Everybody sees the same elephant (says Carlo Rovelli)

[vanesch]

Of course if you make the superimposed states (mathematically represented by vectors in a Hilbert space) "real", then you certainly have no problem with reportablility. But don't you run into the basis problem? If we restrict ourselves to the spectra of the interaction operators it seems we avoid that, no?

Yes, sure, because it is now put in by hand in RQM. The "answers to questions" now specify (as in von Neumann/Copenhagen) *intuitively* what are the pointer states, or the basis states in which we should "project" to obtain the probabilities for answers. Nothing wrong with that, but then one shouldn't point this out as a *problem* for another approach, if you need to go to irreducibly intuitive concepts yourself to answer the same question.

That said, I'd say - and I'm not 100% clear about this myself - the hope of an MWI/decoherence approach is that there is some natural solution to the basis problem when we split the universe in observer/restofuniverse.
If that doesn't work, we can still put it in by hand (by defining what exactly are the "states of the observer that are to be associated with definite awareness") - just as does RQM when positing the basis states which give the "answers to questions", or von Neumann/Copenhagen which specify the Hermitian operator corresponding to a "measurement apparatus" entirely intuitively. MWI would not be worse off this way than the alternatives (which do the same, without any hope for reducing this intuitive input), but surely would be uglier than when there would be a natural appearance of "basis" - as decoherence does seem to suggest, but I'm not yet clear about how much of this is "marketing" and how much is hard results from decoherence. I thought the situation was rather promising, but I'm not sure.

 

[selfAdjoint]

Yes, sure, because it is now put in by hand in RQM. The "answers to questions" now specify (as in von Neumann/Copenhagen) *intuitively* what are the pointer states, or the basis states in which we should "project" to obtain the probabilities for answers. Nothing wrong with that, but then one shouldn't point this out as a *problem* for another approach, if you need to go to irreducibly intuitive concepts yourself to answer the same question.

That said, I'd say - and I'm not 100% clear about this myself - the hope of an MWI/decoherence approach is that there is some natural solution to the basis problem when we split the universe in observer/restofuniverse.
If that doesn't work, we can still put it in by hand (by defining what exactly are the "states of the observer that are to be associated with definite awareness") - just as does RQM when positing the basis states which give the "answers to questions", or von Neumann/Copenhagen which specify the Hermitian operator corresponding to a "measurement apparatus" entirely intuitively. MWI would not be worse off this way than the alternatives (which do the same, without any hope for reducing this intuitive input), but surely would be uglier than when there would be a natural appearance of "basis" - as decoherence does seem to suggest, but I'm not yet clear about how much of this is "marketing" and how much is hard results from decoherence. I thought the situation was rather promising, but I'm not sure.

Well I take (I hope not too quickly) as a major concession from you that RQM is now seen as a contender with MWI, sharing some of the same kind of problems, rather than as simply wrong, which I took to be your earlier position.

The key datum we need to clear everything up, the true nature of the amplitudes, remains beyond us, and we wind up putting some feature or other of their phenomena "in by hand". The differences are all in which feature we pick and which we try to get rid of. For you and MWI it's projection; you want the wavefunction to be real and evolve unitarily. For RQM, projection is OK because they take the information view of the amplitudes (with all the accepted problems of doing that, which they try to finesse), but they try to eliminate nonlocality.

 

[vanesch]

Well I take (I hope not too quickly) as a major concession from you that RQM is now seen as a contender with MWI, sharing some of the same kind of problems, rather than as simply wrong, which I took to be your earlier position.

If you go back, you'll see that I never claimed RQM to be wrong, I claimed from the start it to be the "single-observer" version of MWI, in the sense that everything done in RQM is what a single observer in MWI would agree upon being "in his branch".
So I saw from the start RQM as the "what does a single observer see" view on MWI. As such, it is left in the middle what could be "ontologically" true, and the total wavefunction is simply not "something RQM wants to talk about". Not because it has, or it hasn't, any meaning, but because we decided to limit ourselves to what is seen by a single observer (and hence describe this single, subjective world of that observer without caring about any objective reality or not).

As such, this is interesting, but doesn't address several issues MWI tries to address. Indeed, RQM has already introduced a "preferred basis" (namely the one of the vector with the yes/no answers), and has already postulated a probability rule. Moreover, RQM does NOT try to give any description of an ontological reality, it only describes a subjective observer world.

I only protested against the wordings which try to introduce "objective reality as agreement between observers" and at the same time "the rejection of the objective existence of an ontology" and all that verbal game, which vehicle the impression that RQM is onto something new, which might *solve* longstanding issues. It doesn't. If you take on the PoV that RQM is a "new way of seeing things" without giving an ontology to things like "the state of an observer" and so on, you create more troubles than you solve, but they are put under the carpet by a subtle redefinition of concepts like objective and subjective, and by dancing around with the concept of the state of a system. So I protested against THIS view on things.

RQM is perfectly fine as the description of the subjective world of an observer - including the appearance, in this subjective world, of other observers and their observed (within this subjective world) agreements.

I tried to outline that by puzzling all these different subjective views together, we can arrive at an ontological, objective description ; which is exactly the one given by MWI (although, there, the path is taken in the opposite direction: we start from an objective system, and we try to deduce (many) subjective worlds from it).
All the statements made in RQM make perfectly sense from this PoV (but are in fact tacitly already assumed).

But the real problem, namely, the preference of one over several branches, (which is refuted in MWI, hence the "many", and which need projection elsewhere, or needs a classical token, as in Bohm), is not more addressed by RQM as it is by Copenhagen or by MWI (which tries to live with it).
The (correct) point RQM makes is that you are locked up in your own subjective world, and within that subjective world, all is fine (but we knew that already from MWI).

 

[selfAdjoint]

the preference of one over several branches, (which is refuted in MWI, hence the "many",

"Refuted?" Surely over the top Patrick? Nobody is proving anything here. "Disputed" would be better.

 

[vanesch]

"Refuted?" Surely over the top Patrick? Nobody is proving anything here. "Disputed" would be better.

I meant "rejected". MWI explicitly rejects the idea that "one branch emerges objectively", while this is exactly what Copenhagen-style interpretations do (= projection), as well as Bohmians (which keep both: they KEEP all the branches in their ontology, but one branch gets a "token", namely a strong non-zero value when we take the in-product with the position-states corresponding to the "classical position variables" - while all other branches give essentially 0 in this in-product ; there's no preferred basis problem here, because in Bohmian mechanics, the position basis is in any case preferred).

What MWI does, however, is to say that *from the point of view of an observer (= state)*, there is of course a branch emerging, but that's *subjectively*. It is the branch which contains said observer state.
What RQM does, is saying that *relative to an observer*, projection occurs or not depending on whether this observer has information.

Now, if you translate "observer has information" into "specific observer state containing the information", then it is really a game of words to say that the viewpoints are really different.

The only difference between MWI and RQM is in some word games around "subjective" and "objective". RQM starts from observers with information (= the MWI observer STATES with information), and accepts the fact that, depending on the information, the description of another system can be different from one observer to another. MWI does the same, and tells you that, according to the branch, relative to an observer state, the factor in that branch corresponding to another system can be different (the *relative state* of the other system depends on the state of the observer to which it is relative).
MWI and RQM both agree (and use the same formal demonstration) to show that the information state of another observer will be in agreement with the information state of the original observer and the information he gathered elsewhere.

But this is interpreted in a subtly different way: MWI shows that the *relative state* of another observer in the same branch as the original observer will be such that both are in agreement, but allows different versions of the COUPLES of observers (agreement within each couple).
RQM uses the same formal demonstration, but now focusses on ONE SINGLE COUPLE, and concludes that other observers agree, upon interaction, with the original observer.

And then the big theatre trick comes in: the different, mutually agreeing couples correspond to different subjective realities in MWI (all part of one objective reality), and the fact that there is agreement (within one couple of observer states) in RQM is called "objective reality".
So what were different subjective realities in MWI, is now, because of MUTUAL AGREEMENT WITHIN, called, "objective reality", and this is where a logical error is made in the exposition of RQM (but not in RQM itself, only in the exposition).

It is shown that there exists an A1 and an A2, and a B1 and a B2 and a C1 and a C2. It is then shown that Ai agrees with Bi agrees with Ci agrees with Ai. And this "common agreement" is called "objective reality".
However, it should have been SHOWN that there is ONLY ONE set possible, because we now have that A1 agrees with B1 agrees with C1 agrees with A1, but we also have that A2 agrees with B2 agrees with C2 agrees with A2 (which is evident in MWI and "hidden" in RQM). The demonstration that RQM uses to show that Ai agrees with Bi agrees with Ci agrees with Ai, under unitary evolution, concludes exactly that: that there are different "sets of agreement".
But the trick to call the "common agreement" of Ai with Bi with Ci with Ai "the objective reality O" (without an index i), is the logical error which allows one to "objectify" suddenly Ai's result, in agreement with O, with Bi's result, in agreement with O, with Ci's result, in agreement with O.

To give a simplistic illustration of the erroneous reasoning used here, consider the following:
we have 3 sets of numbers, A, B and C.
Now, we can show that if b is in B, then b/2 is in A, and if c is in C, then c/3 is in B. Moreover, we can show that if c is in C, then c/6 is in A.
Conclusion: we now know the element of A, B and C: namely a, 2a and 6a respectively. So there's now one element in each.

Counter example: A = {1,2,3}, B = {2,4,6}, C = {6,12,18}
The properties hold, but there's no unique element in A, B and C.

Nevertheless, this reasoning is exactly what is done in the exposition of RQM to introduce "objective reality". From the demonstration of the "agreement of observations" (the if b is in B, then b/2 is in A etc...), it is concluded that there is an objective (and hence unique I presume) reality of agreement.

 

[selfAdjoint]

The only difference between MWI and RQM is in some word games around "subjective" and "objective". RQM starts from observers with information (= the MWI observer STATES with information), and accepts the fact that, depending on the information, the description of another system can be different from one observer to another. MWI does the same, and tells you that, according to the branch, relative to an observer state, the factor in that branch corresponding to another system can be different (the *relative state* of the other system depends on the state of the observer to which it is relative).
MWI and RQM both agree (and use the same formal demonstration) to show that the information state of another observer will be in agreement with the information state of the original observer and the information he gathered elsewhere.

I don't think the RQM "observers" at all correspond to MWI states. Maybe I am differing from the authors of the Relational EPR paper but I rely on their statement that only the interaction is "real". State-vectors, kets, even density matrices (I think), all those are a kind of knowledge or conceptual thing. a "shape that the interaction conforms to" in less human-centered language. And the resultant real value observed has to be preserved for later comparison, which I think can only be done, if this picture is to be sustained, by successive self-interactions (cf. my analogy of the boy tossing the ball from hand to hand because he was unable to just hold it). None of this in my view corresponds to what I take to be the MWI reification of the state-vector. And hence the discussion of results later in RQM as I understand it is no stage trick but a completely supported physical interaction. Correct me if I am wrong.

 

[TopcatBV]

Hi,
I read your posts and in my opinion RQM is a MWI variant, thanks to vanesch stubbornness.
Though, I'd like if you can repeat the experiment presented in Rovelli's paper and explain again what observer C will see (in both theories) about the two detectors (A&B) measurement of two entangled particles (a+b=0) generated by a single source. I understood that, in both theories, a-A and b-B interactions will transfer the a-b entanglement to A and B.
Say the C observer will see this superposition, 50% randomly a+b- or a-b+, before A sees B's measurement and B sees A's measurement.
A will "measure" B and there is 50% chance to get a+b- or a-b+
B will "measure" A and there is 50% chance to get a+b- or a-b+

In QM everything is fine, but I don't think I understood how the classical world is arising.

Can be said that:
-A sees B as a quantum object until he gets information of what B measured. B turns in a classical object (relative to A) when A has the information of what B measured?
and
-For C, A and B are quantum objects until C gets the information from them. After C sees A and B, they (A and B) transform (relative to C) in classical object having one particular value?

Thanks

Hope I made some intelligible sentences