Vortex reconnections unravelled.

When two vortices in superfluid helium-4 get close enough together, their cores are very likely to "reconnect". That is, the vortices collide, coalesce, and re-emerge as two new vortices, with the head of one of the original vortices joined to the tail of the other one and vice versa (figure 1). Vortex reconnection has long been assumed to be possible in this exotic liquid, despite the absence of any firm theoretical basis. But now Joel Koplik of the City College of New York and Herbert Levine of the University of California, San Diego, have unravelled the microscopic details of this interesting topology-changing process through numerical simulations of the relevant nonlinear Schrodinger equation (Phys. Rev. Lett. (1993) 71 1375)