String fluid from unstable D-branes

We consider Sen's effective action for unstable D-branes, and study its cla ssical dynamics exactly. In the true vacuum, the Hamiltonian dynamics remai ns well-defined despite a vanishing action, and is that of massive relativi stic string fluid of freely moving electric Aux lines. The energy (tension) density equals the flux density in the local co-moving frame. Furthermore, a finite dual Lagrangian exists and is related to the Nielsen-Olesen field theory of "dual" strings, supplemented by a crucial constraint. We conclud e with discussion on the endpoint of tachyon condensation. (C) 2001 Elsevie r Science B.V. All rights reserved.