### 7.7 Emergent gravity

One of the more fascinating approaches to “quantum gravity” is the suggestion, typically attributed to
Sakharov [540, 628], that gravity itself may not be “fundamental physics”. Indeed it is now a relatively
common opinion, maybe not mainstream but definitely a strong minority opinion, that gravity
(and in particular the whole notion of spacetime and spacetime geometry) might be no more
“fundamental” than is fluid dynamics. The word “fundamental” is here used in a rather technical
sense – fluid mechanics is not fundamental because there is a known underlying microphysics,
that of molecular dynamics, of which fluid mechanics is only the low-energy low-momentum
limit. Indeed, the very concepts of density and velocity field, which are so central to the Euler
and continuity equations, make no sense at the microphysical level and emerge only as one
averages over time-scales and distance-scales larger than the mean free time and mean free
path.
In the same way, it is plausible (even though no specific and compelling model of the relevant
microphysics has yet emerged) that the spacetime manifold and spacetime metric might arise only once one
averages over suitable microphysical degrees of freedom.