Physics And Cities: View From The Street

Jul 3, 2012
Originally published on July 5, 2012 6:23 pm

I was born and raised in the vast urban ecology that is the New York region's metropolitan jungle. As a young man my love for the city and my love for physics grew together. Now I am old enough to see that the pairing of those two romances was no accident.

Beginning this week I will be exploring the relationship between physics and cities. It's part of NPR's most excellent project on Cities. The questions I want to explore concern both the physics of cities and cities as examples of physics.

To tackle these questions, we decided to look at cities on three different scales: from the street, from the roof top and from space. Each scale teaches us something different about how physics shapes the form and function of a city.

We start with city streets, which may just be the best physics lab ever.

For a physicist the world is a theater of forces and energy. Untangling the way forces and energy work in the natural world can be complicated. But cities are, by design, lots of straight lines and right angles. They were built out of an understanding of physics.

Where do you find street-level physics? Everywhere. The lurch of a subway car will teach you more about Newton's laws then a thousand textbooks. Want to experience a (seemingly) variable force of gravity? Step into an elevator and feel it accelerate.

Cities exist to turn energy into useful work. They are machines, engines of the human endeavor. Just look a the plumbing!

On the street you can see evidence of all the many channels bringing energy into the city, and managing its flow. Steam rising from manhole covers speaks of heat flushing through hidden arteries. Subway gratings speak of vast systems of circulating air drawn into the underground and back again. Streetlights hint of power lines snaking beneath the street and transformers buzzing in basements.

Everywhere you look on the street you see the dance of energy flowing in its many different forms: motion, current, heat and light. That glorious choreography is nothing less than a collaboration between our human genius and the natural laws that govern our universe.

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.

Copyright 2017 NPR. To see more, visit


Take away the glamour and the garbage, and cities reduce to systems, complex physical systems. Or at least that's how astrophysicist and NPR blogger Adam Frank sees it.

Adam, hi.

ADAM FRANK: Hi, it's great to be here.

BLOCK: And let's explain that you are out on the street today in Rochester, New York. You're a professor at the University of Rochester. And you're going to give us a street-level view of the city as a physics lab. What do you mean?

FRANK: Well, essentially, I grew up in metropolitan New York, that urban jungle there. And I was learning physics pretty much at the same time I was falling in love with the city. And the beautiful thing was I was able to use the city as my physics laboratory. Because everywhere you walk in a city, you see examples of physics. All of these straight lines and right angles are an example of the way cities were designed on purpose to take advantage of sort of abstracted physics.

So, you know, as I was walking around watching steam come out of a grate, I was learning about the thermodynamics of gases. Or watching traffic flow, I was learning about traffic as a fluid system. So really, in many ways, I found the city to be the best physics laboratory I could imagine.

BLOCK: And tell me, as you stand there in Rochester right now and you look around, what are you seeing that says to you, aha, this is a grand physics experiment right in front of my eyes?

FRANK: I'm actually standing right now at a corner and I'm watching traffic flow. And one of the best places to see the theater of forces and energy - which is, if you're a physicist, that's what you're interested in - to see them applied is through traffic. Now, if you're in a car, you never really get a chance to feel the force and energy 'cause you're not expending it. You step on the gas, you go. Who cares?

But if you're on a bike, you see why bicyclists don't want to stop often. It's because if you have - if it's your muscles that are going to go into getting yourself started again, which is essentially the physics of it, you see why on a bicycle - if you come to a red light, it's easy for a car to stop and start again. But for a bicyclist, it's their force and their energy that has to get you going.

BLOCK: So that bike in motion wants to remain in motion.

FRANK: Right. Newton's First Law, one of the best examples of physics in a city that you can see is Newton's First Law of Motion.

BLOCK: Adam, look at that intersection where you are right now and tell me if you're seeing other laws of physics that are coming to bear in traffic.

FRANK: So, I'm looking at a red light and a bunch of cars all stacked up right now. And what's going to happen is when the light turns green, the cars are going to get going. But they all don't get started at once. First, the first row of cars starts moving and then the second row of cars behind them can get moving. And then the third row of cars can get moving.

So, what's actually happening is there's a wave which is moving backwards, a wave of rarefaction or cars spreading out. The cars moving forward is the wave actually that moves backward.

And that same kind of physics is exactly what happens in an exploding star, for example, the same kinds of fluid motion of what physicists call a rarefaction wave occurs both in traffic when the traffic is beginning to accelerate as occurs in an exploding star when the star - the gas in the star is blown outwards.

BLOCK: So you're looking at the traffic in front of you and you're not seeing, you know, Nissans and Hondas. You're seeing exploding stars on some level?

FRANK: I do, actually. That is - and when I see it, I'll get so excited because it's like, oh, my God, there's an example of a rarefaction wave.

BLOCK: Adam, we've been talking about Isaac Newton and, of course, Newton had a lot of things to say about gravity. How does a city help us see the forces of gravity at work, besides everywhere?

FRANK: Right. The best place to see what Newton was telling us and, actually, even to go beyond it is elevators, because gravity and elevators really go together beautifully. So I'm now in front of a very tall building here and we're going to walk inside and go over to the elevator.

So, when the elevator starts, you feel your legs buckle a little bit as the elevator is rising. And when the elevator comes to your floor, you feel a little bit light as the elevator decelerates. Now, the beautiful thing about that is that is exactly where Einstein started in thinking about gravity. When he went beyond Newton, he was doing all of these thought experiments about what happens in an elevator as you accelerate and decelerate.

So, when I was a foot messenger in New York City - I had a job doing that for a year or so - I used to carry a little red ball with me and I would try and catch that moment. I'd throw the ball up in the air and I'd try and catch the moment when the elevator either accelerated or decelerated and then you could see the ball in space. It would either slow down as it rose or speed up or, sometimes, right in mid-motion, it would hang there for a second.

And I would get so excited because there I was reading the same physics, the Newtonian physics and the Einstein physics, I was seeing it real - the stuff that I had just been reading beforehand. And, of course, the people in the elevator all thought I was a little crazy, but I didn't mind.

BLOCK: They might have just been waiting to see when you were going to start juggling.

FRANK: Right. If I'd only had a few more balls and some more talent, then maybe I might have been able to do that and fund my graduate work, too.

BLOCK: Well, Adam, it's been fun. Thanks for the lesson in the city as physics lab. Appreciate it.

FRANK: Oh, it's been a great pleasure. Thank you.

BLOCK: That's University of Rochester astrophysicist, Adam Frank. He writes for NPR's Cosmos and Culture blog. And, next time, he'll bring us a rooftop view of the city. You can follow the Cities Project on Twitter @NPR Cities.


And next Tuesday, the Cities Project returns with a story about what's worth preserving in our older neighborhoods. Also on Tuesday, we'll be asking a group of urban thinkers for their ideas about the issues we have been covering. They'll be posting their thoughts on Twitter. You can subscribe to the NPR Cities roundtable. It's a list on our Twitter account and you can join the conversation. All tweets will be tagged #NPR Cities. That's next Tuesday, 6 p.m. Eastern time, 3 p.m. Pacific. Transcript provided by NPR, Copyright NPR.