Observations about the universe, life, Lausanne and me

Saturday, April 4, 2009

Ants dont have traffic jams

I love reading the citation alerts of the Physical Review Letters. PRL is a very high quality journal of course, which only accepts papers that are a maximum of four pages of length and report something significant - for a quite stringent value of significant as determined by the editors and reviewers.

So if I find a paper pertaining to my research I can be sure that it will be a good one - but that's not why I love PRL. I love PRL because between articles like "Valence Bond Glass Theory of Electronic Disorder and the Pseudogap State of High-Temperature Cuprate Superconductors" or "Entanglement, Nonlinear Dynamics, and the Heisenberg Limit" you will find gems like

Trafficlike Collective Movement of Ants on Trails: Absence of a Jammed Phase 
 John, A; Schadschneider, A; Chowdhury, D; Nishinari, K
We report experimental results on unidirectional trafficlike collective movement of ants on trails. Our work is primarily motivated by fundamental questions on the collective spatiotemporal organization in systems of interacting motile constituents driven far from equilibrium. Making use of the analogies with vehicular traffic, we analyze our experimental data for the spatiotemporal organization of ants on a trail. From this analysis, we extract the flow-density relation as well as the distributions of velocities of the ants and distance headways. Some of our observations are consistent with our earlier models of ant traffic, which are appropriate extensions of the asymmetric simple exclusion process. In sharp contrast to highway traffic and most other transport processes, the average velocity of the ants is almost independent of their density on the trail. Consequently, no jammed phase is observed.

Not your typical physics paper! I especially like the idea of a bunch of theoretical physicist crouching on the parking lot of the Max-Planck Institute and observing ants...

The paper itself is quite interesting. They observed the traffic flow of ants on parts of unidirectional trails without branches, which you could compare to a piece of straight highway without exits or on-ramps. Surprisingly, they found that unlike humans, ants don't have traffic jams. The average velocity of cars on a highway decreases with increasing density - the cars bunch up and slow down. Not so ants: They scurry along happily, all with the same speed - with increasing density their speed distribution funcion just becomes a steeper gaussian - no one slows down, and no one overtakes!

The root of this phenomenon lies in their different "driving" behaviour. At low densities, humans speed along the highway seperately, each at his own speed. Ants, on the other hand, will assemble into small platoons or convoys, with each ant maintaining a fixed distance to the ant in front of it. So when the ant-density increases, the platoons will just get larger, and nobody has to slow down. Go ants!


  1. As we physicists like to maintain (especially when talking with those pesky chemists): Everything is physics! ;)

    Also, who am I to contradict the PRL? If they say it is physics, than it must be...