If one is worried by the quantum measurement problem,a natural question to ask is: Does the quantum-mechanical description of the world retain its validity when its application leads to superpositions of states which by some reasonable criterion are _macroscopically distinct_? Or rather, does any such superposition automatically get "collapsed", even in the absence of "measurement" by a human observer, into one or other of its branches? Scenarios which predict the latter (for example the GRWP theory) may be denoted generically by the term "macrorealistic".
Even if one believes that QM remains the whole truth at the macrolevel, it is clear that to the extent that environmental decoherence destroys the delicate phase relations characterizing the superposition, the predictions of QM will be indistinguishable experimentally from those of the class of macrorealistic theories (a remark which is often taken, in my opinion quite erroneously, as "solving" the measurement problem). Thus, to
distinguish experimentally between QM and macrorealism one needs a system in which decoherence is low enough that (given that QM is correct) one has a realistic chance of observing _quantum interference of macroscopically distinct states_ ("QIMDS"). Over the last few years, a surprising variety of candidate systems has emerged; however, while all experiments to date have been consistent with the continued validity of QM, none has so far refuted macrorealism outright. In this talk I review the systems in question and discuss the prospects for a truly definitive experiment.