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

Padmanabhan and spacetime microscopic degrees of freedom

  1. Mar 30, 2010 #1
    In his latest paper below, he discusses the microscopic degrees of spacetime and entropy, and how the emergent thermodynamic features of spacetime do not require details of the microscopic degrees of freedom.

    Still, I wonder what sort of microscopic degrees of freedom of space time would be compatible with an emergence ? Is time and space on the same footing or distinct? Does it have to be 3+1 or something else?

    http://arxiv.org/abs/1003.5665
    Surface Density of Spacetime Degrees of Freedom from Equipartition Law in theories of Gravity
    T. Padmanabhan
    20 pages
    (Submitted on 29 Mar 2010)
    "I show that the principle of equipartition, applied to area elements of a surface which are in equilibrium at the local Davies-Unruh temperature, allows one to determine the surface number density of the microscopic spacetime degrees of freedom in any diffeomorphism invariant theory of gravity. The entropy associated with these degrees of freedom matches with the Wald entropy for the theory. This result also allows one to attribute an entropy density to the spacetime in a natural manner. The field equations of the theory can then be obtained by extremising this entropy. Moreover, when the microscopic degrees of freedom are in local thermal equilibrium, the spacetime entropy of a bulk region resides on its boundary."
     
  2. jcsd
  3. Mar 30, 2010 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

  4. Mar 30, 2010 #3

    turbo

    User Avatar
    Gold Member

    His review paper was my pick a couple of years back, and it came out on top. He's always at some "interesting" places. I wish he would consider a dynamic (as opposed to static) model of the vacuum's role in gravitation, but maybe that's just me.
     
  5. Mar 30, 2010 #4
    The ideal gas law and 19th century thermodynamics postulated a toy model of atoms with point masses bouncing around in a vaccuum.

    Id like to see a "toy-model" of atomic spacetime that gives rise to GR via entropy mechanism. Do the current QG candidates offer the capability to serve the requisite microscopic degrees of freedom?
     
  6. Mar 30, 2010 #5

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    If you look more closely at the paper I think that he keeps saying "spacetime" when he actually means "space". Space IS dynamic in his analysis. For example he talks about temperature. A thermometer held by an accelerating observer, or by someone outside the horizon of a BH. Thermometers are in space (at some moment of time) and measure the temperature of space at that moment, around them. They don't measure the temperature of a whole spacetime. It sounds static, the way he talks, but it's just his way of talking.

    I wish you would bet on Padmanabhan again, in the current poll. I remember when you made that choice in another poll, sometime back.
     
  7. Mar 30, 2010 #6

    turbo

    User Avatar
    Gold Member

    I rarely participate in these polls regarding the relative importance of papers because I am not adequately familiar with literature in the relevant fields. Astronomy and Astrophysics is more my bag. Padmanabhan is willing to explore some backwaters that others have abandoned, though, which is something absolutely necessary (IMHO) if there are fundamental advances to be made. When Paddy said that he would model the vacuum as an elastic solid with a role in gravitation, I was encouraged. When he wrote that the vacuum was not dynamic, but a static background against which gravitational interactions played out (just paraphrasing) my heart fell.

    I may have understood, and he may have published more about the dynamic nature of the vacuum in gravitation that I have missed. That would make me happy.
     
  8. Mar 31, 2010 #7

    turbo

    User Avatar
    Gold Member

    I have re-read the paper a couple of times Marcus, and I agree that there appears to be some ambiguity in this respect. If space (vacuum) is a player in an emergent force (gravitation), it stands to reason that space can be polarized/densified/rarified by the presence or absence of embedded matter and the conditions of the space can vary with time. That would allow the development of a dynamic theory of quantum gravity.

    I will still abstain from voting this time, since I have insufficient time to evaluate the candidate papers. It was a bit more cut and dried in a previous poll.

    Cheers.
     
  9. Apr 1, 2010 #8

    Fra

    User Avatar

    Not that my opinion makes a difference but I think that the emergence of space in the "entropic" sense that has become much discussed lately, due to also verlinde an others should probably not be best thought of in this mechanistic sense of an objective microstructure (such as objective atomic structures of spacetime).

    I think the sensible way is to think of the microscopic complexions where space does emerge is better thought of as beeing constrained by the complexity of the observer. Thus I doubt it makes sense to have emergent space from a statistical entropic scheme where there is an objective timeless microstructure and an objective measure of entropy.

    The difference then is that an effective locally objective microstructure of space is then also emergent due to interactions tuning the observers by evolution.

    I think if we look at "comoslogical time", time and space are not on the same footing, they can't be. But if we look at "clock time" as judged from "test clocks" in various subsystems of our environment, those clocks should be more on the same footing with space. Conceptually this doesn't need to be a conflict.

    This is also interesting as it related to smolins ideas of when he talked about the "reality of time" vs "evolution of law". The question is, where judging from smolins own reasoning my guess is that himeself is not sure about this, wether you need an objective (ie. observer invariant cosmological time) in order to make sense out of evolving law? In some of his phrasings I think smolin thinks the answer is yes, but I'm not sure. Anyway, I do not think it's necessary. So I think there is a way to combine evolution of law, with putting time and space on equal footing, since every actual assessment of timeless law, is process that takes place in a finite history, so has been the case every case in the past, and so will be in the future. This one can only speak of timeless law, to the extend that it's decidable. At some point, one simply can't decided wether tha law evolves, or our information of initial conditions was wrong and the laws was different. This is the point where we are forced to treat information about laws and information about intial conditions on equal footing.

    So I think the "toymodel" here should take the form where the complexion structure which is to encode space, can be thought of as a "memory structure" of the observer. When these observers interact, then due to their actions upon each other, an evolution in the population could occurs which collectively allows an coherent emergence of space. The structure emergent in all the distributed memory structures would all encode the same structure in a holographic sense. This is how I'm convinced it must be done, but to my knowledge it's not done yet.

    Edit: I this sense I think it's probably confusing or wrong to think of the structure of spacetime as something actually "sitting in space" ie. like atoms that you would see by zooming into the void large enough, where the microstructure of space might instead sit at the boundary, encoded in matter(observers), and when you do zoom into empty space, the "picture" actually emerges on the boundary where the information is collected and encoded..

    /Fredrik
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Padmanabhan and spacetime microscopic degrees of freedom
Loading...