The Quantum Universe

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• Essays on Physical Theory
• Decoherent Histories QM
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• Probability in Quantum Mechanics
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• Arrows of Time
• Anthropic Reasoning in QC
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Quasiclassical Realms
A Measure for Classicality

The discussions of the quasiclassical realms on the previous page assumed that they were defined by coarse grainings defined in terms of familiar quasiclassical variables. But can classicality be characterized abstractly, in information theoretic terms, without reference to a specific set of variables? Is there a measure for classicality that can distinguish between different decoherent sets of alternative histories? With such a measure we could investigate whether quantum mechanics and a quantum state of the universe lead to distinct realms of high predictability, or whether the one defined by quasiclassical variables is essentially unique. The program to find an abstract measure of classicality has not been successfully completed. But the following papers describe the efforts to find one.

Adaptive Coarse-Graining, Environment, Strong Decoherence, and Quasiclassical Realms [155]

(with Murray Gell-Mann) Three ideas are introduced that when brought together characterize the realistic quasiclassical realms of our quantum universe as particular kinds of sets of alternative coarse-grained histories defined by quasiclassical variables: (1) Branch dependent adaptive coarse grainings that can be close to maximally refined and can simplify calculation. (2) Narrative coarse grainings that describe how features of the universe change over time and allow the construction of an environment. (3) A notion of strong decoherence that characterizes realistic mechanisms of decoherence.

A Measure of Classicality [measclass]

(with Murray Gell-Mann) Excerpts from a paper with Murray Gell-Man on the problem of providing measure for classicality stressing the importance of augmented information.

Equivalent Sets of Histories and Multiple Quasiclassical Domains [103]

(with Murray Gell-Mann) This is a largely technical paper showing how a given quasiclassical realm (called a `domain' here) can be represented in equivalent ways by means of unitary transformations and passive field redefinitions.

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