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

Paper: Rovelli's What is observable in classical and quantum gravity?

  1. Mar 9, 2008 #1

    Fra

    User Avatar

    Paper: Rovelli's "What is observable in classical and quantum gravity?"

    I'm reading Rovelli's QG book and found a reference to his own article: "What is observable in classical and quantum gravity?"

    I fail to find it on arxiv - anyone know if this is available or do you have to buy it from some download service?

    It's not that I expect this papers to contain the ultimate answer, but while trying to understand rovelli's logic I think this paper would be relevant. Because what's observable and what's not really is head on the problem IMO. I can't figure out why he needs to quote his own papers in a 445p book?

    /Fredrik
     
  2. jcsd
  3. Mar 9, 2008 #2
  4. Mar 9, 2008 #3

    Fra

    User Avatar

    I suspect that perhaps that paper don't contain anything radically different after all, judging from the abstract. If it was essential to his logic I figure it should have included it in the book to start with.

    Thanks.

    /Fredrik
     
  5. Mar 10, 2008 #4

    f-h

    User Avatar

  6. Mar 11, 2008 #5

    Fra

    User Avatar

    Hey, thanks alot f-h!!! :tongue2:

    I'll check that paper out and see what is says.

    I much appreciate your help!

    /Fredrik
     
  7. Mar 11, 2008 #6

    Fra

    User Avatar

    f-h, what search technique did you use to find that paper? :) I failed to find it on some plain googlings and now I skimmed that paper quickly it contains further refences to another Rovelli paper from 1991 "Quantum reference systems" which seems to contains the I assume more interesting, and the conclusive part of the quest that was initiated in this paper.

    /Fredrik
     
  8. Mar 11, 2008 #7

    f-h

    User Avatar

  9. Mar 12, 2008 #8

    Fra

    User Avatar

    Thanks alot for that search link! I'll save that for the future.

    /Fredrik
     
  10. Mar 12, 2008 #9

    Fra

    User Avatar

    Spires is en excellent search tool.

    If anyone else is interested here are the direct links to three entangled Rovelli papers.

    "What is observable in classical and quantim gravity?", Rovelli
    -- http://ccdb4fs.kek.jp/cgi-bin/img_index?200034022

    "Quantum reference systems", Rovelli
    -- http://ccdb4fs.kek.jp/cgi-bin/img_index?200034021

    "Quantum mechanics without time: a model", Rovelli
    -- http://ccdb4fs.kek.jp/cgi-bin/img_index?200034015

    The first paper is mostly the classical part, and it's not much to say about that. The other two I just skimmed and I'll have to read it carefully. My first impression is that while doing away with an external reference, he replaces it with another one which is more subtle.

    It seems to be that desire to maintain some observer invariance, begs for an abstract observer invariant reference after all. I'm not sure i like this. I need to read it carefully. There assumption that there exists an observer invariant structure, is the "external reference" I see. And it takes the form of an assumption. This effectively works as a rule of reasoning when constructing the theory. This is strange. It seems like the standard birds view -> frog view issue. The hypotetical birds view is the external reference that bothers me.

    But I'm not sure if this is really what he tries to say in the end... anyway, it was nice to get the papers.

    /Fredirk
     
  11. Mar 12, 2008 #10

    Fra

    User Avatar

    To me it's the similar type of construction that's used in standard QM. The observerinvariant hilbert space structure is the ad hoc reference. Unitarity is not a result of measurements - it's somehow an external assumption.

    But i assume he treats this in the last paper. I'll try to read the last two papers mroe the upcoming days.

    If anyone else has any short opinions on this meanwhile I read the rest it would be interesting to hear your preferred reasoning on this.

    /Fredirk
     
  12. Mar 12, 2008 #11

    Fra

    User Avatar

    In the conclusion of that last paper he writes

    "First, we have made use of quantum mechanics and it's standard (Copenhagen) probabilistic interpretation. We did not question this theory. We do not see any reason for the problem of the absence of an exact time in the fundamental theory to be related to the old difficulties of quantum mechanics. Perhaps it is related, but we believe, with Heisenberg, that a good physicist has to be conservative, and change the rules only if force by experiment. The absecence of an hamiltonian in gravitational physics forces use to consider quantum systems without a well defined time; nothing, up to now, suggests that quantum mechanics itself is wrong"
    -- Rovelli, Quantum mechanics without time: a model, 1990

    I like alot of his writings! but I can't help disliking this statement. There is still too much structure that is not attached to observable processes. It's like the theorist himself considers himeself to be outside the world. I like to compare my own understand and mankinds science, with a ant trying to chart up a manifold, while trying to figure out where to actually store the chart information when he realises that he can not make use of a database that exists in another universe.

    I guess I must me missing something.

    /Fredrik
     
  13. Mar 12, 2008 #12

    f-h

    User Avatar

    Or maybe you're just disagreeing with CR on this particular point. :) Have a look at his relational QM papers to find out!
     
  14. Mar 13, 2008 #13

    Fra

    User Avatar

    Maybe. I've read some of it already, but I've swapped back and forth between papers and chapters in the book. What has bugged me is that I find some of what he says to be really nice, and elsewhere I don't seem to agree. It's that fact that some things he says make sense, and some don't, that is interesting. The confusion is not on standard GR or standard QM, it's how he argues they connect the two views. I have a feeling that his view of QM is close, but that his relational view, still sort of maintains a hidden obective reference.

    The other thing that confues me is that he has repeatadly in various places said that he does not address the fundamental issues of QM itself. But the way I personally prefer to think of it, an analysis of that leads to things that really does give rise to things that seem close enough to gravity that I don't think it's not connected. I had some preliminary hunches that there may exists an alternative interpretation of rovelli's spin networks that may prove to be the formalism I'm looking for. so I'm going through this to learn if that is so or not. I'm still undecided.

    /Fredrik
     
  15. Mar 13, 2008 #14

    Fra

    User Avatar

    I have a feeling that he tries to find the right angle in classical GR, where he can sort of just apply standard QM. But I'm not sure if that's unfair. Maybe I haven't understood his view yet.

    What I try to do, is to improve QM in the way I subjective think it should be in the first place :shy: and use the apparent "problems" appearing when doing so, to introduce gravity from a new first principle information view.

    So I guess my view is that gravity and inertia is a "phenomenon" that appear spontaneously when trying to analyse the measurement process, taking in one step further and not just consider the measurement process, but also what happens with the information obtain from the measurement, how that remodels the platform where the measurement was fired from.

    I was hoping that the observer microstructure that I like to think in terms of, could possibly be identified with a spinnetwork. But it seems this is not quite what rovelli does and it might not allow such interpretation without revision.

    /Fredrik
     
  16. Mar 13, 2008 #15

    f-h

    User Avatar

    "I have a feeling that he tries to find the right angle in classical GR, where he can sort of just apply standard QM."

    I think that's a reasonable one sentence summary. This philosophy is very unpopular today: Try to be as unradical as possible, don't change anything until it's proven (not necessarily rigorously of course) that it needs changing.

    Personal philosophical unease, or the fact that changing this aspect leads to a cool mathematical structure do not count as good reasons. ;)
     
  17. Mar 13, 2008 #16

    Fra

    User Avatar

    I'm glad I'm not the only one to have that impression.

    /Fredrik
     
  18. Mar 13, 2008 #17

    Fra

    User Avatar

    To try the most plausible thing first. And place your bets according to your odds, is just common sense I think. It's a conservative approach in the sense of maximum gain/risk.

    IMO the problem is that the odds are subjective. All due respect to everyone who has dedicated their lives to developing our current theories, I have no sensible choice but to use my own subjective judgement, which is of course influenced also by the collective judgement by menas of feedback. I'm sure those who was along for FORM what is today part of the collective view, indeed also used their own subjective judgement for guidance.

    /Fredrik
     
  19. Mar 13, 2008 #18

    f-h

    User Avatar

    I think he has said so himself on occassion. According to CR there is no indication so far that we need to modify QM to write down a theory of QG.
     
  20. Mar 14, 2008 #19

    Fra

    User Avatar

    Then it seems at least relative to my current incomplete understanding that I disagree. Of course I don't know if it's needed, but to me it seems more likely that a revision is needed, than the opposite.

    I was able to start skim his paper "Relational Quantum Mechanics" last night but fell asleep. I need to continue this weekend.
    -- http://xxx.tau.ac.il/abs/quant-ph/9609002v2

    But I conclude that in the beginning it looks excellent.

    Rovelli writes:

    "I consider a reformulation of quantum mechanics in terms of information theory"

    "The notion rejected here is the notion of absolute, or observer-indepdent state of the syste; equivalently, the notion of observer-independent values of physical quantities"

    So far I'm right with him!! I like his subjective bayesian spirit! Thumbs up! :approve:

    But then he mentions shannon entropy as the measure of information, and that makes me react. But then I fell asleep and remember nothing more :zzz:

    He is very clear in his writing so it should not be hard to find out if he really uses the shannon measure. My fear is that he is trying to use an absolute measure of information, to hide his relational thinking behind another objective - but unobservable - structure.

    IMO, the shannon entropy is probably not the ultimate measure of information, it's not relational. To me the measure of information usually is closely related to the concept of probability, or the microstructures. One can use absolute measures to defined differential measures, but the issues is then that the choice of the absolute measure is arbitrary.

    I hope to find out if I'm wrong when I get time to resumereading..

    /Fredrik
     
  21. Mar 14, 2008 #20

    Fra

    User Avatar

    Probability

    I continued reading his "Relational Quantum mechanics" paper and I might as well comment on it here, to see if someone would disagree.

    "First of all, one may ask what is the “actual”, “absolute” relation between the description of the world relative to O and the one relative to P. This is a question debated in the context of “perspectival” interpretations of quantum mechanics. I think that the question is ill-posed. The absolute state of affairs of the world is a meaningless notion; asking about the absolute relation between two descriptions is precisely asking about such an absolute state of affairs of the world. Therefore there is no meaning in the “absolute” relation between the views of different observers. In particular, there is no way of deducing the view of one from the view of the other.

    Does this mean that there is no relation whatsoever between views of different observers? Certainly not..."
    -- Rovelli, Relational Quantum Mechanics


    Sounds great so far!

    "There is an important physical reason behind this fact: It is possible to compare different views, but the process of comparison is always a physical interaction"
    -- Rovelli, Relational Quantum Mechanics


    Also Excellent IMO.


    "Suppose a physical quantity q has value with respect to you, as well as with respect to me. Can we compare these values? Yes we can, by communicating among us."
    -- Rovelli, Relational Quantum Mechanics


    Again, I'm all in with this! Extremely sound reasoning.

    "But communication is a physical interaction and therefore is quantum mechanical. In particular, it is intrinsically probabilistic."
    -- Rovelli, Relational Quantum Mechanics


    Here I start to get a little bit reserved. He is referring to "quantum mechanical interactions", and the notion of "probability", at the same time he in various places avoids discussing the physical meaning of probability. And I wonder if "quantum mechanical interactions" implies some constraining ideas of what an interaction is. I'll put a marker here and keep reading.

    Now he pulls out the quantum measurement formalism and here his previously plausible reasoning looses me,

    An observer P(observer2), observes S(System)-O(observer1).

    "but a quantum property of the quantum S-O system, that can be investigated by P, and whose yes/no answers are, in general, determined only probabilistically."
    -- Rovelli, Relational Quantum Mechanics


    Again, he uses the probabilistic concept. Where is the physical meaning with probability?
    Is the probability itself "observerd"? If so how? If not, then what is it doing in here and what the line of reasoning that brings in that concept?

    My issues is not with "only probabilistically", because a deterministic probability is not just "only" to me. It's a strong statement.

    It'd say it's used here as a choice of rule of reasoning more than anything physical. This choice comes out as formally arbitrary to me. I think there must be another way to continue that initiated spirit of Rovelli that I agree with up until this point.

    He seems to make a semi-synthesis here, but still in the middle apply the same old QM (more or less).

    Here, I personally feel that Rovelli's plausible and excellent reasoning appears somewhat broken?

    What do you think, I am missing something? or maybe this is to be seen as the best we can do "so far", and Rovelli himself is still looking for improvements? In that case it makes more sense. Maybe I'm asking too much of this.

    I personally do not feel confident in building further onto this platform. I'd prefer to step back to the point where is excellent reasoning seems to get fuzzy, and to me the unclear point is the physical meaning of probability. This is also a common issue for choosing information measures.

    My opinion is that there are at least two issue with QM.

    First is the objectivity issue. And here I think rovelli's idea of relative information is right on, in the sense of subjective bayesian probability. I also share his view on emergent objectivity. I think it's plain brilliant.

    But there is another issues that I at least can't see that he deals with. All of his reasoning is built on the probability concept. Wether bayesian or non-bayesian, the question of it's physical meaning still persists. Here the issues of information capacity also becomes the factors. I have a feeling that he refers to some information concept, which again if we are talking about shannon is more or less an arbitrary measure, still based on an underlying probability measure.

    I am not sure I got his spirit with the supposedly plausible information postulates he made, I need to read them again, but my spontaneous reaction was doubt.

    So to the point of choosing observables, in this context, what is the status of probability?

    I'll keep reading later...

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

Have something to add?



Similar Discussions: Paper: Rovelli's What is observable in classical and quantum gravity?
Loading...