Quantum mechanics versus macroscopic realism: Is the flux there when nobody looks?

It is shown that, in the context of an idealized ‘‘macroscopic quantum coherence’’ experiment, the predictions of quantum mechanics are incompatible with the conjunction of two general assumptions which are designated ‘‘macroscopic realism’’ and ‘‘noninvasive measurability at the macroscopic level.’’ The conditions under which quantum mechanics can be tested against these assumptions in a realistic experiment are discussed.