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The principles of thermodynamics are cornerstones of our understanding of physics. But they were discovered in the era of steam-driven technology, long before anyone dreamed of quantum mechanics. In this episode, the theoretical physicist Nicole Yunger Halpern talks to host Steven Strogatz about how physicists today are reinterpreting concepts such as work, energy and information for a quantum world.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
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STEVEN STROGATZ: In the mid-1800s, engineers were grappling with questions at the forefront of the Industrial Revolution: how to convert steam into mechanical work, translate rushing streams into electrical energy or pump water out of mines. Their inquiries and observations built the groundwork of a new science: thermodynamics.
By the early 1900s, we had not one, but three laws of thermodynamics. These laws have since become ubiquitous and proven fundamental to our understanding of physics in everyday life. But as our knowledge of the physical world continues to grow, the limits of these old mechanical notions become more apparent — especially as we approach the quantum scale.
I’m Steve Strogatz, and this is “The Joy of Why,” a podcast from Quanta Magazine where I take turns at the mic with my cohost, Janna Levin, exploring the biggest unanswered questions in math and science today.
In this episode, we’re going to ask, what do concepts like work and heat mean on an atomic or even subatomic level? And can our laws of thermodynamics be reinterpreted in quantum terms?
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We’re joined by Nicole Yunger Halpern. She is a theoretical physicist at the National Institute of Standards and Technology and an adjunct assistant professor at the University of Maryland. Her research lies at the intersection of quantum physics, information processing and thermodynamics. She’s also the author of an award-winning book, Quantum Steampunk: The Physics of Yesterday’s Tomorrow.
Nicole, it’s a great pleasure to have you with us on “The Joy of Why.”
NICOLE YUNGER HALPERN: It’s a delight to be here. Thanks for having me.
STROGATZ: Well, thank you for joining us. I really am excited about this. I was just asking my wife on the drive over to the studio about the word “steampunk.” I have to admit that I’m not familiar with this. She mentioned to me that even jewelry can be done in steampunk style.
YUNGER HALPERN: Steampunk comes up in costumes and conventions and jewelry and film, in books and short stories all over the place. It’s combines the aesthetic of the 1800s. So the Victorian-era people in waistcoats and petticoats and top hats, and also the American wild, wild west and Beijing, Japan, as well as futuristic technologies.
STROGATZ: Huh, excellent. Well, it’s a very intriguing title, Quantum Steampunk. I hope our listeners will check out the book.
So let’s start talking about the Victorian era and the thermodynamics ideas that grew out of that. We now think of it as classical thermodynamics, and I mentioned the laws of thermodynamics. There are so many things to unpack — the three laws, ideas like work, heat, energy, entropy, efficiency. Take us through some of those ideas, at least, to start with.
YUNGER HALPERN: Thermodynamics is something that we all have a sense of, but maybe we kind of take it for granted. And maybe that’s because thermodynamics is so general. It’s the study of energy, period. The forms that energy can be in and the transformations amongst those forms.
Energy can be transmitted in the form of heat and in the form of work. Work is coordinated, directed energy that can be directly harnessed to do something useful, like power a factory or charge a battery.
Heat is random, uncoordinated energy. It’s the energy of particles jiggling about randomly. Heat engines turn this random heat into coordinated, useful work.
And heat and work feature in the laws of thermodynamics. The number has been growing a bit, depending on whom you ask. There might be three, there might be four, there might be five.
The zeroth law of thermodynamics was actually developed after the first three, but people thought it was so important that it should be given precedence. And so it tells us that there are thermometers. Suppose that you have a cup of tea and I have a cup of tea. We want to be able to compare their temperatures. How can we do that? We can do it using a thermometer.
The first law tells us that the total amount of energy in the world remains constant. The second law tells us that the entropy of a closed, isolated system remains constant or increases only, at least on average. And the third law tells us that you can’t actually cool any system down to the lowest conceivable temperature, absolute zero — zero Kelvin — in any finite number of steps. So that is a very brief history of thermodynamics.
STROGATZ: Excellent. Great. That is a good summary. The second law always feels to me like the really deep one.
YUNGER HALPERN: Yes.
STROGATZ: Right? I mean, the concept of entropy, it’s often phrased as some measure of disorder in a system. Do you want to talk to us about entropy just for a minute?
YUNGER HALPERN: Sure, I think of entropy as a measure of uncertainty. And all sorts of things can have entropies associated with them. For instance, the weather on any given day in the Boston area is extremely random. It could be sunny or rainy or cloudy or snowy. And suppose that we learn on some given day what the weather is. So we’ve learned some amount of information and the amount of information we learn can be seen as an entropic quantity. And then suppose that we average this amount of information that we learn over all of the days. That’s another entropic quantity, a pretty common one.
And we can translate the story into thermodynamics by saying we have physicists’ favorite thermodynamic system — a classical gas in a box. And suppose that we know only large-scale properties of a gas, like the total number of particles and the volume. There are lots of different microstates or configurations associated that are consistent with this large-scale macro state. By a microstate, I mean a list of all of the particles’ positions, all of their momentum, and maybe some other properties, depending on what sorts of particles we have.
So, if we know just these large-scale properties, how ignorant are we about the microstates? And that’s essentially the thermodynamic entropy.
STROGATZ: It’s amazing, this idea of the gas in a box. Because it’s true, that is the universal example. And I’m actually, at this very moment, sitting in a box called a studio. The door is closed. There is gas in here. It’s the air around me.
YUNGER HALPERN: I’m very glad.
STROGATZ: I mean, as far as I can tell. Now, I suppose it’s conceivable that all the air molecules could spontaneously go into the corner, and you might hear me gasping. But that would be a very rare event.
Would that be a state, if all the molecules were in the corner? That would be, what? Very low entropy, I suppose?
YUNGER HALPERN: Right, so there’s a lot of debate, especially in the philosophical community, about how to define thermodynamic entropy. But the way that we’re often taught to reason in statistical physics classes, we would say tend to say, yes, the state in which the particles are all clumped together in the corner of the box is indeed a low-entropy state.
STROGATZ: There’s a concept that comes up a lot: equilibrium. Can you remind us, what does that mean? Like when we speak of a system being at thermodynamic equilibrium, what is that? Why does it matter?
YUNGER HALPERN: Equilibrium is a rather quiet state of a system. It’s a state in which large-scale properties like the total temperature and total volume remain approximately constant over time, and there’s no net flow of anything like heat or particles into or out of the system.
So suppose that we have had a hot cup of tea. We have let it sit on the counter for a long time. It has come to have the same temperature as the rest of the room, and a little bit of the water has evaporated away. At this point, the tea is at thermal equilibrium with its environment.
STROGATZ: And so it always seems kind of like an artificial thing that happens in a chemistry lab or in this famous tea cooling off on the kitchen counter. Whereas in real life, you know, I’m eating food all day long, it seems — just had a cookie before coming to the studio. I can’t relate to thermodynamic equilibrium very well. Is that fair to say? In our everyday life, what things are at equilibrium and what things are not?
YUNGER HALPERN: A great deal in our lives, including life itself, as you point out, is far out of equilibrium.
Organisms keep themselves far out of equilibrium by doing just what you said, by eating so that they consume energy in a well-organized form and expel it in a very highly entropic form. So you radiate lots of heat. This helps keep us far out of equilibrium.
If you have run a bath and let it sit around too long, you might have experienced equilibrium unpleasantly. Or made a cup of coffee and gotten distracted by your work, so that you end up having to drink cold coffee, you might have experienced equilibrium.
STROGATZ: I see. So it does seem like a sort of final state. It’s like after everything settles down. There’s no drive for anything to change anymore, it sounds like.
YUNGER HALPERN: Exactly, there is no drive.
STROGATZ: So when you mentioned the different laws of thermodynamics, what kinds of systems do the laws apply to? What other caveats do we need to make about those systems in order for the laws to apply?
YUNGER HALPERN: Well, the laws of thermodynamics were originally formulated by people who had in mind large classical systems. They didn’t necessarily think of these systems as consisting of many, many particles. The theory of atomism was not entirely accepted by the Victorian era. But they were thinking of systems that, at least now we will all acknowledge, consist of lots and lots of particles.
Around the turn of the 20th century, people discovered Brownian motion, which is random jiggling of particles that’s observable with a microscope, and it led people to accept very broadly that, in fact, materials do consist of very small particles. They jiggle around randomly, and occasional jiggling in the wrong direction led to some minor changes in at least the second law of thermodynamics.
But what’s really surprising to me is that the laws of thermodynamics seem to be going strong, even though we’ve learned a great deal since even the turn of the 20th century about small systems, biological systems, chemical systems and even quantum systems.
STROGATZ: Well, so we’ve been talking so far from the point of view of these particles that you keep mentioning — the atoms or molecules — systems made up of enormous numbers.
