Daniel and Jorge Explain the UniverseDaniel and Katie warp their minds around the question of what gravitational information really is and what we know about a black hole.
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Hey Katie, have you ever been to Las Vegas? I have once, and so what did you think about America's favorite adult playground?
Well, I can't really tell you.
Is that because it was so much fun you remember nothing?
I mean, it is a bit of an information black hole.
You mean like you don't want to talk about it. Like what happens in Vegas stays in Vegas.
It is a singular experience for sure.
All right, we'll leave that past the Katie event arride.
Like they're just handing out margarite?
Is there? What am I supposed to say?
Hi?
I'm Daniel. I'm a particle physicist and a professor at UC Irvin, and I have definitely been to Vegas.
I am Katie Golden, I am made out of particles. I host a podcast about animals, and I mostly liked the food from Las Vegas, not so much the casinos.
I recently went to Vegas with my thirteen year old daughter, which was quite an experience to see it through her eyes. Definitely a place to go at least once.
It is a fascinating city. I regret that I have not been there since they have put the giant orban.
Yeah, the orb is cool. There's definitely a lot of stuff you can do even if you're not eighteen or twenty one plus, though some of the stuff she wanted to do, like drive tanks through the desert and shoot automatic weapons were definitely age restricted.
Yeah. I have a friend who I think shot guns from a helicopter in Vegas. There's a lot of things things you can do.
There's all sorts of crazy stuff you can do in Vegas, stuff that helps you understand who you are and who you aren't. And Welcome to the podcast Daniel and Jorge Explain the Universe, in which we try to explain to you what the universe is and what the universe isn't. We try to pull back the curtain and help you understand what's going on out there in the deepest, darkest regions of.
Space, such as Las Vegas.
There is no Vegas casino yet called the black Hole, but I am assuming one it will eventually open up.
That is a missed opportunity, or like a mini golf course called the black.
Holes that just sucks in all the balls.
Well.
On this podcast, we do try to explain to you black holes and also how everything works on the smallest scales, because we don't just want to describe the universe to you, we want to explain it. We want you to get an understanding, a microscopic picture of how everything around you actually works. We want to dive deep into the nature of reality come out with a bottom up understanding of how the universe actually works. My friend Jorge can't be here today, but I'm very glad to be joined by one of our regular co hosts. Thanks Katie for joining us on this black Hole of an episode.
I am excited because no matter how many times you explain to me what a black hole is, I still feel like I cannot wrap my head around it.
So surely, surely.
This will be the episode where I understand black holes.
Well, I would say there's probably nobody out there who fully deeply understands black holes, because to fully deeply understand something, I think we need a particle level explanation for it. You know, we need to be able to moot up from the tiniest little bits and say, here's what's really happening at the smallest scales when you get sucked into a black hole or when photons try to escape. And the reason I say that nobody understands them is that we do not have a particle level understanding of black holes the way, for example, we can understand and what happens when photons go through glass. They interact with the atoms, the electrons and all that stuff in there, and it bends their path and we have some way to tell a story microscopically if what's happening to those photons and related to what we see actually macroscopically. We can't do that for black holes because we don't understand particles and gravity. We have no theory of quantum gravity that tells us what happens to particles when they feel gravity, and that's what black holes are all about. Gravity. So we don't have this sort of deep down, microscopic understanding of black holes. So probably nobody understands, but that doesn't stop us from asking questions and from trying to grapple with what we have learned about black holes. And on this podcast we encourage everybody to think deeply about the universe and to try to fit these ideas into their heads, to ask themselves questions to see does this make sense to you? Is it possible for you to grapple with what we do and do not know about black holes?
The problem, Daniel is I have a finite number of neurons, a finite sort of number of synaptic pathways in my brain, and it is very difficult for those little guys to understand all we know about black holes and what we don't know about black holes. If we were somehow able to measure everything about a black hole, do you think we could even fully understand it? As humans?
It's a deep question in philosophy, like are we even capable of describing the universe in our minds? Can we understand the universe? And not a question I know the answer to or I think anybody knows the answer to. Amazingly, so far we have been able to do it, to write these little mathematical models, tell ourselves these little stories in our minds, and use them to explain everything that we see and everything we experience, everything our experiments tell us. Is it possible for us to understand them? The smartest human I don't know. I think there probably is a limit to how smart the smartest human is, and so it might be that the universe is just more complex than that, and that no human, regardless of how smart they are, could ever understand it. But you know, we've been making a lot of progress in recent years asking and answering questions about the deepest nature of the universe and about black holes. So I'm kind of bullish on the possibility for some human someday to understand it and that's not just limited to like the Albert Einsteins of the world. I think it's possible for basically everybody out there to get some kind of intuitive grasp or how black holes work. One thing I love about hearing from listeners to the podcast is that it's been teaching them to think like a physicist, you know, like, what does that mean? To think like a physicist? It means to try to understand the world around you, to tell yourself stories, to ask yourself questions, to say, do I understand how this works? To put these models in your head and like turn them around and say, well, if that's true, doesn't it mean this? Or how does this connect with this other thing I do understand? And that's especially important for weird things like black holes. So we get lots of questions from listeners who're doing just that. We're trying to think like a physicist about black holes, to try to incorporate it into the mathematical stories in their minds.
This podcast is Daniel's secret weapon of trying to convert everyone into physicists. We are onto you, Daniel. You are trying to create an army of physicists. There are so many questions that I have about black holes. It's hard for me to even think about how to phrase my confusion about black holes into a question. So just being able to ask a distinct, coherent question about black holes, I think is impressive.
I mean, your question about black holes is like black holes, what's the deal with that?
Yeah? My question is like, huh what?
That's the first step, right, is to be confused, and then the second step is to try to weave together a few bits of information and say, like, what parts specifically of the story don't make sense to me? Which parts do I need to understand better in order to have something in my mind that does work where the understanding does click. Because what I'd love is for people to have that moment where they're like, oh, I get it. This connects with that thing, and now it kind of makes sense to me, even though it's pretty weird. And a lot of people out there are trying to do that, and many of them have run into the same stumbling block. And so today on the podcast, I want to answer a very common question that we get about black holes. Can gravity escape a black hole?
I feel like I'm going crossside just trying to think about this question.
I love this question because it reveals that people are being physicists. They're thinking about what a black hole is, how everything gets sucked into it, how nothing can escape. But they're also thinking, hold on a second, we feel gravity from a black hole. How's that possible? How can the gravity get out out of the black hole? And right there, that's being a physicist, that's saying I was told this, But there's also that how do I make this and that work together? How do I put it together in my head to tell a story that makes sense.
So what you're asking us to do is to listen to what you're saying and then go, hold on, Daniel, you just said this. Are you lying to us?
Everybody's doing their best, nobody's lying to anybody. But in the end, learning is a very personal experience, and an explanation that makes sense to one person doesn't work for somebody else. So that's why in this podcast we often try two or three different analogies or explanations or ways to communicate an idea. And that's what you're here for, Katie, also to make sure that what I'm saying makes sense to you.
My method of learning is definitely food based metaphors, I'm very food motivated.
Is your appetite something like a black hole?
It's something like that. Yeah, one could say I'm a bit of a black hole, but yeah. I mean it's a very interesting question because it's hard for me to think of gravity as like a thing, right, like a thing that can escape something. Gravity to me is like, well, I don't even know exactly what gravity is. I remember from previous podcasts the things that you've taught me about gravity, but it is so hard for me to conceptualize gravity because it feels like it is like a kind of thing woven into like the universe, that is not necessarily like, you know, a physical matter thing, but it has everything to do with physics, so it's it's a very confusing kind of concept. And then you add to that the black hole, which itself is quite confusing, and so this question feels no pun intended, but very heavy.
Exactly well, you put your finger on really the concepts here that are entertangled. You know, what do we know about gravity? How is gravitational information communicated?
Like?
How is it that you are feeling gravity from the sun even though the Sun is super dup far away and not touching you. Does that count as information you're getting from the sun. What kind of stuff is captured by a black hole and what isn't captured by a black hole? Is that information? How does that all work?
You know?
Do you need to pass little particles back and forth in order to feel gravity or what. So one of the reasons I love this question is that it combines all of these fun, interesting, fascinating and difficult questions together and going through it I think is really helpful to clarify for people what a black hole is and what a black hole isn't and how physicists think about it. But before we hear about how physicists think about it, I wanted to hear what everybody else out there was thinking about it. So we have a nice little team of volunteers who answer your questions for the podcast before we dig into them. Helps me understand what people are thinking and helps you calibrate your thoughts against the other listeners. So thanks very much to everybody who participates. If you want to jump in for a future episode, we would love, love love to have your voice on the team. Right to me two questions at Daniel Aandjorge dot com. So think about it for a moment before you hear these answers. Do you think gravity can escape a black hole? Here's what people had to say.
I feel like, since it kind of depends on the mass of an object, it just moves with that object, so it would be dependent on the object being able to escape the black hole.
Well, it really matters what we think of as gravity. If we think it's graviton gravitons, like with the quantum theories, then I guess it escapes. I think, yeah, gravity can escape black holes, otherwise I wouldn't be stucking.
Everything in gravity is a distortion in space time. So I would say that gravity itself cannot escape. Actually it's being created by the black hole itself. But if we are talking about gravitational waves, I believe it can't escape a black hole if it is outside the event horizon. I even think that gravitational waves were discovered by watching collisions of black holes and stuff like that. So my guess is, yes, under the right circumstances, gravitational waves should be able to escape a black hole.
I can't even wrap my head around the question.
I didn't even know those words could go together.
I guess it can escape because isn't that kind of what gravitational waves are.
I love all of the references to gravitational waves because we did an episode on that recently.
We did absolutely an episode on gravitational waves, and that's going to turn out to be crucial in understanding what is gravitational information and what is trapped within the black hole and what is not trapped within the black hole. Absolutely so, gravitational waves are a really helpful way to think about that.
I think like at the root of this question is this seeming paradox, right, the idea that nothing can escape from a black hole, and yet we do know about black holes. We get information about black holes, But how can we receive any information about a black hole if it is something that is all consuming and nothing can escape it?
Boom exactly. Adie, you were officially a physicist because you are putting together those two ideas that you understand and saying, how can these two things be compatible, how can we weave them together into a singular understanding of black holes? And what it's going to take is a little bit of a refinement of the idea of what a black hole is and what a black hole isn't to make this all work together. In your head.
Well, now that you have proclaimed I am a physicist, I am just going to waltz into like the hard drung collider and be like physicists coming through. Let's smash some particles.
Guys.
That's right, you have officially a PhD in podcast physics from the Daniel and Jr.
University worth its weight in gold. So yes, I would love some clarification on what a black hole is. You know, I know it's a big sucky thing. I understand that it has something to do with just an incredible density of matter, and furthermore that gravity is very much a defining characteristic of a black hole.
Then let's make sure we know what it is we're talking about. So a black hole is a region of space where the curvature of space is so powerful that nothing that's inside that region can escape to outside. And that region is defined by this threshold, not really like a barrier, not like a physical wall. There's not got a dotted line somebody draws in space that says beyond this point, if you pass, then you will never escape. You will be trapped within that region forever.
Someone's got to put a sign up, a warning, you know, warning, do not pass this region, you will be spaghettified into a black hole. But I mean when you say space is curved, you know, this is definitely a concept I've been introduced to before, but it's still something that I struggle to understand because when I think of curvature, right, I think of like a physical object that is warped, or like say, I think of fabric and I feel like, you know, I think of pulling down on the fabric and then they're being sort of a curvature and things falling into it, and then that's how I think about gravity. But I believe that is not really exactly the correct understanding of it. Space is not like a piece of fabric that you pull on and things fall into that sort of like dip. But what exactly does curvature mean in terms of space?
Yeah? Good, It's important to think about in terms of curvature because I think a lot of people think about black holes and sort of Antonian way where gravity is a force and the force of gravity is so strong that it's sucking stuff up, right, And that cartoon model of a black hole breaks down very quickly because it can't explain to you, like, well, why does light get trapped by a black hole. Why can't a photon not leave a black hole? Because in the Newtonian picture of gravity, gravity is a force between objects with mass, and photons have no mass, so they should feel no gravity, and so they should be able to escape. In order to have an understanding of what black holes are, you really have to move your thinking of what gravity is from this idea of forces between objects with mass to an apparent force something that actually results from, as you say, the curvature of space itself. So what do we mean by the curvature of space itself? Means that space has a characteristic to it that's invisible, something you can't see when you look at it. You look at some chunk of space and you can't tell whether or not it's curved. But if you shine a laser beam through it, it will either go in a straight line or it will not. It will curve, It will bend this way or will bend that way, because space has this additional weird property that Newton never imagined. And on the smaller scale, what it means is that the relative distances between two points can get changed. So when we say that like space is curved or space is bent. It means that you can take two locations in space, you can effectively make them closer together, or you can make them further apart. And what that does is it changes the path that light will take through that space, because light always takes the shortest path between two points. Now you're changing what the relative distances are between two points, and so you're changing the path that light will take. But I think the most intuitive way to think about it is that like space has this additional bit to it, this characteristic so when a photon is passing through it, basically the space tells the photon where to go. It doesn't just pass through blindly.
It's so interesting because I think the reason it's such a difficult concept is that as humans we deal with physics all the time. But we can observe Newtonian physics. We can observe that type of physics. So it's I think easier to think of particles when even like particles that you can't observe with the naked eye, when they are behaving according to forces, because we can, on the more macro level observe forces. You can see a ball knocking into a ball, you can see something fall down a hole, but with gravity. Even though yes, it's true we can observe gravity, we see it all the time, right because you know, you jump up, you fall down. It's one of the main problems that we have to encounter as a physical being in the world, and yet we can't actually see or kind of experience this like secret framework behind gravity, which is that what you described, like the relative distance between two things like becoming shorter and like the actual like you know, space, like reality sort of warping in a way. It's like so easy to fall into that trap of thinking of it in terms of the type of physics that we see and we observe.
It's confusing precisely because it's invisible. You can't see this curvature of space. You can only see its effect. And the effect of the curvature of space looks in almost every case as if there was a force there. It looks like there is this force we call gravity that's pulling on things, when really things are just following the curvature of space. The shape of space itself is changing the direction in which things move. And if you can't see space doing it, it's like a bunch of stage hands wearing black nudging stuff. Then you imagine that there's a force there, and so that's why we call gravity and the parent force. It's not an actual force the way like electromagnetism is, or the weak force or the strong force. It's just the effect of the invisible curvature of space.
So when I hop down from say a safe height, and land on the ground, I am not really getting sucked into the ground because this is an effect of gravity, right, The gravity of Earth is affecting me, and so I'm not really getting sucked to the ground, but i am like sort of just following where space is curving.
Yeah, that's a great example. Let's walk through it in the sort of Newtonian intuitive picture, and then let's move over to the Einstein space curvature picture. You find that it's a very different story about exactly the same thing. So, in the Newtonian way of thinking, where gravity is a force, you are standing on a chair and you imagine that there's a force of gravity pulling you down, and there's a force of the chair pulling you up. And when you stand on the chair, all forces are balanced, so you're not going anywhere. You jump off the chair and then you fall down because all you have is the force of gravity pulling you down. There's nothing pushing you up until you hit the surface of the Earth right in the surace of the Earth, and it's now pushing back up on you, and so the forces are now balanced and you're not moving anymore. Cool. So that's the Newtonian picture. Einstein says, actually that's all wrong, that jerk. There is no force of gravity, and that when you're falling, when you jump off that chair, what's happening is you're following the curvature of the Earth. You're in free fall. There is no force on you at all. And in fact, if you are carrying a little accelerometer with you, something which can measure whether you're accelerating essentially if there's any force on you. Example of an accelerometer is just like a ball in a box, like is the ball pushed towards one side or the other where like a bowling ball in a truck, will tell you whether you're breaking or accelerating. If you jump off that chair and you're holding an accelerometer, you can do this experiment. You will not measure any acceleration. You will not measure anything because in that case you are not accelerating. Einstein says that you're actually accelerating when you're standing on the chair. That what you're doing there is you're accelerating against the curvature of the Earth. And also when you're standing on the earth, you're accelerating upward. It's against your natural path, which would be following the curvature of gravity towards the center of the Earth.
The chair is essentially blocking where you should go naturally go by following where space is telling you to go.
Exactly, if you jump off the chair, you're in free fall. You're not accelerating at all. You see people who are still on their chairs as accelerating upwards against the natural direction gravity wants to take you.
So, if I'm holding something like a feather, say, and I jump off a chair, the feather goes slower than me. Is the only reason that happens because of wind resistance?
Yeah, in that case, it's the air is accelerating the feather up. It's preventing the feather from following the curvature of space. The air itself is pushing on the feather.
And that's why an ant can survive really high falls, whereas a person could not, even though the ant seems much more structurally delicate than a human because it's so small. The air is basically like this huge you know resistance, this huge force that is, you know, blocking the ant from I guess following the curvature of space.
Yeah, I think there's also something there about the structure of ants when they land, but I'm not a biologist.
They just stick the dismount amazingly. If you zoom in on an ant, it does a little summrsalt lands and then takes a little bow.
So this picture of gravity is this curvature in space rather than a force, gives us a new way to think about a black hole. It's not a very dense mass that has very strong sucking gravity because of all the mass in it. It's a region of space where the curvature is such that there is no path outwards, where every direction is towards the center of the black hole. Anything that's flying along is going to follow the curvature of space, including photons. Photons in Einstein's gravity can be bent because they follow the curvature of space, not because of this Newtonian force between masses. So if hodan that goes past the event horizon is trapped by the structure of space itself, because every path forward now takes it towards the center of the black hole. And that's where this apparent mental paradox comes from that you mentioned earlier. If a black hole traps everything, if everything is encapsulated inside the black hole because of the structure of space, then how is it possible for us to even know about it? How's that gravitational information leaving this trash can of space time.
I do want to hear about sort of this idea of information not being able to escape a black hole, because that just it sounds weird right to like say like information is trapped somewhere, because when we think about information, you know, it's like, wait, so I can't use Google in a black hole? What does that mean? But maybe we should take a quick break before we do that, digest the curvature of the space. See, I need food metaphors for me to learn, and then when we get back, I would love to learn more about what it really means to say that information cannot escape.
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All Right, So, I think I'm starting to get a grasp on this idea of gravity being sort of a curvature of space and not a four. But I do want to hear more about what it means when information can't escape a black hole, because, like, again, when I think of information, I'm like, well, I'm looking on Wikipedia about how ants can stick the dismount. What does it mean when information cannot escape a black hole?
Well, fortunately, if you fall into a black hole, you can still access Wikipedia because information can go into Wikipedia. You can't request specific pages because we can't hear from you, but we can send you information. We could just like randomly send you Wikipedia pages even after you've fallen in.
I would like to get some Netflix and stuff if I'm in a black hole because it seems if I'm still alive, it seems really kind of like a lot of time I'm going to spend in there. I need some entertainment, you know what.
I think maybe nobody does. What everybody should do is prepare. They're like, if I fall into a black hole, what Netflix shows should you bean to me? The way people like you know, prepare A will other life information, Like I'd like to know this about people in my life. I know what to beam to them.
Right, Yeah, exactly, I need to start working on a list. But yeah, so information can't escape a black hole? What is going on there? And what do we mean by information?
Really? Yeah? Great, And here's where we need to zoom back down again to the particle level. We talk about information, Like, imagine that you are inside the black hole and you want to request a Wikipedia page about something. You want to make a soup fla and you want a recipe or something. In order to request that information, you have to send something physical outside of the black hole. The way information works is that it's transmitted physically. The way you are hearing this podcast right now is wiggles in electrons or photons being beamed across the Earth. All information is transmitted as particles or some kind of a wave, and so if particles cannot escape a black hole, then you cannot send information outside the black hole because to send that information would require sending something physical, photon, electron, something outside of the black hole. So that's where this idea comes from, that no information can leave a black hole because no thing can leave a black hole, and you need things to transmit information. Information in the end is physical.
So I cannot order a pizza from a black hole, which seems a little scary.
So get your pizza order in before you fall into the black hole. That's the key. It's just a little bit of planning.
I am writing it in sort of my black hole instruction list, that please send pizza into the black hole. Should I go in there.
I think you should ask for spaghetti, actually, because everything we send in is getting spaghetti fied anyway, so you might as well start with spaghetti.
You are what you eat, especially in a black hole, if you're eating spaghetti.
But let's get specific about what we mean by information, because this is the crux of the issue. This is how we're going to understand later why you can feel gravity from outside a black hole, but you can't order pizza from within the black hole. And what we mean by information is sort of like an update, a change of state, so you're communicating something is different. Like if Katie and I are across the room from each other, she can send me information by like shooting photons at me. Right for example, if she doesn't shoot any photons at me, I can assume like, Okay, Katie's still there. I haven't heard from her in a while. But if I want any new information from Katie, if you want any updates on her situation, she wants to warn me about something, or tell me about the pizza she wants, or change her list of Netflix shows she wants to watch, then she would need to send me some information, some electrons or some photons. So information is sort of like an update. It's like something new that.
You're learning, right, So it kind of makes me think of like, uh, you know the basis behind things like computers. You have to have little on off things, and you know this system inside of a computer like a zero one an on or and off, and that change from off to on is a sort of bit of information that can be expanded into of course the complexity of computer it's the same thing I think with you know, in terms of the brain, where you have either a synapse has fired or it has not. There's a little more complexity because it's it's a large biological process. So you have you know, some states that are somewhat in between, you know. But essentially it's like this on off state. You need to be able to clearly have an off position and an on position and have that reach from one you know, like be able you know, even with like say a neuron, you are shooting neurotransmitters from one end of a neuron to the other end of another neuron. So it's like it's very much like in our day to day life we experience this concept of information exactly.
And so you could also think about information in the sense of like affecting the future. Information is something that but in the future can use to make a decision. Like if I could tell you what the Powerball winning numbers are, you can be a billionaire tomorrow because you would know exactly what numbers to put in. So if I have information, I can send it to somebody and they can use that to make a decision. It can like change the future if I'm sending them information. This is closely connected with another concept we've talked about on the podcast a lot, which is this causal link, right, like causes and effects, how they can be connected, because there's already a limit to how you can affect the future in the universe, Like, for example, I can't change the future in Andromeda tomorrow. Andromeda is so many light years away, and any information I send to Andromeda will not arrive for millions of years, and so nothing I do today can get any information to Andromeda by tomorrow.
So we cannot get a pizza from or to Andromeda anytime soon.
That's right. We can't even tell Andromeda about what kind of pieces they should order, which in principle could travel at the maximum speed of information, which is light speed. And so we have this concept we call a light cone, which tells you where in the universe your information can reach. You can send signals, you can turn on flashing lights, but people outside your light cone will not see it, and so the light cone expands as time goes on. You send a flare. Now that information propagates through the universe and eventually will reach the edges of the universe. Your light cone is expanding in time from every moment, but there is a limitation there, right, there are already parts of the universe that your information cannot reach at an arbitrary time, So there's a limit to the information. You can't send any information to people outside your light cone.
I don't know if this is a pretty rudimentary question, but from Earth, right, we can't see every star in existence? Can we or are we able to see like get light information from every star in existence? From Earth?
We can't because the universe is not old enough.
Right.
The universe is finite age, so there are some stars that are so distant that light from them has not had time to arrive here. Plus you've got a factor in the expansion or the universe. It gets much more complicated. But no, there definitely starts who have sent photons towards us but have not yet arrived.
So it is a matter of timing where because like all the stars that we see right now, like that information is really old, that light information that we're receiving now.
That's exactly right, Yeah, that information is very old, and it's a matter of timing. And if space was simple, space was always flat, if there was no curvature. Photons always traveled in a straight direction and at the speed of light. Then the only limited information would be timing. But the curvature of space changes things. And in curved space, your light cone is not a simple cone. It gets bent, it gets distorted. Near a black hole, or anywhere where space is curved, your light cone gets twisted because it changes where photons go. Right, you send out a flash of light, Now they're going in some directions and not in other directions. They get twisted, they get bent. Once you pass over the event horizon, your light cone is now just pointed towards the center of the singularity. Any flare you set off, all photons are going to end up going towards the singularity. So your light cone is now sort of trapped behind this barrier. And that's what we mean when we say information cannot escape a black hole. We mean that nobody passed that threshold. Can ever send you a photon that arrives to you.
If a photon approaches a black hole but doesn't pass the event horizon, can its light cone still get bent but not fall into the black hole.
Oh yeah. In fact, there's a region near black holes where photons can orbit, they can get trapped and forever loop around a black hole, which is pretty cool, or they can do other weird things like pass around the backside of a black hole and then come back. So you can shoot it like a laser beam just above a black hole and kind of come around the backside and app you in the eyeball from below the black hole.
It's like a laser boomerang.
So the key thing to understand there is that information is about changes of state, how things change, getting updates about what's going on, decisions that are made, new facts that arise. Information is about changes of state.
So with gravity, Now, I thought that we had talked about a previous episode about how with gravity there is a phenomenon known as gravitational waves, which would indicate that gravity can change. Right, You can have changes in gravity. So why would it be considered that gravity is not a form of information.
Yes, great example, So gravitational wave is a great example of gravity changing. Right. We talked about gravity in terms of the curvature of space. So imagine space has some curvature because of the arrangements of masses in it. So space has that curvature, and that curvature is just fixed. It's constant. Right. Now, take one of those masses, the Sun maybe, and wiggle it. You put your finger on it, you shake it back and forth. If you wiggle the Sun, you're changing where the mass is, You're changing how space is bent. So what's happening to there is the curvature is changing. You're moving the Sun to the left, and you're moving it to the right, and you're moving it to the left again. You're moving it to the right, and that's information and that information propagates out at the speed of light. And so if you're far away from the Sun when somebody's wiggling it, you don't feel those changes instantly. It takes time for that information to get to you. So Katie wiggles the Sun. Eight minutes later, I feel the gravity on Earth shaking a little bit. It gets stronger, it gets weaker, it gets stronger, it gets weaker. That's a gravitational wave. That's information moving through the gravitational field. So I think that's sort of a long answer to your question, which is can you send information through gravity? And the answer is yes, And that's what gravitational waves are. There's information sent to the gravitational field. The crucial thing is that that's all about a change in the gravitational field. There's no information being propagated. When the Sun is static, the field is not changing. There's no information there. When you move the Sun, when you've wiggled it, that's sending information.
So Earth has gravity, obviously, but when the Earth moves, you're getting some change in gravity, and so you're getting gravitational waves. And so that change of the Earth moving is sending out information in terms of the change in gravity.
Absolutely, And when the Earth orbits the Sun, because it's effectively moving through space and changing the Earth's gravitational field, we are generating gravitational waves. Our orbit around the Sun generates gravitational waves, and it actually contributes to the decay of Earth's orbit because gravitational waves contain energy, so we're radiating away some of our energy every time we go around the Sun. You just did an episode about this about how stable is the Earth's orbit, and it turns out that's a very very small effect of the stability of the Earth's orbit. But in principle, yes, we are generating gravitational waves which contain information. And if you had a sensitive enough gravitational wave detector, you could pick up those wiggles in space time that are telling you about how the Earth is moving.
So gravity can convey information. It can be considered a form of information if it is a change in the gravity. So with a black hole, why aren't we seeing a change in gravity? Because I would assume if we're not getting gravitational information from a black hole, that would mean that there is not a change in the gravity of a black hole.
Yes, exactly, And so now I think we have all the pieces in place to try to answer this question, which is a black hole does of course have gravity, and you can feel its gravity from a distance. You are a light year from a black hole, you will feel its gravity. But what you can't feel are changes to its gravity. So if there's something going on inside the event horizon, Katie's in there and she's doing a dance, or she's rearranging her living room or whatever, you can't get that kind of information if you change the configuration of the masses inside the black hole. In principle, that generates gravitational waves, but those are trapped within the event horizon. So nothing that's happening within the event horizon is generating any gravitational information that's escaping. All you're feeling is the static field of the black hole, the one that was created when it was formed. So you have a black hole and it has a static gravitational field. Nothing is changing, and anything that happens inside the event horizon is going to generate gravitational waves, but they don't leave the black hole. So you can have gravity outside the black hole from its original formation without knowing what's going on later inside the event horizon, without getting any information from inside the event horizon. The only information you have is the existence of the black hole from when it was created, which is not information from inside the event horizon. Another way to think about it is you create the black hole, you're then freezing that gravitational field. Nothing that happens pas the event horizon can then change the gravitational field outside the black hole. Now that's all if the black hole is not moving and so it has a static, unchanging gravitational field. If you wiggle a black hole or if black holes merge, that motion creates ripples in the gravitational field, just like it does when the Sun or the Earth moves. But that's motion of the event horizon. It's not information from inside the event horizon about what's going on within it. Another way to say it is that from the outside you can see if the black hole is wiggling, but that's not information from inside the event horizon. You can't tell if things inside are wiggling, only if the whole thing is moving.
I mean that's interesting because it's like, I suspect this might be the wrong way to conceptualize it, but in my head, I'm thinking of sort of a Russian nesting doll, where it's like the black hole is the main one, the main sort of gravitational field, and inside you could have like the little tiny doll dancing around doing stuff, but none of that is reaching outside of this nesting doll because like the bigger one is essentially this like static large gravitational field, and nothing is getting outside of it.
Yeah. I think that's a great way to think about it. Another useful thing to think through is how a black hole gets formed, the moment in which a black hole is created. I think that can really help people crystallize how that information is propagated through the universe and why no more information is ever sent out from that object.
All Right, let's take a quick break. As I mentally prepare my mind to be blown, I'll lay out a tarp and then when we get back, please tell me how a black hole is created.
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All right, So I am officially ready for my brain to explode from the information of how a black hole is formed.
So all you have to do to form a black hole is just order enough pizzas. Really, that's all it takes. Heay, So we know that there are black holes out there in the universe and that they are created right there are black holes that have not existed since the beginning of the universe. So there has to be some moment when there wasn't a black hole and when there is a black hole. And I think the thinking through that moment really can help people understand what we're talking about when we say a black hole, what it means for information to be trapped within it. And so there's lots of ways that a black hole can be made in the universe, but let's try to think about like the simplest possible configuration and in general relativity, one very frequently used example is like a sphere of rocks. Imagine you have like tiny little bits of dust or pebbles, and you have a perfectly symmetrical sphere of it. Okay, and now gravity is pulling it, and so it's collapsing. It's going to pull towards itself and get denser and denser and denser. Now, how do you make a black hole. You make a black hole by having enough stuff in a small enough space. It's all about the density of matter. You could take the Earth and squeeze it into a peanut to get a black hole. You could take the Sun and squeeze it into like a three kilometer wide object to get a black hole. You could take a huge blob of stuff and squeeze it. The point is to get high enough density. So now you have this hollow sphere of pebbles, right, everything's like the same distance from the center, and it's collapsing slowly. At some point it's going to be dense enough to pass this threshold and become a black hole. It's going to be a moment before it passes that threshold where it's just like a sphere of stuff that has gravity, and then a moment after passes that threshold, now it's a black hole.
How does the incredible density cause Essentially, what I'm imagining is like pinching the curvature of space such that everything is going towards that point, Like, how does density cause that to happen?
We talked about how gravity is actually just the curvature of space. We didn't talk about how you make that curvature, why bits of space are curved here or not curved there. So the missing piece is density energy density. If you have a chunk of space with a lot of energy in it, be it mass or photons or any kind of energy, then that space gets curved. So the simplest version of general relativity is mass tells space how to bend. Space tell's mass how to move, and so it's that first bit that mass tells space have to bend. The more energy density, usually in the form of mass, that you pile into a little chunk of space, the more curvature you get. When you cross past some threshold, then you have so much curvature that nothing can escape anymore. So that's the moment you become a black hole. So you take this sphere of dust again, it's a hollow sphere. It's not filled in the center. If it's really really big, then you have a lot of empty space. Also, you're including density is very very low, you don't have a black hole. As you collapse it, the radius gets smaller and smaller, all the bits get closer and closer together. Eventually you pass over the short stiled radius the radius of an event horizon for that amount of stuff, and then you have enough density that there's enough curvature that you've got a black hole.
So mass, which we can observe, is sort of tied to this hidden world of gravity which we cannot see. We can feel the effects of, but we cannot see the curvature of space. But that curvature of space has direct connections to all the mass that we can directly observe.
Yeah, that's exactly right. And remember that's not just mass. Right in Newton's physics, it's just mass, But in Einstein's relativity it's energy density. That's why photons and other massless things can actually contribute to the curvature of space. You can actually just make a black hole by overlapping powerful laser beams. Pure photons can make a black hole in theory.
Okay, how do you know that, Daniel, Are you working on that? Are you doing that?
I can neither deny nor confirm those experiments.
That's incredible that just through light, if you cram enough light into a small enough area, not only are you going to like blow out your retinas, you're going to create a black hole.
So now let's go back to our collapsing sphere of pebbles or dust or rocks or whatever. What happens when it creates a black hole? What is changing in space the moment before it creates a black hole? What do you have? What you have gravity? Right, if somebody's going to shoot a photon or a particle near it, it's going to be bending space, and it's going to affect the path of that stuff. Right, You're going to feel the effective force of gravity from this sphere of rocks, right, because it has mass, it's curving space. It's giving you that effect. No big deal, no surprise there. What happens when it crosses that threshold and becomes a black hole? Well, nothing changes from that point of view, right, because the mass hasn't changed, the overall energy hasn't changed. So if you're on the outside, when this sphere becomes a black hole, nothing changes gravitationally, the gravitational field is still the same. And that makes sense, right. It's not like the gravity disappears when the thing becomes a black hole. It changes from a big sphere of dust to a black hole. But if it has the same mass, it has the same gravity as before it became a black hole.
Okay, So you have this amount of gravity that just before the black hole is created, and then once the black hole happens, that gravity is the same. So can you not add any more gravity to a black hole? Like you cannot? Like, say, you put more stuff into a black hole, you put more particles into that black hole. You shoot some lasers there's no more gravity that you can add to that.
That's a great question. Actually really illustrates this point very nicely. Absolutely you can. Right, if you shoot a laser into the black hole, or you throw a more mast in the black hole, you are going to increase its gravity, and that information will propagate outwards. Right, you add to a black hole, you add to its gravity. You can feel more gravity now somewhere else. That information will propagate outwards as a gravitational wave. Is a change in the gravitational field of the black hole. But that's external, right, that's not information coming from within the black hole. That stuff you're adding from the outside. If you have just a black hole and it's collapsed and you're not changing its gravitational field, then what does it mean for information to not be able to escape if you're still feeling its gravity. What that means is that anything that's happening to that stuff that fell into the black hole, you can't tell. Basically, the gravitation field is just frozen from that stuff. If that stuff goes in and does a funny dance, or becomes a singularity, or becomes a quantum fuzzball, or does whatever, you can't ever see, it can't ever tell you anything about it. You can't ever learn anything about the configuration of the mass within the event horizon. The gravitational field of that stuff still exists, but it's now frozen in time. You will get no more updates. If you take that stuff inside the black hole and wiggle it around, generating a bunch of gravitational waves within the event horizon, those waves are trapped within the event horizon. Outside you will still see the same exact gravitational field. And that's why we made the distinction earlier about information being about updates, because that gravitational field can be static. There's no information escaping from within the black hole. It's just sort of like think about the event horizon itself as having that gravity, or the last information you got about this stuff was just before it falls into the black hole. It's that gravitational field you're feeling, no updates, no information means that gravitational field is effectively frozen.
So would it be fair to say that we can get information from the existence of a black hole, and we can get information by shooting information into a black hole and then that sort of changing the characteristics of the black hole. But we cannot get any information from anything that is already inside of the black hole.
Exactly because that would require communicating something physically. Right, if you wanted to send information from within the black hole, you need to like take a bowling ball and wiggle it, which would generate gravitational waves. But I can't feel that from the outside. What I feel is the gravitational field of the stuff the moment it became a black hole. That's what it got frozen in time, although gravitational waves generated inside of it and the updates to that gravitational field are now trapped the same way like if you shoot photons or if you shoot electrons. Right, Remember, information is physical. None of that physical information can leave all right.
But now hear me out. If you had a really long string and you're sitting here on Earth with a dixie cup attached to your end, and I'm all the way in a black hole with that string and a dixie cup on my end, no, I cannot communicate any information to you, even if somehow that string was strong enough to survive the black hole.
Right.
Well, there, you're creating a new paradox, because there is no string strong enough to survive a black hole. Right, because as you zoom in microscopically to that string, it's really just a bunch of particles transmitting information to each other using electrons or using photons, and those photons cannot escape the black hole. But you know, electrons and photons are a great way to think about this information. Also, like say you take an electron, and you just have an electron in space. Electron has an electric field, right, a static field all through space. It gets stronger as you're closer to it, and weaker as you get further away. Now, what happens if you shake that electron. You shake that electron, you're changing the electric field, and that change is a photon. It's physical information. It's changing the field of the electron that is transmitting the information. The static, frozen, constant existence of the field is not information, which is why stationary electrons don't generate photons in the same way that like, the existence of the black hole and its gravitational field is sort of like old information from before it became a black hole, when it was created. Any new information created within it by moving this stuff inside is not going to be transmitted. So information gravitational waves or photons really are about changes in the field, not about the existence of the field itself.
So in my string and cup analogy, of course the string would not survive. But you know, with a hypothetical string, I could be shaken it. I could be talking into the Dixie cup. But as soon as that information, you know, is basically the part of the hypothetical string outside of the event horizon is getting nothing from the string inside from within the event horizon, exactly.
Your string is going to be dangling towards the center of the black hole. All the information you create is going to get funneled towards the center of the black hole.
Wow. Well, I'm going to really emphasize that if I am in a black hole, Pepperoni pizza is good. Also, do a nice carbonara, just keep them common for eternity. I suppose that sounds good.
It's good to get that information now before you fall into the black hole.
Right, could happen any day. We are sponsored by black Hole Insurance. Make sure to set up your affares before you get sucked into a black hole, or should I say, not sucked into a black hole, but follow the curvature of space into a black hole.
So you should think about black holes as Berry to information. But I hope this podcast has helped you understand a little bit more what that means, what information really is, because what's inside a black hole can affect what's outside a black hole. The creation of the black hole itself affects space time near the black hole. But sort of in a simplified way, we can only know a few things about the black hole. We can know it's mass, we can know it's charge, we can know its spin. Anything else that happens within the black hole can't change what's happening on the outside. The changes behind the barrier, wiggles and rearrangements of the stuff within the black hole can't influence things on the outside. So I think the crucial concept there is that information really is about changing the internal arrangements. That's what you're prevented from knowing, not the existence of the black hole, but what's going on inside of it.
So I could be inside of the black hole, drunk off of Martini's standing on the blackjack table and screaming out the lyrics to a Katie Perry song, and no one would ever know.
Only people within the black hole and closer to the center. But hopefully what happens inside a black hole stays inside a black hole.
Yeah, well that's good information to know, because I know now where I'm going to do my next Vegas trip, and it's not in Vegas.
All right. Well, I want everybody out there to keep thinking like a physicist, trying to bring pieces of understanding together and weave them into a deeper, more profound model in your head about how the universe works, how these things all interact, and when things don't jive together, when they don't make sense to you, or when you feel like you have hit upon a contradition right to us, we will help you figure it out. Questions at Danielanjorge dot com. Our goal is to turn everybody into a physicist.
You and your army of physicists. What are you planning?
I can neither confirm nor deny our plan. Thanks very much Katie for joining me on this path towards the center of understanding, following the curvature of explanations.
Thank you for all the information. I'm glad we're not broadcasting from a black hole where nobody would know.
And thanks everybody for listening. Tune in next time.
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You can get it in stores, online, and at Great Big Universe dot net.
Thanks for listening, and remember that Daniel and Jorge explain the universe is a productive oft iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio Apple Apple Podcasts, or wherever you listen to your favorite. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact, but the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as Dairy dot COM's Last Sustainability to learn more.
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