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Information *between* two systems:what counts in physics (FQXi essay)

  1. Jun 27, 2013 #1


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    Rovelli's FQXi essay contest entry argues for a simple proposition
    Whether it is information or entropy you are defining you essentially always need TWO systems.
    The interaction Hamiltonian will indicate which variables matter to system B.

    That's what determines which are the macroscopic variables that system B responds to and are thus the relevant ones. It determines the map of macrostates comprised of B-indistinguishable microstates.

    The interaction between the two begins to look a like a Shannon communication channel.

    So the relevant variables for B might be the volume temperature and pressure of gas in a box, in which case we recover some familiar thermodynamics. The entropy then turns out to be the usual entropy. But the essay considers radical departures from that paradigm.

    An important thing to realize is that entropy is not an absolute. It depends very much on the two systems A and B and how they interact.

    Thanks to PF member John86 for spotting this essay and contributing it to our Loop-and-allied QG bibliography. It was post #1982, quite a recent one. I did not know that the 2013 FQXi essay contest entries were already on-line.

    Relative information at the foundation of physics by Carlo Rovelli
    I observe that Shannon's notion of relative information between two physical systems can effectively function as a foundation for statistical mechanics and quantum mechanics, without referring to any subjectivism or idealism. It can also represent the key missing element in the foundation of the naturalistic picture of the world, providing the conceptual tool for dealing with its apparent limitations. I comment on the relation between these ideas and Democritus.
    Last edited: Jun 28, 2013
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  3. Jun 29, 2013 #2

    Simon Bridge

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    Well sure - what I usually tell students is that it is the relaionships that are important.
    This is the same statement in terms of information theory.
  4. Jun 29, 2013 #3


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    Already knew about his relational QM. His application to thermodynamics is really interesting.

    I like how the irreversibility of the shattered cup is to do with how the states our perceptions take correlate with this cup system because of the type of creature humans are - it implies there could be other creatures that view the world differently.
  5. Jun 29, 2013 #4


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    I am always impressed by the simplicity and clarity of Rovelli's insights into what are the key principles of physics/reality and what notions we should discard to resolve our apparent difficulties understanding reality. And think that they he puts forward these ideas to spark off all new lines of research that could change the way we understand fundamental physics. I personally think it is a shame that his generally covariant statistical mechanics wasn't taken up by that many people, even though some progress has been made.

    Of course there will be people who will think that Rovelli's insights are of purely aesthetically value and so physical implications should be investigated and practical calculations should be made. I think Rovelli thinks this relative information idea insight could have wide ranging implications. It would be interesting to find what might come of it, and also what already established lines of research already parallel his ideas.

    Going off on a tangent, I do remember a while ago looking at a book where it stated current theories of consciousness were tending toward explanations that involved relational elements. Maybe physics and theories of consciousness are converging to similar notions.
    Last edited: Jun 29, 2013
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