Konwersatorium im. Leopolda Infelda

In general relativity gravitational field of bounded sources is described by metric tensor which tends to the Minkowski metric far away from the sources. The best known example is the Schwarzschild metric. Due to conformal techniques one can define a null or space-like infinity at a finite distance. Surprisingly an asymptotic symmetry group at null infinity is much larger than the Poincare group. Regardless this fact one can define total energy of a gravitational system. It diminishes in time due to gravitational radiation. Mathematically, a black hole is represented by an asymptotically flat metric which admits a future event horizon. It is not sure that what is observed in astrophysics corresponds to such horizons. Recently, semi local definitions of black holes have appeared and question of forming an event horizon was raised.