Monday, January 29, 2018

Interstate Traffic Jams and Galactic Structure

Last fall I drove up to Williamsburg and Washington with a couple of my kids. It was a pleasant trip, except for the stretch of I-95 between Richmond and Washington. There, on a sunny Sunday afternoon, we encountered a series of sporadic traffic jams. Each time we hit a slowdown, I expected to see an accident by the side of the road, but no such accident ever appeared. Instead, the traffic simply speeded up again a few miles down the road for no apparent reason. So what was the origin of this mysterious roving I-95 traffic jam? I suspect it's the very same thing that produces these beautiful structures in spiral galaxies:


The stars in a galaxy revolve around the center, so it's natural to think that the whole thing spins around like a pinwheel. But that's not how it works. Instead, as a star circles the galaxy, it sometimes encounters a region where the density of the other stars is a bit higher than average. The gravitational attraction of these other stars slows down the star, so it tends to get trapped there along with the other stars, forming a spiral arm. Then when the star leaves the spiral arm and enters a region where the density is lower, it can speed up again and go on its merry way.

And I think exactly the same thing was happening on the interstate -- a random fluctuation in traffic forced a few cars to slow down, which made the density of cars higher, which forced more cars to slow down, and soon you had the traffic equivalent of a spiral arm -- a traffic jam for no apparent reason. The big difference is that the stars get stuck in traffic for about 10 million years. Even I-95 isn't that bad. At least not yet.

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