Nothing escapes a black hole … or does it? In the 1970s, the physicist Stephen Hawking described a subtle process by which black holes can “evaporate,” with some particles evading gravitational oblivion. That phenomenon, now dubbed Hawking radiation, seems at odds with general relativity, and it raises an even weirder question: If particles can escape, do they preserve any information about the matter that was obliterated?

Leonard Susskind, a physicist at Stanford University, found himself at odds with Hawking over the answer. In this episode, co-host Janna Levin speaks with Susskind about the “black hole war” that ensued and the powerful scientific lessons to be drawn from one of the most famous paradoxes in physics.

Listen on Apple Podcasts, Spotify,  TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Transcript

[Theme plays]

JANNA LEVIN: Historically depicted as inescapable voids, black holes have terrorized the popular imagination. Anything and everything that falls into a black hole is lost forever. Or so the story goes, according to Einstein’s general theory of relativity. This defining character of black holes came under scrutiny in the 1970s with a surprising challenge posed by a young and brilliant but ailing British physicist, Stephen Hawking.

Hawking realized that through a remarkable and subtle quantum process, black holes could evaporate, eventually exploding entirely in a burst of radiation. Even in this explosion, nothing can escape. The black hole seemed to take everything it had consumed with it into oblivion, including all quantum information. But where did it all go?

I’m Janna Levin and this is “The Joy of Why,” a podcast from Quanta Magazine where my co-host, Steve Strogatz, and I take turns exploring some of the biggest unanswered questions in math and science today.

[Theme ends]

LEVIN: Few understood the significance of Hawking’s results initially, but one scientist immediately recognized the crisis that would become known as the information loss paradox. He is here with us today, the famed physicist Leonard Susskind — Lenny to anyone who knows him. In today’s episode, Lenny leads us through the Black Hole War as we ask: Is there a quantum escape hatch from black holes? And will we ever know for sure?

Lenny is a professor at Stanford University and the founding director of the Stanford Institute for Theoretical Physics. He’s widely regarded as the father of string theory, has authored a number of phenomenal books, including The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics, and he’s well-known for his research on quantum field theory, quantum statistical mechanics and quantum cosmology.

Lenny, thanks for joining us on “The Joy of Why.”

LENNY SUSSKIND: Hello, Janna. Long time no see.

LEVIN: Long time no see, such a pleasure to have you here.

SUSSKIND: It’s good to see you again.

LEVIN: So, let’s start at the beginning here.

SUSSKIND: The beginning, the beginning. OK, the beginning.

LEVIN: Well, the beginning for us is going to be the 20th century, when black holes were originally discovered. And I would love it if you could take us through the general relativistic understanding of black holes, without the complications of quantum mechanics.

SUSSKIND: All right, well, a black hole is so attractive gravitationally that it just pulls everything in. The original idea of a black hole was due to [Pierre-Simon, Marquis de] Laplace?

LEVIN: Oh, [John] Michell, I think.

SUSSKIND: Michell and Laplace.

LEVIN: OK, we’re skipping the 20th century.

SUSSKIND: Eighteenth century, 18th century.

LEVIN: Yeah, exactly.

SUSSKIND: Anyway, these ancients, a Frenchman and I guess an Englishman, had the idea that if a star was heavy enough, it would be so attractive — not in the sense of I’m attractive or you’re attractive, but in the sense of gravitational attraction — that it would pull everything in and not even light could get out. They actually made a calculation of how heavy it would have to be for a given size. They worked out what is called today the Schwarzschild radius. And they called it a dark star.

LEVIN: And so this introduces this idea of an event horizon.

SUSSKIND: They didn’t quite have that idea. The idea came out of Einstein and the general theory of relativity, that if you had such a heavy object, there would be a surface around it, where anything that’s inside that will fall into the singularity. This surface of last escape, where if you’re inside it, you’re doomed; if you’re outside it, you have a chance. And that’s called the horizon of the black hole.

LEVIN: Is there anything at the horizon?

SUSSKIND: That is the whole question. Somebody observing the black hole from the outside, doing measurements, doing telescopic observations, and also being allowed to lower probes down to the black hole — let’s say on a fishing line; I used to be a fisherman. You lower your worm down to the surface of the black hole. What would happen is, you would see the surface of the black hole, the event horizon, as being very, very hot. The poor worm would be roasted very quickly. So somebody on the outside would reckon that, yes, there is something at the horizon and whatever it is is extremely hot — so hot that, let’s just put it this way, you wouldn’t want to be there.

LEVIN: This is what you would say was Hawking’s result, that it would be hot.

SUSSKIND: Yes, that’s correct, Hawking and to some extent, a predecessor of Hawking, [Jacob] Bekenstein. Hawking’s results were clearer, they were more precise. And Hawking would have agreed completely with me that that worm will get fried at the horizon long before you get to the singularity. On the other hand, if you just cut the string of your fishing line and let the worm fall through the horizon, the story goes that the horizon would be a non-event for the worm. The worm would just fly through, observing nothing special at the horizon. Yes, Hawking would agree with that.

The problem with it is, it violates a principle of physics. The principle of physics is “nothing ever gets completely lost.” You say, well, that’s crazy. If I take a piece of charcoal and I burn up the charcoal, having maybe written a message on the charcoal, you’ve lost the message. But that’s not true. Whatever you wrote on that piece of charcoal is encoded in the smoke and the products of combustion.

On the other hand, Hawking was saying, “Wait a minute, that information falls through the horizon of the black hole. And from everything we know from general relativity about the structure of black holes, it simply can’t get out.” And so the result was, according to Hawking, that information is permanently lost at the horizon of the black hole. Stuff comes out of the black hole. Hawking radiation it’s called. But that Hawking radiation cannot carry any information because that information was from behind the horizon, and nothing can get out.

LEVIN: So at this time, Hawking came down squarely in favor of pure general relativity and the absence of quantum mechanics, claiming absolutely nothing can get out? So the information that fell in, even if the black hole evaporates, it’s like you’re yanking a curtain up, but the stuff is gone, and there’s nothing you can do about it. And he fell down on the side of “information was lost.” But you said, “Wait, there’s no way.” Why was it so important to you to say information cannot be lost? What’s so bad about that?

SUSSKIND: Well, the conservation of information is at the root of some of the most far-reaching principles of physics, in particular the principles of thermodynamics. The second law of thermodynamics, the first law of thermodynamics, the conservation of energy, the principles of statistical mechanics, the properties of radiation — all of that is 100% dependent on a set of principles that include the zeroth principle of physics, that information is conserved. It’s called unitarity in quantum mechanics. And what it says is that, if there are little differences in what you begin with, those little differences will remain afterwards. Hawking was saying, what comes out of the black hole will be absolutely independent of what fell in.

If you follow that line of reasoning and ask what it implies, it implies chaos. Nothing makes sense anymore. I just felt that couldn’t be right.

LEVIN: So here, you’re at this conflict. But you actually don’t know how to resolve the paradox. And so one of the first ideas that you came up with very early on was this concept of complementarity, which is a concept in physics, but you extended it to black holes. Can you tell us about that?

SUSSKIND: It didn’t say anything more or less than, the information comes out encoded extremely subtly in the Hawking radiation, much too hard to ever reconstruct. In classical physics, it was impossible.

It became a question of what is called complexity. Complexity is a genuine concept in physics and in mathematics, and it’s just a measure of how hard it is to carry out a task. If you ask how hard it is to carry out the task of reconstructing what comes out of the black hole, you’ll find out that it is exponentially complex. The number of little operations that you would have to do in order to reconstruct what fell into the black hole was so extraordinarily high that, for all practical purposes, Stephen was right, the information was lost. But in quantum mechanics, it just becomes very, very complex to do it. So, the principle of complementarity was really just saying to Stephen, “You’re wrong.”

LEVIN: How contentious, how heated did this become, this debate?

SUSSKIND: In a personal sense, not at all. Stephen and I were good friends. We remained good friends throughout the war, or what I call the Black Hole War. There were times when my wife and I and whoever he was with at the time would have dinner together. He would challenge me. There was never a period in which it was at all personal. He was 1,000% certain of what he was saying. Of course, I was also 1,000% certain of what I was saying.

LEVIN: If you don’t mind me quoting you, I believe it was in The Black Hole War, you said Stephen was also a very arrogant man. And then you said, “So was I.”

SUSSKIND: All physicists who are ambitious and really want to get somewhere in the subject have to have a certain degree of arrogance, for sure. You have to believe not only that the human brain is smart enough to unravel these incredibly sophisticated ideas. You have to believe that your human brain is smart enough to do that, to be able to figure out quantum mechanics, smart enough to be able to figure out how the universe works. On the other hand, you also have to be very clear about what you don’t know and what you’re very, very far from. So I suppose in that sense, it takes some humility. Humility to know what you don’t know and what you may never know. Yes, Stephen was arrogant. Yes, I was arrogant.

LEVIN: So there wasn’t a proof or a calculation yet, and yet you had these very original creative ideas which led to calculations, like holography.

SUSSKIND: Yes. Incidentally, there was another person who was also very, very much involved. That was Gerard ‘t Hooft, the Nobel Prize winner, one of the great physicists of 20th century. The holographic principle, that was the thing that ‘t Hooft and I put forward. It came from Bekenstein’s calculation of the entropy of a black hole. Entropy is hidden information, encoded in microscopic details that you don’t have access to.

Bekenstein’s entropy said that the amount of information in a black hole is proportional to the area of the horizon. That was radical. Normally, the amount of information encoded in a structure is proportional to the volume of the structure. That led me and ‘t Hooft to the idea that what falls into a black hole never really falls in, but is encoded on the surface of the horizon in a kind of hologram. A hologram is a two-dimensional image of something which is really three-dimensional. So, the idea was that what falls into a black hole never really falls in and is encoded on the surface in the form of a quantum hologram.

The idea of the holographic principle was more general. Every region of space, not just a horizon of a black hole, is encoded. This room, my room has walls, it’s bounded, and the claim of the holographic principle is that everything taking place inside it — like me, for example, or like the picture behind me — all of that is encoded in a holographic description on the boundary of the room, on the walls of the room.

That seemed crazy to a lot of people. I was sure it was right, but mostly the community said, “Those guys have lost their marbles. They used to be good physicists, what is this holographic idea?”

LEVIN: Now, when some of your friends are saying you’re crazy, does that bother you? Do you just press ahead?

SUSSKIND: It doesn’t bother me in a personal way. It’s frustrating. Why can’t they see what I see? On the one hand, it tells me that if it’s right, it’s worth pursuing, because if everybody thinks it’s wrong and it turns out to be right, that’s a big deal.

So, this idea, it kind of languished for a while. Nothing happened until the young physicist Juan Maldacena discovered an extremely precise version of the holographic principle. It had to do with a kind of space called anti-de Sitter space. It’s, roughly speaking, a solution of Einstein’s equations. And what Maldacena discovered is that it was exactly governed by this holographic principle, that the things inside the room, inside what we call a bulk, were exactly described by a quantum field theory on the boundary of the system. That was exactly the holographic principle. So, it was really Maldacena’s construction and Maldacena’s very precise version of the holographic principle that led to its acceptance.

LEVIN: Just to sort of try to say it in simple terms, it’s kind of a universe in a box. And you might talk about a universe in the box as having gravity and black holes and this information loss crisis, but it is exactly equivalent to an entire universe described only on the boundary, which not only has fewer dimensions, but has no gravity, no black holes, and therefore no information loss.

SUSSKIND: It has no gravity, in the theory, on the surface.

LEVIN: On the surface, yes. So, no information loss.

SUSSKIND: Yeah. We talk about the bulk and the boundary. There’s gravity on the bulk, but no gravity on the boundary.

LEVIN: So you have to conclude, if there’s no gravity on the boundary, then there can’t be information loss.

SUSSKIND: Exactly.

LEVIN: The problem goes away. But you still don’t know how exactly to calculate the information coming out. Is that true?

SUSSKIND: Yeah, that’s true. But that’s not terribly surprising. From the outside perspective, the black hole is very hot. It’s doing what this piece of charcoal would do. It’s very hot. It’s evaporating. And there’s no chance that we could reconstruct the smoke, or the products of combustion, that we could reconstruct what the little bit of writing was on the piece of charcoal. Information gets thermalized. It gets scrambled. So badly scrambled that to reconstruct it is complex beyond imagination but in principle possible.

LEVIN: We’ll be right back.

[Break for ad insertion]

LEVIN: Welcome back to “The Joy of Why.”

So in principle, if somebody does fall into the black hole, becomes completely vaporized, you could reconstruct them outside the black hole.

SUSSKIND: From the Hawking radiation.

LEVIN: From the Hawking radiation.

SUSSKIND: Right. But you could ask, how long would it take? How many quantum operations would it take? And the answer is exponentially large in the entropy of the black hole. Now the entropy of an ordinary black hole is very large in itself, I don’t know, 10⁷⁰. So, we’re talking about times to reconstruct it, which are 10 to the 10⁷⁰ years. That’s what quantum mechanics would say. The right statement, that Hawking should have made, is not that it’s impossible, but that it’s extraordinarily complex once you fall through the horizon.

LEVIN: If I’m an astronaut jumping in and you’re far away, you see me encoded, I never make it into the black hole. All of my quantum information is smeared on this hologram. What’s my experience?

SUSSKIND: Your experience is just that you fall right through.

LEVIN: I sailed right through. So, these are two conflicting realities.

SUSSKIND: Are they conflicting? That was the question. I was saying no, they’re not really conflicting. The person on the inside who falls in simply cannot communicate it to the outside.

LEVIN: Well, you kind of disprove the existence of God.

SUSSKIND: No.

LEVIN: You say there’s no omniscient being. There’s nobody who can simultaneously have the perspective of knowing that there’s —

SUSSKIND: There’s no omniscient being who can see both what’s inside and what’s outside. Physics is an operational subject that has to do with what can be seen, what can be measured. So as long as you believe there’s no being that can see what’s inside and what’s outside, there’s no conflict.

LEVIN: Right, there’s no conflict.

SUSSKIND: And, from that point on, it became obvious that Hawking was wrong, that all of the information was stored on the boundary of the system and never would be lost.

LEVIN: Amazing.

SUSSKIND: Yeah, it is amazing.

LEVIN: So he capitulates, doesn’t he?

SUSSKIND: He did.

LEVIN: Didn’t he give, famously, John Preskill an encyclopedia of American baseball because he conceded the bet?

SUSSKIND: Apparently, yes.

LEVIN: Why didn’t he give you something?

SUSSKIND: I would have liked the encyclopedia.

[LEVIN laughs]

SUSSKIND: I don’t know. He did give me dinner and wine, but I would have preferred the encyclopedia of baseball.

LEVIN: I thought it was kind of him as a Brit to give an American gift.

SUSSKIND: You have to understand, Stephen was both a kind person and an extremely witty person. He was funny. The only problem with funny is that he couldn’t express it easily. But, you know, every once in a while, a word would come out of his machine, and everybody would crack up. He was incredibly witty, incredibly kind — and stubborn!

LEVIN: Who remains unconvinced at this stage? Where are we now?

SUSSKIND: I don’t talk to those people, so I don’t know, you see. Well, at least until recently, the older generation of relativists, but even they sort of have given up, I think. Let’s put it this way, I think you judge what people think not by what they say they think, but what they do. How many people are actually working on a theory of information loss? How many young people? There’s a very brilliant young generation of theorists now. None of them. And if they are, they’re ignored.

LEVIN: You’re describing that all of the information that a black hole can possibly contain is distributed on the horizon. It doesn’t require the entire volume. So what would happen in any region of space if you tried to pack a lot more information in the volume?

SUSSKIND: You could never pack in more than what would exist on the surface. So if you think the walls of your room, for example, are tiled with little tiles, enough to encode everything in the interior of the room, if you try to create more information in that room, guess what would happen? It would form a black hole which was bigger than the room. So it’s impossible to put more information in. And so it must be that the room itself is describable proportional to the area of the room, not the volume.

LEVIN: That’s astounding. That says that not just these exotic objects, black holes, are holograms, but the entire world is a hologram.

SUSSKIND: Exactly. As I said, there was more than one person who thought we were a little bit nuts, but that ended.

[LEVIN laughs]

LEVIN: Why don’t we see black holes evaporate if so much tremendous energy is coming out of black holes?

SUSSKIND: Near this horizon, the only particles that can escape are the ones that are moving radially outward in almost the exact radial direction. So it’s as if the black hole was a very hot box, but with a tiny pinhole letting out the radiation. Only this tiny, tiny subset of the photons can get out. So if you ask, how long does it take a single photon to get out of the black hole? The answer, let’s say, for a solar mass black hole, is about 10^-3 seconds. But that’s one photon. How many photons have to get out? 10⁷⁰. So it’s a very, very, very slow process, one photon at a time.

As the black hole shrinks, the process speeds up. The time that it takes a single photon to get out is proportional to what’s called the transit time across the black hole. That’s the time that a light ray would take to cross the horizon of the black hole. As the black hole shrinks, that time gets shorter and shorter. But it’s only at the very, very last minute that it speeds up enough to create what you called an explosion before.

LEVIN: So at the end of the universe, when everything that can has fallen into black holes, they’ll all be exploding. And then what?

SUSSKIND: Well, we don’t have to wait for those black holes to evaporate. As far as we can tell, the universe is described by an exponentially expanding de Sitter space. This just means that everything is going to fly away from everything else. In about a trillion years, the only thing that will be within our ability to detect will be our own galaxy. All the other galaxies will recede. Why doesn’t ours recede? Well, because we’re in the middle of it, and it doesn’t make any sense to say it recedes. But all the other ones will all just disappear out through a cosmic horizon. We’ll be dead alone.

Supposing all of the astronomical information that we have about the world was lost. And now we wait this trillion years, a new collection of brilliant astronomers and physicists arise. They’re going to look out at the world, and they’re going to say, “We are truly alone.” Emptiness out beyond their own galaxy. How would they ever reconstruct the true story, that there were all these galaxies which just flew apart, and they’re out there, but we just have no access to them. So, if you wait a few trillion years, that’ll be what happens. If you wait orders of magnitude longer than that, everything will form black holes and the black holes will evaporate, and there won’t be anything. A little scary.

LEVIN: So another question I have is, if I can redescribe a universe with gravity as a lower-dimensional universe on the boundary without gravity, does that mean gravity is somehow not real? Is gravity not fundamental?

SUSSKIND: That is a good question. But physicists like to use the word “emergent,” that gravity emerges out of some set of rules that are not fully understood. The quantum mechanical description is the surface of the room with no gravity on the surface. What emerges out of the quantum equations is gravitation in the bulk. Now, does that mean that gravity is not real? I wouldn’t have put it that way.

LEVIN: Reality is overestimated.

SUSSKIND: Well, just do your thing.

LEVIN: Talk about it how you have to talk about it.

SUSSKIND: Talk about it the way you have to talk about it in order to make precise description of it. When I was a young physicist, the attitude was, “shut up and calculate.”

I never liked that. I thought you should shut up and understand. But there are limits. We can’t visualize four-dimensional space. I don’t think we can visualize the principles of quantum mechanics. We know how to deal with them. We know how to encode them in mathematics. But we don’t know how to close our eyes and see the world in a quantum kind of way.

So I think what people tend to mean by “real” is that which you can visualize because your species evolved to be able to recognize certain things. Is a four- or a five-dimensional sphere real? No such thing. I close my eyes; all I can see is a three-dimensional sphere. I can visualize it; it must be real. But I think the idea of what’s real and what’s not real, a physicist has to give that up.

LEVIN: Reality is overrated.

SUSSKIND: No, no, no. Realism is overrated.

LEVIN: Do you think that the key to understanding quantum gravity is here on the terrain of black holes?

SUSSKIND: Yes, I think it’s in the domain of horizons. There are other kinds of horizons besides black hole horizons. If there’s a black hole, let’s say over here, then we surround the black hole, the cosmic horizon surrounds us. So it’s a kind of inside-out horizon surrounding us. When things move outward, away from us, because of the expansion of the universe, they eventually fall through this cosmic horizon, or they eventually approach it, and they disappear out of what we can see. So there are still puzzles about these inside-out de Sitter horizons that I think we’re not necessarily near solving yet, which is exciting. I would say the worst thing for somebody like myself would be if all of the problems were solved.

LEVIN: Yes.

SUSSKIND: What do you do then?

LEVIN: It would be very boring.

SUSSKIND: It would be very boring. I used to like to go fishing, used to like to go fly fishing.

LEVIN: We’ll hang that up on your office door, “gone fishing.”

[Theme plays]

LEVIN: We’ve been speaking with renowned theoretical physicist Lenny Susskind. Lenny, so lovely of you to join us. I will come to Stanford next time.

SUSSKIND: Oh, excellent. Excellent. Very good.

LEVIN: Great to talk to you. Such a pleasure.

SUSSKIND: It was very nice to talk to you, Janna.

LEVIN: It’s been too long.

SUSSKIND: Been too long.

[Theme plays]

LEVIN: “The Joy of Why” is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests or other editorial decisions in this podcast or in Quanta Magazine.

“The Joy of Why” is produced by PRX Productions; the production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Morgan Church and Edwin Ochoa provided additional assistance.

From Quanta Magazine, John Rennie and Thomas Lin provided editorial guidance, with support from Matt Carlstrom, Samuel Velasco, Nona Griffin, Arleen Santana and Madison Goldberg.

Our theme music is from APM Music. Julian Lin came up with the podcast name. The episode art is by Peter Greenwood and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and Burt Odom-Reed at the Cornell Broadcast Studios

I’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.