MANY WORLDS AT 50
101 years ago William James wrote this about the Hegelian movement in philosophy: \'The absolute mind which they offer us, the mind that makes our universe by thinking it, might, for aught they show us to the contrary, have made any one of a million other universes just as well as this. You can deduce no single actual particular from the notion of it.
This talk follows on from Wayne Myrvold\'s (and is based on joint work with Myrvold). I aim (and claim) to provide a unified account of theory confirmation that can deal with the (actual) situation in which we are uncertain whether the true theory is a probabilistic one or a branching-universe one, that does not presuppose the correctness of any particular physical theory, and that illuminates the connection between the decision-theoretic and the confirmation-theoretic roles of probabilities and their Everettian analogs.
Much of the evidence for quantum mechanics is statistical in nature. Close agreement between Born-rule probabilities and observed relative frequencies of results in a series of repeated experiments is taken as evidence that quantum mechanics is getting something --- namely, the probabilities of outcomes of experiments --- at least approximately right. On the Everettian interpretation, however, each possible outcome occurs on some branch of the multiverse, and there is no obvious way to make sense of ascribing probabilities to outcomes of experiments.
The most common objection to the Everett view of QM is that it \'cannot make sense of probability\'. The \'Oxford project\' of writers such as Deutsch, Wallace, Saunders and Greaves seeks to meet this objection by showing that the Everett view allows some suitable analogue of decision under uncertainty, and that probability (or some suitable analogue of probability) can be understood on that basis.
Orthodox thinking about chance, choice and confirmation is a philosophical mess. Within the many-worlds metaphysics, where quantum chanciness engenders no uncertainty, these things come out at least as well, if not better.
In \'Everett Speaks\' I will detail Everett\'s involvement in operations research during the Cold War. He was, for many years, a major architect of the United States\' nuclear war plan. I will talk about his family life and his personal decline. We will hear a portion of the only tape recording of Everett in existence, in which Everett and Charles Misner talk about the origin of the Many World\'s interpretation--twenty years later at a cocktail party.
A fundamental question for Everettians is whether they can formulate a many-worlds interpretation of quantum theory which explains why, amongst all possible types of intelligent creature with all possible types of evolutionary and experimental history, we find ourselves among those whose histories apparently confirm Copenhagen quantum mechanics. Since the theory clearly allows that we could have found ourselves otherwise, the answer has to be probabilistic. Everettians then need to supply some account of how probability is or can be attached to an apparently deterministic theory.
One of the most remarkable features of our quantum universe is the wide range of time, place, scale, and epoch on which the deterministic laws of classical physics apply to an excellent approximation. This talk reviews the origin of such a quasiclassical realm in a universe governed fundamentally by quantum mechanical laws characterized by indeterminacy and distributed probabilities. We stress the important roles in this origin played by classical spacetime, coarse-graining in terms of approximately conserved quantities, local equilibrium, and the initial quantum state of the universe.
I analyze a series of common objections to Everett\'s Many Worlds Interpretation. I discuss which ones are unique to quantum mechanics, and which have nothing to do with quantum mechanics per se as they can also be debated in the context of other areas of physics