  There exists a commonly accepted viewpoint that a movable mirror in an
interferometer should cause interference breakdown due to a quantum jump to one
of the two components of a photon mode. That effect goes back to Dirac. We
argue that the conventional reasoning is inadequate: First, it would be more
circumspect to interpret interference breakdown as being due to the
entanglement of the photon with the mirror, not referring to quantum jumps.
Second--and crucial--even in such an interpretation, the reasoning does not
take into account the uncertainty of the mirror momentum. The effect of the
entanglement and interference breakdown would take place if the uncertainty
were much less than the recoil momentum, which is of the order of the photon
momentum. However, an examination leads to the conclusion that for an actual
mirror the opposite situation occurs. Thus there should be no such effect.
