How are white holes different from black holes? What would happen if you were stuck inside of one? Why don’t they appear in the universe? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

Black holes get all the attention. They get all the love. They're on the cover of books and magazines, countless articles, Nobel prizes in their honor. They star in movie after movie. They are the subject of fierce debate.

The holy grail of quantum gravity, the focus of an overly large percentage of episodes of this very podcast. What about their twins, the white holes? Where are they? Both in the universe and in discussions about the universe. In math, in theory, in reality, the white holes, which are just as improbable as black holes, don't appear to exist.

And so nobody cares about them, and they are forced to live in theoretical shadows, hidden from the daylight of inspection and study. To be fair though, black holes do have an epic rags to riches story. I mean, nobody was even looking for them to exist. And in 1915, Einstein published his general theory of relativity. And months later, another scientist just months later, another scientist by the name of Karl Schwarzschild, who was at the time fighting in the trenches of World War I, got a copy of Einstein's paper, read it, understood it, and found something magical.

He found a solution to Einstein's equations which is a big deal because general relativity, Einstein's theory is just a machine. It tells you how to solve problems but it doesn't tell you what those problems are. You have to plug in various scenarios like planets orbiting a star and general relativity tells you what will happen. There's just one problem. General relativity is enormously complicated.

It is a set of 10 equations that are all linked together and are all individually extremely difficult to solve, and then they're all linked together so you have to solve all 10 equations at once. Contrast that with, like, Newton's law of gravitation which is just one simple equation that applies all over. That's it. General relativity is a nightmare. And to get his own results, Einstein had to play a lot of tricks in simplifying assumptions in games.

And he thought that no exact solutions could be found. That you couldn't ever write down the answer, an exact answer for a given scenario. But that's exactly what good old Carl did. He found a series of simplifying assumptions and math tricks to make everything not just a little bit easy, but super duper easy and not just approximately correct, but all the way correct. An exact solution that he could write down with pencil and paper, again, in the trenches of World War 1 for one very specific case, but a very useful case of a spherically symmetric large mass sitting in the middle of a solar system, like, you know, the sun.

Karl sent letters from the trenches of World War one to Einstein who was very surprised, even Einstein didn't realize that this was possible, and he published them in Carl's name. And then shortly thereafter, Carl got a nasty disease in the trenches and promptly and tragically died. Very quickly, people noticed something funky in Carl's solution which was an exact solution that you could write down with pencil and paper that described the behavior of objects in orbit around a very large spherical mass. In those equations, there was a particular distance, a special radius away from the center of that central massive object where the math went all topsy-turvy, roundy, upside downy. People really didn't know what to do with this Schwarzschild radius as it came to be called.

And so they pretended it didn't exist in the fine tradition of physicists everywhere. When you see a problem in your equations, you say, I don't know. We'll deal with we'll deal with it later. And sure, this special distance, the Schwarzschild radius, where everything seemed to go haywire, was very small compared to the size of, say, the sun. So if you're trying to model the solar system, this weird distance, the Schwarzschild radius is just like a mile across, and the sun is a lot larger than than a mile, and then the orbit of the planets is even larger than a mile.

And so you don't really care about it. So you're like, whatever. Sure. It's in the math, but it's not an obligation for it to exist or to be physically important or meaningful. It's just a weird funky thing that happens in the math and we can ignore it.

But people being people got curious and wondered what would happen if matter were to compress below this special radius. Like, what if it did matter? What if we were to take the sun and squeeze it below this Schwarzschild radius? What would happen? Cue decades of furious debate as to whether these black holes, as they would eventually come to be known and named for an infamous prison in Calcutta where supposedly nobody ever escaped from or was let loose from.

Once you entered the black hole of Calcutta, you never left. People debated whether these black holes actually existed in nature. The debates would circle around and around and around. Yes. They appeared in the mathematics of general relativity, but was that math simply pathological or was it pointed to a real entity?

Could matter really collapse and compress below that threshold? Or would some other force of nature come to stop complete catastrophe? And so you can never form these because there would always be something if you try to squeeze the sun down below the special radius. There would always be something to stop you. Or are we just missing something altogether?

And really, the math was telling us something else that we were simply not clever enough to understand. Nowadays, we recognize black holes as real physical objects, not just weird pathologies or ghosts hidden in the difficult math of g r. And why do we believe that? Well, one, there's nothing in the math itself that says it they can't exist. There's nothing in general relativity itself that says, oh, yeah.

Yeah. Yeah. Once you compress matter below this state, don't worry. Everything is just nuts, and it and and it stops itself. Like, there's some weird trick of gravity that prevents it.

Nope. General Relativity says, Yeah, if you compress an object below the Schwarzschild radius, which would eventually become to be known as the event horizon, it just keeps collapsing forever and you get a singularity at the center of it. 2, there was no other law of physics that said we couldn't do this. It's not like the strong force steps is a wait a minute wait a minute wait a minute. No.

Once you get near near the Schwarzschild radius, the strong force becomes important and prevents total collapse. It does for a while. This is how we get neutron stars, but you can overwhelm that. And then lastly, we got curious enough that astronomers started to find evidence for them in the real universe. We were able to predict what these black holes would act like, and then we saw stuff that acted like that.

I mean, black holes are pathological. The singularity is a point of infinite density where general relativity does break down, and we do not understand what happens at the center or exactly what happens at the event horizon of a black hole. So in that sense, they are pathological. They are ghosts. There is an improved understanding of gravity somewhere out there in the mind verse that will tell us what black holes actually are, but so far we have not found that.

And so this is the best we got. They do exist. Whatever these objects are, whether we have a complete description of them or not, they do exist in the universe. And there's nothing in the math, in the theory, in the physics, or other fields of physics, other areas that prevent them from existing. So theoretically, they're allowed.

And you know the saying, if it's not expressly forbidden, then it must happen. And we find actual evidence for them, and we were able to generate plausible mechanisms. Like, it's one thing to say, well, if I were to squeeze the sun down below this special radius, something horrible would happen. It's a different thing to say, well, nature does that automatically. And we discovered a mechanism that when giant stars die, they do compress, and gravity does take over and overwhelm the other forces.

And so we found a pathway for black holes to form in the universe as weird and troubling as they are. But also in the 19 sixties, right around the time that we were realizing that black holes just might be real, some theorists discovered that they might not be alone, that they might have mirror twins with the exact opposite properties like a a minus black hole, like a negative black hole, an opposite day black hole, a white hole. Now, to understand what a white hole is, or rather what a white hole might be, let's keep going about black holes because remember they always get all the attention. I want you to imagine that you're standing on one of those moving sidewalks, like at an airport or something. This moving sidewalk is a metaphor for the flow of space time around a black hole.

You see, gravity we're we're used to thinking of gravity as like this this force that emanates from all things and pulls objects in and that's that's a fine view of gravity. That's the Newtonian view of gravity. We can also think of gravity through general relativity where we think of a massive object like the earth or the sun or a black hole as causing a dent or a warp in space time. And you've and when you encounter this large gravity object, you start to fall inwards towards that large dent. That's perfectly valid.

And here's another perfectly valid way to look at it. You can imagine that gravity affects space time by sucking in space time by causing space time itself to move around an object. So if you're say approaching the earth, the gravity of the earth literally pulls on space around you and pulls you towards you because you're you're in space. You're like right there and the space underneath your very feet, just like a moving sidewalk, is getting moved towards the Earth. It's a perfectly valid way.

It's not a common way that we use to think about gravity, but it is a perfectly valid way to do it. So gravity is a moving sidewalk of space time. It's a flowing space time. And the closer you get to a massive object, the faster and faster this moving sidewalk goes. Also, the more massive the object is, the faster the sidewalk goes.

And the problem with a black hole is that eventually this moving sidewalk gets so fast that you can't outrun it. Like, if you wanna escape the Earth, if we talk about the escape velocity of the Earth, that means that is the speed of the moving sidewalk that is moving towards the earth. And if you want to leave the earth, you have to turn around and start running against the moving sidewalk. Next time you're at an airport, go go ahead and try it. It's fun.

See if you can beat the speed of the moving sidewalk. The moving sidewalk is moving in one direction at a certain speed. If you go in the opposite direction at a greater speed, you will overcome that movement. You will escape. That is your escape velocity.

The problem with black holes is that at a certain distance from that black hole, that short shield radius, the speed of that moving sidewalk reaches the speed of light, which is fine because we're talking about motion of space, not motion through space. And so to turn around and leave, you have to run-in the opposite direction faster than the speed of light. You can't, and the moving sidewalk of doom carries you beneath the event horizon and towards the singularity. The end of that moving sidewalk is the singularity. Once you cross the event horizon, you must end up in the singularity.

All moving sidewalks terminate at the singularity, and you can't leave, you can't escape, you you die in a finite amount of time because that moving sidewalk is carrying you towards it. So we can take this image of a moving sidewalk of gravity, and we can use it to imagine the formation of a black hole. We can see the formation and evolution of a black hole play out before our very eyes. Maybe that movie that we're gonna watch is called Event Horizon and where we're going, we don't need eyes. We can imagine a giant star, It dies.

It collapses in on itself. That that moving sidewalk, that flowing space time gets faster and faster and faster as this material compresses and compresses. The moving sidewalk gets faster and then it compresses below the short shield radius and that moving sidewalk starts traveling at the speed of light at that radius, you have now formed a black hole. You have created a singularity surrounded by an event horizon. Other bits of matter occasionally wander too close.

They get caught by the moving sidewalk. If they're outside the event horizon, they can turn around and run really, really fast and escape, but at that event horizon, at that Schwarzschild radius, the moving sidewalk is too fast, they're caught, and they will be plunged towards the singularity. This is all neat and tidy and orderly and governed by the mathematics of general relativity. Yeah. It breaks down at the singularity, but everywhere except the singularity we we we got it.

We have this movie, this picture in our head of the formation and evolution of a black hole. But here's the thing, general relativity, those those complicated equations, they don't care about time. The math works whether you're playing the movie forwards or backwards. General relativity, in fact, and in fact gravity itself is time symmetric. For example, let's say I take a movie of me holding a ball, tossing it up in the air, and then catching it, and I just that's all I record, that motion.

Boom. Boom. Boom. And I play that movie for you. Can you tell if that movie is being played forwards or backwards?

Can you tell? The the answer is no. You can't. I raise my hand up, the ball leaves my hand, the ball goes up, reaches an apex, comes down, returns to my hand, and then I lower it back to its starting position. You cannot tell if that movie is being played forwards or backwards in time.

Gravity is symmetric in time. The equations that describe gravity from Newton to Einstein don't care, don't know, they are blind to the evolution of time. That movie I just described to you of a star collapsing and forming a black hole, and the the the moving sidewalk of doom ramping up to the speed of light and then trapping objects. That's one particular direction. I can play that in reverse and general relativity doesn't care.

You say, well, what if the reverse were to happen? General relativity I says, fine by me, I guess, or whatever. General relativity doesn't care. A scenario makes just as much sense in general relativity moving forwards in time as it does backwards in time. So what if we took our movie of the formation of a black hole and ran it in reverse?

What would we see? Well, it wouldn't be event horizon. It would be no zero to neve. We would play that movie. We would see particles streaming out of the event horizon.

The black hole wouldn't be black. It would be blazing white with radiation. And then this object would explode in a fury, and a star would emerge from it. It wouldn't be a black hole. It would be a white hole.

This is how we get the idea of a white hole. They are the time reversal mirror image of a black hole. And and these kinds of symmetries happen in physics, like, all the time. Like, if I take that movie of a formation of a black hole, and I take its mirror image, like, I just hold a mirror up to it, and instead of looking at the real thing, I look at the reflection. The laws of the equations of general relativity hold just the same, except instead of swirling left, now it's swirling right.

Who cares? Einstein doesn't. There's a symmetry there that relativity doesn't care about. There is also a time reversal symmetry that relativity doesn't care about. This mirror image of particles streaming out of an event horizon and then the object exploding and a star forming are perfectly allowed by general relativity.

It's true. All the math runs just fine. No kinks, no jams, no weirdness. I mean, there's still a singularity, but there's not much we can do about that. It just works.

We're just flipping the switch on the moving sidewalk. It's just going in the opposite direction. There's a big lever here, and time forward direction is one way on the moving sidewalk when we flip the lever to the other side. And now the moving sidewalk is going in the opposite direction. Who cares?

If you thought black holes were weird, then you're not even prepared for white holes. We've got a moving sidewalk in reverse. There's still a singularity in the center. There's still an event horizon surrounding it, but everything's opposite. With a normal black hole, you can enter it from the outside, but once you enter into a black hole, you're trapped inside of it forever, and you will be doomed to strike the singularity.

Once you cross the event horizon, you cannot leave. A white hole is the opposite. If you are inside the white hole, you will be ejected from it faster than the speed of light. The moving sidewalk is now flowing out of the white hole, not in. It's moving everything away.

So anything that was trapped inside of the white hole will now be ejected, and that event horizon marks, again, the time or the place where this the speed of ejection slows down to the speed of light, not up to the speed of light, down to the speed of light. Particles stream away from the white hole. With a regular black hole, once you're inside, you can't escape. With a white hole, once you're outside, you can't get in. It's not the black hole of Calcutta.

It's like the exclusive club bed of Calcutta. You can never get in. With a black hole, to escape would require infinite speed, infinite energy, which is impossible. To enter a white hole requires infinite speed because now the moving sidewalk is moving away from the white hole, and you're trying to run towards it. And to reach the event horizon of a white hole, you have to travel at the speed of light in order to overcome the speed of the moving sidewalk of gravity.

And you can't, so you don't. The math of general relativity tells us that white holes can exist. It's fine. Nothing in Einstein's math says the oh, no no no. Actually when you do this time reversal symmetry there's this breakdown here.

No. General relativity is silent. Says, yeah. That's fine. Just as fine as black holes.

Just as weird. And just mirror images of each other in time. Just like a mirror image in space is fine. Okay. Your black hole rotates left instead of right.

Who cares? Okay. Your black hole now evolves backwards in time instead of forwards, and this is what it looks like. It looks like a white hole. It is now our job as physicists to ask, is this real or just a ghost?

We've encountered in physics these kinds of situations before where we get some sort of symmetry in the math, and we don't know if that symmetry, holds up. For an example, if I want to calculate the trajectory of a thrown baseball if I throw a baseball at you, and I wanna calculate the the path it takes to you, I actually get 2 solutions. I get the familiar solution where it arcs up, reaches an apex, and then falls down into your hands. I also get an opposite solution, a mirror image solution, where the the ball goes down and reaches a low point and then curves up into your hand. That is allowed by the equations of gravity.

Both Newton and Einstein give the same result because it's just a baseball. Both of those solutions are allowed. But only one exists. The other is a ghost. On the other hand, we've seen examples like if I have the mirror image of me throwing a baseball at you, like you throwing the baseball back at me, that's allowed.

Or if I look at the situation in the mirror, well, that mirror one is allowed. Particle physics interactions obey time reversal symmetry too. In fact, almost all laws of physics obey time reversal symmetry. If I have some particle interaction where it say 2 electrons collide or sorry. An electron and a positron collide and create a bit of light, a photon.

If I run that in reverse, the time reversal of that is a photon splitting into a positron and electron. And guess what? Both of those happen. Both the original interaction and its time reverse symmetry interaction happen. They both happen.

So sometimes these symmetries give us ghosts in situations that don't happen and sometimes they do. So that's that's not very helpful. To solve this, we're going to have to appeal to some authority outside of Einstein. We have to go beyond general relativity to see if this is allowed. We did the same thing with black holes.

We said, okay. General relativity allows this, but, do the other we gotta check-in with the other laws of physics to see if they they're cool with it too. So white holes. Einstein says it's great. We gotta check-in.

And who, who might you ask, who would be more powerful, more wise, more authoritative than Albert Einstein himself and his general theory of relativity? Who? God? No. Entropy.

General relativity doesn't care about time. You can run the movie backwards or forwards, black holes or white holes, it's all the same. But g r, general relativity, is not the only law of physics that we have access to in this universe. And those other laws of physics have a lot to say about the nature of time and the correct flow of time. And thermodynamics tells us that entropy, which is very very roughly a measure of disorder in a system, must go up with time.

One direction only. For an example, I can toss a cat No. Wait. That's way too grim. I can toss a piano into a wood chip.

If you wanna save the cat, go to patreon.com/pmsutter. Contribute to the show. You get early access to the episodes. You get ad free versions, platform where we can chat with each other, and I will not throw a cat into a witch. I'm not gonna do that anyway.

But either way, go to patreon.com/pmsutter. I I truly do appreciate all of your contributions. I throw a piano into a wood chipper. The wood chipper does its job. The piano is now in a 1000000000 pieces.

There's more disorder. The god of entropy has been satisfied. And the reason you can't just run this into in reverse, you can't shove a bunch of wood and string and lacquer into a reverse wood chipper and out pops a fully functioning piano is because that would lower entropy. That would decrease disorder, which is a big no no. If you're wondering how anything orderly, like actual pianos or human beings can exist, it's because there's an entropy cost somewhere else.

In this case, in the case of the Earth, it's the sun. Any amount of extra order that appears on the Earth comes at even more disorder inside of the sun. No matter what, someone pays the piper. But we can't just throw a bunch of stuff into a woodchipper and have a piano pop out. We can't just have stars magically appearing in poofs of explosions when white holes explode.

You can't just run a black hole in reverse and have all the particles flying out, and then out comes a fully functioning star. It's the same way you can have the earth crash into the sun and disintegrate into a bajillion particles and have a really cool explosion, but a random explosion from the sun won't create a fully formed Earth. Sorry Einstein, but the entropy police have put a stop to this nonsense. It's not general relativity that tells us that white holes can't exist. It's entropy.

It's thermodynamics. This time reversal story of the formation of a black hole, and you run it in reverse and you get the idea of a white hole. Thermodynamics comes in and says, no. No. No.

No. No. No. No. No.

Don't. Just stop. But that's not the end of the story. There is one one way around this. We form black holes through the deaths of stars.

That's how we know that they exist. That's their pathway. And you can't make a white hole through the death of a star because you can't run this process in reverse, but the universe has plenty of other tricks up its sleeve for building stuff. There's this idea called an eternal black hole. An eternal black hole is a particular solution again, a solution of the equations of general relativity, which doesn't tell you by itself what exists or doesn't exist in the universe.

It just tells you how to solve problems. An eternal black hole is a black hole and a white hole that have always been linked together like conjoined twins at birth. Well, how would you get an eternal black hole? You couldn't get it from astrophysics. Collapsing stars don't create white holes.

It violates entropy because, you know, they're the universe is just a bunch of wood chippers. You can't get spontaneous order like that. The only way to get an eternal black hole, a black hole and a white hole linked together, is to have them baked into space time from the very beginning of the universe, just kinda there, always a part of reality. I know it sounds crazy, but there's a lot we don't understand about the early universe. And the early universe is perfectly capable of doing all sorts of weird things, so we have to let the possibility stay open.

It could be that these eternal black holes, a black hole and a white hole bound together since the beginning of time essentially have not formed through some physical process, but are baked into space time itself through something. Even that unfortunately is not an especially tenable situation. And the problem is that this setup is fantastically unstable. In order for an eternal black hole to work, this conjoined twin idea to work, you need a totally empty universe. No matter, no radiation, no nothing except the white hole black hole pair.

Why? Well, let's say there is a white hole that has existed since the beginning of time, and it's connected to a black hole. Or let's say you finally cook up in the laboratory some very carefully placed arrangement of matter in order to create a white hole. Let's say you just make a white hole. Whatever.

You you ignore everything I just said about entropy and you do it anyway, brat. Unfortunately, the white hole is not going to last long in this universe because any speck of dust, any cosmic ray, any bit of radiation that happens to wander near the white hole will be gravitationally attracted to it. It still has mass. It still pulls with gravity, but there's this conveyor belt. There's this there's this moving sidewalk that is flowing outwards.

These compete with each other. The gravity of the white hole it still has mass. So it still gravitationally attracts objects. If you're a little speck of dust, you start wandering. You start getting closer and closer to that event horizon.

And the closer you get, the stronger the gravity of the white hole becomes, but you can't reach the event horizon because the moving sidewalk underneath you is trying to pull you away. What happens is that as soon as one particle, one particle, one bit of radiation, one anything, as it approaches the white hole, it tries to cross the event horizon of the white hole, and it can't. These competing forces, the flowing out of space time and the gravitational attraction, the flowing in of space time, these meet. Now it wasn't perfectly accurate with my analogy earlier. It's not just a moving sidewalk coming out of the white hole.

There's also a moving sidewalk coming in towards the white hole because gravity is still there, and they meet, and they are fighting with each other, and they meet at the event horizon. And when that happens, one particle the energies go up, they start approaching a fin infinity, and you end up with so much energy surrounding the white hole. And energy is mass, and it triggers the formation of a black hole. As soon as you create a white hole and one little speck of dust approaches that white hole, it collapses to form a black hole because the energies go to infinity at the event horizon, which is, to be perfectly clear, super awesome to think about, but definitely not, like, supporting the existence of white holes. As far as we can tell, white holes are ghosts.

They appear in the math of g r, but not in the universe. Unlike black holes, we have no plausible way to form them. They contradict the knowledge we have from other kinds of physics. And if even if we did somehow make a white hole, we would have no way of keeping it stable, as in existing. This is the complete opposite of black holes.

We do have a plausible way to form them. There is no law of physics that prevents their formation, and once they exist, they they tend to hang around for a really long time. But let me end end this episode with a caveat, and it's a big one. We don't fully understand black holes. We do not fully understand what's happening in this in the singularity.

We know that general relativity breaks down at the center of a black hole, and we don't know for sure the fate of any matter that falls below the event horizon. We simply don't know. Perhaps that matter funnels into a different portion of the universe because maybe white holes can exist in with an improved understanding of gravity. We would like to have a better understanding of gravity. We don't.

We know it breaks down at the singularity. And maybe once we introduce that new version of gravity, there'll be, like, an an a a way to get around all this entropy's business and instability business. Maybe the matter that falls into a black hole does get funneled into a portion of the universe or to an entirely new universe. And the most radical ideas, what we perceive to be our universe is actually a white hole. Think about it.

Like, white holes. If you're inside the event horizon, everything is always flying outwards away from the singularity, and that's like the universe. Right? Everything is flying away from the big bang singularity. All galaxies are away from each other.

There's an event horizon, a cosmological event horizon that's really far away. Kind of looks like a white hole. Right? 2 small problems with that. 1, the singularity of a white hole is a point in space, and the big bang singularity is a point in time, so that doesn't quite match up.

And 2, white holes. By definition by definition, in order to construct a white hole, you need to have an outside. It's a part of their mathematical structure. In order to create a white hole, there must be an inside part and an outside part. That is how they are constructed in general relativity.

And the universe, doesn't have an outside. The universe is just the universe. So the universe can't be a white hole as at least a white hole as we understand it in general relativity because those white holes have an inside part and an outside part, and they're separated by the event horizon. But the universe just has the universe. There is no such thing as the outside.

And I know you're saying, Paul, Paul, Paul, Paul, Paul, that sounds like such a technicality. Oh, my sweet summer child, this is physics. It's all technicalities. Thank you to Charles r for the question that led to today's episode, and thank you to my top Patreon contributors. Everyone contributes to Patreon.

It's it's so amazing. It's patreon.com/pmsutter. But special shout outs today to Justin g, Chris l, Barbara k, Duncan m, Corey d, Justin z, Nalia, Scott m, Rob h, Justin Lewis m, John w, Alexis, Gilbert m, Joshua, John s, Thomas d, Simon g, Aaron j, Jessica k, and Valerie h. Thank you so much, and to everyone else. Please keep those questions coming, hashtag askaspaceman, send them to askaspaceman@gmail.com, or visit the website askaspaceman.com.

Just ask a spaceman in general. That's that's the idea. And I will see you next time for more complete knowledge of time and space.

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