  We develop a Hamiltonian formalism which can be used to discuss the physics
of a massless scalar field in a gravitational background of a Schwarzschild
black hole. Using this formalism we show that the time evolution of the system
is unitary and yet all known results such as the existence of Hawking radiation
can be readily understood. We then point out that the Hamiltonian formalism
leads to interesting observations about black hole entropy and the information
paradox.
