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
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 ... "
...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]. "
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.
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."
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
Yes, well, elephants are rather large. And by the way, Johnstone is using the word in just the same way as Rovelli.
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.
**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) truely 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 recieve 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
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.
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.
**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
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.
** 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
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
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
nonlocal super-being The Little Prince~Antoine de Saint-Exupéry [...]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 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 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. Best, B.
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.