ABSTRACT
This chapter investigates several reasons why most realistic measurements are non-ideal and are extended to nonideal measurements. The need for a measurement theory that takes into account the possible nonideality of the actual measurement processes acquires its full importance in the light of a remark first made by Wigner and then generalized by Araki and Yanase. Examples of nonideal measurements, that is, of measurement processes that do not conform to the pattern of, are of course quite easily found. The analysis shows that for most observables the notion of ideal measurements is an idealization indeed, in the sense that it is a limiting case, which real measurements can never exactly reproduce. A model for measurement with an apparatus has been put forward by Green and discussed by Furry. These arguments are based on the postulate that the density matrix is in principle measurable.
