Questions of Physical Theory

Abstract

Prosperi points out that microscopic experiments on a macroscopic body are very difficult to carry out. He implies that only the values of macroscopic variables are meaningful if one deals with macroscopic objects, such as measuring devices. It is not clear whether he considers the values of microscopic variables to be unobservable in principle. If this were so, quantum mechanics should surely be modified and the quantities which are measurable on given bodies specified. To be sure, the statement that only macroscopic variables are meaningful for macroscopic objects will be a clearly defined statement only after some definition of « macroscopic variables » and « macroscopic objects » has been provided. The phenomena of permanent currents in superconductors, of spontaneous magnetization in different directions, show that the behaviour of large objects can be greatly influenced by quantum effects which are outside the realm of ordinary macroscopic theory. As to the difficulty of observing phases, it might be recalled that the difference between dextro and levorotatory sugar is based on a phase relation—a difference surely observable.