selfAdjoint said:
Check on the Beyond the Standard Model board, the thread "Everyone sees the same Elephant (or so says Rovelli)" It is about Relational QM and Vanesch has contributed to it. He is not a believer in it, but I am attracted to it for the same reasons you cite. The thread was triggered by a paper by Matteo Smerlak and Carlo Rovelli (one of the big names in Quantum Gravity research), entitled Relational EPR, quant-ph/0604064 on the arxiv.
Well, I think there have been misunderstandings about my claims in that thread: I DO like (a part of) RQM. What I don't like is some wordings about its possibilities. The way I understand RQM is as the "solipsist" or "single" observer version of MWI. By this, I don't mean that RQM cannot handle multiple observers, but RQM assigns a different "reality" to each observer (just as MWI does). The only difference I really see is that RQM denies the existence of an objective reality, while MWI does give that (although it sounds crazy).
However, in the articles discussed in the thread SA mentionned, it is my impression that there is a big verbal exercise to burry the fact that there's a denial of objective reality (nevertheless, that's about the contents of "hypothesis 2" in one of the two papers).
So, my idea is that RQM is fine from a single observer PoV, where the "subjective" reality of an observer is described (although this is called an "objective observer-dependent reality" or something of the kind).
What's real for A has "no element of reality" for B.
Now, this is exactly also the PoV of MWI. What MWI adds, is also an objective reality (in casu the overall wavefunction), which then explain the different subjective realities of different observers.
So, I'm not against RQM "as a first mental exercise in MWI" say :-)
However, RQM as a "complete vision" hurts deeply my Platonic concept of the world, with a denial of the existence of an objective reality.
Nevertheless, if it were clearly stated that THIS is the fundamental idea (there's no objective reality, only one's subjective reality counts), I wouldn't mind. But the word "objective" is sprawled all over the place in order to hide this subjective character - and this, I find regrettable, because it lures in people who might at first think that this denial isn't so deep, but that it is only a matter of "switching coordinate systems" or something.
The difference is a bit as in relativity. In relativity, there is an "objective reality" which is described by the spacetime manifold (or the equivalence class of isomorphic manifolds for nitpickers) and there are of course the different coordinate representations (say, observers) which assign different numbers to each event.
Now, of course a coordinate description is dependent on an observer, but things like the statement "the explosion of the firecracker took place at coordinates (x,y,z,t) for observer O1" is an objectively true statement, which can be acknowledged by all observers. That is, observer O2 will have of course (X,Y,Z,T) as coordinates for that firecracker, but, knowing the coordinate transformation to O1, he'll be able to conceive that O1 has coordinates (x,y,z,t) for the event. So observer O2 has no difficulties taking the statement that O1 has coordinates (x,y,z,t) for the event as an objectively true (or a false) statement.
This is slightly as in MWI: there's a description of an objective reality (in casu the wavefunction, or better yet, the unitary structure), and then there are the different states corresponding to different observers.
There too, one can say that for observer state O1, who observed a pink elephant, and observer state P1, entangled with observer state O1, BOTH can be in agreement that observer state O1 saw a pink elephant.
They can also agree that observer state O2 has seen a blue elephant, although they'll never interact with it. But the objectively true statement that O1 observed a pink elephant is clear for everybody.
In RQM, however, things are different. Here, in O1's reality, O1 DID see a pink elephant, but in P1's reality, O1 DIDN'T SEE a pink elephant until P1 interacted with O1. P1's description of O1 is still that O1 is in a superposition of having seen a pink or a blue elephant, while this is NOT the case for O1 itself. This is not only because P1 has no information yet: P1 cannot assume that O1 did see one OR did see the other - he still needs to entertain O1 in a quantum superposition, while O1 did clearly see one of both. So the truth value of the statement "O1 saw a pink elephant" is dependent on who's saying so. There is no "objective truth value" to be assigned, EVEN to statements regarding the relationship between an observer and a system.
This is necessary in RQM in order to be able to deny Bell's theorem: O1 "didn't have a result yet" from P1's PoV (while from O1's PoV, O1 DID have a result of course).
This can only be the case if we're talking about two different realities: the one related to O1 and the one related to P1. "Realities" in which IDENTICAL STATEMENTS have different truth values.
So RQM is a web of subjective realities with no objective reality to link them, while MWI is an objective reality from which subjective realities can be deduced.
And RQM needs to cheat a bit to make the different subjective realities coincide (see my post about that in the other thread).
In a nutshell, they regard interacting quantum systems as real, and the basic makeup of reality, and discard the classical observer, which seems to be the block in every quantum physicist's mind. For the authors, "observation" is just a specialized task for a prepared quantum system, just another interaction. Eigenvalues are not objective reality but only relative to the other interacting system, and they work out how all of this comes out right (that is, quantumly) in the EPR case.
This is exactly what can be said about MWI in fact. Only, in RQM, the term "reality" has become observer-dependent, and hence subjective, with a denial of the existence of an objective, observer-independent reality (or at least, the possibility to describe it logically).
So, RQM as the "single observer/solipsist" viewpoint of MWI, I'm all in favor. RQM as a complete vision, I think it is even more bizarre than MWI.
In other words, I regret a bit the verbal marketing around RQM: there's no miracle, and it is a bit sold as if there were.
