Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The 'time oriented coarse graining' hypothesis -- "Rovelli"

  1. Jul 20, 2014 #1
    http://arxiv.org/abs/1407.3384

    Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis

    Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.

    Is Rovelli saying that the low entropy state of the universe plusminus 13.7 bilion years ago is not the whole story ? I don't fully understand what he is trying capture here.
    Could t be that the discrete strutcture of the low entropy state universe thet whe vision after singularity or Bounce or whatever the universe state was 13.7 billion years ago, has something to do with our macroscopic ensemble view.
     
  2. jcsd
  3. Jul 20, 2014 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Yes. Entropy is not an intrinsic/absolute quantity. It depends very much on the choice of macroscopic variables.
    So you are right: the fact that we reckon the entropy of the universe, then, to have been low has VERY MUCH to do with our natural macroscopic variables (those through which we, by our nature, couple to the universe).
     
    Last edited: Jul 20, 2014
  4. Jul 20, 2014 #3

    But than Marcus, macroscopic statements as singularity continuüm or discreet states will also be generalized.
     
  5. Jul 20, 2014 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    My problem is with my own imagination. I cannot imagine anything more interesting and important than what we call "life forms" and I cannot imagine a life-form that does not couple to the universe via macroscopic variables we call "frequency" and "energy".

    The "time oriented coarse graining" paper is primarily about that: about defining alternative sets of macro variables, alternative maps of macro states, if you can do that then obviously the definition of entropy changes. Entropy of where you are is just the "size" of the macro state you are in (i.e. log of number of microstates it comprises). If you change the map of macro countries, you change all their sizes and it is a new game. TOCG paper sets up a toy model balls-in-box universe where we can easily SEE alternative macro variables. My difficulty is in imagining how to extend that to the real universe we see and live in: what other macro variables could IT have?
     
    Last edited: Jul 20, 2014
  6. Jul 20, 2014 #5

    atyy

    User Avatar
    Science Advisor

    Is Rovelli's paper related to this paper which also investigates whether the second law of thermodynamics can be derived by examining whether subsystems of an isolated quantum system can come to local equilibrium?

    http://arxiv.org/abs/1402.3380
    The approach to equilibrium in a macroscopic quantum system for a typical nonequilibrium subspace
    Sheldon Goldstein, Takashi Hara, Hal Tasaki


    Golstein and colleagues trace their work back to von Neumann. Here is a commentary on von Neumann's paper.

    http://arxiv.org/abs/1003.2129
    Long-Time Behavior of Macroscopic Quantum Systems: Commentary Accompanying the English Translation of John von Neumann's 1929 Article on the Quantum Ergodic Theorem
    Sheldon Goldstein, Joel L. Lebowitz, Roderich Tumulka, Nino Zanghi

    "For a "typical" finite family of commuting macroscopic observables, every initial wave function ψ0 from a micro-canonical energy shell so evolves that for most times t in the long run, the joint probability distribution of these observables obtained from ψt is close to their micro-canonical distribution."
     
    Last edited: Jul 20, 2014
  7. Jul 20, 2014 #6
    Yes i see, this surpasses my horizon of imagination to.
     
    Last edited: Jul 20, 2014
  8. Jul 20, 2014 #7

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    If I understand the gist, I think you are making a good point. What about the "start of expansion"?

    How do we define macro variables on it? Who is coupled to it, interacting with it, experiencing it?
    What are the relevant macro variables? It is a very dense state, as we understand dense. A lot of energy, as we understand energy. Perhaps there is a bounce. Does the bounce happen very quickly? Who or what is keeping time?

    There have been several papers recently arguing that in the quantum regime at bounce there is a signature change---from Lorentz (with lightcones and extended present) to Euclidean (as if the speed of light suddenly for a flickering instant became infinite). This troubles me, this suggested "signature change". Only one or two people are talking about it, so perhaps for the time being one should simply be aware that it is being mentioned in the literature but not pay very close attention.
     
  9. Jul 20, 2014 #8

    atyy

    User Avatar
    Science Advisor

    I just came across an intriguing paper about signature change from a non-LQG group.

    http://arxiv.org/abs/1403.0580
    Emergent Lorentz Signature, Fermions, and the Standard Model
    John Kehayias, Shinji Mukohyama, Jean-Philippe Uzan
     
  10. Jul 20, 2014 #9

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    In reply to post #5, "is Rovelli's paper related to [a paper by Sheldon Goldstein]?"
    I think probably not. Both use the word "entropy" but otherwise don't see much overlap.
    If interested, here's an interview with Goldstein (b. 1947, math dept Rutgers)
    Here's a less technical, less specialized paper of his to give an idea of his turn of thought:
    http://arxiv.org/pdf/cond-mat/0105242v1.pdf

    I think Rovelli's concern is with the perception of low entropy at start of expansion---e.g. at the LQC bounce. And our perception that change (time) has a direction. Why don't we remember the future? The TOCG hypothesis (rightly or wrongly) assumes that different coarsegrainings are possible, different choices of macro variables, and that our set of macros is part of our identity, so that the direction of time is somehow rooted in who we are. This idea comes as a shock and goes against one's strongly held preconceptions.

    I'm not sure that either signature change or signature emergence (two different ideas) is relevant to the TOGC paper. I mentioned the former (brief episode of Euclidean signature RIGHT AT the bounce) because it might be relevant to the TOGC hypothesis. It might turn out that, like a horse that throws off its rider, no coarsegrainings are possible in that moment when the signature is Euclidean and the U throws off all its macroscopic variables. Then a moment later it is back to normal, Lorentzian signature is resumed.

    The assumption is that underlying spacetime reality has Lorentz invariance (except according to some people right at the bounce).

    This is different from the "emergent Lorentz signature" idea where people study models in which underlying reality is Euclidean forever and the Lorentz signature somehow constantly emerges from that. E.g. http://arxiv.org/pdf/0806.4239v2.pdf (Girelli, Liberati, Sindoni)
    "In the first section, we have considered fields that live in a Euclidean space, and showed that there exists a class of Lagrangians such that the perturbations around some classical solutions ψ ̄ propagate in a Minkowski spacetime. In this case ψ ̄ is essentially picking up a preferred direction, so that we have a spontaneous symmetry breaking of the Euclidean symmetry. The apparent change of signature is free of the problems usually met in signature change frameworks since the theory is fundamentally Euclidean. Lorentz symmetry is only approximate, and in this sense it is emergent.
    The main lesson we want to emphasize here is that Lorentzian signature can emerge from a fundamental Euclidean theory and this process can in principle be reconstructed by observers living in the emergent system. In fact, while from the perturbations point of view it is a priori difficult to see the fundamental Euclidean nature of the world, this could be guessed ..."
     
    Last edited by a moderator: Sep 25, 2014
  11. Jul 20, 2014 #10

    atyy

    User Avatar
    Science Advisor

    Perhaps the two papers are not related, but the part of Rovelli's paper that caught my eye was the quantum section "The point here is not to assume the tensorial structure of H a priori. Instead, given a generic state, we can find a tensorial split of H which sees von Neumann entropy grow in time." (Entanglement Entropy :smile:)

    This seems similar to Goldstein et al's "We prove that, for a typical choice of "nonequilibrium subspace", any initial state (from the energy shell) thermalizes..." Actually, it may be closer to an earlier paper from Goldstein, Lebowitz, Tumulka and Zanghi http://arxiv.org/abs/cond-mat/0511091, as well as the similar paper of Popescu, Short and Winter http://arxiv.org/abs/quant-ph/0511225, both of which use the entanglement entropy explicitly. But I don't think either of those had an argument for the arrow of time.
     
    Last edited by a moderator: Sep 25, 2014
  12. Jul 20, 2014 #11

    atyy

    User Avatar
    Science Advisor

    Here are some bits of the Goldstein paper http://arxiv.org/abs/1402.3380 in which the notion of "equilibrium state" or "non-equilibrium state" is said to depend explicitly on the choice of macroscopic observable, which I think is similar to Rovelli's point.

    "Consider the simplest situation where one is interested in the behavior of a single macroscopic quantity ##\hat{O}## , whose equilibrium value is ##\bar{O}##. Then one can define Heq as the subspace spanned by the eigenstates of the nonnegative operator ##(\hat{O} − \bar{O})^{2}## corresponding to sufficiently small eigenvalues." (p5, footnote 14)

    "Given the energy shell H, the nonequilibrium subspace Hneq, in reality, is determined not in a random manner, but through the values of macroscopic quantities that we use to characterize the system." (p10)
     
    Last edited: Jul 20, 2014
  13. Jul 21, 2014 #12

    atyy

    User Avatar
    Science Advisor

    Here are two papers by Sugiura and Shimizu that further explain the role of macroscopic observables in determining whether a state is an equilibrium state or not.
    http://arxiv.org/abs/1112.0740v4
    http://arxiv.org/abs/1302.3138v2

    From the introduction of the first:
    "It was shown in Refs. [1–4] that almost every such vector gives the correct equilibrium values of a certain class of observables ##\hat{A}## by ##\langle\psi|\hat{A}|\psi\rangle##. This property was proved in Refs. [1, 2] for observables of a subsystem, which is much smaller than the whole system. The case of general observables, including observables of the whole system (such as the total magnetic moment and its fluctuation), was analyzed in Refs. [3, 4]. It was shown that the above property holds not for all observables but for observables that are low-degree polynomials (i.e., their degree ##<< N##) of local operators [3]. We here call such observables mechanical variables."
     
  14. Jul 21, 2014 #13

    MathematicalPhysicist

    User Avatar
    Gold Member

    Cause the future never occurs in actuality, the past had occured and that's why we remember it.

    Kindergarten question really. :-D
     
  15. Jul 22, 2014 #14

    Remembering historical events or measuring them, is quite patchy or grainy if you like, never exact in a way that al information is available.
     
    Last edited: Jul 22, 2014
  16. Jul 22, 2014 #15
    But would a bouncing state of the universe in the past or the future not also inhabit the same marcroscopic variables questions as discretenes states and continuüm problems whe have now in our theoretical physics descriptions of these situations.

    It would be interesting to see if there is a way to generalize them.
     
    Last edited: Jul 22, 2014
  17. Jul 22, 2014 #16

    MathematicalPhysicist

    User Avatar
    Gold Member


    Well, if this measurement is quite patchy then all our measurements are in the same way, "quite patchy".
     
  18. Jul 23, 2014 #17

    Berlin

    User Avatar
    Gold Member

    It is amazing that Rovelli gets us thinking with every single paper he writes. Very original thoughts to me. The responses here however, seem weird to me. Why talk about 'Who we are' or what 'IT' will be? Don't we allready know that we, defined as the known particles and energy in the universe, are only the 4% or so discovered so far? My physics question would be: all the dark mass and energy only seems to couple with us only through gravity. Is there room (or 'space' :) ) in rovelli's thinking to speculate about that? Can we give boundaries in physical, or new, parameters describing those other forms?

    Berlin
     
  19. Jul 23, 2014 #18

    Demystifier

    User Avatar
    Science Advisor

    The circularity problem

    In my opinion, the main problem with the Rovelli's idea is circularity. To see this, one must ask obvious questions which Rovelli, for some reason, refuses to ask. When the obvious questions are answered by obvious answers, one gets the following logical loop:

    - Entropy increases because humans define macroscopic variables in a specific way.
    - But why do humans define macroscopic variables in such a specific way? Because that's how their mind works.
    - But why their mind works so? Because that's how their brain works.
    - But why their brain works so? Because the brain, as a part of nature, obeys the laws of physics.
    - But what these laws of physics say? Different laws say different things, but here the crucial law is the second law of thermodynamics which says that entropy increases.

    In other words, Rovelli's argument says that entropy increases because entropy increases. Circularity!
     
  20. Jul 23, 2014 #19

    atyy

    User Avatar
    Science Advisor

    But what if we take Goldstein et al's version of the argument? QM is already an instrumental theory, in which macroscopic observers and detectors are necessary, so that doesn't seem a problem - except for the measurement problem. In that sense, one could say that Goldstein et al merely push the problem back to the establishment of Bohmian quantum equilibrium?
     
  21. Jul 28, 2014 #20

    Demystifier

    User Avatar
    Science Advisor

    I don't see how it helps. In Bohmian mechanics particle positions and wave function are fundamental (not merely instrumental), while observers and detectors are made of these fundamental particles. The second law implies that the initial condition for the wave function must be special, which cannot be explained by referring to observers and detectors.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The 'time oriented coarse graining' hypothesis -- "Rovelli"
  1. Coarse graining (Replies: 1)

Loading...