Daniel and Kelly’s Extraordinary Universe - Are black holes hot or not?

Episode Date: August 4, 2022

Daniel and Jorge talk about whether black holes have a temperature and if we can ever know! See omnystudio.com/listener for privacy information.

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Whether you're trying to invest for your future, navigate a toxic workplace, I got you. Listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. Hey, Jorge, I've heard that you are a fan of campfires. Yeah, you know, I'm always up for smores. Well, do you worry about like far future generations of humanity? Space settlers living under domes or folks living on colony ships getting to have that same experience? Yeah, it doesn't sound like a good idea to start a campfire on a spaceship. So how are they going to capture that essential human experience or is it going to be lost forever in the endless depth of time?

I'm sure they can have VR campfires maybe. Only if VR extends to having smells. It might in the future, you know, you plug something into your brain and it gives you the experience of being in front of a campfire smelling fumes. Or you can just fly too close to the sun. Yeah, the sun is sort of like a campfire, right? It's burning and you can toast marshmallows with it if you get close enough. Maybe that's the wave of the future. Plasma toasted marshmallow smores.

Yeah, and you can do your stargazing right there looking at the flames. Seeing stars up close. Yeah, just don't get too close. We'll have plasma toasted podcast listeners. Hi, I'm Jorge. I'm a cartoonist and the co-author of Freakantly Asked Questions about the universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine. And I've just realized that neither of us on this podcast speak English as their first language. And you can probably tell, I'm sure, on every episode. We speak science instead.

That's right. Science was our first language. I mean, we were writing papers, I'm sure, out of our cribs, right? Yeah, my first words were F equals M.A., question mark. My first words were P-value of 05. Nature paper. Those were my first words. But anyways, welcome to our podcast, Daniel and Jorge,

Explain the Universe, a production of I-Hard Radio. in which we try to translate the incredible mysteries of the cosmos into your native language we think it might just be possible to decode the incredible frothing quantum insanity of our universe into a language that humans can understand and we can build some sort of mental model in our heads that describes the incredible vastness and mysteries of the universe that we can pour all of the incredible knowledge and ignorance of humanity into your brain through your ears That's right. I think of this podcast as the duolingo of physics and science, translating all those amazing discoveries and things we've learned about the universe into, you know, bad jokes and short sound bites. Are we going to start assigning homework? I think duolingo has homework. Oh, does it? I think the whole point is that it doesn't have homework. Oh, I thought I had exercises. People are like doing their duolingo challenges and stuff. Oh, yeah, it has like challenges and rewards, but I'm not sure that's homework. Should we give people rewards for listening to the podcast? Yes, we should

definitely give people a reward. We'll mail them bananas. We'll mail them sun toasted marshmallows. But that is interesting that neither of our first languages were English. And look at us today. We have a podcast. I mean, how many people get to have a podcast these days? Nobody.

That's right. That's right. We are super duper rare in the universe. But it is amazing how you can learn different languages as a kid. And even if English isn't your first language, if you learn it when you're young enough, you can become totally fluent in it. Yeah, and that's because you were born in Israel, right? And so you spoke Hebrew?

That was your first language? Yeah, I was born in Israel and spoke Hebrew as my first language. And I didn't really start speaking English until we moved to the U.S. And it turned out you needed to use English to communicate. And that's when I started to pick it up around age five. And it's interesting to me to think about like how people think what languages are intuitive to them. Because we also think about physics sort of the same way. There's certain ideas and physics that are intuitive to you because you learned them as a young kid.

temperature, velocity, this kind of stuff. And then as we grow up, it's harder and harder to learn crazy new concepts and to really feel fluent in them. You always feel like in some sense, you're translating. Yeah, I guess things like quantum physics and the multiverse, those are all very sort of alien to our intuition because, you know, as when you're kids, you're used to like bouncing a ball and holding a ball and those things aren't quite true down to some molecular level. And it's fascinating to look at the history of physics because when these ideas first came

out, the old guard were very against them. It's very difficult for the gray-haired physicists to accept some of these ideas. Even folks like Einstein were resistant to some of the crazier concepts. But now it's the kind of thing that we teach in college. And there are even books like, you know, quantum physics for babies. So I feel like one way that we make progress is by introducing these crazy ideas younger and younger so that physicists become fluent in them and then can become like virtuosos in these ideas rather than always feeling like they're working in some translated alien language of the

universe. Interesting. So I guess if you're pregnant and you're carrying a baby, you should maybe play our podcast to your baby, the womb, just to get them used to these ideas, right? So it's not a shock to them later on when they're 20, 25, and they learn about it in grad school. Exactly. Get them hooked early. I think there's some ethical questions that Jorge about pushing our podcast on kids, you know, getting them addicted when they're young. I'm not sure how I feel about that. Yeah, I think there might be some government regulations. not children's content, but maybe not fetus's content, you know? Maybe that's different.

Wow, way to find the loophole there. But yeah, I do think it's good to teach children to think about the questions of the universe in a different way. But I think fundamentally we're always going to struggle to really understand the nature of the universe in a way that's intuitive to us because it is just weird and we don't have the immediate experience of some of that like quantum weirdness, for example, or relativistic weirdness, unless we are living among Bose-Einstein. condensates and in the vicinity of a black hole, we may never find that stuff intuitive.

Well, I guess it's kind of hard because, you know, you need sort of Newtonian regular old school physics intuition to just like get around the world, right? And play baseball and play basketball with your friends. And that's sort of how the immediate world around us works. But if you train a kid to be a quantum physicist from the beginning, would they be really sort of like awkward or out of touch with the world around them? You know, are you setting this kid up for being very unpopular? Well, I can't speak to the social aspects of it.

But it is an interesting question. If somebody had intuition for relativity, could they still, like, operate in everyday life? The amazing thing about relativity, though, is that it does reproduce our intuition in our scenarios, right? Like, yes, you can use Newtonian physics to describe motion and gravity. But if you use relativity, you get the same answers. It's just sort of a lot more complicated. And so Newtonian physics is, like, a lot faster. So, like, somebody will, like, correctly predict where the baseball is going to go.

but it's going to be like three hours ago. You know, like, okay, I can tell you exactly where that curveball went three hours ago, but I missed the pitch. Yeah, just don't put that kid in right field or left field or center field or any part of the field. Exactly. Yeah, like still doing the calculations. Hold on. I'll get back to you.

Games over like two hours ago, dude. Physics, that kid, that quantum kid. Sounds like a fun, interesting plot for a novel maybe or a movie. Like, you know, you train a whole generation of children. to be quantum thinkers because of some special science project or something where we're sending people to the quantum world. Yeah, maybe. And then they can be in like two emotional states at once.

I hate this movie and I love this movie at the same time. I laughed. I cried. Wait, maybe that's what movie criticism is. It's all quantum mechanical. Yeah, there you go. People will love it and hate it. And this isn't just about Jorge coming up with plots for novels.

This is about asking questions about the nature of reality and trying to understand it or trying apply our intuition, our limited toolbox of ways to think about the universe to probe deep, dark questions about the nature of space and time. Yeah, because there are still big questions that are unanswered out there in the universe and huge pockets of the unknown that we have yet to figure out how it works. And sometimes we like to think about what it might be like to touch those things. I don't know about you, but I've looked at the sun and I've wondered like, how close could you get to the sun. How hot is it really? You know, if I put my finger on it just for like a microsecond when I get burned, maybe I'm the only one who's ever like that. I think walking

around wanting to touch everything is what gets you in trouble as a kid, usually. It maybe also turns you into a scientist. You're like, hmm, I wonder what happens when I press this button. Sounds like a very dark way to get rid of a lot of future scientists. That's right. Those of us who survive, learn a valuable lesson about what not to touch. But it is interesting that there are pockets of the unknown out there in the universe and that we may want to touch them just to see what they're like. And there is probably not a bigger hole in our knowledge of the universe as big as black holes. That's right.

The deepest, darkest mystery of the universe is what's inside a black hole. What's really going on over there? And so, of course, the physicist in me wants to reach out and touch one. Yeah. And so there are many questions we can ask about black holes. Today we're going to focus on one, I guess, that it's all about the black hole's appearance, right? In some sense, I guess you could say it's appearance. I guess I would say more of like its physical description.

It's more to me about black hole temperature than about black hole's ability to find mates, for example. So today on the podcast, we'll be tackling the question. Are black holes hot or not? Isn't that a subjective judgment there, Daniel? You know, like, are you saying, am I attracting to black holes? Isn't everybody attracted to black holes? Black holes are very attractive, even to other black holes. Yeah, you know, they say opposites attract, but not in physics, right?

In physics, masses attract. What if it doesn't have an opposite? Anyways. I like big masses and I cannot lie. That might be taking us into a hole. We don't want to go into, but it is an interesting question to think about whether black holes are hot or not. And I'm guessing you mean temperature, not like hotness. That's right.

Yeah, I make no evaluation of whether black holes are attractive or not. But I am interested in the question of whether or not black holes are hot or black holes are cold. It's really fascinating because black holes certainly do contain a lot of energy. Yeah, yeah. They have a lot of energy for sure. But I feel like it's energy that's like trapped somewhere that you can't access. So therefore, it should feel really cold. Mm-hmm.

You know, that's good intuition. Sometimes, though, our intuition for these things breaks down. Like, it's true that the sun looks hot and it is hot, but there's other stuff out there in space like the interstellar medium, which can be like millions of degrees Kelvin, but if I dropped you inside of it, you would freeze to death. So sometimes these things can be a little bit counterintuitive. Interesting. So something can be hot, but also cold at the same time? Is that like quantum temperature? In the case of the interstellar medium, it's because it's very dilute.

So the individual particles are moving at very, very high speed. So technically it's hot, but there's not a lot of it. So it wouldn't be enough to keep you warm. You would freeze to death, even as you're being smashed into by these high speed particles. I see. So you're saying like a black hole could be hot and actually hot or hot and actually cold or something in between. Yeah, it's very weird. Think about what happens when you shoot a laser beam at a black hole.

In theory, it grows. Does it also get hotter? Is it possible to shoot a laser beam? at something and cool it down. Or what if you throw it like a campfire into a black hole? Does the temperature go up or down? A lazy beam is basically just like a high-tech physics version of a campfire. Yeah.

Or what if you throw Brad Pitt into a black hole? He's pretty hot. I don't know. Is he still hot though? Like as time goes on, is he getting hotter or colder? What do you think? Definitely some of those movies stars stay pretty hot into their old age for sure. Helen Mirren, you know, you get through her.

into a black hole, I'm sure it'll get hotter. Well, Helen Marion is so hot, she breaks the laws of physics, apparently. Let's not throw her into a black hole, please. But this is an interesting question. Are black holes or not? What would happen if you touch and would your hand get burned? Or would it freeze or get sucked in, I guess? Would it file a restraining order against you?

So as usually, we were wondering how many people out there had thought about this question and whether they think black holes will burn you or freeze you. So thank you very much to all those who participate in answering these random questions. If you'd like to hear your voice speculating baselessly for the podcast, please don't be shy. It's a lot of fun. Write to us to Questions at Danielanhorpe.com. So think about it for a second. Would you swipe right or left on a picture of a black hole? Here's what people had to say. I guess that would depend on how you're defining temperature. So temperature generally is defined as the amount of movement of a particular.

set of matter. So maybe the spinning of a black hole would mean that it has lots of energy, which would mean that it might be hot. And then if the black hole wasn't spinning, it might be cold. I guess if they were hot, they would be giving out lots of high energy radiation. And I guess that would make them easier to spot. I know that they give away hawking radiation, but that seems to me to be low energy and low density radiation. So my guess would be black holes are cold, as the name suggests. So there's a lot of matter. It gets compressed down into a black hole, and it gets all smashed in one point.

The other thing that's kind of like that's like a star, right? That's like really hot because there's all this like energy. And maybe if it's like hot as in temperature, which is kind of a standard for energy, We're talking about a lot of matter in a small part. I'm going to call that hot. This sounds very hot to me. I would say black holes have to be cold because when they suck in the stars, the stars

obviously circulate around the black hole until they disappear into it. You can kind of see their light on the horizon. And I feel like if they're hot, then the star would have to then get hotter as it approached it, and I feel like that would then cause more of a supernova or just some kind of explosion happening to the star

as it entered the black hole. Definitely inside, black hole like the object itself the pressure that it has must be had that energy that it's right there but like outside when you go close by by the black hole a black hole that doesn't interact with anything and what I want to say is like no star close to it to feed on that star nothing no object close to the black hole to interact with to make it bigger.

So in this case, if the black hole doesn't let the light go out, probably doesn't let the heat go out either. So if you're right there close to it, it wouldn't be hot. But inside, right there, when the pressure, it might be hot there. I believe black holes must be really hot because they're radiating energy in form of parking radiation. they must be really hot, at least on the surface. That's what I think.

My intuition says cold, so I'm going to go with hot. All right. A lot of pretty hot takes on this question. I think my favorite one is my intuition says cold, so I'm going to get hot. That person has learned some lessons about how the universe works right there. Yeah, a person must have been listening to our podcast in the womb. Is every physics question really a trick question? Is that what they've learned?

Yeah. I feel like the universe is a trick question. Why, it's not really a universe. It's actually just a simulation. That's the trick. Yeah, it's something else, right? That seems to be the lesson. Everything you thought is not really the way it is. Even your thoughts. Maybe there is no truth, man. Maybe there is no truth. But I think there are a lot of really interesting ideas here. There's ideas about how we can think about black holes because they radiate energy and also thinking about how black holes absorb energy. so they should be hot or the high pressure inside a black hole that might make it hot. It's a lot of really good ideas.

Yeah, and I guess it sort of depends on what you mean by hotness, right? Like, is it related to something like pressure or how much energy it emits or what happens if you touch it, right? Sort of maybe depends on what you mean by the question and what it means to be hot. Yeah, and as we learn, the whole concept of temperature is a bit of a slippery topic. Yeah, also whether or not you slip on it also. Well, let's start at the beginning and let's recap for listeners. Daniel, what is a black hole? Like, how do physicists think about what a black hole is? So we have several pictures of what a black hole might be, but the truth is that we're fundamentally not really sure what a black hole is.

And that's why this question is really interesting. A black hole temperature really goes to the heart of what we do and do not know about black holes. So we have a couple of ideas about black holes. One is what we call a classical picture of a black hole, which means that it comes from general relativity. If you just follow Einstein's rules about how mass bends space time and space time tells mass how to move, then you end up in this really puzzling, amazing sort of weird prediction, which is that gravity has this strange runaway effect, right? Mass pulls on mass, which means it's more massive, and then it has more gravity, which means it pulls harder on other masses, which means it has more gravity. And that just keeps going.

And eventually, you get this weird thing, a singularity where space is curved so much that you, you have created an event horizon, a region past which, if something gets too close, it can never leave. No information can leak outside this event horizon. That's sort of the classical view of what a black hole is, a singularity, a point of infinite density surrounded by a threshold called the event horizon, like a marker. If you go past it, you can never escape. You'll be trapped forever. Your future is that singularity. Yeah, I like classic black holes. I feel like the classics are always the best. But I think what you mean is like a classic black hole was basically physicists thinking about like what happens at the extreme levels. Like what if I take a whole

bunch of mass and I cram it into the smallest possible space or even like a tiny dot, like what happens if a dot has almost infinite mass, right? Because gravity is very strange compared to the other forces. As you were alluding to earlier, there's no negative gravity. There's no repulsive gravity. I mean, there's dark energy and is the cosmological constant due to potential energy and fields, perhaps. But when you're talking about just masses and objects, you only have a traction. And so there's no balance, right?

It just keeps going. And so if you have enough mass somewhere, then it's just going to keep getting denser and denser and stronger and stronger until eventually you get infinity, right? This thing diverges. It actually goes to infinite density if you give it enough time. Right, because I think one of the things about gravity is that it gets stronger, the closer you get to it, right?

Like the formula for it has the distance, to it in the denominator in the bottom. So like if you can cram a lot of mass in a small amount of space, then you can get really close to it, which means that your denominator kind of goes to zero, which means that your force sort of like goes almost to infinity. Yeah. And so gravity left to its own devices will always generate black holes.

The reason that you're not a black hole and I'm not a black hole and the earth isn't the black hole is that there are other forces out there that can oppose gravity, right? The electromagnetic force and the strong force give us structures that balance out gravity, at least for a time. And then eventually, if those things fail, then, you know, you collapse into a black hole. But that's the prediction of classical general relativity, this incredibly strange phenomena of an infinitely dense point.

You said the word sort of inevitably, and I wonder if that's how you think about it. Like, is it inevitable that a bunch of mass, if you leave it out in space, will become a black hole if you have enough of it? Like, things will sort of crunch down down to an infinite point. Or is it possible even in the classical sense that things won't crunch down forever?

Like the Earth is not crunching down to a black hole, right? Ooh, cool question. If there was only gravity, then it would be inevitable. If you have a bunch of particles floating in space and the only force in the universe is gravity, then it will suck them together into a black hole. The only thing that could resist that if there are no other forces is angular momentum. You can imagine like some blob of stuff coalescing sort of like the Earth and then orbiting a central black hole but not falling in. Right. So you might imagine maybe that's stable. Maybe a blob of stuff orbiting a black hole could avoid it forever. But you know, even if you're orbiting, even in class, classical general relativity, you are radiating gravitational waves. So you're losing energy. So eventually you will spiral into the black hole. So in a universe with only gravity, then eventually, yes, black holes are totally inevitable. But as you say, the Earth is not a black hole and probably will not become a black hole because the other forces are stronger than gravity and can oppose it for a while.

All right. So that's the sort of classical view of a black hole. Then what's the other view? The new black holes. The other view is that we don't know what a black hole. is, but it can't be that. It's basically, it says, this can't be right. Oh, my gosh, it's ridiculous.

Infinities are not predictions of physical theories. They are failures of physical theories. It says something is wrong here, and it's got to be replaced by some other idea, some quantum version of general relativity. We would call that quantum gravity. We don't have that theory, but we can criticize the current theory, general relativity, for not being consistent with quantum mechanical ideas. And that's because the idea of a singularity, you can't have that in quantum mechanics?

Well, in general, we don't think you can have singularities or infinities in physics at all. Like, whenever you get infinity as a prediction of a physical theory, it's usually a failure. I mean, look back to history, for example, the whole discovery of quantum mechanics was trying to unravel the source of the ultraviolet catastrophe. This prediction from classical physics that objects as they get hotter would radiate infinite temperatures. Like, it was nonsense, right? It was a sign to us that there was something wrong, something missing. in our calculations that gave this crazy prediction. And so anytime you get an infinity,

it's a sign that probably you're doing something wrong. Like we don't see infinities anywhere in the universe. Maybe the universe itself is infinite, but we never measure or observe infinity directly. Seems like a mathematical failure rather than a physical prediction. But more than that, a singularity is a really strange thing in the universe, right?

We talked about singularities on the podcast several times, but in some sense, they're like a path that ends. You know, quantum mechanics says that, time goes from the infinite past to the infinite future and you can predict what happens to an object. You know, it's deterministic in the sense of like evolving its wave function. But a singularity is where like a point ends. Something falls into the singularity and you can no longer predict what's going to happen. Predictability of physics, which is like the foundation of physics, what we try to do is predict the future given the present. That fails at a singularity.

You can't predict what happens after a singularity. There's no like paths out, you know, everything that falls in, no matter how it started, has the same fate. It's just like is there in the singularity. And so that's inconsistent with like quantum information theory and basically everything we know about how physics works. It just seems like a breakdown more than a prediction. I guess that's why they call it a singularity. It's singular.

Exactly. And it's sensicalness. All right, well, I have some questions about that. And so let's get into more of this idea of a quantum black hole. And then let's get to the question, whether it's hot or not. But first, let's take a quick break. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers, the pilot is having an emergency and we need someone, anyone, to land this plane.

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we get to the bottom of questions like these. Join us as we talk to the leading expert on overconfidence. Those who lack expertise lack the expertise they need to recognize that they lack expertise. And then, as we try the whole thing out for real. Wait, what? Oh, that's the run right. I'm looking at this thing.

See? Listen to No Such Thing on the IHeart Radio app. Apple Podcast. or wherever you get your podcasts. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness the way it has echoed and reverberated throughout your life,

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I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. All right, we are talking about black holes and whether or not they are hot or not. I guess whether or not you would go on a date with a black hole because you're definitely attracted to a black hole. Yeah. On the other hand, I also think black holes are pretty cool.

I'm not sure how to feel about that. Oh, boy. Now we're mixing all kinds of metaphors here. So you're saying there's a classical view of black holes. It means that there's sort of a singularity in space because it's sort of the natural extension of what happens if you just have gravity in the universe. But then there's a quantum mechanical view

which says that a singularity is not possible in a quantum world. But I guess my question is, does it have to be possible? Like, isn't the point of a singularity that you never actually get there? Oh, that's interesting. I mean, we are interested in what's going on in the universe, even if we can't observe it directly. So we would definitely want like a physical model for what's happening microscopically inside a black hole. Because that will allow us to understand, you know, the nature of space and time.

It might be that we can ever see inside them. And so, you know, the observability is a question. It's sort of like thinking about the multiverse. It's useful to think about the multiverse, even if you may never observe or go to the multiverse. It's still a question we want an answer to, right? Well, you say that a singularity is inconsistent with quantum mechanics. Maybe it gives a sense of why that is. Like, you know, maybe the singularity is not consistent,

but maybe getting or approaching the singularity is consistent all the way to infinity. Some of the problems with quantum mechanics and black holes we laid out a minute ago, but there are other ones, as you say, you know, quantum mechanics says that things can't be localized in momentum and in location perfectly well, right? But a singularity violates that. It's an infinitely well localized point in space. And so quantum mechanics says if particles are in an infinitely well localized point, they should have an infinite uncertainty in their energy, which would mean some of them would

have basically infinite energy, right? And so it's sort of like physically impossible. There's a minimum quantum fuzz to the universe, which has singularity violates. And so we just don't think it's possible. That doesn't mean that black holes aren't real, right? It might mean that black holes are there, but that at their heart, there isn't a singularity. There's some like weird quantum fuzz happening. And you might not be able to tell the difference from the outside because from the outside it's all just strong gravity. But you know,

we want to know what's going on inside that black hole. And it does have consequences for whether the black hole is hot or not. You're saying like a black hole is still a hole. It could still look like a hole to us. But at the very center of the black hole, like what's going on at the very, very center does it actually get to a singularity or is it kind of fuzzy down there? And I guess just to be contrarian, could it be both? Like, could there be a singularity? And as you get closer to the singularity, you know, space is crunched down so much that you're still fuzzy, but also kind of a singularity. Do you know what I mean?

No, but I like to hear more about that idea. How can you be a singularity and fuzzy? Well, gravity bends space, right? In some ways that you're sort of compressing space and in the middle of a black hole. So as you approach a singularity, maybe you're still fuzzy in a quantum sense, but space is crunched down so much that you're kind of fuzzy in a very small space. That's basically a singularity. I see. I mean, it could be something you could never approach or something that sort of remains that

a certain observable size, no matter how close you get to it without actually being a point. And that's true. That's interesting and maybe a possibility for a quantum mechanical description. But I don't think that would qualify as a singularity. It's definitely not what general relativity predicts, for example. And you might think, well, why do we care what general relativity predicts? And we care because general relativity seems to be accurate about everything else in our universe. You know, the way that space expands, the way the space ripples, the way that objects move through space and time, very, very precisely.

It seems to get everything else exactly right. And the math of it is beautiful. It's gorgeous. I mean, theoretical physicists who learn it, they, like, fall in love with the equations. And so it seems to, like, speak to something deeply true about the universe. So to discover that it's not, that there's something that it fails to describe is an opportunity to learn something really fundamental about the nature of space. Yeah, I guess it's kind of like Newtonian physics we thought was great because it predicted the like the path of the planets and the orbits and whether or not you can catch a baseball. But it sort of breaks down at a different scales kind of right.

You're saying that relativity also sort of works for everything else that we see around us. But when you get down to the smallest levels, it doesn't have quite a solution that works. Exactly. And we would like to know what the fundamental nature of space is, you know, at the smallest level is gravity actually a quantum quantum. force mediated by gravitons or is space itself quantized into this little foam. It's more than just an academic question of what's at the invisible heart of a black hole. Understanding the nature of space and time might give us great power over it. You know, let us develop warp drives and wormholes and explore the universe.

And so it's definitely a question worth asking. Yeah. Well, okay, let's get back to our main question of this episode, which is whether black holes are hot or not. And I guess we mean in the sense of temperature, do black holes have a high temperature? or do they have a very low temperature? And so let's start maybe by talking about what temperature means in this context. Yeah, so temperature is really slippery and tricky. Sometimes when we talk about temperature, we mean like how much energy is in the particles inside

something. So you have a banana in front of you. You can ask, is this banana hot or not? What you really mean is how much internal energy is there? Are the particles inside the banana wiggling a lot or are they frozen in place and not wiggling a lot? And you can actually do cool calculations to connect like. how much those guys are wiggling to the apparent temperature that you would experience if you like touch the banana. That's your intuitive sense of temperature. And the way you measure temperature

usually is by like sticking a thermometer in something and the energy transfers from the banana to the thermometer and you read it out on the thermometer. That's your intuitive sense of temperature. But that doesn't really work with something that you can't touch. So if you want to measure the temperature of something you can't touch, there are other ways to do it. Like we can measure the temperature of the sun, even though no human object has ever. touch the sun and then come back out again. Right, or like even now they have contactless thermometers, right? You just kind of like hover over your forehead and it somehow measures your temperature

without touching it. Exactly. So the way they do that is by measuring the radiation that you generate. Not like, you know, you're shooting off alpha particles and creating hulks everywhere you go or Spider-Man, but you generate energy. Like you radiate photons everywhere you go because everything in the universe that has a temperature does radiate energy. Like the sun radiates energy

in the visible spectrum because it's pretty hot. And the earth radiates energy in a much, much longer wavelength because it's much colder than the sun. But it still does. And you generate energy at a higher wavelength than the earth does,

which is why if you put on like infrared goggles, you can see if there's somebody hanging out in your backyard at night because they're generating a different kind of light, a light that's invisible to your eyes, but can be picked up by infrared goggles. And that's a pretty cool burger. or trespasser, you can't see them.

But I guess maybe a basic question is, why does that happen? Like, why do hot things emit infrared light? Yeah, it has to do with energy just liking to get spread out. You know, microscopically imagine like a blob of stuff. And that blob of stuff isn't inactive, right? It's constantly radiating stuff and reabsorbing it. So electrons within your banana are shooting off photons and then other atoms are reabsorbing that. But near the edges, it doesn't always get reabsorbed.

Some of it just shoots out. So that energy is just like getting spread out through the universe, partially by radiating stuff. And what's really interesting is that the spectrum you emit is controlled by your temperature. So if you have a really high temperature, you tend to emit at shorter wavelengths at higher frequencies. If you have a really low temperature, you tend to emit at lower frequencies or longer wavelengths. So for example, really, really hot gas emits in the x-ray or the ultraviolet. And really, really cold objects like asteroids floating in space emit in the infrared. right yeah because we've talked about in this podcast about how temperature is related to the kinetic energy or like how fast the particles or something are moving within you know the volume that you're measuring the temperature of but then i guess that question is how does that relate to the wavelength of light that's emitted like first of all why do they emit light at a higher frequency

So we've been talking about them emitting light. And by that we just mean photons, right? Remember that by light, we don't always mean things that you can see. So we're talking about something emitting light. You shouldn't expect it to be glowing in a way that your eyeballs could pick it up. You sit in a dark room with a banana and stare at it. You're still not going to see that banana glow because while these are photons and they are hitting your eyeballs, your eyeballs can't see them.

So it's emitting light because it's made out of charged particles, you know, electrons and atoms. and when those things move and accelerate, they always emit light. That's what electrons do for an electron to accelerate, for it like change direction, for it to slow down or speed up, it has to emit a photon because that's the only process that allows it to do it, right? It needs to like push off of something. The same way, if you're in a rocket ship and you want to accelerate, you've got to throw something out the back, right?

You need some mass, some rocket fuel or some propellant to toss out the back. That's the only thing an electron can do to change its direction to accelerate or to deceleration. accelerate is to emit a photon. So because a banana is made out of charged particles, it tends to emit photons. It also emits other stuff, you know, emits neutrinos and it emits other kinds of radiation from the other kinds of forces that it interacts with. But that's why it emits light. That's why it emits photons because it's made of charged particles. And why does it emit different frequencies based on its temperature? Well, you know, higher energy stuff emits higher

frequency photons. You have like electrons whizzing around your atom with more energy. They have more energy, they will jump down more energy levels and they will emit higher energy photons, which have higher frequencies and shorter wavelengths. So there's just sort of like more energy available because everything moving around has more energy. You get on average higher energy photons. Not purely. It's not like a spectral line, right? There's like a distribution. So you get some of the high frequencies, some of the low frequencies. But the shape shifts as you get to higher temperatures, the peak moves and the most likely frequency gets higher and higher as the temperature gets higher and higher.

Interesting. It's almost like you have a bunch of atoms and they have some charge to them and they have electrons and set them. And they're all sort of trapped together by the electromagnetic forces that are holding them together, kind of right?

Or sometimes they bump into each other and you're saying every time, you know, a atom gets pulled into the banana or a gas particle bumps into another gas particle, you know, stuff happens in the electromagnetic force and so that causes photons to shoot off.

Yeah, exactly. Every time two electrons talk to each other, that's a photon. So you have a blob of charge stuff. There's photons everywhere. It's just like a huge mass of photons and particles constantly interacting with each other. And some of that just bleeds out the signs. It's like if you're in an apartment next to a party, right? And everybody's shouting at each other, then you're going to be hearing some of those conversations.

Right, right. Yeah. Unless you're in the party, then it's a good time. And as they drink more and more, they tend to get higher and higher energy, right? And then this analogy breaks down. Yeah, physics is a party. but the main point. That's right.

So in terms of black holes, I think what you're saying is that temperature is normally defined as, like the kinetic energy or how fast the particles of something are moving, but it's sort of related to different things in different ways that we measure it. Like we can measure it by touching in

and some of that kinetic energy moves to you or you can measure the light that comes from it because of this radiation. So that also sort of tells you the temperature. And so it kind of sort of depends on what you mean by temperature of a black hole. Do you mean like the energy it radiates or do you mean like what would

actually happen if you touch it. Yeah, well, the fascinating question about black holes is that we don't know what's going on inside them microscopically, but we can ask questions about their radiation. Do they give off any radiation? And if we use radiation as a way to measure temperature, then we can talk about the temperature of something without having any idea about what's going on inside it microscopically. Like we can measure the temperature of the sun without knowing like, how do those plasma tubes work inside the sun? What is going on at the heart of it? We can still measure its temperature. The same way they measure your temperature on your forehead without touching you, without knowing like, you know, are you sick or not? What's going on inside your brain?

They don't know any of that, but they can still measure your temperature. So it's a bit of an extrapolation to say, we don't know what's going on inside something. We measure its radiation and then we talk about what temperature it has. Right. I think what you mean is like if you take a step back from the black hole and, you know, not worry about what's going on at the very center, you know, how hot does it look from afar? Yeah, we use this concept of temperature a lot in physics in a way that might be confusing otherwise. Like we talk about the temperature of the cosmic microwave background radiation. We say that's 2.73 Kelvin.

What does that mean? Does it really have a temperature? What it means is that the cosmic microwave background radiation is radiation at a wavelength that would be emitted by an object at 2.73 Kelvin. That's the black body radiation for an object at that temperature would be the frequency of the CMB. In the same way, we can look for radiation coming off of black holes and use that to measure its temperature and then try to deduce what might be going on inside of it from that. Well, I guess it's kind of weird because, you know, I imagine that a black hole has particles inside of it moving in a certain speed and it does have sort of like a kinetic internal kinetic energy. But you're saying like let's not even worry about that because we don't really know what happens beyond like the black holes event horizon. So let's just take a step back and see how hot it looks.

Yeah, well, let's start there because that's something we can do from the outside. And then let's turn that around and try to understand based on its temperature, what might be going on inside. So let's use that as a probe, right? Because temperature is like a message. It tells us something that's going on inside from afar. So without touching something, you can get a little bit of information about what might be going on inside. And in the end, that's the goal is to try to understand what's going on inside the black hole. Right.

But I guess it's messed up because, you know, the definition of a black hole and nothing can escape of it, not even light. What does it mean to measure the light coming off of black holes if light cannot escape a black hole? Yeah, that's true. Nothing can escape the event horizon, nothing from inside can ever come out. But that doesn't mean that black holes don't radiate.

But there's a difference in how you describe the radiation from black holes in classical GR, like Einstein's pure singularity black holes, and more modern attempts like Hawking's view of what a black hole is. So they give us very different senses for what might be going on inside the black hole. Interesting. All right, well, let's get into whether Einstein and Stephen Hawking are also hot or not, and whether or not they're right about the temperature of black holes. We'll dive into that and what it all means. But first, let's take another quick break. Imagine that you're on an airplane, and all of a sudden you hear this.

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daniel how it's weird to think of the heat radiated like the the light hit radiated by a hot black hole because black holes are black. By definition, things that don't radiate light or trap light, how can something be both radiating heat and also trapping heat forever? Well, you know, Einstein would agree with you. Einstein's general relativity says that black holes are perfect absorbers. They do not radiate any light, anything that hits them falls in, and they radiate nothing. And so from the general relativistic point of view, black holes are at absolute zero.

They have zero temperature. What is the expected radiation from an object at absolute zero? It's nothing. And so if black holes radiate nothing, then therefore they must have zero temperature. That's the GR version, the classical view of a black hole. All you can know is it's mass, its charge, and it's spin, not its temperature. Because that would be like information about what's going on inside the black hole. So according to Einstein, black holes are cold.

They're like infinitely cold or perfectly cold. That's right. According to Einstein, you can shoot a laser beam out of a black hole and never heat it up. It will just keep eating that laser, but never get hotter. All right. Well, then, but what does Stephen Hawking say? So Stephen Hawking says, actually, if we live in a quantum mechanical universe, then that cannot be true. So Hawking did his famous calculation where he thought, let's think about a black hole. And now, instead of thinking about what's going on inside the black hole, because we can't and we have no idea how quantum gravity works, Let's just try to put the black hole in a quantum universe and say, how do you do quantum mechanics when you have an event horizon?

If there's something there which like eats all the information. And so he did a bunch of fancy calculations in quantum field theory. And he discovered that the only way this works, the only way you could have like a boundary condition like a black hole is if you have radiation coming out of the black hole. So he found like solutions to the quantum field theory that require outgoing waves from the black hole. And so that's really interesting. that says that like if you have black holes and the universe is quantum mechanical, then they must be emitting something. And he went a step further. He said, well, you know, how does this radiation work? What does it look like? And what he discovered, and this is sort of the fascinating

moment, is that the radiation spectrum that you expect from the vicinity of a black hole follows exactly the spectrum you get from black bodies. Black bodies like we talked about earlier don't just emit at one number, right? It's not like the sun emits at only one frequency. There's this shape to their spectrum. What he discovered is that this radiation that he was predicting black holes emit also has a shape and that shape matches exactly the shape you expect from black bodies at a certain temperature, at a non-zero temperature. So he concluded there must be some radiation from black holes and black holes must have a non-zero temperature. Well, I feel like maybe that's not quite what he said, right? Like I think what you're saying that he's saying is that black holes kind of have to leak

stuff out like they can't trap stuff in there forever stuff leaks out and the stuff that the energy that leaks out sort of looks like something that would have a temperature but that doesn't necessarily mean it's the temperature of the black hole right you're exactly right and in the paper he even said like be careful about interpreting this literally as a temperature it's more like an effective temperature it's an attempt to describe what might be going on inside the black hole but we don't know microscopically we have no idea you know is everything inside the black hole totally frozen is this like quantum space wiggling. We don't understand what's generating this radiation,

but it's a way to describe the black hole sort of thermodynamics and statistical point of view, right? And so he says, you're right. It seems to like it should generate a spectrum as if it had a temperature. What that temperature means in terms of like the microscopic wiggles inside, Hawking didn't say, couldn't say, and we still don't know.

But I guess can you step us through a little bit of how he did it or how he reached this conclusion? Like what does it mean that you can't have a perfect hole in the universe? Like why does it have to leak because of quantum mechanics or what? Yeah, because of quantum mechanics. And so you take a black hole and you put it in the universe. And the universe also has fields in it, you know, fields for fermions, fields for bosons, you know, the electromagnetic field, the electron field.

And now you want to quantize those fields is what we do in quantum mechanics to get to quantum field theory. We say the universe is filled with space and the space has fields in it and those fields are quantized. They can only have certain solutions, right? Not any arbitrary continuous set of solutions, but only specific solutions, like a ladder. Just like electrons whizzing around an atom have a ladder of solutions. And so the thing that he ran up against is that you add a black hole to that, that changes how you can quantize these fields. And for that quantization to work, there's various consistency conditions on this quantization. You need like unitarities.

You're not predicting things would have probability more than one. You need things to not contradict themselves. Various internal consistency conditions on the quantization. of the fields in the presence of a black hole requires this radiation to exist. Right. I wonder what it means is that, you know, he's saying that like you can't have a perfect event horizon. Like you can't have like this perfect, perfectly smooth boundary where if you cross one, you

know, tiny little bit, you're in the black hole or whether you step out a little bit, you're out of the black hole. Maybe what he's saying is that, you know, we live in a quantum universe. You can have that kind of like stark boundary. or certainty. And so as a result, you can have a fuzzy event horizon in which will tend to leak. I think that's one way to interpret it. But I think it's dangerous to try to come up with a microscopic interpretation. I mean, I love that your brain immediately goes to like, what's happening right there at the edge? What does that really mean for an individual particle?

The truth is to understand that, we'd need a theory of quantum gravity. We'd have to understand how gravity affects quantum particles. And we just don't. You know, quantum particles can do these weird things like have a probability to be in two different places at the same time. How does gravity affect that? Does it tug halfway on both of the places where the particle is? We just don't understand the microscopic picture of quantum mechanics and gravity at all. So we can't really tell a story about what happens right at the edge of the event horizon for quantum objects. But we can tell a story about the temperature of these black holes sort of in this indirect way. Well, I guess I feel like you're saying there are two answers.

One is Einstein saying from afar, a black hole will seem like it has zero temperature. And from afar, Stephen Hawkins says that black hole should have a little bit of a temperature and almost like it was kind of hot. But neither of those really tell you what the real temperature is inside of the black hole, right? Like I'm thinking of the black hole as this like perfect cooler that doesn't let any heat out. And so if you stick something hot or something cold inside of the cooler, like it has a temperature inside of there. but from the outside, it's going to look totally different, right?

From the outside, a perfect cooler will seem super cold. Or if you were Stephen Hawking, that cooler will leak out or look a little bit warm. But that doesn't tell you how hot or cold it is on the inside of the cooler. No, you're totally right. Neither of these tell us what's going on inside. And the program, the way to make progress, is to try to build up from the ground up, a theory of quantum gravity that would describe the kind of radiation we would see from afar.

And so people are working on that. They're trying to do that. You know, the loop quantum gravity folks are trying to do that. They're trying to describe this radiation is like the shaking of the vibrations of the quantum of space, you know, this foam. In the other direction, people in string theory are trying to describe what's going on at the heart of the black hole in terms of like vibrating strings. And they've actually had a lot of success.

These super symmetric black holes have been able to predict very well this expected distribution of radiation from a black hole. It's like one of the biggest successes of string theory. as a theory for quantum gravity. So, you know, if we can make careful measurements of the black hole from the outside and then come from the other direction and try to predict those measurements, we might be able to get some insight as to what's going on inside. Right, because it is sort of, as we talked about before,

it is possible to go into a black hole, right? Like for certain black holes, you can go past the event horizon without getting shredded to bits. And so it is sort of possible to go into a black hole and measure the temperature inside of a black hole. I mean, you wouldn't be able to tell anybody on the outside. side, but it is technically to like know, possible to know that the temperature of a black hole inside of one, right? Yeah, that's true. You can fall inside a black hole and for large enough

black holes, you can survive the tidal forces just past the event horizon. Eventually, you will get shredded. But yeah, you can fall past the event horizon and do experiments. You know, if it's a classical black hole, then you're not really going to learn anything until you get to the singularity anyway. If it's a quantum black hole, then there might still be an extended region and so you could do some experiments there. Yeah. Like not quite at the core of the black hole, but like in the rind of the black hole. The crust. The crust. There you go. The crust. But I think the calculations are also super fascinating because the numbers themselves are really weird. If you ask, all right, what is the temperature of a black hole that has the mass of the sun, for example?

You know, Einstein says, oh, it's zero. Hawking says, no, it's not zero, but it's 0-000-0-0-0-6. Kelvin. So like black holes, they're not zero, but they're definitely not hot. It's still a pretty cold object. You mean from afar? Like outside of the cooler, outside of the cooler, the coolers are pretty cold. Yeah, we're always talking about from afar because we don't know what's going on inside this. So these black hole temperatures are always what you measure from the radiation that might be coming off the black holes. The leaking, you mean, right? Yeah, the leaking. The thing that's really weird is that this temperature is inversely proportional to mass. We talked about this before that Larger black holes radiate less and smaller black holes radiate more, which means smaller black holes are warmer.

So as black holes get larger, they get colder. Einstein says you shoot a laser beam at a black hole. It just stays at zero Kelvin. Hawking says you shoot a laser beam at a black hole. It gets colder. Whereas if I shoot a laser at you, you would get hotter and more annoyed, kind of. Yeah, I would get louder. Yeah.

Well, that's interesting. So it's almost like you're putting more energy into the black hole, but from the outside, it's looking like it's losing energy. Yeah, it looks like it's getting colder and colder, which is hard to understand. But think about like what might be going on inside the black hole. It's gravity is getting stronger and stronger. And so time is slowing down inside the black hole. And so things are moving more slowly, which you can kind of understand as maybe having lower temperature, right? So again, we don't know anything about what's going on inside of black hole, but trying to put together like a rough intuition for how,

black holes could be getting colder as they get larger, time dilation kind of helps that picture. Oh, I see. Or could it be that, you know, as you add more mass or energy to the black hole, like it gets stronger, so it pulls stuff in more and so there's less leakage. And so it looks colder. You know what I mean? Like the more mass the black hole has, the better the cooler is that is keeping it hidden from us. And so maybe it just looks colder to us. Yeah, in which case, maybe black holes don't follow these laws of black body radiation that everything else in the universe does, right? Because they have this event horizon. Black body radiation can describe everything in the universe that has charged particles. So, for example, dark matter also doesn't follow that rule because dark matter doesn't have electric charge. And so it doesn't

radiate photons based on its temperature. Maybe the event horizon of a black hole also prevents this kind of radiation. And so maybe there's a mismatch then between the temperature we observe from afar and the actual temperature inside a black hole in a more intuitive sense. I call it the cooler theory. It's definitely cooler than any theory I've heard before. Yeah. Should we come up with a great acronym for it? Better than JCT, Jorge's cooler theory. There you go. Well, I feel like what you're saying is that to answer the question, whether a black hole is hot or not, the answer is we don't know. You know, it's sort of like asking, is that dress blue with black stripes or what with gold stars? It's like, it depends on who you ask. Yeah, because we don't know what's going

on inside a black hole microscopically, we don't know what its true temperature is. We have this weird clue that black holes do radiate a little bit. And that tells us something about what might be happening inside of them. But we don't know if it really is a clue about the microscopic nature of the black hole or just the black holes leak in a weird different way than anything else. Right. Because even this leakage theory, this Hawkins radiation, is still theoretical. Like, we haven't seen that hawking radiation in actual life. That's right. We have never ever seen any hawking radiation from anything. We've looked for black holes evaporating rapidly and giving bright flashes at the end of their life, but we've never seen. If we could create black holes at the

Large Hadron Collider, then we could see them evaporate on short time scales. That would be exciting. But nobody has ever seen hawking radiation. So is a black hole hot or not? I feel like you can't really swipe right or left here. You have to just close the app? I don't know. What do you do? I think black holes are either zero or very, very, very, very cold. There's no chance they're actually hot. I see. You're saying, does it matter

if they're hot or not, they're pretty cool. Yeah, exactly. They're still pretty cute. You would still date them, I think is what you're saying. I definitely want to hang out

with black holes, yes. Forever. All right. Well, it's an interesting question to tackle. And I think it tells you a lot about what we still don't know about the universe.

You know, there are still places that we know exist and our author and we have pictures of, but where our theories about the universe break down and we need to come up with new theories that maybe people out there are working on or that will work on. And our efforts to understand the universe are usually through the lens of

concepts that we understand, motion and temperature. And sometimes those very concepts break down in the weird contexts that are our universe. All right. Well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. Stuff Podcast Season 2 takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission. One Tribe, save my life twice.

Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart radio app, Apple Podcast, or wherever you get your podcast. Every case that is a cold case that has DNA. Right now in a backlog will be ideas. identified in our lifetime. On the new podcast, America's Crime Lab, every case has a story to tell, and the DNA holds the truth. He never thought he was going to get caught, and I just looked at my computer screen. I was just like, ah, gotcha.

This technology's already solving so many cases. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, we're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. Avoidance is easier.

Ignoring is easier. Denials easier. Complex problem solving. Takes effort. Listen to the psychology podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast. Thank you.