Black holes and the nature of quantum gravity

Hawking and Wald have recently argued that the process of quantum black hole evaporation requires the violation of the fundamental physical law which asserts that the time evolution of quantum states is governed by unitary operators. It is shown that this violation can be avoided by a change in the global boundary conditions. It is remotely possible that astronomical observation could establish whether or not the universe has these boundary conditions in which quantum mechanical time evolution is governed by unitary operators. (author)