What drives the time evolution of the spacetime geometry?
  I show that in a general, evolving spacetime, the rate of change of
gravitational momentum is related to the difference between the number of
degrees of freedom in the bulk and the boundary of a region. This expresses the
gravitational field equation in the thermodynamic language which is the natural
description, if gravity is an emergent phenomenon. In all static spacetimes,
the number of degrees of freedom in the boundary is equal to the number of
degrees of freedom in the bulk; i.e., these spacetimes maintain holographic
equipartition. It is the departure from holographic equipartition that drives
the time evolution of the spacetime. This result, which is equivalent to
Einstein's equations, provides an elegant, holographic, description of
spacetime dynamics.
