Introduction

In a very elegant and rigorous paper, K. Hepp has discussed quantum measurement theory. He uses the C* algebra description of infinite quantum systems. Here an attempt is made to give a more popular account of some of his reasoning. Such an attempt seems worthwhile because many people not familiar with the C* algebra approach, and even somewhat intimidated by it, have been intrigued by the following statement in Hepp's abstract:

This could look like a clean solution at last to the infamous measurement problem. But it is not so, nor thought by Hepp to be so. Here we will take one of his models and analyse it in elementary text-book terms. It will be insisted that the ‘rigorous reduction’ does not occur in physical time but only in an unattainable mathematical limit. It will be argued that the distinction is an important one.

We will work at first in the Schrödinger picture, but later, with the extension to relativistic systems in mind, it will be argued that such considerations become particularly clear in the Heisenberg picture.