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arXiv:1505.01125physics.hist-ph

Is Time's Arrow Perspectival?

Carlo Rovelli

Abstract

We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depends on coarse-graining. Therefore the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. Low past entropy may be due to the fact that this coupling (rather than microstate of the universe) is non-generic. I argue that for any generic microstate of a sufficiently rich system there are always special subsystems defining a coarse graining for which the entropy of the rest is low in one time direction (the "past"). These are the subsystems allowing creatures that "live in time" ---such as those in the biosphere--- to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.

arXiv comments
6 pages, 4 pretty figures. substantial text overlap with arXiv:1407.3384. in revision references added
Journal reference
DOI
Subjects
physics.hist-phHistory and Philosophy of Physics gr-qcGeneral Relativity and Quantum Cosmology hep-thHigh Energy Physics, Theory

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Linked by 2 identifiable commenters in 3 comments across 3 threads, from Nov 2018 to Nov 2023.

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Discussed in comments3 threads

  1. Constraints on physical computers in holographic spacetimes 1 comment

    • Thanks for the paper. Brownian computers are a cool idea, they seem like exploitation of the ratcheting paradigm. Entropy increasing is a illusion based on the specific selection of macroscopic observables / slices of configuration space that we use as inputs for entropy. There…

      goldenkeyView on HNParent context

  2. Maybe the spaghetti code conjecture is false 1 comment

    • Entropy itself is somewhat dubious as Carlo Rovelli gets into with the idea of relative entropy and course-graining. Entropy only exists with regard to a representation / set of observable operators. When you change your basis, you get a new entropy. So the cold ant, looks like…

      goldenkeyView on HNParent context

  3. A Binary Star Is About to Go Supernova and Could Produce a Gamma-Ray Burst 1 comment

    • I haven't read Rovelli's pop-sci book. Could you glance through the table of contents and/or the index and look for, among other things, "thermal time" and "time emerges in a world without time"? I would be astonished if he did not discuss these ideas at length in his book. >…

      raattgiftView on HNParent context

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