  In the spirit of Sakharov's `metric elasticity' proposal, we draw a loose
analogy between general relativity and the hydrodynamic state of a quantum gas.
In the `top-down' approach, we examine the various conditions which underlie
the transition from some candidate theory of quantum gravity to general
relativity. Our emphasis here is more on the `bottom-up' approach, where one
starts with the semiclassical theory of gravity and examines how it is modified
by graviton and quantum field excitations near and above the Planck scale. We
mention three aspects based on our recent findings: 1) Emergence of stochastic
behavior of spacetime and matter fields depicted by an Einstein-Langevin
equation. The backreaction of quantum fields on the classical background
spacetime manifests as a fluctuation-dissipation relation. 2) Manifestation of
stochastic behavior in effective theories below the threshold arising from
excitations above. The implication for general relativity is that such
Planckian effects, though exponentially suppressed, is in principle detectable
at sub-Planckian energies. 3) Decoherence of correlation histories and quantum
to classical transition. From Gell-Mann and Hartle's observation that the
hydrodynamic variables which obey conservation laws are most readily decohered,
one can, in the spirit of Wheeler, view the conserved Bianchi identity obeyed
by the Einstein tensor as an indication that general relativity is a
hydrodynamic theory of geometry. Many outstanding issues surrounding the
transition to general relativity are of a nature similar to hydrodynamics and
mesoscopic physics.
