Daniel and Jorge Explain the UniverseHave we solved the black hole information paradox?
Daniel and Jorge break down recent advances in quantum gravity!
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How do we utilize the opportunities that we have that they don't right? And a lot of that is educating ourselves, educating ourselves on how to not make the same mistakes they did.
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Hey Daniel, what's the best place in the universe to hide something?
Mmmm? 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 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?
Wait? 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, everything comes back out except for the page that says out.
Hi am Orham, a cartoonist and the creator of PhD comics.
Hi.
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 it eats spaghetti? What happens to spaghetti 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 iHeartRadio.
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 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 there's spaggattini, right, and angel hair pasta. 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's quantized eventually, right.
If you get down to a single chain of pasta atoms, Yeah, right, nano pasta. Yeah, that's a whole new field of fabrication quorkinis.
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 that solution to the age old conflict between general relativity and quantum mechanics might lie right at the heart of black holes. But of course 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 physicists bunkers 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, 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 make it sound like physicis are like paparazzis of the universe, like you're trying to catch the universe. You know, sunbathing in its ports 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 there's surprise to being a d universe.
We're just 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 to sell to the National Inquirer or Science magazine, you know, one of those two, whoever will take it.
Is this part of the free Britney 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 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 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, are 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 they're just like an endless playground for theorists to think about what the universe means and how it behaves in really strains circumstances, and so their continual source of mystery and also opportunities to learn things about the universe, because that's what mysteries are. There are ways for us to unravel a thread and figure something out.
Right, because I guess anything that happens in the universe tells us 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, that there are physical laws and that 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 podcasts for that. But the goal of physics is to take advantage of that fact and say, let's find opportunities where the things that happen reveal what the rules must have been.
You make it sound like we should rename the podcast Daniel to Daniel and Jorge Exposed the Universe.
That sounds a bit too riskque, and we're trying to be family friendly here.
So black holes are mysterious, and they're sort of super extra mysterious. I feel like, you know, I feel like there's not just kind of the questions what happens inside or what happens does 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, and another theory of physics says 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 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 for decades, about the stuff that goes into the black hole.
Yeah, and it seems like very recently scientists have made some pretty possibly interesting progress towards solving one of these big mysteries. So have they been, you know, parked outside their favorite coffee shop, you know, just waiting to take those pictures the physics paparazzi?
Exactly when will that black hole finish its spa appointment?
So to the end of the program, we'll be asking the question, have we solved the black hole information paradox?
I love that phrase, 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 the last few months, maybe physicists have solved this paradox.
That's right. 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 going to learn something deep about the universe. Or they imagine that maybe in a one hundred yar 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, And apparently recently something has happened, so people have polished 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, a new theoretical way to attack black holes without of course actually going over there and measuring anything.
All right, Well, as usually we were wondering how many people out there had heard about the black hole information paradox, or that they might have found a solution 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 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 danielanhorge dot com. It's fun.
Here's what people had to say.
I don't even know off a paradox information wise in black holes, so I don't have the solution.
I'm very sorry.
I believe it is that all the information is imprinted on the surface like a what they call it a hologram, and some theories actually say 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 stored at the event horizon of the.
Black hole, more like black hole disinformation.
The black holes 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. But or to me, the information is just hidden in the black hole and the 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.
Sounds lucky.
It's something to do with Obviously, nothing can ever leave a black hole, but Hawking ratiet radiation says that if left alone, nothing else goes into it, Eventually they could evaporate it in nothing.
That sounds pretty paradoxy couldn't be so.
I had seen that a solution had been found recently or in the past year or so. But apparently it's something going in leaves an imprint on the event horizon, which somehow is represented in the radio in the Hawking radiation. But that kind of seems wrong because of the no hair theorem. I mean, I think that's the gist of it.
I have heard of the black hole information paradox, but I'm not sure exactly what it is.
I believe it is related to Hawking's 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 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.
No, 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 in black holes. Though, I guess our listeners have done their homework.
Yeah, apparently our episode didn't fall into a black hole. It actually made it to people's spaghetti place.
And felling in people's ear holes.
Yeah all right, Daniel, Well 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 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. They are this place in space where no information can escape. Anything that falls past the event horizon should be inside the event horizon forever until the end of the universe. And you can't know anything about 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 know anything about it because that would be allowing you to get in 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 a banana instead of an apple. You can only measure the mass that they threw into the black hole.
Right. That's an interesting concept because I guess, you know, maybe we're all used to the idea that black holes can sort 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 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, Daniel, can gravitational waves escape a black hole.
Gravitational waves cannot escape a black hole, But a black hole itself can create gravitational waves.
But when it creates it doesn'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 of 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 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 diction. It says like if you 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 it's sort of scattered now and harder to reconstruct.
Right. This is 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 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 in quantum mechanics, Like that's at the basis of quantum mechanics. It's 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 union, that information is not.
Lost because we think kungum mechanics is pretty real.
We think it's pretty real. We've tested it pretty well. We test it all day every 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, 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 not a problem. The information is still inside a black hole. It'll just sort of stuck there. And that's okay, except that we don't think that black holes live forever. We think, thanks to Hawking radiation, that black holes are not actually black. They give off 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. It's called black hole evaporation, and eventually they disappear. 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 evaporates, Is it is it in the air now.
Stephen Hawking did a really careful and sort of famous calculation where he showed 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 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 believe 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, then that violates 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 there.
Yeah. Oh no, now I really have a headache.
Now.
It feels like the Grandfather paradox. Like if you put two theories into a 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, you end up with the Avengers movie. Maybe.
In our podcast episode about this, we talked about a few possible solutions. 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 sort of the most popular theory until last year, until this recent progress was made, was that maybe somehow there is information in that 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 horizon somehow contain the information and influence the hawking radiation that was sort of the direction people were going until recently.
Interesting until 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.
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All right, we're talking about the black 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't it be that? For example, quantum mechanics does allow energy to always be preserved except at a black hole, which is sort of the end of the line. Do you know what I mean? Like, 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, your rules and your household are only meaningful if your kids always follow them. Otherwise, you know, what do they even mean? So this is a bedrock principle of quantum mechanics, and to have an exception at black holes means there might be an exception somewhere else. And 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 could have no record of it ever having existed. That would just be a pretty different universe from the one we thought we lived in. Wouldn't 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 kind of cool. That's like we say on this podcast all the time, it would be awesome to knock down a basic tenet 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 equation.
You know.
The Shorteninger equation is unitary. It says 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 imagined. So we're definitely keeping our minds open to that, but considering other possibilities also.
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 particle 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.
The problem with that idea of the event horizon not actually being smooth is that 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 region where nothing can actually go. Then like what's actually in the black hole? Right? Nothing? That's sort of weird, and it requires this really strange 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 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 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 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, 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 that combines the two, and that's really 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. 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 handwavy calculation where he 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 because they just evaporate. That's how are you saying that's just a big maybe there because Stephen Hawking's stort fudgeted.
Yeah, but the other half of that calculation that it would be producing black holes relies on the same So if we're producing them, then they are also evaporating.
Oh icee, all right, so maybe it's not producing black holes.
Ic see, yeah, exactly.
All right, Well then so that doesn't work the idea that information 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 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 have to 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 spins have to be opposite in order to balance. Right, you start out with zero spin, you 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, and when you measure one of them, it determines the spin of 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, 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 the quantum entanglement is a way to send information 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 anti particle pair created just outside the event horizon. One of them falls in and the other one flies out, which is the Hawking radiation. If those two were created together, then they are entangled. So now this Hawking radiation is somehow entangled with a 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 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 Hawkings calculation. It says look by itself, the Hawking radiation, these particles that fly in 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 some relationships between the Hawking radiation and what is inside the black hole. You can't tell what's inside the black hole just by looking at the Hawking radiation, but it might be hidden in there, 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. Your password's in there, but you can't see it. So maybe this information is in the Hawking radiation. It's 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 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, its information is still connected to you somehow.
Like I still remember the banana.
Is what you're saying, exactly, you are the hawking radiation.
In that case, my dear race 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's entangled with each other and so you never really lose it when it goes into the black hole. Is that the basic outline of the solution.
That's not 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 it 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 the black hole is disappearing, So where's 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 it's evaporating, its 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's students, Don Page, came up with, and that's sort of the crevice that these new groups attacked in order to try 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 eu well.
And so then the bana goes into the black hole. Inside the black hole, it gets entangled with particles being evaporated, 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 banana is somehow in the black hole, right, and then as the black hole is evaporating, you wonder, 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 disappearing whereas that entanglement go does.
The entanglement then gets passed on to the evaporating particle? Is that the idea?
That's the question, right, how do you somehow go from having entanglement to getting zero entanglement? Like, the black hole is definitely creating entangled pairs because things are falling into the black hole and they're entangled with whatever they came from maybe 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 a bunch of smart young theorists 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 to start from zero for a black hole that it has to grow, 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, 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 they apply this tool. Then they put in wormholes and they do this calculation. It's like a real like toorative 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, peaking, and then coming 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 to 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 could use wormholes that they knew how to calculate. Oh, I don't know how to integrate this thing, but I'll transform it to 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. One they said, well, let's just take it down a couple notches and think about you know, two dimensional 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 there actually are wormholes connecting the inside of the black hole to the outside, But for now, just think of it as sort of a mathematical trick that they did in order to get to a number. And the thing to understand about this calculation is like the we 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 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 this 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 their mathematical and how they relate to physicality is complicated, and the cosmologists I talk 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 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. 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 holey anymore.
What, Yeah, how can a black hole not be a black hole inside of a black hole?
It's a double decker, it's like multi layer. So the idea is that, like, it's just like a normal classical black 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 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? But first, let's take a quick break.
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All right, Daniel, it sounds like maybe they have a new way to attack, finding a potential solution to a potentially imaginary, not quite 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 could 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 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 of 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 this is this special moment halfway through the lifetime of a black hole when the entanglement just starts to drop. 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've thrown in black hole are not lost forever.
That's when the black hole gets desperate and absorbs a Ferrari or something, an expensive sports car or something.
Yeah, where it leaks out of a 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 out bananas and Ferraris 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 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, 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 work quickly for small black holes, so you get more evaporation near the end of the life So the beginning of a life cycle like a little bit. But then as the black hole shrinks and shrinks, it gets brighter and brighter.
All right, So then this potential solution says that maybe information is not completely destroyed. It sounds like media is that you throw a banana in. The banana somow 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 still carries a little bit of the banana 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, it's a shell now and it just basically shrinks down to surround this quantum island and then disappear. So then all that information is still there. Again, cosmologists are really hesitant to 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 some of this recent progress, Professor Netta Inglehart at MIT, to tell us a little bit more about it. Nedda, thank you very much for.
Joining us, my pleasure, thank you for having me.
So my first question is, I think we'd like to get a mental picture, if possible, of what happens when halfway through the life of the black hole the enterpiece suddenly starts to decrease. Your papers talk 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 extreme al surface is and what may be starting with what we mean by an extreme surface to be in 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 of the surface does not change. Nowhere does this quantum bit come in. What does that quantum extremal surface. Well, it's a little bit like a quantum corrected area. So what is a quantum corrected area like the area plus a contribution from quantum fields, a contribution from the entropy of quantum fields that are just living in 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 quantum fields, this quantum contribution to the area, this quant corrected area doesn't change when you slightly juggle the location of this surface. So that's the sort of definition of a quantum extremal surface, and so indeed in the black hole information paradox, in the recent developments, what we have found is that the difference between extremal and quantum extremal is exactly what it takes to allow us to see the signature of information conservation. It is this critical difference between whether the area can be increasing even though the overall quantum corrected area is not. That is, this is critically new ingredient in a very frenzy new developments over the past two years that have been some since in the renaissance in the black hole information paradox.
Can we think of this as a physical surface or is this sort of like an intellectual dividing point for you to New year calculations. Is it a real physical thing where the space is different inside and outside the surface.
It's a real physical surface in desises that it lives in space time. It's physical in another way, which is also nice in that if you had a family of observers who were sitting closely spaced along the surface, and 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 you had observers all around the ball, and each one of them measured in their 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 is something which is imprinciple 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 extremal surface help us understand the decrease in entanglement of the black hole? The interior of the black hole, with the Hawking radiation 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 actually understand the entanglement entropy? And the fact of the matter is that we now have two ways of computing the entanglement entropy of Hawking radiation, either Hawking's original calculation, which tells you that you have information loss, or the quantum extremal surface prescription, which gives you information conservation. The quantum extremo surface prescription has some grounding given the holographic correspondence. It has derivations coming from the holographic correspondence which is very well motivated, but nevertheless to conjecture and in particular, but 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 different occult to put on a same footing Hawking's calculation and the quantum extremal surface prescription and 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 pourt 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, but I should also say that's a very active area of investigation. So given how quickly things 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 one regime into this other 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 extremal surface calculation. And I should preface this by saying that this is a wild speculation that is not at all currently backed by anything that I have derived or done, which is that there's a very beautiful paper by Harlow and Hayden in which they discuss how complicated it is to decode the Hawking radiation, and that the assumption that hawking radiation does sort of purify itself so that the end the information is conserved in the process, then it's still that I would say, exponentially difficult in terms of computational complexity to decode the other Hawking radiation to understand exactly to see unitarities exponentially complicated. We might speculate that Hawkings calculation in some sense did not include those exponentially complicated operations, this especially complicated information. When you look at Hawkin's calculation, did you see that he missed anything? No, So then there's a question, how would we see this exponential complexity? Where is it hiding? How did Hawking miss it? In 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 corners of the universe. Really appreciate your time.
It was my pleasure.
So that was my fun conversation with Nita Engelhart who tried to help us get a mental picture of what's going on. And it's sort of amazing 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, of someone looking at our black holes evaporating, they like, you know, make oh there's four his banana there.
That's the question, you know, can you decode this information? That was the subject of this fun TV show called devs, you know that basically built on this whole concept that could 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 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 propagate 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 in principle you could decode it. 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 in 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. 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 your real one, then you can peer inside of it and see this must have been what was inside the black hole?
What?
But you would 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, because then that information is now being revealed from the core of the black hole, the real black hole, to the one in your simulation in your quanta 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, 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 Bukatini 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 does this sort of side step that conflict.
It sort of side steps the conflict, right, it doesn't really get to the 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 Supper week, So we can't like really do gravity experiments with tiny particles, so it's pretty rare we get to even like explore this regime experimentally to like see what happens, which is why black holes and 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 big question is that it's a bit disappointing and exhilarating. Like it's disappointing because people thought you needed quantum gravity to answer this question. So now answering it without the getting the theory of quantum gravity is sort of like, oh h darn. You know, we thought maybe this was like a thread we could pull on to reveal the true theory of quantum gravity. Instead we only sort of like you know, pulled 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.
We got some entangled there.
And also remember 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 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 three D surface around that four D space, And in the three D surface there is no gravity, it'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, h maybe that's cheating. How people feel like, no, that connection is real. It's a mathematical trick, but it's real and it's reflective of 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 physicists into the black hole and let them duke 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 to similar results, and so that's sort of reassuring. But 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 a black hole. All this theoretical calculation is fun and that tells us what questions 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 home.
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, we hope you enjoyed that. Thanks for joining us, see you next time.
Thanks for listening, and remember that. Daniel and Jorge explain the universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce least conserve natural resources and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digesters to turn the methane from maneure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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