principle,
that says that basically any time, you think you had a three-dimension collection of stuff,
like us here in this room.
All of the information needed to describe us can be compressed into one two-dimensional surface.
In a black hole, it becomes frighteningly real.
In a black hole, the two-dimensional event horizon really does contain all the information you need,
to talk about what's happening inside, according to the holographic principle.
But it should be true even in this room, or the galaxy or the universe.
And if that is true, locality is being dramatically violated,
because there is a lot less that can possibly happen in this room than you thought could.
You thought that something could be happening here, and something could be happening there,
and different things could be happening at every point.
But the holographic principle says: No, that's not true.
One of the arguments for it, is if you imagine all of the different possible things that could happen
most of them would have a lot of energy and would collapse to make a Black hole.
So there is an upper limit on the number of things that could happen in this room,
and the size of the upper limit is proportional to the area of the walls around this room.
So there is this hypothesis that all of physics really lives in a world
that is one dimension lower than the world we actually see.
And again, we are trying to make sense of this idea. We are making progress, but we are not completely there yet.
The other idea that has come out of Black holes and argues against locality is called Black hole complementarity.
Remember I said that, from the point of view of Bob from far away,
he sees radiation coming out of the Black hole, and he says: well if I trace it backwards,
it must have been very high energy radiation when it left the event horizon.
Whereas Alice, in the conventional way of thinking about things,
passes through the event horizon and sees nothing there
just empty space.
So they had incompatible ways of describing the same situation.
Bob thinks the event horizon is bubbling with high-energy radiation;
Alice says there's nothing there.
Black hole complementarity says: they are both correct.
Black hole complementarity says they are different-sounding ways
of giving equivalent descriptions of the same fundamental underlying reality,
and that two things that are seen by two observers can look very very different,
as long as the observers can never get together to compare notes.
So what happens is, if you give Bob enough time to collect the Hawking radiation,
and figure out what he thinks the horizon looks like,
and you give Alice enough time to fall into the horizon.
If Bob then says: alright, I've got some data; I know what's coming out of the Black hole.
I am going to fly into the Black hole and tell Alice what I saw.
It is too late. She has been spaghettified and crushed into the singularity.
So these two observers see a very different thing happening in the world, but hey can never talk about it.
Only we - God-like physicists, looking at the whole thing from afar,
can give the bird's eye view on everything that is going on.
That is the principle of Black hole complementarity.
It's borrowed from the early days of quantum mechanics when Niels Bohr pointed out that
you are allowed to measure position, OR you are allowed to measure velocity.
You are not allowed to measure both at the same time.
That was quantum complementarity; this is Black hole complementarity.
So again, it's a violation of locality in some sense.
It says that the right way to describe the world isn't what's happening here, and what's happening there,
and what's happening there, and what's happening there, separately.
What's happening right there can depend in a very dramatic way
on who's looking at it and from what perspective.
So somehow, all the information about what's going on in the world
is not simply located in individual points in space.
It's encoded in some cryptic way that we don't yet understand,
and that's what we are trying to get at, by doing these thought experiments.
The problem is, these two types of non-locality,
don't seem to be enough to solve the Firewall puzzle.
Remember in the holographic principle, you say: OK, all the information is there on the horizon.
That doesn't tell you anything at all about what you see, when you fall into the horizon.
And in the complementarity, you explicitly say that Alice, when she falls in, doesn't see anything at the horizon
but it's just an assertion.
You are not explaining how the entanglements can actually work.
The force of the Black hole Firewall paradox,
as in the amount of non-locality you seem to need to make physics work,
is even worse than that given to us by the holographic principle or complementarity.
So what could be going on, and again, nobody knows what's going on,
so we'll tell you two crazy-sounding ideas.
The hope is that they are just crazy enough to be right.
One idea is that, there really isn't any such thing as "inside the Black hole".
This idea is that, what you think of as "inside the Black hole",
is just a copy of certain things going on outside the Black hole.
This is a little bit strange to get it straight, because in fact, in quantum mechanics,
there is a rule that says: you can't copy things.
There is what's playfully called the no-Xerox theorem, or the no-cloning theorem.
You have the quantum bit of information. You can move it，
and you can evolve it and so forth, but you can't just copy it without destroying it.
But this is saying something more profound than that.
It's not that you fall in and there is a copy of you that goes in and a copy of you that goes out.
It is saying that the interior of a Black hole really is exactly the same thing,
as the exterior secretly,
secretly encoded in some weird way.
So the difference is that if I can take a quantum mechanical object,
and clone it, then I could separately do things to this one and then do separate things to this one.
But this is saying that literally everything that happens inside the event horizon,
is mirrored in some exactly equivalent way by things going on outside the event horizon.
That is a set of words that we can say.
Those are all English sentences that are more or less grammatical.
We don't really have a quantitated theory about how this would work.
It would in fact solve the Black hole Firewall puzzle,
because this thing that we thought was a photon falling in, labelled Alice,
was exactly Bob all along, or one of the other photons.
It seems dramatic but it might actually be true.
And then just to go one step weirder, we have what is playfully called ER=EPR.
This is an idea of again Leonard Susskind and Juan Maldacena.
EPR, we already mentioned - Einstein-Podolsky-Rosen, are the ones who first took quantum entanglement
put it front-and-centre.
ER was just Einstein ad Rosen; Podolsky didn't get involved, but it was a paper written I think the same year
as the EPR paper, but on a totally different subject,
about the idea that, given general relativity, given the idea that space can be curved and dynamical,
you could imagine building little bridges in space-time that we call wormholes.
So Einstein and Rosen, what we call an Einstein-Rosen bridge,
is just a connection between two widely separated pieces of universe,
maybe that look like Black holes from the outside.
The trick here is that, you think the distance between this Black hole and this Black hole is very large,
if you travel this way, but secretly it can be very short if you take the shortcut through the wormhole.
This was the Einstein-Rosen suggestion.
It was a little bit speculative,
but it went into the toolbox of theoretical physicists.
And it had nothing to do with entanglement or quantum mechanics,
and Maldacena and Susskind was saying is that, well actually maybe it does.
Maybe when two particles are quantum entangled,
secretly, there is effectively a little tiny wormhole that is connecting them,
that for certain purposes,
you can think of quantum entanglement as being enforced by a little bridge in space-time,
that brings those two particles together.
So the way that information can get out of a Black hole, in this idea,
is that Black hole is connected with by little virtual wormholes
to all the particles that are being emitted as Hawking radiation.
Really, the importance of this suggestion is that space-time is not fundamental.
Space-time is an approximation
that comes out of thinking about quantum states and how they are entangled with each other.
we live in space-time; we tend to think that space and time are kind of important.
This point of view is saying: they are just good ideas.
They are nothing absolutely fundamental; they are just a good approximation.
And finally of course, there is the other possibility that Firewalls are real.
That we've been wrong for the past 50 years
when we are thinking about what would happen when you fall into a Black hole.
It would not be painless until you got spaghettified.
It would be painful long before then.
This is the old picture of Black holes. You would just fall in and nothing would happen.
And maybe that's completely wrong. Maybe in fact, there is a wall of ultra-high energy radiation
that incinerates you to bits long before you fall into the event horizon,
and would be spaghettified.
Again, that seems weird to us,
because the horizon in classical general relativity, not only is nothing going on there,
but the gravitational field isn't even that strong.
We think of, if you are right next to a Black hole, the gravitational field better be pretty strong.
But it's really just, it's so big that you don't notice.
You would not be spaghettified at the event horizon.
You are not spaghettified until you are well inside, an astrophysically large Black hole.
So this idea is saying that, in a region of space-time, that should seem perfectly benign and happy,
something really dramatic is going on.
That may be true; if so that would be a very important discovery.
So to conclude: why is this important? Why does this matter?
And we could have started by telling you why it matters, but you are already here,
so we figured that you'd go along.
The point is that trying to quantise gravity
is probably the most important outstanding puzzle in fundamental theoretical physics,
the ultimate description that we have of how nature works at a deep level.
We have a very very good theory of particle physics, quantum mechanics, and space-time.
The quantum mechanics part doesn't play well together with the space-time part
and that's the search for quantum gravity.
But to get a theory of quantum gravity, we need to know what it is that we are quantising.
We think that we understand classical gravity; Einstein told us what it is. it's the curvature of space-time,
but all of these ideas from Black holes, getting the information out, the holographic principle, complementarity, Firewalls;
all of them are telling us that space-time itself is not fundamental,
that when we talk about space-time, it's like talking about the air in this room as some kind of fluid,
that has a temperature, a pressure, and a velocity, and a speed of sound and so forth.
But we know, and you and I know that if you look closely enough at the air in this room, it is no longer fluid,
it resolves into different atoms and molecules.
This kind of idea is saying, sure, space-time is a nice approximate way to talk about the world,
but when you really push it, it breaks down and needs to be replaced by something else.
We have no idea what that something-else is. That's the problem.
If quantum gravity is not a theory of space-time curvature being quantised,
it is a theory of something else being quantised,
something that is related to the space-time we observe every day in some mysterious, non-local way,
that we haven't fully understood.
We are some provocative clues; we have some toy models, that seem to give us useful information,
but we are nowhere near the final answer yet.
That is what makes it fun; that is why this is a fun thing to think about.
It is not going to cure cancer or build a better iPhone,
thinking about the holographic principle and Black hole Firewalls.
All that it will help us do, is understand at a deeper level what the fundamental nature of reality is.
And I think that is something that is well worth thinking about.
Thank you.
[APPLAUSE]
So now we have plenty of time for questions. I am going to bring up my lovely wife again.
We were told - I don't know if this is true -
we were told that the last time as husband-and-wife team spoke at the Royal Institution,
it was Marie and Pierre Curie.
And they didn't let her talk.
Come a long way. Alright, way up, right in the middle.
MAN: Thanks. I have a question about the human Alice and Bob, which is probably just misunderstanding.
When Bob is watching Alice enter the Black hole,
she basically freezes in space, and he doesn't actually see her
well, and infinite amount of time would possibly pass before she enters the Black hole.
When I was young, there were still ideas about the Big Crunch,
and I was thinking Alice can never get to the singularity,
because time is basically slowed down so much from the point of view of Bob and the rest of the universe
that the Big Crunch will happen before she ever gets there.
Now obviously the Big Crunch might not happen because of dark energy,
but as the black hole evaporates,
would the evaporation of the Black hole not sort of overtake Alice reaching the singularity?
I don't really know if this is a question. It really is a request for evaporation.
SEAN: And the question is: don't you agree?
I do agree, actually you are completely right.
Your younger self was not right, because in a Black hole that would not have evaporated away,
from Alice's point of view,
the clock is still ticking in an absolutely normal way,
when she crosses the event horizon.
There is a finite number of seconds before she hits the singularity.
If the Black hole is one solar mass in size,
between the event horizon and the singularity, it takes about one millionth of a second from her point of view.
But now with the Black hole evaporating, we honestly don't really have a very straight answer to that question.
There's still going to be tremendous gravitational fields in there.
You are probably still going to be spaghettified,
but there is probably not a singularity. That is probably an artefact of classical general relativity.
So it's more like you all get mixed up, and somehow your information comes out again.
MAN: Would the Black hole not evaporate?
From Bob's point of view, he's seen the Black hole evaporate, in external time.
Alice is almost frozen from his point of view,
so why isn't it that the Black hole evaporates before anything happens to Alice?
It seems to tie in with your idea that
the event horizon, which is this massive radiation, there's actually nothing inside.
SEAN: Well you know, Alice's point of view is also valid.
And in her point of view, she enters the event horizon.
The point is that, there is a tiny bit of exaggeration,
when we say that from Bob's point of view, Alice stops moving.
If you see somebody, that means they are giving off radiation.
That means that time is passing.
So even though Bob sees Alice move more and more slowly,
Alice also appears redder and redder, and fainter and fainter
because she is embarrassed that she is falling into a Black hole,
and because the photons are being red-shifted.
Bob only gets a finite mount of information, about what happened to Alice,
and that information is not complete.
The actual fact about Alice is that she enters into the Black hole.
MAN: Is there any potential research, aims and instrumentation
that could give you evidence about what you are talking about?
JENNIFER: Don't you love the theory? The pureness of the theory?
SEAN: yeah the theory should be enough!
I think that the most we could possibly hope for is evidence of Hawking radiation.
The evidence for Firewalls and crazy wormholes is a bit much to ask.
If a Black hole is big, like if you make it from a star at the centre of a galaxy,
the weird thing about Black holes is that the bigger they are, the fainter they are,
the lower the temperature is.
so for an astrophysical large Black hole, it's actually a lot colder than the surrounding space.
And the amount of Hawking radiation given off is a lot fainter than anything else in the universe.
The only hope is to find a really tiny Black hole, a primordial microscopic Black hole.
and those could be exploding today.
It's possible that we've been observing them in terms of gamma ray bursts or something like that.
It would just be very hard to tell whether or not you are actually observing an exploding Black hole,
and probably there aren't any.
But that would be the biggest hope to find something.
Actually Jennifer is an expert on the actual experiments in the lab for the Bill Andrews Black holes.
JENNIFER: Ohh. I am not an expert.
Expert is a strong word.
Yes, there has small analogs that Bill Andrews was doing,
using what's a little bit like water going down a drain.
He is basically been able to create sonic analogs of Black holes, or with mechanical water waves
where you actually do get a kind of trapped effect similar to light not being able to escape,
suddenly sound can't escape.
He's actually had a lot of very interesting hands-on experimental work with us.
The maths isn't exactly the same as for a Black hole,
but it's essentially what amounts to a toy model with some experimental evidence.
So the hope there is that through the small-scale experiments,
they can actually get some insights into the larger full Black hole equations.
SEAN: Yes.
MAN: I have two questions,
both unrelated and related depending on your point of view.
The first question is that...SEAN: complementarity! MAN: Exactly.
So normal Black holes, supermassive, and these theoretical micro ones,
are they different sized animals of the same species
at least from a behaviour perspective?
or is it different species? First question.
Second: in terms of Bob and Carol and Ted and Alice,
As far as information transfer, is there 100% fidelity of the conservation of information,
or is there noise in the system?
SEAN: the first answer is that they are certainly the same species.
They are the same animal; in fact, Black holes are arguably the simplest things in the universe.
If you tell me what the mass of a Black hole is, what its electric charge is, and how fast it's spinning,
you have told me everything there is to know.
It's not like a planet where you have to tell me the topography and what it's made of and all those things.
Black holes are all very simple and basically identical to each other.
The information laws, you know, again we think an absolutely iron-clad rule of physics,
that information is 100% super-duper conserved.
It's like being a little bit pregnant, a little bit of information loss is very noticeable.
We could be wrong about that, but that is the thought.
That is the idea that we are working on.
JENNIFER: I think I would just add one thing, which is Black holes of different sizes actually radiate faster.
If they are smaller they disappear almost instantaneously.
If they are larger... So you know, the rate of which they evaporate is tied to their size,
and that's the only difference. I mean, they are pretty much the same species
but there are different size scales,
and the does affect the rate of which they evaporate away.
SEAN: Someone right here had a question.
BOY: Thanks. That was a wonderful lecture.
Going back to the holographic effect, if you end that door there...yeah that one...
and there's no space between you and the door,
what would happen to the information of the air between you and the door?
SEAN: Well it's not...that's a very good question, but you have to understand exactly what's being said.
If you think about a hologram, have you seen a hologram? BOY: Yeah.
So it's a two-dimensional piece of film that you shine light on and a three-dimensional image appears.
So the holographic principle is saying that this room is like that.
So it's not saying that there's no air or no space in between me and the door.
What it's saying that all of the information needed to talk about the air in between me and the door,
is embedded in some secret invisible two-dimensional space.
But it is all there.
There are fewer ways that there could be air in between me and the door,
than you might have thought,
because there isn't as much room in the two-dimensional hologram to put them
as there would be in a three-dimensional space.
BOY: Thanks.
MAN: This follows on, actually from talking about this principle of two-dimensional space,
in which three-dimensional information is encoded.
I just wondered if you could talk about how this relates to the prediction of the string theory,
which predicts a lot more dimensions, I believe,
and what we actually mean by dimensions in both circumstances.
JENNIFER: That's on you. SEAN: Yeah, you would think that they would get their story straight,
and decide whether there are more or fewer dimensions than we thought there were.
But it's actually quite compatible.
In the string theory, there are supposed to be extra dimensions of space,
which are invisible to us for a completely different reason.
Those dimensions are invisible to us because they are curled up, into a little tiny ball,
and we can't see them; they are much smaller than atoms,
OR we are stuck on some three-dimensional surface inside some bigger space and we just literally can't get there.
This is saying that, all of those descriptions are ways of talking about curved space-time,
but the right way to talk about the fundamental nature in curved space-time
is just to go one dimension less.
So however many dimensions there really are, in the fundamental description of reality,
curved space-time with gravity always appears like it has one dimension more.
That seems to be the principle that holography works on.
So it might be that the real number of dimensions of space-time is eight,
and therefore gravity works in nine-dimensional space-time; that's one of the theories you can write about.
MAN: is there any suggestion that there is a link
between quantum gravity and dark energy or dark matter?
SEAN: Any question that begins with "is there a suggestion", the answer is Yes.
People have suggested that.
Dark matter is probably quite parochial and conventional by these terms.
It's probably just another particle that we haven't found yet.
People have tried more outlandish ideas: it' a modification of gravity; something holographic,
something like that.
Those ideas literally don't fit the data very well.
Whereas the idea that dark matter is just another particle seems to fit the data very well.
Dark energy, which is more subtle, it's something that is smoothly distributed throughout the universe,
and makes the universe accelerate.
We have less idea where the origin of that is, so it's absolutely possible that when we do,
it will in some way involve ideas like holography,
but I really can't tell you precisely how.
SEAN: Right there.
MAN: Hi, you spoke already about how quantising gravity
was one of the greatest challenges in modern physics.
Could you comment on how cosmic inflation helps explain that?
SEAN: Don't think it does, is the short answer.
But it might help us learn something about what works.
so inflation is this idea that in the first, a very very tiny fraction a second after the Big Bang,
the universe underwent a huge fast amount of expansion that smoothed everything out,
and left us with the universe we see today.
The models that we have of inflation are basically classical,
are basically not involving quantum gravity.
There is a little bit of quantum field theory in there, but gravity is still classical in that regime.
So you don't need quantum gravity to make inflation work, nor does inflation rely on quantum gravity in any way.
However, if inflation is real and we can get observational evidence about it,
it is a phenomenon that happens at an energy scale much closer to the energy scales of quantum gravity
than any other phenomenon that we have any way to observe in the universe today.
So literally the difference in energy between the temperature of the air in this room,
and the protons that you smash together at the LHC, the Large Hadron Collider,
is less than the difference in energy between those large hadron collider protons and the inflation energy.
So if we can make that leap up to inflation, which maybe we have with recent data about microwave background,
that's as much of a leap forward in energy as all of the previous rest of the history of physics.
SEAN: This side. Do we have any...ah good.
MAN: Hi, thanks again for the talk. Fantastic.
I wanted to ask, would you comment on what these paradoxes might mean
for our understanding of cosmology in the sense of the origin of the universe, and its future.
In particular, I think I remember reading that holography may link what we think of eteranl inflation
and the many-worlds interpretation of quantum mechanics.
What we think are completely different things,
are somehow linked via holography, but no idea if I got that right,  or How.
SEAN: Yeah, you do actually have that right.
So, complementarity. The question is, what is this thing got to do with cosmology?
It's interesting, because as a cosmologist, my goal in choosing my career was
to go into the simplest possible area of science, cos the universe is actually very simple.
You ignore all the details and just look at it on large scales.
And yet, I went wrong, because Black hole are actually even simpler than the universe
So applying these ideas to cosmology is hard, cos the universe can do more things than a Black hole can do.
It's like that question; Black holes just can't do many...there is just not that many possible Black holes.
Having said that, let's imagine taking complementarity very very seriously.
Complementarity says that the point of view of someone outside the horizon is different than someone inside the horizon.
So because our universe is accelerating out, because there is dark energy,
there is a different kind a horizon that we are inside.
If distant galaxies move far enough away, they are accelerated away by the expansion of the universe
and we will never see them again.
So they are outside the horizon.
So certain ambitious physicists have said, that maybe there is this certain complementarity
that works for cosmological horizons,
as well as Black hole horizons.
And if that were to be true, what we will be saying is that, the entire description of the universe,
is our observable universe plus our horizon.
Everything outside is secretly embedded on our horizon.
So you don't need to talk about the entire universe outside of what we can see.
It's all secretly encoded in this local bit.
OK.
And if that's true, then quantum mechanics says the universe is like any other quantum mechanical system,
it can be a superposition of different possibilities.
So this observable bit of universe can be a superposition of different states of being.
and that would be the cosmological multiverse.
So the many-worlds in quantum mechanics and the cosmological multiverse would be brought together
in a happy relationship, and it's all completely speculative at this point.
JENNIFER: Is there one up there?
SEAN: I haven't seen any up there but there are a couple of plucky folks over here.
Is there anyone up there?
MAN: I just wanted to talk about dark energy and inflation.
If you imagine a speedboat going across water, and the speedboat itself is inflation,
the ripples behind it might be dark energy,
because dark energy says that things that are furthest away from us are moving further away fastest,
and those that are closest to us are moving away less quickly.
So that would be the same thing with the speedboat, with the ripples coming out from the speedboat.
As you get further away from the actual speedboat, the ripples themselves move slower.
So are we looking for something that doesn't actually exist? It's just inflation.
SEAN: So, my personal point of view is that the speedboat analogy is probably not the best one,
because inflation is not the movement of something through something else.
It is literally the size of the universe changing with time.
It is true that when we talk about the acceleration of the universe and dark energy,
distant galaxies are moving away from us faster than nearby galaxies.
But that's only because the universe is expanding.
That was still true even before we knew about dark energy or the acceleration of the universe.
Edwin Hubble's famous law says that the apparent velocity of a galaxy,
is its distant times a constant called the Hubble's constant.
So as the distant gets bigger, the apparent velocity gets bigger and bigger.
That's really just explained by the fact that there is more space,
in between you and the distant thing.
So, it's a tricky thing, because we very naturally fall back on analogies,
that relate what the universe is doing to things that we are familiar with,
all of which are things inside the universe, doing things, subsets of the universe doing things.
But in some ways, the universe is a unique kind of beast.
I think that dark energy really needs to be some kind of energy.
I don't think it's sort of automatic or anything like that.
MAN: Sure.
MAN: Thank you. I'm still having a little trouble understanding what you were saying when...
SEAN: What?
Alright...
MAN: Back early on with you Alice photon falling into the Black hole, and your Bob photon being emitted.
Why is it that Bob is emitted? Why can't he just hang around outside the event horizon for a little while,
and then follow Alice in just a little bit later?
And sort of following on from that,
I also don't quite see where this Firewall comes from.
Does that come from the same phenomenon as the original Hawking radiation?
Or is that just something else that would give us a similar prediction?
SEAN: So to the first question, why couldn't Bob just hang out?
Have you ever tried hanging out right next to the event horizon of a Black hole?
I thinks as Jennifer mentioned, most of the virtual particles near the event horizon do just stay there.
They get created and they get annihilated very quickly; they don't escape to infinity.
So when we draw these pictures, we are just picking out the plucky ones that manage to escape.
So most of the virtual particles are just sitting there near the horizon.
The important thing about those particles is that,
if the combination of the states of all those particles looks like empty space,
it must be that all those particles are entangled with each other,
in a very intricate way, in a very intimate way I should say.
The Firewall paradox is just saying that,
if you want to get the information out
from Bob and all the other particles coming on later,
then it can't be true that Bob and Alice are entangled at the same time.
If Bob is entangled with Alice, he is not entangled with anything else,
and therefore the information is getting lost every time  he leaves.
But as soon as you disentangle Alice and Bob down there,
it's no longer empty space.
It's a wall of high energy particles.
That's the Firewall claim in a nutshell.
JENNIFER: Finally someone back here.
MAN: Yeah, sorry...OK so just another question relating vaguely to entanglement,
and I'm sorry if this was covered in the lecture and I maybe dropped off and missed it.
It was very interesting. That could be the beer I had earlier.
So when you talk about entanglement,
in my mind, that is a way of you describing a supposition, which is a way of you saying that
something you observe infers the truth about something else.
So for example, if you observe that the ball is blue,
that implies that the other ball is red.
But regardless of whether you observe one truth or another truth,
both of those truths still exist, whether you observe them or not.
so a ball is still blue and a ball is still red, somewhere.
So as soon as you observe whether one ball is blue or red or whatever,
then that entanglement, in a way, does that cease to exist?
Because you observed that one ball is red,
the other ball becomes blue; they are entangled.
But does that relationship maintain, so if you were able to change the ball you have observed to red,
does the other one become blue?
Or does that entanglement cease at some point of observation?
This is kind of my question.
I don't know if that makes sense.
SEAN: It is a perfectly good question, and there are two things.
One is that, yes the entanglement does persist.
When you observe one box, or one ball, the entanglement that was there, stays there.
So one way to be entangled is,
this ball is definitely red, and this ball is definitely blue.
It's a kind of entanglement.
More interestingly, there is a fraction of the superposition that is red and blue,
and the opposite that is blue and red over here.
But the point is when you make an observation and make this one red,
that one instantly becomes blue.
They are still entangled, just in a more trivial way.
And that is an experimental fact.
That is not just a theory; we have done that experiment.
We have put them in superpositions, move them apart and observe them,
and then checked, and it always works out exactly as we predicted.
MAN: So can you then sort of reboot the supposition with the same two particles?
SEAN: You can...once have observed the particle and you have seen that it's red or blue,
it stays red or blue. So that is what's called collapse of the wave function, in quantum mechanics.
and it's very mysterious, collapse of the wave function.
Before you saw, it was in a superposition,
afterward, it's one or the other,
and that's what it's going to be from now on.
You have come very close to inventing what is called many worlds interpretation of quantum mechanics,
which says that, MAN: Thank you very much.
SEAN: when I...You are late!
MAN: I'll take that. I'll call or text my mom. She'd be very proud.
JENNIFER: about a hundred years too late.
SEAN: A little bit too late, and the guy who invented it kind of went crazy.
Hugh Everett, who invented the many-worlds interpretation,
said look, when you talk about these experiments, you have to realise that
you, a human being are made of quantum mechanical particle,
so you obey the rules of quantum mechanics.
You have a quantum state that can be in superposition.
When you open the box, and look to see if the box if red or blue,
it's not that you see it red, and the blueness disappears, or vice versa.
It's that you evolve into a superposition of the ball was red and you saw it red,
plus the ball was blue and you saw it blue.
There are now two different copies of the world in which you saw different things going on.
They are both equally real, and they will never communicate with each other ever again.
They are for all intensive purposes, separate worlds,
multiple worlds in reality.
If you believe that, which I do actually, I think that it's probably true,
you should go to the APP store and download something called universe splitter.
It is an iPhone APP, where it gives you two options and you can say,
I would like to study physics in graduate school, or study philosophy in graduate school.
It will send a signal to a lab that sends a photon and splits it then observe it,
and it comes back and tells you which branch of the wave function of the universe you are in,
and you should o study philosophy.
And if you promise you'd do what the phone tells you to do,
there is another branch of reality in which you became a physicist.
MAN: And when do I go on the X-factor... SEAN: That's right!
MAN: Thank you!
SEAN: One more question, good right in the middle right there. You can just move it right there.
WOMAN: I read earlier this year that Stephen Hawking , I think, probably in connection with the Firewall,
wrote an issue, with saying something about maybe Black holes didn't exist,
as such as having an event horizon.
and maybe had more of an apparent horizon instead.
Can you sort of say a bit about that? Sort of explain it a bit, sort of how it adds to the debate?
JENNIFER: Well I can say something as a science writer, that the new covered was terrible of that.
That's actually not what he said.
The headlines and some of the covers of that were not very good, I think,
really hadn't looked very deeply into what Hawking was actually saying.
If you ask Sean, Sean will tell you that he thinks Hawking is possibly not correct.
I will let you elaborate. SEAN: It's one of the few times in my life,
where I thought to myself: of course Stephen Hawking.
Because he wrote this paper, and it was really just a conference proceedings.
He had given a talk, and he typed it up in his computer
And he ended it off. It was very short; there were no details, no equations.
it was just like a suggestion.
But because his name is Stephen Hawking, and he's done amazingly wonderful things in the history of physics,
and everyone pays attention to him,
people latched onto it, and there is this one sentence in his paper, which says that:
Therefore, Black holes do not exist in the sense that  blablabla. instead, they exist in this sense.
And the headline in Nature the next day was
Stephen Hawking thinks that Black holes do not exist.
So, if you go back the picture Jennifer showed of the centre of our galaxy,
remember, this is data.
The orbits of these stars that are plotted here; it's done in a computer program,
but they are based on observations of the stars,
around a big plus sign.
If you believe, it's a Black hole,
because it's enormously massive and enormously small and enormously dark.
And Stephen Hawking, if you asked him: is there is Black hole in the centre of our galaxy? He would say yes.
So I think that, he is gesturing in the direction of
a certain kind of resolution of the Black hole Firewalls problem,
to take the idea of an event horizon a little bit less seriously, given that Black holes eventually evaporate.
But there is a long way to go before enough meat is put on those bones to call it even a proposal,
much less a theory.
And I think I'm being told that we are out of space and time, so we will...
we have come to the end of space-time; thank you once again all for coming.
[APPLAUSE]
