Cities may be the defining element of human civilization.
The path from hunter-gatherers in the Paleolithic era 25,000 years ago to the high-tech, high-wonder jumble we inhabit today runs straight through cities. In traveling that path, our construction of cities has always been a dance with physics. In some cases, that physics was explicitly understood; in others, its manifestation was only recognized in hindsight.
As our cities have become more complex the physics embodying their behavior and organization has also become more nuanced, subtle and profound.
About a month ago I walked down the streets of my hometown of Rochester, N.Y., to discuss a street-level view of physics and cities. From the street a city is all about the physics of "simple" machines: bikes, buses, streetcars and the plumbing of fire hydrants. Once you get up high enough, however, your physics-colored glasses lets you see cities in a whole new way.
Instead of just basic machines, the city becomes a vast interconnected system designed for turning energy into work. Seen through that lens, cities are really giant heat engines, and that makes them creatures subject to one of the most profound principles in all of physics: the omnipresent Second Law of Thermodynamics.
Before we hit the second law, it would be good to remember the first law, which tells us energy is always conserved.
Climb a flight of stairs and the chemical energy in your morning's oatmeal is turned into your muscles' energy of motion. The oatmeal's chemical energy originated, of course, in sunlight via photons streaming across space. Taken as a whole, energy is neither gained or lost. It is only transformed from one form to another.
In a city you see this every day, all day. Electricity runs through wires and transforms into the movement of a fan or the illumination flowing from a streetlight. The first law is relatively easy to understand, and it makes a kind of innate sense to us.
The second law is something else entirely. It speaks to the consequences of those energy transformations mandated by the first law. There are rules for how energy is transformed — rules and limits. Most importantl the second law tells us that anytime energy is used to do useful work — like lift an I-beam to the top of a new building — then some of that energy must end up as waste, garbage, pollution or just plain crap.
Useful work creates unusable waste, always and forever. That is the law, the second law. That universal link between energy, work and waste and is what makes the second law such a profound, universal principle.
Use a load of coal to drive a steam-powered locomotive and some of the coal's energy ends up heating the locomotive itself, leading to wear, tear and eventual decay. Physicists call this waste entropy, but what it really means is disorder.
Entropy is a beautiful word and an even more beautiful idea. Systems, like a box of gas, can be in many states, with some more ordered than others. All the atoms in the box could be neatly packed into a corner. That is a high order and low entropy state. The atoms could also be bouncing around everywhere in the box. That's a low order and high entropy state.
What does this have to do with the view of cities from the roof? On the roof you can literally hear the city acting like a giant engine. The sounds of traffic, music, construction and sirens all merge together into a cacophony, into a clamor, into noise. But what is noise in this case? It's an acoustic measure of the second law at work. It's the city's entropy made audible!
Every moment of every day, vast quantities of energy stream into the city through all that plumbing we saw on the street. The city then uses that energy to do work, to organize itself into vast architectures of order. But the second law will not let the story end there. Waste, pollution and disorder must follow.
There is a word that applies to the sound of cities which almost never gets applied to nature: "Din." The din of cities heard on the rooftop as a rising wall of noise is a testament to the true nature of cities as engines of organization and dynamos of disorder.
The first time I became aware of this din of acoustic entropy, I was sitting across from Manhattan on the cliffs of Weehawken, N.J. It was night and the great city was blazing from horizon to horizon. Its low rumble of noise flowed like a breeze blown at me from a mile away across the dark river. There it was, the second law made real in sound and light. The great might of that city — its vast converging streams of energy, creating a singular nexus of activity — was no more stunning than the visceral realization that it was also radiating disorder like a mad star.
The second law is a thing of great beauty, because it is just as true for a box of atoms as it is for a star, or a single cell or a great city. The dynamic balance of energy and entropy is a universal law of systems in whatever form they take.
But it has its darker side, too. The second law is a kind of warning to cities and civilization. No matter how clever we are, there will always be disorder, waste and pollution following in the wake of our work organizing societies into cities. There is another way of putting the second law that states the entropy of the universe always increases. So the work we do to create and maintain cities means we are also raising the level of disorder, waste and pollution for the planet as a whole.
The second law tells us that doing work always leads to unintended consequences. That is what we have seen happen with our relentless city building. What else is global warming but the unintended consequence of burning fossil fuels to power the highly organized culture we have created?
We live at a moment when cities are poised to become the dominant mode of human habitation on the planet. But we don't yet know if such a mode can be made sustainable for more than a century or two. Coming to grips with that question can only mean coming to understand the physics of cities — the physics of thermodynamics and its ever-present second law.
You can keep up with more of what Adam Frank is thinking on Facebook and on Twitter @AdamFrank4. His latest book is About Time: Cosmology and Culture at the Twilight of the Big Bang.
AUDIE CORNISH, HOST:
Cities are like engines, vast systems for turning energy into work. That's how Astrophysicist and NPR blogger Adam Frank puts it.
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. Transcript provided by NPR, Copyright National Public Radio.