Quantum Reality: Beyond the New Physics

by Nick Herbert

in the Copenhagen interpretation, M devices enjoy a special classical-style status. On the other hand, in von Neumann’s all-quantum model measurement devices are not special but are represented by proxy waves like everything else.

In von Neumann’s all-quantum description of the world, the quantum measurement problem boils down to one question: where in fact does the wave function collapse?

When dice come up seven, for instance, we do not regard this as a “collapse” of spread-out dice possibility into one particular actuality. Why then should we regard the quantum probability collapse as any more real than the realization of a dice event?

there’s no natural boundary line between measurer and measuree.

the fact remains that no one has succeeded in showing how random activity by itself suffices to turn quantum ignorance (where the desired information simply doesn’t exist) into classical ignorance (where the desired information exists but is hidden).

The gist of von Neumann’s argument is this: in order for an all-quantum world to work, a special process—the quantum jump—must be present in all measurement acts and nowhere else.

“A coupling even with a measuring device is not yet a measurement. A measurement is achieved only when the position of the pointer has been observed. It is precisely this increase of knowledge acquired by observation that gives the observer the right to choose among the different components of the mixture predicted by theory, to reject those which are not observed, and to attribute henceforth to the object a new wave function, that of the pure case which he has found. We note the essential role played by the consciousness of the observer in this transition from the mixture to the pure case. Without his effective intervention, one would never obtain a new Ψ function … Thus it is not a mysterious interaction between the apparatus and the [quantum] object that produces a new Ψ for the system during measurement. It is only the consciousness of an ‘I’ who can, by virtue of his observation, set up a new objectivity.”