  We comment on the interpretation of absolute rotation measurements with
superfluids, which involve the Sagnac effect. This effect, first observed in
rotating optical interferometers, has now been seen in a host of different
physical systems, ranging from atomic clocks to electrons, neutrons, atomic
beams, ... We show that the time-honored explanation of this effect based on
general relativity provides a common explanation for all these systems,
including superfluids, in contradistinction with the views expressed in a
recent review article.
