08/09/15 11:00H Talk @ UFRJ – Theo M. Nieuwenhuizen- Int. Inst. Phys., Natal, and Inst. Theo. Phys. University of Amsterdam


On quantum measurements, the measurement problem and the interpretation of QM


Since the only point of contact between the quantum formalism and reality lies in measurements, it is mandatory to model them and analyze the dynamics. A rich but still solvable model is the so-called Curie-Weiss model, where the z-component of a spin is measured by a Curie-Weiss magnet, consisting of many spins, mutually coupled in the z-direction. The pointer variable is its magnetization in the z-direction, which at low enough T can point upwards or downwards.

The magnet starts in a metastable paramagnetic state, and is triggered by the measurement towards either its up or its down stable state. We work out the dynamics of the measurement, postponing interpretation as far as possible. 

Three dynamical regimes emerge: 

1) off-diagonal elements of the density matrix (Schroedinger cat states) decay quickly in a cascade of small multiparticle correlations;

2) next, the distribution of the pointer moves and widens, to become narrow again around the final up or down state. The final state for the total ensemble emerges. 

3) Near the end of the measurement a relaxation process takes place which causes arbitrary, mostly unphysical, decompositions of the density matrix to relax towards the ones on the measurement basis. 

At this point we postulate that the stable decompositions are related to physical subensembles and that pure subensembles occur too. For the latter, each member yields the same measurement outcome, hence we can connect the quantum formalism to individual measurements, thus narrowing down the measurement problem. 

It is noticed that quantities like |\psi_i|^2 are “quantum probabilities” that have no a priori physical meaning, while true probabilities are related to outcomes of the macroscopic pointer, in the frequency interpretation.

The emerging picture of the ensemble interpretation is confronted with many worlds and minds; the GHZ-paradox, Schroedinger cats, Bell’s theorem, the black hole information paradox and the wavefunction of the Universe.

UFRJ- Centro de Tecnologia do Fundão, Bloco A, sala 343


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