2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48333We derive the special and general relativistic hydrodynamic equations of motion for ideal fluids from a variational principle. Our approach allows to find approximate solutions, whenever physically motivated trial functions can be used. Illustrating this, a Rayleigh-Plesset type equation for the relativistic motion of a spherical (QGP) droplet is obtained. Also the corresponding general relativistic effective Lagrangian for spherically symmetric systems is presented.8 pages; LaTex, uses sprocl.sty - Invited talk presented by HTE at the "Fourth Workshop on Quantumchromodynamics", 1-6 June 1998, American University of Paris (France); H. Fried and B. Mueller, eds. (World Scientific), proceedings to be publishedHigh Energy Physics - PhenomenologyAstrophysicstext