Have we solved the black hole information paradox?

Daniel and Jorge break down recent advances in quantum gravity!

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, what's the best place in the universe to
hide something? Like? What do you want to hide? You know,
treasure or secrets anything. I guess you could tape it
to a rock and hide it in the asteroid belt
among millions of other rocks. Yeah that's pretty good. But
you know, technically somebody could still find it. Yeah, that's true.
I guess you could drop it in the sun. I

(00:31):
said hide it, not destroy it completely. All right, this
is pretty tricky. What if you put it into a
black hole? It depends can you wait like two zillion
years to get it back out again? But you mean
you can get things out of a black hole? Well,
you know, technically depends on the definition of out. But
if you want to throw a dictionary into the black hole,

(00:52):
everything comes back out except for the page that says out. Ye.
Hi am Jorge Immad, cartoonists and the creator of PhD Comics. Hi.

(01:14):
I'm Daniel. I'm a particle physicist, and sometimes I feel
like my mind has been thrown into a black hole.
Oh yeah, is it that nothing comes out of it?
That would be the case if my mind was a
black hole. But sometimes I feel like everything I've learned
has been slurped up and dropped into a black hole
because I can't remember any of it. It gets spaghettified.
And what if he eat spaghetti? What happens to spaghetti

(01:34):
when it gets spaghettified? It becomes spaghetti squared, becomes a
string theory maybe, but welcome to our podcast Daniel and
Jorge Explain the Universe, a production of I Heart Radio,
in which we tear our minds into little pieces of
spaghetti and then stretch those out into tiny little strings
in a vain and futile attempt to understand everything in

(01:56):
the universe. We take it all apart, we tear it
in a little bits, and we try to put it
back together in a way that we hope you can understand.
That's right, because it is a long and tasty universe
out there, with lots of mysteries and unknowns and exciting
things discover, and lots of different sauce options. You know. Actually,
I just realized they're spaghettini, right, and angel hairy pasta.

(02:19):
Although I don't know the difference. There must be a
whole spectrum, right, It's not like pasta is quantized. There
must be an infinite number of kinds of pasta. Well, eventually,
pasta is quantized. Everything is quantized eventually, right if you
get down to a single chain of pasta atoms, right,
nano pasta. Yeah, that's a whole new field of fabrication quarkinis.

(02:39):
But yeah, the universe is full of interesting and amazing things,
and none more so mysterious than black holes. That's right.
Black holes are these weird corners of the universe where
we think secrets lie. We suspect the solution to the
age old conflict between general relativity and quantum mechanics might
lie right at the heart of black holes. Of course,

(03:00):
nobody can see inside them, so it's incredibly frustrating to
think that the answers to some of our deepest questions
exist but are hidden from us. Does that drive physicist
bonkers to know that you know there are answers out there,
but maybe they're trapped in a place where they can
never get out. That's essentially the project of physics, right,
The answers are out there. The universe does contain these answers,

(03:22):
and if you can come up with the right experiment,
you can force the universe to reveal those truths. That's
basically how we accumulated all the knowledge that we do have.
But yeah, it's frustrating to think that there might be
places in the universe that nobody could ever look, where
information could hide forever, escaping from even our most clever
efforts to reveal them. You may get sound like physicists

(03:43):
are like paparazzis of the universe, Like you're trying to
catch the universe, you know, sunbathing in its supports or something.
I used to think of us as more like murder
mystery detectives, But yeah, paparazzi is a more positive way,
because the universe is beautiful. It deserves our attention. Positive. Yeah, yeah,
I guess they're surprised to being d universe. We're just

(04:03):
here trying to enjoy the beauty of the universe, you know,
and get its signature once in a while so we
can sell it on eBay and so to the National
Enquirer or Science magazine, you know, one of those two
whoever will take it. Is this part of the free
Brittany movement. I don't think there's a free universe movement
where they're like, hey, physicists, leave the universe alone. Keep
some of the mystery. You know, maybe doesn't want to

(04:25):
reveal its mysteries. You're being pushy. The mysteries are wonderful,
but the incredible part of this journey of physics is
that the truth is always more amazing and bonkers and
beautiful than we even possibly imagined, and so it's always
worth trying to dig into it and figure out what
the truth is. And so recently we've taken pictures of
black holes. You sort of know where they are, you

(04:46):
can see them gravitationally, and now we know what they
actually look like. But they still remain a pretty big mystery,
maybe even the biggest mystery we have in the universe, right,
that's right, And new mysteries keep popping up. You know.
Originally people were like, wow, our black holes real, and
then we discovered that they are actually out there and
that was kind of mind blowing. But then more questions
arose about them, and more questions and more questions, and

(05:08):
they're just like an endless playground for theorists to think
about what the universe means and and how it behaves
in really strange circumstances, and so they're continual source of
mystery and also opportunities to learn things about the universe,
because that's what mysteries are. They are ways for us
to unravel a thread and figure something out, right, because
I guess anything that happens in the universe tells us

(05:30):
a little bit about how the universe works, especially the
really extreme situations like black holes. Yeah, because we think
that the universe follows rules right there. There are physical
laws and everything in the universe always has to obey them.
Why that is is like a deep and fun question
in the philosophy of science, So check out some other
philosophy podcast for that. But the goal of physics is

(05:52):
to take advantage of that fact and say, let's find
opportunities where the things that happened reveal what the rules
must have been. You make It's not like we should
named the podcast Daniel to Daniel and Jorge Exposed the Universe.
That sounds a bit too risk, ay, And we're trying
to be family friendly here. So black holes are mysterious,
and there's sort of super extra mysterious. I feel like,

(06:13):
you know, I feel like there's not just kind of
the questions of what happens inside or what happens as
you go in, or how they can possibly exist in
the universe. But there's also sort of some very interesting
theoretical questions. Yeah, it's sort of weird that black holes
even exist. You know. One theory of physics say that
they can't exist, but they have to have certain properties,

(06:34):
and another theory of physics is that those properties are impossible.
So there's like a conflict at the heart of physics
between our idea of space and time, which is governed
by general relativity, and our idea of how the universe
works at the smallest scale, which is governed by quantum mechanics,
And those two disagree about what's going on inside a
black hole, whether black holes can live forever, and all

(06:55):
sorts of stuff, and so there are really fun mysteries there.
And so there's one mystery in particular that people have
been wondering about four decades about the stuff that goes
into the black hole. Yeah, and it seems like very
recently scientists have made some pretty um possibly interesting progress
towards solving one of these big mysteries. So have they been, um,

(07:17):
you know, parked outside their favorite coffee shop, you know,
just waiting to take those pictures the physics paparatti Exactly
when will that black hole finishes spa appointment. So to
the end the program, we'll be asking the question, have
we solved the black hole information paradox? I love that phrase,

(07:40):
black hole information paradox. It's got some poetry to it. Yeah,
it's a little too long for me. I kind of
struggled to get that one out. Black hole information paradox?
Have we solved it? That's the question? That's the question,
all right. So there's a paradox about black holes. It
involves information, and we think that maybe in their last
few months, maybe I physicists have solved this paradox. That's right.

(08:03):
It's a puzzle that's been outstanding for decades and to
one of those questions that people always thought, wow, if
you figure that out, then you're gonna learn something deep
about the universe. Or they imagine that maybe in a
hundred years somebody will know the answer to that. So
have some progress made on it to have people feel
like maybe they've even found the solution. It's a big deal, right,

(08:24):
And apparently recently something has happened, so people have appolished
the paper or they what happened? You know, there's been
a whole series of papers in this past year where
people have found a whole new way to attack this
problem and made what they thought was pretty exciting progress,
and some people even think that they may have solved it.
Oh interesting. They found a new method of attack, yes, exactly,

(08:46):
a new theoretical way to attack black holes, without of course,
actually going over there and measuring anything. All right, Well,
as usual, we were wondering how many people out there
had heard about the black hole information paradox, or that
they might have found a solution in for it. So
Daniel went out there into the wilds of the internet
to ask folks, what is the solution to the black
hole information paradox? That's right, So thanks to everybody who

(09:10):
volunteered their minds and their time. If you would like
to participate for a future episode, please do not be shy.
Right to me two questions at Daniel and Jorge dot com.
It's fun. Here's what people had to say. I don't
even know of a paradox information wise in black holes,
so um, I don't have the solution. I'm very sorry.

(09:31):
I believe it is that all the information is imprinted
on the surface like a that's what they call it,
a hologram. And some series actually said that how your
whole universe could be something like that. The paradox is
about what happens to information that gets sucked into a
black hole. I think what happens is that it gets

(09:52):
stored at the event horizon of the black hole, more
like black hole disinformation coles are trying to trick us. Well,
I guess this one refers to the fact that information
can get sucked into the into a black hole and
then you cannot get access to it anymore. Um but

(10:13):
ultimate information is just hidden in the black hole and
they destroyed it. So maybe the solution is that there's
no paradox after all. I don't think I know the
answer to the black hole information paradox, but I do
know that information can't be destroyed, so it's got to
be in the black hole somewhere. How that information gets extracted,
I'm not exactly sure. It sounds lucky. It's something to

(10:36):
do with Obviously, nothing can ever leave a black hole. Um,
but Hawking radiation says that, um, if left a lot
of nothing else goes into it, eventually they could evaporate
into nothing. Um that sounds pretty paradoxical to be so.
I had seen that a solution had been found recently
or in the past year or so, But apparently it's

(10:59):
something and going in leaves an imprint on the event horizon,
which somehow is represented in the radiation in the Hawking radiation.
But that kind of seems wrong because of the uh
no hair theorem. I mean, I think that's the gist
of it. I have heard of the black hole information paradox,

(11:23):
but I'm not sure exactly what it is. I believe
it is related to Hawkings radiation. Well, I suppose I
would ask why does it need to be conserved in
the first place? So why would it information disappearing be
such a problem. Perhaps it's not a problem at all,
especially if information seems to be created as new space

(11:46):
is created. Maybe it's not a problem that information goes
away when it enters a black hole. All right. Not
a lot of people had heard of this, you know,
a lot of good speculation, but yeah, no concrete answers there,
and some people seem to know what it is though.
That's pretty impressive. Yeah, well, we covered it on an
episode dug deep into the nature of this paradox about
information and black holes. Though I guess our listeners have

(12:09):
done their homework Apparently our episode didn't fall into a
black hole. It actually made it to people's spaghetti places.
It fell into people's ear holes. Yeah, all right, Daniel
will step us through this. Then let's start with what
it is first. What is the black hole information paradox.
This is a really fun concept because it brings together
these two different theories, general relativity, which tells us something

(12:31):
about black holes, and quantum mechanics, which tells us something
about information. So we start with general relativity because that's
where black holes come from. General relativity tells us, you know,
how space is curved in the presence of mass and
stuff like that, and it's sort of the genesis of
black holes, right there are this place in space where
no information can escape. Anything that falls past the event

(12:53):
horizon should be inside the event horizon forever until the
end of the universe. And you can't know anything out
what's inside the black hole other than the total mass
of the black hole, whether it's spinning, and whether it
has electric charge. Everything else is hidden from you. So
anything that's going on inside the black hole, you can't

(13:13):
know anything about it because that would be allowing you
to get information out of the black hole. This is
called the no hair theorem, tells you that's a very
limited amount of information you can have about a black hole.
So that means that if your friend threw a banana
into the black hole, you couldn't tell that it was
banana instead of an apple. You can only measure the
mass that they threw into the black hole. Right, That's

(13:35):
an interesting concept because I guess, you know, maybe we're
all used to the idea that black holes can sort
of things, and things can't get out, not even light,
But that also I guess applies to information itself. You know,
if light can't get out, then really nothing can get out,
not even like ones and zeros or you know, just anything. Right.
That's right, because we live in a physical universe and

(13:56):
information has to be represented physically. So to get information
from one place to another, you have to send a message,
which means like a particle or a wave of some
kind of physical thing. So if no physical thing can
cross that boundary, then no information can get out technically,
then you can gravitational waves escape a black hole. Gravitational
waves cannot escape a black hole, but a black hole

(14:17):
itself can create gravitational waves. But when it creates it
isn't it sort of like sending a signal out. Now,
that's just the information about the black hole itself. Like
you can know the mass the black hole from the outside,
and a gravitational wave is just a change in like
the location of a mass. And so you can know
about where a black hole is and what mass it
has and how it's moving, and that motion can generate

(14:38):
gravitational waves without knowing anything about what's going on inside
the black hole. All right, So that's a general relativity.
And then how does quantum mechanics figure into it. Well,
quantum mechanics says something really important and powerful about information.
It says that you can always reconstruct the past of
our universe based on what's going on right now. It's
sort of like a post addiction. It says, if you

(15:00):
could scan the universe finally enough, you could figure out
what has happened. It's sort of like saying, if I
burn a book, then you should be able to from
the ashes and the smoke and everything that comes out
of it later tell me exactly what that book was.
That all the information from that book is still somehow
imprinted in the universe, even if sort of scattered now

(15:20):
and harder to reconstruct, right does it kind of related
to that idea that we've talked about before, which is
the idea of cost and effect. Like if a particle
does this and it affects another particle, and that particle
affects another particle, you can always backtrack kind of what happened.
That information is not lost, like what one particle does
to another particle they remember, right, yeah, exactly. That information

(15:41):
is not lost. And our present state of the universe
is unique, right, it maps back to a single past.
You can't have two different pasts that produce the same
present because then you wouldn't be able to tell which
was which. And so you have all the information you
need in the present to reconstruct the past. And this
is a really deep and powerful important thing and quantum
mechanics like that's at the basis of quantum mechanics. It's

(16:03):
called unitarity. And if we didn't have it, if it
wasn't true, then we would have to question like quantum
mechanics itself and come up with another theory. So it's
like something every physicist out there believes is true about
the universe. The information is not lost because when you
think clancal mechanics is pretty real. We think it's pretty real.
We've tested it pretty well. We tested all day every

(16:24):
day at the Large Hadron Collider and in lots of
other different ways, so we're pretty sure it's true. Although,
you know, black holes are an extreme situation, so maybe
something gets broken. You never know. That's kind of where
the paradox part comes in, right, there's a paradox about
what happens between these two concepts of the universe inside
of a black hole exactly, because you might ask, well,

(16:46):
what happens to the information about my banana if I
throw my banana into the black hole and now it's
inside the black hole and I can't learn anything about it,
where is that information? Well, general relativity says black holes
live forever, and so that's a problem. The information is
still inside a black hole. It was just sort of
stuck there. And that's okay, except that we don't think

(17:08):
that black holes live forever. We think, thanks to Hawking radiation,
that black holes are not actually black. They give off
a little bit of radiation, they glow a tiny little bit,
and when they do that, they lose their mask because
they're giving off energy, So they get smaller and smaller
as it's called black hole evaporation, and eventually they disappear.

(17:28):
And so if your information about your banana was inside
the black hole and then the black hole evaporates, where's
the information? Did it disappear from the universe, you know?
Or did it somehow get leaked out in the Hawking radiation?
Is there a smell of banana once a black hole evaporations?
Is it in the air now? Stephen Hawking did a
really careful and sort of famous calculation where he showed

(17:51):
that the radiation itself has no information about anything inside
the black hole, only about the mass of the black hole.
So like, you can't tell what was inside the black
hole based on the Hawking radiation. So that's sort of
a mystery. Like the black hole disappears and the Hawking
radiation has just basically noise in it, So like, where
is the pattern for my banana? And that's the paradox

(18:13):
I see. The paradox is that it seems like information
hits the end of the road in a black hole,
whereas quantum mechanics would have you believed that it never
ends exactly, So somebody must be wrong, right, If information
is lost, then quantum mechanics is wrong, and that really
weakens the foundations of physics. If the information somehow escapes,

(18:34):
then that violens general relativity. Right, that's like information leaving
a black hole. How can that possibly happen? So it's
a super fascinating like sort of test bed to crash
these two theories together and say, well, you know, it's
like a cage match. Two theories go in and only
one comes out, and it depends on what happens next. Right, Yes, exactly,
it depends on what happens. No, No, I really have

(18:56):
a headache now. It feels like the Grandfather paradox, Like
if you put two theories into the black hole, but
only one of them can come out, but then the
black hole evaporates. What happens one of them is left
eating spaghetti? Yeah, the the Avengers movie maybe, And our
podcast episode about this, we talked about a few possible solutions.

(19:16):
You know. Hawking himself famously said that he thought information
was lost, that it just disappeared from the universe. But
most physicists don't believe that. Most physicists think that must
be wrong. That information is not lost. We just have
to figure out how it leaks out and So the
most popular theory until last year, until this recent progress
was made, was that maybe somehow there is information in

(19:38):
the Hawking radiation because the black hole surface is not
entirely smooth, like when you throw your banana onto the
black hole, maybe it gets like stretched out over the
event horizon but doesn't actually make a perfect sphere, and
that those little wiggles in the event horizons somehow contain
the information and influence the Hawking radiation. That was sort
of the direction people were going until recently interest ing.

(20:01):
So maybe you figure it out. Who comes out of
the cage? All right, Well, let's get into what this
potential solution is to the black hole information paradox and
most important, what does it mean. But first, let's take
a quick break. All right, we're talking about the black

(20:28):
hole information paradox and a potential solution to this paradox. Now, Daniel,
I guess maybe I'm having some trouble understanding why it's
a paradox, Like, you know, why can it be that,
for example, quantum mechanics does allow energy to always be
preserved except at a black hole, which is sort at
the end of the line, Do you know what I mean? Like,

(20:50):
why does it need to work everywhere and all the time. Well,
it really only means something if it works everywhere. You know,
momentum conservation is always meaningful if it's always conserved. You
know You're rules in your household are only meaningful if
your kids always follow them. Otherwise you know, what do
they even mean? So this is a bed robed principle
of quantum mechanics, and to have an exception at black
holes means there might be an exception somewhere else. And

(21:12):
basically it means it's not a foundational principle of quantum
mechanics that like information can disappear, stuff can be deleted
from the universe and you can have no record of
it ever having existed. That would just be a pretty
different universe from the one we thought we lived in.
Would be a catastrophe. We just have to figure out
some new theory that accommodates that, and a lot of
the assumptions we've been making would be wrong. So that's

(21:34):
kind of cool. That's like we say on this podcast
all the time, it would be awesome to knock down
a basic tenant of physics and discover something new. It's
just that that's a pretty big tenant, and so we're
pretty skeptical. You're pretty skeptical that there might be exceptions exactly.
I mean, it goes right back to the shorting or equation.
You know, the shorting or equation is unitary. It says

(21:55):
that probability is conserved. If you have a certain amount
of probability, it has to go somewhere, It doesn't just disappear.
And so that would be kind of weird but also fascinating,
like that would be a really interesting place to live,
and it wouldn't be the first time in the history
of physics that the universe was totally different in a
really basic way than the one we imagine. So we're
definitely keeping our minds open to that, but considering other

(22:16):
possibilities also, Right, all right, well, you say, one potential
solution is that maybe the black hole is sort of
leaking information when it evaporates like that somehow the evaporating
particles somehow carry away some of the information about the
black hole. That seems like a pretty good solution. But
what's the problem with that idea. Problem with that idea
of the event horizon not actually being smooth is that

(22:39):
then the information doesn't really go into the black hole.
It's just like stuck on the outside of the event
horizon so that it can radiate out, but then it
doesn't actually go into the black hole. So then you
need this like weird firewall, this like region where nothing
can actually go. Then like what's actually in the black hole? Right? Nothing?
That's sort of weird and require is this really strange

(23:01):
boundary that people can't really make sense of. The Other
problem with that is that it's hard to reconcile because
remember there's two ways to think about things falling into
a black hole. One from the outside when you're watching,
and you can never actually see the thing fall in
because of the gravitational time dilation. It takes infinite time
to fall in. You never actually see it fall in.
But the other point of view is from the point

(23:22):
of view of the thing falling in, and that just
passes right through the event horizon and heads to the singularity.
So it doesn't really make sense from the point of
view of the thing falling in for things to be
stuck on the event horizon. So it's not really a
workable solution, all right, I guess I mean trouble understanding
why exactly are you saying that you can have when

(23:42):
it evaporates, it can be sort of both coming out
and coming in at the same time. That's weird. Well,
it either needs to fall into the black hole or not, right,
And if it's information is encoded on the outside of
the event horizon, that means it hasn't really fallen into
the black hole. It's something like we're preventing it from
falling into the black hole. So then you need this
other thing, this firewall, preventing it. But we don't understand

(24:06):
what that would be or why that would be, and
it contradicts our view that you can fall into a
black hole from the point of view of the thing
falling in. So you need some whole other like idea
of what a black hole is to make that work.
So is this whole idea of evaporation theoretical or something
we understand thoroughly. Evaporation is not something we understand thoroughly,
and it is theoretical. Like, nobody's ever actually seen Hawking radiation,

(24:30):
So we say that black holes are not totally black,
but we've never actually seen one radiate anything. So this
is purely theoretical, and you know, it's an approximation like
Hawking did these calculations, but we don't really have a
theory to back them up. Like we have general relativity,
we have quantum mechanics, we don't have a theory of
quantum gravity when the combines the two, and that's really

(24:50):
what you need when things get both really really small
and really powerful gravitationally. Currently, we can do quantum mechanics
by ignoring gravity because gravity is very very small for
little particles that are affected by quantum mechanics. And when
we do gravity calculations we can ignore quantum mechanics because
quantum mechanical effects are small for the kinds of things
where we do get gravity calculations that are pretty big.

(25:11):
So inside black holes, we think you need a new
theory quantum gravity. Stephen Hawking didn't have this theory, but
it is sort of like a hand wavy calculation where
added a little bit of quantum mechanics to gravity, and
he came up with this calculation. But no, we've never
seen Hawking radiation, so we don't know that it's actually real.
I see. So when you say that the large Hadron
collider is producing black holes, but not to worry about it,

(25:33):
because they just evaporate. That's are you're saying. That's just
a big maybe there because Stephen Hawking sort of fudgeted. Yeah,
but the other half of that calculation that it would
be producing black holes relies on the same calculation. So
if we're producing them, then they are also evaporating. Oh,
I see, all right, so maybe it's not producing black holes.

(25:54):
I see, yeah, exactly. All right, Well then so that
doesn't work the idea that information can can leak out
through Hawking radiation. But now there's maybe a new theory
that could explain this paradox of what happens to the
information that goes into the black hole. And the approach
here is to add more quantum mechanics to the calculation,
to think, like, let's make this more quantum mechanical. Let's

(26:14):
add another concept and maybe they'll give us some insight.
And one that's been really appealing for getting information from
one place to the other is the idea of quantum entanglement.
We talk about this on the podcast, that two particles,
if they have a shared history, can have their fates intertwined.
Like if you have a particle with no spin and
it makes two particles that do have spin then those

(26:35):
spins have to be opposite in order to balance. Right,
you start out with zero spin have to end up
with zero spin. Quantum mechanics says that those two particles,
whether they're spin up or down, is not actually determined
until you measure one of them, So they're entangled even
if they're far away from each other in the universe.
So if one is created on Earth and the other
one is now sent out into space and they're kilometers apart,

(26:56):
and when you measure one of them, it determines the
spin at the other because they have to be opposite.
So that's cool because it feels sort of like a
way to get information from one place to another without
actually sending a message, right. It feels sort of like
this non local information transfer that you might want to
get information out of a black hole. Oh, I see,

(27:16):
So is the idea that then when something falls into
the black hole, it's actually entangled with something else outside
of it. Exactly. That's sort of the basis of the idea.
Before we dig into a little bit more, I just
want to add a caveat that for quantum entanglement, you
can't actually use entanglement to send information. You need also
some other sort of mechanism. And a lot of people
think that quantum entanglement is a way to send information

(27:38):
faster than the speed of light. But you can't do that.
But there is an important way that we need to
think about entanglement in order to attack this black hole
information paradox problem, and that's the Hawking radiation. Remember, Hawking
radiation is created when you have a particle antiparticle pair
created just outside the event horizon. One of them falls
in idea, another one flies out, which is the Hawking radiation.

(27:58):
If those two were created together, then they are entangled.
So now this Hawking radiation is somehow entangled with the
particle inside the black hole. And that's fascinating because it
tells you that, like maybe there's information there. Somehow, the
Hawking radiation has information about what's inside the black hole.
It's like at the moment of creation, it puts a

(28:19):
little bit of information into the black hole, but it
also takes away some information with it in the evaporation. Yeah,
and it doesn't contradict Hawking calculation. It says, look by itself,
the Hawking radiation, these particles that fly on from the
black hole, they don't have any information. You can't look
at them and tell what was inside the black hole,
but there might be some correlations there. There might be

(28:39):
some relationships between the Hawking radiation and what is inside
the black hole. And you can't tell what's inside the
black hole just by looking at the Hawking radiation, but
might be hidden in there. It's sort of like encrypted, right.
It's like if somebody can see how the computer represents
your password, but they don't actually know what your password is. Right,
they can just see those little dots on the screen

(29:00):
your pathwords in there, but you can't see it. So
maybe this information is in the Hawking radiation. It is
just sort of like encrypted by quantum entanglement. Interesting, but
that applies to the particle that gets evaporated and that
comes out. But what about my banana that I threw in?
What happens to my banana information? Oh, that's a good question.
What happens to your banana information? Well, your banana has

(29:21):
a quantum history, and so it is entangled with you,
who threw it in, and so it's much harder to
talk about quantum mechanics and macroscopic objects like bananas. But
in principle, your banana is entangled with you, and so
even if you throw it into the black hole, it's
information is still connected to you somehow, Like I still
remember the banana. Is what you're saying exactly, you are

(29:43):
the hawking radiation. In that case, my dear memories of
that banana are somehow still preserving the information about in
the years. Is that true? Is that kind of what
this new solution is about. It's just that you know,
everything is entangled with each other and so you never
really lo is it when it goes into the black hole?
Is that the basic outline of the solution. That's not

(30:03):
the solution. It's another way to look at the problem
because it doesn't actually solve the problem. It just sort
of restates it. But it restates in a way that
we then can attack. And here's why it's not actually
a solution to the problem. Think about what happens as
the black hole evaporates. As the black hole gets smaller
and smaller, you have all this hawking radiation that was
created entangled with stuff inside the black hole. But now

(30:24):
the black hole is disappearing, So where is that entanglement going.
Then the hawking radiation is like entangled with something that
has disappeared, and so you sort of have the same problem, right,
it's just sort of stated in a different way. So
what we need is a way to figure out how
entanglement for a black hole can start at zero, and
then during the black hole's lifetime it can grow. As

(30:45):
it's evaporating, it's giving off Hawking radiation, which is now
getting entangled with the stuff inside of it somehow. Then
that entanglement has to get back down to zero. All
those entanglements have to get broken or somehow resolved before
the black hole evaporates. So that's this newer way of
looking at the problem. This way that one of Hawking students,
Don Page came up with, and that's sort of the

(31:05):
crevice that these new groups attacked in order trying to
figure out what's going on. Oh I see it is
that like I'm entangled to my banana. I remember it fondly.
I remember the curvature and the just how many spots
and how sweet it was, Oh banana, I eat eu well.
And so then the banana goes into the black hole.
Inside the black hole, it gets entangled with particles being evaporated,

(31:28):
which means that the you know, evaporating particle that comes
out is somehow entangled to the banana which was entangled
to me. Yeah, Or more directly, if you throw it
into the black hole, you are still entangled with that banana.
You don't need hawking radiation for that. But then your
information about the bananas somehow in the black hole, right,
and then as the black hole is evaporating, you wonder,

(31:50):
like what happens to that entanglement? Instead of thinking about
like this information stuck inside the black hole, think about
in terms of the entanglement, how can I be entangled
with something which is then just appearing. Where does that
entanglement go? Does the entanglement and gets passed onto the
evaporating particle? Is that the idea? That's the question, right,
how do you somehow go from having entanglement to getting

(32:11):
zero entanglement? Like, the black hole is definitely creating entangled
pairs because things are falling into the black hole and
they are entangled with whatever they came from hawking radiation
or Jorge throwing a banana, And the question is then
how do you get a black hole to break those
entanglements somehow? All right, so then it's somehow preserved and
so what's been the progress in the last year. So

(32:33):
a bunch of smart young theories attack this problem and
they came up with a new way to do this calculation. See,
the problem is that nobody knows even how to calculate
this entanglement. Like we say, entanglement has a start from
zero for a black hole, that it has to grow,
that it has to come back down to zero. Problem is,
we don't have a theory of quantum gravity, so we
don't understand how to calculate entanglement, which is a quantum concept,

(32:54):
in the environment of a black hole, which is a
gravitational concept. So until now, nobody even knew like how
do we calculate this thing. We know it has to
start a zero, go up and come down, but we
don't know how to calculate it. So what they did
is they came up with a clever new approach theoretically
to do this calculation. And if you read their papers,
it's like a bunch of crazy mathematical tricks they do.
They take the problem, they transform into something else and

(33:16):
they apply this tool and they put in wormholes and
they do this calculation. It's like a real like tourtive
force of mathematical tricks. And what they've done is they
figured out a way to sort of calculate the entanglement
of this black hole over its lifetime. And in their calculation,
they see the entanglement going up, peeking, and then coming

(33:37):
back down. Interesting, what do you mean they put in
some wormholes. You can just do that. You can sprinkle
in some wormholes. Yeah, exactly. They use wormholes as just
a sort of a way to do these calculations. You know,
sometimes you attack a math problem, you don't know how
to do it, so you transform it into something else.
And so what they did is they transformed the problem
from one kind of problem into another one where they

(33:58):
could use wormholes that they knew how to calculate. I
don't know how to integrate this thing, but I'll transform
into this other thing that I do know how to integrate.
There's a lot of details there about how they did
this calculation. For example, nobody knows how to calculate entanglement
in four dimensional space with gravity. So they did two things.
When they said, well, let's just take it down a
couple of notches and think about, you know, two dimensional

(34:20):
black holes instead of four dimensional black holes. Does that
mean that you have to assume that there are wormholes
inside of the black hole. These wormholes are more of
a mathematical trick than actual physical wormholes. But in a
minute we could talk about what the solution means and
whether they're actually are wormholes connecting the inside of the
black hole to the outside. But for now, just think
of a sort of a mathematical trick that they did

(34:41):
in order to get to a number. And the thing
to understand about this calculation is like, they don't understand
how the information is getting out. It's not like they
found some mechanism where they're like, oh, look, here's some
little holes in the event horizon. They just figured out
a way to do this calculation and it has the
shape they expected to have. If black holes are leaking information,
that doesn't mean they know how it's happening. It's like

(35:03):
if you see your fridge temperature is rising, you don't
necessarily know where the leak is in your fridge. You
just know that somehow somewhere heat is getting in. I see.
The solution also sort of supposes that black holes are
not perfectly smooth inside. Right, there's some crazy interpretation of
these calculations, and I say interpretation, because these calculations are

(35:24):
their mathematical and how they relate to physicality is complicated,
and the cosmologist I talked to really disagree about whether
you can make any physical interpretation about like what's happening,
because sometimes this information exists in this sort of abstract
space we call Hilbert space for quantum mechanics, not in
a physical space, but there is sort of a cartoon
picture that you can draw to try to get an

(35:46):
understanding of what's going on, and it involves really crazy
things like perhaps inside the black hole, it's not actually
all black hole. Maybe there's like an island inside the
black hole where radiation can fall, and it's sort of
no longer part of the black hole. It's part of
this like quantum radiation island. It's physically inside the black hole.

(36:09):
But now the black hole, instead of being like a sphere,
is sort of like a shell. You know, it has
like a hollow core, and that hollow core is not
black holy anymore. What Yeah, Well, how can a black
hole not be a black hole? Inside of a black hole,
it's a double decker, it's like multilayer. So the idea
is that like it's just like a normal classical black

(36:29):
hole in the shell part, but then the core of
it has this other weird quantum gravity thing going on
that we don't yet understand. But we think that maybe
stuff inside the black hole is like falling into this
quantum island, and that quantum island is entangled with the
stuff that has come out, and so this is how
things can like sort of leave the black hole without

(36:50):
passing through the classical event horizon. Again, people disagree about
whether this is like a cartoon picture just for sort
of understanding the calculations, or what's really going on inside
a black hole. All right, well, let's get into what
this potential solution might actually mean, like have we actually
solved the paradox or are we still a long ways away?

(37:10):
But first let's take a quick break. All right, Daniel,
it sounds like maybe they have a new way to attack,
finding a potential solution to a potentially imaginary, not quite

(37:35):
sure paradox about black holes. Right, Did I get that right? Yeah?
I think you did. There's enough qualifiers there. You know,
this is exciting because people for a long time thought
we wouldn't make progress on this for decades until we
came up with a theory of quantum gravity. How can
we even calculate the level of the entanglement of the
inside of the black hole and the outside of the
black hole if we can't do quantum gravity calculations. So

(37:58):
this is exciting, but it's not exactly like a full answer, right.
It's sort of like we were able to without coming
up with the theory quantum gravity. We still don't have one.
We were able to do this one calculation, and that
calculation says something amazing. It says that the entanglement decreases,
and it's a special moment halfway through the lifetime of
a black hole when the entanglement just starts to drop.

(38:21):
It's like the midlife crisis of a black hole. And
that suggests if this calculation is correct, then it means
what we think. It means that information does leak out
of black holes. The things you throw into a black
hole are not lost forever. That's when the black hole
gets desperate and absorbs a Ferrari or something and expensive
sports car or something. Yeah, where it leaks out of

(38:42):
Ferrari right exactly. It's been hoarding its stuff for all
of its lifetime and then it decides, oh my God,
I gotta spend all this mass before I evaporate, and
it starts leaking up bananas and Ferrari's and everything. I
guess this idea that black holes have a lifespan depend
on the idea that black holes at some point stop
eating stuff, right, Like, you have to starve a black

(39:02):
hole for it to disappear. Oh, absolutely, yeah. This is
only the case of an isolated black hole. If you
kept feeding a black hole, it would just keep growing.
This is what would happen if you had a really
big black hole and then you left it totally isolated.
General relativity says it will sit there forever. Quantum mechanical
modifications to general relativity by Stephen Hawking say no, actually,

(39:23):
they will radiate a little bit, leak out mass, and
eventually disappear and evaporate in a blinding flash of light
at the very end. Because the evaporation happens more quickly
for small black holes, so you get more evaporation near
the end of the life So the beginning of the
life cycle like a little bit. But then as the
black hole shrinks and shrinks, it gets brighter and brighter,

(39:43):
all right, So then this potential solution says that maybe
information is not completely destroyed. It sounds like the idea said,
you throw a banana in. The banana SML falls into
a pocket of non black holiness inside of the black hole,
in which it could get maybe entangled with a particle
that's evaporating from the black hole. Therefore, then the evaporating
particle it still carries a little bit of the banana

(40:06):
with it, Yeah, exactly. And they don't really have an
understanding at all for how this information leaks out, Like
as you say, the banana falls in, it's entangled with
you somehow. That banana's information then moves from the pure
black hole state into this weird quantum island, and so
it's not really counted as part of the black hole anymore.
And then as the black hole is evaporating, is a

(40:28):
shell now and it's just basically shrinks down to surround
this quantum island and then disappears, and so then all
that information is still there. Again, cosmologists are really hesitant
to like tie the location of this information with anything physical,
So there are some really tricky questions here, and so
I reached out to one of the theorists who is
actively working on these problems and has been involved in

(40:48):
some of this recent progress. Professor Netta inglehearted m I
t to tell us a little bit more about it. Netta,
thank you very much for joining us pleasure, thank you
for having me. So. My first question is I think
we've I think at a mental picture, if possible, of
what happens when halfway through the life of the black
hole the entropy suddenly starts to decrease. Your papers talk

(41:10):
about this quantum extremal surface that divides the black hole
into these two regions. Can you tell us a little
bit about this surface, how it appears, and then what
the two regions are like. Is it possible to get
a mental picture? I can certainly try. So let me
maybe say a few words about what a quantum extremal
surface is and what what maybe starting with what we

(41:31):
mean by an extremal surface to begin with, So, why
does an extremal surface why do we call it an
extremal surface. So an extremal surface is a surface whose
area doesn't change when you jiggle it a little bit.
So if you slightly, ever so slightly modify the location
of the surface just a little bit, then the area

(41:54):
of the surface does not change. Now, where does this
quantum bit come in. What is that quantum extreme more surface. Well,
it's a little bit like a quantum corrected area. So
what is the quantum corrected area like the area plus
a contribution from from quantum fields, a contribution from the
entropy of of quantum fields that are just living in

(42:17):
your universe, and a quantum extremal surface is kind of
what it sounds like. It means that the sum of
the area of the surface with the entropy of of
quantum fields, this quantum contribution to the area's count corrected
area doesn't change when you slightly jiggle the location of
this surface. So that's the sort of definition of a

(42:40):
quantum extremal surface. And so indeed, in the black hole
information paradox and the recent developments, what we have found
is that the difference between extrememal and quantum extreme all
is exactly what it takes two allow us to see
the signature of information conservation. It is this critical difference

(43:02):
between whether the area can be increasing even though the
overall quantum corrective area is not. That is this this
critical and new ingredient in a very frazy new developments
over the past two years that have been some sense
in a renaissance in the in the black hole information paradox.
Can we think of this as a physical surface or

(43:24):
is this sort of like an intellectual dividing point for
using New Year calculations? Is that a real physical thing
where the space is different inside and outside the surface.
It's a real physical surface in the sisse that it
lives in spacetime. It's physical in another way, which is
also nice, and that if you had a family of
observers who were sitting closely spaced along the surface, and

(43:45):
they were able to each of them could measure what
light rays are doing at the surface. So, in other words,
if you had a ball and he had observers all
around the ball, and each one of them measured in
their own the own little small neighborhood, whether light rays
are expanding or away from the ball or contracting, and
you put all those observations together, you would be able
to tell this is a quantum extremal surface. So this

(44:08):
is something which is in principle observable the location of
that surface. Of course, we expect that if you're seeing
a quantum extremal surface, it is already too late to
communicate that to someone who is standing outside of the
black hole. So then, how does this quantum extremeal surface
help us understand the decrease in entanglement of the black

(44:29):
hole the interior of the black hole, with the Hawking
rediation already produced in the first half of the black
hole's lifetime, does it break that entanglement or do later
particles now get entangled with earlier particles? How does the
entanglement actually decrease? That's an excellent question. How do we
actually understand the way in which information gets out? I
think this is what you're really asking, how do we

(44:51):
actually understand the entanglement entropy? And the fact of the
matter is that we now have two ways of computing
the entanglement entropy of hawk mediation, either Hawking's original calculation,
which tells you that you have information loss, or the
quantum extremal surface prescription, which keeps you information conservation. The
quantum extremal surface prescription has some grounding in the holographic correspondence.

(45:16):
It has derivations coming from the holographic correspondence, which is
very well motivated, but nevertheless still like conjecture, And in particular,
when I say that not thinking that it might be wrong,
but thinking that we don't have a derivation of it,
and so it is difficult to put on the same
footing Hawkings calculation and the quantum extremal surface prescription and

(45:39):
simply ask where did Hawking go wrong? Because right now
we have two completely different calculations of the same thing
that don't talk to each other, and we don't know
how to poured over those insights from the quantum extremal
surface prescription into the same model that Hawking was working with.
Now I say we don't know how to do that,

(45:59):
but I should also say that's a very active area
of investigation. So given how quickly thinks have moved in
the past couple of years, I expect that our ignorance
of this is not going to last much longer. All right,
So do you have like a cartoony picture in your
mind of you know, what's happening to these particles? You know,
I understand it's not easy to transport your calculation from

(46:21):
one regime into this this sort of picture that we
have in our heads. But what picture do you have
in your mind when you think about where the information
is going? Where the information is going is complicated, but
I can tell you what my opinion is on what's
missing from say, Hawking circulation that's included in the quantum

(46:42):
extremal surface calculation. And I should preface this by saying
that this is a wild speculation that is not at
all currently back by anything that I have derived or done,
which is that there's a very beautiful paper by Harlowe
and Hayden in which they discussed how complicated it is
to decode the Hawking radiation, and then the assumption that

(47:04):
Hawking radiation it does sort of purify itself, so at
the end the information is conserved in the process. Then
it's still i would say, exponentially difficult in terms of
computational complexity to decode the Hawking radiation, to understand exactly
to see unitarities exponentially complicated. We might speculate that Hawking

(47:28):
calculation in some sense did not include those exponentially complicated
operations exponentially complicated information. When you look at Hawking calculation,
do you see the team missed anything? Now, so then
there's a question, how would we see this exponential complexity,
where is it hiding? How did Hawking miss it? In

(47:49):
all honesty, that's very much an open area of research
right now, we just don't know. All right, Well, thanks
very much for sharing your thinking about this really fun
and fascinating puzzle about these corner of the universe. Really
appreciate your time. So that was my fun conversation with
Nada Englehart, who tried to help us get a mental
picture of what's going on. And it's sort of amazing

(48:10):
because it means basically quantum mechanics was right. It means that, yeah,
information is not destroyed. It's there. It will not disappear
from the universe, even if it has to like hide
in some little weird quantum island at the core of
a black hole. It will persist, right, I guess the
question is could you decode that information? Like if you,
you know, trillions of years from now in the future,
someone looking at our black holes evaporating, could they like,

(48:33):
you know, make up, Oh there's more banana there. That's
the question, you know, can you decode this information that
was the subject of this fun TV show called Dev's
you know that basically built on this whole concept that
couldn't you take a scan of the Earth now and
use that to reconstruct history? Could you know, see Jesus
on the cross or important historical events just based on

(48:54):
you know where air molecules are today, and in principle
if possible, but in practice it would require knowing the
quantum state of all of these particles so you could
properly get them back in time, and that would be
essentially impossible even with a quantum computer, but in principle
it is possible. And so if this information does leak
out of black holes in terms of Hawking radiation, then

(49:16):
in principle you could decoded. And the physicist actually did
a really fun sort of thought experiment to how you
could make that work That involves like weird informational wormholes. Oh,
great sprinkle and more wormholes? Why not? The way they
think you might be able to do it is to
run a simulation of a black hole on a quantum computer.

(49:37):
To take a quantum computer which can represent these quantum
states fairly naturally, and run a simulation of the black
hole and compare the Hawking radiation you get from the
real black hole to the simulated ones. So then if
you find a simulated black hole that has the same
pattern of radiation as you're a real one, then you
can peer inside of it and see this must have
been what was inside the black hole. What, but would

(50:00):
have to match like every particle that comes out of it, Yeah, exactly,
It would be tricky. But if you do that, then
something really weird happens. In their calculations, they suggest that
a wormhole is essentially created between the real black hole
and the simulated black hole in your quantum computer. What
it's like an informational black hole, like a virtual black hole. Yeah,

(50:24):
because then that information is now being revealed from the
core of the black hole, a real black hole, to
the one in your simulation in your quantum computer. And
so there's like, how does that information get from there
to there? It's this weird non local effect, and so
non local information transfer can really only be done by wormholes,
which connect different places in space and time. So like,

(50:46):
that's pretty weird. If you run a program which creates
a wormhole between your computer and a real black hole,
whoa creates like a little bookatiny channel or something between
the two. All right, well, I guess the big question
is what does this mean? Does this mean that we've
solved or we think we may have solved this kind
of like conflict between quantum mechanics and general relativity or

(51:10):
does this sort of side step that conflict. It sort
of sidesteps the conflict, right, it doesn't really get to solution.
The solution would be a full theory of quantum gravity,
one that tells us what happens really small distances and
powerful gravity, which is not something that's easy to test,
right because small distances you only have little particles, and
little particles have very small amounts of mass and gravity

(51:32):
super duper weak, so we can't really do gravity experiments
with tiny particles, so it's pretty rare we get to
even explore this regime experimentally to like see what happens,
which is why black holes in their interiors are such
a valuable test. Ben So we think we need like
a theory of quantum gravity to really resolve this. And
the fascinating thing about making progress like this on a

(51:53):
big question is that it's a bit disappointing and exhilarating.
Like it's disappointing because people fought you need a quantum
gravity to answer this question, So now answering it without
the getting the theory quantum gravity is sort of like,
oh darn, you know, we thought maybe this was like
a thread we could pull on to reveal the truth

(52:13):
theory of quantum gravity. Instead, we only sort of like,
you know, pull the thread and got the end of
the thread. We didn't really like get the whole thing
to unravel. Basically, physicists are never happy. You find a solution,
you're happy and unhappy, And you don't find a solution,
you're happy and unhappy. We're in a superposition of happy
and unhappy at all entangled happiness there. And also remember

(52:35):
that not everybody really believes in this solution. There are
folks out there that are just like, no, there's too
many mathematical tricks there. I don't really buy it. It's
sort of a little bit controversial still, and you know,
there's reasons to be skeptical. Like to do this calculation,
they did this weird trick where they said, I don't
know how to do a calculation in four dimensional space

(52:57):
with gravity, but I can do a similar calculation on
a three dimensional space that has no gravity. And there's
this famous result in physics from about twenty five years
ago showing that those two calculations should be the same.
You take a four dimensional space that has gravity in it,
and you should be able to instead get all the
information about that space just by looking at like a

(53:17):
three D surface around that four D space, And in
the three D surface there is no gravity, there's just
quantum mechanics. So they did their calculation on this like
three D surface, not in the actual gravity of a
black hole. And so some people feel like maybe that's cheating.
Other people feel like, know that connection is real. It's
a mathematical trick, but it's real and it's reflective of

(53:39):
the way our universe works. So there's still sort of
a lot of controversy about whether this will hold up.
Just throw all those physics into the black hole and
then them doke it out. Yeah, and you know, we
have the East Coast versus the West coast. There's two
different calculations, two different strategies for doing this calculation that
got dissimilar results, and so that's sort of reassuring, but

(54:00):
it's also sort of fun because you get like East
Coast versus West Coast. Who wraps better? They're all wrapped
it up inside a black hole. All right, Well, it
sounds like maybe stay tuned. It seems like there's some
interesting progress that's been made, but not everyone is convinced
about this new approach. That's right, and my personal hope
is that we do eventually get to do experiments near

(54:21):
black hole. All this, the theoretical calculation is fun and
it tells us what question is to ask, but we
never really know the answer until we actually get to
go do experiments near a black hole or maybe even
in a black hole. Until then, we just have to
smell the banana in the air and remember it and
remember and remember both our dearly departed lunch. All right, well,

(54:44):
we hope you enjoyed that. Thanks for joining us, see
you next time. Thanks for listening, and remember that Daniel
and Jorge explained. The unit Verse is a production of
I Heart Radio or more podcast from my heart Radio.
Visit the I Heart Radio Apple Apple Podcasts, or wherever

(55:08):
you listen to your favorite shows. H