Must-Read: What does it say about me that I find "we have used a Stern-Gerlach magnet with a field gradient in the direction z to measure spin in the direction y..." to be the funniest thing I have read this month?
We are all, potentially, the Friends of Wigner. It has always seemed to me that anyone with the empathy and imagination to think of him or herself as one of the Friends of Wigner is then driven inescapably to either "quantum mechanics is totally wrong wrong wrong wrong and just predicts well for incomprehensible reasons" or "many-worlds". There really are no other alternatives, or at least what alternatives there are are even stranger.
I am reminded of Sidney Coleman's joke--or was it--that he concluded that he and he alone could cause the reduction of wave packets, and his advisor's natural question: "Before you were born, could your father collapse wave packets?":
W. H. Zurek (1981): Pointer basis of quantum apparatus: Into what mixture does the wave packet collapse?: "Hence, there is also a 100% correlation between the state of the atom and the spin in a basis completely different...
...We have used a Stern-Gerlach magnet with a field gradient in the direction z to measure spin in the direction y. Moreover, we can choose what we shall measure on the atom long after the spin has ceased to interact with it. Counterintuitive predictions of quantum mechanics for this bit-by-bit measurement correspond closely to the nonseparability of the Einstein-Podolsky-Bosen paradox. For what is measured on the state of the atom influences the state of the spin. Moreover, even though the spin has been split inside RSG into the two well-separated beams, one definitely carrying |UP> and the other |DOWN>, once these beams are recombined the atom can supply definite and correct information about the spin's alignment (i.e., parallel or antiparallel) with respect to other directions. Wigner's friend--to use the analogy of Ref. 10--not only ends up in a superposition after taking a look at the single spin passing through RSG, he can also be led to admit that he has seen-—inside the RSG magnets with magnetic field gradients in the z direction-—spin aligned along the y axis.
This conclusion appears preposterous. For quantum systems it has been nevertheless convincingly verified in experiments stimulated by the EPR paradox. Furthermore, if one denies (in disagreement with original proposals of von Neumann, London and Bauer, and Wigner) any special role to consciousness, there is seemingly nothing that could keep one from describing an arbitrary system, no matter how large, by a state vector and Schrödinger equation. After all, there is nothing in the laws of physics that would make quantum mechanics applicable to a few-body system but render it invalid for a truly many-body system, even if it contains 10^25 or more atoms as long as it remains isolated...
