The paper first scrutinizes thoroughly the variety of compositions which lead to the same quantum-mechanicalmixture (as opposed tostate orpure state). With respect to a given mixtureevery state has a definite probability (or mixing fraction) between 0 and 1 (including the limits), which is calculated from the mixtures Statistical Operator and the wave function of the state in question.

A well-known example of mixtures occurs when a system consists of two separated parts. If the wave function of the whole system is known, either part is in the situation of a mixture, which is decomposed intodefinite constituents by adefinite measuring programme to be carried out on theother part. All the conceivable decompositions (into linearly independent constituents) of the first system are just realized by all the possible measuring programmes that can be carried out on the second one.In general every state of the first system can be given a finite chance by a suitable choice of the programme.

It is suggested that these conclusions, unavoidable within the present theory but repugnant to some physicists including the author, are caused by applying non-relativistic quantum mechanics beyond its legitimate range. An alternative possibility is indicated.

†Proc. Camb. Phil. Soc. 31 (1935),555–63.Google Scholar

‡ The valuable conception of a mixture and the appropriate way of handling a mixture by the Statistical Operator is due to Johann von Neumann; see hisMathematische Grundlagen der Quantenmechanik, Berlin, Springer,1932Google Scholar; especially pp. 225ff.

†Schrödinger,E.,Annalen der Physik (4),83 (1927),961Google Scholar.Collected Papers (Blackie and Son,1928), p.141.Google Scholar

‡Dirac,P. A. M.,Nature.137 (1936),298.CrossRefGoogle Scholar