For all their variety and variation, cities are, at their root, physical systems. That means, at some fundamental level, they are also expressions of the laws of physics. In physics size matters (or "scale" as we call it). Physicists learn different things about an object by looking at it from different scales. In our first exploration of physics and cities we stayed at the street level. At that scale we saw cities as machines: cars and elevators, pipes and plumbing. Then we went up to the roof. At that scale we saw cities as engines, vast systems for turning energy into work. Today we want to take the final step in our journey and look at cities at the largest scale. Today we get the 20,000-foot view: the city from the sky!
When we climb into jet-planes, flying high over the land, we see physics and cities from the grandest perspective: the city as a force in planetary evolution.
Take a plane, especially at night, and the view from 20,000 feet lets you see far more than the layout of any individual city. Instead you see vast, sprawling networks of interconnected human habitation. Cities are the nodes of those networks.
At night you see a spider-web pattern of light spreading across the planet. It's densest in the cities, thins out in the suburbs, finally transforming into delicate tendrils that link one metropolitan area to another. From the sky you see that entire regions, hundreds of miles across, have been colonized by aggregations of cities and suburbs. From horizon to horizon, the view from the air shows individual cities spreading until some areas become one, an almost seamless urban axis (think of the coastline from Boston down to Atlanta). It's only when we fly over landscapes that are truly far removed from cities that the surface finally becomes dark, dotted only with the occasional outlines of small, faint towns.
So what does this tell us about the physics of cities?
From 20,000 feet (or better yet from orbit) our perspective actually shifts from physics to astrophysics because now we begin to get a kind of planetary view of cities. I think its best to call this an astrobiological perspective. Astrobiology is relatively new field where researchers ask about life in a planetary context. It combines astronomy and biology, of course, but also involves lots of other fields like geochemistry and atmospheric science to create a truly global view of life and planets.
Astrobiology asks how the evolution of a planet can affect conditions for life. For example, how do changes in a planet's orbit over time make it more or less hospitable to life? Thinking this way has really helped folks look at the problem of the origin and evolution of life in some new and interesting ways. But for me what's really remarkable about this perspective is that it's also shown that, just as planets affect life, life affects planets too!
A bunch of times in Earth's history, the evolution of life has completely changed big important characteristics of the planet. Things like, say, the chemistry of the atmosphere. Where does the oxygen in our atmosphere come from? It's pretty important right? Well, in the beginning, billions of years ago, the atmosphere had almost no oxygen. Tiny microbes, breathing in other gasses and then breathing out oxygen, pumped this key gas into the atmosphere! Their "waste" gas gave us the oxygen-rich atmosphere we know and love. And, like I said, these kinds of changes have happened more than once.
Sometimes life has driven the planet into a warmer and wetter state. Sometimes it has resulted in a so-called "Snowball Earth" phase where the whole planet almost froze.
Some researchers have a name for this back and forth between life (the biosphere) and the planets other systems (the atmosphere, oceans, ice and land). They call it co-evolution. That means the planet and life are changing the course of each other's history.
So what does this have to do with physics and cities? Well those nighttime views from really high up show you something really remarkable happening and I would argue that its co-evolution again.
That vast web of geometries traced out in light shows you cities as a kind of infestation. They're like living networks colonizing the planet. Far from the network nodes – the cities – we see energy being harvested in forms as diverse as food and petrochemicals. Then through those tendrils of roads, rail and pipelines the network moves energy (and information) into, around and between the cities as they grow. And all that activity we can see as lights and land use from 20,000 feet is changing the planet in ways we can't so easily see.
Our relentless city building is really at the heart of our changing the planet in ways that include increasing atmospheric levels of CO2 and methane, in the warming of the land and the oceans and the lowering the planets total ice cover. Life – in this case us and our city building – is once again driving the planet in new directions. There is even a new word for this.
Every geological epoch gets a name. Looking back over millions of years, you have your Eocene, your Pliocene, your Holocene epoch (that was the most recent one). Now folks are suggesting that we are entering a new epoch, the Anthropocene.
The Anthropocene means it's our activity which is the main driver changing the planet. But you know what? The Anthropocene is really the epoch of the city. It's our city building — on a massive, planetary scale — that is at work here. The idea of a single world-girding city pops up a lot in science fiction. I doubt we are heading towards that anytime soon. But it is truly amazing to recognize that an organizing principle that began some 4,000 to 6,000 years ago has exploded into, perhaps, the dominant driver of planetary history. That is both pretty impressive and pretty scary at the same time.
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ROBERT SIEGEL, HOST:
And I'm Robert Siegel.
A little bit of the science of cities now, as part of the NPR Cities Project.
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SIEGEL: Cities are built on physical systems: buildings, streets, plumbing, wires. And as NPR blogger and astrophysicist Adam Frank has been telling us, modern cities are great places to see the laws of physics at work. Out on the street, you can see traffic flowing like fluid. From a rooftop, you can see heat shimmering off the buildings as energy is consumed by the people working inside. Adam has taken us on those excursions in past conversations, now the view of the city from 20,000 feet up.
Adam Frank, urban physicist, is at the airport today. Welcome once again.
ADAM FRANK, BYLINE: Oh, it's great to be here.
SIEGEL: And you're at Rochester International. Tell us about the principles of physics and the aerial view.
FRANK: Well, what I'm thinking about today is really the view of cities from the grandest perspective. And in that sense, it's the city as a driving force in the evolution of the planet. So, why am I here at the airport? Because I can see all these travelers and what are they going to do? They're about to get on airplanes and they're going to, you know, fly up to 20 or 30,000 feet.
And it's from that view that they're going to be able to see not just the structure of individual cities but they're going to see how cities are interconnected, how they're basically vast sprawling networks of interconnected human habitations. And it's the cities that are the nodes of those networks.
So I'm going to walk over to the flight and departure board - the arrival and departure board that we're all familiar with - and take a look at some of the flights we have. OK, so there's one going to looks like D.C., right?
FRANK: So what do the people who are flying to Washington, D.C. from Rochester going to see? Well, particularly if they're flying anywhere near the coast, what they're going to see is the entire eastern seaboard is almost one giant axis of urban area.
SIEGEL: But, Adam, you're here to talk with us about physics. So what does that 20,000-foot view of the cities have to do with physics?
FRANK: Right, when you look at cities from that view what you do is you shift from physics to astrophysics, a planetary view of cities. And what I think is best to start thinking about astrobiology. And what astrobiology shows us is the way planets and life go together. Some researchers call this back and forth co-evolution. And what they mean by that is that life and the planet can change the course of each other's history.
SIEGEL: So what you're saying is that when we're flying over cities at night and we're seeing these clusters and ribbons of lights, we're seeing a form of co-evolution. We're seeing cities as an extension of organic beings.
FRANK: To me, that's the way I look at it. I really do see it as co-evolution. And so, those vast geometries that you see traced out in light, in some sense, they're showing us a kind of infestation. They're like living networks that spread across the planet. You know, far from the cities, what's going on? You're harvesting energy, whether it's food and petrochemicals to power those cities.
And then, through those tendrils - the pipelines, the roads - you're bringing that energy into the cities and that's what's fostering the growth of those cities. So what you see from 20,000 feet with all those lights and that land use, is the way we're changing the planet in ways that we can obviously see and ways we can't see, like climate change.
SIEGEL: Yeah, you referred earlier to life and us as an infestation, which isn't very positive. Is it appropriate? Are you talking about life as a parasitic invasion? Do you mean to be that negative?
FRANK: No, it's really - really the question is negative for whom, right? This has been going on for a long time. And from the astrobiolgical perspective, whether it's good or bad doesn't change the fact that it's happening. So, you know, some scientists looking at the way life and planet evolve have even coined a new term for what's going on right now. There's lots of geological epochs, right - the Pliocene, the Holocene.
And now folks are suggesting that what we're entering is a new epoch, the Anthropocene; when all of our activity actually pushes the planet and its systems in new directions. And when you think about it, the Anthropocene really is the epoch of the city. So that spider web of lights that you're seeing from 20,000 feet is a real marker. It's the physical manifestation of life, meaning us and our cities, changing the planet.
And what I think is really remarkable is this practice of city building, which started 4,000 or 6,000 years ago, has exploded to become such a powerful force on the planet. That's both really impressive and pretty scary at the same time.
SIEGEL: Some big thoughts about cities as seen from up high and as seen from Rochester, New York, from Astrophysicist Adam Frank. Thanks, Adam.
FRANK: Oh, it's been a pleasure.
SIEGEL: And you can find our other conversations with Adam and other stories from the NPR Cities Project at NPR.org/nprcities. Transcript provided by NPR, Copyright NPR.