Quasiclassical Coarse Graining and Thermodynamic Entropy

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dc.creatorGell-Mann, Murray
dc.creatorHartle, James
dc.date2006-09-25
dc.date2007-05-16
dc.date.accessioned2026-07-07T11:44:35Z
dc.date.available2026-07-07T11:44:35Z
dc.descriptionOur everyday descriptions of the universe are highly coarse-grained, following only a tiny fraction of the variables necessary for a perfectly fine-grained description. Coarse graining in classical physics is made natural by our limited powers of observation and computation. But in the modern quantum mechanics of closed systems, some measure of coarse graining is inescapable because there are no non-trivial, probabilistic, fine-grained descriptions. This essay explores the consequences of that fact. Quantum theory allows for various coarse-grained descriptions some of which are mutually incompatible. For most purposes, however, we are interested in the small subset of ``quasiclassical descriptions'' defined by ranges of values of averages over small volumes of densities of conserved quantities such as energy and momentum and approximately conserved quantities such as baryon number. The near-conservation of these quasiclassical quantities results in approximate decoherence, predictability, and local equilibrium, leading to closed sets of equations of motion. In any description, information is sacrificed through the coarse graining that yields decoherence and gives rise to probabilities for histories. In quasiclassical descriptions, further information is sacrificed in exhibiting the emergent regularities summarized by classical equations of motion. An appropriate entropy measures the loss of information. For a ``quasiclassical realm'' this is connected with the usual thermodynamic entropy as obtained from statistical mechanics. It was low for the initial state of our universe and has been increasing since.
dc.description17 pages, 0 figures, revtex4, Dedicated to Rafael Sorkin on his 60th birthday, minor corrections
dc.identifierhttps://arxiv.org/abs/quant-ph/0609190
dc.identifierhttp://arxiv.org/abs/quant-ph/0609190
dc.identifierPhys.Rev.A76:022104,2007
dc.identifierdoi:10.1103/PhysRevA.76.022104
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/201652
dc.subjectQuantum Physics
dc.subjectStatistical Mechanics
dc.subjectGeneral Relativity and Quantum Cosmology
dc.subjectHigh Energy Physics - Theory
dc.titleQuasiclassical Coarse Graining and Thermodynamic Entropy
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