don@allegra.UUCP (D. Mitchell) (05/03/84)
Occasionally, physics cannot avoid confronting philosophical issues. The Copenhagen interpretation describes how the "probability" of the outcome of a measurement can be calculated. But what is probability? The frequency of success when an infinite number of experiments are performed? Is it, as Bayesians believe, a measure of how much you subjectively expect something to happen? J. von Neumann and G. Birkhoff claimed that Quantum Mechanics defines a new logic. Because points in classical phase space obey standard set theory, statements about classical systems obey standard logic. In a quantum mechanical system, you cannot always manipulate statements this way. If X is the statement "The Position of the Electron is 10" and Y is "The Momentum of the Electron is 3", the conjunction of X and Y is meaningless. Quantum Logic has been a hot topic since, but many philosophers have objected that such a formalism does not "explain" anything and does not suggest new paths of thought. (and therefore is not a "good theory") There are terribly complex issues involving measurement and the question of how to separate reality into "system" and "observer". What if you want to include the observer in the system? If the universe is governed by quantum mechanics, why are all measurements ultimately described by classical terms (e.g. the position of a needle on a dial?)