Are black holes the key to unlocking a quantum theory of gravity?

I'm Brian Cox. I'm a professor of particle physics at the University of Manchester and the Royal Society Professor for Public Engagement in Science

And amongst other things, I'm the author of several science books, including Black Holes, the Key to Understanding the Universe

Chapter 1. Black Holes and the Edge of Physics. Could black holes be the key to a quantum theory of gravity, a deeper theory of how reality, of how space and time works

Well, I think so. It's interesting. These objects, which have been known, I would say, for 40 or 50 years, but theoretically for the best part of a century, have always been fascinating

fascinating. They're odd things. The simplest way to describe a black hole would be a region

of space from which even light can't escape. Predictions that such objects existed go all

the way back to the beginnings of relativity back at the turn of the 20th century. But actually

really, I would say into the 1960s, perhaps even into the 1980s, many physicists felt that because

of the intellectual challenges that these predicted objects pose, many physicists felt

that maybe nature would not create them. I even saw the great physicist Steven Weinberg say that

he, in some sense, hopes that these things would not exist because they're so confusing. But we now

know that they do exist, and so we have to face the challenges that they pose

Black holes are interesting because going back to the work of Stephen Hawking in the 1970s

it turns out that they demand that we think about both quantum theory and general relativity

together and the quest to unify those two great pillars of 20th and 21st century physics

into what's often referred to as a quantum theory of gravity is in some sense a holy grail for theoretical physicists

But the problem has always been, well, is there anywhere in nature that we can look

to observe something that requires us to merge those two theories together

And black holes really are the unique place, as far as we can tell, in nature

where we can see a thing sitting there in the sky that demands that we consider those two theories

working together to hopefully reveal a deeper theory. The idea of black holes goes back a long way, actually, back into the 1780s and 1790s. There

were two physicists, mathematicians, natural philosophers, whatever you want to call them

working at the time that had the same idea apparently independently of each other. One

was a clergyman, an English clergyman called Mitchell, and the other was the great French

mathematician Laplace. And they were both thinking in terms of an idea called escape velocity. So the

escape velocity is the speed you have to travel to completely escape the gravitational pull of

something, a planet or a star. For the Earth, for example, the escape velocity from the surface of

the Earth is around 8 miles a second, 11 kilometers a second. If you go bigger, you make a bigger

more massive thing, let's go up to a star, for example, like the sun, then the escape velocity

increases because the gravitational pull at the surface increases. And actually for the sun

it's somewhere in the region of 400 miles a second. It's really fast. What Michel and Laplace

thought, and I think it's a very beautiful idea, is they imagined in their mind's eye, well, can you

go bigger? Can you imagine more and more massive stars, giant stars, such that the gravitational

pull is so large at the surface that the escape velocity exceeds the speed of light? And then you

wouldn't be able to see them. There's a wonderful quote, actually, in Naples's paper, where he says

that the largest objects in the universe may go unseen by reason of their magnitude. And this is

back in the 1780s or 1790s. So he's imagining stars where the gravitational pull is so vast

that even light can't escape and you couldn't see it. Dark stars, I think he referred to them as

Now we know that such objects do not exist in the universe in that sense, in the sense that

Mitchell-Lund-Plas meant. But actually, they missed something, which is not surprising because

it sounds almost paradoxical. But you can also increase the escape velocity, the surface of an

object, by squashing it. And it turns out that if you take the Earth and you squash it down and

squash it down and squash it down until it's about that big, the radius just less than a centimeter

then the gravitational pull at the surface would be so great that light couldn't escape

And that is essentially the modern concept of a black hole. Now Mitchell and Laplace's

calculations or imaginings were based on Newtonian physics, so pre-Einstein. We come forward to 1915

Einstein published his General Theory of Relativity, which is a different model

a better theory of gravity. And it turns out that black holes also exist in general relativity

Now, if we come forward to 1915, Newton's theory of gravity is replaced by a better model, a better

theory, which is Einstein's general theory of relativity. But the idea that there can be objects

that can be compressed such that they trap light is also present in Einstein's theory

The first physicist to predict such a thing, or at least derive the mathematics that describes such a thing as a black hole, although he didn't know that they existed, was a man called Carl Schwarzschild

What Schwarzschild did was provide an exact solution to Einstein's equations that describes space and time, the distortion of space and time in the region of a star, or at least an idealized star, which is a perfectly spherical, non-spinning ball of matter

It's a model, a simple model of a star. Now, Schwarzschild described or discovered the solutions to Einstein's equations that describes what happens to space time outside such a thing way back, actually 1916, just after the theory was published

What Schwarzschild's solution also describes, though, although he didn't think in these terms at the time, was what that space looks like if you completely remove the star, but leave its imprint in the fabric of the universe behind

And that is essentially the theoretical description or the model of a modern relativistic black hole

Now while Schwarzschild solution and by solution I mean we to picture a distortion in space and time a distortion in the fabric of the universe While Schwarzschild solution does indeed

describe the simplest possible black hole that we can model in the universe, people didn't really

think in those terms at all until later. You see in the 1930s, Einstein and a colleague of his

Rosen, for example, exploring that space-time and building models of what that space-time

might look like. But I think it's true to say that really, certainly until the late 1930s, and actually

arguably post-war until the 1960s, most physicists thought that such things would not exist in nature

So they were theoretically interesting, perhaps not practically interesting. The reason is that you have to create such a thing

So it's one thing to have a model of space and time that describes this object called a black hole from which not even light can escape

But it's another thing for nature to actually make it. So if you go through the 1930s, there's a lot of papers

Actually, Robert Oppenheimer and a student of his Snyder are a very famous paper just before the war

which explored whether a real star in the universe at the end of its life could collapse and collapse without limit to form this geometry, this thing that we call a black hole

Just before the war, Oppenheimer and Snyder showed that under certain assumptions, a star could behave in such a way

But it wasn't really until the work of people like Roger Penrose and Stephen Hawking and several others in the 1960s that it really began to look as if nature would build these things

There's this great quote I remember with some fondness, actually, Arthur Eddington, a colleague of Einstein's, who was very English, very proper physicist

And he said, nature will prevent such absurdities from existing. That's it. Nature will prevent it

Well, it turns out nature doesn't prevent it. We now know, and we've observed them, that stars do collapse to form black holes

And then theoretical physics moves on. So people accept that these things should exist

although it's true to say that we haven't actually imaged one until the 21st century

But in any case, people accepted these things should exist. More and more evidence mounted that they do exist at the centres of galaxies

and at the sites of collapsed stars. But then we have to face the consequences

What does it mean for our understanding of the universe if there are these objects where space and time behave in a very strange way

where light is trapped and where it would seem that anything that falls in

is at the very least locked away from the universe forever. So to understand the conceptual problems that are faced if these things exist

then it might be worth just describing very briefly the Einsteinian description

the pure description in general relativity. A black hole, what do you see from the outside

Well, there's an event horizon surrounding the black hole. In some sense, it defines the boundary between the external universe and the interior of the black hole

The event horizon is very simply and a bit hand-wavingly, but it's a reasonable description

is just if you can imagine a sphere in space and if you go across the boundary into the interior

of this sphere then even if you can travel as fast as the speed of light you can't escape

so the event horizon separates the interior of the black hole from the external universe

we'll see a bit later why that's a bit of a hand wavy description but another description of the

event horizon, which confused people all the way through the history of black hole research

actually, certainly to the early papers in the 1930s and perhaps even post-war, was the idea

that the event horizon, when viewed from the outside, is a place in space where time stops

And that's a direct prediction of Einstein's theory of relativity from the external perspective

If you watched, for example, an astronaut falling in towards a black hole, then from your external perspective, you'd see their time pass more slowly, slower and slower and slower as the astronaut approached the black hole until on the horizon you would see their time stop

That suggested to many people in the early days that a star couldn't collapse to form a black hole

It confused them. They thought, well, if a star's collapsing, then does it not freeze forever in some sense on the horizon

So all sorts of initial early conceptual problems, which ultimately were solved

The thing about relativity is the one sentence thing to understand is that time can stop from one perspective

but time can pass at the usual rate from another perspective. And indeed, from the perspective of an astronaut falling into a black hole, then for a sufficiently large black hole, like the ones that we find at the centres of galaxies, the astronaut would notice nothing at all as they fell across the horizon into the interior of the black hole

So time passes at one second per second on the watch of an astronaut falling in

But from the external perspective, time freezes on the horizon. So black holes are full of these apparent conceptual challenges, which are actually not conceptual challenges at all

They're just a central part of Einstein's general theory of relativity. So that confusion was eventually dealt with and solved and people understood, certainly by the 1960s, what these things are and how general relativity models them

There is a central problem, though, which is still not solved, which is, put it this way, what lies at the centre, and I'll be careful with my language, what lies at the centre of a black hole

Now, in pure, just in Einstein's general theory of relativity, actually, it's not right to talk about the centre of a black hole, really

So what are we picturing? It's this thing called the singularity. You might think of it as an infinitely dense point to which this massive star collapses

It's kind of the natural way to think of it. But actually, just even in pure general relativity, when you look at a nice map of a black hole

the so-called Penrose diagram named after Roger Penrose, what you see is that the singularity is not really a place in space at all

it's a moment in time and actually it's the end of time so one way of picturing what's happened

when a star collapses to form a black hole is that space and time are so distorted that in a sense

their roles swap and so what we thought of as an infinitely dense point a place in space

at the center at the center of the collapse of the star if you like actually becomes a moment in time

and the end of time, the singularity. But the nature of that thing was not

and is still not understood So that a great mystery And it been long accepted that we will need a so quantum theory of gravity a deeper theory of gravity in order to explain the singularity

And for many, many years, actually, until quite recently, then people thought, well, there we are

we have a problem with the singularity, we don't really have any access to it, we don't have the

conceptual tools to explore it. So it may remain a mystery for a century to come, it's not clear

what to do. The great revolution in black hole research was to notice, and it began with Stephen

Hawking's work in the mid-1970s, was to notice that actually there are conceptual problems at the

event horizon of the black hole, not in this extreme place, not only in this extreme place

the singularity, whatever that might be, but at the horizon. Now that was a real challenge

and extremely interesting because we think, we strongly expect the laws of nature that we

understand now, laws of nature that we have full mathematical and conceptual control of

to apply at the event horizon. And so that's why black holes have become so interesting

It's because we have a place where we think, we assume, and indeed we're right to assume it seems

that we have full control of the physics. We understand what's happening, but there is a

fundamental clash of principle between our two basic theories of nature, general relativity and

quantum mechanics. And that is ultimately why the event horizon of a black hole and black holes

themselves have become so fascinating and so important. The modern revolution in our thought

which is still ongoing, by the way, in understanding black holes and quantum gravity

really does begin with work that Stephen Hawking did in the mid-1970s. Stephen Hawking, in his own

words, showed that black holes ain't so black. So we've described them as prisons, in a sense

We're picturing them as a region of space from which nothing can escape. What Stephen Hawking

showed in a landmark couple of papers, a tour de force of calculation actually, is that if you

consider quantum theory, quantum mechanics in the vicinity of the horizon of a black hole

then you find that they glow, they produce particles, they have a temperature. This thing

that we pictured in Einstein's theory as pure geometry, as just distorted space and time, actually emits particles

It's called Hawking radiation. And one way to picture that, and Stephen in his 1974 paper gives this

he says it's a hand-wavy description. So it's not a full description

You need the mathematics for that. But he gives this hand-wavy description

which is kind of a nice way to picture what's happening. The idea is to imagine, to zoom in on space

in the vicinity of the event horizon of a black hole. If you zoom in on any piece of space

the piece of space in front of your nose right now, if you could zoom in and slow time down with a big microscope

you can picture what's happening as a series of particles coming in and out of existence all the time

so-called entangled particles. so it's a picture of what the vacuum of space looks like just to emphasize again it's not a

complete picture it's not supposed to be precise but it's a reasonable model of what's happening

so particles in and out of existence all the time and that's happening everywhere in space

everywhere that you look in the most empty piece of space you could imagine that's what's happening

in the vicinity of the horizon of a black hole you can have the situation where one of those

particles, those pairs of particles, is on the inside of the event horizon and one is on the

outside. And then it can happen that the one on the outside, instead of merging back with its

partner again, can escape into the universe. It's essentially made real by the presence of the black

hole. So the other partner is interior to the black hole and this particle heads off into the

universe, removing energy from the black hole as it goes, this rain of particles, this glow of

particles is called Hawking radiation. That has profound implications. So it's kind of a simple

picture of what's happening. But the upshot is, imagine this thing. It's a black hole. It's glowing

It's just space-time geometry, but it's emitting particles, losing energy, therefore it's shrinking

which means that one day it will be gone. So black holes are not eternal prisons, they have a lifetime

One day whatever's in there is returned to the universe. The question was, the central question

that was immediately raised by those calculations is this, what happened to all the stuff that fell

in? The way I've described it, the way Einstein's theory describes it, is somehow that stuff goes to

the singularity, whatever that thing is, the end of time, a region of space-time that's so convoluted

and distorted that we don't understand how to describe it at all. But then, one day, the whole

thing is gone. All that's left in the far, far future is Hawking radiation, those particles that

were produced in the vicinity of the event horizon. The question is, is it possible if you could

collect all that radiation, all the Hawking radiation through the whole life of the black

hole. Is it somehow possible in principle that the information about everything that fell into

the black hole throughout its history is imprinted in that radiation in the far future? Is that true

or is it not true? You might say, why did I ask that question? Seems like a bit of a random question

It's a very important question. Let's say that I take anything here, in this room, a book, a table, the camera, anything at all, and I set fire to it

I incinerate it. I destroy it in any way that I can

I could throw it into a furnace. I could put it in the heart of a nuclear bomb and explode it or whatever, just completely incinerate it

In physics, in basic fundamental physics, then it turns out that if you could collect every piece of that thing that I detonated or incinerated, every quantum of radiation, every photon, every particle, everything

In principle, if I could just collect it all and I was clever enough, then I could reconstruct the thing that I had destroyed

information is conserved in the universe as far as we know. So every law of nature that we have says that information is conserved The problem was that Stephen Hawking initial calculation of the way those black holes evaporate away

said that information is not conserved, said that black holes are erasers of information

To put it very bluntly, the calculation said that once that black hole had gone

then even in principle, there is absolutely no way you could learn anything

reconstruct anything about the things that fell in, including the star that collapsed to form it

So information erasers, the only information erasers that we know of in nature

That was the initial picture of black holes as Stephen Hawking understood them back in the 1970s and 1980s

This became known as the black hole information paradox. So we have a situation, the 1980s, 1990s, 2000s, where it appears that there's a fundamental problem with our understanding of something

Black holes, quantum mechanics, general relativity. When you put them together, you have this apparent prediction that these things erase information from the universe

Some physicists, Stephen Hawking initially actually, felt that that was the case

Maybe these things do erase information. Maybe we don't care about that

Maybe that's the way the quantum theory of gravity is. And other physicists, people like Leonard Susskind initially

for example, Gerard Zahouft, the great Nobel Prize winning particle physicist and others

felt that, no, there's something wrong with our understanding. There's no way in which we can allow these things to erase information from the universe

thus challenging our understanding of the basic laws of nature. So that debate was vigorous and went on for decade after decade after decade

The reason it's interesting is because there's a very precise problem posed

So it's not some kind of thing like trying to understand the singularity or understand the Big Bang

or where you can just say, well, we're miles away from understanding. The challenges were posed in a region of space

the vicinity of the event horizon, where we thought we had control of the physics

And so that's really useful because it means that it should be the case

that calculations can be performed to resolve. The reason this challenge, this apparent contradiction

this fundamental problem, garnered so much attention, is because it occurred, the problem came from physics

that everybody thought they understood. So calculating in a region of space under conditions in the universe

where we thought and assumed that we had full control of the mathematics of the theories, that's really interesting

because it still means you have a chance catching a glimpse of some kind of deeper theory by resolving this contradiction

So that's why more and more people got interested in the black hole information paradox. It turns out, if we fast forward to the present day and a series of papers are still being written

So this is still research that's happening as we speak. But it turns out now that the general view, I would say the generally accepted view

is that black holes do not erase information from the universe. So in principle, if you could collect all that Hawking radiation emitted over eons

the lifetimes of some of these black holes, by the way, 10 to the power of 120 years plus for some of the big ones

It's one with 120 noughts after it. And the universe is only one with 10 knots after eight years old at the moment, 10 billion years old or so

So we're talking about timescales vastly longer than the current age of the universe

But when they've gone, then in principle, we now think you could collect all that radiation in principle, put it in some quantum computer, carry out some operations and all that radiation and reconstruct the information about everything that fell in

But the implications of that, the mechanism by which that happens, I would say is it's profoundly exciting because it really does seem to be given as a glimpse of a deeper theory of gravity

The simple way to say it, it seems that space and time are not fundamental

So one view, and I emphasize that there are other views, we're at the cutting edge of research now, but one view is a field which has become known as emergent space-time

So what does that mean? It means that space and time themselves are not fundamental

In Einstein's picture, we assume there is such a thing as space-time

This four-dimensional surface manifold is the technical term. But you assume it. It's part of the model

There are such things as space and time woven together into the fabric of the universe

What they are, Einstein has nothing to say. It seems that there is a deeper theory underlying what we think of as space and time, which is a quantum theory

And so the idea is that space and time themselves emerge from what

From quantum entanglement, from some kind of smaller parts or pieces. We don't know what those things are

We don't know the nature of them. But it does seem that there's a deeper underlying theory from which space and time emerge

That is what we call the quantum theory of gravity. But the key thing is, the key thing to understand is we've been driven to that picture by asking very clearly defined, precise questions about how black holes behave

Which goes all the way back to, well, to Einstein and then to Stephen Hawking and many others' calculations through the 70s and 80s

Well, supermassive black holes are fascinating things. things. So these are the black holes that we have images of, radio telescope images. We have two

images from a collaboration called the Event Horizon Collaboration. One of a black hole

it was the first one they acquired, of a black hole in a galaxy called M87, which is about 55

million light years away. This is a big galaxy, something like a trillion suns. And in the centre

of that, we've long suspected there was a black hole. Now we have an image of the black hole

It's a black hole that is so supermassive. So over six billion times the mass of our sun

a big black hole by any measure. We also have an image, by the way, of the rather smaller

supermassive black hole at the centre of the Milky Way galaxy, which is only about just over

four million times the mass of our sun, so it's a little one, but still big

We think that virtually every galaxy has a supermassive black hole at its heart

A caveat a little bit, because a couple of papers have been released recently suggesting

that there may be galaxies observed where there are no supermassive black holes. There are lots of galaxies in the universe, maybe there are exceptions, but it's fair to say that virtually every galaxy has a black hole at its centre

That's an interesting observation because we don't know how those form. One of the reasons is we don't really fully understand how the first galaxy is formed in the first place

It's current research, live research. One of the goals of the James Webb Space Telescope is to observe the formation of the first stars and galaxies

It can do that because it can detect very faint, very long wavelength light, which is what you need to detect if you want to look far out into the universe and therefore far back in time towards the origin of the first stars and galaxies

So the JWST, new instrument that's looking at that. There are also a series of new radio telescopes being built

The Square Kilometre Array, for example, in Australia and South Africa, which is a radio telescope array aimed at observing the formation of the first stars and galaxies

So this is live research. It's a fundamental question. How does structure form in the early universe

Not only are black holes interesting from a theoretical perspective, allowing us to peer or uncover deep questions or ask deep questions about the structure of space and time itself

but they're also key to understanding how galaxies form and how structures form in the early universe

How many black holes do we know of? So direct observation, in terms of radio telescopes, we have a direct observation of two of them from the Event Horizon collaboration

The one in our galaxy, the Milky Way, and the one in the galaxy M87

We also have what I would call direct observation of black hole collisions

This is a completely different technology, a different approach. It's called gravitational wave astronomy

So gravitational waves are ripples in the fabric of the universe. Let's describe them like that

Although I once heard Kip Thorne call them a storm in time

which I think is also a very beautiful idea. And so the idea is that what is a ripple in space and time

They be passing through everywhere now. So through you and me, through the space in which we sit

And they're ripples in space and time. So time speeds up and slows down a bit as these things go through

distances kind of shrink can expand a little bit as well so they're real distortions in space and

time and they're caused by well actually pretty much everything that happens in the universe at

some level but they're very very faint and difficult to detect the ripples we've been

able to detect come from the collisions of black holes or the collisions of neutron stars so these

are highly energetic events in the universe. And the observatory is called LIGO and Virgo

These are essentially laser beams, so-called laser interferometers, about two and a half miles

actually long, four kilometers long, at right angles to each other. And they can detect little

shifts in the distance between the mirrors between which the laser beams bounce, far, far smaller

than the diameter of the nucleus of an atom. So tiny, tiny ripples in the fabric of the universe

By observing those ripples, we can see the collisions of black holes. We tend to see the collisions of big ones

and that's what's called a selection effect. Big ones are more energetic

and so the gravitational waves are easier to detect. But the collisions we're seeing are between black holes

that are, for example, around 30 times the mass of our sun

And the number of those collisions is, I think it's fair to say, larger than anyone would have imagined before the observations were made

So it seems there are quite a lot of these black holes floating around that are very massive indeed, 30, 40 times the mass of our sun and upwards

And I think it's fair to say the formation of those things is a mystery as well

It's presumably that they're formed from the collapse of very massive stars

But the number of them, I think it's fair to say, took everybody by surprise

So we've detected, I don't know what the current number is, actually. I'm speaking now, June 2023, there are collisions being detected reasonably often

But it's tens of collisions, maybe more, actually. But we have multiple observations of collisions of black holes with black holes

and indeed black holes and neutron stars as well. So it's quite a few

So we're very certain now, as certain as we can be, that these things really exist

Instead, we have images and we have these wonderful observations of the collisions of them

Now, how many black holes might there be out there? There'll be billions of them, billions

So all massive stars, stars that are, you know, So three, four, five, six, seven, ten times the mass of the sun

There are lots of stars that are above the limit, beyond which we understand that at the end of their lives

when they run out of nuclear fuel, they will collapse without limit

Nothing will stop the gravitational collapse, and they will inexorably form black holes

So black holes are the natural, we used to say, end point in the lives of stars above, what, three, four times the mass of our sun

We now know, of course, that ultimately those black holes will evaporate away again

But, you know, that really is an in principle statement in the far future

So the way to think about black holes is that any star more than, let's say, three or four

times the mass of our sun and upwards will form a black hole when it runs out of nuclear fuel at

the end of its life. The numbers are kind of debated, but let's say something like two trillion

galaxies, large galaxies and smaller galaxies in the observable universe, 2,000 billion galaxies

pretty much every one of those will have a supermassive black hole at its centre as well

So we're talking about, not to steal Carl Sagan's phrase, but billions and billions of black holes

Carl Sagan said, I never said that. So I'm not stealing Carl Sagan's phrase, actually

because he says that he never said billions and billions. Let me give you some glimpses into why black holes are so fascinating

and so perplexing and wonderful. So if we go back right to the beginning of the work on black holes in the 1970s

Jacob Bekenstein, a colleague of Stephen Hawking's, actually, one of the first researchers to really begin working on black holes alongside greats like

John Wheeler for example. Bekenstein noticed in a simple calculation, which was initially pretty much a back of

the envelope calculation actually he noticed that you can ask the question you can answer the question how much information can a black hole store That a strange thing to say because the model of a black hole is pure geometry pure space

So you might think, well, it's not capable of storing any information. Now, how does something store information? You need some structure, you need atoms

or something that can store bits of information. Well, it turns out that you can calculate that a black hole stores information

And how much? This is the fascinating thing. So it stores in bits the information content is equal to the surface area of the event horizon in square plank units

What's a plank unit? It's a fundamental distance in the universe that you can calculate by putting together things like the strength of gravity, Planck's constant, speed of light

It's the smallest distance often described as the smallest distance that we can talk about sensibly in physics as we understand it, the Planck length

This is a bizarre result. There's so many things that are bizarre about that

The questions it raises, what's storing the information? How is information stored

Why is the information content of a region of space equal to

or why does that have anything at all to do with, the surface area surrounding that region rather than the volume

If I asked you how much information can you store in your room, the room that you're sitting in now, let's say it's a library

then you would say, well, it's to do with how many books I can fit in the room or hard drives or whatever it is, right

It's to do with the volume of the room, the space. But black holes seem to be telling us that there's something about the surface surrounding a region which is fundamental

This is the first glimpse, I think, of an idea called holography, which now seems to be correct in a sense

So holography, what is that? It's the idea that there are different descriptions of our reality

There's one description which is the description that we're all comfortable with and familiar with I would say which is that we live in this space, the three dimensions of space and time is a thing that ticks and Einstein told us that they're kind of mixed up but still you have this picture of space being this, the thing in which we exist

There's an equivalent description, it seems. It's been proved, by the way, for a very specific model called ADS-CFT by a physicist called Maldacena

There's an equivalent description, which is of a theory that lives purely on the boundary of the space

And it's absolutely equivalent. It's an absolute perfect description, a dual theory of the description of the space itself in the interior of this region

That's called holography because if you think about what a hologram is, then at the very simplest level, it's a piece of film

But that piece of film contains all the information to make a three-dimensional image

So all the information about a three-dimensional image is contained on a two-dimensional piece of film

It's basically a hologram. And it seems, from our study of black holes, and the hints go all the way back to the 1970s, to Jacob Bekenstein's work

that our universe is like that, in a way that's not fully understood yet

The picture is that, and physicists, I always like to joke, when they don't really know or the language isn't present

then physicists often say in some sense and wave their hands. So in some sense, there's an equivalent description of our reality

which lives on a boundary surrounding it. It's perfectly equivalent. And you might ask the question, well, what's the real description of reality

And the answer is we don't know, but they're equivalent. So it's strongly suggestive that there's a, let me say a deeper theory

but at least a difference theory of our experience of the world, of space and time

that does not have space and time in it. And that's one of the wonderful surprises that's really emerged from

at least in part, from the study of black holes and the attempt to answer the very well-posed questions that black holes pose

I should say that the work done by Maldesina, the ADS-CFT correspondence, was purely mathematical

So it wasn't framed in the study of black holes, although the questions ultimately seem to be intimately related

So that's number one. So the study of black holes seems to be strongly suggesting that these ideas of holography, holographic universe, which came from a different region of physics, actually, from trying to understand other things, those descriptions may be valid, may be useful, may be in some sense true

The other remarkable thing for me, I think, in the study of black holes, is an intimate relationship between black hole physics and quantum computing

This was wholly unexpected, I think it's fair to say. The relationship comes by saying, OK, so there's a theory on a boundary, a quantum theory, which is a perfect description, a dual description of the physics of the interior of a region of space

You might say then, well, how is information encoded on this boundary

How does that relate to the physics of the interior space and time

And it seems that we're beginning to glimpse and answer, at least in very simplified models

And it seems that the information is stored on the boundary redundantly

which means that you can lose a bit of it and still fully specify the physics of the interior

redundant storage of information. Now we go to quantum computers there's an engineering challenge

in building quantum computers and just to emphasize we have these things they're not theoretical

they exist in labs across the world. There's a challenge which is how to store information in

the memory of the quantum computer safely robustly because quantum computer memory is

notoriously susceptible to any interference from the outside environment. If any of the environment

in which the memory sits interacts with the memory in any way, then the information is destroyed. So

it's a tremendous challenge. And there are deep problems associated with the fact that you can't

copy information in quantum mechanics, which is basically the way that your iPhone or whatever it

stores information and prevents errors entering into the memory of the computers that we're all

familiar with. It's basically copying information. You can't do that in quantum mechanics. Fundamental

It's called the no cloning theorem. Engineers have had to develop very clever algorithms and

ways of trying to store information in quantum computer memory and build the memory such that it resilient to errors And it turns out that the solutions that are being proposed and explored look like the solutions that nature itself uses

in building space and time from the theory, the quantum theory that lives on the boundary

It's really strange. And I just emphasize, you're not meant to understand what I've just said, because I don't understand what I've just said, because nobody understands what I've just said

We're catching glimpses of this theory. And that's where the research is at the moment

It's tremendously exciting. There's papers being published all the time that are digging more deeply

They're suggesting pictures of reality, physical pictures of what's happening. But there's no sense, and I emphasize it, in which everybody agrees on these pictures

So I'm giving you an interpretation, and there will be other people who have different interpretations

But it does seem, it does seem that whatever this quantum theory is that underlies our reality

then there's some redundancy in the way the information is stored in that quantum theory

And it does seem that that's akin to or similar to the way that we will in the future

encode information in the memory of quantum computers to protect them from errors

It's called a quantum error correction code in the jargon. I find that fascinating

It's tantalizing. It's a glimpse of something Einstein said. There's a beautiful phrase which actually the physicist Sean Carroll used as a title of one of his books on this

Einstein said that if you look at nature really carefully and keep pulling at the intellectual threads and keep going and just keep delving down into what nature seems to be trying to tell us, then if you're lucky and persistent, you can catch a glimpse of something deeply hidden

It's beautiful, isn't it? A glimpse of something deeply hidden, which is the deep underlying structure of nature

And it does seem that we're beginning to glimpse something deeply hidden

From the study of black holes, the related, surprisingly, field of quantum computing and quantum information

we're glimpsing something deeply hidden, the deep underlying structure of reality itself

And I think it's very beautiful. No one really knows, I think it's fair to say, where this is going

But they're hints of something deeply hidden. black holes may well help us understand a different but related question which is what

happened at the beginning of the universe so i think black holes are at the moment

the most interest in naturally occurring objects in the sense that they're helping us understand

or forcing us into a deeper understanding of what space and time are and my view is that if we're

going to talk about the origin of the universe, even ask questions such as, did the universe have

a beginning in time, which we don't know the answer to, then it surely seems to me that we

have to understand what space and time are before we can ask and have any chance of answering such

questions. And black holes, it turns out, are the objects that we can see, that we can observe

that actually exist in nature, that force us down that route to ask questions, sharp, well-defined

questions actually, about the nature of space and time themselves. It surely must be the case that as

we get a deeper understanding of what the, let's call it, the fabric of the universe is, then we can

begin to get a rather more, a rather deeper understanding or more insight into questions

about the origin of the universe itself, should it have one? I mean, why do I keep saying should

it have one, by the way? Because you might be watching this saying, well, it's a Big Bang. We know there's a Big Bang. And that's true. We do know there was a thing called the Big Bang

What do we mean by that, though? We mean that 13.8 billion years ago, the universe was very hot and

very dense everywhere. The region of space that now forms the room in which you sit was very hot

and very dense 13.8 billion years ago. We know that all the galaxies are receding from each other at the moment

And just put very simply, if we make the measurements of how they're receding

and run time backwards, then you find that they're all on top of each other

13.8 billion years ago. So we know something interesting happened back then

We have a measurement to it. It's not up for debate. Often when people say, well, how do you know about this Big Bang thing

The vast amount of evidence that such a thing happened, the best evidence probably is you can see it

So we see something called the cosmic microwave background radiation, which is the universe as it was about 380,000 years after the Big Bang

What we see there, we have a photograph of it, is a universe that was very different from the one we see today

There were no stars, no planets, no galaxies. It's just a hot, glowing mass of gas, primarily hydrogen and helium

And we have a photograph of that. So we know something interesting happened

But do we know that that was the origin of the universe? No, we don't

We have theories, strong physical pictures of the universe before the Big Bang

in a very precise sense, before the universe was hot and dense

theories called inflation, which say that space was still there and it was expanding very fast

And then that fast expansion drew to a close and all the energy driving that expansion got dumped

into space, made particles, and that's what we call the Big Bang, the so-called hot Big Bang

as we term it today. What happened to start inflation off? The answer is we don't know

When did inflation start? We don't know. We have a minimum time that it needs to go on for

but we don't know what or even if that phase of the universe constitutes a beginning

There are so many reasons to study black holes. There's fascinating, beautiful things that we know to exist in the universe

pose fundamental questions. But one of the reasons that we're really interested in them is that, put it this way

there's singularities at the end of time. The Einstein description says that inside a black hole, there's a singularity

and locally, at least in that region of the universe, that constitutes the end of time

There's a singularity, maybe, at the beginning of time, the other end of time, if you like, that we're also interested in

You might call it the Big Bang singularity, the origin of the universe

And it seems to me to understand that singularity at the beginning of time, the Big Bang singularity

then we need to understand the singularities at the end of time. And the great benefit is we can see those, so we can watch them bump into each other

we can see them at the centers of galaxies, we can make observations of them, and we can do calculations

Understanding black holes I think will be the key to a deeper theory of our universe So in that sense I think it is fair to say that black holes are the keys to understanding the universe

Chapter 2, Alien Life and the Fermi Paradox. Enrico Fermi is one of the great physicists, legendary Italian physicists, who laid many

of the foundations of modern 20th century physics, so-called Fermi-Dirac statistics and all sorts of

things. If you do a degree in physics, then you will spend a lot of time revising equations and

theories that Fermi did. One of the great legends. The Fermi paradox is probably the thing he spent

least time on, actually. It's almost one throwaway remark that he delivered. And the question is

where are they? By they, I mean aliens. The heart of the Fermi paradox is this. We know that we live

in a big old galaxy, in a big old universe. And let's, for the purposes of this discussion

confine ourselves to the Milky Way galaxy. The Milky Way galaxy, we now know, has something like

400 billion suns. And we now know that most of those suns have planetary systems around them

So trillions of planets. The galaxy has been around for pretty much the age of the universe

10 billion years plus. And so there's a lot of real estate and there's been plenty of time

for civilizations to develop in the galaxy. The Fermi paradox at its heart is the statement that

notwithstanding the fact that there have been billions of years on billions of worlds

for civilizations to arise, we see no evidence of any of them in the galaxy at all. So the paradox

is why. It's a paradox. And I think it's a very good question. It's an extremely good question

And there can be many answers. And the great fun or the great, I would say, the intellectual value

of the Fermi paradox. So if we accept that we haven't seen any, so let's accept that. Let's

accept that, you know, there isn't a UFO sat in some warehouse in Roswell or something like that

Let's accept that we haven't seen any. Let's accept, well, we have to accept the picture that

I've just given you about the Milky Way galaxy and its age, because that's a measurement. So we know

that. The question is then, why? How do we resolve these apparent contradictions and paradoxes

so one answer to the fermi paradox this is the idea that we don't seem to see anyone

is that no one ever evolved right so life didn't get complex why might we think that

well what did we need to produce our civilization on this planet well on this planet

one thing we needed was time we have good evidence that life was present on this planet

3.8 billion years ago, perhaps even earlier. The planet is four and a half billion years old

So we know, as a matter of fact, as an observation on this planet, that life was present 3.8 billion

years ago, but it took 3.8 billion years, give or take a few tens of thousands of years

to go from the origin of life to a civilization. Let's say from cell to civilization. Four billion

years. That's one third of the age of the universe. So a possible answer to the Fermi paradox

the question of why there are no civilizations, is because the Earth is pretty much unique in the

Milky Way galaxy in that it was stable enough, the climate, the conditions on Earth were stable

enough for long enough for life to go from cell to civilization. If you think about it, that's a big

ask. What I'm saying is on this planet, an unbroken chain of life existed for almost 4 billion years

notwithstanding the fact that we live in a violent universe. The sun must have been stable enough

for long enough. We know the output of the sun has changed over those billions of years

But it's not changed so radically that it managed to erase, destroy life on Earth

We know that we live in a violent universe. We know there are supernova explosions all over the place

It turns out that there have been no stars massive enough to explode as a supernova in

a way that would damage life on Earth or erase life on Earth in this vicinity for 4 billion years

We know that nothing has happened. We know that there have been impacts on the Earth. We know that the famous impact that wipes out the large dinosaurs, there's been no impact being enough to destroy the unbroken chain or break the unbroken chain of life for 4 billion years

So maybe, maybe it's the case that whilst there are billions of planets which may have liquid water on the surface, may have oceans that can support life, it may be that none of those planets in the Milky Way galaxy have been stable enough for long enough to produce a civilization

So that would be a property of the planet itself, the so-called rare earth hypothesis

Actually, I should say it's a property of a solar system. It's not really just a property of the planet

It's a property of the parent star. You could ask the question, well, if it was a binary system, for example, a binary star system

Many stars are binary star systems. Is it possible to have a planet with a stable climate, stable enough orbit in a binary star system to support an unbroken chain of life for four billion years

Perhaps not. When we talk about rare Earth, I think I would like to talk about rare solar system

Another possibility with the Fermi paradox is that it's not a paradox

They are here. So there are intelligent civilizations out there and they are present in the solar system

It's possible. Let's think, for example, what such an intelligence might look like

Well, who knows? They could have sent nanomachines to our solar system

There could be probes all over the place in the solar system, but if they're the size of an iPhone, then we'd have no way of detecting them

So it could be the technology of a sufficiently advanced alien species

a civilization, is so beyond anything we can comprehend or detect that we haven't seen it and we've been fooled into thinking

that there are no advanced civilizations in the galaxy. And that's certainly entirely possible

Another possibility, another possible resolution to the Fermi paradox is just that the galaxy is so big

The distances between stars are so great that if you imagine there's another civilization, let's say, on the other side of our galaxy

let's say 20 million, 30 million, 40 million light years away, even if they had the most powerful radio transmitters you could imagine

or even if they'd spread out to neighboring solar systems, then it may just be

Distances are so great that the signals are diluted, that we can't detect them because they're too weak

or that it's just very, very, very difficult in an engineering sense to build interstellar spacecraft

Now perhaps you can build a spacecraft that can hop a few light years away to the nearby solar system

four light years in our case to Alpha Centauri, but you can't build spacecraft that can traverse a galaxy

That's a possibility. One of the arguments against that, for me, is the argument, it's called the space travel

argument against the existence of extraterrestrial life. And it's often framed in terms of self-replicating machines, so-called von Neumann machines

So imagine it's possible for us, for a civilization, to build a machine, some kind of AI, that's

sufficiently smart and capable that it can fly to a nearby solar system, reproduce itself, copy

itself, and then send the copy out to the next solar system, and so on. So you have, if you build

a one successful replicator, you have an exponentiation of replicators. You have one

and then two and then four and then eight and so on. Exponential growth. And you can show that even

given our rocketry technology, you can cover a galaxy like the Milky Way in a reasonably short

space of time. By reasonably short, I might even mean 100 million years, right? That's reasonably

short on galactic timescales. But the key point is once a single successful replicator has been

launched, then it is inevitable that over a few tens of millions of years, the galaxy will be

covered with replicators and we don't see any evidence of them. It's possible that we can infer

that if we assume that we could detect them, then the absence of them may allow us to infer

that no civilization has ever got to that point. And I think that's quite a persuasive argument

Now it's possible that there are many civilizations out there, but the advanced civilizations choose to remain hidden

Sometimes called the dark forest hypothesis, the quarantine hypothesis. We've been asked to make moral judgments or judgments on how a civilization will behave, what they will choose to do

And that's, of course, impossible to judge. But let's imagine civilizations, when they get technologically advanced, also get intellectually and morally advanced

And let's say that they choose, perhaps for good reason, let's say they choose to remain hidden because they don't want to draw attention to themselves

Let's say it's inevitable that if you think about it carefully and you think there are other advanced civilizations out there, then you choose to remain silent

You hide yourself as best you can. It's possible that that's the way that a civilization would think. Maybe that's a logical thing to do

I find it difficult to believe, given human history, that that's the way that intelligent civilizations behave

We certainly haven't made any attempt to remain hidden so far. We've broadcast radio signals out to the stars, the Arecibo message, for example, albeit weak ones

We've launched on our space probes, like Voyager, maps, pulsar maps in that case, which shows the location of our solar system should any other civilization find it

So at least at the moment, we haven't come to the view, which may be a wise view, but we haven't got there, that we should remain silent

Quite the opposite. We've tried at every opportunity to broadcast our existence

Maybe that's because Carl Sagan argued, I think, that a sufficiently advanced civilization, a civilization that can build interstellar spacecraft and communicate across interstellar distances, perhaps is wise enough to have overcome those primitive instincts, the instinct to cause trouble, to fight wars, to colonize, to walk over other civilizations

civilizations Perhaps it inevitable that with technological advance ultimately comes wisdom But it a hypothesis Maybe it is Maybe it just anybody sufficiently clever to build an interstellar spaceship will be also sufficiently clever to hide it

and not draw attention to themselves. Maybe it's immoral, maybe it's like Star Trek

Maybe it's the prime directive. Maybe it's morally certain that if you're sufficiently advanced

you decide to take the position that you will never introduce yourself or interfere with

another civilization. Maybe that becomes a kind of law of nature for sufficiently intelligent beings

maybe that's conceivable as well. Another explanation for the Fermi Paradox might be that

civilizations live and die, they rise and then they fall, and because of the sheer time scales

involved and the sheer size of the galaxy, no two civilizations ever overlap. I once had the great

pleasure of meeting Frank Drake, the Drake equation, a legend in his house. And he also

grows orchids. And I arrived at his house just coincidentally on the day that this rare orchid

flowers. And it flowers for, I think, one or two days and then goes away again for the year and

then flowers again the next year for one or two days. And he used it as an ogy. He said

Or maybe civilizations are like that. So maybe civilizations are like rare orchids

And so they flower and die and flower and die. And just because of the sheer timescales involved, none of them ever overlap

And so there could be the wreckage, the ashes, the fossils of civilizations out there

But of course, we would have no way of knowing until we explore the galaxy and maybe find the ruins of these other civilizations

Who knows? I mean, it's quite plausible if you think about it. are we going to exist in 10,000 years' time? It's, to a large extent, in our hands. Maybe we're

sufficiently stupid that we won't exist beyond the next century. There's an idea in this field, I'm trying to explain the Fermi paradox, called the Great Filter

Now, the Great Filter can lie in our future or our past. So let's think about what it would mean for

a great filter to line our future. That would mean that civilizations do arise in the Milky Way

galaxy and get to somewhere like the position that we're at now. So they develop rocketry

they develop nuclear power, nuclear weapons, for example, they industrialize. But then there's a

filter in the future that stops them becoming true space-faring civilizations. So stops them

becoming multi-planetary species and stops them ultimately traveling between solar systems to begin to colonize a galaxy. So why might that be? Why might there be a filter waiting for us

in our not too distant future that's going to stop us going on to Mars and stop us escaping

our solar system? What might stop us from becoming an interplanetary species and ultimately traveling

out beyond our solar system. I don't think it's technology. As far as I can see, I don't see

anything in the laws of nature in principle that would stop us from becoming an interstellar

species. It might be a thousand years in the future, 10,000 years in the future

might be a hundred thousand years in the future. Even that, right, a hundred thousand years

it's a blink of an eye in the lifetime of the universe, in the lifetime of a galaxy

So I don't see any reason in principle why we couldn't become an interplanetary, interstellar species other than potentially our own stupidity

And I think that probably it could be one of the reasons why we don't see any other civilizations around

It could be that our knowledge, our scientific prowess exceeds our wisdom, exceeds our political skill

It could be that once a civilization develops the means to destroy itself in the form, for example, of nuclear weapons or biological weapons or maybe some kind of a lack of control of AI

who knows, it may be that once a civilisation acquires that technical

know-how, then it goes ahead and destroys itself essentially inexorably, because it's just too difficult politically to run a civilisation that has the power to

destroy itself If you look back through our recent history there have been several occasions that we know about that I know about and you know about where we came very close to destroying ourselves or at least setting us back to the Stone Age basically

The Cuban Missile Crisis, well-documented events in the 1980s, for example, where there could have been nuclear launches and weren't

I'm sure there are many others that we don't know about. There's the challenge of climate change

We're completely incapable of coming together at the moment as a global civilization to address that challenge

That could set our civilization back. Biological weapons, the threat of AI, we seem to be completely incapable of regulating those threats

So it might just be almost a law of nature. Things like us, things that can build an industrial civilization

are just inherently too stupid to get out there to the stars

And I wouldn't put that past us. My favorite is the other one, so I'll do the other great filter

If I was to guess, and this is a guess, if I was to guess why we see no evidence of other civilizations out there

the so-called great silence is what astronomers call it, is because there aren't any and there never have been any

That's my guess. The reason I guess that, and I emphasize it's a guess, is biology

So if you look at the history of life on Earth, then we see that life began 3.8 billion years ago, let's say

But then we see for the best part of 3 billion years on this planet

that there's nothing more complex than a single cell. 3 billion years

It's only in the last billion years or so, perhaps a little bit less, that multicellular life has existed on this planet

And there could be good biological reasons for that. One that springs to mind is the evolution of what's called the eukaryotic cell

which is the cell with the cell nucleus and all little organelles and chloroplasts and plants and all those things

which form all multicellular living things on the planet. Those cells, which seem to be prerequisite for complex multicellular life

evolved once on this planet, as far as we can tell. It's pretty widely accepted. It's called

the fateful encounter hypothesis. And so it seems that there is a very unusual evolutionary event

at some point, maybe a billion, a billion and a half, even two billion years ago, that laid the

foundations for us. If that's typical, if it typically is the case that it takes four billion

years from cell to civilization, then I think there may be very few planets in a typical galaxy

which are stable enough for long enough for that process to proceed. And we could be, for all we

know, on the fortunate end of evolutionary timescales. We don't know. Let's imagine that

actually we were on the lucky side. And really, on a typical planet, if there is such a thing

then it takes three or four times as long. That would exceed the current age of the universe

My guess is that whilst I think there might be microbes all over the place, I wouldn't be

surprised, I'd be delighted, but I wouldn't be surprised if we found evidence of microbes on Mars

Europa, Enceladus, even in the subsurface oceans of places potentially even as far out as Pluto

right in subsurface lakes. If there exists liquid water below the surface, who knows

I wouldn't be surprised if we find microbes all over the place. But a galaxy full of complex

living things, other planets with not only complex life, but sentient life, things as smart as us

things smart enough to build rockets and head out to the stars. My guess is that a typical galaxy

may have less than, on average, less than one civilization per galaxy. Let's put it that way

But actually, just to say, there's a very famous book I'd strongly recommend, Barrow and Tipler

called The Anthropic Cosmological Principle. It's a great book, one of the books I grew up with

And in that book, Barrow and Tipler say that in their view, there might be one civilization

in the observable universe, which should be us, right? So who knows whether we should go that far

But I think civilizations are very rare. And I think it's the biology. So I'm guessing, I'm giving you a guess

I'm saying that in my opinion, I think there's one civilization in the Milky Way galaxy, and there only has ever been one

And there might only ever be one and that us Which by the way means that we have a tremendous responsibility responsibility not to mess this up Because as I said to the I was asked to give a video introduction to the climate summit the COP26 climate summit in Glasgow recently Give the world leaders one minute

they said, you've got a video, say something to them. And I said, I think, let's assume that

we're the only civilization currently in the Milky Way galaxy, perhaps the only civilization

there will ever be. That means the Earth is the only island of meaning in a sea of 400 billion

suns. And so if we destroy this, we might destroy meaning in a galaxy forever. Discuss. So that's my

guess. However, that's a hypothesis. I will be delighted if it turns out that's not true. And

that's not just because it removes some responsibility from me and you and everybody else

to preserve intelligence in a galaxy of 400 billion suns, as the weight of responsibility

is heavy. But also, every scientist should be delighted if they are shown to be wrong

Because the moment you're shown to be wrong, it means you've learned something. And that's the way

that knowledge progresses. So nobody should be worried about making a guess, advancing a hypothesis

an educated guess, or even an educated guess. Don't worry about doing that. As long as the moment

it turns out you're wrong. And me being wrong, by the way, would constitute a flying saucer landing

and some aliens coming out like E.T. and saying hello. So that would be brilliant

But it would be doubly brilliant because it would turn out that I'd learned something about the

universe, which is that complex civilizations are not as rare as I think they are, or civilizations

aren't as rare. So that would be a good thing. So there's a lesson. I think the biggest questions

are in physics, how does space and time emerge from a deeper theory, if indeed space and time

emerge from a deeper theory? I want to know what time is, and I want to know what space is

I want to know how likely it is that life begins on a planet that has the potential to support it

So I think a deep question is the question of the origin of life

How does a planet which is geologically active but devoid of biology become a planet that has biology on it

What is the transition from geochemistry to biochemistry? How does that happen? How likely is it

I want to know how likely it is that given microbes on a planet

those microbes will get together into complex multicellular things. I want to know how something as complex as the human brain evolves in the universe

How common are brains in the universe? I'd like to know what it is

How does this little blob of matter, which is just a pattern of atoms

which is a temporary pattern of atoms, how does that give rise to the feeling of existence

the feeling of consciousness. It's often called a hard problem in neuroscience

and it is a very hard problem. So I want to know the origin and nature of consciousness

I want to know whether it is possible for a computer to be conscious

not only to pass the Turing test, but actually to be conscious. What do I mean by that question

My guess is that it is possible, but I don't know. I want to know whether the universe has a beginning in time

or whether it's eternal. I want to know what the origin of the laws of nature is

Why is gravity so much weaker than all the other three forces of nature? Why does the Higgs field

produce masses for top quarks and up quarks and down quarks, but not photons? I mean

I know the answer to the question, it doesn't interact with photons, but why? What's the

origin of those laws of nature is there only one way the universe can be is there only one logical

construct for a universe and it's this one or are there other possible universes do other

possible universes exist is it possible that you can have universes which do not support life

is it possible how rare are universes that have laws of nature that can support living things

it could go on want to support the channel join the Big Think members community

where you get access to videos early ad free