 
Summary: Published as: Aerts, D. and Durt, T., 1994, "Quantum, classical and intermedi
ate: a measurement model," in the Proceedings of the International Symposium
on the Foundations of Modern Physics 1994, Helsinki, Finland, eds. Montonen,
C., et al., Editions Frontieres, Gives Sur Yvettes, France.
QUANTUM, CLASSICAL AND INTERMEDIATE: A MEASUREMENT MODEL.
Diederik Aerts* and Thomas Durt**
Free University of Brussels
Theoretical Physics, Pleinlaan 2,
B1050 Brussels, Belgium.
Abstract: We present a measurement model where the origin of the quantum
probabilities lies in the presence of fluctuations between the measurement ap
paratus and the physical system. First we make a reasoning where we show
that the measurement process cannot be described by the unitary Schršodinger
evolution only. Afterwards we present our model of measurement and show the
necessity of developping a more general structure than orthodox Hilbert space
quantum mechanics to resolve the measurement problem.
1. Introduction.
In conventional textbooks on quantum mechanics, the classical limit is often pre
sented through Ehrenfest's theorem: if the wave function representing a quantum
* Senior Research Associate of the Belgian National Fund for Scientific Research
