Many Worlds at 50 - 2007

I shall present an overview of quantum mechanics in the Everett interpretation, that emphasises its structural characteristics, as a theory of what exists. In this respect it shares common ground with other fundamental theories in physics. As such its appeal is conservative; it makes do with the purely unitary equations of quantum mechanics as exceptionless and universal. It also makes do with standard methods for extracting \'high level\' or \'emergent\' ontology, the furniture of macroscopic worlds, from largish molecules on up.

Everett explained “collapse of the wavepacket” by noting that observer will perceive the state of the measured quantum system relative to the state of his own records. Two elements (missing in this simple and compelling explanation of effective collapse) are required to complete relative state interpretation: (i) A preferred basis for states of at least some systems in the wholly quantum Universe must be identified, so that apparatus pointers and other recording devices can persist over time.

I will review the current state of the probability problem. My main focus will be on the attempts by David Deutsch and myself to provide a proof of the Born Rule starting from Everettian assumptions, but I will also attempt to locate these attempts within the more general framework of the probability problem.

I will rehearse and try to sharpen some of the perennial worries about making sense of probabilities in Everettian interpretations of quantum mechanics, with particular attention to the recent Decision-Theoretic proposals of Deutsch and others

Probability is often regarded as a problem for the many-worlds interpretation: if all branches of the splitting wavefunction are equally real, what sense does it make to say that the branches have different probabilities? In the decision-theoretic approach due to Deutsch and Wallace, probabilities acquire a meaning through the preferences of a rational agent. This talk reviews the decision-theoretic approach to probability in classical physics and quantum mechanics and shows that its application to the many-world interpretation creates a new difficulty for the latter.

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

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.

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.

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.

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.