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I recently came across the paper Relation between the psychological and thermodynamic arrows of time (arXiv).

Their argument makes sense to me, however their concept doesn't seem to address what 'now' is.

If each person exists in an unchanging state in the block universe, and the flow of time is an illusion caused by the way memory functions, then how exactly does this illusory sensation of 'now' work? Are there an infinite number of me's at every point in time, all experiencing their own now? Or is there one me that 'travels' though the block universe, experiencing the illusion of time? And what is it that is experiencing this illusion of movement?

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  • $\begingroup$ You might want to check the paper at the arXiv link I added to your question. If you get the answer you want from it, go ahead and post it. $\endgroup$
    – David Z
    May 8, 2014 at 21:36
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    $\begingroup$ Also, by way of a partial answer (which someone who posts an actual answer can incorporate) the "block universe" idea you mention is a cornerstone of special relativity. So it's not only a widely held view; essentially all serious physicists take it as a given. $\endgroup$
    – David Z
    May 8, 2014 at 21:38
  • $\begingroup$ Just edited it. Turns out they didn't answer the question in the paper. $\endgroup$
    – iRoygbiv
    May 8, 2014 at 22:34
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    $\begingroup$ Notice also that this "now" is completely private. As soon as one assumes the relativistic viewpoint there is no longer room for a common universal "now" as in classical physics. There is a very interesting book ("What is time?") written by an Italian philosopher of science (M. Dorato) about various problems related with time in relativistic theories, technically (mathematically) examining several problems including the one about the notion of "now". Sorry, I do not think there is an English translation yet. $\endgroup$ May 9, 2014 at 7:31

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Even if there is no agreement in the physics community about what is special about "NOW", I believe that most physicists that believe in a block universe would agree with your statement that there are an infinite number of me's at every point in time, all experiencing their own now. Not only that, there is an infinite number of mathematical universes (see Max Tegmark's work) in which different "you"s experience different alternative realities. The issue of when the "you" becomes someone else (that is, when he is different enough to be considered an entirely different person) is a matter of debate in philosophy too (I'll try to find references to the articles that deal with these issues).

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  • $\begingroup$ Thank you for a constructive answer! One of the very few I've gotten since asking this question in several places. $\endgroup$
    – iRoygbiv
    Jul 9, 2014 at 18:32
  • $\begingroup$ Thanks! but to be fair, I should add that one of the star physicists of today, Lee Smolin, believes just the opposite, see en.wikipedia.org/wiki/Lee_Smolin. $\endgroup$
    – user16007
    Jul 10, 2014 at 0:08
  • $\begingroup$ He thinks that the NOW is very important and should be incorporated in physical theories, which would result in a revolutionary change in physics. I myself do not find any of his arguments (he doesn't really have anything resembling a theory beyond that idea) convincing. $\endgroup$
    – user16007
    Jul 10, 2014 at 0:11
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In note [5] of your paper, check the recent paper The physics of 'now', James B. Hartle. Hartle builds models of simple information gathering and utilizing systems and explains how past, future and now may be concepts describing IGUSes process information. It is interesting to point out different IGUSes can have drastically different and bizzar notions of "now" as we conventionally understand. All are consistent with 4 dimentional fundamental physics. So the notion of now as we understand is not really a necessary consequence of fundamental physics but are the results of laws of physics and particular mechanism of the human IGUS, according to Hartle.

In note [4], In Memory Systems, Computation, and the Second Law of Thermodynamics, Wolpert distinguishes between two kinds of memories, one that remembers past and future and one that remembers past only (like the human brain). Wolpert's argument makes more sense to me than your paper. Maybe you want to check it out.

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To address the issue of "...then how exactly does this illusory sensation of 'now' work", let's consider the mathematical multiverse mentioned in Julian Fernandez' answer above. If you take this seriously, then you should consider yourself to be a mathematical algorithm. A mathematical algorithm can be defined by consdering how it acts on certain states, but it cannot be construed as being such a state that it is acting on. Restricting ourselves to a universe described by quantum mechancis, this means that it is wrong to associate an observer to a wavefunction (or density matrix), rather one should describe an observer by an effective Hamiltonian. Obviously, this implies that the flow of time is encoded in the description of the observer as he exists at any moment (the Hamiltonian is the generator of time translations).

The formalism developed in this article fits in well with this idea, although the authors use it to address different issues.

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  • $\begingroup$ It seems like saying 'now' represents initial conditions, on which a time translation is constantly acting.. $\endgroup$
    – CriglCragl
    Jul 2, 2020 at 21:24

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