  In this paper, we successfully derive the Bekenstein-Hawking entropy for
Schwarzschild black holes in various dimensions by using a non-trivial phase
space. It is appealing to notice that the thermodynamics of a Schwarzschild
black hole actually behaves like that of a $1$-dimensional quantum mechanical
system. Our result suggests that black hole should be viewed as a system with
the equation of state $P=\rho$, and it also suggests that a holographic stage
should exist in the early universe.
