# Fish gotta swim, birds gotta fly, I gotta do Feynmann graphs 'til I die: A continuum theory of flocking

#### Fish gotta swim, birds gotta fly, I gotta do Feynmann graphs 'til I die: A continuum theory of flocking

- Event time: 1:00pm
- Event date: 4th September 2015
- Speaker: John Toner (University of Oregon)
- Location: Higgs Centre Seminar Room, Room 4305, James Clerk Maxwell Building (JCMB) James Clerk Maxwell Building Peter Guthrie Tait Road Edinburgh EH9 3FD GB

### Event details

### Abstract

Flocking - the collective motion of large numbers of organisms or other self-propelled entities - exhibits a number of strange and baffling phenomena. The most baffling is that it exists at all in two dimensions: a long-known theorem of statistical mechanics called the Mermin-Wagner theorem implies that it would be impossible for an arbitrarily large collection of creatures in a 2d plane to all *point* in the same direction. Yet, apparently, they *can* all *move* in the same direction with no difficulty.

In this talk, I'll show that all of these mysteries can be explained by a very general "hydrodynamic" theory of flocks, which summarises the behaviour of ALL flocks in much the same way that the well-known Navier-Stokes equations summarise the behavior of all simple fluids. I`ll then discuss this theory's explanation for the apparent violation of the Mermin-Wagner theorem. In hindsight, this proves to have a simple hand waving explanation, which I'll present.

### About Higgs Centre colloquia

The Higgs Centre Colloquia are a fortnightly series of talks aimed at a wide-range of topical Theoretical Physics issues..