### 7.10 Other pieces of the puzzle

Sakharov had in mind a specific model in which gravity could be viewed as an “elasticity” of the
spacetime medium, and where gravity was “induced” via one-loop physics in the matter sector [540, 628].
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.