Daniel and Kelly’s Extraordinary Universe - What is Entropy?

Episode Date: April 15, 2025

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

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And it's not helped by the fact that often physicists have done. 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 power isn't usually clearly explained. Today in 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. Hello, my name is Kelly Weiner 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. Hi, I'm Daniel. I'm a particle of physicist, and I think that there's always one person in the marriage who's more orderly and one person is more chaotic. Yeah, it's pretty easy to identify who's who in my marriage. Are you the more orderly or the more chaotic in your marriage? Katrina seems pretty orderly, but so do you. 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. Aw, 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. Exactly. Well, the yin and yang is what makes it exciting, isn't it? 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? 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 nodes doesn't do it for me. I need a little bit of a rhythm and some beat to it and whatever. So I'm more of a blues guy than jazz. How about you?

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. But yeah, 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. 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 balls of line 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 it be to 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'm 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. 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've 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. 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, the 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? 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. Oh, 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. Well, I can tell you moths, not very good on pizza. I mean, they're delicious, but really not a popular topping. 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. That's right.

Let's try to beat back the chaos and stay on topic today, Kelly. Good luck. 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. the tendency of a closed system to kind of go from order to disorder unless you add energy to go from order to disorder I really don't have my head around it entropy is a measure of the

remaining free energy in a specific volume homogenous milk toast a description of the state of a system in terms of its energy the more different configurations a system can have and be in the same state the higher the entropy it has. Matter being either organized or being in a chaotic state. The degree of disorder in a physical system. The amount of energy in a system that can't be recovered. Describes the level of organization. As time progresses, entropy progresses. I think it's the tendency of energy to spread out. Slow crawl towards simplicity that the universe imposes on everything. Disorder, aos, the absence of order.

Things degrade over time. It feels weird because it seems made up what disorder is and what order is. If there are more configurations, the entropy is higher. Number of microstates of the system, which can result in the current macro state. Things want to move from a higher embodied energy state to a lower one. Well, no pizza in those answers, but actually a lot of variety in the answers there. 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. I do think this idea has escaped into like the general consciousness and maybe it has disconnected from its physics definitions in that process. Yeah, I think you see a lot of tech pros on social. media using entropy as if they know what it means.

They really don't. I mean, it's nowhere near as bad as the word quantum, but let's clear things up today. Exactly. It's getting there, though. It's getting there. All right. So tell me about the first time the word entropy was used. 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. The things that bring us all together. Yeah. So you can go back and look at the Sumerians thinking about the path of the planets and the 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. 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? So it's the early 1800s and there's the Carnot cycle. And so this French physicist Carneau, actually a father-son team, were thinking about engines and heat. And it was sort of a mystery at the time. Like, well, 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 there's 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 electric charge where, like, there were two of them and there 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 used to produce work. That's Carnot's big insight in the early 1800s. Did we have the steam engine at this point yet? 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 Carneau understood that engines can do this. Engines can turn heat differences into work. But he describes sort of a perfect engine, one where you can turn the heat differences into work, and then you could 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, but you get a little bit less, sort of like the way if you drop a ball, in principle, the potential energy, 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, and 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 describe this with equations and sort of more mathematical concepts. Okay. But even today, we don't have a perfect engine, right? All of our engines are imperfect? All of our engines are imperfect, exactly. And it was Clausius, who sounds like he should be an ancient Greek guy. He does, like Claudius. 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, of entropy to help us understand why some engines are imperfect. And he thought of it as this thing which flows to 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 Ghiblions 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? But sometimes you invent a number and it means something in the universe. It like describes someone 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' 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.

And so at this point, he's thinking about the move. movement 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? Not disorder. Disorder comes later with Boltman, 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 so energy doesn't flow completely perfectly. But I also love the stuff. 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 trope 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 it to connect it with like ancient languages and ancient thoughts. And he said, quote, I prefer going to the ancient language. for the names of important scientific quantities so that they may mean the same thing in all living tongues. 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. Now, I am imagining him saying that in a toga. He's carving it into marble, right? That's right. And has history judged this to have been a good decision? I don't know. I mean, listen to the listeners.

is 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 superconductivities so like a dude knows his stuff

and he says that Clausius quote succeeded in coining a word that means the same thing to everybody. Nothing. I hope that's not my legacy. 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? I've never been sure about that, but okay. 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. The 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 the 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 truer. And, you know, we'll talk about that 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 at work to describe the

macroscopic without understanding the microscopic. The universe gives us that, that access to many different layers of reality. So if every layer is useful, what I'm here, 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. 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 the

chemistry is terrible. It's just that I can't do it. Same. Same. Yep. No, I appreciate you too. 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 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 Boltzman'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 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. Why is that so rare? 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 its wings 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 that there are in a drop of water, right?

Like Avagadja'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 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. 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.

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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 1023, Avogadro's number, 10 to the 20. third. 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. Confession time here. Dropping this on me now. We've been working together for so long. I felt like I had to tell you. I can't live this lie anymore, Daniel. Was this burning a hole in your psyche all these times? 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. Yeah.

So what we end up with is a description of entropy from several different perspectives. We have like Carnot and Klausius. Their description of entropy is like something in the same level is like temperature and energy, a macroscopic course. 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 they're actually more. 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 and we have 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. times 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. We have these

approximate, limited views into the universe, and you've 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. And as a biologist, I'm thinking about the definition of species, of the word species. Same sort of situation. We'll have a whole episode on that at some point. Please. That sounds fascinating. 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.

Also, and then I get to have fake outrage and how ridiculous you guys are. That's fair. That's fair. Taste of my own medicine. All right. Let's start with this statistical definition. 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. Antropy 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 can observe, like temperature or energy or density or something that you can measure sort of at 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 macro state. 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 macro state that you measure.

I would love an example. 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 this 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 average quantity, right? So that's your macroscopic information. And then we'll define the microscopic as like actually which coins are heads, right?

So microscopically, like maybe it's the first fiber heads and the second fiber tails or whatever. Okay, so we've defined a macro state and a micro state. And entropy is a measure of how many microstates you can have for a given macro state. So say, for example, my macro state, 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? One. They all have to be tails.

Yeah. 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 macro state is that one of the coin has heads. How many microstates are there that can give? give you one coin having heads. 10.

Exactly. 10 choose one for the mathematicians out there. Now if I say, okay, we do it again. And this time we got five heads. How many microstates are there? I'm going to give you the middle finger because I can't calculate that on air. It's a big number, right? It's like 10 times nine times eight, whatever.

It's a big number. So the point is each macro state has a different number of microstates. 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 microstates that are consistent with that macro state, it's low entropy. If you have a lot of microstates 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 microstate and the macro state. Okay. I get that. 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 microstates 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 microstate 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 microstates. 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 fives 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. 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? 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. You know, 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. 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. Yeah, I don't think you need to connect disorder with badness, you know, I think like maybe the universe is just getting jazier as time goes on. That sounds good. That sounds good, depending on my mood. You know, we're getting rid of the melody. We're kicking out inching ideas about keys and whatever.

We're just wandering up and down the scales without a plan. The universe is just getting jassier. Is it less planned or is it just you don't have as good a handle at the micro scale 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 plany state to be in than five heads. Is that what we're saying? 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, a hundred because there's a hundred particles. And you can't tell which particle is which, but there's a hundred 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 macro state where the energy is smoothly shared between them, now you have a lot of different particles that have energies.

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 location. of particles. If your macro state 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 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 all right now i'm with you that definition landed better for me i think or that example 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 macro state. If you can observe more fine green details than I can, we have different macro states. 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

Klossius invented. So could we go through this example again, but think about how Boltzmann identified the macro and the microstates? 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 a thing about Boltman. Before we talk about entropy, let's just talk about temperature.

Because temperatures 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 in different like vibrational states, but just imagine a box of particles and it's a simple gas and the particles are whizzing 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,

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 is what Boltzman did. He connected these two senses of temperature microscopically and macroscopically. So just to nail the point home, high temperature is more entropy. Is that right? Because they're moving around and more disordered? Oh, no. Great question. And entropy is a slightly more subtle connection to temperature.

When energy flows to erase a temperature difference, like when energy flows to erase a temperature difference, Like when something goes from hot the cold, entropy is the stuff that's being transported. And sort of the same way that 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. Charges 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 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.

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 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. 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 thermodynamics. 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. So you don't have to have an 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 DEDS is the same. That's the definition of temperature thermodynamics. 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 the ratio of energy to entropy.

It turns out you can derive one from the temperature. derive one from the other, right? You can start with the mathematics of kinetic energy of particles and derive this definition. That's what Boltzman did. It's incredible mathematical bridge of temperature from one to the other and also helps us 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. 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. A foot washed up a shoe with some bones in it. They had no idea who it was. Most everything was burned up pretty good from the fire that not a whole lot was salvageable. These are the coldest of cold cases, but everything is about to change.

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Presented by Capital One, founding partner of IHeart Women's Sports. And we're back. So now we have two different definitions of entropy. Let's talk about some application. of this knowledge. 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, I'm 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, 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. 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. 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? 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 that people make to 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. 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 physics tells you why time runs forward. 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. So maybe I've missed this, but I think that's the first time we've used the phrase second law. The second law is just a statement that entropy increases as time goes on. Okay. The delta s is greater than zero, right? S is the symbol for entropy. And so it just says the entropy increases as time marches on. All right. So if I can hijack the conversation for a second. Oh, please do. 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. That really bends the meaning of the word debate also. 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? Yeah, I'm thinking about it for a minute first because I'm wondering what is meant here by

complexity, you know, like, and whether even is connected to entropy, statistically, order, disorder, microstates, macro states, 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. Yeah, okay. I'm not even really sure that 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. And 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 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. It's like system 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, is 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 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 thermodynamics at all or really says anything about micro versus macro states. Interesting. That's not an answer. I heard in 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 biologist 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 definition of life? That's a good question. I think it would be.

I once had a conversation with Sir 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? I did. I did. 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 unearth much, much, much, much, much, much, much, much, much, much, much, much, older than 6,000 years old. Yeah, wow, fascinating. So thinking about deep time and the history of the universe and 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 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 would be useful to go through these a little bit and help untangle some of the confusion. Go for it. 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 high, 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 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. 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 going to stop trying to finish your sentences because it reveals how little I'm understanding. But I think I'm getting this. 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 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 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 is the macro and the microstates. 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 an 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 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. I 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 clumps 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 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. 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.

what happens to the particles. We only have a macroscopic understanding of hawking radiation because, as we said many times in the show, we don't understand quantum gravity. 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 disorder has a negative connotation to me. And I think that maybe that's been holding me back this whole time. I'm not going to comment because I feel like that's going to put me in the middle of your marriage. Okay, let's move on then. I want Zach to like me.

All right. Let's move on to heat death. 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 end point of the universe is those black holes evaporate into photons. And the universe is just filled with this hawking. radiation, sometimes photon, 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 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 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. Daniel, that was a perfect explanation. And so we end up with a 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 of it's like death, freezing. But it's not absolute zero. It's

It just means there's no way to get anything done. The way like Carnot 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.

And so that's why you can't have life or anything else. Interesting because everything's maximally spread out. You can't take advantage of any energy differences because there aren't any anymore. We know what temperature the heat death will be if it's not absolute zero. 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. 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. And now I'll start thinking about jokes about how no work can get done. I'm going to start working on that. 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.

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

and not feel self-conscious about having it wrong. Absolutely. 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. 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 questions at danielandkelly.org. We'll ride back to you. We respond to everybody. Looking forward to hearing from you. All right. Until then, everybody, keep it jassy. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you.

We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at Daniel and Kelly.org. 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. Don't be shy. Write to us. I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. avoidance is easier ignoring is easier denials easier complex problem solving takes effort listen to the psychology podcast on the iheart radio app apple podcasts or wherever you get your podcasts it's important that we just reassure people that they're not alone and there is help out there the good stuff podcast season two takes a deep look into one tribe foundation a non-profit fighting suicide in the veteran community september is national suicide prevention month so join host jacob and ashley shick as they bring you to the front lines of one tribe's mission one tribe save my life twice welcome to season two of the good stuff listen to the good stuff podcast on the iheart radio app apple podcast or wherever you get your podcast do we really need another podcast with a condescending finance brof trying to tell us how to

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