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Cities are like engines, vast systems for turning energy into work. That's how Astrophysicist and NPR blogger Adam Frank puts it.

Right, Adam?

ADAM FRANK: That's right.

CORNISH: So, we'll remind listeners you're a professor of astrophysics at the University of Rochester. And, as part of the NPR Cities Project, you're giving us a physicist's view of the city. And today, I gather you are on a roof?

FRANK: Yes, I'm actually on the 11th story of the Wilder Building here in Rochester, New York. This is actually the first steel-frame skyscraper that was built in Rochester. Right now, I'm in the elevator room but I'm about to step outside onto the roof right now.

CORNISH: OK. And may I ask why you're on a roof?

FRANK: Well, last time we talked we looked at the city from the street level. And we saw that there, we could think of the city as a machine or simple machines - wheels and plumbing. So, we're out on the roof today because, once again, we're looking at the city as a physics lab. And from this level, what we see is that the city is not a simple machine but it's actually a vast interconnected system, an engine for turning energy into useful work.

And that way, it is directly a creature of the profound Second Law of Thermodynamics.

CORNISH: Oh, OK. All right, wait a second - second law. We need to actually remind us all about the First Law of Thermodynamics.

FRANK: OK, the First Law of Thermodynamics tells us that energy is always conserved. When you climb stairs, like if you wanted to climb up these 11 stories, the food that you ate turns into the energy of your muscles. So you can never lose energy, you just transform it from one form into another. Now, when you look out on this city you see the thing happening all the time.

There's electricity running through wires and that's turning into movement of a fan or it's turning into the illumination street lights. So energy is always conserved but it can be transformed from one form to another. So that's the first law.

CORNISH: OK, energy into action. Got it. So that second law?

FRANK: Well, the second law shows us that there are consequences to those transformations. You can't just take 100 percent of energy and use it for useful work. When you do useful work with energy, you always generate waste. And physicists call this entropy. And entropy is a great word. I really love it. I think it's quite beautiful. And what it really means is disorder. And so, essentially as we try and use energy to create cities, we're generating some disorder.

CORNISH: All right, consume energy give off waste. I get it. So why are you on the roof?

FRANK: Well, the great thing about being on the roof is you can literally see and hear the city acting as a giant engine and, therefore, fulfilling the second law. So, when I look out right now, I can see heat shimmering off the buildings and that heat really is the waste that comes from energy being pumped into those buildings to keep the people inside comfortable and productive.

Now, the other thing from the rooftop is I can hear the entropy, as well. So can we listen for a second? So what do I hear? I hear traffic noises below. I can hear an air conditioning unit on the building next door and all of this is a cacophony and that noise really is acoustic waste energy. A din is the measure of the disorder, the entropy, the mess that comes from pumping all this energy to the city and getting it to be fruitful and useful.

CORNISH: But, Adam, lots of people complain about the din of cities. So what's exciting about it to you as a physicist?

FRANK: Well, the great thing about the din is it's an abstract physical principle that I can actually hear manifest in the world around me, but the other interesting thing is that the second law is really a warning for cities, right? The second law is, essentially, telling us that if you try and organize a society into cities then you're going to have to account for waste and disorder.

So what that really means is that with all of our relentless city building, there's going to have to be unintended consequences. Really, what else is global warming but the unintended consequence of burning all those fossil fuels to power our cities? So this question - this really intense question of - is it even possible to have a sustainable society of the kind that we have, really turns on whether or not we can reconcile cities and the second law.

And, of course, the really amazing thing is we don't really know what the answer to that question is right now.

CORNISH: University of Rochester astrophysicist, Adam Frank, thanks for that understanding of the city as an engine.

FRANK: Oh, it was my pleasure.

CORNISH: Adam's next view of the city, from the sky. You can follow the Cities Project on Twitter @NPRCities.

MELISSA BLOCK, HOST:

And, next week, the NPR Cities Project will hold another roundtable discussion about some of the issues we've been covering, such as sustainability and resilience in the face of environmental disaster. We'll be having that conversation on Twitter and we invite you to contribute. Use the hash tag, #NPRCities and, as always, you can send us your city's sounds and pictures. You can find out how, see what others have sent and listen to other stories from the series. It's all at NPR.org/NPRCities.

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