Sakharov's Gravity: Could Gravity Arise from the Fluidic Properties of Spacetime?

Emergent Gravity

"One of the more fascinating approaches to "quantum gravity" is the
suggestion, typically attributed to Sakharov [332, 393], 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 timescales 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. Sakharov had
in mind a specific model in which gravity could be viewed as an
"elasticity" of the spacetime medium, and was "induced" via
one-loop physics in the matter sector [332, 393]. In this way Sakharov
had hoped to relate the observed value of Newton's constant (and the
cosmological constant) to the spectrum of particle masses.

More generally the phrase "emergent gravity" is now used to
describe the whole class of theories in which the spacetime metric
arises as a low-energy approximation, and in which the microphysical
degrees of freedom might be radically different. Analogue models, and
in particular analogue models based on fluid mechanics or the fluid
dynamic approximation to BECs, are specific examples of "emergent
physics" in which the microphysics is well understood. As such they
are useful for providing hints as to how such a procedure might work in
a more fundamental theory of quantum gravity."

http://relativity.livingreviews.org/...ticlesu25.html


Double-A