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Everybody sees the same elephant (says Carlo Rovelli) 
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#19
Apr1206, 03:22 PM

P: 198

The Little Prince~Antoine de SaintExupéry
[...]after some work with a colored pencil I succeeded in making my first drawing. My drawing number 1. It looked like this: I showed my masterpiece to the grownups, and asked them whether the drawing frightened them. They answered me: "Why should any one be frightened by a hat?" My drawing was not a picture of a hat. It was a picture of a boa constrictor digesting an elephant. Then, I drew the inside of the boa constrictor, so that the grownups could see it clearly. They always need to have things explained. My drawing number 2 looked like this: The grownups' response, this time, was to advise me to lay aside my drawings of boa constrictors, whether from the inside or the outside, and devote myself instead to geography, history, arithmetic and grammar. That is why, at the age of six, I gave up what might have been a magnificent career as a painter. I had been disheartened by the failure of my drawing number 1 and my drawing number 2. Grownups never understand anything by themselves, and it is tiresome for children to be always and forever explaining things to them. I admit not having any idea what the status on the field is. So could someone explain me what is new about the Rovellipaper? I kind of like it, it goes along with my believe that there is no paradox, but I don't see how it helps in any other regard. Is the central point that the observers need to actually interact to compare their information? The elephant issue seems to me a rather philosophical one. As scientists, I agree, we should stick to what we can say about nature, but does that really answer the question why we see what we see? You might claim, that's not a good question to ask, but I would like to know nevertheless. Indeed, it's more like we see the prince's hat  or is it an elephant inside a boa? That's the question we can't answer. To be pragmatic, it's a question that we most likely don't need to answer. Can't avoid hoping to finally make some sense out of the quantization. Best, B. 


#20
Apr1206, 03:43 PM

P: n/a




#21
Apr1206, 03:59 PM

P: 198

I am quite flexible with my opinion what physics should or should not be, and I don't mind philosophy. It might be useful in several regards, to discuss the foundational issues of physics, esp. when being stuck at the front of research. I have no idea what Rovelli is aiming at, but it seems to me like an attempt to question what you call 'physical intuition'. Unless you assume that we are born with a natural connection that tells us what the truth about reality is, most of our intuition comes from the education. And so far, it seems to be a pretty dead end  at least I don't want to be stuck with the 'unfinished revolution' for the rest of my scientific life. Best, B. 


#22
Apr1206, 04:12 PM

P: 1,667

Cheers, Careful 


#23
Apr1206, 05:33 PM

P: 198

well, that discussion here is definitly a benefit for me



#24
Apr1206, 10:31 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

Relational QM is not a novel explanation of quantum mechanics
http://plato.stanford.edu/entries/qmrelational/ but more a way of adapting one's ideas of the world and of nature to reflect the lessons learned from ordinary usual QM. Instead of changing QM to fit one's ideas, one adjusts one's ideas to fit QM. the link is to an article in the Stanford Encyclopedia of Philosophy specifically about Relational Quantum Mechanics dated July 2005 that was written by Rovelli and a friend. ( I guess it can be considered authoritative ) the summary begins: "Relational quantum mechanics is an interpretation of quantum theory which discards the notions of absolute state of a system, absolute value of its physical quantities, or absolute event. The theory describes only the way systems affect each other in the course of physical interactions..." For more info, there is the main paper on it (Rovelli 1996) http://arxiv.org/abs/quantph/9609002 Relational Quantum Mechanics Carlo Rovelli Int. J. of Theor. Phys. 35 (1996) 1637 


#25
Apr1306, 04:47 AM

P: 102

Virasorolike extensions in N dimensions are encoded in the Lie algebra cohomology group H^2(vect(N), (Z_N1)^), where vect(N) is the algebra of vector fields in N dimensions and (Z_N1)^ is dual to the module of closed (N1)forms. In particular, when N=1, a closed 0form is a constant function, so the Virasoro extension is central in 1D, but not otherwise. The number of independent nontrivial extensions, dim H^2 = 1 if N = 1, and dim H^2 = 2, N >= 2. This is a mathematical fact, which no philosophy (or lack thereof) in the world can change. 


#26
Apr1306, 08:16 AM

P: n/a




#27
Apr1306, 08:49 AM

Astronomy
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PF Gold
P: 23,124

Stanford Encyclopedia of Philosophy (July 2005) 


#28
Apr1306, 08:55 AM

P: 102

Then let me try again. It is widely appreciated that spacetime diffeomorphisms play a crucial role in GR, although there is some confusion about what the terms general covariance/diffeomorphism invariance/diffeomorphism covariance/background independence really mean. However, this is a question which belongs to philosophy, or perhaps semantics.
What is completely clear, however, is that the multidimensional (especially the 4D) Virasoro algebra is related to spacetime diffeomorphism invariance in exactly the same way as (twice) the ordinary (1D) Virasoro algebra is related to conformal invariance in 2D. Some of us think that it might be a good idea to know about the Virasoro algebra when one studies conformally invariant theories such as string theory. In the same sense, it is a good idea to know about the 4D generalization of the Virasoro algebra if one studies diffeomorphism invariant theories such as GR; it is simply the correct quantum form of the constraint algebra (in covariant formulations). The outstanding lesson from the multidimensional Virasoro algebra is that in order to construct representations, one must first expand all fields around an operatorvalued curve, which is naturally identified as the observer's trajectory in spacetime. Hence background independence on the quantum level forces us to explicitly consider the observer. It is quite remarkable that one arrives from this mathematical starting point to a need for observer dependence, which is very similar to what Rovelli finds on purely physical grounds. 


#29
Apr1306, 11:46 AM

Emeritus
PF Gold
P: 8,147

But TL, if your observer (I remember your making this same argument years ago on spr) has a trajectory, then he isn't a "global observer" but a "local one", no? He may be outside the system constrained by the Virasoro algebra, but he is not like God or Laplace's demon.



#30
Apr1306, 12:15 PM

Astronomy
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PF Gold
P: 23,124

BTW a young QG student at U. Nottingham has made a long blog post about the Rovelli paper
http://realityconditions.blogspot.co...mechanics.html The student (or maybe postdoc) name is Alejandro Satz. Nottingham is a good place, I think it has John Barrett of the BarrettCrane spinfoam model and also Kiril Krasnovwho started the GFT (group field theory) treatment of spinfoam QG along with Laurent Freidel IIRC. I would say it is interesting what Alejandro Satz has to say about Rovelli's paper. ============= Alejandro also gives a link to the QG blog of Christine Dantas called "Background Independence" where he says there is currently opportunity to discuss the Rovelli paper http://christinedantas.blogspot.com/...evolution.html 


#31
Apr1306, 02:16 PM

Astronomy
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PF Gold
P: 23,124

It turns out that Alejandro Satz is a second year PhD student at Nottingham.
His blog is called REALITY CONDITIONS http://realityconditions.blogspot.co...mechanics.html and his explanation of Rovelli's paper seems to me to give a description of Relational QM that more clear and accurate than some I have seen (even if by older commenter's) so I will quote a piece of it: from Reality Conditions blog ...The main idea of the relational interpretation is that a quantum state is not an "absolute" description of a system, but only relative to a given observer, and that a same system may be described at the same time by many different states. For example, in the "Wigner’s Friend" version of the Schroedinger Cat paradox, one observer inside a box makes a measurement of a quantum system and sees a definite result, while for a second observer outside the box the whole system including the first observer is still in an indeterminate "superposition" state. The relational interpretation has a simple description of the situation: the state is collapsed relative to the first observer and superposed relative to the second observer. (In contrast, the better known "many worlds" interpretation would say that the "true" state is the superposed one and that the first observer’s impression is a kind of illusion produced by the "branching" of his consciousness. The relational interpretation is more "democratic"; none of the descriptions is privileged.) A key feature of the relational interpretation is that according to it any quantum system can be called an "observer"; conscious beings have no special status, and any interaction can be a "measurement"... endquote 


#32
Apr1306, 02:40 PM

Emeritus
Sci Advisor
PF Gold
P: 6,236

I'd say that a little philosophy can save you sometimes a lot of useless troubles. In the same way as a little philosophy in social life can make you accept a priori unfairness in life and avoid you to lose your energy in trying to set up a revolution and try to exterminate all sources of unfairness (which is in any case a lost cause and will bring you lots of useless troubles), in the same way a little philosophy in physics can make you accept certain aspects of physics you deem "unfair" and save yourself a lot of intellectual and career trouble (I'm not thinking of anybody in particular here ). But of course you won't be Napoleon then... 


#33
Apr1306, 04:03 PM

P: 1,667

**Then let me try again. It is widely appreciated that spacetime diffeomorphisms play a crucial role in GR, although there is some confusion about what the terms general covariance/diffeomorphism invariance/diffeomorphism covariance/background independence really mean. However, this is a question which belongs to philosophy, or perhaps semantics. **
I do not agree, your ideas about covariance are technically different from the LQG type of quantum covariance and certainly different from string theoretical QFT ideas. Also, they differ from Hartle and Sorkin's approach within the context of decoherence functional quantum mechanics. I must say however that I find your paper ``manifestly covariant canonical quantization I´´ quite interesting and have spent today something like one hour studying it. I have some questions and some silly (technical) remarks  since I looked a bit in the details I shall also give some of the typos. (a) the first remark concerns the computation of the cohomology on page 10  there you say that each function which contains pi is not closed, that is not true, a counterexample is pi*e + (psi*)*K*(pi*), however this one is in the image of the KT derivative. (b) in general, your idea is to quantize first and then impose the dynamics, but are you not running then in similar problems as canonical quantization in the interaction picture for non linear theories? (c) in formula 4.8, the second psi* should be \bar{psi} and similarly in 4.10, it is correct again in 4.18 (d) On page 17, the purpose of your splitting of the Hamiltonians, that is the constraint H_0 and the ``observer´´ H is to define the time derivative relative to the quantum worldline of ``the observer´´ and associated to this, the definition of the Fock vacuum state relative to the worldline and the parameter time t. However, t by itself is just window dressing and should have not any physical significance, this calls the question for reparametrisation invariance of the measured quantities. This issue is adressed in section 8 where you mention that extra matter coupling is necessary to make sense of this (did I get that right?). Now here I am confused in the beginning, since at page 28 you mention that every bosonic pjet bundle contributes 2(N+p,N) to the central charge (and minus for the fermions) while in formula 8.6 I suddenly get entirely different numbers. (e) Also, in the latter construction , one would expect the relative energy to be a measurable quantity only in case the worldline would interact with the matter fields. How would this reflect upon your relative positive energy condition? (sorry, did not really think about this ) Another comment/remark, since *t* is some unphyiscal parameter, it becomes obscure what happens to your equal time commutators, and more in particular to causality itself. Actually, I see no reason why field operators corresponding to causal separations should commute. Could you elaborate more upon the relationship between the standard Fock QFT quantisation and your framework? That's all for now... Cheers, Careful 


#34
Apr1306, 04:07 PM

P: 1,667




#35
Apr1306, 11:36 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

quote from Satz blog I think this is a quite elegant solution to the measurement problem. It does not involve any change in the testable predictions of QM, unlike the models with a physical wavefunction collapse; it does not involve extra physical baggage like hidden variables models do, and it does not involve the extra ontological baggage of the many worlds interpretation. (From the point of view of the relational interpretation, the many worlds interpretation would seem to privilege as the only "true" state one which is not relative to any particular observer; God’s point of view, so to say. I think that the relationist should deny that there is any such state, like a "wavefunction of the universe". This should have implications for quantum cosmology.) Even more attractive, for me at least, is that the interpretation is not instrumentalistic: quantum mechanics is not merely a tool for calculating and predicting but a true description of how the world works; the description must be done from the "point of view" of some physical system, but there is no privileged choice for the reference system (much like the situation with reference frames in special relativity). endquote 


#36
Apr1406, 02:29 AM

P: 1,667

**
I think this is a quite elegant solution to the measurement problem. ** It is not a solution to the measurement problem (remember the born rule is still in there), neither is MWI from the strict reductionist point of view. **It does not involve any change in the testable predictions of QM, unlike the models with a physical wavefunction collapse;** It remains to be seen whether that is a good or a bad thing **it does not involve extra physical baggage like hidden variables models do, and it does not involve the extra ontological baggage of the many worlds interpretation. (From the point of view of the relational interpretation, the many worlds interpretation would seem to privilege as the only "true" state one which is not relative to any particular observer; God’s point of view, so to say.** There is no need for God in MWI, and if you do the counting MWI is actually equally economical as the relative state interpretation (remember, any observer needs his own wavefunction). ** I think that the relationist should deny that there is any such state, like a "wavefunction of the universe". This should have implications for quantum cosmology.) Even more attractive, for me at least, is that the interpretation is not instrumentalistic: quantum mechanics is not merely a tool for calculating and predicting but a true description of how the world works; the description must be done from the "point of view" of some physical system, but there is no privileged choice for the reference system (much like the situation with reference frames in special relativity).** It is still instrumentalistic wrt to the observer though. Cheers, Careful 


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