What is Entropy?

Daniel and Kelly try to organize their thoughts about the disorderly topic of entropy. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

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• 2:19Daniel Whiteson?

  There are so many topics in physics that are hard to grapple with to deeply understand, and it's not helped by the fact that often physicists have given these things confusing names. Electrons, for example, have quantum spin, but they're not actually spinning. Moments of inertia have nothing to do with moments of time. Work and power have very different meanings in physics and in English. But sometimes, even when physicists invent a brand-new word to convey a new idea, it's still slippery to grasp. So today on the pod, we're going to try to grab hold of one of the trickiest concepts in physics, one that's often tossed about and attached to a simple explanation, but whose subtle

• 3:03Daniel Whiteson?

  power isn't usually clearly explained. Today on the pod, we are tackling entropy. What is it? Does it explain why our teenagers' rooms are so messy, or why coffee spills out of cups but not back into them? Does it tell us about the fate of the universe or the nature of time? Is it about order and chaos? Why did physicists even devise this concept? Welcome to Daniel and Kelly's extraordinarily disorderly universe.

• 3:44Kelly Weinersmith?

  Hello. My name is Kelly Wiener-Smith, and I often make jokes about entropy increasing in my home. And today, I'm going to find out if that joke is scientifically accurate or not.

• 3:55Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist, and I think that there's always one person in the marriage who's more orderly and one person is more chaotic.

• 4:04Kelly Weinersmith?

  Yeah, it's pretty easy to identify who's who in my marriage.

• 4:09Kelly Weinersmith?

  Are you the more orderly or the more chaotic in your marriage? Katrina seems pretty orderly, but so do you.

• 4:15Daniel Whiteson?

  I don't want to slander my wife on air, especially if she's not here to defend herself. So I'll say that in some categories, I'm more orderly and in some categories, she's more on top of stuff. And that's why we're such a good team.

• 4:26Kelly Weinersmith?

  Oh, I'm going to slander my husband. He's the chaotic force in our family. He absolutely is, but he's also, you know, a lot of the creative force. So it works out.

• 4:35Daniel Whiteson?

  Exactly. Well, the yin and yang is what makes it exciting, isn't it?

• 4:38Kelly Weinersmith?

  It keeps things fresh, that's for sure. So I was thinking about the topic for today, and when I think about entropy, the first thing that comes to mind is jazz music. And when I Googled entropy and jazz, actually, a lot of things came up. People have like studied jazz music through the lens of entropy. Do you like jazz music? What do you think?

• 4:58Daniel Whiteson?

  Wow, that's fascinating. I never connected entropy and jazz music. There's some jazz music I like, but I like things that are a little bit more melodic. So just like a wandering sprinkle of notes doesn't do it for me. I need a little bit of rhythm and some beat to it and whatever. So I'm more of a blues guy than jazz. How about you?

• 5:17Kelly Weinersmith?

  It depends on my mood. There are some moods that I'm in where jazz is exactly what I want and it's exactly what I need to listen to while I'm writing. Then there's other moods where it does make me feel kind of frustrated and overwhelmed. So, but you know, I feel that way for a lot of different kinds of music. I've got like very particular kinds of music for different kinds of moods.

• 5:35Daniel Whiteson?

  I appreciate the jazz nerds though, because they help me understand how almost any human endeavor, there are so many layers to it. You know, like people who appreciate wine and they can tell the difference between like a $500 bottle of wine and a $5,000 bottle of wine. Whereas like, I can't tell the difference between $10 and $20 bottles of wine. Or like how many levels of skill there are to chess, you know, where like a level 20 person will be the level 18 person will be the level 17 person consistently. I feel that same way about jazz because people talk about it and they're like, wow, this guy is a genius and so amazing in this way and that way. And I just like at the beginning of a journey of appreciating that. But I love when human culture goes really deep on something and people could appreciate the fine nuances of it.

• 6:17Kelly Weinersmith?

  Yeah, you know, if I had endless time and money, I would love to just like jump into different cultures and just sort of like appreciate and absorb, you know, the various features that are exciting to them and just sort of enjoy their culture for a little while. And I very briefly jumped into jazz as I've had different friends who do jazz things. And it's a very cool culture, but I am still at the early phases of like, what is good? I don't know. I just know that this makes me smile.

• 6:43Daniel Whiteson?

  Well, something I do have very strong opinions about because I spend a lot of time perfecting my tastes about it is pizza making. I'm an at home pizza maker and I have like strong opinions about like, you know, stretchiness of the dough and the puffiness of the crust and the darkness of the sauce and whatever, because I've made a lot of pizzas in my house. So I know like exactly what I like in a pizza. What have you nerded out on? What are you like a level 20 expert in?

• 7:09Kelly Weinersmith?

  Well, so first I'll say that my husband also nerds out on pizza, which is making me wonder if we should make pizza when you're here to visit or if we shouldn't.

• 7:18Kelly Weinersmith?

  Zach and I will have a discussion about that. What am I a level 20 nerd on? I don't know. I guess I'm sort of a generalist. I like lots of things. I mean, I'm really into the moths in our area. And I spent two years learning about Russian and Russian culture and like the language. But I don't know if I'm like a level 20 on either one of those things.

• 7:40Daniel Whiteson?

  Well, I can tell you moths, not very good on pizza. I mean, they're delicious, but really not a popular topping.

• 7:45Kelly Weinersmith?

  No, I totally believe that. But I've pulled us far from our topic. Today, we're talking about entropy. And, you know, I think first, we should find out what our audience thinks entropy means.

• 7:57Daniel Whiteson?

  That's right. Let's try to beat back the chaos and stay on topic today, Kelly.

• 8:01Kelly Weinersmith?

  Good luck.

• 8:01Daniel Whiteson?

  So I went out there and I asked folks what they thought entropy was because I was curious what connections people had made in their minds. Maybe they'll also connect it to jazz or blues or pizza making or moth loving. Let's hear what folks had to say.

• 8:16Speaker 07

  The tendency of a closed system to kind of go from order to disorder unless you add energy. To go from order to disorder.

• 8:24Speaker 08

  I really don't have my head around it.

• 8:26Speaker 05

  Entropy is a measure of the remaining free energy in a specific volume.

• 8:31Speaker 09

  Homogenous, milky toast.

• 8:34Speaker 10

  A description of the state of a system in terms of its energy.

• 8:39Speaker 11

  The more different configurations a system can have and be in the same state, the higher the entropy it has.

• 8:47Speaker 12

  Matter being either organized or being in a chaotic state. The degree of disorder in a physical system.

• 8:55Speaker 09

  The amount of energy in a system that can't be recovered. Describes the level of organization. As time progresses, entropy progresses.

• 9:03Speaker 13

  I think it's the tendency of energy to spread out.

• 9:06Speaker 07

  Slow crawl towards simplicity that the universe imposes on everything.

• 9:11Speaker 14

  Disorder, chaos, the absence of order. Things degrade over time.

• 9:17Speaker 15

  It feels weird because it seems made up what disorder is and what order is.

• 9:21Speaker 14

  If there are more configurations, the entropy is higher.

• 9:24Speaker 16

  Number of microstates of the system, which can result in the current macrostate.

• 9:29Speaker 17

  Things want to move from a higher embodied energy state to a lower one.

• 9:34Kelly Weinersmith?

  Well, no pizza in those answers, but actually a lot of variety in the answers there.

• 9:38Daniel Whiteson?

  Yeah, I think we really capture something here that there's a general sense that entropy goes up and that it has to do something with order and disorder, but also that there are multiple concepts of entropy, right? There's senses of energy and entropy and a sense of organization related to entropy. And so I think that captures like the big chaotic confusion that is most people's understanding of what entropy actually is.

• 10:01Kelly Weinersmith?

  I do think this idea has escaped into like the general consciousness and maybe it has disconnected from its physics definitions in that process.

• 10:10Daniel Whiteson?

  Yeah, I think you see a lot of tech bros on social media using entropy as if they know what it means.

• 10:17Kelly Weinersmith?

  I mean, it's nowhere near as bad as the word quantum, but let's clear things up today.

• 10:20Daniel Whiteson?

  Exactly. It's getting there, though. It's getting there.

• 10:24Kelly Weinersmith?

  All right. So tell me about the first time the word entropy was used.

• 10:27Daniel Whiteson?

  Yeah, entropy is a fun topic because it's not an ancient topic, right? People have been talking about motion and velocity and energy and time since people have been like smoking whatever and sitting in caves and looking up at the night sky and wondering how the universe works.

• 10:42Kelly Weinersmith?

  The things that bring us all together.

• 10:44Daniel Whiteson?

  Yeah. So you can like go back and look at the Sumerians thinking about the path of the planets and length of the year and, you know, the structure of the solar system and stuff like that. But entropy is a recent concept. It's less than 200 years old. It's a word that was invented fairly recently as people were puzzling over engines and wheels and energy.

• 11:05Kelly Weinersmith?

  Oh, so was the idea that like it's hard to keep an engine working because entropy sort of over time makes the system less reliable? Or tell me more about this history.

• 11:15Daniel Whiteson?

  So it's the early 1800s and there's the Carnot cycle. And so this French physicist Carnot, actually a father-son team, were thinking about engines and heat. And it was sort of a mystery at the time, like, what is heat anyway? People had the sense that the universe had a microscopic explanation that could help you understand the macroscopic view. Like, you know, this is around the time when we're about to get Dalton and thinking about the existence of atoms. And so that idea is out there that like, maybe this microscopic stuff, you know, and also biologically, right? Like the germ theory is coming around this time, you know, within decades at least. So people were wondering, like, what is heat? Is heat like a particle? Is it a substance? Does it flow from one thing to the other? Remember, early ideas of like

• 12:00Daniel Whiteson?

  electric charge, where like, there were two of them, and they were a liquid and they flowed? So this is sort of like an idea that was out there. People were wondering, how do engines work? What is energy? What is heat? How are they all related? And in 1824, Carnot sort of put his finger on this idea, that differences in heat can be used to do useful stuff. Like, if you have something that's hot and something that's cold, energy will flow from the hot thing to the cold thing, and you can use that to do stuff. Sort of like the way water flows from uphill to downhill. And if you capture that flow, you put like a water wheel there, you can use that to do stuff, like grind your wheat into flour, right? Very useful. But if the water is all flat, you can't use it to do anything. And so heat differences can be

• 12:44Daniel Whiteson?

  used to produce work. That's Carnot's big insight in the early 1800s.

• 12:49Kelly Weinersmith?

  Did we have the steam engine at this point yet?

• 12:51Daniel Whiteson?

  Yeah, steam engines existed at this point. And that's a great example of how you use heat, right? You heat the steam, it rises, it turns your turbine, you can use that to do work, or these days, to make electricity, right? And so Carnot understood that engines can do this. Engines can turn heat differences into work. But he described sort of a perfect engine, one where you can turn the heat differences into work, and then you can Use that work to create heat differences. So back and forth and back and forth and sort of perfectly without any loss. But he also had this idea that, well, sometimes you have imperfect engines. That something is lost and something creeps out of your cycle. This is an imperfect engine. Eventually it'll wind down. You'll turn a heat difference into work and then you'll turn that work into a heat difference.

• 13:36Daniel Whiteson?

  But you get a little bit less, sort of like the way if you drop a ball. In principle, the potential energy of the ball turns into kinetic energy. And then it bounces back up and regains all of that potential energy. But in practice, there's a little bit of friction. There's losses in the system and the ball doesn't bounce forever. Right? In the same way, he understood that this happened, but he didn't describe it mathematically. It wasn't for a few more decades that people sort of described this with equations as sort of more mathematical concepts.

• 14:04Kelly Weinersmith?

  Okay, but even today, we don't have a perfect engine, right? All of our engines are imperfect?

• 14:09Daniel Whiteson?

  All of our engines are imperfect, exactly. And it was Clausius, who sounds like he should be an ancient Greek guy.

• 14:16Kelly Weinersmith?

  He does, like Claudius.

• 14:17Daniel Whiteson?

  I know. Yeah, exactly. I imagine him in a robe, even though he probably just wore a suit and had a top hat. But Clausius defined it mathematically and he thought of it as the stuff that's flowing. Like entropy is leaving the system. It's moving through. It's like a physical thing. And he connected it to temperature. And so his contribution was essentially invent this concept of entropy to help us understand why some engines are imperfect. And he thought of it as this thing which flows through the system, like a real physical thing. Not just like, hey, here's a number, we're calculating it, we define it. Like you could invent anything, right? You could invent the Gibrilions and define it as like the number of apples in the universe minus the number of ice cream cones. And that doesn't necessarily have to mean anything, right?

• 15:02Daniel Whiteson?

  But sometimes you invent a number and it means something in the universe. It like describes something which actually exists or is important. And so he connected entropy not just to heat, but also to temperature. Right? And temperature also something people were trying to understand. And so we're going to get into the mathematics of what that all means and how it works a little later on. But Clausius's other big contribution was the word. He created the word entropy. Before this, we had heat and we had temperature. But Clausius created the word entropy to think about energy flow.

• 15:34Kelly Weinersmith?

  And so At this point, he's thinking about the movements of heat or the movement of energy, but like, so when I think of entropy, I usually think of like stuff getting lost. Was the idea of stuff getting lost or getting disordered part of this idea? Or is he just tracking the flow of things?

• 15:50Daniel Whiteson?

  Not disorder. Disorder comes later with Boltzmann, but we'll get to there in a minute. But he's thinking about the energy flow and where does it go? And he's using that to understand why engines will wind down, right? For sure. Because entropy is one of the reasons why energy doesn't flow completely perfectly. But I also love the story of how he came up with the word. So entropy is like a word he invented. He took the letters E N from energy, because it's related to energy. And then he took the word tropey from the Greek word for change. So he's like, oh, this is cool entropy. And, you know, he was cognizant of the fact that this is something he was inventing, and it was kind of a recent idea. So that's why he reached all the way back to Greek, because he wanted to connect it with like ancient languages and ancient thoughts.

• 16:35Daniel Whiteson?

  And he said, quote, "I prefer going to the ancient languages for the names of important scientific quantities, so that they may mean the same thing in all living tongues."

• 16:44Kelly Weinersmith?

  Ooh.

• 16:44Daniel Whiteson?

  I think he was hoping that like if he uses a Greek root, then even like Romanians and Bulgarians and English speakers and everybody is going to have some understanding intuitively of what this concept means.

• 16:55Kelly Weinersmith?

  Now, I am imagining him saying that in a toga.

• 17:00Daniel Whiteson?

  He's carving it into marble, right?

• 17:02Kelly Weinersmith?

  That's right. That's right. And has history judged this to have been a good decision?

• 17:06Daniel Whiteson?

  I don't know. I mean, listen to the listeners, like entropy is very confusing. I don't think anybody has the same idea of what entropy is. And there's a famous physicist, a few decades ago, Leon Cooper, who won the Nobel Prize for superconductivity. So like dude knows his stuff. And he says that Clausius, quote, "succeeded in coining a word that means the same thing to everybody: nothing."

• 17:29Kelly Weinersmith?

  Oh, I hope that's not my legacy.

• 17:33Daniel Whiteson?

  I don't know. If I invent something so pervasive that people are griping about it, then like, hey, you know, I've done something. All publicity is good publicity, right?

• 17:40Kelly Weinersmith?

  I've never been sure about that, but okay.

• 17:44Daniel Whiteson?

  And so at this point, we have sort of these macroscopic handles on it. We're describing temperature and energy and entropy as things we can measure about the stuff we experience, right? And macroscopic, that means like stuff in our world, the human scale, you know, like you can take a thermometer and you can put it in your water and you can measure the temperatures. And number, you can measure about like large amounts of physical things. But people again, wanted to understand these things microscopically, like what happens down below? If you're understanding the particles, what does this all mean? And on this show, we do that a lot. And people are often writing in to ask me like, well, what's really happening at the particle level during Hawking radiation or when light bounces off a mirror or something? Like what's really going on? As if like the reductionist explanation reveals something

• 18:28Daniel Whiteson?

  truer. And, you know, we'll talk about that in a minute, but I want people to understand that like, there are many layers of the universe of reality. None of them are more true than the other. We have this sense that like, as you go deeper, maybe you're approaching some fundamental layer of explanation. But every layer is useful. You know, the macroscopic view, the human level view of the universe is just as valid and just as useful, even if there is another layer underneath. Because the amazing thing is that you can write equations that work to describe the macroscopic without understanding the microscopic. The universe gives us that, that access to many different layers of reality.

• 19:03Kelly Weinersmith?

  So if every layer is useful, what I'm hearing you say is that it's okay that I skip chemistry. I can focus on the other useful layers. They tell me just as much.

• 19:13Daniel Whiteson?

  I'm saying, and unfortunately you can quote me on this, chemistry has its uses. There are places where there are problems you can't solve using biology or using physics. You need that intermediate step where you're like, you're thinking about the stoichiometry and whatever. So you're like, yes, chemists out there, I appreciate you. It's not that chemistry is terrible. It's just that I can't do it.

• 19:32Kelly Weinersmith?

  Same, same. Yep. No, I appreciate you too.

• 19:35Daniel Whiteson?

  All right. So then late 1800s, we have Boltzmann. He's one of the guys that founded statistical mechanics and thinking about things in terms of the particles. He really wanted to understand like, what is temperature in terms of the particles? Like if I have something hot and something cold, what's going on microscopically? And he made this huge contribution connecting temperature to microscopic motion and specifically defining entropy in terms of what's going on microscopically. This is a huge leap forward and really one of the only places in physics, or maybe even in science, where we have a mathematical bridge between two different layers of reality, where you can take the microscopic understanding of like particles whizzing around and use

• 20:18Daniel Whiteson?

  it to derive the macroscopic rules, right? Different levels of realities, like when you have different kinds of laws, like particles have different behavior than liquid flowing. But we know liquids are made of particles. And so liquids are this thing that emerge from particles. And usually we don't know how to derive it. We can like find the laws for liquids and find the laws for particles, but we don't know how to connect them, right? Like you can't derive fluid mechanics from the standard model. But here, he developed an understanding of what's going on for particles and he built the mathematical bridge. Like you can derive the ideal gas law from Boltzmann's description of what's happening with these particles. It's amazing. It's like the only place I've ever seen this kind of connection

• 20:58Daniel Whiteson?

  where like, not only do you have a reductionist ability to see the lower level, but also there's like a mathematical bridge that shows you why it works. It's kind of incredible.

• 21:08Kelly Weinersmith?

  Why is that so rare?

• 21:10Daniel Whiteson?

  Because the universe is complicated, you know, like to go from microscopic to macroscopic, you have to describe a lot of stuff. And usually there are two hurdles, chaos and approximations. Chaos, because like sometimes the tiny little details matter, you know, like butterfly flaps, it swings in China, hurricane goes a different direction. Like, so you can't ignore those little details, which means you have to keep track of lots and lots and lots of little details. Like remember how many atoms there are in a drop of water, right? Like Avogadro's number is a big, big number. Calculating all these details is essentially impossible. So you end up making approximations. And sometimes those approximations work, like Boltzmann's big contribution was finding ways to calculate these averages that work mostly, but sometimes they don't. And maybe

• 21:55Daniel Whiteson?

  we just don't have the right kind of math. So in principle, it should be possible, but the approximations we make along the way and the sensitivity to the little details make it really, really hard. That's why we need chemistry.

• 22:06Kelly Weinersmith?

  All right. Well, on that note, let's take a break so that we can sort of absorb the fact that we need chemistry and come to terms with that. And when we get back, we'll talk about the different actual definitions of entropy.

• 22:29Speaker 01

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• 22:57Speaker 05

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by rising star Sienna Spire. What is so fascinating to you about impermanence?

• 23:19Speaker 18

  Everything about us, everything we do is temporary. And I just got terrified of that. You know, what we do with our time and how I feel like the way the world is today and how fast paced it is and how much we don't even acknowledge anything around us. How I felt temporary to the people around me and how that made me scared and how much I wanted to last and make an impact on people. And I guess to change the way I looked at things and to change myself and to change how I guess I approached everything in my life. It's important that things are impermanent because nothing would feel special. And enjoying, you know, learning to love with knowing it's not going to last.

• 23:53Speaker 05

  Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.

• 24:03Speaker 02

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• 24:41Kelly Weinersmith?

  All right, we're back. We've all come to terms with the fact that we need chemistry in our life. And I have a confession to make. I actually minored in chemistry and my birthday is October 23rd, which is 10/23 Avogadro's number, 10 to the 23rd. So when I was in college, all of my parties were chemistry themed parties. We would play like periodic table games. I know, I know. It's just confession time here.

• 25:03Daniel Whiteson?

  You dropping this on me now? We've been working together for so long.

• 25:06Kelly Weinersmith?

  I felt like I had to tell you. I can't live this lie anymore, Daniel.

• 25:11Daniel Whiteson?

  Is this burning a hole in your psyche all these times?

• 25:14Kelly Weinersmith?

  But I cannot tell you how large my bill was at the glassware depot because I broke so much glass in my chemistry journey. I decided there's no way I can make a living out of this. I can maybe just become bankrupt. But anyway, okay, let's get back to entropy. We've talked about how you can study heat to do work and how that heat sort of moves around. Tell me about the relationship between those phenomena with entropy.

• 25:39Daniel Whiteson?

  Yeah, so what we end up with is a description of entropy from several different perspectives. We have like Carnot and Clausius. Their description of entropy is like something in the same level is like temperature and energy, a macroscopic quantity, right? A thermodynamic quantity that relates to like temperature and energy flow. And then we have Boltzmann. He describes entropy statistically in terms of like the little particles and how you average those up and how that emerges from those tiny details. Later on, a guy named Shannon creates an idea of information entropy, which is probably what people were talking about when they connect jazz to entropy. So we have three different definitions of entropy, and there are actually more.

• 26:21Daniel Whiteson?

  There's like five different definitions of entropy, and they're all related. And we'll talk about the statistical and the thermodynamic definitions of entropy today. But they're connected. They're not the same thing, but they are similar. And you might think like, what are you talking about? How can you have different definitions of the same thing? Right? Well, we already have that like for temperature. Temperature, we have a statistical view of temperature. Maybe a thermodynamic view of temperature. These things are a little bit fuzzy. And just like we have different levels of understanding of the universe, some of which are useful sometimes and not others, like particle physics, great when you're at the LHC, not so useful when you're pouring liquid into beakers. Right? They're useful in some cases and not useful in others because we don't have a complete understanding of the universe.

• 27:06Daniel Whiteson?

  We have these approximate, limited views into the universe. And you got to pick which toolkit you use. So that's why we end up with several different definitions of entropy. But you know, they are connected. And today we're going to show you some of those connections.

• 27:19Kelly Weinersmith?

  And as a biologist, I'm thinking about the definition of species of the word species.

• 27:23Daniel Whiteson?

  Right.

• 27:24Kelly Weinersmith?

  Same sort of situation. We'll have a whole episode on that at some point.

• 27:27Daniel Whiteson?

  Please, that sounds fascinating.

• 27:30Kelly Weinersmith?

  Yeah, we could drink and discuss this for like weeks on end if we have enough biologists together. That and somehow we'll start talking about poop. But anyway.

• 27:37Daniel Whiteson?

  Also, and then I get to have fake outrage at how ridiculous you guys are.

• 27:42Kelly Weinersmith?

  That's fair. That's fair. Taste of my own medicine. All right, let's start with the statistical definition.

• 27:48Daniel Whiteson?

  Okay, so statistical definition is a good entry point because I think it connects to a lot of people's intuitive description of entropy as related to chaos or disorder or something. And you often hear people say entropy is a measure of disorder in the universe. But that's missing a lot of really important nuance that I really want people to grab hold of. Entropy does have to do with order, but specifically it's a relative quantity. It's not an absolute thing where you're like, you measure the disorder and you get a number. It has to do with how much information you have about two relative levels of the universe. And it requires you to define those two levels. So we have like a macroscopic view, things you could observe like. Temperature, or energy, or density, or something that you can measure sort of at

• 28:33Daniel Whiteson?

  the human level, and then microstates, things that you can't observe, arrangements of the particles, or something in observable that would give you that same macrostate. So you have to pick these two levels, right, macro and micro, in order to even define entropy. Entropy has to do with how many different microstates you can have that are consistent with the same macrostate that you measure.

• 28:56Kelly Weinersmith?

  I would love an example.

• 28:58Daniel Whiteson?

  All right, so let's do an example. Let's say, for example, you have 10 coins, and you flip them, they're either heads or tails, okay? And let's say that macroscopically, because you have limited information about the universe, all you can know is how many heads there are. You can't tell which coin is heads and which coin is tails, you can just know how many heads there are. So maybe there's five, maybe there's 10. Macroscopically, you always have limited information. Like, when you measure the temperature of your coffee, you're not measuring the speed of every individual particle, you have some big, overall, averaged quantity, right? So that's your macroscopic information. And then we'll define the microscopic, because like, actually, which coins are heads, right? So, you know, microscopically, like, maybe it's the first five are heads, and the second

• 29:40Daniel Whiteson?

  five are tails, or whatever. Okay, so we've defined a macrostate and a microstate. And entropy is a measure of how many microstates you can have for a given macrostate. So say, for example, my macrostate, which is just how many heads there are, I flip all the coins, and I tell you there are zero heads. Well, how many microstates are there that can give you zero heads? How many arrangements of those coins can give you zero heads?

• 30:06Kelly Weinersmith?

  One. They all have to be tails.

• 30:08Daniel Whiteson?

  Exactly. So, that's a very small number of microstates. What if I do it again, and this time, I can tell you, well, the macrostate is that one of the coins has heads. How many microstates are there that can give you one coin having heads?

• 30:23Kelly Weinersmith?

  10.

• 30:24Daniel Whiteson?

  Exactly. 10 choose one, for the mathematicians out there. Now, if I say, okay, we'll do it again, and this time we got five heads, how many microstates are there?

• 30:32Kelly Weinersmith?

  I'm gonna give you the middle finger, because I can't calculate that on air.

• 30:38Daniel Whiteson?

  It's a big number, right? It's like 10 times 9 times 8, whatever. It's a big number. So the point is each macro state has a different number of micro states. Some of them have only one arrangement of the coins that will give you the same macro state. That's low entropy. If you have few micro states that are consistent with that macro state, it's low entropy. If you have a lot of micro states that are consistent with your macro state, like if your macro state is five heads, then there's lots of different arrangements of that. That's high entropy. Okay, so the key here is it's not just disorder, like how scrambled are the heads and tails? It's relative lack of knowledge between the micro state and the macro state.

• 31:16Kelly Weinersmith?

  Okay, I get that.

• 31:18Daniel Whiteson?

  If you hold on to that in your head, it actually makes it very easy to understand why entropy tends to increase in the universe. You just need one more piece of information. If you assume that all the micro states are equally likely, like any particular arrangement is equally likely. And that's true in this example of the coins because like, you know, every coin toss is independent. You're just as likely to get all heads as all tails or any other particular arrangement, like heads, tails, heads, tails, heads, tails. If I'm specifying them exactly, every micro state is equally likely. What does that mean? Well, if you just slip all the coins, you're more likely to get a macro state that has high entropy. Because the macro states that have low entropy by definitions are the ones with few micro states.

• 32:02Daniel Whiteson?

  Like it's hard to get all heads or hard to get all tails. There's lots and lots of ways to get five heads and five tails. So if you keep flipping coins, right? Then on average, you're going to get higher entropy than lower entropy. And so the universe does this not with coins, but with quantum states. If each quantum state of the universe is equally likely, then the universe tends towards higher entropy. Because as you keep flipping coins, you're more likely to get microscopic configurations that give you higher entropy, just because there are more of them.

• 32:37Kelly Weinersmith?

  So so far I haven't heard anything that would suggest that that makes the universe more disordered or anything like that. And is that right?

• 32:45Daniel Whiteson?

  So here we get a little slippery because we have a nice crisp mathematical definition of microstates and macrostates and numbers and entropy is mathematically the log of the number of microstates. So what do we mean by disorder? Disorder is like one of these intuitive words that we use that we don't have a crisp definition of, but we can try to connect it. So for example, if I told you all the coins are the same, you'd be like, oh, that's nice and ordered. If I told you, oh, it's a scrambled heads, tails, tails, heads, whatever, that would seem more disordered. Right? And so in that sense, it connects with that intuitive definition. But I think there are other examples that are maybe more intuitive.

• 33:22Kelly Weinersmith?

  When I hear disorder, my connotation of disorder is that it's something bad is happening or like we're moving towards a state of more badness. But what we're really saying is that as entropy increases at like each individual point of interest, it's just harder to predict what's happening at each of those spots.

• 33:45Daniel Whiteson?

  Yeah, I don't think you need to connect disorder with badness, you know, to think like maybe the universe is just getting jazzier as time goes on.

• 33:52Kelly Weinersmith?

  That sounds good. That sounds good, depending on my mood.

• 33:55Daniel Whiteson?

  You know, we're getting rid of the melody. We're kicking out ancient ideas about keys and whatever. We're just wandering up and down the scales without a plan. The universe is just getting jazzier.

• 34:06Kelly Weinersmith?

  Is it less planned or is it just you don't have as good a handle at the microscale of what's going on? Like, does that necessarily result in something less planned? I guess you would say that zero heads is a more plan-y state to be in than five heads. Is that what we're saying?

• 34:28Daniel Whiteson?

  I have a little bit of trouble with the intuitive concept of disorder again, because it's not very well defined. I think it's maybe easier to think about it in terms of where stuff is physically rather than heads and tails. So let's take another example that's maybe more intuitive. So let's say, for example, you have 100 particles in a box and instead of just knowing like the average energy of those particles, your macroscopic measurement can tell you the energy distribution. So you can tell the difference between like all the energies in one particle or the energy is shared. Okay, and so if all the energy is in just one of those hundred particles, one of them is like going crazy whizzing around, the other ones are just sitting there. How many microstates are there that are consistent with that? Well, 100, because there's 100 particles.

• 35:13Daniel Whiteson?

  And you can't tell which particle is which, but there's 100 ways you could give all the energy to just one particle. On the other hand, if you share the energy, right? If you're in a macrostate where the energy is smoothly shared between them, now you have a lot of different particles that have energy. So there's like 100 times 99 times 98, whatever. There's lots of different ways to arrange those particles so that they share the energy. So, you know, that seems more disorder because now you have more particles whizzing around than rather than just one particle. And you can make sort of similar arguments about physical locations of particles. If your macrostate is to measure the distribution of the particles, right? Then having all the particles in one corner of the box gives you very few ways to arrange

• 35:56Daniel Whiteson?

  the particles, whereas having the particles all the way through the box is lots of different ways to arrange those particles. And so the reason that happens is that those have higher entropy. Or another way to say that is that there are just more ways for that to happen in the universe. So if all the microstates are equally likely, the ones with more entropy are more likely. And in that sense, entropy is connected to disorder because it tends to share energy, spread energy out, and also spread particles out. So it tends to make things smooth and even rather than like clumped and tight together.

• 36:30Kelly Weinersmith?

  All right, now I'm with you. That definition landed better for me, I think, or that example.

• 36:35Daniel Whiteson?

  All right, cool. Yeah, but it's important to understand that the statistical definition of entropy really requires you to define these two levels. And so there is no absolute sense of entropy. Like you and I could look at the same system and have different numbers for entropy if you have a different macrostate. If you can observe more fine-grained details than I can, we have different macrostates. If we're thinking about different microstates, we have different entropies. Entropy is a relative thing, like velocity, right? So it's not some fundamental thing in the universe. From a statistical point of view, it's this relative thing. But it's also connected to energy and temperature and this thermodynamic sense of entropy that Clausy has invented.

• 37:14Kelly Weinersmith?

  So could we go through this example again, but think about how? Boltzmann identified the macro and the micro states?

• 37:23Daniel Whiteson?

  Yeah, absolutely. Let's do that. And then let's think about how that gives us a handle on energy. And it's going to take us to understanding why energy flows and the macroscopic sense of energy. And so let's back up and think about Boltzmann. Before we talk about entropy, let's just talk about temperature, because temperature is this other thing where we have like a definition of it microscopically and macroscopically. Temperature macroscopically is like, well, you put a thermometer in something, right? Or you touch something, you can feel it's hot or it's cold. But we also amazingly have this microscopic sense of temperature. Microscopically, we think about just particles and their velocities. Like, it gets more complicated. You're talking about solids and liquids and whatever, and different like vibrational states. But just imagine a box of particles, and it's a simple gas, and the particles are whizzing

• 38:06Daniel Whiteson?

  around. What happens when something is hot is those particles have higher speeds. And when something is cold, those particles have lower speeds. And I think a lot of people already have a sense of this. But what maybe you don't appreciate is that this really is a mapping between the microscopic and the macroscopic. You're like, hot equals high-speed particles, cold equals low-speed particles. That's amazing. It's incredible that we have this connection, right? And those are two different definitions of temperature. Definition one, statistical, microscopic sense of like particles moving. The other, macroscopic, thermodynamic definition of temperature, where you're like, it's hot, it's cold, right? I feel hot, and therefore, heat is flowing from me, right? This is incredible. And this

• 38:48Daniel Whiteson?

  is what Boltzmann did. He connected these two senses of temperature, microscopically and macroscopically.

• 38:55Kelly Weinersmith?

  So just to nail the point home, high temperature is more entropy. Is that right? Because they're moving around in more disordered?

• 39:02Daniel Whiteson?

  Ooh.

• 39:02Kelly Weinersmith?

  Oh, no!

• 39:05Daniel Whiteson?

  Great question. And entropy is a slightly more subtle connection to temperature. When energy flows to erase a temperature difference, like when something goes from hot to cold, entropy is the stuff that's being transported. And sort of the same with it like, If you imagine an electric circuit and you have a voltage difference, what happens when you have a voltage difference? You have flow of current, right? You have charges flowing from one to the other to balance that out. Charge is the stuff that's transported. When energy flows to erase a temperature difference, you can think of it entropy is like the charge of that system. It's the stuff that's being transported. And so there's this connection between energy and heat and temperature and entropy.

• 39:48Daniel Whiteson?

  That's a little bit subtle, but I think it's important to understand. So we're familiar with the idea that energy flows, right? Things flow from hot to cold. Why does that happen, right? Why do things flow from hot to cold? The answer is that when things flow from hot to cold, the number of microstates tends to increase, right? Just like the example we talked about a minute ago with the particles. If you have one really, really hot particle and 99 cold ones, that has less entropy than sharing the energy among the particles. There's more ways to arrange it. If you can give it to all the particles and just give it to one, there are more microstates. So what happens when you have a temperature difference? Is even opportunity to increase the entropy. So energy moves to maximize the microstates,

• 40:32Daniel Whiteson?

  not because like, oh, the universe likes energy to be spread out or something like that. It's because all the microstates are equally likely and energy flows in a way that increases the number of microstates, right? Maximizing entropy is what causes energy to flow. Or another way to say it is like, energy flowing is increasing the entropy, right? And energy stops flowing when it no longer will increase the microstates. If you have like two systems next to each other, A and B, energy will flow from one to the other if it increases the number of microstates, right? So now as energy is flowing from A to B, A is losing energy. It's going to have fewer microstates. You're losing microstates, but B is gaining them.

• 41:15Daniel Whiteson?

  And this will happen as long as the gain in B is greater than the loss in A, right? So as soon as that equalizes, as soon as moving energy from one system to another will not increase the total number of microstates, energy stops flowing. And that's what we define to be temperature. Temperature is this relationship between energy and entropy in a material. If the temperatures are equal, there's no gain in energy flowing. It does increase the energy. So that's the definition of temperature thermodynamically. If the temperatures are equal, no energy flow. And mathematically, we define temperature to be this ratio of a change in energy to a change in entropy. Chemists out there probably know it's D E D S.

• 42:00Daniel Whiteson?

  So you don't have to have equal energy between systems. What you need is equal temperature, which means that any energy that moves will make an equal change in the entropy. And so when the temperature is equal between the two objects, no heat will flow because D E D S is the same. That's the definition of temperature thermodynamically. Right? We have the microscopic definition of temperature. It's like particles whizzing around. It's their speed. Now we have this weird thermodynamic definition of temperature as a ratio of energy to entropy. And it turns out you can derive one from the other. Right? You can start with the mathematics of kinetic energy of particles and derive this definition. That's what Boltzmann did. It's incredible mathematical bridge of temperature from one to the other and also helps us

• 42:45Daniel Whiteson?

  understand entropy from one to the other. And so that's sort of the thermodynamic sense of entropy. And I think it's amazing because it tells us why energy flows. Energy flows to increase the number of microstates and it will stop flowing when the number of microstates will not be increased.

• 43:03Kelly Weinersmith?

  Okay, I exactly kept up with that explanation. So my brain has no questions yet. So let's take a break. And when we get back, we'll see what Kelly's brain has to offer.

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• 45:28Kelly Weinersmith?

  And we're back. So now we have two different definitions of entropy. Let's talk about some applications of this knowledge.

• 45:38Daniel Whiteson?

  Yeah, entropy has a lot of really deep consequences and it touches so many topics in physics. From time to life to black holes, to the Big Bang, to the future of the universe. It's really incredibly pervasive. One reason is that it's so simple. It just tells us about how systems evolve. They evolve from less likely to more likely. And what does that mean, the consequences? And one of the deepest connections with entropy is what we heard the listeners say that somehow entropy is responsible for why time moves forwards. And I remember hearing this for the first time and thinking, what? That's crazy. That would explain such a deep mystery, right? Like, we have these three directions of space and one of time.

• 46:20Daniel Whiteson?

  And we know space and time are related and connected by relativity, but time is different. You can always revisit a location in space, but you can't revisit a location in time. And you can move positive and negative in the X axis, but only positive in the time axis. And why forwards and not backwards? And like, what does this all mean? So there's some deep mysteries here about what time is.

• 46:40Kelly Weinersmith?

  The idea here that if entropy is increasing, you're never going to get all the particles back in that same configuration. And that's why you can't go back in time? Oh, no, you're laughing.

• 46:50Daniel Whiteson?

  I love that. I think that is sort of a summary of how physicists put it. The argument is a little bit more elaborate. It's that when we look at the laws of physics, so many of them are reversible. They don't seem to refer one direction. You know, for example, in a vacuum, if you bounce a ball, it hits the floor, it comes back up, it'll come back up to the same height. And so that path is reversible, right? If you play a video of that happening in a vacuum where there's no energy loss, then you can't tell if the video is being played backwards or forwards. The same laws of physics apply and describe it perfectly. Same with particles almost completely. And so people were wondering for a long time, like, well, if the laws of physics don't care if time goes forwards or backwards, they work both ways. Why does time go forwards?

• 47:34Daniel Whiteson?

  And so entropy seems to be one of the places where physics has a preference for one direction or the other, right? Like it likes the microstates to increase. So energy increases with time. And then there's this leap people make. They say, oh, energy and time increase together. Therefore, that's why time moves forwards. And I'm not sure I follow that leap of logic. Like, I will accept that entropy and time are connected. Entropy increases as time goes up. But that same law could tell you like, well, the universe could still run backwards. It just wouldn't be symmetric. It would just mean that if the universe ran backwards, entropy would decrease. That law predicts that also. It predicts that if time ran backwards, entropy will decrease. Why don't we live in that universe? I don't know.

• 48:20Daniel Whiteson?

  It's consistent with the second law of physics, right? As long as time runs backwards. So I don't believe that the second law of physics tells you why time runs forwards. It connects time and entropy, but it doesn't tell you why time goes forward or backwards. There could be folks out there living in another universe where time runs backwards and entropy is decreasing, and they're claiming that entropy is the reason time runs backwards. Of course, they don't call it backwards. There's definitely some interesting hints there, but I don't believe that it conclusively shows us why time goes forwards.

• 48:50Kelly Weinersmith?

  So maybe I've missed this, but I think that's the first time we've used the phrase "second law."

• 48:54Daniel Whiteson?

  The second law is just a statement that entropy increases as time goes on.

• 48:58Kelly Weinersmith?

  Okay.

• 48:58Daniel Whiteson?

  That delta S is greater than zero, right? S is the symbol for entropy, and so it just says that entropy increases as time marches on.

• 49:05Kelly Weinersmith?

  All right, so if I can hijack the conversation for a second.

• 49:08Daniel Whiteson?

  Oh, please do.

• 49:09Kelly Weinersmith?

  So, you know, as someone who spends a lot of time with evolutionary biologists, I've been to a couple creationism-evolution debates, and those get spicy.

• 49:21Daniel Whiteson?

  That really bends the meaning of the word "debate" also.

• 49:24Kelly Weinersmith?

  I felt like I learned a lot by thinking through the arguments. But anyway, so a common point that is brought up during these discussions is that the second law says that things should be getting more disordered with time. So how do you have evolution creating more complex organisms over time? And what is the answer that you would give them? The answer I heard, if this is helpful, is it has to do something with the second law being about if you're in a closed system, but the Earth isn't a closed system because energy is coming in from the Sun. And so if you're not in a closed system, none of that holds. Is that right?

• 50:00Daniel Whiteson?

  Yeah, I'm thinking about it for a minute first because I'm wondering what it meant here by "complexity." You know, like, and whether even is connected to entropy statistically, order, disorder, microstates, macrostates, or if it's just this sort of like intuitive, this seems similar to that. So let's use one word in place of the other.

• 50:20Kelly Weinersmith?

  Yeah, okay.

• 50:20Daniel Whiteson?

  I'm not even really sure the complexity is connected to entropy at all. But it's definitely true that life and entropy have a close connection because living things tend to decrease their local entropy. My body is a system that decreases its entropy. You might wonder like, well, if entropy is supposed to always increase, how does that happen? Well, as you say, I'm not isolated, right? I have a huge environment around me. And so I exchange energy with the environment, do all sorts of complicated stuff to locally make my entropy go down. Overall, I'm increasing the entropy of the environment I'm interacting with more than my entropy is decreasing. So overall, second law is fine. The point of the second law is you can't pick out a one part of a system and say the entropy

• 51:03Daniel Whiteson?

  always has to increase for every subpart of the system. It's just the whole system where entropy has to increase. The same way like you can't apply conservation of energy to just half of a system and say, oh, the energy is flowing out of this. And so energy is not conserved, like energy is conserved for the whole system. Entropy increases for the whole system. So actually, this is one way that some physicists think we should define life as like, systems that decrease their local entropy at the expense of the environment. Because, you know, biologists spend hours arguing like, what is life anyway in a system that can reproduce? Is it something that passes on genetic information, whatever? There's all these definitions of life. And this is sort of a physics based definition of life. It's something that decreases its local entropy. It doesn't violate the second

• 51:48Daniel Whiteson?

  law of thermodynamics to decrease your local entropy at all. And I don't know how to think about evolution in terms of complexity. Like, I guess, evolution has produced systems that tend to decrease their local entropy more and more as time goes on. But to me, that's not a violation of the second law of thermodynamics at all, or really says anything about micro versus macro states.

• 52:10Kelly Weinersmith?

  Interesting. That's not an answer I heard at any of the debates that I attended. I was raised. We don't need too much of Kelly history here, but I was raised Catholic in a family where, and I know Catholicism is okay with evolution, but I was raised in a family where the young Earth hypothesis, that Earth is only 6,000 years old, was held pretty tightly. And so I went to a lot of these debates to try to figure out how I felt about it on my own. And the answer I always heard from the evolutionary biologists was, well, we're not a closed system or whatever. So anyway, that was really interesting. Would a virus be alive according to the physicist's definition of life?

• 52:44Daniel Whiteson?

  That's a good question. I think it would be. I once had a conversation with Sarah Amari Walker, and she wrote a fascinating book last year about this whole question. So I should ask her that, but I think so. But let me ask you, what was it that convinced you that the Earth is not young, that it's billions of years old and not thousands of years old, assuming, of course, that you got there?

• 53:05Kelly Weinersmith?

  I did. I did. It was, I mean, I took enough classes where I learned about the various ways we date rocks and about the fossil record and how complete it was and just sort of engaged more with what we actually know in the science. And it became pretty clear to me that the data was pointing to an Earth much, much, much, much, much, much, much, much, much older than 6,000 years old.

• 53:28Daniel Whiteson?

  Yeah. Wow. Fascinating. So thinking about deep time and the history of the universe, the future of the universe, entropy is also connected to these ideas of like the Big Bang and the future of the universe and black holes. And there's a lot of confusion out there about what entropy tells us about these things. I think partially because people are thinking about entropy from a temperature point of view or a gravitational point of view, which are actually a little bit different. And people are thinking about no entropy, meaning no temperature. So I thought it'd be useful to go through these a little bit and help untangle some of the confusion.

• 54:00Kelly Weinersmith?

  Go for it.

• 54:00Daniel Whiteson?

  So let's start at the very beginning with the Big Bang, right? If the universe is increasing in entropy all the time, then as you go back in time, the universe is decreasing in entropy. And that means that entropy gets lower and lower and lower, which means, you know, if you go back to the very first few moments that we can think about, what we call the Big Bang, when the universe was very hot and very dense, then that must have been very low entropy, right? Because entropy is increasing. So entropy must have been very, very low. But it's hard to get your head around, like, I'm imagining a hot, dense gas, and it's pretty smooth, right? It's not very clumpy. That doesn't seem to me like a very low entropy situation. In fact, it seems like kind of disorganized and everything's flying around. How is that low entropy? This is confusing to people, but the key thing

• 54:44Daniel Whiteson?

  to understand is the dominant force there is gravity. So instead of thinking about entropy from a point of view of like, the temperature of the particles, think about the arrangements of the particles, and what's a more likely arrangement?

• 54:56Kelly Weinersmith?

  So gravity is pulling everything to one spot, whereas without that, it would have been all over the place and much more disordered and spread out. Is that the idea? I'm gonna stop trying to finish your sentences because it reveals how little I'm understanding. But I think I'm getting this.

• 55:12Daniel Whiteson?

  From a gravitational point of view, being really spread out is very low entropy, and being clumped together is higher entropy, right? Because gravity is not the same as heat. Gravity tends to clump things together instead of spread things out. And so from a gravity point of view, being very spread out is rare. Like, if you have a bunch of matter and you let it sit there, like, it's very rare for it to be perfectly spread out. Like, for that to happen, everything would have to be in perfect balance, like a universe that's completely smooth, where there's no perturbations. That's what would be required for gravity to not be able to clump things together. So gravity likes to clump things together. Clumping things together increases their entropy from a gravitational point of

• 55:55Daniel Whiteson?

  view. Remember, we said entropy is relative. It's not like there's a certain number for the universe, or a certain number even for a system. It depends on the arrangements, and what you define as the macro and the micro states. And so from a gravitational point of view, an initial state where everything is very spread out is quite unlikely. And actually, one of the deep mysteries of the universe is, why did the universe begin in such a low entropy state? If we're going from low entropy to high entropy, and it turns out there's actually going to be a maximum entropy of the universe, then the sort of the time it takes to get to the maximum entropy, which we'll call the heat death, which we'll talk about in a minute. Defines a sort of interesting period of the universe. Once we get to the heat death, the universe isn't really very interesting anymore. So how low the entropy is when we begin sort of defines how long we can do interesting

• 56:40Daniel Whiteson?

  stuff, right? And we're sort of lucky the universe began with very, very low entropy. Like if it started with very, very high entropy, not much would happen. It would just sort of continue that way. And so one of the mysteries of entropy actually is like why it started with such low entropy. And as gravity continues to do its work, it makes black holes, right? It can clump these things together and black holes actually have the maximum entropy. Like there's no way to arrange a mass with higher entropy than a black hole. It's like the maximum entropy arrangement of a system. And the fact that black holes have an entropy is really fascinating. And it was one of the ways that Hawking and his collaborators figured out that black

• 57:21Daniel Whiteson?

  holes must glow a little bit because having entropy means you can define temperature for black holes. And if you can define temperature for black holes, then you can think about them glowing like everything else in the universe that has temperature. So you can derive Hawking radiation thermodynamically saying like, well, if it has entropy, it's got to have some temperature and then it should glow in the universe. And you can think about the temperature of a black hole and actually really, really massive black holes have lower temperature, which is why they glow less than small black holes that have higher temperature and they glow brighter. And a lot of people think that Hawking derived his idea of Hawking radiation from like thinking about the little particles near the edge of the black hole. But that's not true, actually. It's not where the derivation comes from.

• 58:04Daniel Whiteson?

  It comes from thermodynamics because we don't understand the gravity for little particles. Like there is no way to think through that little example microscopically of what happened to the particles. We only have a macroscopic understanding of Hawking radiation because as we've said many times in the show, we don't understand quantum gravity.

• 58:22Kelly Weinersmith?

  I'm trying to decide if I can rescue my comparison about my house being a mess with entropy. Could I say my house is like a black hole? Because that's where the maximum entropy is, or I really need to let this comparison go, I think. This is where disordered has a negative connotation to me, and I think that maybe that's been holding me back this whole time.

• 58:41Daniel Whiteson?

  I'm not going to comment because I feel like that's going to put me in the middle of your marriage.

• 58:44Kelly Weinersmith?

  Okay, let's move on then.

• 58:45Daniel Whiteson?

  And I want Zach to like me, so.

• 58:46Kelly Weinersmith?

  All right, let's move on to heat death.

• 58:48Daniel Whiteson?

  Yeah, so what's going to happen at the end of the universe? Well, you know, gravity clumps things together into black holes eventually, but those black holes also glow, right? And so you get the universe increasing its entropy, you get black holes, and those black holes glow out photons. And so the final endpoint of the universe is those black holes evaporate into photons, and the universe is just filled with this hawking radiation, sometimes photons, sometimes other particles. And that's the state of maximum entropy. So how do we understand the entropy in this whole story? It's low when the universe begins with matter mostly spread out and then grows as the universe gathers together things into massive objects like black holes, and then keeps growing as the universe converts those black holes back into a bath of matter and radiation

• 59:33Daniel Whiteson?

  from black hole evaporation. How does that make sense in terms of our definition of entropy and microstates and all that? Yeah, the answer is it really doesn't because we don't have microstates for gravity. That would require understanding how gravity works for particles, and we just don't, not until somebody cracks quantum gravity. So this concept of black hole entropy is not statistical, it's thermodynamic, as we mentioned a minute ago. It's derived from arguments about temperature and about energy. We know that a black hole entropy grows with its surface area, and that lines up with our understanding that entropy grows because gravity will gather stuff together to make black holes larger. But then, how does it make sense for entropy to keep growing as those black holes evaporate

• 1:00:18Daniel Whiteson?

  away to something that resembles the early universe again? But it turns out the heat death bath of radiation is not the same as the early universe conditions. We think that the quantum information is still there. The whole history of the universe is encoded. And so the particles that evaporated from the black holes are all entangled together in a complicated way people are still figuring out that holds that information. So the answer is, we still aren't sure about a lot of this. Black hole information and entropy is a very active area of research. And that's the best explanation I can give you.

• 1:00:50Kelly Weinersmith?

  Daniel, that was a perfect explanation.

• 1:00:52Daniel Whiteson?

  And so we end up with this situation where energy is sort of surprisingly spread out again. But it's not cold. People confuse the heat death of the universe and think, oh, nothing is moving. Right. They think it's like death, freezing, but it's not absolute zero. It just means there's no way to get anything done. The way like Karno was saying, that you need energy differences to get stuff done. Like to run your temperature, you need hot and cold. So the energy can flow from hot to cold. You need water to flow downhill so you can capture it with your water wheel. If everything is flat and smooth, then there's no way to do anything useful. Right. And so that's what the heat death is. Not when everything is frozen, not when particles can't move, but when there's no way to do anything useful in the universe.

• 1:01:35Daniel Whiteson?

  And so that's why you can't have like life or anything else interesting. Cause everything's maximally spread out. You can't take advantage of any energy differences because there aren't any anymore.

• 1:01:44Kelly Weinersmith?

  We know what temperature the heat death will be if it's not absolute zero?

• 1:01:47Daniel Whiteson?

  Yeah, it's a great question. It depends on how long it takes because as the universe expands, it cools. And we don't know actually how quickly the universe will be expanding in the future. It's accelerating, but you know, the mechanism for that acceleration is dark energy, which is not understood. So unfortunately I can't give you a number for that today.

• 1:02:03Kelly Weinersmith?

  Well, one of the things I love about our conversations is that so often halfway through a lesson, which is what some of these end up feeling like. I feel like I enter the conversation thinking, okay, I know what we're talking about. And I leave thinking, wow, that was a lot different than what I thought the answer was going to be. And so I always leave and think about the conversation like much longer into the day. It sticks with me for a while. And so, thank you for helping me realize that I shouldn't be making entropy jokes about my house.

• 1:02:35Kelly Weinersmith?

  And now I'll start thinking about jokes about how no work can get done. I'm gonna start working on that.

• 1:02:42Daniel Whiteson?

  Yeah, or, you know, use jazz instead. Hey, Zach, can you jazz up the kitchen a little bit? Or, the kitchen's gotten a little too jazzy.

• 1:02:49Kelly Weinersmith?

  A little too jazzy, I think, is the problem with the kitchen. It needs more melody.

• 1:02:55Daniel Whiteson?

  I love thinking about these topics, and especially helping people understand how they really work, because so often, the real understanding of it is more fascinating and more interesting. We're not throwing a wet blanket on people's idea of entropy. We're showing them how exciting, how jazzy it actually is.

• 1:03:10Kelly Weinersmith?

  Yes, and I'm trying to make myself feel a little bit better about the multiple times in this conversation where I got the answer exactly opposite of correct. And I guess I'm hoping that this is a place where people can come with their incorrect, preconceived notions.

• 1:03:24Daniel Whiteson?

  Oh, yeah.

• 1:03:24Kelly Weinersmith?

  And not feel self-conscious about having it wrong.

• 1:03:28Daniel Whiteson?

  Absolutely.

• 1:03:29Kelly Weinersmith?

  Because if Kelly can be wrong so often, and can continue to move through her life with any degree of confidence, then you should also feel welcome to ask us anything. So, reach out.

• 1:03:39Daniel Whiteson?

  Yeah, and if one of our explanations didn't make sense to you, please do reach out. It's not just Kelly who gets to ask questions, and not just me who gets to ask Kelly biology questions. We want to hear your questions. Please do write to us, to [email protected]. We'll write back to you. We respond to everybody.

• 1:03:55Kelly Weinersmith?

  Looking forward to hearing from you.

• 1:03:56Daniel Whiteson?

  All right, until then, everybody, keep it jazzy.

• 1:04:06Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:04:11Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 1:04:18Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 1:04:24Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 1:04:31Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 1:04:40Daniel Whiteson?

  Don't be shy. Write to us.

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