On the Interpretation of Bohmian Mechanics

Abstract

Terms like “complementarity,” “potentia,” the “collapse” of the wave packet, “phenomena” identified with the whole experimental arrangement, and so forth, mark the standard interpretation of quantum mechanics. Despite different public faces there is a core unifying theme. The theory is probabilistic and, in contrast with statistical mechanics, the standard interpretation regards the probabilities as objective, in the sense that it does not ground them in human limitations concerning knowledge of the finer details of things. The objectivity of the probabilities makes for indeterminism and, more fundamentally, for some sort of irrealism; since, according to the standard view, in significant situations there just are no finer details of things. The irreducibility of the probabilities might be thought to constitute a realism of a higher order (with respect to the probabilities themselves) except that in the standard interpretation the probabilities are entirely instrumental. They express a relation between a physical system and acts of measurement; they are probabilities for measurement outcomes. Thus, in a curious turn about, despite the objectivity of the probabilities the observer enters quantum theory in a fundamental way. On Bohr’s view we are required to divide each experimental situation into an observer part, that is treated classically and to which we do not apply the quantum formalism, and a quantum part, to which we do. On Heisenberg’s view the probabilities in the wave function somehow objectively represent both real “potentialities” and also subjective knowledge. Standing outside the causal order, an act of measurement “actualizes” a potentiality and, when we take account of this actualization, our changing knowledge is again objectively represented by a new “collapsed” wave function. Either view makes a mystery of how any object ever comes to possess any property; that is, of how anything at all actually happens. Standardly, we are cautioned not to inquire further. Physics stops here.

Now the concept of “physical reality” presents itself as problematic and the question arises as to what, then, it really is and what theoretical physics (through quantum mechanics) tries to describe and to what its established laws refer. Certainly this question could be answered in different ways. (Einstein 1953, 35. My translation)