  An increasingly popular approach to quantum gravity rests on the idea that
gravity (and maybe electromagnetism and the other gauge fields) might be an
'emergent phenomenon', in the sense of representing a collective behaviour
resulting from a very different microscopic physics. A prominent example of
this approach is the condensed matter scheme for quantum gravity, which
considers the possibility that gravity emerges as an effective low-energy
phenomenon from the quantum vacuum in a way similar to the emergence of
collective excitations in condensed matter systems. This condensed matter view
of the quantum vacuum clearly hints that, while the term 'ether' has been
discredited for about a century, quantum gravity holds many (if not all) of the
characteristics that have led people in the past to label various hypothetical
substances with the term 'ether'. Since the last burst of enthusiasm for an
ether, at the end of the 19th century, was brought to the grave in part by the
performance of a series of important experiments in interferometry, the
suggestion then naturally arises that maybe interferometry could also play a
role in the current discussion on quantum gravity. We will highlight some
aspects of this suggestion in the context of the condensed matter scheme for
emergent gravity.
