Macroscopic Quantum Objects and their Interaction with External Environments

Abstract

To gain some appreciation of why any quantum phenomena on the macroscopic scale should be regarded as potentially important to developments in quantum mechanics we have to look back to the origins of the subject. Quantum mechanics today stands pre-eminent in Physics, being as important in calculating the subtleties of chemical bonding at roughly 10⁻⁸cm as describing the behaviour of quarks and gluons at 10⁻¹⁵cms. However, the unifying factor is always that these quantum objects are microscopic; not surprising since quantum mechanics was created originally to describe processes occurring in the atomic domain which were totally inexplicable in terms of the classical physics of the nineteenth century. This has imposed an operational structure on quantum mechanics which, although infrequently stated explicitly, may have limited its possibilities for development as a theory. This structure is simply stated. In quantum physics we have always been concerned with observing the behaviour of extremely small objects using experimental apparatus which is definitely macroscopic, i.e. on a length scale of roughly one centimetre to a few metres. This experimental situation carries with it two problems intimately connected with the interpretation of quantum mechanics. First, the effects of single microscopic events need to be amplified sufficiently to register (that is, make an irreversible record) in a large scale apparatus. This implies that a hierarchy of levels exists between an event occurring in a microscopic quantum object and the macroscopic environment — here, the apparatus to which it is coupled. Inevitably, as Feynman has emphasised¹, this creates an intrinsic uncertainty in our knowledge of the event. Second, and equally important, there is no possibility of following the detailed time evolution of a single quantum object, given the disparity in size between this object and the external apparatus. In general we have to be satisfied with an average view of the behaviour of a large number of identically prepared microscopic quantum objects. Of course, it is perfectly proper to ask why the macroscopic apparatus, which, after all, is a condensed matter system, should be treated as classical when we know full well that condensed matter is composed of microscopic quantum objects (atoms)? This is a very good question which, to date, cannot be answered satisfactorily. All that we can say with certainty is that in a quantum physics experiment we always appear to require an apparatus which we can describe as classical, i.e. one which will not go into superposition of states with the quantum object of interest but will act irreversibly to make a permanent record of information concerning that object. It is a matter of fact that very complex condensed matter systems (with an enormous number of uncorrelated internal microscopic degrees of freedom) fit the bill perfectly.