What's the densest thing in the Universe that's not a black hole?

00:00:07
Speaker 1: Gravity is a monster. Not the kind of monster that jumps out and slashes you. It's the kind that shuffles slowly, never racing, but always getting closer. It's patient, happy to wait billions of years until everyone else has had their turn. But it never ever ever gives up. It's always there, ready to tear you to shreds or crush you into a speck. And in the end, it's gravity that dominates our cosmic destiny. We rely on quantum forces, like the structural integrity of the Earth or the radiation pressure from fusion that keeps the Sun from collapsing. They keep gravity at bay for now, but inevitably they will fail us, and gravity will overcome each of our defenses, making white dwarves and neutron stars, and eventually victorious in creating black holes, Gravity's ultimate trophies. But where exactly is the quantum limit? How close can one get to the black hole threshold without collapsing? What is the last line of quantum defense against the plotting unavoidable onslaught of the gravitational monster. We'll dig into all of that on today's episode. Welcome to Daniel and Kelly's extraordinarily crushing universe.

00:01:36
Speaker 2: Hello.

00:01:36
Speaker 3: I'm Kelly winder Smith. I study parasites and space, and today we're going to learn that physicists love noodles. Where do you go from there, Daniel?

00:02:00
Speaker 1: Hi, I'm Daniel. I'm a particle physicist and I do worship at the arm of his newly appendage.

00:02:06
Speaker 3: Oh man, that is a throwback. I feel like the Pastafarians were a big deal when I was an undergrad and master student.

00:02:15
Speaker 1: Raw men brother ram, Oh my gosh, I.

00:02:17
Speaker 3: Never heard that. Okay, all right, so let's go way back. So I'm wondering what young physicist Daniel was, like, what is the first science project you did? And I'm thinking, like science fairies when you were a kid.

00:02:33
Speaker 1: Ooh, that's a great question, and I think it's kind of revealing, but not very romantic. Actually, young physicist Daniel had no idea what physics was actually like and was doing it because number one, I seem to be good at it, and number two people told me it was hard, and I was like, I don't really understand how the world works, and I'm kind of confused and socially awkward, but this is something I'm good at that people seem to like. So I'll just do that for a while. And honestly, my sincere just in physics wasn't really kindled until pretty late in undergrad when I found particle physics and I realized, oh, my gosh, physics can actually be fun. Research can be something that touches a passion deep inside you. I was going through the motions for a long long time, including my first ever science project, which was an experiment to measure how much light mirrors absorb. So we took a little laser beam and we bounced it back and forth between some mirrors. Then we had a censor at the end, and I measure the intensity of the light after a bunch of bounces and before, and did it as a function of the number of bounces, and so from that you can extract the fraction of the light that's absorbed by every bounce in the mirror.

00:03:42
Speaker 2: Whoa cool?

00:03:43
Speaker 3: What year was that?

00:03:44
Speaker 1: It was pretty cool? I think I was in middle school, and you know, it didn't take any fancy math or any fancy equipment. But I remember the judge being like, did your dad do this experiment? I was like, what which part do I need him for? Like the dividing the count It's just like a pen laser and a couple of mirrors. But I had a lot of fun. I thought it was really cool because I didn't realize until then the mirrors don't perfectly reflect. They do absorb some light.

00:04:11
Speaker 3: So your hypothesis was that you were going to get the exact same value from the laser as you got after it bounced.

00:04:16
Speaker 1: I was curious it was measurable. I thought maybe I was going to get something where it was like nine percent, But it turns out to be mirrors are quite absorbent. I mean, you can't use them to wipe up your kitchen or anything, but they do they do drink some light. How about you, Kelly, what was your first ever piece of science data?

00:04:33
Speaker 3: Yeah, so I Kelly, did not look like a promising scientist. In sixth grade for the first science fair, I put the project off until the last second, and it was like a couple of days beforehand. And I was into like conservation and protecting the environment, but I didn't really know how to do science, and so I pretty much like took a tiny little fish bowl and I stole some motor oil from the garage, and I like dumped it in and I was like, that doesn't look good. Oil spills are bad.

00:05:05
Speaker 1: That was no whole project, and what was the hypothesis? Is oil? Icky? Answer?

00:05:13
Speaker 3: Yes, uh yeah, I don't actually think I had a hypothesis. I was just like, gross, that's not good. And I remember getting a pretty uh negative review from my teacher who was get you know, who probably pointed out that there was no hypothesis being tested here. Kelly just dumped some oil and water and brought that to school and uh so, so anyway, I appeared pretty dense as a sixth grader, and today we're going to be talking about dense things.

00:05:40
Speaker 1: Ooh wow, what a crushing transition. Nice job, thank.

00:05:44
Speaker 3: You, thank you. I planned ahead. Well.

00:05:48
Speaker 1: The crushing power of gravity is fascinating and one of the enduring mysteries of physics. How does it work? How do we interface it with quantum mechanics? Is Einstein's theory real or is it just some weird emergent approximation of something else that's happening deep down? And the best way to get the answers to those questions are the places where gravity and quantum mechanics connect, where gravity is so extreme it can actually overcome quantum forces. So that means thinking about black holes and how they form and how basically the whole universe inevitably is going to collapse into a black hole. And that means you and everything.

00:06:20
Speaker 3: You love ah existential dread, my old friends that that friend tends to visit more when Daniel's are.

00:06:28
Speaker 1: Out, But you know, it's existential dread with a life sprinkling of dad jokes and mom.

00:06:35
Speaker 3: Jokes yay and knowledge right.

00:06:39
Speaker 1: And something that's always fascinated me about this question is that we can resist gravity sort of temporarily. We're like a weightlifter holding some crushing weight above our heads, but our knees are shaking and our thighs are vibrating, and eventually you know you're going to drop that weight, but you can for a while. And what's really fascinating to me about holding off gravity is that we have these series of defenses, these places where chemistry and quantum mechanics pushes back. But I've always wondered, what is the last line of defense, What is the densest thing that can exist in our universe that hasn't quite yet given up the ghost to gravity?

00:07:14
Speaker 3: Oh, and when it gives up, it becomes a black hole.

00:07:16
Speaker 1: Right, Okay, I've learned something that's surrender. Becoming a black hole is surrender?

00:07:21
Speaker 3: All right? Well, that is a fantastic question. I honestly don't know the answer, but let's see if our extraordinaries know the answer.

00:07:28
Speaker 1: Thanks very much to everyone who participates, and if you would like to join this elite crew of speculators, please write to us two questions at Danielankelly dot org. So think about it for a minute yourself. What do you think is the densest thing that can exist in the universe that's not a black hole?

00:07:44
Speaker 4: Outside of black holes? I always thought that neutron stars were the densest things in the universe, but I guess we could also say the core of a neutron star would be the densest thing. Of course, given you're asking, it's probably neither of those dnsiest thing that's a black hole, that would be.

00:08:03
Speaker 2: Neutron star. It should be a neutron star.

00:08:06
Speaker 3: I know those are the dancest objects in the universe which are not black holes. Either a neutron star, a movie by David Lynch, or does chocolate cake come about to it? I like science, so I know a few things and I can easily say confidently that the dentsst thing not a black hole would be a neutron star.

00:08:31
Speaker 2: Is the densest thing that's not a black hole, a neutron star, like a sun that died and didn't become a black hole because it was too small, so it came a neutron star. Maybe a neutron star spinning like a nine percent speed of light for some reason, just because it seems I can give it a little more, get up and go.

00:08:49
Speaker 3: Well, Daniel, it sounds like the answer is neutron star. Shortest episode yet, Thanks for Thanks for playing.

00:08:56
Speaker 1: Everyone, And this is why science is not democratic, right, just boat on stuff and move on. But it's cool that everybody's heard about neutron stars and they know the neutron stars are dense. But spoiler alert, neutron stars are not the theoretically most dense objects in the universe.

00:09:14
Speaker 3: What okay, wait, and so just to clarify, you said theoretically, So at the end of the day, we're going to be talking about what we think might be the most dense thing next to black holes, but we're not totally sure, is that right?

00:09:25
Speaker 1: Stick around for the end and you'll find out.

00:09:28
Speaker 3: I'm here for it. Let's do this all right. So you, before we started going to the listener responses, you were talking about how, you know, like a person holding up a weight, and I guess I really when I did CrossFit for a while, I really loved weightlifting. I totally miss it. But anyway, so like a person trying to hold up a heavyweight, they're you know, shaking and quaking and they're pushing back against the weight before they collapse. What is the equivalent for a star? Why doesn't it just collapse?

00:09:57
Speaker 1: I love that in this analogy, Kelly, is all of QUI forces holding back gravity. What's the heaviest thing you were ever able to lift?

00:10:05
Speaker 3: Kelly, I don't remember what my pr was is that I think that stood for personal record. I just know that I really liked taking heavy stuff from the ground and then putting it over my head. I thought that was great.

00:10:17
Speaker 1: Well, there is something very satisfying about that, because you're overcoming gravity, and stars when they collapse, they collapse due to gravity. But before we dig into that, let's just remind ourselves the basics of gravity and why black holes are inevitable. Gravity is one of the fundamental forces but it's different from the other fundamental forces. It's not a quantum force. We don't have a quantum explanation for it. And it's also different because it's super duper weak. So if you compare these things equivalently, like look at the forces between two protons, gravity is like ten to the thirty times weaker than any of the other fundamental forces, and so you might think, well, gravity should be relevant, right. It's like if you're balancing your checkbook and one expect is like ten to the thirty times smaller than the others, you can basically ignore it and still get the answer right to the penny. Right. But the thing about gravity is that it's inevitable because it cannot be canceled out. Gravity is only an attractive force. Masses only get pulled together. There is no repulsive gravity. I mean, there is expansion in the universe, which we don't quite understand entirely, but the force of gravity and over short distances can only attract things, which means that it can't be neutralized. For example, particles feel very strong electromagnetic forces, but then those are rapidly neutralized when they form neutral atoms, and hydrogen doesn't feel electromagnetic forces anymore. So vast clouds of hydrogen only feel gravity. Gravity is inevitable because it cannot be neutralized because it's only attractive.

00:11:46
Speaker 3: All right, So let's see if I've absorbed all of that. So you said that gravity is only attractive, but you also mentioned that the universe is expanding, and we don't really know why that does seem to be overcoming gravity. So does does that mean, as we understand it right now, gravity is only attractive, or does the expansion of the universe just suggest that we understand gravity, but something different is happening that's overcoming gravity because it's such a whim.

00:12:12
Speaker 1: Yeah, we don't really understand the expansion of the universe, especially it's accelerating expansion, and that can overcome gravity, but only we're very very large distances, like between galactic superclusters, where gravity gets weak because the distances are large. Over smaller distances like the cluster of our galaxy, gravity overwhelms dark energy or the expansion of the universe and holds things together. That's why, even though the universe is expanding, you're not flying apart, or our solar system is not flying apart, or our galaxy is not flying apart. So while it's inevitable for things to collapse into a black hole, it's not going to be one huge, single universe black hole. It's like every little neighborhood where gravity dominates is going to pull everything together into a black hole eventually.

00:12:53
Speaker 2: Okay.

00:12:54
Speaker 3: And then just to remind me where we are with gravity, we haven't found like gravity fields or anything. We don't really kind of understand what's happening with gravity, but we can very clearly measure it, and we know that it's like a definite thing.

00:13:05
Speaker 1: We have an excellent classical theory of gravity that ignores quantum mechanics but makes perfect predictions everywhere we can test it. So it's an excellent theory, but we think it's probably wrong.

00:13:15
Speaker 3: All right, Well, at least it's helpful, okay.

00:13:18
Speaker 1: But because gravity can't be canceled out the way like electromagnetism can, you can't make an object which is neutral in gravity eventually, even though it's weak, it's the only thing left on the playground. So like, that's why the structure of the Solar System is mostly due to gravity. The structure or the galaxies mostly due to gravity. The weakest force but everything else gets canceled out because they're so powerful and because they have like positive and negative charges. Equivalently, gravity basically only has positive charges, and so things only attract and so it's the only thing left over. It can't be neutralized, which is why gravity, though it's super weak, shapes the cosmos.

00:13:56
Speaker 3: That's interesting. I hadn't thought of gravity as being the like winning worse because everything else canceled out. I thought of gravity as being the winning force because there's just so much mass.

00:14:06
Speaker 1: Well, it's not hard to overcome gravity. Like you can overcome gravity with your legs, right, you hold up those enormous Kelly pr weights and the whole Earth is pulling on them, and you're overcoming them with your like admitted the impressive muscles. But you know you're small compared to the Earth, yet you're still able to overcome its gravity. And so that's an example. The quantum forces of your muscles et are resisting gravity, and you can do that temporarily, but you can't hold that weight up forever. And eventually, if you add enough mass, gravity can overcome any quantum force. It can pull stuff together and eventually collapse into a black hole and that's sort of the destiny of everything in the universe. But we can hold it off temporarily, Like our Sun is not yet collapsing into a black hole.

00:14:49
Speaker 3: And what is the Sun's equivalent of my massive bulging rippling muscles? What is what is keeping things from collapsing.

00:14:58
Speaker 1: Yeah, it's an amazing story because the beginning of the life of a star is gravitational. You have these vast clouds of hydrogen and dust and grains from previous stars, etc. And a little bit of gravitational over density somewhere in that cold cloud has to be like ten to twenty Calvin will start a collapse. It will create a region of higher density there for higher gravity, the pole stuff which makes higher density, which means higher gravity. And you get this runaway effect where you get a huge accumulation. You start from very very low density cloud into a very high density object a protostar. And what stops the collapse is quantum mechanics, is fusion. You get such a high density at the core of this star that the temperature goes up because of all the pressure from the outer layers. And when you have high temperature and high pressure and you have protons, they start to fuse, and that emits light. It releases energy, and that energy comes out as photons. We call this radiation pressure. So fusion ignites when the star gets big enough to get hot enough, and that balances the star. So you have this initial rush in due to gravity, and then fusion stands up and says, hold on a second, I'm going to burn for a little bit here, And for millions or billions of years, the star is incredibly in balance where the radiation pressure sort of cancels out the force of gravity, and the star is able to hang out there and just like emit light for billions of years.

00:16:21
Speaker 3: And is it like amazing that they balance out or does it make sense that they balance out because the amount of fusion happening is like proportional to the extent that it's getting squeezed.

00:16:32
Speaker 1: It's sort of amazing to me that it balances out and that it can balance for so long that the balance is stable for billions of years. I can imagine lots of other settings on the universe knobs where stars are very very brief, right, And in our universe, the length of star burns depends on its size. So, for example, if you get a really really big star like early universe, we think we had collapses of matter like two hundred times the mass of the Sun. Well, the bigger the star, the more massive it is, the more gravitational pressure you have, the higher the temperature of the core, and fusion is very sensitive to temperature, so as the temperature goes up, the rate of fusion increases dramatically. So counterintuitively, a bigger star doesn't last longer because it has more fuel. It lasts much shorter because it burns that fuel at a much higher temperature and it burns through it much more quickly. So super huge stars only last a few million years, whereas tiny stars can last for like many many billions of years, longer than the age of the universe. We think, Wow, our star is slightly on the bigger end. We think its life cycle is going to be about ten billion years, but red dwarves can last for much much longer. Small cold stars just above that fusion threshold can be in balance for billions and billions and billions of years.

00:17:46
Speaker 3: Amazing, it's incredible. We've had conversations where I've learned that mass is more complicated than I thought it was. When you talk to a physicist, does density require some similar unpacking or does density for a Physicists pretty much mean what we sort of imagine it.

00:18:01
Speaker 1: You mean, how we like sneakily redefined density to mean something else and not means you didn't keep using the same word. We would never do that, except we do it all the time. Yes, you would, Yes, And we're gonna do that with the word pasta later on. No, density means the same thing. It's mass over volume. But again counterintuitively, the densities of stars is sort of surprising, Like the smaller stars red dwarfs are actually much denser than the bigger stars. So to calibrate, water is a thousand kilograms per cubic meter, that's the density, that's its density, and the Earth is like five times that, So like five thousand kilograms per cubic meter, a red dwarf is like fifty to two hundred times that density, much much denser than the earth or water at the core. And you might expect that it's the opposite, right, that smaller stars are less dense because they're not as hot and there's not as much pressure at the core. But the bigger stars that burn hotter, they're creating a lot more radium pressure, so they puff the star out. So the Sun, for example, has a density of like one to two thousand kilograms per cubic meter. That's like about the density of water.

00:19:10
Speaker 3: I could float on the Sun. I mean I die, but I could float a little.

00:19:15
Speaker 1: We are not doctors. We do not recommend that you take a swan dive into the Sun. But in principle, yes, it isn't the density that would kill you.

00:19:23
Speaker 3: That's right, something else would.

00:19:25
Speaker 1: But it's weird to think about the density of the Sun being density of water. But as you go to bigger stars, hotter stars, like blue giants, they have a density less than water, like two hundred to five hundred kilograms per cubic meters. That's like the density of our atmosphere.

00:19:41
Speaker 3: Okay, so bigger stars are less dense.

00:19:46
Speaker 1: Yeah?

00:19:47
Speaker 3: Is that because if a big star was more dense, it would squish down. Yet, why couldn't you have I think I'm still not quite following why you couldn't have a really big dense star. What apps prohibits that from happening.

00:20:01
Speaker 1: If you have a really big dense star, then it's going to have an incredibly high temperature and fusion is going to be super intense and it's going to blow that star out, okay, And so gravitationally nothing prevents you from having a big, dense star, but the quantum mechanics of that star are going to push back. And that's what happens. You can't have a star that's stable, that's that big and that dense.

00:20:22
Speaker 3: Okay, got it, totally makes sense. It's stuck in my head now for now.

00:20:26
Speaker 1: So that's about as dense as we can get with actively burning stars because of the fusion. Right, here's quantum mechanics pushing back on gravity. But that requires having fusion at the core. And sometimes you get objects of form that can't fuse, and so you might wonder could they be even denser than stars?

00:20:43
Speaker 3: And that question would keep me up at night, But fortunately you are going to have an answer after the break, all right, Daniel. So fusion is the rippling muscles of the universe pushing back against gravity. But what happens when you don't have fusion.

00:21:20
Speaker 1: Yeah, we talked about the collapse of stars, and you have these huge Maluculier clouds that collapse, but you get lots of different collapses, right, You get a cloud doesn't collapse just into one star has lots of different stars at different masses, and sometimes you get stars whose mass is too small to raise the internal temperature to the level of fusion. Right, So, like below, a red dwarf is something we call a brown dwarf, something that has less than eighty times the mass of Jupiter doesn't reach the internal temperature to begin fusion. And this is also true of like Jupiter itself or Earth. Right, Earth is a big blob of stuff. It's hot at the core, it's not hot enough to fuse, and that's why planets can get denser than stars.

00:22:02
Speaker 3: Oh and so does that have a bit to do with what the insides are made of and like whether it's good fusion material or not, or is it all just about like how freshed it gets and how hot that makes things.

00:22:15
Speaker 1: All of those things what comes into play is what it's made out of. And also it's structural integrity. Right, So, we don't have fusion to protect the planet from collapsing into a black hole. But the Earth is not collapsing into a black hole right now as far as we know. Why is that? Right? Well, what's saving the Earth is chemistry actually, so thank you to chemistry.

00:22:35
Speaker 3: Because man, it's a dark day for you and me.

00:22:39
Speaker 1: I mean, we don't have to understand it, but we can be grateful for it. But you know, all of those atoms on the inside of the Earth are resisting being collapsed by gravity. They have these forces between them, the Vanderwall's forces and the electric repulsion, and they form bonds and those things are still more powerful than gravity. But yeah, the density of the planet depends on what they're made out of. And again, the dense is sort of counterintuitive. You'd be hard pressed to guess. For example, Kelly, what do you think is the densest planet in the Solar System? Bitter Although you sound so confident, but.

00:23:14
Speaker 3: You know, you just told me that the big suns are the least dense, and so now I'm wondering if something similar is happening with the planets. But I think probably not. So I'm gonna say Jupiter.

00:23:23
Speaker 1: Well you double counterintuitive yourself.

00:23:26
Speaker 3: Oh man, wait, so Venus Venus is like has a lot of lead, Maybe it's Venus.

00:23:31
Speaker 1: Well, both directions are actually wrong. You might think I'm going to go for the biggest mass like Jupiter, but Jupiter has created a huge amount of gas. Right, it's a massive planet because it gobbled a lot of gas, and in the early formation of the Solar System, it's out beyond the snow line, and so it can accumulate not just rock and metal, but also ice. And so we don't have a perfect theory for how these giant planets form, but one of the leading theories is that they start from like a gravitational over density, like a mini version of a solar collapse, and then they grab the rest of the gas in the outer Solar System. But because there's a lot of hydrogen there, it's not actually that dense. So then you think, well, what about an inner Solar system. Inner Solar system, all the water is vaporized and all the gas is either gobbled by the Sun or blown out by the Sun's early radiation, and so you're left with things like iron and rock, and like that's mostly what the Earth is made out of, right, iron and rock and all sorts of crazy heavy dense stuff like that. And mercury actually has the highest percentage of these heavy elements, like eighty five percent of the interior of mercury is a metallic core, compared to just fifty five percent for Earth. Mercury has like a very very thin mantle.

00:24:41
Speaker 3: I think the lesson here is never go with Kelly's intuition.

00:24:46
Speaker 1: But mercury is also not the densest planet in the Solar system, right. The densest planet in the Solar System is actually Earth because you have a balance here. In order to get density, you need gravity to make it dense. And so a mercury that has a lot of metal in it doesn't have enough mass to compress the core to get to the same density that Earth has. And so you can take mercury, for example, and add to it, add more stuff to it, and it's radius grows, but also the pressure grows, so it gets more collapsed, and so it becomes denser and denser. And this is actually sort of a maximum size to a rocky planet, which is about the radius of ten thousand kilometers, not much bigger than Earth. If you took Earth and you like added a whole bunch more stuff to it, it wouldn't actually get much bigger. It would just get denser. Why because of gravity, right, it would just keep compressing it. Because the stuff inside the Earth does get compressed, it still resists gravity. It's not yet collapsing to a black hole. Eventually that would happen if you added enough mass. But as you keep adding mass to the Earth, for example, it gets denser and denser and denser, And so in the Solar System, the Earth is sort of at the extreme of all of these balancing factors.

00:25:53
Speaker 3: Oh man, it's nice that for once, Earth is special in some way.

00:25:57
Speaker 2: You know.

00:25:58
Speaker 3: Wait, once we got decenter from the universe, you know, that was a little bit of a bummer. But this is making me feel good.

00:26:03
Speaker 1: So Jupiter's density is just above that of water. Mercury is like five point four times that, and Earth just tops out mercury at five point five. Venus is close at five point two. But Earth is definitely king of the Solar System in terms of density. Yay, we are the densesto.

00:26:22
Speaker 3: Hey man, I'll take what I can get. Oh wait, does that mean we've quit? Like we're not pushing back as much anymore?

00:26:30
Speaker 2: Is this? No?

00:26:30
Speaker 3: Never mind, don't think too hard about it. It's not a sign of failure. It's a sign of success exactly.

00:26:35
Speaker 1: But planets are also not the densest things in the Solar System. So stars which are actively fusing, they have a lot of radiation pressure keeps them from getting very dense. Planets you can make them denser than stars, but if you make them too dense, they start to fuse.

00:26:50
Speaker 3: And then once we fuse, we start pushing back out again and density goes down again. Yeah, exactly, I've been listening exactly.

00:26:55
Speaker 1: But you could also play another game and say, well, I'm just gonna wait for fusion to run out. Right, what happens when fusion burns itself out. We've been talking about how stars have a certain lifetime, and eventually fusion runs out because it relies on fuel in certain conditions.

00:27:10
Speaker 3: And I'm going to guess this is where our brilliant listeners come in and is the next stage of neutron star?

00:27:17
Speaker 1: Not yet?

00:27:18
Speaker 3: Oh man, almost right.

00:27:19
Speaker 1: We're not quite there yet. But think about the life cycle of our star. What's going to happen. Well, we have a fairly low mass star. It's going to burn and burn and burn, and it's burning mostly hydrogen and it's accumulating helium at its core, but it's not hot enough to burn that helium, so that helium is just sort of like ash. It gets in the way of fusion, and so as the core builds up helium, the fusion gets pushed to the outside, so we have fusion instead of at the core, now you have it in the middle and then in the outer layers. And so the star blows up to become a red giant as the fusion starts happening in its core. And this is why people say the star is going to absorb the Earth, because the radius of our sun is going to get enormous as the helium core heats up. But eventually, for a brief moment, the star will go across that threshold be able to burn helium for like literally a minute, and there'll be a helium flash where the helium fuses, and that'll blow out the star and you'll be left with a nebula, like you know, the outer edges of the star blown out, and that the core will be a remnant, which for our star is a white dwarf, and a white dwarf is just that core of unburnable stuff, leftover stuff from fusion where the star was not massive enough to fuse it. So it's just sort of like unfusible fuel left over, but very hot and very dense. And that's a white dwarf.

00:28:36
Speaker 3: Oh my god. Okay, So I imagine that that one minute period that you were talking about where the helium is burning is going to differ depending on like what kind of star you're talking about. So but still, I imagine this a very narrow amount of time during which you could catch this happening. But that sounds amazing. Have we caught this like on any of our telescopes.

00:28:56
Speaker 1: No, because unfortunately it's mostly internal. The helium flash is absorbed by the star, And we've looked for these things on other stars, but we've never actually seen them because again it's mostly internal and absorbed. People are trying to study it by doing astro seismology, looking at like periodic changes in a star's brightness to see if maybe something is happening internally, or looking for other kinds of indirect evidence, but we've never directly seen a helium flash. So currently it's still theoretical.

00:29:24
Speaker 3: Okay, but it's definitely awesome.

00:29:27
Speaker 1: Yeah, it's definitely awesome. And so what's left over and the white dwarf depends on how big the star was. The bigger the star, the hotter the core, the more elements you confuse, and the biggest star confuse all the way up to iron. Smaller stars like ours can only form things like carbon and oxygen. But that's what the white dwarf is going to be made out of. And these things are incredibly dense. They can have a mass of up to the mass of our Sun. Our sun won't leave a white dwarf the mass of the Sun because some of the mass is blown out, but bigger stars up to like eight times the mass of our Sun can leave a white dwarf. And the core there is like the mass of our sun.

00:30:03
Speaker 2: Wow.

00:30:04
Speaker 1: And the radius is very small, it's only like ten thousand kilometers. And so these things are incredibly dense. Density of like ten to the ten kilograms per cubic meters earlier we were talking about like thousands of kilograms per cubic meters. This is like ten billion kilograms per cubic meters. Really incredibly dense.

00:30:24
Speaker 3: Holy cow. Okay. And the reason even though it's so dense, it's not fusing because it's already burned up all of its fusion fuel, so fusion can't happen anymore.

00:30:32
Speaker 1: Yeah, exactly, it's not dense enough to have any more fusion. But if something comes along and leaks a little bit of mass to the white dwarf so it can compress further and overcome the quantum forces. Then does suddenly trigger fusion in the whole star and blow the thing up as a type one A supernova. That's how type one A supernovas are formed.

00:30:49
Speaker 2: What that sauce?

00:30:51
Speaker 3: Oh, I kind of wish I could see this stuff. I mean, i'd be dead, I know, but wow, Well we.

00:30:57
Speaker 1: Can see them across the galaxy, which is why they're so power for telling us about the expansion of the universe. But the reason that their wife d warf is stable. Like you might ask, what's keeping this thing from collapsing into a black hole anyway? And it's not like the structural integrity of iron or carbon the way the Earth is, And it's not fusion the way a star is. It's something else called electron degeneracy pressure. Instead of thinking about this object as made of individual atoms, think about it like a metal. What happens in a metal is the electrons all sort of flow around, and you have like electron energy levels across the metal, like conduction band, valence band, all that kind of stuff. Well, what's happening here is that the electrons, because they're fermions, they're the kind of particle where you can't have two of them in the same state. They can't collapse to low energy levels because those are occupied, and so the electrons are forced to stay in higher energy levels because they can't go down to those occupied levels, and that means they have higher energy, which means they're flying around, bouncing against stuff, and that's where electron degeneracy pressure comes from. People often write and ask me, like, what is the poly exclusion principle? What force is acting on it? It's not one of the quantum forces. It's not electricity, magnetism, it's not the weak force, not the strong force. What force are we talking about here? It's not any individual force. It's this quantum rule that prevents electrons from going to lower energy. So they have a higher energy, so they bounce off the stuff and apply pressure.

00:32:20
Speaker 3: Huh okay, and that's what keeps the white dwarf white dwarf fyet.

00:32:24
Speaker 1: Yeah, exactly. That's the thing that prevents gravity from collapsing into a black hole. Those electrons do not want to go down into that lower energy, so they keep having high energy and they push back in the same way that radiation pressure keeps a star from collapsing, electron degeneracy pressure keeps a white dwarf from collapsing.

00:32:41
Speaker 3: All Right, So I'm on the edge of my seat now because i know that at some point we have to get to neutron stars, and I'm guessing we're going to get there through white dwarfs, and so maybe something goes wrong with electron degeneracy pressure. But let's go ahead and take a break and find out when we get back, all right, Daniel. At the beginning of this episode, the Extraordinaries pointed out that neutron stars are very dense. And we've explained how you get to white dwarves and how electron degeneracy pressure keeps the white dwarf from collapsing even further into a black hole. How do we get to neutron stars.

00:33:32
Speaker 1: Yeah, so to get to neutron stars, you have to overcome this electron degenescy pressure, which means either adding more mass to a white dwarf or just having more mass in the star. Initially, this leads to a type two supernova, when that last moment of fusion in our star would have been a helium flash, but in a bigger star, that's a supernova, and it blows out the outer edges much more dramatically, and instead of getting a white dwarf at the core, you get a neutron star. And what happens here is that you've overcome the electron degenerously pressure just by having more mass, and you squeeze those electrons into the protons and made neutrons. This is inverse beta decay. Beta decays when a neutron decays into an electron and a proton. This is like you squeeze the electron back into the proton and you form a neutron. And so that's what a neutron star is. When you've said screw you, electrons, I'm pushing you into those states.

00:34:24
Speaker 3: Holy cow, eat it all right, that's intense. So are there any electrons or protons left or they're all smooshed into neutrons. There's probably an equal number of them.

00:34:33
Speaker 1: Neutron star is mostly made of neutrons. There are going to be some protons in there as well, and a few electrons. Especially at the edge. We don't know exactly what's happening, and then at the core we really don't understand but these things are incredibly dense. They're still like one to three times the mass of the Sun, but the radius is only ten to fifteen kilometers.

00:34:53
Speaker 3: Wow.

00:34:54
Speaker 1: The white dwarfs the radius was like ten thousand kilometers. This thing is a radius of ten kilometers the size of Los Angeles, right, and its density is ten to the seventeen kilograms per cubic meters, so like ten million times the density of the white dwarf.

00:35:11
Speaker 3: Wow. Okay, but still somehow that's not enough to become a black hole.

00:35:15
Speaker 1: That's still not enough to become a black hole because at the core quantum mechanics is still pushing back, and these neutrons are neutral electromagnetically, but they still have the strong force. And when you get at the core, even the neutrons themselves start to merge, and you don't just get individual neutrons to get pushed into something called like a cork gluon plasma or these other hypothetical states of matter, for example nuclear pasta.

00:35:40
Speaker 3: Oh, I've been waiting all episode for us to get to the nuclear pasta. Okay, tell me about the Noki phase. How do you pronounce that?

00:35:51
Speaker 1: So if you have a few protons left over and you do the calculations of what happens when you compress neutrons really really far. You get these weird blobs. So this called the Niolki phase. We have these semi spherical blobs, and this is just what emerges from the calculations. You like, run your simulations and you get these blobs. But sometimes instead you get the spaghetti phase where you get like long rods of neutrons form, or they have a phase with the like sheets of neutrons they call the Lasagna phase because of course, or another phase they call the anti spaghetti phase where you roll those sheets up back into rods.

00:36:27
Speaker 3: I feel like they're losing the plot there.

00:36:32
Speaker 1: So we don't know if this is what's actually happening inside the neutron star. But here remember neutrons are also fermions, so the polyexclusion principle applies to them as well. So even though they're neutral, they resist collapse because they don't want to be in the same state either. And so that's quantum mechanics, like last ditch effort to avoid being turned into a black hole. But if you take a neutron star and you add more mass to it or equivalently if you started with a much bigger star, more than forty times the mass of our Sun. Then when you have that super nova and then the collapse, you don't get a neutron star. You get a black hole.

00:37:06
Speaker 3: And I'm just trying to imagine, like add more mass to it. Might mean that like dust from nearby gets sucked in, or like a comet that's passing by gets sucked in, and that adds a lot more mass. Is that how you get the mass?

00:37:18
Speaker 1: Maybe, but that's probably not going to be enough. I think more typically you'll have a binary star system and it'll absorb one of its neighbors. And so you see these systems where like one of them is a white dwarf for a neutron star, and the other one is a still burning, puffy star and its partner is like slurping on it. You see there's like tendrils of gas space exactly.

00:37:38
Speaker 2: Yea, we get.

00:37:41
Speaker 1: Job. But the question of this episode is can you go beyond a neutron star? Is there something denser than a neutron star which can still resist the force of gravity?

00:37:51
Speaker 3: And what's the answer, Daniel.

00:37:54
Speaker 1: So we're trapped between a neutron star and a black hole, So let's put some numbers on it to make a concrete and counterintuitively, also the density of black holes is confusing. As black holes get more massive, their radius goes up, and so they're also not super dense. For example, a black hole the size of our solar system has the density of about water, but smaller black holes have a smaller radius, they're incredibly dense. So let's imagine our neutron star scenario. We have a mass of like two times the mass of the Sun, a radius of ten kilometers. That's a density of like ten to the eighteen kilograms per meter's cube. If you collapse it to a black hole, would require squishing that ten kilometer radius star down to about six kilometers, and that would make a density of five times the neutron star. So we're wondering if there's a place between the density of that neutron star and the five times density of a black hole where something can survive and not collapse gravitationally.

00:38:49
Speaker 3: So, just to make sure that I'm understanding, so you said that some black holes could have the density of water, and we talked earlier about how our sun has the density of water. So does that mean that there are blue giants out there that are denser than some black holes.

00:39:04
Speaker 1: There are blue giants out there that are denser than some black holes. Absolutely, because a black hole that's super big is not very dense. And here we're defining density as the massive thing divided by the event horizon. We don't know inside the black hole how that mass is distributed, if there's a singularity of infinite density, if it's smoothly distributed, if there's weird quantum stuff happening, if there's a black hole squid pasta. We have no idea, but we're just sort of assigning the average density of the black hole.

00:39:29
Speaker 3: Okay, all right, So we've got a range of densities, and we're trying to figure out if there's anything between a neutron star and the least dense black.

00:39:39
Speaker 1: Hole, well, between a neutron star and the black hole it would eventually collapse into, which would be quite dense. Okay, And I chose a neutron star because the black hole it would collapse into would be quite small six kilometers and very very dense. And so we're wondering if there's a possibility in between. And so there's an Australian physicist Hans Bookdall, who thought about this stuff several decades ago. He came up with this really clever calculation and he proved this limit that said that nothing can be smaller than twelve percent larger than the radius of a black hole. So if you have a certain mass, you can calculate what the black hole radius would be. For our case, for two times the mass of the Sun, our neutron star with a radius ten kilometers, the black hole radius is six kilometers, and he says that nothing can survive less than seven kilometers without collapsing into a black hole. But then in principle it is possible. So he was thinking about in terms of the radius of a black hole, which is a short siled radius, and for that given mass, he said that nothing can be smaller than a radius of nine eighths of the sort chiled radius, so like twelve percent bigger than the short siled radius. If you get smaller than that, then you have to collapse into a black hole. But theoretically it's possible to have an object that's like nine eighths the radius of the short filed radius, which would not yet be a black hole.

00:40:56
Speaker 3: Okay, but like, my hat is smaller than that, and so by like, by nothing do you mean know celestial bodies?

00:41:05
Speaker 1: Well, your hat has a tiny mass, and so the swart Child radius for your hat is very, very small. And book Doll is saying that your hat, in principle could resist becoming a black hole by staying at nine eighths of the swart Child radius of your hat.

00:41:20
Speaker 3: Oh, okay, all right.

00:41:21
Speaker 1: And so in the case of our neutron star, it's short Chiled radius is six kilometers and its book Doll radius is about seven kilometers. So book Doll did this cool calculation. And the way he got this number is he said, well, let me make some assumptions. Let me assume that pressure in the object is finite, and that density increases monotonically from the inside to the outside. So you have this sort of simplified object. It's a perfect fluid with isotopic pressure. And in a Newtonian world, where gravity is just like a force between two objects, there is no limit here on the density of an object. But in general relativity pressure contributes to gravity, so as something gets denser, it has more pressure pushing out. Actually, gravitates also increases the gravitational pull on the object, and so Oakdahl discovered this limit where the pressure essentially becomes infinite. So you can't have an object smaller than nine eighths of their short stiled radius without that pressure blowing up and becoming infinite. And therefore that thing would have to collapse into a black hole. It couldn't survive under those conditions.

00:42:27
Speaker 3: Okay, So Bechdahl did these calculations, and I'm guessing the implication here is that he found that there is an intermediate density that could exist between a black hole and a neutron star exactly. But have we ever seen that.

00:42:45
Speaker 1: We have not ever seen that, and this remains theoretical. It's fascinating to understand that there is a limit to the density. There is a maximum possible density you can achieve before you get to a black hole. Neutron stars do not achieve that density, And in order to achieve that density, you'd need some kind of force which is capable of resisting gravity more powerfully than neutron degenerousy pressure. We don't know what that is, but in theory, if it did exist in the universe, it could create what they call a book doll star. This object with a radius of nine eighths the short Child radius, not yet a black hole, but something with a surface that you could actually like land on that Kelly could swim in, or you stand on the surface of and try to lift her weights. But you know, we've seen black holes in the universe, but we're not one hundred percent sure they actually are black holes. Like we see objects out there in the universe that are smaller and denser than neutron stars can be, and so people say, oh, well, therefore they must be black holes. And we see them gravitating and we see things getting close to them, so we can measure their radius. But our measurements are not precise enough to actually tell us whether these things are black holes or book doll stars because we can't see the event horizon directly. We've observed the accretion disk, we've seen the indirect effects, but that's a very small difference between a book doll star and a black hole. So in principle, some of the black holes we've seen could actually be book do all stars.

00:44:13
Speaker 3: Okay, so what do we need to know to differentiate between those?

00:44:16
Speaker 1: We need a more precise measurement of the radius of these things. So, for example, the black hole of the center of our galaxy, we don't know precisely what its event horizon radius is. We've seen stuff go near it and get sucked in, we've seen stuff go near it and not get sucked in. That lets us measure it, but it's not precise enough to distinguish between a black hole or a book doll star. What we really need is direct evidence of an event horizon. See something fall in and red shift and act the way you would expect something falling into a black hole to behave.

00:44:45
Speaker 3: I volunteer, Zach, that's so nice.

00:44:49
Speaker 1: Of you, really just for the sake of science. Or the other direction you could go is you could think hypothetically about what these objects might be. What is capable of creating on our at that density or maintaining that density and resisting the black holes collapse. And there's some ideas out there fuzzballs or quark stars or weird hypothetical objects. We have earlier episodes about them if you want to learn more about them, and some of those have specific predictions that you could look for that differ from just like, hey, this is a very dense object that like short lifetimes or other weird predictions. So you could try to make predictions for what a theoretical object like this would look like that's different from a black hole, and then look for those signatures.

00:45:32
Speaker 3: Wow. Okay, So where we are is we've definitely seen neutron stars. We suspect there's this intermediate thing, and maybe we've even been looking at the intermediate thing. He's being the Bechdal stars. Every time I say it, I say it a little different. That's fine.

00:45:46
Speaker 1: He passed away ten years ago, so you don't have to worry about it, all right.

00:45:48
Speaker 3: He's not going to call me on it. That's good. But so we might have already seen that kind of star and black holes, and maybe one day we'll be able to tell which is which, but at the moment we can't.

00:45:58
Speaker 1: Yeah, that's right. We don't have direct evidence for the existence of black holes, which leaves the door open to wondering exactly what are these objects because the argument for black holes is essentially there's something else we can think of that's this massive and this small, But that doesn't mean it's not out there amazing.

00:46:15
Speaker 3: All right, So time to get into science, kids, because there's a lot of problems left to solve.

00:46:20
Speaker 1: So the listeners were mostly right that the densest thing that's not a black hole that we have observed and have confirmation of is a neutron star. But in principle, there could be something denser in our universe.

00:46:31
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00:46:44
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00:46:50
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00:47:03
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