Ep. 391: Entropy

Astronomy, Forces, Physics | 2 comments

Have you ever been doing thermodynamics in a closed system and noticed that there’s a finite number of ways that things can be arranged, and they tend towards disorder? Of course you have, we all have. That’s entropy. And here in our Universe, entropy is on the rise. Let’s learn about entropy in its specific, thermodynamic ways, and then figure out what this means for the future of the Universe.

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Show Notes

Entropy as Time’s Arrow


Transcription services provided by: GMR Transcription

Dr. Pamela Gay: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Frasier Cain: Astronomy Cast episode 391, Entropy. Welcome to Astronomy cast, your weekly fact-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. My name is Frasier Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Everettsville, and the director of Cosmo Quest. Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well, how are you doing Frasier?
Frasier Cain: Doing very well. People walking the Weekly Space Hangout and the live show learned that I got married on Friday afternoon.
Dr. Pamela Gay: And that is awesome, and congratulations to you and your new awesome life.
Frasier Cain: Thank you so much. Yeah you met Carla at Dragon*Con, so we finally tied the knot.
Dr. Pamela Gay: So congratulations. I have nothing that exciting here. We dressed up in costumes, we handed out candy, we terrified small children, yeah, the only thing I have to add is I did catch that we’ve been doing something slightly sideways with our recording. Our good friend Uncle Bob who is one of the sponsors of this show switched which of his companies is sponsoring us this year and we’ve been running the wrong ad, and we love Uncle Bob and I want to state it’s Clean Coders who is our current advertiser, and we love Clean Coders and we love Uncle Bob and we’re really sorry that he has so much awesome that we screwed up and promoted the wrong awesome.
Frasier Cain: Oh no. I have no idea how that could have happened.
Dr. Pamela Gay: That’s okay. It’s entirely on me. Yeah.
Frasier Cain: Okay. All right, well let’s move on to this week’s show.
Female Voice: This episode of Astronomy Cast is brought to you by 8th Light Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember that’s www.8thlight.com. Drop them a note. 8th Light, software is their craft.
Frasier Cain: So have you ever been doing thermodynamics in a closed system and noticed that there’s a finite number of ways that things can be arranged, and they tend towards disorder? Of course, you have. We all have. That’s entropy, and here in our universe, entropy is on the rise. Let’s learn about entropy and its specific thermodynamic ways and then figure out what this means for the future of the universe. All right Pamela, in preparing for this show, it’s quite surprising sort of how many people try to explain entropy and then how many people tell those people that they’re doing it wrong. So can you explain entropy and then can we figure out why people think that other people are doing it wrong? And I guarantee that someone’s going to say you’re doing it wrong, but that’s just the internet.
Dr. Pamela Gay: No we have stalwartly avoided this topic for nine years, and you called me on it last episode or two episodes ago.
Frasier Cain: Yeah, “Haven’t we ever done entropy?” “Oh god.” It’s like centripetal force, right? Like, oh, here come the pedants.
Dr. Pamela Gay: I’m good with that one. That one is fine.
Frasier Cain: All right, well here come the pedants.
Dr. Pamela Gay: So entropy is, scientifically, the way we describe how well we understand a system. So, if you look at an atom; if one of its electrons got itself into a higher energy level, you have to describe the energy required to get it there, you can describe its spin, you can describe all of these little tiny factors, but if it’s at its lowest energy level, suddenly the amount of different things that could have gotten it there are so much higher that that actually is harder to describe, so, you have more unknowns, and the fact that you have more unknowns means that it has more entropy, and this really makes very little sense when I first say it, but sadly that is reality. It’s one of those things in science that our stomachs go, “No.” Another way of looking at this is if you have a frozen ice cube and you stick it on your counter, that frozen ice cube has one set of energies, the waters are all locked up in a certain way in a very specific shape, and then the room around it has gas and heat and molecules and stuff and things that are presumably warmer than the ice cube in this particular example.
And over time, the energy in the room will go into that ice cube, and as the energy in the room drops, the entropy in the room drops, but in that frozen blob, as it melts, as more and more disorder goes into its system, as the atoms start to go into a variety of different states that can’t be precisely defined the way they were when they were frozen, the amount of entropy in the ice goes up and it actually goes up more than the amount of entropy in the room went down. And so in the room’s energy, melting that ice cube, it increased the entropy for the room plus the ice cube that is no longer ice.
Frasier Cain: Right, and so one of the important things when you’re thinking about entropy is that you need to consider it in terms of a closed system, right? And so in this case we are thinking of the room, which also happens to contain the ice cube, and that’s the entropy that is sort of moving, tending towards increasing, right?
Dr. Pamela Gay: And a lot of times this is looked at in terms of the energy in the system. The history of where the idea of entropy came out of was actually the way people were trying to understand thermodynamics in the late 1800s, where they were looking at the relationships between pressure and volume and temperature in a gas, and starting to define heat engines and idealized systems such as what’s called the Carnot cycle, where a gas will expand and contract as you change the temperature, but it actually does this in a variety of different steps as the system outside the gas does work on it, and then as the gas does work on the system outside of it as it expands into that system, pushing out, and then as it contracts allowing the system outside to push in. In order to try and explain all of this they came up with this concept of entropy where as the system expands out, the entropy changes and as it expands back in, the entropy changes again.
Frasier Cain: But and for thermodynamics and doing the kinds of math that’s required to get you through understanding what’s going on in the system, that’s one thing, but entropy has become this kind of catch all way of describing chaos and disorder and things falling apart, and that’s where I think a lot of people go wrong, right?
Dr. Pamela Gay: Yeah. And so you’ll hear things like, human beings must defy entropy because we create order out of disorder. Well, we put a lot of energy into the system when we do this, and so this is where you have to look at the whole interplay. So, one of the standard examples that people use is if you grab a piece of wood that is a well-ordered state that is well-defined and by being able to define the different states is locked in, it’s low-entropy, and then you burn it; you release that energy that was holding the molecules and chemical bonds all together, and the entropy in that system goes down because now things are in more of a chaos. And then people try and use this to explain why rooms become messy over time and they use it to try and explain all sorts of things, and then they come around and say, “But life defies entropy, therefore entropy is wrong because life,” and they forget that energy is involved, and yeah.
Frasier Cain: But you have your messy room and you move all, you take the nice, neat, orderly room and you move all the stuff around in the room making it more messy, but the reality is from a thermodynamic standpoint, actually the entropy hasn’t really changed that much. You’ve maybe moved the position of some things and maybe used a little heat energy in your body, but it’s kind of like, I always think of this idea. People talk about you take a deck of playing cards and you drop it on the ground and now they’re all mixed up and jumbled, increasing their entropy, but actually from a purely particle standpoint, nothing much has changed because the configuration of the cards is still the same, the atoms are all in the same places, the amount of entropy that you’ve increased is actually fairly low.
Dr. Pamela Gay: And the potential energy for gravity doesn’t really have a lot to do with entropy so the whole idea of everything falls down to a lower energy state, therefore that must discuss entropy is starting to confuse similar but not identical concepts. And so people just want to, any time you have energy states being discussed, they want to evoke entropy as the catch all idea.
Frasier Cain: So, one of the concepts, and we learned about this back in physics class, and of course you’re a physics instructor, so this is very convenient is this idea that energy can neither be created or destroyed, it can only be changed, and with every sort of process that happens, a certain amount of that energy is converted into heat. And bringing back to thermodynamics. So how does this transfer of heat play into or be part of this concept of entropy, right? Where the heat is kind of more and more of the energy or is becoming, can be used for work, right? As it turns into heat.
Dr. Pamela Gay: So when you start trying to talk about entropy, this is where you get into this really confusing temperature versus heat discussion, and this is one of those things that is really at the root of why I have avoided discussing this for so long. So, one way to think about it is if I have a metal rod that is exposed to one temperature on one end, so I am adding energy to it on one end, and the other end is a cold reserve, so that means the other end has ice cubes on it or something, then what you will have is that high temperature heat reserve will cause heat to move along the pipe, energy is trying to bring the system into equilibrium, and that idea that you have this change in heat and you have two things at different temperatures, it’s in trying to discuss temperature versus heat that leads to a lot of chaos.
Frasier Cain: Right, and so then, and that’s a great segue. So can you talk about kind of the chaos? How does that play into this concept of entropy?
Dr. Pamela Gay: Well in this case it’s emotional chaos that I was relating to rather than chaos theory.
Frasier Cain: Oh I see, but the concept of disorder, chaos and disorder.
Dr. Pamela Gay: Right, so mathematically chaos is very specific and we’re not going down that particular rabbit hole in this episode. But in terms of order versus disorder, what is being discussed is there are certain states where in order for the atoms and molecules in them to be in that state. Their state has to be extremely well-defined, and that extremely well-defined, this must be up, this must be down, this must have this energy, this must have this energy, for instance with degenerate gasses, this extremely well-defined state, this phase of matter, in some case, as something goes from a solid to a liquid to a plasma, this well-defined state is part of what entropy describes. So when you have something that is a solid, it has a lower entropy than when it becomes a gas, and so as you go from phase state to phase state, the entropy in the system, how well-defined all of the aspects of the system are, changes. And entropy isn’t an absolute construct in thermodynamics the way, for instance, a joule is in most cases. So I can say if I’m going to exert this much force, it’s going to have this many joules of energy that is required to do that.
With entropy, it’s more until you start getting into statistics, with regular everyday thermodynamic entropy where we started defining it, it’s more a matter of if one observer looks at one set of variables, they’re going to get one entropy. But if when you’re doing your experiment some other observer uses a different set of variables, they might see different changes in the entropy based on what variables they’re using.
Frasier Cain: And also what they consider to be the closed system, right? Like if they’re just looking at the metal bar, for example, and you’re considering the whole room, then where that entropy is could be vastly different.
Dr. Pamela Gay: And this is where you start looking at defining your closed-systems and what defines your borders and you have to have a closed-system, but you can have closed-systems inside of one another, so it gets tricky.
Frasier Cain: And can you, you can’t practically physically have an actual closed-system in this universe, right?
Dr. Pamela Gay: Well for thermodynamics, you can get pretty darn closed with good insulation. So there will never be a perfect Carnot cycle where everything is perfectly conserved, but you can get fairly close, and we like spherical cows, although instead of spherical cows, we should probably discuss spherical dogs because those actually happen.
Frasier Cain: What? What are you talking about?
Dr. Pamela Gay: So in physics – I was trying to make a joke. I failed. So in physics we always make jokes about using spherical cows because it’s easier to calculate if it’s a sphere than if it’s actually cow shaped, and then the joke was made when it became popular to groom dogs so that they have perfectly spherical anatomies, that we should start using spherical dogs instead because they actually exist. It was a bad joke.
Frasier Cain: Right, I understand, let’s see if we can pull this together. So really the joke is let’s not calculate with cows based on how they actually are, let’s instead calculate using a shape that’s a lot simpler for the math. It doesn’t matter the actual cows are.
Dr. Pamela Gay: Right, and with thermodynamics we make assumptions all the time to make the calculations rational.
Frasier Cain: Right, and so how does that lead into, I don’t know about you, but I sure get a lot of emails from people who are certain they’ve come up with a perpetual motion machine, right? And the laws of thermodynamics are what they’re fighting against.
Dr. Pamela Gay: And this is where the actual system is always going to be slowly losing energy. You have friction, you have things that aren’t perfectly reversible, is the phrase that comes up in thermodynamics, so in classical thermodynamics, again going back to our Carnot cycle, we talk about how the amount of entropy that has to go in as the system goes from a high pressure to a low pressure at constant temperature has to be equal to the amount of entropy that goes out as at constant temperature it goes from low pressure to high pressure. The reality is that it will always end up gaining entropy more than it loses entropy.
Frasier Cain: Right, I’m thinking of an example, as you said, like a piston or something, and you push the piston down, the gas heats up, entropy increases, you pull the piston open, the gas cools down, entropy increases, like that one you can’t get away from it always moving in that one direction.
Dr. Pamela Gay: There’s going to be friction in a system that generates heat. This is one of those problems, and that extra heat comes out of the system and radiates away. There’s, everything has something wrong in how it functions in the practical realities. Yeah, so we’re just stuck with entropy always goes up.
Frasier Cain: Now there is one closed system that is completely closed, we assume, and that’s the entire universe.
Dr. Pamela Gay: Which may be infinite, which is a special kind of not actually quite closed, but we’re going to pretend that we don’t know that.
Frasier Cain: Right, either way I don’t know what you do with entropy in a closed system when that closed system is infinite in size. But let’s even just assume that the universe is finite for now. So how do physicists think about entropy on that large a scale?
Dr. Pamela Gay: This is where you have, where I said earlier, that an atom in its lowest energy state actually has maximum entropy, because there are so many different ways that it can be in that lowest energy state, by having all of these different ways, this unconfined way of being in its lowest state, it has maximum entropy.
Frasier Cain: And so sorry, just to continue, so if I imagine an atom in its maximum entropy state, what’s it doing? Is it floating around the universe without bumping into anything? It’s not in any kind of lattice or structure?
Dr. Pamela Gay: Yeah, it’s a free range atom that is good and chill. But in order for something to be in a Bose Einstein condensate or something crazy like that, it has to be locked into specific energy states. So I’m not talking about absolute zero here, I’m talking about unconfined lowest energy state.
Frasier Cain: Okay, okay, and so you started to talk about, before I so rudely interrupted, you started to talk about that kind of matter, and so sort of in larger scales, how does that turn in?
Dr. Pamela Gay: So when we look around the universe, a lot of things are in defined phase states, defined atomic states, defined configurations, and over time that stops being true. Black holes evaporate, white dwarfs cool off, and maybe if protons decay evaporate as well or at least decay away, and as things cool and they go from higher energy states to lower energy states and as the universe expands, which is related to the cooling, we’re headed more and more towards that free range atoms in their lowest energy state configuration of the universe.
Frasier Cain: And so what does that look like at the grandest scale? If we could imagine this sort of universe where everything is at its maximum entropy, what would the universe look like?
Dr. Pamela Gay: Well, so if protons do decay, we actually go from being free range atoms to being free range, well I guess electrons, as far as we know, don’t decay. So free range, the particles that are fundamental and don’t decay, so quarks, electrons, that’s pretty much what we’d get that’s stable, everything else is unstable, so we’re looking at a very boring universe of random particles in basically a sea of vacuum energy.
Frasier Cain: And exactly the same temperature.
Dr. Pamela Gay: Yeah, everywhere.
Frasier Cain: Right. What do you think that temperature would end up being? Would it essentially be just shy of absolute zero?
Dr. Pamela Gay: Well you can’t get to absolute zero because that requires confining the atomic states so that everything stops or is close to stopping, but yeah, it’s going to be cold, it’s just not going to be absolute zero cold.
Frasier Cain: It’s funny when they call it the heat death of the universe, when it’s actually the opposite of heat. You imagine like everything getting cooked when you hear the heat death.
Dr. Pamela Gay: Well, I think it comes from if you remove heat from the system everything dies, but it is a confusing phrase, so we have the cold death, but it all comes back to Robert Frost, by fire or by ice, everything is going to die, and it’s an icy death but referred to as a heat death.
Frasier Cain: So when are we, would we see the heat death, the maximum entropy, of the entire universe?
Dr. Pamela Gay: It all depends on if protons decay or not, and this is one of those big confusing things in physics. When I started studying physics, everyone was like, “Yes, we know,” no, that the proton is going to decay into a different thing, it’s going to decay into energy basically, but we keep watching for this to happen; we keep watching for the telltale flashes that are consistent with it happening in some of the super sensitive underground experiments that are going on, some of the ones that are looking for neutrinos, and we keep not seeing protons decay. And we thought that they probably decayed in about 10 to the 30 years, which means that if you watch 10 to 30 protons for one year you should see one decay, and we haven’t seen any decays, so last time I looked they pushed it out to stable for at least 10 to the 33 years, and every time I look they’ve pushed that number out a little bit further, so, it’s looking more and more like our whole notion that protons decay is wrong, in which case the heat death is going to be a bit less dramatic, but whenever and however it occurs, we’re looking at trillions and trillions and trillions and trillions of years in the future.
Frasier Cain: And so if protons don’t decay, then you’ll just end up with balls of protons.
Dr. Pamela Gay: You’ll still have atoms. So, the cool thing is that with the expansion of the universe, in all likelihood, most stuff will end up eventually wandering close enough to a black hole that it falls in, but then the black holes will start decaying, and so what you’re left with is the energy that’s given off by the black holes and then anything that didn’t fall in. So there’s the potential that there’s going to be the random neutron star that avoided death, there’s going to be the random very very cold white dwarf star that avoided falling into a black hole, and who knows? There might even be the rogue planet or other miscellaneous piece of mass that simply became very cold, very very very cold.
Frasier Cain: Cooled down to the background temperature of the universe.
Dr. Pamela Gay: But if protons don’t decay, there might be random bits of mass that avoid the statistical probability of falling into something.
Frasier Cain: Right, and you could then imagine some far far distant future when two of those objects crash into each other and, for a moment, a little bit of thermodynamic work is getting done, until even those blips are removed. It’s a very sad and depressing end to the universe. It’s funny though, I wonder why. When you think of the options we have available to us, where the universe tears itself apart or the universe crashes back in on itself in some kind of fiery ending or this one, this one seems like the sad one.
Dr. Pamela Gay: Human beings want steady state. We want a universe that goes on forever that allows every civilization to build megastructures and not necessarily just the one around KCS whatever whatever whatever, and the truth is that eventually all the civilizations are going to have to die, and the idea of a dramatic death by rip or dramatic death by crush is so much easier to think about than the slow, running out of energy and being the last one alive.
Frasier Cain: Yeah. I hope everyone has a happy and cheerful day. Thank you Pamela.
Dr. Pamela Gay: The universe is going to kill us, all of us.
Frasier Cain: Yep, in the end. Thanks Pamela. Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Frasier Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at info@astronomycast.com, tweet us at astronomycast, like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org. If you enjoy Astronomy Cast, why not give us a donation? It helps us pay for bandwidth, transcripts, and show notes. Just click the donate link on the website. All donations are tax deductible for US residents. You can support the show for free too. Write a review, or recommend us to your friends. Every little bit helps. Click “Support the show” on our website to see some suggestions. To subscribe to this show, point your podcatching software at astronomycast.com/podcast.xml or subscribe directly from iTunes. Our music is provided by Travis Serle, and the show is edited by Preston Gibson.

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  1. susie

    Apologies – it should be all fixed now!

  2. Ralph

    I’m pretty sure that if you burn a piece of wood in a closed system consisting of the wood and the air to burn it, the entropy of the system goes up, not down as stated. The system after burning is in a more disordered state than the state before burning.
    I’d also suggest that in such discussions that it’s better to describe what a concept, e.g. entropy, is than what it’s not.


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