What is the Problem of Measurement

UANTUM MECHANICS, TOGETHER WITH RELATIVITY, are the basis of most of what we know of the very large, the very small, and the very fast. And in all of these areas our knowledge is extensive. It would be hard to say how good these theories are; Q arithmetic, one might say, is good mathematics. For all that, there is a problem, a strange and amorphous difficulty: it is the problem of measurement. This problem has become much more interesting, and much more pressing, as a result of some recent developments. Hitherto the difficulty has been so peculiar, and so formless, that physicists have on the whole thought it a matter of philosophy. But that is not quite right (or that is what I shall argue). It is certainly philosophical, but it is also a matter of methodology, a question of what physics is to do; and now the matter has come to seem quite pressing. In this sense the measurement problem has become a problem for physics. But what is the problem of measurement? As a first stab, we can say this: on the basis of quantum mechanics, there is a difficulty in accounting for the