Everybody sees the same elephant (says Carlo Rovelli)

[marcus]

It is clear that everybody sees the same elephant. More precisely: everybody hears everybody else stating that they see the same elephant he sees. This, after all, is the best definition of objectivity.

Page 6 of this paper:
Relational EPR
Matteo Smerlak, Carlo Rovelli
7 pages
"We argue that EPR-type correlations do not entail any form of "non-locality", when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."
http://arxiv.org/abs/quant-ph/0604064

 

[marcus]

I think it could be beneficial to try and understand what this paper is saying. I will look for some more quotes. Here is one:

DEFINITION OF LOCALITY (a principle---a feature that physical theories can have or not have)

"2.3. Locality. We call locality the principle demanding that two spatially separated objects cannot have instantaneous mutual influence. We will argue that this is not contradicted by EPR-type correlations, if we take the relational perspective on quantum mechanics. In fact, locality is at the roots of the observation that different observers do not describe the same reality. As emphasized by Einstein, it is locality that makes possible the individuation of physical systems, including those we call observers 7 ... "

 

[marcus]

...Bohr’s epistemological position, as presented for instance in [26]:

"It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

[26] Petersen, A.: The philosophy of Niels Bohr, Bulletin of the Atomic Scientist 19, n 7, 8-14

I guess I would paraphrase it by saying that nature is not a collection of THINGS, but a collection of FACTS.

Do you not think this is an adequate paraphrase? Then please give your own. The article also quotes the famous saying of Wittgenstein from the Tractatus. See footnote 8 on page 3:

"8. We can take this observation as an echo in fundamental physics of the celebrated: “7. Whereof one cannot speak, thereof one must be silent” [25]. "

 

[marcus]

Fascinating stuff. Here is what he has to say about Einstein's supernatural realism (realism carried to what I judge to be an impractical or unrealistic extreme):

"What is missing in Einstein’s quotation above is the distinction between “elements of physical reality” (quantum events) relative to A and “elements of physical reality” relative to B. Observer A can of course measure the state of B (or, for that matter, beta), but only when A is back into causal contact with B [14]. This is, needless to say, in the future light-cone of A, and therefore poses no challenge for locality. In other words, Einstein’s reasoning requires the existence of a hypothetical super-observer that can instantaneously measure the state of A and B. It is the hypothetical existence of such a nonlocal super-being, and not QM, that violates locality."Actually I do not believe in such a superobserver super-being. And so, for me, there is no one official set of facts.

I think Rovelli is saying that each observer is a quantum animal like everything else (there are no classical systems, or classical clocks, or classical observers) and his state Phi in his Hilbert space of states represents all he has learned about the world so far---all the facts which are fruits of his experience.

Perhaps my natural tendencies---to be skeptical of self-appointed authorities who tear down everybody and everything that is not them----to be suspicious of Official dogma----perhaps my own nature prepares me to find Rovelli's message acceptable.

But so far I have not made up my mind about this by Rovelli et al---except that it is a very readable and charming short paper.

=======================

Another thing is, it reminds me of Padmanabhan's recent paper (the one from the Paris Einstein Centennial). Because for Padmanabhan EACH OBSERVER HAS A BOUNDARY. to describe all the observers you describe all the boundaries. there is no one superbeing superobserver who instantaneously can observe all the other ones. all observers are morally equal.

 

[marcus]

Found a beautiful article in Rovelli's citations

http://arxiv.org/abs/quant-ph/0310010
Einstein, Podolsky, Rosen, and Shannon
Asher Peres

"The EPR paradox (1935) is reexamined in the light of Shannon's information theory (1948). The EPR argument did not take into account that the observers' information was localized, like any other physical object."

 

[Kea]

Hee, hee. The Elephant is a book on Category Theory by Johnstone.

 

[marcus]

Hee, hee. The Elephant is a book on Category Theory by Johnstone.

Neat title. Is it a good book about Category Theory, and entertaining?Hmmm it seems the real title is "Sketches of an Elephant" and the Wikipedia article on Topos has this to say:

Peter T. Johnstone: Sketches of an Elephant: A Topos Theory Compendium, Oxford Science Publications, Oxford, 2002. Johnstone’s overwhelming compendium. As of early 2006, two of the scheduled three volumes were available.

http://en.wikipedia.org/wiki/Topos

 

[Kea]

Johnstone’s overwhelming compendium...

Yes, well, elephants are rather large.

And by the way, Johnstone is using the word in just the same way as Rovelli.

 

[Thomas Larsson]

Rovelli is undoubtedly one of the clearest thinkers around. I especially like this paper, presumably because it is philosophically very close to my own position. Five years ago, I tried (presumably unsuccessfully) in http://www.arxiv.org/abs/hep-th/0110166 to express the need for observer dependence as follows:

"The resolution forced upon us by representation theory is as radical as it is simple: do not consider space-like separations at all. After all, a quantum theory should only deal directly with observable quantities, and space-like distances can not be observed; no observer can be in two different places at the same time. To introduce two observers does not help, because the second observer belongs to the system being observed by the first observer. Of course, I do not propose that space-like distances do not exist, only that they are not described explicitly within the formalism. What I do propose is a very strong notion of locality. Not only should all interactions be local in spacetime, but the theory should only deal directly with quantities that are local to the observer, i.e. objects on the observer’s trajectory. A drastic example: a terrestial observer does not observe the sun itself, but only photons and other particles that reach terrestial detectors, including the naked eye."

This is apparently the same idea as "Whereof one cannot speak, thereof one must be silent."

However, whereas Rovelli arrived at this position by thinking deeply about physics, I had a completely different starting point; I needed a physical interpretation of the mathematical structures arising in the multi-dimensional Virasoro algebra, which is the mathematical statement of background independence on the quantum level.

 

[Careful]

**Rovelli is undoubtedly one of the clearest thinkers around. I especially like this paper, presumably because it is philosophically very close to my own position. **

There are many clear thinkers around with equally different points of view. It is not such a big deal to come up with a story which avoids the need for non-local collapses if you allow for distinguished elements obeying different dynamics (see MWI - Rovelli's version is a clear follow up of that story). The entire difficulty of physics is to find a unifying story which (a) gives the correct predictions (b) is maximally economic (c) truly allows for a unified dynamics (or at least for an extension in that direction). Rovelli's interpretation violates (c) and (b) and perhaps also (a) - cfr. cosmological constant, dark matter, pioneer anomaly, etc... . And again, why to think that causality (in the sense cause -> effect) is restrained within the (dynamical) lightcone ? So in that sense Rovelli's clarity is the consequence of a clear historical embedding. For example : 't Hooft is a very sharp and quick thinker but probably less clear for many people.


**
This is apparently the same idea as "Whereof one cannot speak, thereof one must be silent." **

I think you still speak about the sun, moon, etc... and I assume you still give them a definite shape even if you do not receive photons from them - and neither can you speak about different worlds since no-one experienced this (and there is no ``proof´´ that these exist). Anyway, anybody can believe what he/she wants to... my bet is that perfect Bell tests do not exist (but at least avoid the citation of claims which can be turned into any direction).

Cheers,

Careful

 

[marcus]

I guess it should be pointed out that Rovelli's relational QM has nothing to do with the "Many Worlds Interpretation" and is not derived from it.Traditional QM allows for several observers, as many as one pleases. According to my reading of the paper this thread is about----"Relational EPR"------Relational QM also allows for many observers. But:

1. THEY ARE REAL PHYSICAL OBSERVERS (part of quantum nature) and
2. THERE IS ONE FEWER OBSERVER than in the traditional QM picture.

To rephrase these differences, which I would say make RQM at once more realistic and more economical.

1. in RQM the observers are not classical.

2. in RQM there is no omniscient super-observer making measurments and statements about the circumstances of all the others observers

==============

in RQM two observers can only compare notes if they are causally connected----that is, if one of them is in the lightcone of the other.

One dispenses with the wish to have supernatural or metaphysical facts which transcend the communication between ordinary observers. Objective reality becomes what these ordinary rank-and-file observers can agree on.

RQM, I would say, is a SIMPLIFICATION achieved by what James Hartle calls throwing out excess baggage.
http://arxiv.org/abs/gr-qc/0508001
One thought that one needed a supreme classical observer to watch all the others, but then one finds one does not----one can keep track of the world without using that unnecessary baggage.

 

[Thomas Larsson]

My point was that the multi-dimensional Virasoro algebra seems to know about Rovelli's relational QM. I find that quite remarkable, and some sort of triumph for both of us.

If anything, my position is more extreme than Rovelli's. The problem with quantum gravity is that QM, or rather QFT, is not completely quantum - there is an residual classical element which causes problems. Obviously, classical physics rears its ugly head in the Copenhagen interpretation - the observer is placed on the classical side of the Heisenberg cut. More seriously, however, is that an assumption about a classical observer sneaks into the formalism of QM itself.

This is most easily seen in the Hamiltonian formulation. Here one starts by foliating spacetime into fixed-time slices. But since time is defined by the observer's clock, this step implicitly assumes the existence of a macroscopic, classical observer. One can of course pick another foliation, which corresponds to a different choice of observer, but once time has been defined, it remains the same independently of what happens.

This is an unphysical assumption. In order to observe a system, the observer must interact with it. This interaction will transfer momentum to the observer, making her undergo a Lorentz transformation, and change the definition of time. Thus, the act of observation changes the foliation. Only if the observer is macroscopic, and thus classical, can we ignore this effect.

This is not so serious if we ignore gravity, since detectors are usually much bigger than the quantum phenomena we want to observe. However, a macroscopic observer has infinite mass. Hence the assumption about an a priori foliation secretely introduces an infinitely massive observer into the universe. Since gravity interacts with this infinite mass, this assumption will most likely wreck havoc in a quantum theory of gravity, in agreement with experience.

One may expect to recover ordinary QFT from observer-dependent QFT in the limit that the observer's mass goes to infinity, in the same sense that one recovers Newtonian mechanics from QM when hbar -> zero and from SR when c -> infinity.

 

[Careful]

**I guess it should be pointed out that Rovelli's relational QM has nothing to do with the "Many Worlds Interpretation" and is not derived from it.**

Calm down Marcus (f-h referred to MWI too in the context of Rovelli's LQG). In MWI, the observers are also QUANTUM (that means, there exist at least an aleph_0 number of copies of them) and there is no superobserver (God, outside of the universe who watches it all) - no foliation and all that. What I mean by uneconomical is that observers are experienced as classical in our world - so why having an infinite number of copies of them and what makes that we observe only one possibility out of an infinity of them?

Cheers,

Careful

 

[marcus]

I guess it should be pointed out that Rovelli's relational QM has nothing to do with the "Many Worlds Interpretation" and is not derived from it.
...

didn't mean to sound uncalm! I just thought the above should be pointed out.
No one has established a connection AFAIK. Anyone who wants to establish a connection should spell it out. Give links and quotes.

I don't think it is actually possible to do this, though. One would come up with only at most some superficial resemblances that don't survive scrutiny.

If F-H actually thinks that RQM is a "follow-up" of MWI, then let him speak for himself. He can tell me this himself and explain in what sense he means it.

 

[Careful]

**
If anything, my position is more extreme than Rovelli's. The problem with quantum gravity is that QM, or rather QFT, is not completely quantum - there is an residual classical element which causes problems. **

Not in the path integral formulation (see later).


**
This is most easily seen in the Hamiltonian formulation. Here one starts by foliating spacetime into fixed-time slices. But since time is defined by the observer's clock, this step implicitly assumes the existence of a macroscopic, classical observer. **

I do not see why (the foliation is a problem for quantum covariance in the Hamiltonian formulation true), doing QFT on any background *should* (at least according to our wishes) not be dependent upon the choice of foliation (that is the theories are expected to be unitarily equivalent) - this is not true in case of the thermal state calculated in the context of the Unruh effect of course, but the latter is due to a singular coordinate transformation.

** One can of course pick another foliation, which corresponds to a different choice of observer, but once time has been defined, it remains the same independently of what happens. **

Ok, but physical measurements should be independent of choice of global foliation in background dependent QFT (they indeed depend only upon the local classical clock of the observer).

**
This is an unphysical assumption. In order to observe a system, the observer must interact with it. This interaction will transfer momentum to the observer, making her undergo a Lorentz transformation, and change the definition of time. Thus, the act of observation changes the foliation. Only if the observer is macroscopic, and thus classical, can we ignore this effect. **

You mean the LOCAL foliation I presume.
Ok, but in an entirely classical theory, it would be ``easy´´ to calculate such backreaction effects and even in the context of QFT one could calculate the higher momenta of the momentum transfer and impose - as a first order correction - accordingly a statistical motion upon the observer (this not a local procedure in the strict sense of course but the same would be in the quantum case). More general: within the framework of Hartle and Sorkin, you basically only need an initial hypersurface and wave function to ask any spacetime question concerning any field observable you want to (on a fixed spacetime background) given a certain notion of coarse graining. This does not depend upon any foliation at all, you can treat everything quantum.

**
This is not so serious if we ignore gravity, since detectors are usually much bigger than the quantum phenomena we want to observe. However, a macroscopic observer has infinite mass. Hence the assumption about an a priori foliation secretely introduces an infinitely massive observer into the universe. **

No, it does not, the foliation is entirely kinematical.

**Since gravity interacts with this infinite mass, this assumption will most likely wreck havoc in a quantum theory of gravity, in agreement with experience.**

No, something like CDT has a classical time notion and some observables come out right.

** One may expect to recover ordinary QFT from observer-dependent QFT in the limit that the observer's mass goes to infinity, in the same sense that one recovers Newtonian mechanics from QM when hbar -> zero and from SR when c -> infinity. **

Classical mechanics cannot be retrieved from quantum mechanics (for N particle systems), taking limits can a be subtle and nasty process.

Cheers,

Careful

 

[Careful]

didn't mean to sound uncalm! I just thought the above should be pointed out.
No one has established a connection AFAIK. Anyone who wants to establish a connection should spell it out. Give links and quotes.

I don't think it is actually possible to do this, though. One would come up with only at most some superficial resemblances that don't survive scrutiny.

If F-H actually thinks that RQM is a "follow-up" of MWI, then let him speak for himself. He can tell me this himself and explain in what sense he means it.

The argument in the paper is almost the same as the one of Vanesch with that difference that in the relational intepretation Vanesch's consciousness is replaced by the usual discrete measurement operation. You can find evidence for the latter claim at page 2, paragraph 2.2

Cheers,

Careful

 

[vanesch]

The argument in the paper is almost the same as the one of Vanesch with that difference that in the relational intepretation Vanesch's consciousness is replaced by the usual discrete measurement operation. You can find evidence for the latter claim at page 2, paragraph 2.2

I was hesitating to jump in (I do not often hang around here), but yes, I have to agree with Careful. I skimmed to the paper, and this sounds seriously as just another MWI variant (but from the point of view of one observer where one simply doesn't talk about the copies).

Look at the thread https://www.physicsforums.com/showthread.php?t=114207
for instance.
In MWI there is NOT this "superobserver which sees all spacelike events at once" either, and, within a branch, all observers agree on seeing the same elephant also.

 

[Careful]

I was hesitating to jump in (I do not often hang around here), but yes, I have to agree with Careful. I skimmed to the paper, and this sounds seriously as just another MWI variant (but from the point of view of one observer where one simply doesn't talk about the copies).

Look at the thread https://www.physicsforums.com/showthread.php?t=114207
for instance.
In MWI there is NOT this "superobserver which sees all spacelike events at once" either, and, within a branch, all observers agree on seeing the same elephant also.

For completion, there are two differences:
(a) in Rovelli's story each observer has his own wavefunction on which he/she alone can perform the reduction rule. So your zombies are by definition merely interactions in his framework. Hence, there is no omnium and conscious beings (those who do the reduction) are by definition all in different universes.
(b) consciousness is replaced by good old fashioned reduction.

Actually, I do not feel like speaking about all this, it is just the 100'th variation to the MWI theme (although Bach certainly proved that variations on the same theme can be beautiful )

Cheers,

Careful

 

[hossi]

nonlocal super-being

http://en.wikipedia.org/wiki/The_Little_Prince"

[...]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 grown-ups, and asked them whether the drawing frightened them.
They answered me: "Why should anyone 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 grown-ups could see it clearly. They always need to have things explained. My drawing number 2 looked like this:

The grown-ups' 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. Grown-ups 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 Rovelli-paper? 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.



B.

 

[josh1]

I admit not having any idea what the status on the field is.

Maybe that's because you prefer physics to, well, not physics. Of course, when you're dealing with issues that require a radical conceptual change, unless you have some specific physical intuition about it that doesn't lead to a dead end, the only recourse you'll have is to philosophy. This is the case with Rovelli.

 

[hossi]

Maybe that's because you prefer physics to, well, not physics. Of course, when you're dealing with issues that require a radical conceptual change, unless you have some specific physical intuition about it that doesn't lead to a dead end, the only recourse you'll have is to philosophy. This is the case with Rovelli.

Hi josh,

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.



B.

 

[Careful]

Maybe that's because you prefer physics to, well, not physics. Of course, when you're dealing with issues that require a radical conceptual change, unless you have some specific physical intuition about it that doesn't lead to a dead end, the only recourse you'll have is to philosophy. This is the case with Rovelli.

I fully agree, either you are putting in new physics like 't Hooft, Leggett and Penrose suggest or else you simply add another slightly different interpretation to the rest of them. Personally, I do not see the benefits of the latter...

Cheers,

Careful

 

[hossi]

well, that discussion here is definately a benefit for me

 

[marcus]

Relational QM is not a novel explanation of quantum mechanics

http://plato.stanford.edu/entries/qm-relational/

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/quant-ph/9609002
Relational Quantum Mechanics
Carlo Rovelli
Int. J. of Theor. Phys. 35 (1996) 1637

 

[Thomas Larsson]

Maybe that's because you prefer physics to, well, not physics. Of course, when you're dealing with issues that require a radical conceptual change, unless you have some specific physical intuition about it that doesn't lead to a dead end, the only recourse you'll have is to philosophy. This is the case with Rovelli.

I am not Rovelli, and I disagree with him (and with everybody else) about certain things (gauge anomalies). However, the explicit introduction of an observer makes a hard mathematical difference. This is because it makes it possible to construct diff anomalies, which generalize the Virasoro algebra from one to higher (in particular four) dimensions. To explicitly introduce an observer is necessary, because in all known representations, the relevant cocycles are functionals of the observer's trajectory.

Virasoro-like extensions in N dimensions are encoded in the Lie algebra cohomology group H^2(vect(N), (Z_N-1)^), where vect(N) is the algebra of vector fields in N dimensions and (Z_N-1)^ is dual to the module of closed (N-1)-forms. In particular, when N=1, a closed 0-form is a constant function, so the Virasoro extension is central in 1D, but not otherwise. The number of independent non-trivial 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.

 

[josh1]

I am not Rovelli

This is the part of your post that made sense to me.

 

[marcus]

http://en.wikipedia.org/wiki/The_Little_Prince"
View attachment 6688
... could someone explain me what is new about the Rovelli-paper? 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?

Yes! that is a key point. There is no absolute overseer who can instantaneously report all the observers' results. (Not even in a Gedankenexperiment! )

... the explicit introduction of an observer makes a hard mathematical difference.

By explicit I understand "labeled". Each observer's results are labeled according to who is doing the observing. I think this highlights the observer's importance and allows for different observers to query/ have information about each other as autonomous quantum systems.

"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..."

http://plato.stanford.edu/entries/qm-relational/
Stanford Encyclopedia of Philosophy (July 2005)

 

[Thomas Larsson]

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 multi-dimensional (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 multi-dimensional Virasoro algebra is that in order to construct representations, one must first expand all fields around an operator-valued 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.

 

[selfAdjoint]

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.

 

[marcus]

BTW a young QG student at U. Nottingham has made a long blog post about the Rovelli paper

http://realityconditions.blogspot.com/2006/04/relational-quantum-mechanics.html

The student (or maybe postdoc) name is Alejandro Satz.

Nottingham is a good place, I think it has John Barrett of the Barrett-Crane spinfoam model and also Kiril Krasnov----who 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/2006/04/unfinished-revolution.html

 

[marcus]

It turns out that Alejandro Satz is a second year PhD student at Nottingham.

His blog is called REALITY CONDITIONS
http://realityconditions.blogspot.com/2006/04/relational-quantum-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---

 

[vanesch]

Maybe that's because you prefer physics to, well, not physics. Of course, when you're dealing with issues that require a radical conceptual change, unless you have some specific physical intuition about it that doesn't lead to a dead end, the only recourse you'll have is to philosophy. This is the case with Rovelli.

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...

 

[Careful]

**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 p-jet 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

 

[Careful]

But of course you won't be Napoleon then...

Elba wasn't such a bad place, he only escaped once !

 

[marcus]

http://realityconditions.blogspot.com/2006/04/relational-quantum-mechanics.html
...

continuing what I said in post #30 and 31, Alejandro Satz (in his blog "Reality Conditions") does an outstanding job reviewing Smerlak Rovelli "Relational EPR". Here is a key quote:

-------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---