Interpretation of Probabilities in Physics

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In classical physics, the role of probability was primarily to express ignorance and opinion. Probability was needed because the physicist simply did not have enough information to make an absolute determination. In principle, however, as with Pascal's "superman," if scientists had sufficient prior information, they would not need probabilities. The quantum revolution of the 20th century, however, denied such certainly is ever possible, even with infinite knowledge of the prior conditions. Einstein objected to quantum theory, because of its denial of ever achieving certainty, that "God does not play dice with the Universe." In this research project, Professor Van Fraassen is examining these two differing aspects of the use of probabilities in physics i.e. that which is due to "ignorance or opinion" given insufficient prior information, and that due to theories like quantum theory which deny the possibility of certainty. The first problem is a general one: what belief is involved concerning the actual world, in the acceptance of a (n irreducibly) statistical theory? To formulate this more precisely, the concept of belief is replaced by that of opinion, represented as personal probability. There is a generally accepted recipe for how opinions about objective chance (physical probability) constrain opinions about what will actually happen. Any solution to the first problem may be required to entail the correctness of this recipe; other requirements may also be laid down. The second problem is specific to contemporary physics: what interpretation can we give to the probabilities it presents with their characteristic features of "probability interference" and "loss of individual identity"? Professor Van Fraassen will investigate this problem with special reference to one interpretation of quantum theory, (the Copenhagen variation of the modal interpretation), and one area in quantum theory (statistical behavior of identical particles).