Listener Questions 60,491: Mini solar systems, greek physics and black holes!

00:00:08
Speaker 1: Hey, Daniel, So we're answering more listener questions today.

00:00:11
Speaker 2: Oh, yes, we are.

00:00:13
Speaker 1: What number is it today?

00:00:14
Speaker 2: You know, I sort of gave up on giving them numbers in order why. I figured, you know, we just go on vibes rather than being limited by the traditional numbering system.

00:00:24
Speaker 1: So we're just doing random numbers now? Or are their secret codes embedded in the numbers.

00:00:29
Speaker 2: I'm not just going to give that answer away right now.

00:00:33
Speaker 1: I see. So you were just picking numbers out of the.

00:00:35
Speaker 2: Air, just the same way we answer questions, picking answers out of the air, and sometimes it aligns with reality.

00:00:42
Speaker 1: You're just a physicist of typing on a typewriter, one of many, and then statistically, eventually one of you will type out the answers to the universe. Maybe you'll be the number of this episode. Hi am Joege, a cartoonist and the author of Oliver's Great Big Universe.

00:01:15
Speaker 2: Hi. I'm Daniel. I'm a particle physicist, professor at UC Irvine, and I hope to one day type in the true explanation for the universe.

00:01:24
Speaker 1: Like you come up with it yourself or are you're just taking notes?

00:01:28
Speaker 2: Well, you know, Mozart said he was basically taking dictation from God. So maybe I can just be a vessel for the truth.

00:01:36
Speaker 1: I say, you're born again? Is that what's going on?

00:01:41
Speaker 2: If I could one day be the Mozart of physics, I feel like that would be pretty good.

00:01:45
Speaker 1: Maybe they'll call Mozart the White Sun of music.

00:01:52
Speaker 2: Oh man, that's so ridiculous. I can't laugh about it.

00:01:57
Speaker 1: I just blew your mind. But you got a dream big.

00:01:59
Speaker 2: That's like at my university, Rice, they got tired of people calling them the Harvard of the South, so they printed up a bunch of shirts saying Harvard is the Rice of the.

00:02:08
Speaker 1: North aspirational t shirts.

00:02:13
Speaker 2: Yeah. Yeah, it's a clever joke, but not really, folks.

00:02:16
Speaker 1: So if you're the most sort of physics, would that make me the Soliary of physics? Or am I the Mozart of podcast hosting?

00:02:25
Speaker 2: No, man, you're the Beethoven of cartooning.

00:02:28
Speaker 1: There you go.

00:02:30
Speaker 2: I think maybe we should get Bach to physics though, hmm.

00:02:34
Speaker 1: Or maybe there's a bag of basket weaving already. But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.

00:02:43
Speaker 2: In which we do our best to compose a symphony of understanding to bring into harmony your brain with the universe. We think that everything that's out there is somehow following physical laws, laws that are rational, that are mathematical. That makes sense to our tiny little human brains. Our goal is to bring you up to speed with a current effort to write down the opera of the universe and perform it for you.

00:03:08
Speaker 1: That's right. We try to belt out the melody of the loss of physics and also singing the praises of the amazing scientists that are out there trying to discover what is the song of the universe and in what key is it in?

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Speaker 2: Do you think when we have the final melodic understanding of the universe we'll find it beautiful and harmonious? What if it's like Philip Glass, just like a bunch of random notes.

00:03:32
Speaker 1: You mean, like, are we gonna like the music of the universe? Yeah? Or is it not going to be of our taste?

00:03:38
Speaker 2: Yeah? Exactly. For the same reason, I wonder why we often find the earth beautiful in random landscapes. We're like, oh wow, look how pretty? Why do we find it pretty? Does that mean we're likely to find the explanations for the universe also to be pretty or beautiful or just sort of subjectively appealing.

00:03:54
Speaker 1: M you mean to give us thumbs down? If you didn't like it, you're like, oh, that's the universe. Swipe left, no thanks.

00:04:01
Speaker 3: Yeah.

00:04:01
Speaker 2: I wonder if there are aliens out there that have evolved on their planet and they're like, man, this place is kind of a dump, and they would also think the universe is just kind of blah, whereas it's sort of human to find our surroundings beautiful, or maybe it's not. Maybe that's universal.

00:04:15
Speaker 1: Who knows, Well, don't I say that beauty and our appreciation of what we find beautiful is maybe genetic, Like we've evolved to like certain colors and certain landscapes just because maybe it helps us survive. Maybe only the physicists who find the universe beautiful are the ones who are going to survive.

00:04:35
Speaker 2: How does it help us survive to like trees and hills and waterfalls and that kind of stuff?

00:04:41
Speaker 1: Well, all of those things are signs of life, right, Like it's helpful to like water and green, right, So.

00:04:49
Speaker 2: What does it say about me that I like desert scenes?

00:04:51
Speaker 1: It means maybe you were born in Israel.

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Speaker 2: Or have very dry tastes.

00:04:58
Speaker 1: All of your ancestors they didn't like deserts. Probably, uh, they didn't quite make it me.

00:05:03
Speaker 2: Yeah, but none of my ancestors were born in the desert, so that doesn't really work anyway. It's not a time to dig into Daniel's evolutionary history.

00:05:11
Speaker 1: Yeah, the whites and line is not our topic here today. Our topic here today is questions. Questions that scientists ask about the universe, that they ask about how things work, and that trickle down to what everyday people are asking about their everyday lives.

00:05:25
Speaker 2: That's right. We are all curious about the music of the universe. Is it possible to make sense of it? Can we boil down everything that's out there into ideas that fit into our heads? And we encourage you to also ask questions about the nature of the universe. And we want to hear your questions. We want to hear from you when things don't make sense, when you need a little bit of help sticking ideas together and explaining everything that you see out there in the universe.

00:05:51
Speaker 1: That's right. Sometimes people send us questions and sometimes we answer them here on the podcast, and so to the on the program, we'll be tackling listener questions. Now, Daniel, you gave this one an odd number. What was the number?

00:06:08
Speaker 2: You gave it six hundred and seventy two, four hundred and ninety one.

00:06:13
Speaker 1: And that number was that random? Or is this your go to random number?

00:06:18
Speaker 2: That's not my go to random number. Last time you were making fun of me for keeping track of these episodes since they'd gotten to such a large number. So I decided, let's just go with vibes. And I felt like, what number does this episode want to be? And that's the number I typed out.

00:06:33
Speaker 1: Interesting, so this episode talked to you even though we haven't recorded it yet.

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Speaker 2: That's right, We had met yet recorded. But of course I thought about it in advance, and I thought, what number does this map to? If I had to pick a number for this episode, what would it be? And that's what came out. I see.

00:06:48
Speaker 1: So it basically it means nothing.

00:06:52
Speaker 2: It's a one dimensional representation of the high dimensional complexity of the ideas in this episode.

00:06:58
Speaker 1: So yeah, nothing, that's right, because we have no big ideas here on the program, only tiny ideas.

00:07:05
Speaker 2: Our big idea is to stimulate crazy questions and ideas in your mind out there. So thanks very much to everybody who sends in your questions. Still be shy, write to us two questions at danieland Jorge dot com. We will answer everybody's questions and we might even pick yours to talk about here on the podcast.

00:07:22
Speaker 1: Yeah, like we're doing here today. And so today we have three great questions from listeners. We have a question about micro's solar systems, but whether the universe really is all Greek to physicists, and also a question about whether a black hole could destroy our planet, so to cheerful.

00:07:40
Speaker 2: Topics cosmic importance, I.

00:07:44
Speaker 1: Am getting six hundred seventy two four one hundred and ninety one vive here.

00:07:48
Speaker 2: Yeah, right, just feeling it, aren't you? You're feeling it?

00:07:51
Speaker 1: No, I was being totally totally fake there. So let's jump into our first question, and it comes from Tim.

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Speaker 3: Hey, Daniel, curious if there is such a thing as a small solar system out there somewhere, maybe a place where the planets are the size of marbles or something. I don't think so. I would imagine for this to happen, the star would have to form, which would require a lot of material. But I was just wondering if physics supported us.

00:08:23
Speaker 2: Thanks all right.

00:08:25
Speaker 1: I think it's kind of ironic that Tim is asking about tiny solar systems. You know, tiny Tim.

00:08:33
Speaker 2: Maybe tiny Tim is an ironic nickname, and Tim is like a very large dude.

00:08:38
Speaker 1: A huge dude like you to John from Robin Hood.

00:08:43
Speaker 2: Exactly. But I love that Tim's question is about scales because I often wonder about that in the universe, like why do things form in the universe at certain sizes and not other sizes? What does that reveal about the fundamental workings of the universe. Can you have solar systems that are super duper tiny and super duper huge, or is there a reason why galaxies tend to be a certain size and stars tend to be a certain size and planets a certain size? What does that teach us about the fundamental rules of the universe and how they come into balance.

00:09:14
Speaker 1: Yeah, pretty interesting question about scales out there. And I think we've tackled like the what's the largest planet? But this question is interesting because it talks about, you know, what is the smallest solar system you could have?

00:09:25
Speaker 2: Yeah, And there's a couple of different angles here, like can you have a miniature sun, What is the smallest possible star you could form? And also how small can you make a planet? So I think both of these are really interesting questions.

00:09:38
Speaker 1: Yeah, pretty cool, So maybe let's tackle them one at a time. I mean, to a solar system, you need a soul as a sun. So what's the smallest sun that you could make or have?

00:09:48
Speaker 2: So if you want your solar system to have a sun in the middle of it, and by sun you mean something that's giving off energy, that's fusing, that's creating light and heavier elements at its core, then there is a minimum size there because you need enough stuff for gravity to have enough power to compress the core of this object to be high pressure and high temperature and for fusion to start. There's a reason, for example, why the Sun is glowing and Jupiter is not. Even though Jupiter is an enormous ball of hydrogen and other stuff which could fuse, there isn't enough gravity in Jupiter to kick star fusion at its core. There's not enough pressure in temperature to get that going. So there is a threshold there. There is a minimum size you can have for a star to begin fusion to begin.

00:10:36
Speaker 1: But could there be one that has already started, or what if you kickstart one, would it just blow everything away?

00:10:42
Speaker 2: Yeah, that's a cool question. Essentially, you're asking could you ignite a star by injecting fusion into it and then let it go the way you can? For example, have a piece of wood sitting in front of you. It's not burning, but if you heat it up, then it will actually ignite and the rest of the fuel will burn. That I don't think will happen with fusion, because fusion needs a certain temperature and a certain pressure in order to be sustained, Like, it'll just fizzle out if you don't maintain those conditions.

00:11:08
Speaker 1: You need a certain amount of density too, right.

00:11:10
Speaker 2: Yeah, yeah, because fusion is hard to accomplish. You know, protons don't like to stick together with other protons. They're both positively charged. They push against each other pretty hard, and so you got to really squeeze them together. You got to make sure it's enough density that have nowhere else to go in order for some fusion to happen. And even if you do that, mostly the protons are not fusing. That's why stars can burn for so long, because there's comparatively little fusion happening. Most of the protons in the center of the Sun are not fusing with other protons. It's a tiny fraction where that's actually accomplished. And that only happens when you have an object like eighty times the mass of Jupiter. So you take Jupiter and you add like seventy nine more jupiters, then you'd have enough mass to create the conditions at the core to make a star.

00:11:57
Speaker 1: Like it will start burning on its own, for sure, or do you still need something to kind of trigger it.

00:12:03
Speaker 2: It'll start burning on its own. And that's how all stars have started. You know, enough mass has coalesced together to create the conditions, and then fusion is inevitable because there's enough protons banging against other protons and they fuse and they create energy and that sustains itself.

00:12:19
Speaker 1: Don't you need sometimes like a supernova to go off somewhere for you to start burning a star.

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Speaker 2: You don't need a supernova for the star to start burning. You do need something to trigger the gravitation or runaway effect to form the star. Like you might have a huge cloud of gas and dust, you know, just like a big molecular cloud. What makes it actually collapse into a star is a little bit of a question. You might just have like a seed of a slightly heavier element from something else, or you could have a shockwave from a supernova that just like triggers the collapse. That collapse still takes millions and millions of years. And then once the star forms, fusion starts at the core.

00:12:58
Speaker 1: Could a supernova trigger a tiny star to start burning or not? Or as you said, it just wouldn't sustain itself.

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Speaker 2: Yeah, it just wouldn't sustain itself because it's not hot enough and there's not enough pressure. So even if you like created your own fusion at the heart of a star, that heat would then just dissipate and wouldn't ignite more fusion. You need that temperature and that pressure, like we do that here on Earth and magnetize fusion and we can get a few fusion reactions to go. But unless you have enough temperature and pressure and density, it doesn't ignice so that it's self sustaining.

00:13:29
Speaker 1: So then what's the smallest star we could get or have in nature?

00:13:33
Speaker 2: So in order to have our kind of fusion, like the kind of fusion in our star, you need eighty times the mass of.

00:13:38
Speaker 1: Jupiter eighty times the mass. But then how big would that sun be? It wouldn't be eighty times bigger than Jupiter necessarily, it might be more compact.

00:13:46
Speaker 2: Oh that's right, it wouldn't be eighty times the volume of Jupiter because it'd be much more dense. But we have lots of small stars in the galaxy. Of fact, our star is quite heavy and quite big compared to the average star, which is a red dwarf. So red dwarfs go down to like ten percent of the mass of the Sun.

00:14:01
Speaker 1: I'm asking, like, how big is it? Is it bigger than Jupiter? Is it smaller than Jupiter? What's the diameter of this smallest sun?

00:14:09
Speaker 2: So these things are not actually much bigger than Jupiter because as you add more mass to them, they just get denser and denser, in the same way that like a rocky planet can't get much bigger than Earth. It just gets denser and denser. There are stars out there whose radius is not much bigger than Jupiter, but they are burning. There are red dwarfs, and they have fusion at their core, so they wouldn't be much larger than Jupiter. They're just a lot more massive.

00:14:33
Speaker 1: Interesting, so the lower limit for the size of a star is about the size of Jupiter.

00:14:38
Speaker 2: Then yeah, order magnitude Jupiter. So you couldn't have a solar system the size of marbles, but you could have a Jupiter sized star at the heart of a solar system.

00:14:49
Speaker 1: But then what is a brown dwarf.

00:14:50
Speaker 2: Yeah, so if you require the kind of fusion that happens at the heart of our star, like true fusion, then you need that much mass. But it is possible to have a different kind into fusion happening at the heart of a star, called deuterium fusion, and this happens sometimes in objects we call brown dwarfs, which are just like big jupiters. You have like ten times the mass of Jupiter, then you can get this other kind of nuclear fusion going where deuterium and protons combined to make helium three and it releases a little bit of heat, but it's not as hot and not as productive as the kind of fusion that happens at the heart of our star. So brown dwarfs are called brown because they don't like glow. They don't give off photons the same way that red dwarfs, and like yellow stars like our sun, dom.

00:15:34
Speaker 1: So they're more like simmering, but they're still burning inside. So what do they look like from the outside of your standing next to one, like a glowy Jupiter or what.

00:15:42
Speaker 2: There's a whole range of course of brown dwarfs from low temperature to higher temperatures. Some of them do glow. They don't glow as bright as the Sun, but they do glow brighter than Jupiter of course, And of course Jupiter gives off a lot of radiation on its own, but not like a brown dwarf. So yeah, it'd be sort of like intermediate between Jupiter and an actual sun.

00:16:01
Speaker 1: And how big would those be?

00:16:02
Speaker 2: Those are again not much bigger than Jupiter, right, because as you keep adding masks, it doesn't get larger, it just gets denser.

00:16:09
Speaker 1: But it would maybe only be like ten times more massive than Jupiter, in which case, like a solar system with a star like that would maybe look a lot like Jupiter with its moods.

00:16:18
Speaker 2: Yeah, exactly, something like that. So definitely not marble sized, but smaller than our solar system for sure.

00:16:25
Speaker 1: And would that sun give enough light to maybe sustain life or I guess it depends on what kind of life.

00:16:32
Speaker 2: There's a lot of debate about whether you can have life around a brown dwarf, whether there is a sort of a habitable zone around there. I don't think we know enough about brown dwarfs to say for sure.

00:16:42
Speaker 1: M I guess it maybe depends on the size of the planet, which is sort of Tim's other part of the question, which is maybe what's the smallest planet you could have.

00:16:53
Speaker 2: Yeah, this is a really interesting question, and it sort of comes down to the nature of like how you define a planet and also like how things get right because there's a lot of stuff out there in the Solar System. You know, there's rocks like the Earth, but there's also tiny little rocks just floating out there all the way down from asteroids to meteoroids to just like grains of dust, And you might ask, like, could you consider all those things planets. Well, we have requirements for what a planet is. Planets have to orbit the Sun, not some other object, which is like why the Moon is not a planet. They also have to have enough gravity to make themselves round, like if you're long in cigar shaped, you're not a planet. This is a way to avoid calling every tiny piece of dust that's orbiting the Solar system of planet. And then the last requirement is that has to clear its neighborhood, has to be like the dominant gravitational thing in its neighborhood. And so that sort of sets an effective minimum size for a planet.

00:17:51
Speaker 1: What would be that size? How many kilometers?

00:17:53
Speaker 2: Yeah, it's interesting. It depends a little bit on what it's made out of. Like, first of all, like why does gravity make things round? Well, gravity makes things round because it likes to pull stuff down. You know, imagine a spherical planet, then you put a very tall mountain on it. Eventually gravity is going to pull that mountain down, especially if there's weathering effects on the planet that shake things or tectonics, et cetera.

00:18:14
Speaker 1: And by down you mean like the center of whatever it is.

00:18:17
Speaker 2: Right mm hmmm yeah, And so sphere is like the gravitationally most stable object. If something is a perfect sphere, there's nowhere for anything to fall further down. So gravity likes to make things round, but it can't always do that, right, Like you take a pencil, a pencil has its own gravity, but not enough for gravity to make it into like a round ball. Right, So the amount of stuff you need in order to become round also depends a little bit on your mass. So as things get bigger, they have more gravity, and then gravity can overcome the internal strength of an object and make it round. And so there's this size called like the potato radius, around which gravity starts to round things out.

00:18:57
Speaker 1: I feel though that maybe that's a like a very human set a definition of a planet, Like we just want our planets to be round. But I wonder if there could be, you know, some body out there is some oddly shaped asteroid or giant rock that maybe has evolved life, and then those aliens would call their little rock a planet, right, or at least they would call it home.

00:19:18
Speaker 2: For sure. Our definition of planet is human centric. We made this thing up, and you're right weak interested in planets because that's the first thing we noticed. We live on one. To us, it's the most important part of the Solar System. In reality, the Solar System is failed the huge, beautiful spectrum of different shaped objects. And this is just like people drawing dotted lines around certain kinds of them and then trying to defend those dotted lines even though they're ridiculous and arbitrary. Yeah, you could probably have life form on not around objects and then those aliens might think that our distinction is arbitrary. But you know, Tim is a human. He's asking questions from a human perspective.

00:19:55
Speaker 1: Is he?

00:19:55
Speaker 2: Now, I'm not veteretan.

00:19:58
Speaker 1: I mean, if I'm so tiny, then maybe he's.

00:20:02
Speaker 2: But the minimum size to make something round if you're like made of ice or water is like four hundred kilometers. That's when things tend to form like potato shapes. If you're made of rock, it's about twice that big, and so you need like eight hundred kilometer wide. Stuff tend to be pretty round. So that's like the minimum size for an object for gravity to make it roundish ooh.

00:20:26
Speaker 1: Well, I feel like maybe also are your definition of a sun is also the human centric because that's what happens in our sun is the regular kind of fusion. But aren't some stars out there at that astronomers call stars, aren't they just like really hot rocks.

00:20:41
Speaker 2: Well, there are objects out there called white dwarfs, which are the remnants of stars. They're not burning anymore, there's no fusion happening at their core. They're leftover bits of stars that have ended their life. And those are called like white dwarfs.

00:20:56
Speaker 1: And you can see them in the night sky sometimes, right.

00:20:58
Speaker 2: I don't know that you can see them with the naked eye. We have detected a few of them, but they're pretty hard because they don't glow the same way that other stars do.

00:21:06
Speaker 1: But you still call them stars though, right, because they're emitting a lot of radiation.

00:21:10
Speaker 2: They do emit a lot of radiation. I don't know if astronomers would call them stars or stellar remnants, but yeah, they're definitely related to stars.

00:21:19
Speaker 1: Because I wonder if you can just have maybe like a super duper hot marvel out there in space, and then have really really tiny marbles orbiting around that hot marble, and then maybe you can call that a solar system, and maybe things have evolved in those teeny tiny marble planets and they're living in their little marble existence, marveling at how small they are.

00:21:42
Speaker 2: Yeah, that's certainly possible. You could take a scoop of neutron star material, right, that'd be a hot blob of stuff, and you could put a few rocks in orbit around it, and you could call that a solar system.

00:21:54
Speaker 1: Sure, right, and then the thing in the middle of the hot stuff in the middle would be admitting light and that might sustain life in those little tiny rocks or being around it.

00:22:02
Speaker 2: Yeah. Absolutely. I don't know how long that would last because you don't have that much stuff, and so it's going to radiate its heat out pretty fast. I don't know if there's really time for life to evolve under those circumstances. But you know, if you're setting up a little vivarium in space, you could put some microbes on there and then you could say, hey, look, I have a solar system with life on it.

00:22:20
Speaker 1: Yeah. Yeah, it'd be like instead of sea monkeys, the space.

00:22:24
Speaker 2: Monkeys exactly, space monkeys.

00:22:27
Speaker 1: Well, it sounds like the answer for tim is that you could have a teeny tiny marble sized solar system. Like that's technically possible if you make it, but maybe naturally occurring. It'd be kind of uh hard to find something like that.

00:22:40
Speaker 2: Yeah. The universe has certain scales because of the forces of gravity and the strength of other materials. Gives us a minimum size for stars and a minimum size for planets. I think that's really interesting and it really does reveal something about the balance of the various forces in our universe.

00:22:58
Speaker 1: Yeah, because in the end, everything is limited by physics, right, not our imagination.

00:23:04
Speaker 2: I'm certainly limited by physics.

00:23:05
Speaker 1: Yeah, all right, well, thanks Tim for that question. Let's get to our next question, and this one is about the Greeks and what they knew about the universe, So we'll dig into that, but first let's take a quick break.

00:23:31
Speaker 3: Right.

00:23:31
Speaker 1: We're answering listener questions here today, and our next question comes from Michael.

00:23:36
Speaker 4: I thought about an off hand comment you made many episodes ago about things we still don't understand that the ancient Greeks didn't understand. So my question tonight is what are the three most opique things we still don't have a good answer for since the Greeks, and what is our best understanding of them now? And what was the most common ancient understanding of them?

00:23:56
Speaker 2: All?

00:23:56
Speaker 1: Right, awesome question from Michael. He's basically asking what is still all Greek to us?

00:24:05
Speaker 2: Yeah, apparently I made some comment about how there are still questions we haven't answered that the Greek's puzzled about, which is super awesome because it just means that some of these questions are timeless. It also excites me to imagine that one day we might know the answers that in the future. Physicists and philosophers could look back at Us and lump Us in with like Aristotle as the clueless, pre knowledge kind of human before we really understood how things were.

00:24:33
Speaker 1: I see. Well, in that case, would being called the Aristotle of the twenty first century be a good thing or a bad thing?

00:24:39
Speaker 2: Oh?

00:24:39
Speaker 3: Yeah?

00:24:39
Speaker 2: Being the Aristotle of anything would be awesome? Well that is super smart? Yeah, yeah, yeah, I wouldn't mind being the Aristotle of baking. I could be the Aristotle of naps. That sounds wonderful.

00:24:51
Speaker 1: You do it in a toga anyways? Right, all right, Well, I guess Michael's question is kind of a little bit straightforward. He saying, what did we not know back then when the Greeks were hanging on and thinking about the universe that we still don't know today?

00:25:06
Speaker 2: Yeah, great question, Michael on The answer is pretty basic stuff, you know, questions that like your kids might ask you. So, question number one is like, how did the universe come to be? Did it come to be?

00:25:19
Speaker 3: Right?

00:25:19
Speaker 2: Really? What is the age of the universe? What is its origin? This is not something we know the answer to, and definitely something the Greeks didn't understand.

00:25:29
Speaker 1: Is there a record of the Greeks asking this question or wondering about it.

00:25:33
Speaker 2: Oh, absolutely, and a lot of Greek mythology tries to explain how they thought the universe came to be. But their answers were not at all scientific, you know, as much as the Greeks made progress in trying to understand the nature of the universe around them, the answers to the question of the origin of the universe really read like pure mythology. You know, they talk about gods that created the air, a god named Chaos, and then another that created the earth, and you know, created Mount Olympus, and then battles between these gods that formed the earth that they know about, and then came forth darkness, and then came forth the night. The whole thing just reads like pure mythology.

00:26:16
Speaker 1: But I guess that's mythology and religion in Greek times. But I wonder if everyone back then believe these things, or did, maybe like Aristotle, think have other questions or maybe other theories about the beginning of the universe.

00:26:30
Speaker 2: It's certainly not possible to know for sure what all the Greeks thought about these things, because remember, we have a tiny fragment of all the Greek writing the Greeks were very prolific, thought about lots of things, wrote about a lot of things. And when we talk about our knowledge of the Greeks, we talk about what has survived, and that's really a tiny fraction of everything that was written. And in many cases we have only like references to discussions about other topics that Aristotle mentioned. I'm sure that there are people thinking about the origin of the universe in Greek times that we have not captured. And actually we live right now at a very exciting moment because we're using new technology to read ancient scroll that have never been read before, ones that were burned in fire and in volcanic destructions, where we have like the Charred Scroll, but we never have read it before because if you try to open it up, it just falls apart. And now to have new scanning technology that can read the scrolls without unrolling them, and so for the first time we're discovering huge numbers of new Greek writings, which is going to change completely our understanding of what the Greeks thought. But that said, even Aristotle and these other guys, they're basically philosophers, and at the time these were the cutting edge ideas for how the universe came to be?

00:27:42
Speaker 1: Right, right, Well, I wonder if he had asked Aristotle so he thought the universe began? What do you think he would say? Would he say zeus or would he say beats me?

00:27:51
Speaker 2: I suspect the Aristotle would say that he didn't know. I mean, Aristotle and the Greeks had trouble even conceiving of the cosmos. Like this question, we're asking, where did the universe come from? They didn't even really understand the universe the same way that we do. They mostly thought about the Earth and the Solar System and a few stars. They had no concept for even like how big the universe was, which we'll talk about in a minute. So it's sort of like asking him a question. It wouldn't even understand the question.

00:28:20
Speaker 1: Right because they just didn't know as much as we do now.

00:28:23
Speaker 2: Yeah, exactly, And yet even today we still don't have an answer to this question. We have a few ideas for where the universe came from. We know at least how old it is, like we can date the evolution of the universe back almost fourteen billion years. That's often quoted as the age of the universe, but we really don't know what came before that. We don't know if there was a singularity and the universe was burst somehow just before that, as general relativity says, or if there was some other weird state that predated that early hot moment that we can describe, and the universe was like infinitely old before that in just some other form that's now beyond our understanding and capacity to describe. So we still don't have any idea really how the universe came to be, how long it's been around. We do have some concept for at least how old it must be, so that's something. But we're still struggling to answer this very basic question that the Greeks wondered about.

00:29:19
Speaker 1: Wait, wait, are you saying that maybe the Greeks could still be right? He said that maybe it's all chaos before the big bank like make them correct.

00:29:29
Speaker 2: I mean, in principle, it could all just be some simulation run by external superintelligent beings and you could call those the Greek gods. So yeah, I guess in that sense, the Greeks could still be right.

00:29:40
Speaker 1: All right, Well, what's another question that we know that they were asking that we're still asking today.

00:29:44
Speaker 2: Another question is how much of the universe is there? Right? How big is the universe? And it's really fun to dig into like what the Greeks thought about the universe, because the Greeks were really careful and trying to understand the nature of the Solar System, the Earth, and the planets in the sun. They're very geometrical, very mathematical about what they saw happening in the night sky. But then it's very difficult for them to think much beyond that, for them to cast their minds and understand how big the universe really was.

00:30:15
Speaker 1: Right, Well, back then, did they believe that the Earth was the center of the universe and that the Sun went around the Earth?

00:30:21
Speaker 2: Yeah? Absolutely, they thought the Earth was the center of the Solar system and everything went around the Earth. And the reason they thought this is not because they were stupid, it's actually because they had a misunderstanding of how big the universe was. Like they considered the idea that maybe the Earth and all the other planets went around the Sun. Absolutely, and they're thinking was, well, if that's true, then we should see the stars move in the sky. And they were right, because as the Earth moves around the Sun, we do see the stars wiggle in the sky. It's called the parallax effect, like we have a different perspective on the stars from one side of the Sun and the other. They looked up at the stars and they didn't see any whigs because they didn't yet have the capacity to see the tiny wiggles of the star. The parallax effect was too small for humans to detect until like the eighteenth century. So they concluded from that the Earth was not moving around the Sun.

00:31:12
Speaker 1: Whoa, they actually had like data to back this up.

00:31:15
Speaker 2: Yeah, exactly. Their mistake was they thought the stars were super duper close and so the parallax should be pretty obvious. Actually, the stars are much much further away than they could even conceive of, and so while the parallax effect is there, the stars are wiggling in the sky, they just couldn't see it. So their mistake in thinking that the stars were much closer than they were is actually what led them to misunderstand the structure of our solar system. But it was very logical and very reasonable.

00:31:43
Speaker 1: Like just one tiny assumption, right, Like one assumption in their thinking and can really throw you off.

00:31:49
Speaker 2: Mm hm. And yet they also knew that the Earth was tiny. We have evidence in their writing, like Marcus Aurelius said that, quote, the whole Earth is a point in space, and Aristotle described the Earth as quote no great size relative to the cosmos. So they knew that the Earth was a tiny dot in space. They just couldn't even conceive of how big that space actually was.

00:32:13
Speaker 1: They knew it was a dot, but actually it's like a super duper tiny micro.

00:32:17
Speaker 2: Dot exactly, It's like a dot of a dot. And today we know that the universe is much vaster than they could conceive of, at least ninety two billion light years across, And yet we don't know if that entire observable universe is just a dot in some much vaster universe, potentially even infinite. So we basically have no clue as to the absolute answer to this question of how big is the universe?

00:32:42
Speaker 1: No, I see so like proportionally like, if the size of the possible universe is from zero to infinity, we're still pretty clueless because infinity is infinity. So we have no idea. Right it could be that we have as much of an idea plus or minus a few percent the Greeks.

00:33:01
Speaker 2: Well, you know, in the same whether we've made progress and understanding the age of the universe. We know at least how old it is, and we know at least how big the universe is, So we've made some measurable progress. But you're right, that could be a zero percent fraction of the actual size of the universe.

00:33:15
Speaker 1: All right, great question for Michael. It sounds like we still have a lot that we haven't figured out since the Greeks.

00:33:22
Speaker 2: Yeah, if we had Aristotle over for dinner, we could teach him a bunch of stuff, But I think in the end he would still be unsatisfied because his fundamental questions have not yet been answered.

00:33:32
Speaker 1: Well, I feel like he would just spend the whole night playing with your iPhone. He'd be like, what is this magic?

00:33:37
Speaker 2: I what now? He'd probably fall for our phishing scheme immediately and send all of our money to Nigeria or something.

00:33:43
Speaker 1: He'd be watching YouTube all night. Probably.

00:33:46
Speaker 2: It makes me wonder if I had dinner with physicists from one thousand or two thousand years in the future, would they have answers to these questions? Would they have made measurable progress but still have no real fundamental answers. That's exhilarating, but also potentially frustrating.

00:34:01
Speaker 1: Nice, and that's why you want to freeze yourself, right, Oh yeah, absolutely, let's do it just for that dinner date in the future, multiple dinner date. Do you have the book it now, I'm gonna leave it. I'm gonna have dinner with your great great great great great great great great grandkids. It's on the calendar. Yeah, don't forget. I sent them a calendar invite. Absolutely, don't forget. To defrost me. Please.

00:34:24
Speaker 2: My daughter sends me calendar invites for like ten years in the future or thirty years in the future.

00:34:29
Speaker 1: Oh yeah, for like asking you for money.

00:34:32
Speaker 2: Yeah, like buy Hazel a car.

00:34:34
Speaker 1: Say hey, Dad, buy me a car in.

00:34:35
Speaker 2: Five years, Yeah, exactly.

00:34:39
Speaker 1: All right, well, thank you, Michael. Now let's get to our last question, which is about whether a tiny black hole could still destroy the Earth. And so let's get it to that question. But first let's take another quick break.

00:35:04
Speaker 3: Right.

00:35:04
Speaker 1: We're answering listener questions here to the and our last question is about the destruction of our planet by a small black hole. The question that comes from Scott. My name is Scott and from medical like Washington, and my question is if a black hole were to suddenly pop into existence about the size of a.

00:35:22
Speaker 2: Pea in the middle of the Earth.

00:35:25
Speaker 1: Would it eat the planet? And if so, about how long would it take? Thank you all right? Another tiny question, I feel like.

00:35:33
Speaker 2: A cosmic question about tiny things.

00:35:35
Speaker 1: And big destruction. Here is Scott's question is that if you suddenly made a pie sized black hole appear in the middle of the planet, would it eat up the planet and swallow us all into this black hole? Or would we survive?

00:35:51
Speaker 2: Yeah? Great question?

00:35:53
Speaker 1: And how long would we be alive if it ends up killing us?

00:35:57
Speaker 2: How much time do you have to pack and board a ship off the planet? Right?

00:36:01
Speaker 1: How long can you taken that before waking up to meet your doom in your toga? Well, Daniel, what's he asked for this question? What are some of the basics of black holes? We need to know?

00:36:14
Speaker 2: So remember that black holes are like a deformation in space and time. They are a place where there's so much energy concentrated in a tiny little spot that space has curved so much that you formed an event horizon, which means nothing can ever escape. An important thing to understand is black holes can basically eat anything. You throw anything into a black hole, it just adds to its energy. You drop into shark, it gets more massive and it gets bigger, right exactly, There's a close relationship between the mass of the black hole and its radius, and so as you add stuff to the black hole, it tends to grow. The only exception to this is if you had a black hole isolated in space, it would emit hawking radiation and gradually disappear. So if you don't feed a black hole, it will eventually fade out of existence. But any black hole that's near stuff, it's going to grab that stuff, pull it in and grow. There's no way for the Earth to hang out with like a black hole inside of it without that black hole eventually eating the planet. So Scott's question number one is like, is it going to eat the planet? And the answer is yes. So do you have a black hole the heart of the planet. That black hole already has a lot of gravity. He's talking about a black hole the size of a pea. Well, a black hole the size of pea doesn't sound very large, like a pea doesn't have much gravity, but to make a black hole of that size would require about as much mass as the Earth. So now you're talking about about as much mass of the Earth concentrated down to a pee that's gonna have a very strong gravitational effect. Put it anywhere inside the Earth, it's going to pull that part of the Earth apart. It's going to feed itself on the Earth, and then it's going to grow and get more gravity, and you have a very rapid runaway effect there where everything eventually falls into this new black hole.

00:37:56
Speaker 1: Right, because I guess if you PLoP it in the middle of the Earth, it has material to eat right away, right, Like, what would happen if you PLoP a p sized black hole in the middle space with nothing around it? Would it just evaporate or would it stick around for a long time?

00:38:09
Speaker 2: Yeah? Absolutely, If you don't feed a black hole, it will evaporate. How long it takes to evaporate depends on its size, because the Hawking radiation depends on the temperature, and the temperature is connected to the size. And actually bigger black holes are colder, tend to radiate more slowly, So the smaller the black hole, the more the radiation. So really big black holes very gently radiate and shrinks very slowly. As they get smaller and smaller, they radiate faster. And faster till eventually they disappear in quite a bright flash of light, which would make evaporating black holes from hawking radiation something we might be able to see out in the night sky. We've never actually seen one before, but people are looking for it. But yeah, you put it in the center of the Earth, it's going to gobble it.

00:38:50
Speaker 1: Up, right, It's going to start eating the Earth, and then it's going to grow. But then I wonder if it would eat all of the Earth, Like I wonder if at some point we'll eat the inside of the Earth, but maybe it'll leave a space between it and the outer shell of the Earth.

00:39:05
Speaker 2: Yeah, it actually gets a little bit complicated. What we often do in physics is just start with like the simplest scenario. Imagine all the mass of the Earth is just like a bunch of balls on the surface of the Earth and they all fall into the black hole. And you can ask, like how long does it take all of that mass to fall into the black hole? And so that would take about ten ish minutes for all the mass of the Earth to fall into this central black hole. That's like the fastest it could possibly happen, you know, like around ten minutes before everything actually reaches the black.

00:39:35
Speaker 1: Hole, Like if it just goes straight in.

00:39:37
Speaker 2: Yeah, if it just goes straight in, And that's the most efficient, the fastest way.

00:39:41
Speaker 1: And then you made this calculation, how just by like measuring how long a ball on the surface of Earth would take to fall into the center of a black hole in the middle of Earth.

00:39:49
Speaker 2: Yeah, essentially simple gravity for a simple object, not interacting with anything else, And that's the fastest possible thing. Nothing is interfering with these objects. But that ignores a lot of it important physics, like the Earth is spinning, and what would happen if the black hole actually did start to eat the center of the Earth is black holes have tremendous tidal forces, and those tidal forces would heat up the stuff near the black hole. Like surrounding every black hole we actually see out there in the universe is an accretion disk of stuff that's swirling around it and hasn't yet fallen in. So this new black hole in the center of the Earth would compress and heat up the stuff around the Earth, and it would create a lot of radiation which would push stuff out away from the black hole. So what we see in reality out there in the universe is that there's a maximum rate at which black holes can grow because they have very strong gravity to pull stuff in, but they also tend to create accretion disks, which create radiation to push stuff away. So black holes cannot form super duper quickly because of this effect.

00:40:49
Speaker 1: Because if things bolt into quickly, then they tend to explode as they go in, which would push the stuff that's in line to fall in away exactly.

00:40:56
Speaker 2: And stuff is also rotating right, so it doesn't fall in immediately. It tends to swirl around the black hole before it falls in. It gets heated up, and it radiates and it pushes out on other stuff. And so while the minimum time for this to happen is like ten ish minutes, in reality it would probably take much much longer because it would form an accretion disk, and it would take a long time for that accretion disc to all fall into the black hole. Eventually, it still would, but.

00:41:22
Speaker 1: It would take much much longer, Like how much, Because I think that's the question.

00:41:26
Speaker 2: Yeah, I don't have a solid answer for that, because that requires calculations that are very complicated and require like really intense gravity and multibody simulations. I'd estimate thousands of years or maybe even millions of years before the last bits of the Earth actually do fall into this new black hole, but the Earth would very rapidly be uninhabitable. Like this is answering the question of when the last tiny bit of earth dust falls into the black hole. That might take thousands or millions of years, but still you would not want to be on the surface of the Earth for more than a few minutes after that black hole is created, because it's going to get very unpleasant very quickly.

00:42:04
Speaker 1: Like it's going to basically break up the Earth right shatter it. Yeah, exactly, they can collapse under its own sort of weight.

00:42:10
Speaker 3: Hm.

00:42:11
Speaker 2: Ten minutes is the fastest possible time for the Earth to disappear, which tells you that the timescale for effects to appear on the surface of the Earth is minutes. So the Earth is going to get pulled apart and distorted and all sorts of craziness. That's going to happen within minutes, even if the complete hoovering up all the last bit of the Earth might take much longer because of the feedback effect from black hole radiation.

00:42:35
Speaker 1: So all benus you want to get on.

00:42:38
Speaker 2: That spaceship asap. That's the answer.

00:42:41
Speaker 1: Isn't there some science fiction story where they some aliens have created a planet with a black hole in the middle, like that's powering it or something.

00:42:48
Speaker 2: I haven't read a book like that. In principle, it's possible to have a stable structure around a black hole, like if it's strong enough and far enough away and thin enough to avoid tidal forces, then yeah, you could build a big spherical structure kind of like a disin sphere around a black.

00:43:05
Speaker 1: Hole and a giant shell, right.

00:43:08
Speaker 2: Yeah, like a giant shell. And if you pour stuff into the black hole, you could use the hawking radiation from the black hole as an energy source. We even talked about using black holes to power a space ship, have a whole episode about that. So, yeah, all these things are potentially possible, but it depends a lot of the details, and you don't build that kind of thing by just plopping a black hole at the center of an existing planet.

00:43:30
Speaker 1: And it's also unstables that I remember about it, but I guess maybe then. An extension of Scott's question is what's the smallest black hole we could survive? Or is it that any size black hole there is plopped in the middle of the Earth would eventually swallow us up.

00:43:47
Speaker 2: Yeah, any size black hole that's plopped in the center of the Earth gets bigger, and so there is no minimum size we could survive because every black hole just grows to be a larger black hole.

00:43:57
Speaker 1: But don't you talk about in the Large Hadron Collider make black holes or you might be making black holes, but they disappear very quickly.

00:44:04
Speaker 2: Yeah, if you made a super tiny microscopic black hole in a vacuum, like we might do with the Large Hadron Collider, then it could evaporate too quickly to absorb any other material. If you put it in the center of the Earth, then you have no chance. Even a microscopic black hole will eventually get bigger. But if you make a microscopic black hole and a vacuum, it'll evaporate before it's dangerous.

00:44:24
Speaker 1: Unless you created knocks on the wall of the Large pattern Collider, would that be bad news?

00:44:30
Speaker 2: Black hole we make at the LHC will actually have a lot of velocity, so it's not just going to stay there inside the LEDC and actually fly off and just go through the Earth or into outer space before it has a chance to eat anything. They've done all these calculations, and they're pretty convinced that it's not dangerous. So you know, I mean, fundamentally, we do not understand microscopic black holes because it requires a theory of quantum gravity to predict what would happen. Because now you're talking about gravity of quantum objects, which are uncertain, and so it's not something we really know how to calculate. And all the calculations done for like black holes with a large Hadron collider, they use a semi classical approximation of quantum gravity in which nobody really believes.

00:45:14
Speaker 1: Anyway, you're not really making me feel more comfortable.

00:45:17
Speaker 2: Here, Daniel. I wasn't the goal.

00:45:19
Speaker 1: Your assurance is only getting worse.

00:45:22
Speaker 2: I'm telling you we don't really understand super microscopic black holes anyway.

00:45:26
Speaker 1: I see, And so therefore we should believe you when you say not to worry about it.

00:45:30
Speaker 2: I didn't say not to worry about it. I said they've done the calculations, and they tell us not to worry about it.

00:45:34
Speaker 1: Uh huh. And now you're telling us.

00:45:37
Speaker 2: I'm saying I'm not worried about it. Oh well, in that case, but remember you can always check the website. Has the large Hadron collider destroyed the world yet? Dot com which we always keep up to date.

00:45:50
Speaker 1: Right, right, although there would be a delay technically right, so it could be wrong for about five minutes.

00:46:00
Speaker 2: I don't know about five minutes. I think you'd be updated faster than that.

00:46:03
Speaker 1: I hear a lot of nervous laughter here, Daniel.

00:46:05
Speaker 2: If you're destroyed by a black hole and you have complains about that website, feel free to email me.

00:46:09
Speaker 1: Yeah, well you get it in time, though.

00:46:13
Speaker 2: I've promised to answer all emails, and so I'm going to do my best.

00:46:16
Speaker 1: Yeah, yeah, you might have to break that promise. All right. Well, I think that's the answer for sCOD, which is yes, if you put a pea sized black hole, which would be the equivalent of a black hole with the mass of the Earth in the middle of the Earth, next to all of the material inside the Earth, then yeah, it would quickly grow and swallow us all up, or at least destroy the Earth and then eventually swallow our remnants up.

00:46:41
Speaker 2: Don't do it, Scott, bad idea.

00:46:43
Speaker 1: Don't do it, Scott, don't do it, Daniel.

00:46:47
Speaker 2: We're not creating pea sized black holes.

00:46:49
Speaker 1: Man, oh right, although you said any size is dangerous, so you're not helping me sleep, not my job, all right. Well, thanks to everyone who sent in their questions. We always appreciate it.

00:47:08
Speaker 2: And thanks everybody who's curious about the universe and shares sarah curiosity with us. This whole podcast project is just us marinating in the joy of your curiosity. Thanks very much everybody.

00:47:20
Speaker 1: You hope you enjoyed that. Thanks for joining us. See you next time.

00:47:28
Speaker 2: For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discord, Instant, and now TikTok. 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.