Hawking Radiation and the Black Hole Information Paradox

<v Speaker 1>Welcome to the quart Side Quantum Physics Podcast, an exploration
<v Speaker 1>of the fundamental structure of reality, where quantum laws govern matter, energy,
<v Speaker 1>and information. Here, uncertainty is a feature, not a flaw,
<v Speaker 1>and understanding begins at the smallest scales.
<v Speaker 2>Welcome back to the show. Today, we're looking at something, well,
<v Speaker 2>it's not really an object or a place. We're looking
<v Speaker 2>at a battleground.
<v Speaker 3>That's a good way to put it.
<v Speaker 2>And I don't just mean a metaphorical debate in a
<v Speaker 2>university hall. I mean a literal, honest to goodness conflict
<v Speaker 2>in the laws of physics themselves.
<v Speaker 3>The conflict where the universe needs to be arguing with itself. Yeah,
<v Speaker 3>and we're caught in the middle trying to figure out
<v Speaker 3>who's right.
<v Speaker 2>It feels incredibly dramatic because as we've been preparing for
<v Speaker 2>this exploration, it's become clear this is a fifty year
<v Speaker 2>old standoff between the two pillars of modern science.
<v Speaker 3>It really is. And this standoff, this paradox, it doesn't
<v Speaker 3>just bend the rules of reality, it seems to snap
<v Speaker 3>them in half.
<v Speaker 2>And it keeps physicists awake at night. I mean, if
<v Speaker 2>you follow the logic all the way to its conclusion.
<v Speaker 2>It might even mean that space and time, well, they
<v Speaker 2>aren't what we think they are at all.
<v Speaker 3>It truly is the ultimate mystery. We are talking, of course,
<v Speaker 3>about the black hole information paradox.
<v Speaker 2>I have to admit when I first heard that term,
<v Speaker 2>I thought, okay, black holes, they're mysterious, big, dark, scary,
<v Speaker 2>We get it. But this is so much deeper.
<v Speaker 3>It's not just about not knowing what's inside right.
<v Speaker 2>It's about a logical contradiction. It's like a mathematician proving
<v Speaker 2>that two plus two equals five. It threatens to bring
<v Speaker 2>the whole structure down.
<v Speaker 3>The gravity of the situation, if you'll excuse the pun,
<v Speaker 3>is immense. Since nineteen seventy four, this specific problem has
<v Speaker 3>been the engine driving so much of theoretical.
<v Speaker 2>Physics, because it forces general relativity.
<v Speaker 3>Einstein's grand theory of gravity, space and.
<v Speaker 2>Time into a case match with quantum mechanics.
<v Speaker 3>The theory of the very small of particles and forces,
<v Speaker 3>and the terrifying thing. The central problem is that in
<v Speaker 3>this specific scenario they cannot both be right. One of
<v Speaker 3>them has to give.
<v Speaker 2>So that's our mission for this investigation. We are going
<v Speaker 2>to try to figure out what happens to information to
<v Speaker 2>the story of something when it falls into a black hole?
<v Speaker 3>Does it get deleted from the universe?
<v Speaker 2>Does it get stored somewhere? Does it I don't know,
<v Speaker 2>sneak back out in disguise, and.
<v Speaker 3>We're going to see why the answer to that question
<v Speaker 3>could literally determine the ultimate fate of the entire universe.
<v Speaker 2>To get there, we have to trace the journey of
<v Speaker 2>the black hole itself. We'll start with the simple classical view,
<v Speaker 2>the gravity prison.
<v Speaker 3>Then we'll move into the quantum revolution that Stephen Hawking started,
<v Speaker 3>and from there we'll get into the really wild proposed solutions.
<v Speaker 2>And some of these sound like science fiction. We're talking
<v Speaker 2>about things called fuzzballs, firewalls, and even the idea that
<v Speaker 2>the universe is a kind of hologram.
<v Speaker 3>It's where physics gets very, very strange. But the strangeness
<v Speaker 3>is necessary to solve the problem.
<v Speaker 2>I love that physics terms can sound like something from
<v Speaker 2>a fantasy novel. I cast fuzzball. But before we get
<v Speaker 2>to any of that, we have to start at the beginning,
<v Speaker 2>the before picture classical view. Exactly before nineteen seventy four,
<v Speaker 2>before Hawking shook the world, what did we think a
<v Speaker 2>black hole was?
<v Speaker 3>You have to go back to Einstein pure general relativity,
<v Speaker 3>and in that picture, a black hole is well. It's
<v Speaker 3>elegantly simple and absolutely terrifying.
<v Speaker 2>It's a region of space time.
<v Speaker 3>Where gravity is so powerful, so unbelievably intense, that nothing
<v Speaker 3>can escape.
<v Speaker 2>And when you say nothing, you mean nothing, not even light.
<v Speaker 3>And since light is the cosmic speed limit, the absolute
<v Speaker 3>fastest anything can travel.
<v Speaker 2>If late can't get out, then forget it.
<v Speaker 3>Nothing else has a chance, exactly. And the key feature here,
<v Speaker 3>the thing everyone's heard of, is the event.
<v Speaker 2>Horizon, the point of no return.
<v Speaker 3>It is the ultimate point of no return. Think of
<v Speaker 3>it like one way door in space time. You can
<v Speaker 3>pass through it going in, no problem.
<v Speaker 2>But you can't come back out.
<v Speaker 3>You can't even try to come back out. Inside the horizon.
<v Speaker 3>The curvature of space time is so extreme that every
<v Speaker 3>possible path you could take, every direction, every single direction,
<v Speaker 3>whether you fire your rockets or just float, leads to
<v Speaker 3>one place. Yeah, the center, the singularity. The direction out
<v Speaker 3>literally ceases to exist. It's like trying to go south
<v Speaker 3>from the south pole.
<v Speaker 2>So it's the ultimate root motel. You check in, but you.
<v Speaker 3>Never check out. And this idea comes from the schwartz
<v Speaker 3>Child metric. Carl Schwarzschild solved Einstein's equations just months after
<v Speaker 3>they were published back in nineteen fifteen.
<v Speaker 2>An incredible achievement.
<v Speaker 3>It really was. And he showed that for any amount
<v Speaker 3>of mass, there's a critical size a radius where if
<v Speaker 3>you crush that mass down into that volume, gravity just wins.
<v Speaker 3>It overwhelms everything and collapses.
<v Speaker 2>I always have trouble getting my head around the densities
<v Speaker 2>we're talking about here. Can you give us a sense
<v Speaker 2>of scale?
<v Speaker 3>Sure, Let's take the Sun, a gigantic ball of fusing plasma.
<v Speaker 3>To turn it into a black hole, you'd have to
<v Speaker 3>crush all of its mass down into a sphere with
<v Speaker 3>the radius of just three kilometers.
<v Speaker 2>It's the entire Sun into the size of a small town.
<v Speaker 3>Or even more dramatically, take the Earth, our whole planet,
<v Speaker 3>every mountain, every ocean, all of it. To make the
<v Speaker 3>Earth a black hole, you'd have to crush it down
<v Speaker 3>to the size of a sugar cube.
<v Speaker 2>Sugar cube, that's that's just unimaginable.
<v Speaker 3>It's a density that defies intuition. And the main takeaway
<v Speaker 3>for our story in this classical view is that black
<v Speaker 3>holes are these stable eternal prisons. They are endpoints. They
<v Speaker 3>eat matter, they grow, and then they just sit there forever, so.
<v Speaker 2>A one way street. They only get bigger, they never
<v Speaker 2>get smaller. That seems, I don't know, a little too
<v Speaker 2>neat and tidy, and physics usually hates things that are
<v Speaker 2>too neat.
<v Speaker 3>It absolutely does, because when you find a rule that
<v Speaker 3>seems that absolute, it often means you're not looking at
<v Speaker 3>the whole picture.
<v Speaker 2>And that's where the first crack in the armor appeared.
<v Speaker 2>It didn't come from quantum mechanics initially, it came from thermodynamics.
<v Speaker 3>That's right, the physics of deam engines, of all things.
<v Speaker 2>It feels like such a clash of eras nineteenth century
<v Speaker 2>industrial physics meaning twentieth century cosmic horror.
<v Speaker 3>It is. Specifically, we're talking about one of the most
<v Speaker 3>sacred laws in all of science, the second law thermo dynamics.
<v Speaker 2>Okay, remind us what that is.
<v Speaker 3>In simple terms, it says that the total entropy of
<v Speaker 3>a closed system can never decrease.
<v Speaker 2>And entropy is basically a measure of disorder.
<v Speaker 3>Yeah, messiness, messiness, disorder, or even hidden information. That's why
<v Speaker 3>a broken egg doesn't spontaneously reassemble itself. The universe tends
<v Speaker 3>toward more disorder, not less.
<v Speaker 2>Okay, so things get messier over time.
<v Speaker 3>Got it?
<v Speaker 2>How does that connect to black holes?
<v Speaker 3>Well, here's the problem. Imagine you have something with very
<v Speaker 3>high entropy, like a burning library. All the ordered information
<v Speaker 3>in the books is being turned into disordered smoke.
<v Speaker 2>And ash, maximum messiness exactly.
<v Speaker 3>Now, what if you take that entire burning library and
<v Speaker 3>you just dump it into a black hole? Okay, according
<v Speaker 3>to classical general relativity, that mess is just gone. It's
<v Speaker 3>been removed from our observable universe. The black hole itself,
<v Speaker 3>in this classical picture is perfectly smooth and simple, has
<v Speaker 3>no hair, as they say, it only has mass, spin,
<v Speaker 3>and charge, no other features.
<v Speaker 2>So if the mass disappears from the universe and the
<v Speaker 2>black hole doesn't get any mess here on the outside,
<v Speaker 2>did I just violate the second law? Did I just
<v Speaker 2>clean up the universe's room?
<v Speaker 3>That was the terrifying question. It looked like a perfect
<v Speaker 3>cosmic crime, a way to delete entropy and break one
<v Speaker 3>of the most fundamental rules of nature.
<v Speaker 2>And nature does not like its rules being broken, not.
<v Speaker 3>At all, And for a long time, physicists kind of
<v Speaker 3>swept this under the rug they figured, well, gravity a
<v Speaker 3>special maybe the rules just don't apply here.
<v Speaker 2>They just ignored it pretty.
<v Speaker 3>Much until the early nineteen seventies, when a graduate student
<v Speaker 3>named Jacob Beckenstein started pushing the issue. He argued that
<v Speaker 3>black holes must have entropy, and if they.
<v Speaker 2>Have entropy, then they must have a temperature, right.
<v Speaker 3>And if they have a temperature, they have to radiate heat.
<v Speaker 2>Which totally contradicts the whole nothing can escape idea.
<v Speaker 3>Precisely Beaekenstein was on the right path, but he didn't
<v Speaker 3>have the mechanism. The person who found the mechanism improved
<v Speaker 3>it with math was Stephen Hawking, and in nineteen seventy
<v Speaker 3>four he dropped a bombshell that changed everything.
<v Speaker 2>This is it the moment that kicks off the whole paradox?
<v Speaker 2>What did Hawking find? What did you do differently?
<v Speaker 3>He did something no one had successfully done before. He
<v Speaker 3>brought the two great theories together. He applied quantum field
<v Speaker 3>theory to the curved space time right at the edge
<v Speaker 3>of the event horizon.
<v Speaker 2>He looked at the quantum world in the most extreme
<v Speaker 2>gravitational field possible, and.
<v Speaker 3>He discovered that black holes are not completely black.
<v Speaker 2>They glow in a.
<v Speaker 3>Very particular, very subtle way. Yes, they emit what we
<v Speaker 3>now call Hawking radiation.
<v Speaker 2>Okay, let's unpack this slowly, because this is the absolute
<v Speaker 2>core of the problem. How does a prison that nothing
<v Speaker 2>can escape from suddenly start spitting things out? Is it leaking?
<v Speaker 3>That's the crucial point. Not leaking from the inside. The
<v Speaker 3>radiation is generated outside the event horizon, right at the edge.
<v Speaker 3>How to understand that, you have to understand the quantum vacuum.
<v Speaker 3>In quantum mechanics, empty space isn't empty at all. It's
<v Speaker 3>a roiling, fizzing sea of energy.
<v Speaker 2>The virtual particles.
<v Speaker 3>Exactly thanks to the uncertainty principle, pairs of particles and
<v Speaker 3>antiparticles are constantly popping into existence out of borrowed energy.
<v Speaker 3>They exist for an infinitesimally small moment.
<v Speaker 2>And then they find each other and annihilate, paying back
<v Speaker 2>the energy loan. Poof.
<v Speaker 3>Right, It's happening everywhere all the time, in this room,
<v Speaker 3>in deep space, everywhere. But normally it's just a background hum.
<v Speaker 2>So what's different At the edge of a black hole?
<v Speaker 3>The gravity is so extreme, the space time is so
<v Speaker 3>warped that something dramatic can happen. Imagine a pair of
<v Speaker 3>these virtual particles pops into existence right on the razor's
<v Speaker 3>edge of the event horizon.
<v Speaker 2>Okay, one is just inside, one is just.
<v Speaker 3>Outside, precisely, and the intense gravity can physically rip them
<v Speaker 3>apart before they have a chance to annihilate.
<v Speaker 2>It separates them permanently, exactly.
<v Speaker 3>The particle that popped into existence inside the horizon is doomed.
<v Speaker 3>It falls for the singularity. But the one on the
<v Speaker 3>outside it's now an orphan. Its partner is gone forever.
<v Speaker 3>It can't annihilate, so it's stuck. It's stuck, and it's real.
<v Speaker 3>It escapes out into the universe, and to a distant
<v Speaker 3>observer like us with a telescope, it looks for all
<v Speaker 3>the world like the black hole just spat out a.
<v Speaker 2>Particle that is just mind bending. The black hole is
<v Speaker 2>conjuring particles out of the vacuum at its edge. Ye.
<v Speaker 2>But physics is all about balance sheets. Nothing comes from nothing.
<v Speaker 2>There's always a cost.
<v Speaker 3>You're absolutely right. Conservation of energy is king. If that
<v Speaker 3>escaping particle carries positive energy away into the universe, which
<v Speaker 3>it must, then that energy has to be paid for,
<v Speaker 3>and it's paid for by the black hole itself.
<v Speaker 2>So the black hole loses mass.
<v Speaker 3>It loses mass. You can think of the particle that
<v Speaker 3>fell in as having a kind of negative energy from
<v Speaker 3>the perspective of the outside universe. When it gets eaten,
<v Speaker 3>its subtracts from the black hole's total mass. Energy. The
<v Speaker 3>books have to balance.
<v Speaker 2>And this was the revolutionary discovery.
<v Speaker 3>This was it. Black holes aren't eternal, they have a lifespan,
<v Speaker 3>they shrink, they evaporate.
<v Speaker 2>That just completely upends the classical picture. And there's a
<v Speaker 2>formula for this, right, the Hawking temperature. It's famous for
<v Speaker 2>being this beautiful mess of letters.
<v Speaker 3>It is. The formula itself is T T A C
<v Speaker 3>three E g KPM.
<v Speaker 2>Okay, don't worry about the numbers, but let's talk about
<v Speaker 2>the letters, because it's like a guest list for the
<v Speaker 2>Party of the century.
<v Speaker 3>In physics, it really is a profound equation. You've got
<v Speaker 3>daybar h bar, which is the symbol of quantum mechanics.
<v Speaker 2>World of the small.
<v Speaker 3>You've got Davoter Newton's constant, which represents gravity. You've got
<v Speaker 3>t bowl order the speed of light from Einstein's relativity
<v Speaker 3>the world, and you've got Keeler ball Er Bill Boltzmann's constant,
<v Speaker 3>which is the cornerstone of thermodynamics.
<v Speaker 2>The world of heat and disorder. It's like the avengers
<v Speaker 2>of physics, all assembled in one equation to describe one
<v Speaker 2>an object.
<v Speaker 3>It's the first real signpost on the road to a
<v Speaker 3>theory of quantum gravity. It proves black holes are where
<v Speaker 3>all these theories are forced to talk to each other.
<v Speaker 2>But there's a weird twist in there. The m for
<v Speaker 2>mass is on the bottom of the fraction.
<v Speaker 3>Yes, it's in the denominator, which means.
<v Speaker 2>The bigger the black hole, the lower the temperature.
<v Speaker 3>Exactly. It's completely counterintuitive. A supermassive black hole at the
<v Speaker 3>center of a galaxy is incredibly frigidly cold, colder than
<v Speaker 3>the background temperature of space. It's barely radiating at all.
<v Speaker 2>But a small one.
<v Speaker 3>A small black hole is scorching hot, and as it radiates,
<v Speaker 3>it loses mass, which makes it even.
<v Speaker 2>Smaller, which makes it even hotter, and.
<v Speaker 3>It radiates even faster. It's a runaway process, so in
<v Speaker 3>its very final moments, a tiny microscopic black hole would
<v Speaker 3>evaporate in a final brilliant flash of high energy radiation. Wow.
<v Speaker 2>So biggers are cold and quiet, small ones are hot
<v Speaker 2>and explosive. That's the opposite of what you'd think it is.
<v Speaker 3>In this discovery, this whole thermoe dynamic picture, it actually
<v Speaker 3>solved that old entropy.
<v Speaker 2>Problem, right the burning library.
<v Speaker 3>Hawking and Beeckenstein show that, yes, black holes have entropy,
<v Speaker 3>and the amount of entropy they have is enormous. So
<v Speaker 3>when you throw your messy library in, the black hole's
<v Speaker 3>own entropy increases by more than enough to account for
<v Speaker 3>what you threw in. The second law is saved.
<v Speaker 2>Okay, phew, so the laws of physics are safe. But
<v Speaker 2>wait a minute. The title of this investigation has the
<v Speaker 2>word paradox in it, So fixing one problem must have
<v Speaker 2>created a much, much worse one.
<v Speaker 3>And that's exactly what happened. In patching the hole in
<v Speaker 3>the wall of thermodynamics, Hawking tore out the entire foundation
<v Speaker 3>of quantum mechanics.
<v Speaker 2>Okay, before we get to that crack in the foundation, though,
<v Speaker 2>you mentioned something important the way the entropy is stored.
<v Speaker 2>You called it the area law.
<v Speaker 3>Yes, this is a massive clue and it's going to
<v Speaker 3>become very important later.
<v Speaker 2>So explain this. Why is it so strange?
<v Speaker 3>Well, think about any normal object, a hard drive of
<v Speaker 3>book your brain. The amount of information you can store
<v Speaker 3>is proportional to its volume.
<v Speaker 2>Right, more space, more stuff. If I buy a bigger
<v Speaker 2>hard drive, I could store more data.
<v Speaker 3>It's a three D storage system, but for a black hole.
<v Speaker 3>Beieckenstein and Hawking found that the entropy the information it's
<v Speaker 3>hiding is not proportional to its three D volume. It's
<v Speaker 3>proportional to the two D surface area of its event horizon.
<v Speaker 2>That's really weird. That's like saying the amount of data
<v Speaker 2>on my hard drive depends on the size of the
<v Speaker 2>sticker on the outside, not how many platters are inside.
<v Speaker 3>That is a perfect analogy. It's as if all the
<v Speaker 3>information of everything that ever fell in stars, planets libraries
<v Speaker 3>isn't stored inside the black hole, but is somehow smeared
<v Speaker 3>out and encoded on its surface, so it's plastered on
<v Speaker 3>the walls precisely. And this was the very first hint
<v Speaker 3>of something called the holographic principle, the idea that a
<v Speaker 3>description of a three D volume of space can be
<v Speaker 3>encoded on a two D boundary.
<v Speaker 2>Okay, let's put a pin in that holography. I feel
<v Speaker 2>like we're definitely coming back to that, but for now,
<v Speaker 2>let's face the crisis. We have evaporation, we have entropy.
<v Speaker 2>Why do we have a paradox?
<v Speaker 3>Okay, this is the heart of it. All boils down
<v Speaker 3>to the fate of the information itself. Yea. And it's
<v Speaker 3>a direct conflict between two concepts unitarity versus.
<v Speaker 2>Thermal radiation unitarity. Okay, that's a big physics word. Break
<v Speaker 2>it down for us. It's basically a core belief in
<v Speaker 2>quantum mechanics, right.
<v Speaker 3>It is the absolute bedrock of the theory. Unitarity means
<v Speaker 3>that information is never ever destroyed in the universe. Never never.
<v Speaker 3>The theory is reversible. If you know the precise state
<v Speaker 3>of a system, now you can run the clock forward
<v Speaker 3>and predict its future, and you can run the clock
<v Speaker 3>backward to perfectly reconstruct its past.
<v Speaker 2>So if I burn a book, in practice, it's gone.
<v Speaker 3>But in principle, in principle, if you could capture every
<v Speaker 3>single particle of ash, every molecule of smoke, every photonic
<v Speaker 3>heat and light, the laws of quantum mechanics, that you
<v Speaker 3>could reverse the process and get the exact book back.
<v Speaker 3>The information isn't deleted, it's just scrambled.
<v Speaker 2>So quantum mechanics has a perfect memory it never forgets exactly.
<v Speaker 3>Now, compare that to Hawking radiation. Hawking's original calculations show
<v Speaker 3>that the radiation coming out is thermal, meaning it's just heat.
<v Speaker 3>It's random, it's a perfect black body spectrum. Its properties
<v Speaker 3>depend only on the mass, charge and spin of the
<v Speaker 3>black hole. It does not depend on what fell in
<v Speaker 3>to create it.
<v Speaker 2>Wait, so you're telling me that if I make a
<v Speaker 2>black hole out of a collapsing star, or I make
<v Speaker 2>an identical black hole out of a giant pile of
<v Speaker 2>encyclopedias or giant ball of rubber ducts, oh in, radiation
<v Speaker 2>that comes out at the end is exactly the same
<v Speaker 2>in all three.
<v Speaker 3>Cases, according to the original calculation. Yes, yes, the radiation
<v Speaker 3>has no memory of what it used to be. It's generic,
<v Speaker 3>featureless heat.
<v Speaker 2>I see the problem, I think I see the collision
<v Speaker 2>course here. Let's make it super concrete with a thought experiment. Okay,
<v Speaker 2>I build a quantum computer. It contains the most complex
<v Speaker 2>information imaginable, the cure for all diseases, the works of Shakespeare,
<v Speaker 2>my personal diary. It's a unique, highly ordered state. I
<v Speaker 2>take this quantum computer and I throw it into a
<v Speaker 2>black hole.
<v Speaker 3>Okay, the black hole gets a tiny bit bigger. Then
<v Speaker 3>over googles of years, it slowly radiates away, gets smaller
<v Speaker 3>and smaller, and then poof.
<v Speaker 2>It's gone, completely gone, and was left behind just.
<v Speaker 3>A uniform, expanding cloud of warm, random Hawking radiation.
<v Speaker 2>And if that radiation is truly random, truly thermal, there
<v Speaker 2>is no way to look at that cloud of heat
<v Speaker 2>and reconstruct my quantum computer.
<v Speaker 3>No way at all. The information, Shakespeare, the cures your
<v Speaker 3>diary has been utterly and completely erased from the universe.
<v Speaker 2>So let's recap the conflict. General relativity says the information
<v Speaker 2>fell past the point of no return. It's inside.
<v Speaker 3>Quantum mechanics says the information must be conserved, it can't
<v Speaker 3>be deleted. That's illegal.
<v Speaker 2>But Hawking radiation says the information was converted into random heat,
<v Speaker 2>and then the black hole disappeared, erasing it forever.
<v Speaker 3>You've just stated the paradox perfectly. We start with what's
<v Speaker 3>called a pure state, the ordered quantum computer, and we
<v Speaker 3>end up with a mixed state, the disordered heat. According
<v Speaker 3>to the rules of quantum mechanics, that transforms is forbidden.
<v Speaker 2>So physics itself is breaking its own laws. And if
<v Speaker 2>information can be destroyed, then then what.
<v Speaker 3>The quantum mechanics, the theory that describes everything from transistors
<v Speaker 3>to stars, is fundamentally wrong. We lose predictability, we lose
<v Speaker 3>the connection between past and future. It's chaos.
<v Speaker 2>Okay, so we have a crisis. The laws of reality
<v Speaker 2>are on trial. Now let's talk about the proposed solutions.
<v Speaker 2>Physicists have spent fifty years trying to find a way
<v Speaker 2>out of this box, and.
<v Speaker 3>There are really only a few possible escape routes. You
<v Speaker 3>either have to modify quantum mechanics or modify general relativity,
<v Speaker 3>or find some clever loophole that lets them both be right.
<v Speaker 2>Let's start with solution A, the original position. What did
<v Speaker 2>Hawking himself think? At first?
<v Speaker 3>Hawking was nothing if not bold. He looked at his
<v Speaker 3>own calculation and took it at face value. He said, yep,
<v Speaker 3>the information is just lost.
<v Speaker 2>He accepted defeat for quantum mechanics.
<v Speaker 3>He did. He argued that gravity is special and that
<v Speaker 3>in the presence of bla black holes, the sacred rule
<v Speaker 3>of unitarity is violated. He famously made a bet with
<v Speaker 3>physicists Kip Thorn and John Preskill. Hawking and Thorn bet
<v Speaker 3>that information was destroyed, that.
<v Speaker 2>Seems almost nihilistic, just saying, oh, well, the universe has
<v Speaker 2>a delete button.
<v Speaker 3>It was a radical stance, but it was consistent with
<v Speaker 3>his math. But over the years this position became extremely
<v Speaker 3>unpopular because if you allow information loss, it creates all
<v Speaker 3>sorts of other paradoxes and problems. You get violations of
<v Speaker 3>energy conservation. Things get very messy.
<v Speaker 2>So physicists didn't like it.
<v Speaker 3>They really didn't, and eventually needed it Hawking in two
<v Speaker 3>thousand and four he publicly announced that he'd been wrong.
<v Speaker 3>He conceded the bet to Priskill and gave him an
<v Speaker 3>encyclopedia of his choice.
<v Speaker 2>And encyclopedia yeah for the preservation of information.
<v Speaker 3>That's clever. Okay. So solution A, information is lost is
<v Speaker 3>off the table. What's solution B.
<v Speaker 2>Solution B is the idea of black hole.
<v Speaker 3>Remnants, remnants like leftovers exactly.
<v Speaker 2>The idea is, maybe the evaporation process, it doesn't go
<v Speaker 2>all the way to zero. Maybe it stops when the
<v Speaker 2>black hole shrinks down to the smallest possible size, the
<v Speaker 2>plank length, which is incredibly tiny.
<v Speaker 3>So it leaves behind this little nugget.
<v Speaker 2>A tiny stable nugget, and this remnant, this microscopic speck
<v Speaker 2>would act as a kind of cosmic vault, holding all
<v Speaker 2>the information of everything that ever fell into the original
<v Speaker 2>black hole.
<v Speaker 3>Wait, hold on. If I threw an entire star with
<v Speaker 3>all of its quadrillions of particles into a black hole
<v Speaker 3>and it evaporates down to a speck smaller than an atom,
<v Speaker 3>how does that tiny speck store the information of an
<v Speaker 3>entire star? That is the big glaring problem. It's sometimes
<v Speaker 3>called the clown car problem. For this to work, this
<v Speaker 3>tiny object would need to have a nearly infinite number
<v Speaker 3>of possible internal states to store all that.
<v Speaker 2>Information, and that causes problems.
<v Speaker 3>It causes huge problems for quantum field theory. Also, if
<v Speaker 3>these remnants are stable and can never disappear, then the
<v Speaker 3>universe should be absolutely filled with them from all the
<v Speaker 3>black holes that have ever evaporated. We've never seen any
<v Speaker 3>sign of them.
<v Speaker 2>So remnants are probably not the answer. Too much information
<v Speaker 2>and too small a package. Okay, what's next on the menu?
<v Speaker 2>Solution C sounds cute. The fuzzball.
<v Speaker 3>The fuzzball paradigm. This one comes out of string theory, of.
<v Speaker 2>Course it does. String theory is answer to everything seems
<v Speaker 2>to be more strings.
<v Speaker 3>Well, in a way. Yes. The fuzzball idea, championed by
<v Speaker 3>physicists Samir Mathur and others, argues that our classical picture
<v Speaker 3>of a black hole is just wrong. They say, when
<v Speaker 3>matter collapses, it doesn't form a singularity and a smooth
<v Speaker 3>empty event horizon.
<v Speaker 2>Wait, no event horizon. That's like the defining feature of
<v Speaker 2>a black.
<v Speaker 3>Hole, not a sharp empty one.
<v Speaker 2>No.
<v Speaker 3>Instead, they propose that the fundamental strings that make up
<v Speaker 3>matter get all tangled up into a giant, messy, vibrating ball,
<v Speaker 3>a fuzzball.
<v Speaker 2>So there's no empty space inside. It's filled with a
<v Speaker 2>ball of cosmic yarn.
<v Speaker 3>Pretty much. The surface of the black hole isn't an
<v Speaker 3>abstract point of no return in empty space. It's a
<v Speaker 3>physical object like the surface of the sun, but made
<v Speaker 3>of tangled strings.
<v Speaker 2>Okay, how does that solve the information paradox?
<v Speaker 3>Well, it solves it beautifully. If you throw your quantum
<v Speaker 3>computer into a fuzzball, it's information doesn't disappear into a singularity.
<v Speaker 3>It gets absorbed into the structure of the fuzzball. The
<v Speaker 3>vibrations of the strings change to encode the new information, so.
<v Speaker 2>The data gets stuck in the fuzz exactly.
<v Speaker 3>Then, when the fuzzball radiates. It's not random thermal radiation
<v Speaker 3>from an empty horizon. It's radiation coming off a complex
<v Speaker 3>vibrating object. The vibrations carry the information away like sound
<v Speaker 3>waves echoing off a drum.
<v Speaker 2>So the radiation is a detailed message from the start.
<v Speaker 3>That's the idea. No horizon, no singularity, no information loss,
<v Speaker 3>no paradox.
<v Speaker 2>That actually sounds surprisingly elegant. What's the catch.
<v Speaker 3>The main catch is that it requires string theory to
<v Speaker 3>be the correct description of reality, and we don't know
<v Speaker 3>if that's true, and it completely rewrites Einstein's picture. It says,
<v Speaker 3>if you were to fall into a black hole, you
<v Speaker 3>wouldn't just peacefully float across an invisible line. You would
<v Speaker 3>hit a physical surface, splat against a wall of strings.
<v Speaker 2>Okay, a very different experience for the astronaut, but it
<v Speaker 2>saves the information. Now, let's move on to the really
<v Speaker 2>scary standing one solution D the firewall.
<v Speaker 3>The firewall. This is a much more recent idea, from
<v Speaker 3>twenty twelve. It's often called the Amps paradox after the
<v Speaker 3>four physicists superposed it. Alm Harry, MAROLV. Pulcinski and Sully.
<v Speaker 2>Sounds like a powerful law firm it was.
<v Speaker 3>A powerful paper. They took a very close look at
<v Speaker 3>quantum entanglement. Remember our particle pair at the horizon, one
<v Speaker 3>falls in, one.
<v Speaker 2>Escapes, and they're entangled linked spooky action at a distance.
<v Speaker 3>Exactly Now, for information to eventually get out of the
<v Speaker 3>black hole, that escaping particle, let's call it particle B,
<v Speaker 3>needs to be entangled with all the other hawking radiation
<v Speaker 3>that came out before it. It needs to join the
<v Speaker 3>collective to reconstruct the message.
<v Speaker 2>Okay, so B needs to be linked to the cloud
<v Speaker 2>of earlier radiation.
<v Speaker 3>But here's the problem. Particle B is already maximally entangled
<v Speaker 3>with its original partner, particle A, the one that fell
<v Speaker 3>inside the black hole.
<v Speaker 2>Can it be entangled with both?
<v Speaker 3>Quantum mechanics says no. There's a fundamental rule called the
<v Speaker 3>monogamy of entanglement. A quantum system can only be fully
<v Speaker 3>maximally entangled with one other system at a time. It
<v Speaker 3>can't have two perfect partners.
<v Speaker 2>Quantum mechanics believes in monogamy.
<v Speaker 3>Good to know, so amps argue that something has to
<v Speaker 3>give if the information is going to be saved. If
<v Speaker 3>Particle B is going to join the outside radiation cloud,
<v Speaker 3>then it's entanglement with particle a inside must be violently
<v Speaker 3>broken the moment it's created.
<v Speaker 2>And breaking entanglement that's not a gentle process.
<v Speaker 3>No, breaking that quantum link requires a huge amount of energy,
<v Speaker 3>and they calculated that if this happens for every single
<v Speaker 3>particle pair at the horizon, it would create an intensely
<v Speaker 3>energetic barrier right at that spot, a literal wall of fire.
<v Speaker 2>So if I'm the astronaut falling in instead of a
<v Speaker 2>gentle crossing into darkness, you would.
<v Speaker 3>Hit this wall of high energy quanta and be instantly vaporized,
<v Speaker 3>annihilated to the horizon.
<v Speaker 2>That seems unpleasant for me, But why is it a
<v Speaker 2>problem for physics?
<v Speaker 3>It's a huge problem for Albert Einstein. The absolute cornerstone
<v Speaker 3>of general relativity is the equivalence principle. It was what
<v Speaker 3>Einstein called his happiest thought, which says it says that
<v Speaker 3>the experience of being in free fall in a gravitational
<v Speaker 3>field is indistinguishable from floating in empty space. For an
<v Speaker 3>astronaut falling into a black hole, crossing the event horizon
<v Speaker 3>should be completely uneventful. It's not a special place in space.
<v Speaker 3>You shouldn't even notice when you cross it.
<v Speaker 2>Right, It's just a line on a map, not a
<v Speaker 2>brick wall.
<v Speaker 3>But the firewall says no, it is a brick wall,
<v Speaker 3>a wall of death.
<v Speaker 2>So to save quantum mechanics and its rule of monogamy,
<v Speaker 2>the firewall has to completely shatter Einstein's equivalence principle.
<v Speaker 3>It really feels like every solution breaks something else. One
<v Speaker 3>breaks quantum mechanics, another breaks the structure of the black hole.
<v Speaker 3>This one breaks general relativity. It's like trying to solve
<v Speaker 3>a Rubik's cube, where fixing one side messes up all
<v Speaker 3>the others. That's a perfect description of why this has
<v Speaker 3>been such a persistent problem for fifty years. But in
<v Speaker 3>the last couple of decades, a new heavyweight contender has
<v Speaker 3>entered the ring. It's not so much a specific physical picture,
<v Speaker 3>but a powerful mathematical argument.
<v Speaker 2>And this is where we get back to holography. The
<v Speaker 2>idea called ad SCFT, Yes, the.
<v Speaker 3>Ad s CFT correspondence, proposed by one Meldadicina in nineteen
<v Speaker 3>ninety seven. It's one of the most influential and cited
<v Speaker 3>papers in the history of theoretical physics.
<v Speaker 2>Okay, ad SCFT is a lot of letters. I need
<v Speaker 2>you to give me the soup can analogy.
<v Speaker 3>I read about the soup can analogy is perfect. This
<v Speaker 3>was called a duality. Meldocinas showed a mathematical equivalence between
<v Speaker 3>two theories that look completely different.
<v Speaker 2>Okay, what are the two theories.
<v Speaker 3>First, you have the physics inside a specific kind of
<v Speaker 3>toy universe called anti de Sitter space or ADS. This
<v Speaker 3>is the soup inside the can. It's a universe with gravity,
<v Speaker 3>it has black holes. It's a three D bulk space.
<v Speaker 3>Got it.
<v Speaker 2>The soup gravity lives there.
<v Speaker 3>Then you have a totally different theory, a normal field
<v Speaker 3>theory or CFT. This is a quantum theory of particles
<v Speaker 3>and fields with no gravity, and it lives only on
<v Speaker 3>the boundary of that universe. It's the two D label
<v Speaker 3>on the outside of the soup can.
<v Speaker 2>So a three D universe with gravity inside and a
<v Speaker 2>two D quantum universe without gravity on the label.
<v Speaker 3>And here is Meldosine is incredible discovery. He proved that
<v Speaker 3>the physics of the soup is mathematically identical to the
<v Speaker 3>physics on the label. There are two different descriptions of
<v Speaker 3>the exact same system, a dictionary between them, a perfect dictionary.
<v Speaker 3>Every event that happens in the three D gravitational world
<v Speaker 3>of the soup corresponds exactly to some event happening among
<v Speaker 3>the two D quantum particles on the label.
<v Speaker 2>So if a black hole forms inside the soup, can.
<v Speaker 3>That corresponds to something like a hot, swirling fluid of
<v Speaker 3>particles interacting on the two D label. And here's the
<v Speaker 3>genius move go on. The theory on the label, the
<v Speaker 3>CFT is a standard quantum field theory. We know for
<v Speaker 3>a fact that it obeys the rules. It is unitary.
<v Speaker 3>Information is never lost in that theory.
<v Speaker 2>Oh, I see where this is going. If the theory
<v Speaker 2>on the label is unitary and never loses information.
<v Speaker 3>And the label is just a different description of the soup.
<v Speaker 2>Then the soup must be unitary too.
<v Speaker 3>Exactly. It proves mathematically that a theory of quantum gravity
<v Speaker 3>must preserve information even if we don't know the exact
<v Speaker 3>physical mechanism of how the black hole gives the information back.
<v Speaker 3>The duality guarantees that it does.
<v Speaker 2>That feels like I don't know a brilliant cheak code.
<v Speaker 2>It's like saying I don't know how to solve this
<v Speaker 2>impossible three D puzzle, but I found the two D
<v Speaker 2>instruction Manual and it says on the last page, this
<v Speaker 2>puzzle is solvable.
<v Speaker 3>That's a great way to put it. It gave physicists
<v Speaker 3>enormous confidence that information is ultimately safe. Now there's a
<v Speaker 3>big caveat which is our real universe is not an
<v Speaker 3>anti de Sitter space. It's a different shape. So this
<v Speaker 3>is a proof of principle in a toy universe, not
<v Speaker 3>a direct map of our reality. But the principle is
<v Speaker 3>believed to be general.
<v Speaker 2>And this idea has led to real progress.
<v Speaker 1>Right.
<v Speaker 2>It led to the calculation of something called Page curve.
<v Speaker 2>This feels like the most recent breakthrough it is.
<v Speaker 3>This is based on work by Don Page back in
<v Speaker 3>the nineties. He thought about the entanglement entropy of the
<v Speaker 3>Hawking radiation as the black hole evaporates.
<v Speaker 2>Okay, so he's basically tracking how messy or how entangled
<v Speaker 2>the radiation is over time.
<v Speaker 3>Right, And he made a prediction. If information is truly lost,
<v Speaker 3>as Hawking first thought, then the entropy of the radiation
<v Speaker 3>just keeps going up and up and up as more
<v Speaker 3>random particles are emitted, until the black hole is.
<v Speaker 2>Gone, a straight line going up more or less.
<v Speaker 3>But age argued, if information is conserved, something different has
<v Speaker 3>to happen. The entropy should go up for the first
<v Speaker 3>half of black hole's life, but then it has to
<v Speaker 3>hit a peak the page time and start coming back down.
<v Speaker 3>Why would it come down, Because in the second half
<v Speaker 3>of the evaporation, the radiation that's coming out is now
<v Speaker 3>deeply entangled with the radiation that came out in the
<v Speaker 3>first half. It starts to purify the system. It's releasing
<v Speaker 3>the key to the scrambled message. By the time the
<v Speaker 3>black hole is completely gone, the total entropy of all
<v Speaker 3>the radiation has to drop back down to zero because
<v Speaker 3>it has reformed a pure state.
<v Speaker 2>The original information is all there, just unscrambled. And recently
<v Speaker 2>physicists actually managed to calculate this curve.
<v Speaker 3>Yes, this was the huge breakthrough. In twenty nineteen, using
<v Speaker 3>tools from the holographic principle, two different groups of physicists
<v Speaker 3>finally figured out how to do the calculation for an
<v Speaker 3>evaporating black hole, and they reproduce the page curve exactly.
<v Speaker 3>They saw the entropy go up and then come back down.
<v Speaker 2>That feels like the smoking gun. It feels like we
<v Speaker 2>finally have concrete mathematical proof that information gets out.
<v Speaker 3>It's the strongest evidence we have to date. It suggests
<v Speaker 3>that somehow, through subtle correlations and entanglement, the information does escape.
<v Speaker 3>But it still leads us with a nagging question, which
<v Speaker 3>is how and why does the radiation look so random
<v Speaker 3>and thermal? If it's secretly carrying this complex message.
<v Speaker 2>Right, if it contains Shakespeare, why can't we just read it?
<v Speaker 3>And the answer seems to be the difference between something
<v Speaker 3>that is truely random and something that is pseudorandom.
<v Speaker 2>Like a secret code.
<v Speaker 3>Precisely, if I take a long novel and I encrypt
<v Speaker 3>it with a powerful modern encryption key, what I get
<v Speaker 3>out looks like total.
<v Speaker 2>Gibberish, just random noise.
<v Speaker 3>You could run every statistical test you want on it
<v Speaker 3>and it would look perfectly random. But it's not. It
<v Speaker 3>contains all the information of the original novel in a hidden,
<v Speaker 3>correlated form.
<v Speaker 2>And if you have the key.
<v Speaker 3>If you have the key, you could decrypt it and
<v Speaker 3>get the novel back perfectly. The modern consensus is that
<v Speaker 3>Hawking radiation is like that. It's not random heat. It's
<v Speaker 3>the most complexly encrypted message in the universe.
<v Speaker 2>So the black hole is not a shredder it's an
<v Speaker 2>ultimate scrambler.
<v Speaker 3>That is the picture that's emerging. It scrambles the information
<v Speaker 3>so completely that for any practical purpose it's gone. But
<v Speaker 3>in principle, the universe holds onto the key.
<v Speaker 2>What an incredible journey. We've gone from a simple eternal prison,
<v Speaker 2>to a leaky radiator, to a fretter, to a hard drive,
<v Speaker 2>to a ball of fuzz, to a wall of fu
<v Speaker 2>and now to a holographic scrambler.
<v Speaker 3>It's a perfect example of how science pushes at the
<v Speaker 3>boundaries of what we know, and how each new idea
<v Speaker 3>reveals a deeper level of reality.
<v Speaker 2>So let's bring it all home. Let's talk about the
<v Speaker 2>big picture. Why should someone listening right now care about
<v Speaker 2>burning encyclopedias in space? What are the real stakes here?
<v Speaker 3>The stakes are everything. This paradox isn't really about black holes.
<v Speaker 3>Black holes are just the laboratory the universe has given us.
<v Speaker 3>This is the crucible where we are trying to forge
<v Speaker 3>a theory of quantum gravity.
<v Speaker 2>The holy grail of physics, the theory that unites the
<v Speaker 2>very big and the very small.
<v Speaker 3>The theory of everything. Solving this paradox will tell us
<v Speaker 3>something profound about the fundamental nature of space and time themselves.
<v Speaker 2>What do you mean by that? How could it change
<v Speaker 2>our idea of space and time?
<v Speaker 3>Well, this holographic principle we've been talking about, it's a
<v Speaker 3>radical idea and suggests that space time might not be
<v Speaker 3>the fundamental bedrock of reality. It might be.
<v Speaker 2>Emergent, emergent like it's made of something else exactly.
<v Speaker 3>Think about temperature. A single atom doesn't have a temperature.
<v Speaker 3>Temperature is an emergent property of a large collection of
<v Speaker 3>atoms moving around. The idea gaining ground is that space
<v Speaker 3>time itself might be like that. It might emerge from
<v Speaker 3>a deeper network of quantum entanglement.
<v Speaker 2>So space is built out of quantum connections between things.
<v Speaker 3>That's the frontier. The slogan some physicists use is it
<v Speaker 3>from quibit, the idea that the geometry of space time
<v Speaker 3>is determined by the entanglement structure of quantum information. If
<v Speaker 3>you bring the entanglement, the space itself falls apart.
<v Speaker 2>That's a lot to wrap your head around. It makes
<v Speaker 2>reality sound like a giant quantum computer.
<v Speaker 3>In some sense, it might be. And the amazing thing
<v Speaker 3>is this isn't just philosophy anymore. We have observational tools
<v Speaker 3>coming online that could actually test some.
<v Speaker 2>Of this, like the event horizon telescope exactly, which gave
<v Speaker 2>us that first incredible image.
<v Speaker 3>We have gravitational wave detectors that can listen to black
<v Speaker 3>holes merging. And there's even the possibility of finding primordial
<v Speaker 3>black holes.
<v Speaker 2>Tiny ones left over from the big bag.
<v Speaker 3>Yes, and if they exist, some of them should be
<v Speaker 3>finishing their evaporation and exploding right now. If we could
<v Speaker 3>detect one of those final flashes.
<v Speaker 2>You could analyze the radiation directly, we could actually see
<v Speaker 2>if it follows the page curve.
<v Speaker 3>It would be the most profound discovery, imaginable, definitive proof
<v Speaker 3>one way or the other.
<v Speaker 2>It's just a stunning reminder that even with all our knowledge,
<v Speaker 2>we're still just scratching the surface. The universe is so
<v Speaker 2>much stranger than we can imagine.
<v Speaker 3>Absolutely, the paradox isn't a failure of physics. It's a
<v Speaker 3>signpost pointing us toward a deeper, more interconnected reality.
<v Speaker 2>So as we close, here is a final provocative thought
<v Speaker 2>for you to take with you. We talked about holograms.
<v Speaker 2>We talked about how all the information of a three
<v Speaker 2>D black hole might just be stored on its two
<v Speaker 2>D surface. Yes, and you mentioned that some theories, like
<v Speaker 2>at SCFT, suggests that the entire universe might work that way,
<v Speaker 2>a three D soup with a two D label that
<v Speaker 2>describes it all.
<v Speaker 3>It's a leading theoretical possibility. Yes.
<v Speaker 2>So if a black hole is just scrambled information painted
<v Speaker 2>on a horizon, and if the whole universe is a hologram,
<v Speaker 2>then what are we? Are we really three D beings
<v Speaker 2>living in a three D space or are we just
<v Speaker 2>part of the two D information painted on the distant
<v Speaker 2>edge of the cosmos having a very convincing hallucination that
<v Speaker 2>we have depth.
<v Speaker 3>That is a question that pushes right up against the
<v Speaker 3>limits of physics and into philosophy. But the math doesn't
<v Speaker 3>say no, we might just be a.
<v Speaker 2>Projection on that slightly unsettling thought. Thank you for joining
<v Speaker 2>us on this exploration of the impossible, Stay curious, and.
<v Speaker 3>Never stop questioning the nature of your reality.
<v Speaker 2>We'll see you next time.