The Most Astonishing Theory of Black Holes Ever Proposed | Curt Jaimungal

At first sight, it sounds crazy and radical. I must say, it was very surprising to us that

this solution works. Standard physics describes black holes with these paradoxical interiors,

these regions that end space-time. They have infinite curvature, information is lost. Now,

Professor Neil Turok is upending this view with black mirrors, a theory which

incorporates something called CPT symmetry and analytic continuation, all of which are explained

in the episode itself. It makes black holes two-sided structures without interiors. The event

horizon becomes a surface where matter meets its antimatter counterpart, from a mirror universe,

and annihilates. We literally replicated Hawking's black hole calculation, and we were very surprised

we could do it at all. It's a pursuit yielding a finite theory. A theory without no infinities. So

that's very exciting. It's brand new. Potentially explaining particle generations. We cancelled

the vacuum anomalies. We explained why there are three generations of elementary particles. It is,

as far as I know, the simplest explanation anyone has ever given. And bypassing trappings like extra

dimensions and cosmic inflation. We don't need to keep inventing new particles, new dimensions,

multiverses. I think the whole field sort of went haywire. We shouldn't overcomplicate physics.

While we touch on abstruse mechanics like non-invertible matrices and null

energy conditions, don't worry, Neil is a master explicator and today's podcast requires no prior

physics background. We even discuss Interstellar's depiction and why he deems ergodicity arguments

for cosmic uniformity to be absolutely wrong. You recently released a controversial paper on

black holes and how they're more akin to black mirrors. Explain the primary idea behind this

result and why it's caused such a stir among a subset of physicists. What we are explaining is

a mathematical solution to Einstein's equations, which describes black holes rather differently

than the conventionally accepted solution to Einstein's equations. It was motivated by our work

in cosmology, where we noticed that the Big Bang singularity is actually not all that singular.

We used a technique called analytic continuation, which is a mathematical method related to complex

numbers. A very powerful, very beautiful method, which often works in physics. We used that method

to traverse the Big Bang singularity and find a mirror universe on the other side.

One of my PhD students was bold enough to say, why not try this for black holes? I myself hadn't

attempted it because I thought black holes are a lot more complicated. But sure enough, he was able

to get the same method to work for a black hole. Strangely enough, it gave a new and alternative

interpretation of black holes themselves. In essence, the point is that the black hole horizon

is a rather special surface in spacetime. You should think about it as a two-dimensional surface

enclosing the black hole. But if somebody inside emits a signal, we will never ever receive it.

You may wonder, is the inside real if we can never receive a signal from the inside? The conventional

interpretation is that it is real. That leads to all kinds of paradoxes. If something falls into a

black hole, the information it carries is lost and can never be received outside. The paradox gets

even worse if the black hole evaporates quantum mechanically, as Stephen Hawking described,

which is widely accepted that black holes will evaporate. Because this information is then

lost forever, that's incompatible with quantum mechanics. Quantum mechanics doesn't allow you to

destroy information. There are other puzzles about black holes. If we watch somebody falling into a

black hole, we as outside observers would never actually see them falling through the horizon.

What we'd see is that their time would effectively slow down. Anything they were doing, anything they

were using, like clocks, would just slow down and freeze. The ultimate picture we would have of them

is that they're just frozen on the horizon. Again, people have wondered, if what happens inside the

black hole is never actually observable, is it really true that the interior of a black hole

even exists? We applied this method of analytic continuation to the metric of a black hole.

We actually did it for ourselves, or my student did it for himself. But later we discovered that

Einstein himself had used the same method before the conventional description of a black hole

was discovered by Martin Kruskal. Martin Kruskal discovered how to describe the transition across

the horizon in a, let's say, kosher mathematical way. I think around 1960. But even before that,

Einstein was puzzled by the black hole horizon. Einstein and Rosen, the same people,

Einstein-Podolsky-Rosen, the famous EPR paradox in quantum mechanics, the same Rosen with Einstein,

solved the equations for a black hole in a different way. Basically, they used this technique

to transition through the horizon. They discovered what is called the Einstein-Rosen bridge. This

connects two exteriors of the black hole, which are really distinct universes. As you go through,

as you follow the solution to the horizon and beyond, you emerge in the other side of the

black hole. In fact, this is absolutely analogous to what happens in our description of cosmology.

We go back to the Big Bang, and we just follow it through, and we come out on the other side,

and there's another Big Bang there. It turns out that all known solutions of GR have this form. All

known black hole solutions and all cosmological solutions, which begin with radiation domination,

as ours seems to, they all have this property of the two-sided character. But what surprised

us is that we found we emerge on the other side without even noticing the black hole

interior. Mathematically, effectively, you hit the horizon surface on one side, and you come out

on the horizon surface on the other side into the other universe without seeing anything in between.

There is no black hole interior in this solution. Now, that seems strange. Something must go wrong,

because we've managed to avoid the singularity, because in the middle of a black hole, inside the

black hole, there's this curvature singularity, which is where the Einstein equations break down.

If you fall into a black hole, you're going to hit the curvature singularity. There's nothing

you can do, and you'll be crushed and stretched infinitely. So the standard description has this

severe problem that inside the black hole, the equations fail. That doesn't happen in our case,

but something else does fail. It turns out that in the usual picture of general relativity,

you have this spacetime metric, which you use to measure distances. And in the normal approach to

general relativity, that's a matrix. This method is a four-by-four matrix, and one of the axioms is

that it must be invertible. You must be able to write down the metric, and it's matrix inverse.

It turns out that in this coordinate system we are using, and which Einstein and Rosen used before

us, the metric fails to be invertible exactly on the horizon. So it's completely analytic,

meaning it solves the field equations, but this one axiom breaks down on the horizon. So

we would say we have a type of singularity. It's in the conventional sense of GR. You can't only

use conventional GR to make sense of this, but it's much milder than the singularity you would

otherwise have if you took the inside seriously. So in other words, we found a way of avoiding all

curvature singularities in black holes, which involves accepting another kind of singularity.

Essentially what happens is the metric is not invertible on this surface. Now,

is that a catastrophe, that the metric is not invertible? No, by no means. There's nothing,

you know, God-given that says that the geometrical description – you see, essentially the idea that

the metric is invertible can be phrased much simpler by saying that locally in spacetime,

if I use a magnifying glass and I zoom in as much as I can, then locally the spacetime just

looks like flat Minkowski space. There's no impact of gravity at all on short distances.

That's the usual way. And if you say that, then when you zoom in on a given point in spacetime,

you can always use the Minkowski metric and just forget about gravity. And the Minkowski metric is

invertible. So that's the usual justification. So we are saying something special does happen

on the horizon, but it's not that bad. It needs a physical interpretation. What special

is happening? Now, the special thing that's happening is to do with CPT symmetry. So CPT

symmetry is charge conjugation, parity, and time reversal, which basically means that you take the

conventional description of it is you take the coordinates in spacetime, which we think about as

numbers – there's the time coordinate and three space coordinates – and you replace them with

minus themselves. Now, probably the nicest way to think about this is if in effect you are rotating

space into time. So if I think of time going up and space going sideways, you do a rotation by

180 degrees, so time goes down and space goes in the other direction. So that is what we call a PT

transformation. It's parity reversing space and T, time reversal, reversing time. Now, in special

relativity, you're not allowed to rotate space into time. We're allowed to rotate space into

space because we see that the world is pretty much invariant under rotations in space, but you can't

rotate space into time. Why? Because in special relativity, you're only allowed to boost, meaning

you can travel faster, and that has the effect of squishing space and stretching time, but you

can't actually rotate them into each other. Now, again, this comes into the mathematics of complex

numbers. So it turns out that in particle physics, when you calculate scattering of particles or any

event involving ingoing and outgoing particles, you are allowed to rotate space into time,

and that's an exact symmetry. So one of the most famous expositors of quantum field theory is

Sidney Coleman, and he has this beautiful book. His lectures at Harvard are sort of a classic,

and his students wrote them all up, and they're the best place to learn about CPT, by the way.

Sidney says, look, if we discover in experiments that charge conjugation is violated, you know,

when you change a particle into an antiparticle, you discover that physics changes, that's no big

catastrophe. If parity is violated, you know, inverting space is not an exact symmetry, that's

not a catastrophe. And same for time reversal. The laws of physics we know do actually violate

time reversal, space inversion, and charge. Each of them is violated separately. But,

he says, if CPT is violated, that is a complete calamity. We would have to start all of physics

again. So CPT is very profound. Now, it changes particles into antiparticles, and the nicest way

to picture this geometrically was realized by a guy called Stuckelberg in 1941. He was a genius

in Austria who was not sufficiently recognized in his lifetime. But he realized that if you think

of space and time, so time goes up, space goes sideways, and now think of a particle. What's a

particle in space-time? So a particle is what we call a world line. So this particle is a curve,

every particle follows a curve through space-time. So if I slice the space-time in the time

direction, I'll see this point moving along in space on different slices, you know, as the slices

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in the description. I'll see this point moving along in space on different slices, you know,

as the slices proceed. So Stuckelberg said, okay, that's the picture of a particle in relativity,

and in classical general relativity, it can't go faster than light. And that always means that

this line going up in time, if the particle is stationary, the line just goes vertical.

But if the particle's moving, it goes at an angle to the time axis, because moving along in space,

it's not allowed to go faster than light. So the slope can never be bigger than 45 degrees from

the vertical. And so Stuckelberg said, wait a second, in quantum mechanics, we have events,

processes called quantum tunneling. And they allow things which are impossible classically,

like particles going through walls, but they're perfectly possible in quantum mechanics. He said,

even though classically a particle can't go faster than light, quantum mechanically, surely it's not

disallowed. So he said, what happens if I have a particle which is traveling forwards in time,

and then it gets faster and faster, and its world line tips over, and it ends up going backwards in

time? And he said, that's got to be allowed by quantum mechanics. And he interpreted, he said,

you see, when it's going forwards, and we do our time slices, we will see a single particle going

up where the line intersects the plane. But when it comes back, we see another particle,

except it's going backwards in time. And that's an antiparticle. And Stuckelberg

realized that quantum mechanics and relativity inevitably predicts that for every particle,

there is an antiparticle. And the interpretation is that an antiparticle is just a particle that

happens to be going backwards in time. Yes, many people attribute this to Feynman. Yeah, that's

not right. Feynman got the idea from Stuckelberg. And Stuckelberg left so-called fundamental physics

and worked on chemistry, mainly because his work wasn't appreciated enough. But as time goes on,

you will find him mentioned more and more and more often. He had incredibly deep insights into what

we now call quantum field theory, actually, long before Feynman. Lyle Troxell Wouldn't that also

show a particle disappear? Oh, no, but that's right. If there was a particle, antiparticle,

right. So the interpretation of this funny curve up and down is that our interpretation,

our picture of it as time proceeds, is we see a particle, an antiparticle,

and they come along and annihilate. And we see that in laboratories all the time. And likewise,

you can have a particle coming in from future time and turning around and going back up again.

And that's pair creation in an electric field. If you switch a strong electric field on, then

it literally pulls an electron out of the vacuum in the direction opposite to the electric field,

and it pulls a positron, a positively charged electron, or the electron's antiparticle. It

also pulls that out, and the two together go flying apart. And Stuckelberg said,

you know, this is inevitable. You can have this process. Now, in fact, the particles annihilating

and the particles being created, the pairs annihilating or being created, those are CPT

conjugate processes. If I just turn the picture upside down, which is the CPT transformation,

the one is exactly the other. The rates of them have to be identical, and that's the CPT theorem.

Our picture of the Big Bang is, in fact, completely the same, mathematically,

as a particle-antiparticle pair being created. We have these two sides of the Big Bang, something

our universe coming out of the Big Bang, and then on the other side, the CPT image or anti-universe,

from our perspective, it's going into the Big Bang with a sort of reverse direction of time.

But from its own perspective, it's just the same as ours. So we see this happening in physics,

the consequences of CPT symmetry are happening in physics we absolutely know and trust. And all

we have done is generalize the same mathematical principles to cosmology and now to black holes.

Now, to come back to the black hole, when you fall into the horizon and you hit the special surface,

what's going to happen? Well, what happens is very dramatic. As you fall in from this side,

the other side is part of the anti-universe, and so there is antimatter. There's an antimatter

version of you falling into the other side at the same time, and both of you will hit the horizon

at once. And what will happen is the particles you are made of and the antiparticles the other

version of you is made of will annihilate into radiation, and that will travel up the horizon

and eventually escape when the black hole evaporates. So it is a complete picture,

not only of formation of what black holes are, but of how they can evaporate and where the matter

that forms the black hole ends up, which is it just annihilates into radiation and runs off to

infinity. Now, I have to say that only the first part of the story, the stationary black holes,

so this would be Schwarzschild, which is not charged or rotating, or charged black holes,

exactly the same thing works, or even rotating charged black holes, which are the most general

case. We've shown that mathematically they all have exactly the same property. But what we have

not shown is that in the time-dependent black hole case, a black hole actually forming by collapsing

star, and then evaporating, that's a much harder problem to describe. And so we're working on this,

and basically this requires... new approaches to solving the time-dependent Einstein equations,

which still need to be developed. So this is still a work in progress, but it's very exciting,

because potentially there would even be signals of this matter-antimatter annihilation

on the horizon. So your innovation and your collaborators as well wasn't just analytically

extending? Right. Okay, because that's been done since the 60s, as you mentioned. Yeah,

no, but the funny thing is that this particular way of analytically extended preceded the work

in the 60s, as I said Einstein and Rosen used it, but they of course only did Schwarzschild,

the simplest solution that was known then. What we've done is use actually the same

analytic extension, but we've applied it to all possible black holes, and we find it still works.

I think the fact that there was an alternative was not noticed by people in general relativity,

because they were insisting that the metric has to locally look like Minkowski space-time,

at every point in space-time. And that does not happen on the horizon. On the horizon,

you have this funny, technically you say that two of the eigenvalues

switch. That's what happens on the horizon. The time-like one becomes space-like, and the

space-like one becomes time-like. So they both go to zero on the horizon. So something, let's say,

different than normal GR, general relativity, does happen on the horizon mathematically. But to us,

it seems like this is easily the most minimal resolution of all the puzzles associated with

black holes. I mean, our whole philosophy is that we shouldn't overcomplicate physics. We

need to always look for the simplest, most minimal resolution of the most profound puzzles. So what

was the Big Bang? We claim we can understand that by this process of analytic continuation.

And there's some new developments on that front, too. When dealing with black holes,

we would say that the conventional description has these pathologies that you lose information, that

you have a curvature singularity, which is just unremovable. I mean, it means the theory fails

irredeemably. Finally, actually, the conventional description is inconsistent with CPT. It's just

inconsistent. And actually, Stephen Hawking, the last paper he ever wrote on black holes was

called something like the Black Hole Information Loss Problem and Weather. It was a funny paper.

He was trying to explain that if black holes evaporate, the information gets scrambled and

it's more like the weather. We can't predict the weather tomorrow, but that doesn't mean we don't

believe the equations. But during this paper, he explained that one of the basic paradoxes with

black holes is the usual description seems to be incompatible with thermal equilibrium. So what is

thermal equilibrium? Thermal equilibrium is where you have stuff, let's say, in a box and it's hot.

And so if it's molecules, they're flying around at high speed and interacting with each other. And

there will be radiation that's bouncing off the walls of the box. This is a very generic physical

situation that you have hot stuff in a box and it's fluctuating into all kinds of configurations.

So imagine you put a black hole in this box. Well, CPT symmetry demands that for every process

forming a structure, like forming a black hole, you're inevitably going to form black holes out

of matter happening to fall in towards itself. So every process in which you form something,

there must be an exactly equal process in which it unforms. That's what CPT symmetry says. Whatever

comes in at whatever rate, there must be an exactly mirror image process where stuff comes out

and unforms that structure. Now, in the usual description of black holes, that's impossible

because stuff falls in and forms a black hole, and that's the end of the story. I mean, you can't

unform the black hole. So he said the conventional picture of a black hole is incompatible with CPT

because we don't have white holes. You know, there's a black hole where things only fall in,

but there is also a white hole solution where things come out. And the problem with the usual

description is that we ignore the white holes and we only include the black holes in our description

of thermal equilibrium. And Hawking said that just doesn't make sense. So our black mirrors, we

believe, are perfectly compatible with CPT. That's how we construct them. And therefore, they're

perfectly compatible with thermal equilibrium. So they seem to have a number of advantages. But as I

mentioned, a lot remains to be done to understand when such a black mirror actually forms,

exactly what is seen from the outside as it settles down. Or in particular, if two black

mirrors interact, that's a very tough problem. And there's such exciting progress in the last

20 years because now we can literally see black holes merging. And as they spin around each other,

they emit gravitational waves. And we see them actually merge into a bigger black hole. So all

of this stuff is now possible to watch happening. And the next few years, there will be literally

movies of black holes merging because the gas which surrounds them is like a tracer. And so we

can see the gas with radio telescopes. And so with powerful enough radio telescopes, we can actually

see all of this amazing physics happening. So that problem of understanding exactly how two

black holes merge was only really solved about 20 years ago. Using powerful computational techniques

and supercomputers, you can put Einstein's equation on a computer and see what it predicts.

But that's the prediction from the conventional picture and includes the black hole interior.

In our prediction, you basically need what is called different boundary conditions on the

horizon than the ones people would normally use. And those will change the evolution of the black

holes. And so that's going to take some time to sort out. It's a harder problem to solve than the

conventional approach. Because in a certain sense, we are putting in a boundary condition

in the future as well as the past. You'll notice that when I turn space-time upside down,

the future becomes the past. And that's one of the appeals of our cosmology picture, is that

we claim that the arrow of time emerges in this picture, because on the two sides of the Big Bang,

you've got time going in different directions. So time goes forward out of the bang on both sides.

And somebody inside the universe would see only one of those two arrows. And so we claim that the

arrow of time emerges from a Big Bang within this CPT symmetry picture and doesn't have to be put

in from the outside. In conventional approaches to physics, the arrow of time is just put in at

the beginning with no explanation, even though the laws of physics don't violate CPT, which includes

time reversal. People just assume that the state of the universe somehow does violate CPT. Now,

when it comes to solving these two merging black holes, usually people would specify the

configuration of the black holes at one time and then just run the equations forward to see what

happens. But in a CPT symmetric picture, it's a little more involved, because what you have

to do is impose conditions not just in the past but in the future. Now, why wouldn't it

be that by imposing conditions on the past, it automatically imposes conditions on the future

if they're symmetric? Good point. That would be true classically. But in quantum mechanics,

quantum mechanics is very different than classical mechanics in the way it treats the past and the

future. In classical mechanics, the world is a machine. You just specify the configuration, like

the particle positions and momenta, at one time and just run it forward. In classical mechanics,

you cannot specify the complete state of the system at two times. You're not allowed to do

that. I mean, if I tell you what the positions and velocities are now, you can't tell me, oh no, I'm

going to freely specify them at some later time. It's inconsistent, because it won't agree with the

evolution of the initial condition. But in quantum mechanics, this is not true. In quantum mechanics,

you are free to specify the wave function at two times. And so I can tell you what the wave

function is at one time. You see, it's only a function of the coordinates. And I'm not allowed

to tell you the velocities if I told you the wave function of the coordinates. So if I tell you

the coordinates, you can either specify the wave function of the coordinates or the wave function

of the momenta. You can't do both. But the upside of that is I can tell you what the wave function

is at one time, arbitrarily, and I can tell you what it is at a different time, arbitrarily,

and then I can predict what happens in between. And this is a point made by Yakir AhAharonov,

who's probably the deepest thinker on quantum foundations today. And in fact,

all he does is think about paradoxes and puzzles and thought experiments. And he does it better

than anyone else. And his point is that in quantum mechanics, it's very natural to have two times.

Our point is that that allows you to impose CPT symmetry on the universe. Because you say,

I take my initial wave function and my final wave function, and CPT symmetry asserts that they are

identical. And then I just figure out what happens in between. And we live in between,

and we can then predict everything that happens in between. So in the case of the black hole,

we would tell somebody who's going to do a simulation of black holes merging, that you should

specify the initial condition, let's say, of the matter falling in, but incompletely. You can only

tell me the momenta of the particles coming in, not their positions, or vice versa. And then

in the CPT symmetric picture, the outgoing state has to be the image of the incoming one. And

those two, when they're adjusted, will give this special behavior on the horizon, which is the same

as you get in the stationary black holes, where everything is, say, analytic on the horizon.

So basically what seems to be required to predict the fate of a black hole is to say something about

the future as well as the past. Now that, at first sight, sounds crazy and radical and so on,

which it is. But in this two-sided cosmology, it's absolutely natural. Because in the two-sided

cosmology, we have the future coming out of the Big Bang, the future universe, the past universe

coming out in the opposite direction. Now, really, these two are mirror images of each other, because

the final condition is the same by CPT symmetry, or it's related by CPT. So the one is literally

the mirror image of the other. So what I can do is fold the lower universe, think about it as a sort

of cone coming out of the Big Bang. So fold it up so that it doubles the upper cone. Now what I have

is what we call a two-sheeted universe. And it's just like the particle-antiparticle pair. Imagine

if you really put those two things on top of each other. This double-sided universe is like the

universe-anti-universe pair, and you can think of them as being parallel to each other. You see, the

picture is very beautiful. It says that the future universe you should think about as a sheet, as

one of two sheets. And there's, if you like, the past universe is the other sheet. Now what goes on

when you make a black hole? Well, literally, you cut a triangle out of the future sheet,

and the same thing happens on the past sheet. And those two cut-out triangles are put on top

of each other like this, and there's nothing in between. It's just a seam where they join, where

the two sheets join. So the black hole horizon is the seam. There's nothing inside the black hole.

There's a hole in this double-sided universe. And then when the black hole evaporates, the whole

thing reglues, and the black hole goes away, and we're left with two sheets again. So the formation

of the black hole is literally just the sticking together of the past and the future universe,

in which the section that's stuck together is just eliminated. It doesn't exist. It's literally a

hole in this double-sheeted picture. But all you have on the sides of the hole are a seam. Okay,

I have some technical questions, but people who are watching, before I get to them, they may be

wondering, what happens to me as I fall toward the black hole? Yes. So what happens to me in the

traditional picture prior to this paper? Right. And then what happens in your view, or in you and

your collaborators' view? Brilliant. Yes, exactly. So the traditional picture is that you would

experience nothing special at all as you cross the horizon. You're sitting in your spaceship,

you know. You see, the matter of when you cross the horizon... There are actually different

definitions of when you cross the horizon, because the horizon is a somewhat subjective notion in the

sense that if I'm trying to communicate from my spaceship to another spaceship that's, let's say,

further out from the black hole, depending on exactly where that spaceship is, I may or may

not be able to send signals. So when I cross the horizon, the usual definition of what's called the

event horizon is that when I cross the surface, I cannot communicate to someone at infinity,

infinitely far away from the black hole. No signal I send will ever reach infinity. But if someone's

nearer, you may be able to communicate with them. And so there's something called the event horizon,

there's something called the apparent horizon. This is a surface which Roger Penrose defined in

his proof that black hole formation is inevitable, and his definition was much more physical.

It was that if you imagine sending out light rays in this spacetime where the black hole is forming,

there will be some of those shells of light rays will start reconverging. And when they reconverge,

they can never diverge again. So basically, when the outgoing light rays start to converge,

you can call that when the black hole is formed locally. And so that's called the apparent

horizon. So there's still this ambiguity about exactly where the horizon would be. Our best guess

would be that in the conventional picture, nothing happens at all. You just fall across the horizon.

Okay, some of your signals, both horizons, doesn't matter if it's apparent or not. It doesn't matter.

In the standard picture, it doesn't matter at all, because locally, you have no idea whether your

signals are ever going to reach somebody. It's not something that concerns you at all. You might

send a signal and nobody ever receives it, but you know, so what? You don't experience anything

in the standard picture. You just fall across the horizon and nothing happens to you at all.

What happens next is very dramatic, because you then inevitably fall into the singularity and

get crushed. So that's the standard picture. Nothing exceptional happens at the horizon,

at either horizon at all. The horizons, by definition, are just where light either

fails to make it off to infinity, or the outgoing light rays start to reconverge. And in fact, that

doesn't really affect you at all, either, because it's a very global property. It's not something

you could measure locally. Okay, so when does this crushing occur that people see in sci-fi movies?

And where's the hypercube from Interstellar? Oh, it's at the singularity. Okay, so in Interstellar,

the assumption was that they went into the black hole, and then something very spectacular happens

at the singularity itself. Now, the truth is that no one has a clue how to make sense of a

curvature singularity in general relativity. What happens is that space shrinks in one direction

and blows up in orthogonal directions. So typically, it shrinks in one and blows up in two,

or shrinks in two and blows up in one. And that's just sort of a catastrophic failure of the theory.

The whole picture of space-time gets stretched and crushed alternately. In fact, there's something

that happens there called mixmaster chaos. And the mixmaster was a machine in the 1960s,

which is a food blender. Okay, so some company, I'm not sure which, maybe it was General Electric,

made mixmasters. And so this phenomenon of this space-time in which things get crushed and

stretched and crushed and stretched alternately is called mixmaster behavior. So that is the

classical expectation. And in Interstellar, that doesn't make any sense. Everything goes haywire.

So in Interstellar, they replace this by somehow time travel and the ability to communicate,

let's say, across time. But nobody really has, I would say, a good physics idea for how to make

sense of what happens to you. There are notable attempts by people who study holography. And they

have a much more radical picture than ours, which is that there are wormholes. I guess it's a little

bit like Interstellar. There are wormholes which connect the interior of different black holes and

share information across these two black holes. But to be honest, I've never been able to make

sense of that picture. It's far more radical than ours. Okay, so in the traditional picture,

you pass these so-called horizons. You don't notice anything as you're passing through. And

then eventually you get squeezed into a tube, and then you reach what is called the singularity,

the curvature singularity, because just like there are different forms of horizons, there

are different types of singularities. Curvature singularity, that's right. So you meet that, and

then no one knows what occurs once you meet that. Okay, that's the traditional approach since the

20s, 30s? That's the traditional approach. I would say no. It became accepted after Kruskal analyzed

the Schwarzschild metric, which is the metric of a non-rotating, non-charged black hole, the simplest

black hole. Kruskal analyzed it and realized that there was a way to analytically continue

across the horizon, which left the spacetime locally Minkowski everywhere, except at the

singularity. So, yeah, the conventional picture was only really began to be accepted in the 60s.

But since then, it's been, I mean, all the general relativity community has essentially

bought the standard picture. Okay, and now you come in. So the person listening is wondering,

they are falling toward a black hole. What do they see as they're going toward it, and what

occurs as they move past the horizon, if they can even move past it? Yes. Good. So essentially,

nothing happens in this picture until you encounter the special surface. And then

something extremely dramatic happens. And this is well before anything would happen in the standard

picture. What happens is that you encounter antimatter. You encounter an anti-version of your

spaceship containing an anti-version of you. Of yourself. Yes. And the two spaceships would meet,

annihilate into radiation, which would then fly up the horizon and off to infinity. So it's extremely

dramatic. It could not be more different than the standard picture. Now, would you even see that

other person? Let's say there is no... No, no, no, you can't. You don't have a chance because

the way light travels in the space-time forbids you from actually seeing any signal. From the

other side. Until you hit the horizon. The horizon is the first surface at which I could actually see

something coming from the other side. I cannot see it before I hit the horizon. Yeah, in your

paper you joined two boundaries, one of sigma plus zero and one of sigma minus zero. Exactly,

exactly. So sigma equals zero is where the two join. And neither side knows anything

about the existence of the other side until you hit that special surface. So, yeah, it's

a very different picture. By the way, some ideas which in a certain way anticipated what we did

also became popular briefly in the string theory community and the I guess 2020s. Sorry, 2000s,

which was called the firewall. People thought, people argued, and this was Joe Polchinski and Don

Maroff and others. They argued that because black hole formation violated quantum mechanics so badly

in the conventional picture, there had to be a different resolution. So they argued there must be

a firewall. There must be something which prevents you from going into the interior. And, you know,

there was a lot of debate. These are very smart people, and there was a lot of debate about it,

but I think it was inconclusive. So our picture, I think, is a more, is a better, I would claim,

a better motivated mathematical description than a firewall. But, you know, something very dramatic

is going to happen when you hit the horizon. And it's important to realize that process is quantum.

As you hit the, you know, the process of pair annihilation, as I described at the beginning,

it cannot happen quantum mechanically. It's just not, sorry, classically, it's not allowed. It

depends on the particles going faster than light for a brief quantum moment. You know, that's,

this curve turns around. That's pair annihilation. And what we're claiming is that is exactly the

process which saves the black hole, in the sense of making it compatible with quantum mechanics,

is that the particles come in from one side, the antiparticles from the other side,

they annihilate and sail off as radiation, and there is no interior to the black hole.

So, I imagine that you checked other invariants to make sure there's no other form of curvature, like

the Kretschmann scaler? Exactly. No, everything is completely regular. All curvature invariants

are regular at the horizon. There's nothing new. But all we're saying is, actually, we found an

analytic solution of the Einstein equations, which extends, as I said, up to the horizon

of the first exterior, and continues onto the horizon of the second exterior, without including

any interior. I mean, I must say, it was very surprising to us that this solution works. We

were expecting to find something on the horizon, like a kink in the geometry, which forced you to

have some kind of stress energy source. This is typically what happens in general relativity.

If you try to make a spaceship, for example, which goes faster than light, or, you know, violates any

of the classic principles, you generally find you have to introduce weird forms of matter,

which kind of allow this behavior. What we found is we didn't have to introduce anything. This

is just naturally there in the Einstein theory. So you don't introduce any odd forms of matter,

but there is an odd metric. Is that what psychologically prevented people from coming up

with this solution? Because CPT symmetry is known, and analytical continuation is known. Combining

them has this, what is it, eigenvalue degeneration on the surface? Exactly. Yes. It's a swapping over

of eigenvalues. So in the space-time metric, one of the eigenvalues is, let's say, negative, and

three are positive. It's a conventional choice, whether you make one positive and three negative,

or one negative and three positive. But let's stick with one negative and three positive. So

what happens when you hit the horizon, the horizon is a two-sphere, and it's completely regular. So

that has two positive eigenvalues, and they're all fine. There's nothing weird in those two

dimensions. They're perfectly regular geometry. There are two dimensions left, and you can think

of them as one of them is the radius, and the other one is the time. And what happens is that

the eigenvalue of the metric in the time-time direction and the space-space direction, so one

was negative, one was positive. What happens is at the horizon, the positive one goes negative,

and the negative one goes positive simultaneously. So space and time effectively switch roles. And

that's what happens. And indeed, I think the reason people miss this, though with hindsight,

Einstein did not miss it, as it turns out. It's in his paper. But the reason people missed it,

starting in the 60s, is that they treated the space-time metric as sacrosanct. It had to be a

4x4 matrix, which is symmetric and invertible. And that fails. Now, actually, you could say,

why does the space-time metric have to have an inverse? I mean, it's something we normally use

in the mathematics of GR. But I realized, and it's only last week, that actually when you – so one

sort of derivation of general relativity from, let's say, quantum field theory principles,

is that all you assume is a spin-2 particle. And actually, this derivation goes back to Feynman.

Feynman said, you know, people are making all this fuss about curved geometry, but actually, if we

have a spin-2 particle, it travels along and it's spinning around with double the spin of a photon,

and we have energy and momentum conservation and relativity. And then we try to see, what

is the most general possible interaction between these spin-2 particles? You can go through various

calculations and you discover, basically, general relativity is the only game in town. Although

Einstein had this amazing picture, which gave the full nonlinear theory out of geometry, you know,

general relativity is all about geometry, Feynman said, actually, this is completely

compatible with particle physics, as long as we have spin-2 particles. And we would end up with

a similar conclusion to Einstein, but on a sort of much more nuts-and-bolts point of view. Now,

from that Feynman point of view, it turns out that to derive general relativity from spin-2 and

special relativity, what you use, and this is a little bit technical, I'm sorry, but what you

use in the action is what's called the densitized inverse metric. Not the inverse metric. Okay, what

does that mean? Basically, you have root minus g, you might remember from the volume element,

gets multiplied by the inverse metric. And that's the only thing which occurs in the derivation.

And it turns out that quantity is not singular in our description of gr. As well as the freedom to

change coordinates, you have freedom to change the variables which depend on those coordinates. So,

in E&M, we have electric fields and magnetic fields, and we also have the spacetime

coordinates. And we never think of any particular choice of those coordinates as being better than

any other choice. You're free to change variables if you want to make the equation, you know,

if you discover the equations are not well-defined or have a singularity, what you should do is

change coordinates either on spacetime or on your field variables to try to make the equations make

sense. And if you can do that, that's perfectly fine. So what we are claiming is that there is a

choice of variables on spacetime, at least as far as the metric is concerned, which leaves

everything regular. I believe what happens is that there's something else in gravity called

the Christoffel symbol. And the Christoffel symbol actually is singular. And that tells you that as a

particle hits the horizon, it experiences a sudden force. And the sudden force forces it to travel

up the horizon. In other words, forces it to go at the speed of light. Because the only way to escape

falling into the black hole is to travel at speed of light, because the horizon is a light-like

surface. And the only way you're going to travel at the speed of light is if you encounter this

antiparticle with whom you annihilate. So there is a singular singularity, but it is

not as simple as just saying, oh, the metric is no good on the horizon. That's too simplistic,

because the metric itself is not a, you know, the inverse metric, I should say, is not a, our

metric is actually fine. It's the inverse metric, which doesn't exist. I see. But there's nothing

sort of sacrosanct about the inverse metric. It's just… Now, if you don't have the inverse metric,

can you even form the Ricci scalar? Yes. So the way you do it is you define the Christoffel symbol

and this densitized inverse metric as your two independent dynamical variables. And all of GR

can be formulated purely in terms of those. So this was done by Stanley Dezza a long time ago,

maybe in the 70s. And what he did is he found a sort of much simpler version of Feynman's, and

more rigorous version of Feynman's argument that spin-2 and special relativity give you gravity,

give you general relativity. Now, how would you say that this metric, the eigenvalue swapping at

the horizon, how does it affect the quantized field propagation across the surface? Great,

great question. We are just beginning to study this. What we can say is that in cosmology,

when you study the Dirac equation across the Big Bang, there is no singularity at all.

The Dirac equation is completely insensitive to the shrinking away of the metric. That's called

conformal invariance. There's a mathematical reason why neither Dirac equation nor the Maxwell

equation sees the Big Bang singularity, although the metric disappears there. In the Big Bang, it's

even worse. All four eigenvalues of the canonical metric vanish for a moment at the Big Bang in our

cosmological version of CPT symmetric cosmology. So, but it turns out that equations that physics

is built from, like the Dirac equation and the Maxwell equations, do not see that singularity.

The equations are still perfectly sensible. Now, why is it that you say that you get

annihilated at the surface instead of redirected to some second exterior universe? Well, because

you have to take a particle, which we're assuming is a massive particle, falling into the horizon,

and you've got to suddenly accelerate it to the speed of light. So, as I said, the Christoffel

symbols do that. They do seem to diverge as you hit the event horizon. But yeah, I mean, maybe

that happens on its own. Maybe it happens as a consequence of meeting your antiparticle. I think,

you know, further study is needed. As I say, it's a quantum process. You can only really describe

it using quantum fields on this space time. And that study has only just begun. Would you then

say that the space time is geodesically complete for causal geodesics that are not radial? Yes,

only if it is possible for a particle with a mass to be accelerated to the speed of light as it hits

the surface. That's what makes it possible for the space time to be geodesically complete. So

it's a big if. Classically, it's very difficult to accelerate a particle to the speed of light.

I don't know, even if the Christoffel symbols diverge, you would say there'd be huge back

reaction and all kinds of complications. But we know the process must be quantum. And the

way to study it is to study quantum fields in this background. And there are already suggestions from

earlier studies of quantum fields on black hole backgrounds that do indicate this kind of behavior

is possible. You see, when you study, it's a funny fact about the conventional description

of black holes is, as I've mentioned, there are two sides, the two exteriors of a black hole. Now,

Werner Israel described this using quantum field theory. And what he was able to do is show that

you can give a complete description of the quantum field on the black hole by only referring to the

two exteriors. It's like our picture. You never mention the interiors. You say, look,

there's a quantum field, and it has some dynamics on the other side and some dynamics on this side.

And then what he showed is that because I can't observe the vacuum on the other side, all I can

do is observe one side of this spacetime. The consequence of that is that I would see a thermal,

a temperature of the black hole. So he showed that he basically argued that the origin of this black

hole entropy, which Hawking discovered, is that you are summing over all the degrees of freedom,

which you're unable to observe. The degrees of freedom on the other side. Interesting. When

was this analysis done? That would have been in the 70s. So following Hawking's papers on

black hole evaporation, Israel gave this kind of interpretation of what does that entropy mean, and

where does the temperature come from? Why is a black hole hot? And the argument is the black

hole is hot because you are only seeing half the spacetime. So that work also is encouraging for

us because it's saying that there is, it does look like it's completely consistent to build a quantum

field theory, which only operates on the exteriors of the black hole. So are there any local energy

conditions that are violated in the black mirror solution at the surface? No. As far as we can

tell, no. I mean, I should say we've not studied this in enough detail. But no, I think what we've

done already shows that there's nothing dramatic happening in the local stress energy before you

hit this special surface. When you hit it, as I say, we expect a signal of particle-antiparticle

annihilation. I assume you're going to say that this is a work in progress, but how do you imagine

the specific CPT identification point, the sigma equals zero, how does it get determined during

something that's dynamic or non-spherical collapse? It's a great question. And yeah,

so the only answer we have is that you have to impose boundary conditions in the future

and in the past, and you have to think of the problem quantum mechanically. You have to look

at what is usually called a path integral. So, you know, what is a classical solution of any theory,

actually? And the way we understand what classical dynamics is, is that it is a saddle point. It is

a stationary point of a quantum mechanical path integral. Basically, you sum over all paths and

some of them interfere constructively. And the ones which do, when they interfere constructively,

that is called a classical path. But the way quantum mechanics works is, let's say,

the way in which quantum mechanics leads to classical behavior, inherently involves data

on the past and the future. How so? Certainly for gravity, because in the case of gravity, the only,

let's say, the only, I think, sensible proposed framework for connecting quantum mechanics and

gravity is the path integral framework. Where you say that I specify, let's say, the geometry,

three geometry and the matter content at one time, and I specify it at a later time. Okay,

I don't tell you the time, I just specify these two, three geometries. And then your job is to

find the classical solution which connects these two. And that is how classical GR emerges from

the path integral for gravity. This was a picture developed by John Wheeler in the 60s,

who was Feynman's PhD advisor. And it's still, it's an incredibly beautiful picture. It's very

technically challenging. But as far as I am aware, it is the only reasonably well-motivated framework

for quantum gravity that makes any sense. String theory, for all its successes, never really tells

you how a space-time is governed by boundary conditions. I mean, string theory, you always

just assume a space-time, and then you scatter strings in it. And so string theory doesn't

really give an answer to this question. But Wheeler did in the 60s, and then his picture was

developed by Claudio Teitelboim in the 80s in some magnificent papers, which were largely overlooked,

unfortunately, because people got very enamored with string theory. But those papers, I think,

are the firmest foundation we have currently for connecting gravity to quantum theory. And

as I say, with the path integral, what I do is I specify an initial state, I specify a final state,

and then I calculate the amplitude to go from one to the other, by summing over all possible

paths with quantum mechanical interference. And so that framework fits our sort of CPT proposal,

fits perfectly within that framework. But it's a bit more difficult than classical GR,

where you simply evolve the field equations forward. And it's not quantum at all, but you just

take Einstein's field equations and evolve them forward in time. That's fine, that's a classical

picture. But it will never make sense of sort of truly quantum phenomena like the ones we expect in

our picture to happen on the horizon. So does that mean the universe is superposed? Yes, yes. Does it

make sense for the universe to be entangled with itself? Yes, it has to be, yes. I mean,

I think quantum mechanics, I mean, all proposed solutions, resolutions of black holes, well,

maybe that's not quite true. There are probably some proposals which are purely classical, but

I think anybody who thinks, well... I know local structure can get entangled,

but global structure? Yes, absolutely, yes. Okay, so I'm now going to appeal to observation. Okay.

We look at the universe, right? And let's say we look at opposite points on the sky. And those

opposite points have never communicated with each other. Obviously, because the light from both of

them is only reaching us now. So they never had a chance to communicate. And yet, they're at

exactly the same temperature. Okay? How amazing is that? Now, one explanation for this fact that the

universe is astonishingly... Uniform? Uniform in all directions, right? Homogeneous and isotropic.

One explanation for that is there was a period of inflation in which the universe was actually

a very small object in which everything was communicating. So it somehow thermalized. And then

it was blown up into this gargantuan universe we see around us today. And they correlated because

once upon a time, they knew about each other. And they did communicate with each other before the

Big Bang, if you like. During the inflating epoch, they did communicate with each other. Now, as you

know, I'm not a believer in that picture. That's a very classical picture, actually. And it's

extremely ad hoc because you postulate a form of matter, an initial condition, which is this kind

of exponential expansion before the Big Bang, in order to explain what we see. I don't think that's

necessary at all. You see, I think the error that's being made is the classic one, which is

that correlation does not imply causation. Right? We see the temperatures correlated on two sides

of the sky. It doesn't mean that one side caused the other one. It just means they're correlated.

So they want to preserve locality, and that's why they came up with inflation? Just a moment. Don't

go anywhere. Hey, I see you inching away. Don't be like the economy. Instead, read The Economist. I

thought all The Economist was was something that CEOs read to stay up to date on world trends. And

that's true. But that's not only true. What I found more than useful for myself personally

is their coverage of math, physics, philosophy, and AI, especially how something is perceived by

other countries and how it may impact markets. For instance, The Economist had an interview

with some of the people behind DeepSeek the week DeepSeek was launched. No one else had

that. Another example is The Economist has this fantastic article on the recent dark energy data,

which surpasses even Scientific American's coverage, in my opinion. They also have the chart

of everything. It's like the chart version of this channel. It's something which is a pleasure

to scroll through and learn from. Links to all of these will be in the description, of course. Now,

The Economist's commitment to rigorous journalism means that you get a clear picture of the world's

most significant developments. I am personally interested in the more scientific ones,

like this one on extending life via mitochondrial transplants, which creates actually a new field

of medicine, something that would make Michael Levin proud. The Economist also covers culture,

finance and economics, business, international affairs, Britain, Europe, the Middle East, Africa,

China, Asia, the Americas, and of course, the USA. Whether it's the latest in scientific innovation

or the shifting landscape of global politics, The Economist provides comprehensive coverage,

and it goes far beyond just headlines. Look, if you're passionate about expanding your knowledge

and gaining a new understanding, a deeper one, of the forces that shape our world, then I

highly recommend subscribing to The Economist. I subscribe to them, and it's an investment into my,

into your, intellectual growth. It's one that you won't regret. As a listener of this podcast,

you'll get a special 20% off discount. Now you can enjoy The Economist and all it has to offer,

for less. Head over to their website, www.economist.com slash TOE, T-O-E, to get

started. Thanks for tuning in, and now let's get back to the exploration of the mysteries

of our universe. Again, that's economist.com slash TOE. So they want to preserve locality,

and that's why they came up with inflation? Yes. They want to preserve, well, I would say they were

stuck on classicality, and a classical notion of causality, right, which quantum mechanics

violates. They were stuck on that, and they wanted to preserve locality. So let me phrase the

question another way, because this is sort of a very basic way of seeing this. Imagine we're doing

statistical mechanics. We're trying to describe the behavior of gas in a room. So it's a perfectly

rectangular room, no doors or windows. We throw a bunch of molecules into it. There's a certain

number of molecules, and they have a certain total energy, kinetic energy. They're just flying around

and bouncing off the walls. So question is, what's a typical state for molecules of gas

in a box or a room? Many people would say, oh, you need ergodicity. You need the dynamics. What

happens is these particles, even if you put them all in a corner, they will spread themselves out,

so that the typical state will be quite uniform, homogeneous and isotropic, just like the universe.

But that takes time, and it requires them to explore essentially all the possible

configurations to find the most probable ones. This argument, I believe, is absolutely wrong,

okay, in principle. If you give me a box full of molecules with certain total energy, what you need

to do, what you can do, if somebody says, what's the typical state of the molecules in the box? You

know the energy. You know the number of molecules. What do you do? Well, you want to count the

states. You want to count all the possible states. So what do you do? You quantize the molecules.

A quantized particle in a box has a certain number of states, and if there are n particles,

I know exactly what all the states are. I find those states which are consistent with the given

total energy, and they basically live on a shell in the space of quantum numbers, and I pick one at

random, okay? That's a typical state. You can't get a better defined notion of typicality than

that. That is 100% kosher, because I quantized everything, so everything's specified by integers.

I'm not biasing the calculation in any way. I'm only telling you the macroscopic variables, the

energy and the number of particles, and you pick at random. And what you'll find is the typical

state is homogeneous and isotropic. You know, that's the explanation. You don't need ergodicity

or dynamics to explain correlations. Correlations are inevitable when you have a well-defined

ensemble, probability ensemble. So the same for the universe. Are we really surprised that one

side of the universe is the same temperature as the other if we know the dynamics, and if we can

show that when we count states, the typical state has the two sides at the same temperature? Now,

Latham and I, Latham Boyle and I, have published papers showing exactly that, that we assume

Einstein's theory of gravity, the path integral for gravity, and then we generalized Hawking's

calculation of the entropy of a black hole, using exact solutions in cosmology. By the way,

you should know that I spoke to Latham Boyle here. The link is on screen and in the description.

It was a presentation on the math of the CPT symmetric universe. And we discovered that the

maximum entropy configuration for a cosmology is homogeneous, isotropic, spatially flat, which our

universe appears to be, and has a small positive cosmological constant. It fits with all the

observations. So you don't need anything else. You just need to count. You don't need a sort of ad

hoc dynamics, which inflationary theorists would have you believe in, in a prior epoch prior to

the standard. But you don't need any of that. You just need the known laws of physics. And indeed,

our whole point is, in all our work on cosmology and black holes, that the laws we already know,

quantum mechanics, general relativity, and the standard model of particle physics, are capable

of explaining everything we see. We don't need to keep inventing new particles, new dimensions,

multiverses. I think the whole field sort of went haywire. And the spirit of our work is to return

to simplicity and foundational principles. And again and again, we've discovered that

certain things have been overlooked, which, to us anyway, appear to be much simpler explanations for

everything we see. So I hope, I mean, we can't be sure our ideas are right. I mean, they seem

to be converging with the data. One prediction we made is that the lightest neutrino is massless.

And just a few weeks ago, the DESI galaxy survey has now put very tight upper limits on the mass

of the lightest neutrino. And it's consistent with exactly what we predicted. And that was a

consequence of our explanation of the dark matter. So, you know, it takes us a bit further afield.

But basically, we are finding that it is possible to explain all observed phenomena in the universe

using these basic principles of CPT symmetry and the standard model, and very little else. Okay,

let's talk about some cosmological data while we're on this subject. So DESI, a few months ago,

I believe they indicated that dark energy can be dynamical. Oh, and just as an aside, for those

who want to know more about your Big Bang is a mirror theory and your whole theory of everything,

in a sense, you and I, Neil, had a conversation that went quite in-depth and it also went viral.

And if people want to learn more about the recent DESI results, I'll put a link to an

Economist article on screen where they explained it as well, which you're about to explain it, so

please. Super. Yeah, so the DESI result, and there have been a number of results along these lines,

is what's pointing to a tension. People usually refer to it as a tension between the, let's say,

standard model of cosmology, which is very minimal and very predictive, and the data. So one of

these tensions is called the Hubble tension, that the most basic parameter in cosmology,

the expansion rate of the universe, is called the Hubble constant, and different ways of measuring

it give slightly different results. Not hugely different, I mean they differ by about 10%,

but nevertheless this seems to be inconsistent with their estimated error bars. So the Hubble

tension has existed for a while, it continues to exist, the new DESI measurements have not shed any

light on that, but the DESI experiment discovered another tension, which is that in the standard

model, the cosmological constant is inserted as a free parameter. And this cosmological constant

is a sort of very, very old theoretical construct, it was invented by Einstein, I think in 1917, when

he wrote down his first model for the universe. The reason he invented it was it is the simplest

conceivable form of matter. A cosmological constant is absolutely smooth in space,

absolutely unchanging in time, and it's also what we call Lorentz invariant, namely if you move

through space, this cosmological constant won't change at all. So it's a strange form of energy,

which you can think of as almost like an ether. It's just a uniform, invariant, unchanging thing.

And Einstein realized that this type of energy, or matter, would be gravitationally repulsive,

that it pushes space to expand. Whereas other forms of matter, like the stuff we're made of,

or dark matter, or radiation, causes space to contract. And so Einstein balanced the

cosmological constant's repulsion against the attraction of ordinary matter to make a

static universe. To him, he didn't know about the expansion of the universe, so he thought he had

to explain why is the universe able to exist when gravity is trying to cause it to collapse. So he

used the repulsive gravity of the cosmological constant to hold up the universe. Sadly,

he didn't realize that this balance was unstable, and so even in this delicately balanced universe,

either you would collapse one way or you would expand to infinity. And so his solution didn't

really work. Nevertheless, we have recently discovered, this was in the 90s, that this

cosmological constant is about 70% of all the energy in the universe. So, it's been called the

biggest problem in physics. Why does even empty space have this energy, the cosmological constant,

which as I say is unchanging and absolutely uniform. Where did it come from? Why is

there a cosmological constant? So, the standard model includes this, and because it's included,

it's able to fit a huge range of data. So it's one parameter, but it explains hundreds

of thousands of observations, so it's a pretty good model. Now DESI comes along and they said,

our data doesn't quite fit the standard model. In the standard model, this cosmological constant is

causing the universe to accelerate its expansion, but they find that the acceleration is not exactly

as predicted by a cosmological constant. It takes a very weird form. So, it was accelerating more in

the past, and then apparently in recent epochs, that additional acceleration is going away. So

it's not a model anybody dreamed up. It's not a theory anybody dreamed up. They're finding their

data fits, and all they do is a fit. They don't have a theory. So they do a fit to it, and they

find that they can fit it by assuming that the cosmological constant, which is one number, is

replaced by two numbers, one of which is the value now of the cosmological constant, and the other,

if you like, is the sort of rate of change in the past as we look to the past of this cosmology. So

they've got a two-parameter model, and they say it fits better. So what's the bet? by using three

different experiments, one of which is theirs, and the other two are not theirs, and these different

experiments have different systematic errors. So if you combine three experiments with their own

systematic errors, which are really difficult, these measurements are very, very difficult in

astronomy, and you end up with something around four standard deviations, you know, it's not very

impressive. And particle physics has learned never to believe a result which isn't five standard

deviations from a single experiment. They're using three experiments, and so anyway, I'm not

convinced. So I said to him, look, what you're doing is proposing a fit. It's not a theory.

You've got a two-parameter fit, and you're saying this is better than a cosmological constant. You

agree that this fit is compatible with, let's say, a thousand theories. You don't even have a theory,

right? As far as I know, there's not even one theoretical model. I'm sure people will come up

with them, but as far as I know, currently, there's not even one semi-plausible. No,

it does the wrong thing, you see. So that's what I said, because in this fit, the lambda's bigger

in the past than now. Quintessence goes the other way. So in quintessence, the field sort of rolling

stops, and so the cosmological constant kind of settles, and you stick with it. In this fit,

the cosmological constant was sort of big, I don't know, redshifts three, four, and then switched off

today. It's a very puzzling behavior. I get the idea. You're not a fan of this. You don't buy it.

No. So I said, you know, there's a thousand models that would fit your data, and there's one model

that fits the standards. One standard model. So I'll bet you a pound against your thousand pounds.

And he's willing to take that? No, he hasn't accepted that, but he should. Well, it depends

on how certain he is. Well, he's not willing to bet a thousand pounds against one. If he's one

to one thousand. So I would say the standard, the cosmological constant is a really well-motivated

theoretical construct, and it fits pretty well. He's saying an ad hoc two-parameter fit fits

better. You know, I'm not impressed. But he may well, maybe it's right. I have the utmost respect

for the observations. They are going to improve. And if it reaches more than five or six or ten

sigma, I will have to accept it. So that's great. This controversy is very good for the field.

Just speaking of bets and certainty, I was speaking with Neil deGrasse Tyson,

and he said about how there's UAPs in the sky, and are they aliens, are they UFOs? He thinks it's a

one in one hundred billion chance that they're aliens. So I said, OK, if that's the case, I will

put up one thousand dollars and you put up one million dollars, and that should be vastly in your

favor. Yes. And he said, no, no, I'll put up one hundred dollars to ten dollars or something like

that. And I'm like, well, then that's expressing you're not as certain as you claimed. Right. I did

this myself, actually. I was a volunteer teacher in Lesotho in Southern Africa before going to

university, and I had a little motorbike. Now, all the villagers used to tell me that there is magic.

There were witches and people who did things at night, and there's something called a tokoloshi,

which is a magical person you make out of various herbs and things, and it will go and kill somebody

you want it to kill. So they told me all these stories, which they genuinely believed. And in

fact, even the nuns in the convent believed it as well. And so I said, OK, I have this motorbike.

You show me one piece of real evidence for magic, and you've got my motorbike. OK? Yes, exactly. So

you were willing to put your money where your mouth is. Absolutely. I'm always willing to do

that. I mean, frankly, with this bet on the DESI result, if pressed, I would put £1,000 against it.

I think there is too much wishful thinking. It's very tempting, as an experimentalist,

to believe that you've discovered something fundamental and shocking. And that's a bias

which is very, very difficult. And again and again, I mean, I'm not holding anything against

these particular experimentalists, but I think that is a bias which they would love. I mean,

as I pressed him, in fact, this is what he said. He said, look, we better hope this is

real. Because if all there is is a cosmological constant, then the field is dead. Meaning that

there's kind of no point in doing any more observations, because the answer is so simple,

because you've solved it. But I have the opposite point of view. That if the observations turn out

to be simple, it is putting right in our face that we don't understand. You know, we don't understand

the Big Bang singularity. We don't understand this mysterious future of the universe dominated by

cosmological constant or dark energy, whatever you want to call it. We don't understand the

arrow of time. These foundational questions about the world, there's plenty to do. We don't need

a glitch in an experiment to tell us that we don't understand what's going on. It's obvious we

don't understand. So I take the opposite point of view. If these experiments home in on an extremely

simple model, that's our best hope. That's our best hope. Because if things are simple,

then they may be comprehensible. You know, Einstein discovered general relativity on

the basis of experiments done over the previous 300 years, which showed that objects of different

composition and masses fell at the same rate under gravity. And he suddenly realized, oh,

this implies that they're all moving in the same arena, because they're all falling in exactly

the same way. So maybe there's something like a curved spacetime, you know, which causes them to

move through it independent of what they're made of. And that was his basic clue, which led him to

general relativity. So I think the simpler things get from the point of view of observations, the

better it is for our eventual understanding. Okay. So, you know, this is a purely emotional point of

view. I'm not saying one is right or wrong, but my point of view is that the simpler the observations

are, the more likely it is that we're going to understand all of them. While we're here on the

cosmos, there's this recent data from the JADES experiment or survey about the spinning galaxies.

Okay, I haven't seen that. I haven't seen that. Is it a correlation of spins? Yeah, it turns out

that two-thirds of galaxies early on rotate in the same direction, and it should be 50-50. I haven't

studied it myself, but I will be very skeptical. People have looked at the alignments of galaxies,

and many, many times, you know, strange alignments have been noticed without… an explanation. And

almost invariably, well, invariably in the past, these alignments have been found to be just a

sort of statistical bias or some other mundane explanation. I think the evidence for statistical

isotropy on the sky is huge. And the best evidence is the cosmic background, that

it's just the same in all directions to basically one part in the temperature, one part in 100,000.

And that's the most distant structure we know, and it's telling us that we're just surrounded by this

almost absolutely uniform sea of radiation. So it's really hard to imagine why there would be

big local structures. People do make claims like this from time to time. In general, they have

not held up. They're always interesting because there's always a chance one of them will turn out

to be right. But yeah, the track record is not good. Okay, let's get back to your black hole

model. Okay. People are probably wondering, what is the physical status of this exterior universe

in philosophical terms? What is the ontological status of it? Of the other one? I mean, we live

in one exterior, and there's another exterior. The way we describe it is as a mirror. It's

like a mirror. So when you look into a mirror, what you're seeing is the light, which came off

your face, bounced off the mirror back into your eye. There's clearly only one side of the mirror,

and you don't know anything what's behind the mirror. There is another mathematical description

of a mirror called the method of images, in which of images, in which you take yourself

and your face, and you make a mirror image of it, where left becomes right. And you put that at the

same distance from the mirror as you are, and you throw the mirror away. And that's what you see. So

that's called a method of images, because mathematically, what you do is take your own

image, transform it, put it at a certain distance behind the mirror, and it tells you exactly what

you'll see. So we believe that this two-sided cosmos is a way of implementing a certain boundary

condition at the Big Bang, which uses the method of images. So the image is merely a mathematical

device to render your calculation consistent with CPT symmetry, and it ends up imposing a

certain boundary condition at the Big Bang, which is therefore compatible with the laws

of physics. The same thing for a black hole. We don't actually think of the mirror image universe

as a real independent universe at all. It is an image of us, but it is allowed. You see, because

the whole construction is quantum, this path integral construction is quantum, fluctuations are

allowed on both sides, which are not necessarily mirror images of each other. If you think about

the creation of a particle-antiparticle pair, you know, the Stuckelberg picture,

the particle and its antiparticle are mirror images of each other, but they're not identical.

They can satisfy the same boundary condition at future time infinity, but the curve can fluctuate

differently on the two sides. So we see it in this way. The two sides would be highly entangled. If

you try to describe it classically, you will find they are exact mirror images of each other, but

if you describe it quantum mechanically, they are not. That's our best guess. I would say it's still

an open question how to fully specify this CPT symmetric construction. I don't think we've done

it. It's something we're working very actively on, and all the clues we're getting from cosmology and

from black holes and from mathematics are helping us kind of build a more precise picture. It's not

very precise yet. I want to end on a couple of questions about the black hole. But first,

I realized that from our previous conversation about the 36 fields, the scalar fields, you

mentioned that people hear that and then they're like, okay, so this is an extremely simple model,

minimal assumptions. We're just adding 36 extra scalar fields that weren't there before,

and they need to be fine tuned or tweaked. Okay, so help the audience understand why

that is not an arbitrary imposition. How is that more simple? Well, the motivation for those fields

are, so yeah, I mean, you're absolutely right to pull me up on this, because

we're assuming the standard model, and then we're bringing in these 36 additional weird

scalar fields for which there is, and I emphasize, no direct experimental evidence yet. Now, let me

phrase it the following way. So we were led to these fields by a real observation, which is the

fluctuations in the temperature in the sky. I said the temperature is the same to 1 part in 100,000,

but it does fluctuate at a level of 1 part in 100,000, and there's a particular pattern in

those fluctuations. Extremely simple pattern, specified by two numbers. One is an amplitude,

and the other is called a tilt, spectral tilt, a very small number. And those two numbers specify

the pattern we see on the sky. So if you ask yourself a question, what kind of field produces

that pattern, then the answer is exactly the kind of field we've postulated, this dimension zero

field. And in fact, in subsequent work, we have explained quantitatively the fluctuations seen on

the sky in terms of that field. Now, we wouldn't believe in those fields, except for another

theoretical piece of evidence. The evidence is the following. You see, when the Big Bang

shrinks away, if you follow the universe back in time, the universe shrinks away at the Big Bang.

Now, in order for our mathematical description, this analytic continuation through the Big Bang,

in order for that to work, we need the theory to have this very special symmetry at the Big Bang.

It's called conformal symmetry. It means that the size can change, but the material contents

of the universe do not care. So the radiation, the particles, are insensitive to the fact that

the size is shrinking away and reappearing. They actually don't see that. Conformal theories only

care about angles, not sizes. And the standard model is conformal in the first approximation.

And so what we discovered, and this was actually amazing, is that if we have precisely 36 of

these rather funny fields, which have four time derivatives, not two, so they sort of violate one

of the basic assumptions in the laws of physics for a long time, these fields would cancel all

of those violations, and they would cancel the vacuum energy. The standard model has infinite

vacuum energy. The zero-point fluctuations in electromagnetic fields, in the Dirac fields,

and all the other fields add up in the standard model to a non-zero number. And what basically

this means is that you can't consistently couple gravity to the standard model, because you've got

this infinite vacuum energy. So it turns out that precisely 36 of these fields cancel the vacuum

energy, and all the violations of this conformal symmetry. So they allow you to describe the Big

Bang. And then in subsequent work, we showed that with this cancellation, when you ask, what is the

predicted pattern of temperature fluctuations in the sky, you get exactly the right number.

Now, still, you should be worried. These 36 fields, surely I have loads of free parameters,

but that's not true. This theory is very, very highly constrained. And in fact, recently,

we realized that with precisely 36 of these fields, we have an indication that the standard

model formulated this way will satisfy what's called maximal supersymmetry. So supersymmetry

is a hypothetical symmetry that relates bosons to fermions. And in supersymmetry, theories that are

supersymmetric, the vacuum energy always cancels, because you have the same number of fermions and

bosons, and one has positive vacuum energy and the other has negative. So we didn't realize

at the time that we were looking at a particular case of supersymmetry. But there's something more.

It turns out that in four dimensions, the biggest supersymmetry you can have

is called n equals 4. And in that symmetry, for one gauge boson, and the standard model has 12,

but for every one gauge boson, you must have four what are called vile fermions. That's, let's say,

a left-handed fermion. You must have four of them, and you must have six bosonic fields,

normal bosons. These are two-derivative bosons. So you end up with this ratio 1, 4,

6. It comes out of supersymmetry. And that's the most beautiful supersymmetric field theory known.

It has no divergences. So all the infinities go away. And it turns out, we hadn't realized this,

but the counting in our theory is exactly the same, because we have 12 gauge bosons. We have

48 fermions in three generations in the standard model. So that's the four, factor of four. And

then we have 36 of these fields, whereas we should have 6 times 12, 72. But each of our dimension

zero scalars actually has twice the number of degrees of freedom of an ordinary scalar,

because it has four derivatives instead of two. So in fact, we end up with 72 scalars. So amazingly,

in our framework, we are finding the signal of supersymmetry. And if that's true, it's going to

tell us that we have no infinities in this theory at all. So it's very exciting. It's brand new. We

haven't written any papers about it. But the other thing, which is, you see, in our framework, we are

not allowed to have the Higgs boson. The reason is that this cancellation of the vacuum energy

and the conformal anomalies, the violations of conformal symmetry, that cancellation, which kind

of happens through almost miraculous numerology in the standard model, that cancellation does

not allow an ordinary scalar field. It does not allow any two derivative ordinary scalar fields.

So the big mystery in our framework is where did the Higgs boson come from? How was it formed? And

it's particularly embarrassing for me, because I hold Higgs chair at Edinburgh, and I'm arguing

there cannot be a Higgs boson. It's inconsistent with conformal symmetry. You mean there can't be

a fundamental Higgs boson? Exactly. But it can be composite? Exactly, exactly. So the only way

out is that the Higgs boson is a composite of these 36 dimension zero scalars. Now, actually,

that is extremely interesting. And what we are studying now is the quantum field theory of

dimension zero scalars. It's getting a little bit technical, but that quantum field theory turns out

to be asymptotically free, meaning that at very high energies, the coupling vanishes. It becomes

a free theory. That's great, because it means that this quantum field theory actually exists

mathematically as a well-defined theory, whereas the usual Higgs theory does not. The Higgs theory

is not asymptotically free. The coupling blows up at large energies. And so that theory, we believe,

is sort of ill-defined. If you probe it with a very powerful microscope, you will find it doesn't

make any sense at all. It just gets sort of worse and worse. The coupling gets bigger and bigger,

and there's no good limit. So the dimension zero scalars have a better limit, and now there's a

chance that we will solve what's called the hierarchy problem. The hierarchy problem is

that the Planck mass, which is about 10 to the 19 GeV, associated with gravity, huge energy scale,

only probable through the Big Bang itself. When we look at observations of what came out of the Big

Bang, we can talk about phenomena due to Planck scale physics. But this Planck scale is 10 to the

19 GeV. The other scale we have to put into the standard model is the weak scale, which is about

100 GeV. That's the mass of the Higgs boson. Those two scales and the cosmological constant are the

three mass scales in the standard model, which have to be inserted by hand. Okay, so far, because

we don't really understand their relationship. But the hierarchy puzzle in particle physics is why is

the Planck scale 10 to the 17 times bigger than the weak scale? This sounds incredibly contrived.

You don't get 10 to the 17 just by playing with pi's and 16's and so on. You might, but it would

require a lot of contrivance. So the hierarchy puzzle was a huge motivation for supersymmetry,

conventional approaches to supersymmetry. They argued you had to have all these super particles,

essentially to cancel quantum corrections that would push the Higgs mass up to the Planck scale.

So what we have with the dimension zero scalars is an opportunity to explain this ratio. In a

much more compelling way. The way you explain it is because in an asymptotically free theory,

the coupling constant runs with energy and goes to zero at large energies. So you say,

imagine the coupling was about 1 30th at the Planck scale, you know, some moderate number at

the Planck scale. When I run it down now, it only runs logarithmically in energy, which is very,

very slow. So let's say it's a 30th at the Planck scale. You can ask what energy scale

does it become one? And that can be 100 GV. So you start at 10 to the 19, but where it's a 30th and

it becomes one at 100 GV. There's no fine tuning in that. You have explained this huge hierarchy

very naturally because it's only logarithmic. In fact, the same explanation works in QCD.

Nobody wonders why the mass of a proton is one GV, whereas the Planck mass is 10 to the 19. And the

reason is that QCD is asymptotically free and the coupling becomes strong at one GV and that

determines the mass of a proton. So with these dimension zero scalars, we have a chance of making

the standard model much more compatible with the facts. Now it's only a chance and we're busy doing

lattice theory computations with dimension zero scalars to see how this Higgs mass would emerge,

how it can behave as a Higgs boson. And if that works, it'll be very exciting because it will

then create a rival to the standard model Higgs, so the two can be tested against each other at

future accelerators. But again, what we stumbled across is a simpler way of solving the hierarchy

puzzle than supersymmetry, which, yes, it involves these weird extra fields, but they don't have any

particle excitations. There's no more particles. All these extra fields do is actually change the

vacuum and they change the vacuum in such a way as to make it consistent with this very profound

symmetry called conformal symmetry. So potentially here is a rival to the standard model, which will

explain the hierarchy and the Higgs mechanism, which broke particle physics symmetries, and also

fit the cosmic microwave background. I mean, it's absolutely a unified theory of the whole cosmos

stretching from the tiniest scale to the largest scale. And it may be within our grasp. I mean,

it's tremendously exciting. And in fact, it feels to us like it's just around the corner. Lyle So,

Professor, there's so many more questions I have for you, and I'll have to save them for next time.

But if you can answer briefly about these two questions, because it seems like your theory,

which I don't recall if it has a name, a moniker. Peter CPT Symmetric Universe. I

think that's probably the simplest. Yes. Lyle The CPT Symmetric Universe. Does it also solve

the measurement problem or the flow of time? Peter Oh, these are great questions. The flow of time, I

would say yes. Lyle Not the arrow of time, but the flow of time. Peter Oh, the flow of time. Why does

time appear to be flowing? Okay, good question. I would say so far, no. But there are real prospects

for doing so. Nobody has even tried to calculate whether there would be an apparent flow of time

within this framework. It's a reasonably well-defined mathematical framework. And,

yeah, indeed, I think it would be very good to try and do calculations to see whether

for macroscopic entities like ourselves, there would be an apparent flow of time. So, possibly,

it will solve that puzzle. What was the other one? The flow of time and? Lyle Measurement.

Measurement. No, my colleague, Latham Boyle. Peter Who I've spoken to, by the way, and a link will be

on screen and in the description just for people who are interested in learning more about this

theory and seeing your collaborator. He gave a presentation. Lyle Yes. So, Latham has a notion

that, you know, in quantum mechanics, things are doubled because we have real numbers and imaginary

numbers. And quantum mechanics works with both, whereas classical mechanics only works with real

numbers. And so, Latham believes and hopes that this doubling of the universe will be in some

ways reflective of the fact that to describe it properly, you need both real and complex numbers,

which means you have double the number of numbers, if you like. And that is not unreasonable because

what happens in this two-sided universe, you could ask, why are there two sides? Why are there always

two sides in black holes and in cosmology? And the reason is a mathematical one, which goes back to

work of Hawking a long time ago, where Hawking noted that in geometry, the sort of simplest

kind of geometry is called Euclidean geometry, in which everything is like space. Whereas Minkowski

introduced Lorentzian geometry, where you have one time and three space, okay? To go from one

to the other, you make time imaginary. It's a very old trick. You have in the spacetime

distance or metric, minus delta t squared plus delta vector x squared. Time comes in with a

minus sign. That's very, very basic in relativity. But if I make time, if I say t is i times tau,

where tau is real and i is the imaginary number, then the metric is plus, plus, plus, plus, four

pluses. So Minkowski realized this, actually, that if you make time imaginary, you're dealing with

Euclidean geometry. So relativity becomes just Euclidean geometry. So Hawking used this fact. He

started with a Schwarzschild black hole, which has one time and three space. He made time imaginary,

and he discovered a Euclidean version of the geometry. And it turns out that Euclidean geometry

is completely non-singular, right? It doesn't have the curvature singularity at all, anywhere.

In fact, that Euclidean geometry pretty much describes the exterior only of the black hole. So

if I have this picture where imaginary time… So in the complex numbers, you have the imaginary axis

and the real axis. And if you describe a solution up the imaginary axis, okay, which is this,

as I say, Euclidean geometry, when you come back to the real picture, there are two ways to go.

You go left or you go right along the real axis. And those are the two sides of the black hole.

And those are the two sides of our universe in cosmology. And so this way of going from real

numbers in Euclidean geometry to complex numbers, through complex numbers, to Lorentzian geometry,

which has a, quote, real time and a direction of time, involves precisely, you know,

and which doubles the time directions, that indeed is related to how you go

between complex and classical mechanics. And so I think it's not an unreasonable hope

that this doubled picture will tell you something about why quantum mechanics uses complex numbers

and hopefully what they mean. So, I mean, there's another factor of two, you know, in quantum

mechanics, the probability is the square of the amplitude. And in our doubled universe picture,

it's just crying out to somehow say that you double things, you square things, they're

two sheets to the universe. So yes, we are hoping that this picture will shed new insights into the

very mathematical structure of quantum mechanics. Before we get to just your advice to students and

your hope for the future of physics, I just have a quick question about the black hole. So given its

horizon structure, does it satisfy certain like uniqueness theorems such as no-hair theorems? Hi

everyone, hope you're enjoying today's episode. If you're hungry for deeper dives into physics, AI,

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Thank you and enjoy the show. Just so you know, if you're listening, it's c-u-r-t-j-a-i-m-u-n-g-a-l

dot org, CURTJAIMUNGAL.org. Like uniqueness theorems such as no-hair theorems. Yeah,

that's a good question. I would say yes, because those uniqueness theorems only use the Einstein

equations, and we are satisfying the Einstein equations. So indeed, I would say they do

satisfy the uniqueness theorems. We don't expect black holes with any hair to emerge from this

construction. But the question of the dynamics of the black holes as they merge and settle down to

those unique stationary states, that's where the difference might be revealed between our picture

and the conventional one. So the stationary states, we would agree on. But in the dynamics,

how you get there, we might be different. If an observer is going tangent to the surface,

do you imagine there would be an infinite tidal force to the horizon? I don't think so. You see,

what we find is that in the stationary case, there is no divergence in the curvature on the horizon

at all. All the curvature invariants are finite on the horizon. So that's in the stationary case. In

the dynamical case, I don't expect it'll be very different, because I think if matter's falling

onto the horizon and then annihilating and zooming off up the horizon and being released at infinity,

I don't expect that to cause infinite anything. But yeah, we shall see. We haven't done those

calculations. And do you expect the Hawking entropy, the Bekenstein-Hawking entropy, to be

recovered from entanglement during evaporation? No. So in the usual picture, the entropy of a

black hole, people like to explain. I mean, the entropy calculation itself uses this imaginary

time picture. It's very elegant and unique, but it doesn't give you much physical insight,

okay? The way Hawking calculated the entropy. By the way, that way is exactly the same way that

Latham and I calculate the entropy of cosmology. It's a very mathematical construction using

imaginary time. We literally replicated Hawking's black hole calculation for cosmology, and we were

very surprised we could do it at all. And that gave the answer for the entropy of a cosmology.

But as I say, it's very mathematical and abstract, and it's quite hard to figure out what it means.

So people are still arguing about this for black holes. Now, what is this entropy counting? In

some sense, people believe it's the entropy of stuff which fell in, and that we cannot… It's

all the states. It counts the number of states of everything that fell in, which we can't see,

okay? So that's how they explain the entropy. But they're big puzzles with that too, you see,

because Hawking's entropy calculation does not depend on the number of particles in the standard

model. You know, the standard model has a certain number of particles, a certain number of forces.

Those just don't come into the calculation. So according to Hawking's calculation, if I doubled

the number of particles so I could make, you know, chairs and tables out of standard model fields or

different versions of standard model particles, according to Hawking's calculation, that would not

change the entropy of the black hole. And that's called the species puzzle. Hawking's calculation

is independent of the number of particle species. Yeah, even if there was less species,

like just one. Yes! If there's only one, it would give the same answer. So I would… Now, people have

trouble explaining this, okay? This is a very profound puzzle. How can it be that the entropy

of a black hole is independent of the number of different types of particle there are in physics?

I think the only sensible resolution is that if his calculation is correct and the answer for the

entropy is unique, then combining gravity with particle physics is much more unique than people

expected. The mere inclusion of gravity forces the number of particles to be some number. And

you just can't consider coupling one particle to gravity, you see. And that's the evidence we're

finding in this cancellation of anomalies and vacuum energy. Again, that's an indication that

you can't just chuck any old particle species into gravity. You have to couple. The fact you want a

consistent theory, including gravity, tells you how many particle species you can have. So… Sorry,

just a moment. Is that formalized yet? Is that a no-go theorem that you all have come up with? Yes.

I would say if you want the conformal anomalies to cancel, we can give you the precise conditions,

and they heavily constrain how many particle species you can have. So we use this to explain

why there are three families of particles. When we canceled the vacuum energy and the trace

anomalies, we explained why there are three generations of elementary particles. It is,

as far as I know, the simplest explanation anyone has ever given. Yeah, so canceling the vacuum

energy and these conformal symmetry violations predicts that there are three generations of

elementary particles. When you postulate the global CPT symmetric boundary conditions,

does that comport with the observed baryon asymmetry? Yes. Yes, that's fine. The reason is

that all of this anomaly cancellation requires 48 fermions, which is three generations of standard

model particles, which have 16 particles each. 16 includes a right-handed neutrino,

and we use one of them to explain the dark matter. So in fact, this is what started us

around this whole journey, is that we found we could explain the dark matter much simpler than

anyone else as being one of those right-handed neutrinos. Now, right-handed neutrinos violate

lepton number. It's just a fact. If you put them into the standard model, lepton number

is no longer a good symmetry. In fact, there are no good symmetries left. Global though, correct?

No good global symmetries left in the standard model. And so lepton number, baryon number are

all violated. And there is this picture, I mean, the simplest picture of how the baryon asymmetry

was created is a scenario called leptogenesis. Basically that these right-handed neutrinos

are just created thermally by high-temperature processes in the early universe, and then as the

universe expands, these right-handed neutrinos, which are heavy, decay, and those decays violate

baryon number. You mean lepton number? Sorry, they violate lepton number. And then, yeah,

so you produce a net lepton number. And then within the standard model, there are these

very beautiful processes, which happen, called B plus L violating processes. They go through

something called a sphaleron, you may have heard of. It's basically a non-perturbative process,

which is now pretty well understood, whereby this lepton asymmetry is converted at the electroweak

scale into a baryon asymmetry. So basically this is quite a long story, which I participated in,

it would be in the 90s. And this is now the simplest explanation of where the baryon asymmetry

comes from. Unfortunately, there's only one number to predict, which is the baryon asymmetry. And in

the standard model with right-handed neutrinos, there are more than enough parameters to dial them

to fit the observed number. So in a certain sense, it's not terribly predictive, it's just, you know,

there are enough parameters that you can fit the observations. So that scenario fits perfectly

within our overall picture. I don't think we're adding anything particularly new to it,

but that picture I think is very compelling. And in fact, there's a new accelerator which will be

operating in two years' time at Brookhaven, where they are going to be able to explore these

sphaleron processes, actually in QCD. But these same non-perturbative processes are going to be

explored experimentally, and that will shed light on exactly how they happen in the standard model.

There's not much doubt they are there, they have been calculated, but so far there's no direct

experimental evidence. But there's definitely an avenue for the future. Speaking about the future,

please tell us your vision of physics in the future, what you hope for physics,

and speaking about physics research, and also if you're speaking right now to physics students,

graduate students, PhD students, new upcoming students, prospective students,

what is your advice? I was just at the Perimeter Institute actually, where you were a director for

11 years or so. And so it turns out this podcast is somewhat viral at the Perimeter Institute. I

felt like a celebrity there. So there are probably many people who are watching from there. Lovely.

No, Perimeter is a wonderful place, and I had the opportunity of a lifetime to go there and

be director for 11 years and to try to shape it. And yeah, so vision for physics. I mean, physics

is an absolutely incredible field. We can write down on one line all the laws of nature we know,

and the suggestions are, and this is the lines I'm working on, that that one line is enough

to explain everything. In nature, at least at a very elementary level. The universe appears

to be incredibly simple on large scales. We've got this standard model, the Lambda-CDM model,

which has only five numbers, fits everything. The universe is also very surprisingly simple

on small scales. The Large Hadron Collider, the most powerful ever microscope, has not

found anything beyond the Higgs. So it may well be that the laws of physics we already know are

more or less the complete story. And putting together these laws into a coherent framework,

which explains the arrow of time, the passage of time, the future of the universe, which is strange

and vacuous, dominated by this cosmological constant, apparently, into the infinite future,

and the Big Bang singularity, even more puzzling that everything came out of a point in our past.

Putting that all together, I think, is an absolutely wonderful intellectual challenge. I

couldn't be more excited about physics. Obviously, new data from experiments is very important, but

if that new data confirms the standard picture, I think that will be a great sign. The minimal

picture, let's say. I think that will be a great sign that we're on the track to understanding

these much bigger and deeper questions. And so that's what I'm hoping for. If they contradict it,

of course, the picture has to be revised, and potentially the whole picture has to be revised,

which you might say is even more exciting. So I think physics has an amazing future ahead. I still

cannot get my head around how successful physics is. I mean, it's just bizarre that Einstein,

with a little guidance from experiment, more or less conceptualized the equations which govern

the expansion of the universe, predict black holes, gravitational waves, everything. That's the

kind of amazing unification which thinking about physics can achieve. And to some extent, Higgs did

the same with predicting the Higgs boson in the 1960s. And so that's the kind of unique property

of theoretical physics, which I don't think there is in any other field of science, that starting

from very coherent, economical, mathematical principles, one is able to explain this kind

of bewildering variety of natural phenomena. So that's really exciting. Now, in contrast

to physics, you have... scientific disciplines like molecular biology, or AI, or computation,

or quantum computing, or whatever, which are looking at complexity. And it seems to be a

fact about the universe that all the complexity is in the middle. It's on intermediate scales.

Nature is very simple on small scales, very simple on large scales. But in the middle, where we live,

we haven't succeeded in understanding it. We don't really know what life is, we don't know what

consciousness is. Those are wonderful challenges, too. But it's difficult to predict when we will

make advances in understanding complexity. Is it all going to end up as just a big mess of

computers with algorithms? I don't know. But that's personally what puts me off working

in that field, is it's too heavily computational. And I don't see the same elegance, economy, and so

on. And maybe that's just inevitable. Nature is not very economical at intermediate scales. And

that's what allowed us to exist. So yeah, that's how I would put physics. If you like simplicity,

if you like powerful predictivity and explanatory power, then nothing beats physics. So it's very

compelling from that point of view. And every day feels a wonder to be involved in a field

like that. It's such a privilege. It's something like, I guess, the Buddhist monks or someone who's

reached some very high level of enlightenment must feel the same way. It's just such a privilege

to feel you're part of this. Now, advice to young people, I would, based on my own career,

my own experience, I would say the time you spend thinking about foundational issues,

the most basic questions, what exactly is going on in the formalism? Is there a more simple

way of explaining it? The questions you try to understand, the interpretation, the meaning of

those equations, that time is never wasted, okay? Because that's always the source, I would claim,

of the most profound insights. So I see young people today very anxious about the future, very

anxious about career in particular. And I think that can be very destructive in terms of making

people work on things which are publishable in the short term, fit within some standard paradigm so

the referees will wave it through. And I think that is disappointing. There's a vast amount of

literature coming out on fields which essentially aren't making much of a contribution except in

volume, okay? In volume of material which doesn't particularly have any novel or useful insight. So

I would encourage young people to think, why did you go into this field if you went into it because

of its beauty, economy, simplicity, power? Stick to that. Don't give up your principles for the

sake of a few quick papers. Of course, you have to be pragmatic, so you do have to find projects

which are doable and worth publishing. But the more time you can spend on foundational issues,

and I'm really trying to do something novel which adds to our understanding, the better you will do

at physics. I think that quality is quite rare, but Perimeter Institute is one of the few places

actually in the world where the culture among the young scientists is of strongly promoting

independent thinking rather than just following established schools. And so I think that's one

of Perimeter's great strengths, and I just wish there were more places like that around the world.

That was my sense as well. Thank you so much, Professor. It's always a pleasure speaking with

you. No, thank you very much for the work you're doing. I think your podcast is pretty unique in

bringing together philosophers and thinkers across the spectrum. It's very unique, and I think it's

really commendable. Because it's accessible to young people, you're going to encourage them to

think, do I want to be a philosopher? Do I want to be a physicist? Do I want to be a mathematician?

And I know for my own part, when I went into science, I never thought about any of this. I

had no idea. It was just a random walk. I wasn't systematic in my approach to my own career at all.

And I think the guidance people can get from online, informal conversation is really very

valuable. They could say, aha, you know, that's an idea that I would like to learn more about. Well,

if your career is an ergodic walk, then it'll certainly be a theory of everything that we'll

have to discuss at some point. That's right. That's right. Okay, thanks very much, Curt. You're

welcome. It's not on Theories of Everything. It's not on Patreon. Also, full transcripts

will be placed there at some point in the future. Several people ask me, hey Curt, you've spoken to

so many people in the fields of theoretical physics, philosophy, and consciousness. What

are your thoughts? While I remain impartial in interviews, this Substack is a way to peer

into my present deliberations on these topics. Also, thank you to our partner, The Economist.

Firstly, thank you for watching. Thank you for listening. If you haven't subscribed or

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