PeterDonis said:
Of course, many people do not think this viewpoint is actually valid.
Btw, one reason for this is that it seems to contradict the friend's viewpoint--the friend thinks that what he does inside the lab
is a measurement, not just an ordinary unitary interaction like two qubits getting entangled. The friend
does apply rule 7 once he learns the result of his measurement.
The only way to make this consistent with Wigner taking the viewpoint that, until he interacts with the friend+lab, everything that happens to the friend+lab is just ordinary unitary interactions, is to claim that, even when we humans
think a measurement has happened, and irreversible traces have been left (marks on a paper, particular values stored in a computer's memory, particular memories of the person who did the measurement about what the result was), that's not really the case: all that stuff
could be reversed by some other entity that could perform the required unitary operations to reverse it. But if we take this viewpoint, then it should also apply to Wigner himself: Wigner has no more basis for claiming that his interaction with the friend+lab is an actual irreversible measurement, than the friend has for claiming that his interaction with the lab is an actual irreversible measurement.
In other words, this viewpoint undermines itself, at least in the opinion of those who make criticisms like the above.