My apologies, I was editing my post while you were replying to it.<p>&gt; the <i>aggregate system</i> containing both continues evolving unitarily.<p>The Copenhagen interpretation is that the <i>measured subsystem</i> collapses in a non-unitary way.  That doesn&#x27;t imply the overall system did.  I have no opinion* on whether a measured subsystem collapses unitarily or not — the answer to that question does not currently seem to be experimentally testable.  But it is testable that the overall system continues evolving unitarily during a subsystem collapse (well, at least up until the overall system is measured), and this has been tested and verified many times.<p>(* Ok fine, I do have an opinion.  I don&#x27;t think unitarity breaks — ever.  I think it <i>appears</i> to break through the process of decoherence.  But I&#x27;m certainly not going to claim that as fact unless experiment can prove it somehow.)