Everybody sees the same elephant (says Carlo Rovelli)

[vanesch]

And of course it goes without saying that, while the differences might be obfuscated by terms like "the same set of events", this doesn't change the fact that O and P measure two different values for two different properties of two different systems at two different times.

But Rovelli then goes on to say, during the formulation of his theory: "The multiplication of points of view induced by the relational notion of state and physical quantities' values considered above raises the problem of the relation between distinct descriptions of the same events. What is the relation between the value of a variable q relative to an observer O, and the value of the same variable relative to a different observer?"

If you ignore his questionable assertion that both observers describe 'the same set of events' differently, you surely cannot ignore that Rovelli has somehow let this evolve into 'the value of a variable'. Remember that O and P never try to measure the same property, not even the same system. Yet Rovelli now suggests that different observers get different values for the same variable of the same system. This is important because it is central to his reasoning that each observer has his own distinct reality.

Upon reading your comment, I think you have formulated my objection in a much cleaner way than I was able to do, but indeed, you put your finger on where it hurts IMO. (which comes down to my criticising this "objective observer-dependent reality" where there's jumping back and forth between "objective" and "subjective").

Indeed, the problem is that, for O, the state description of S went from:
a|s1> + b|s2> to |s1> through projection (standard QM as you say).

Fine. But for P, he still needs to view O and S quantum-mechanically, and so in P's description of "reality" (which is entirely subjective to P), the state before O measured S:
|O0> (a|s1> + b|s2>)

and after:
a |O1>|s1> + b|O2>|s2>

where O1 is the state of the QUANTUM system O: "O measured 1" and O2 is the state "O measured 2".

And now, the problem is of course, when P measures this O-S system, *why the hell should he pick the first term*, from HIS PoV.

I thought that this RQM was a "single-observer" version of MWI, because in MWI, we simply do:

a|P1>|O1>|s1> + b |P2>|O2>|s2> (eq 2)

and then we fall upon the same reasoning as in the paper, where it is shown that P1 can only interact with O1 and P2 can only interact with O2, so there is internal consistency in that P1 will not be contradicted by an O2 record - but the well-known price MWI has to pay for this is the "multiplication of observers", that there are now TWO P observers, which hurts of course one's intuition and makes MWI unacceptable for many.

So I was curious how RQM got around this - and I think it does it by cheating and jumping back and fro between "objective" and "subjective".

Because from (eq 2), there's no way for P to guess that in O's subjective world, he found 1, if he's not allowed (and he's not) to consider that 1's state *objectively* changed into OR |O1> OR |O2> (although he ignores the result before measurement).
So, for a small lapse of logical time, this a|O1> + b|O2> which subjectively changed into |O1> in "O"'s acount of of reality" and which SHOULD NORMALLY NOT leak into "P's account of reality" if it is still a quantum description, does, for this "lapse of logic" to make things come out "correctly".

As such, there's some verbal exercise to make you believe that there is "no reality to O's measurement" from P's PoV, but nevertheless enough of it for it to make P make the right choice in equation (2).
The "no reality" part is used to refute Bell's theorem afterwards.

At least, that's my critique of it - a rethorical jumping back and fro of objective and subjective, and reality and no reality, as things have to be argued for the sake of the argument at hand - I think you formulated it better.

Since P makes no measurement of the S-O system between t_1 and t_2, why does her description of S-O change? Unless she actually asks the question: did you measure S-O, she does not even know the measurement took place.

exactly ! For P, the S-O description is still given by equation 2.
But it is needed to sneak in the right result of S in the S-O measurement, to make P pick out the right term - although he's not supposed to know about this, and although the S-O "reality" for P should not take into account the S-O reality for O.

 

[El Hombre Invisible]

I can't help but thinking that the solution of this dilemma is in abandoning the whole concept of observer as a special class of quantum system. Observation is one kind on interaction between systems, but interactions, including observational ones, are symmetric in effect between their two component systems. No, a system can't interact with itself, and the issue has nothing to do with sentience. Quantum systems far from any sophonts will still interact and reduce each others' states.

I agree with all you say, which is why I found the paper initially appealing. I can't disagree with Rovelli's stance that all systems are equivilent, and all systems are quantum systems. I understand the opposing view to the former originates from our inability to know what is happening in interactions that we ourselves are not taking part in, therefore we have to treat ourselves differently. Rovelli's theory is a logical extension of this: that each observer can only assign states to systems he himself, and no-one else, measures. But it seems everyone is usually quite happy assigning states to unobserved systems. The opposing view to the latter is, I assume, the presumed necessity for QM to ultimately give classical values. It's one thing that drives me mad about QM: that we don't examine the actual mechanism by which measurements take place, but insist that the classical world magically appears. Again, Rovelli's stance is very sympathetic in this respect. Unfortunately he just can't stop himself doing exactly what he says others do wrong (i.e. forcing the classical world to appear, describing states with no measurement, etc).

 

[El Hombre Invisible]

And now, the problem is of course, when P measures this O-S system, *why the hell should he pick the first term*, from HIS PoV.
...
So, for a small lapse of logical time, this a|O1> + b|O2> which subjectively changed into |O1> in "O"'s acount of of reality" and which SHOULD NORMALLY NOT leak into "P's account of reality" if it is still a quantum description, does, for this "lapse of logic" to make things come out "correctly".

First off Patrick, thanks for the thumbs up on my wee critique.

That touches close to one of my questions, though I think the point Rovelli makes is that P will not neccessarily pick the first term from his PoV even though O did... that he will, with probability b, pick the second (if I have understood you right). My question in this arena was why, if P measures, say, state |O1> of the system must she also measure state |1>? I don't think this is unique to RQM though (is it not also true in MWI?), but I don't understand the mechanism enforcing this.

**EDIT: This is an entanglement issue I understand. That is, I understand it is the issue. I don't understand quite how entanglement "fits into" the QM model. Is it an extra postulate, or is it derived from QM laws? **

exactly ! For P, the S-O description is still given by equation 2.
But it is needed to sneak in the right result of S in the S-O measurement, to make P pick out the right term - although he's not supposed to know about this, and although the S-O "reality" for P should not take into account the S-O reality for O.

I agree there seems to be some forcing of expected values into Rovelli's descriptions that form the basis of his theory... no wonder he derives QM correctly! ;o)

 

[marcus]

...I can't help but thinking that the solution of this dilemma is in abandoning the whole concept of observer as a special class of quantum system. Observation is one kind on interaction between systems, but interactions, including observational ones, are symmetric in effect between their two component systems...

...Again, Rovelli's stance is very sympathetic in this respect. Unfortunately he just can't stop himself doing exactly what he says others do wrong (i.e. forcing the classical world to appear, describing states with no measurement, etc).

I must acknowledge that it nearly always leads to trouble and ultimately fails when one tries to buck the vernacular. Always safest to use words just as one hears others do, or sees in the dictionary.

But I don't like calling every little seaweed or shellfish in the ocean an "observer"----just because they are all interacting in various ways with the moon. So I don't like how many people, perhaps a majority, use the word "observer".

To me, everybody is quantum systems and morally equal on that footing---liberté égalité fraternité as quantum systems.

An OBSERVER, for me, is a character in a QUANTUM STORY. to describe an observer you have to describe an interaction, or a particular type of measurement. You have to implicitly isolate and specify. And you have to provide the designated observer a Hilbert with his name on it.

there has to be a designated transfer of information (probably across some designated boundary) in order for me to recognize that one of the creatures in the picture is acting as an observer.

it troubles me that occasionally I hear other people apparently calling any point of reference, or any bit of material, an observer. Mainly I worry where are we going to get funding to supply the Hilberts for this vast superfluity of observers?

Hilberts, in my view, do not exist in nature. They are a human invention, temporary until made obolete by some new invention, provisional, conventional, artificial. So when I tell a Quantum Story, I only want to put a FINITE NUMBER of these Hilbert gizmos into the picture---as needed to clarify the flow of information.

If there were any chance of reforming the terminology to make it more sensible (whether in the way selfAdjoint suggested, along the lines I indicated, or in some other fashion) it would be cause for celebration.

As for ontology BTW, my view is that it is quite simple---there is obviously one world, one objective reality---because I have never seen any indication of more than that---and the proof is that reasonable good faith people always eventually agree when they get together and talk about it. So, in effect, my ontology is more or less the same as that of a huntergather savage or a classical greek----that is, of people who don't wear UNDERWEAR as well as some who do.

 

[marcus]

"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."

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

We have it from the Bohrses' mouth that quantum mechanics is about INFORMATION.

and that it is not about making a math (structural, tinkertoy) model of Nature, or whateverelse people call it, the World, the Universe. Judging from what Bohr said, it is about making STATEMENTS, or as I would say it, about telling quantum stories.

I understand from Bohr's saying that it would be naive to think one was constructing a working mock-up of the universe--his QM was about flows of information and what true statements can you make.

Not sure how closely alligned this view is with others expressed in this thread. May be in some agreement or could be in complete disagreement.

 

[marcus]

...
My main concern (forget all that stuff above) was the rules determining the correlation between measurements of a third observer. That is, why if second observer P measures the system S, the value she gets has to be the value she measures O to have got. Actually, my question was more: how is this derived from Rovelli's three postulates.
...

Hombre, we have not yet talked about the THREE POSTULATES although you have referred to them several times IIRC.

the first two are a kind of charming faux paradox

1. for any system there exists a number N such that it can be completely described by answering N yes-no questions. ( i.e. N bits, N dimensionality)

2. you can always get fresh information about the system

this sounds contradictory---you can completely describe the system (so as to quantumly predict future)-----and yet you can always ask some different questions and get fresh information that you didnt know already


because of a cheerful and poetic OVERFLOW CONDITION, where asking the N+1 question destroys some of the previous answers.
thus, a sparkling overflowing fountain of novelty, just as we should always desire of Nature is we are wise and love her. BUT stated as rovelli postulate 1 and 2 a seeming contradiction.

maybe these two postulates are a cliche that other people have used to describe QM, but to me they were new when i saw them RQM

 

[El Hombre Invisible]

maybe these two postulates are a cliche that other people have used to describe QM, but to me they were new when i saw them RQM

I would expect that it were already known that you can give a complete quantum description of a system after a measurement, then measure it again and get fresh information. For instance, you could measure the state of excitement of an atom and say: 'tis in its first excited state. You can then measure it again and discover it to be in its ground state. Is that not precisely the kind of thing Rovelli means?

**EDIT: last question a genuine, not rhetorical, question - that was my understanding of Rovelli's first two postulates. **

 

[marcus]

.. measure the state of excitement of an atom and say: 'tis in its first excited state. You can then measure it again and discover it to be in its ground state. Is that not precisely the kind of thing Rovelli means?
...

I think you are right that he does mean this. the example i had in mind is measuring spin in a northsouth direction-----OK now the system is completely described----OK now I want to get MORE information about it so I will measuring spin in an eastwest direction-----Great now I have some fresh information, but OOPS I have meanwhile spoilt the first information.

there is a limit to the amount of quantum numbers or degrees of freedom ----but i can keep making new measurements to my hearts content----only after a while it starts messing and UNdoing the earlier answers
==================

and yes you are right again! I guess this has always been known from the beginings of quantum mechanics. what I think is charming is that rovelli has taken this seeming paradox and expressed it clearly as two postulates, and then----after adding a third postulate---attempted to DERIVE THE FORMALISM from these postulates. Again, this part of it may not be new but it seemed to me a useful crystalization----which you also mentioned in an earlier post

 

[vanesch]

That touches close to one of my questions, though I think the point Rovelli makes is that P will not neccessarily pick the first term from his PoV even though O did... that he will, with probability b, pick the second (if I have understood you right). My question in this arena was why, if P measures, say, state |O1> of the system must she also measure state |1>? I don't think this is unique to RQM though (is it not also true in MWI?), but I don't understand the mechanism enforcing this.

Well, the claim of RQM is that P WILL pick the first term (that's how I understand his statement that everybody will see the same elephant!).

In MWI, P doesn't have to "pick" the first term even though "S did", simply because there are now 2 P's and 2 S-es, and "to be one of the P's" (randomly, through the Born rule) doesn't mean you have to be in sync with "to be one of the S-es" (also randomly, through the Born rule).

However, if you pick randomly ONE P state (= 'to be one of the P's') then you WILL interact with a SPECIFIC S, namely the one that is in agreement with your other observations. So if you pick this P state, and "you see a pink elephant", then you will automatically be in potential interaction only with the specific S that also saw a pink elephant. BUT (and that's the difference between MWI and RQM if I understand it), MWI acknowledges also the existence of another P and another S, both who saw a blue elephant. What can be shown, and Rovelli takes over this reasoning (and SA quoted it down this thread somewhere) is that in the overall state, containing BOTH the P-blue and S-blue state on one hand, and the P-pink and S-pink state on the other hand, P-blue can never interact with S-pink and vice versa (if the states are fully decohered). As such, P-blue will be convinced that S only saw the blue elephant ; in the same way as P-pink will be convinced that S only saw the pink elephant. As I said, this is the MWI view (and what people don't like about it is that P is now "dedoubled" into a P-blue and a P-pink). I first thought that RQM wanted to limit ourselves to just one P picked out in this picture - as such it was the "one observer PoV" of MWI.

Note that the above statement FOLLOWS (in MWI) from the basic unitary machinery of QM ; it is not a postulate or anything. It simply follows from the linearity of the time evolution operator U:

If the initial state is |P-pink> |S-pink>, and one let's U act upon it, then this will do something (like, say, change the state of P-pink into a state where he talked to S or something):

U |P-pink>|S-pink> = |P-pink-and-talked-to-S-pink> |S-pink>

Same for |P-blue>|S-blue>:

U |P-blue>|S-blue> = |P-blue-and-talked-to-S-blue> |S-blue>

and if we keep BOTH in the overall wavefunction:

|psi> = a |P-pink>|S-pink> + b |P-blue>|S-blue>

we will get, from U |psi>, from the linearity, that the final state will NOT get a mixture of |P-blue> and |S-pink>.

So the machinery of QM already contains this separation, simply by the linearity of the time evolution operator.

**EDIT: This is an entanglement issue I understand. That is, I understand it is the issue. I don't understand quite how entanglement "fits into" the QM model. Is it an extra postulate, or is it derived from QM laws? **

It is entirely part of the mathematical machinery of QM:

You have to assign a basis state in the Hilbert space to each different configuration of your system you're modelling.
That's why the basis states of the Hilbert space of a point particle are given by |x,y,z>.
(for each different x, y and z value, which is a different configuration of your system = point in 3-dim space, there is a different basis vector in Hilbert space - I'm jumping here over all problems with continuous spectra and so on).

If you consider now as system S2 the union of two subsystems, Sa and Sb, then the configuration of S2 is the juxtaposition of the configurations of Sa and those of Sb. In other words, if you consider the Hilbert space of the system "two points in 3-dim space", then (x1,y1,z1) is the configuration of the first, and (x2,y2,z2) is the configuration of the second, and the configuration of S2 is then (x1,y1,z1,x2,y2,z2).
If we want to assign a basis state to each of these configurations,
|x1,y1,z1,x2,y2,z2>, then it is easy to show that this can be written as the tensor product of the basis of Ha and Hb (the Hilbert spaces of the individual subsystems):
|x1,y1,z1,x2,y2,z2> = |x1,y1,z1> x |x2,y2,z2>
and the resulting space is the tensor product space H2 = Ha x Hb

Now, SOME vectors in H2 can be written as |A> x |B> where |A> belongs to Ha and |B> belongs to Hb, but MOST vectors in H2 are NOT of this form, but are COMBINATIONS of such vectors, such as:
|A>x|B> + |C>x|D>

This is the "most common" state in H2 that corresponds to a state of our combined system. When the state does NOT take on a form |A> x |B>, then we call this state ENTANGLED. Most states are entangled. The specific states |A> x |B> (which are rare) are called product states. These are the only states in which a clear state is assigned individually to system Sa and to system Sb.
Usually, when systems interact with each other (described by the unitary operator which is the solution to the Schroedinger equation), they end up in an entangled state.

 

[El Hombre Invisible]

Well, the claim of RQM is that P WILL pick the first term (that's how I understand his statement that everybody will see the same elephant!).

Ah, then we've been attacking the beast from two different angles. To me, it is quite clear that, in RQM, second observer P may observe a different state for the observed system S than the first observer O did. This is the point about 'different observers can see different things', to me the entire point and basis of this paper.

Then in resolving the correlation issue Rovelli points out, rightly in my opinion, than in order to check the result observer O obtained, observer P has to measure, by fundamentally quantum mechanical processes, observer O and it is at this point that consistency demands that P measure from O what she also measured from S, BUT NOR NECESSARILY WHAT O MEASURED! O's reality is not of consequence to P's reality. And there is no contradiction or paradox because there are no real objective observers like we are pretending to be.

This is, IMO, the entire point of the paper. That observers CAN measure the same system as being in different states. My question was then about the entanglement issue which you've answered in part... but my specific question was: can what you've replied on entanglement be derived from Rovelli's three postulates? (That is, did the process you outlined demand new assumptions or did it follow absolutely from existing QM laws that can in turn be derived from the three postulates?)

 

[vanesch]

Ah, then we've been attacking the beast from two different angles. To me, it is quite clear that, in RQM, second observer P may observe a different state for the observed system S than the first observer O did. This is the point about 'different observers can see different things', to me the entire point and basis of this paper.

That would indeed be a logical conclusion, that everybody is living in his own "reality" disconnected from others.
In other words, there is a real P observer, which has seen a pink elephant, and, in "his reality" has also seen an S observer which has seen a pink elephant.
And then, there is (eventually, but of no importance to P), also a real S observer, which has seen a blue elephant. We could guess that in S's reality, there is even a P observer which will agree with him on the blue elephant (but that's clearly not the P observer of the previous sentence).

The funny thing is that this comes extremely close to MWI, where it is simply acknowledged that there are now TWO P and TWO S observers, and they "pair up" as needed, upon interaction. So if this is what was meant, I'd say that this is just paraphrasing MWI, without wanting to state explicitly that the S-observer in the P world is potentially a different S observer in the S world.

However, when it is stated that "everybody sees the same elephant" and when it is stated that "objective reality is what everybody agrees upon", I have the impression that there's a need for S and P to have seen the same (say, pink) elephant, and that there is no room for another S observer having seen a blue one.

Then in resolving the correlation issue Rovelli points out, rightly in my opinion, than in order to check the result observer O obtained, observer P has to measure, by fundamentally quantum mechanical processes, observer O and it is at this point that consistency demands that P measure from O what she also measured from S, BUT NOR NECESSARILY WHAT O MEASURED! O's reality is not of consequence to P's reality. And there is no contradiction or paradox because there are no real objective observers like we are pretending to be.

Well, I think that is not what Rovelli wants us to accept, but maybe I'm wrong on Rovelli's intentions. However, if THIS is what is to be taken, then really I don't see the difference with MWI, because after all, we now have TWO realisations of each observer: the "O-fiction" in P's world, where he *thinks* that there is an O-observer having seen a pink elephant and hence who agrees with him, and the "real" O-observer who has actually observed a blue elephant (and who will meet a P observer who will agree with him). Considering simply that "objective reality" is then the combination of both (a superposition of both) we are in EXACTLY the MWI scheme.

But, as I said, I don't think that this is what Rovelli intends, and then there IS an issue of how come that O and P "took the same decision" when collapsing their wavefunctions.

This is, IMO, the entire point of the paper. That observers CAN measure the same system as being in different states.

As I said, if that's Rovelli's point, then I really fail to see the difference with MWI.

My question was then about the entanglement issue which you've answered in part... but my specific question was: can what you've replied on entanglement be derived from Rovelli's three postulates? (That is, did the process you outlined demand new assumptions or did it follow absolutely from existing QM laws that can in turn be derived from the three postulates?)

Well, entanglement is part of the unitary part of quantum theory, so as long as a system is considered a quantum system (and it's understood that the entire unitary machinery is then valid for the system at hand), this is automatically part of it.

 

[selfAdjoint]

I think what "Everybody sees the same elephant" actually means in the paper is that when and if everybody gets together afterward to compare notes, they will agree on what they saw. But as long as they stay spacelike the simply have no relationship at all, and what each of them saw is only defined for that individual, and strictly undefined for the others. This is all that their math really supports.

If this is what MWI boils down to, that spacelike related individuals live "in different worlds", and it is acknowledged that those worlds can in principle be eventually brought into timelike relation and unified, then I see nothing mystical in it.

 

[vanesch]

I think what "Everybody sees the same elephant" actually means in the paper is that when and if everybody gets together afterward to compare notes, they will agree on what they saw. But as long as they stay spacelike the simply have no relationship at all, and what each of them saw is only defined for that individual, and strictly undefined for the others. This is all that their math really supports.

If this is what MWI boils down to, that spacelike related individuals live "in different worlds", and it is acknowledged that those worlds can in principle be eventually brought into timelike relation and unified, then I see nothing mystical in it.

No, this is not what MWI is about. But I'm affraid we're talking next to each other in a way.

As I said before, there's a difference between being fundamentally ignorant about something and claiming that that something has no objective existence - at least if objective existence is not denied.
If Q looks at an elephant, then Q can see a pink one or a blue one. Now, P can be ignorant of Q's observation (or even, but that's an extra complication which is not needed here, whether Q did perform his observation of not). But that doesn't change the fact that Q did have an observation.
It is as if we're doing Bayesian probability here:
from a Bayesian PoV, P assigns different probabilities to the statements:
"Q didn't look at anything" -> p0
"Q saw a pink elephant" -> p1
"Q saw a blue elephant" -> p2

and assuming that this is an exhaustive list, p0 + p1 + p2 = 1.
The numbers (p0,p1,p2) describe the knowledge P has about Q's looking at an elephant.
When Q tells P about his elephant experiences, this knowledge changes, and hence the numbers change (for instance, to 0,1,0, in the case Q tells P he saw a pink elephant).

But all this has nothing to do with the "objective reality" of the truth of the above statements: one is true, and the two others are false. Only, Q has this information, and P doesn't (yet).

The Bayesian probabilities describe P and Q's knowledge of certain events, and it is taken that they are "realities" in RQM. Then of course P's and Q's "realities" are different, and can be set equal when they meet (their interaction equalises their knowledge of events), but we're in fact only talking about their KNOWLEDGE.
Also, when equating bayesian probabilities with (subjective) "realities" there is an underlying objective reality which contains the truth values of each statement, and when all is said and done, and all observers come together and exchange all of their information, then the only probabilities that remain in all these subjective realities are the same, and only take on 1 or 0, 1 for objective truths and 0 for objective falsities. So if this was the game, there's no point in denying an objective reality (which assigns objective truth values to statements), and derived "subjective" realities which are in fact sets of limited knowledge, and where the Bayesian probabilities are real numbers, changing each time upon "interaction".

But all this would not be worth writing a paper, because all this is classical probability or information theory ; there's just a renaming of "realities" for "knowledge".

In this framework, however, Bell's theorem would still be valid, and RQM's entire construction, denying "reality" to spacelike separated observations, has as sole and unique goal to be able to say that Bell's theorem is not applicable to an event that "has no reality".
There would, in this framework, still be an entirely objective truth value to the outcome of the measurement at B (even though A is ignorant of it, and hence has no knowledge of it, and hence assigns still 50/50 to the potential outcomes until he learns about it). It would be this objective truth value which "synchronizes" the "outcomes" of P and of O, when they talk to each other.

But we cannot work in a Bayesian probability system in quantum theory. Otherwise, this would be nothing else but a local hidden variable theory, as forbidden by Bell's theorem!
Saying that A is in the state |a> + |b> is NOT equivalent to saying that we have partial information about A, and give it (Bayesian) probabilities 50% / 50% to be in a or in b. If A is in the state |a> + |b> then this is a unique, well-defined state which is DIFFERENT form saying that A is in the state |a> or A is in the state |b>, with 50% chance. It's the entire difference between a superposition and a mixture.

If P would say that O is in a MIXTURE, with 50% chance to have seen a pink elephant, and 50% chance to have seen a blue elephant, we could interpret this as a Bayesian statement about the information P has about O, and we COULD STILL ASSUME THAT O IS, IN FACT, IN ONE OF BOTH STATES. But then, collapse DID happen, only P DOESN'T HAVE THE INFORMATION. O DID objectively have one or the other result, but P doesn't know that yet. But then we're in Copenhagen ! And then, Bell's theorem is valid and nothing has been gained.

However, RQM claims that O is a quantum system, and hence IS STILL IN SUPERPOSITION and NOT in a collapsed state with 50% chance for each, in other words, in a mixture which is saying that O *is* in a definite (objective) state, but we don't know in which one, and it is entirely essential to RQM, in order to be able to deny the application of Bell's theorem.

And when starting from the quantum-mechanical superposition, there's no reason for P, upon measurement, to find the SAME value as S. There's no "little devil" in the superposition which says that the "true" value is the pink elephant "as seen by S" (while there IS such a little devil in the mixture: it is the objectively true value, which just gives rise to an "update" in the Bayesian probabilities describing our information).
Given that a superposition is a unique state (and not a probabilistic mixture of states, such as in a Bayesian information description), there's no extra element which can make the balance switch for one term or another. And given that RQM claims to be complete, there's no possibility for an underlying mechanism doing so (like a hidden-variable theory).

So, again, the "agreement" can only happen if there WAS an objective collapse at O and P's "reality" is nothing else but a Bayesian description of his knowledge, in which case RQM reduces to Copenhagen ; or whether there are different parallel worlds, matching up the agreements (such as in MWI), or when everything is only seen from the PoV of one subjective reality (which makes the thing "solipsist") at a time, and there is no "agreement" between different subjective realities.

 

[selfAdjoint]

However, RQM claims that O is a quantum system, and hence IS STILL IN SUPERPOSITION and NOT in a collapsed state with 50% chance for each, in other words, in a mixture which is saying that O *is* in a definite (objective) state, but we don't know in which one, and it is entirely essential to RQM, in order to be able to deny the application of Bell's theorem.

RQM does not say this, for it would require the abandonment of the basic relational position to talk about "STILL IS" between spacelike related measurements. That is your objectivist way of thinking, what Smerlak and Rovelli say requires a privileged classical observer. At the end of section 3 they say:

"Of course the price to pay for this slution to the puzzle (i.e the relational solution of the EPR puzzle) is that the sequence of events which is real for B is different from the one which is real for A."

This is a flat denial of objectivism.

A sees a system in superposition and measures it getting a result which is real for A. B also sees a system in superposition and measures it, getting a result which is real for B. These facts cannot be combined or put in time order because the measurements were spacelike related. Different relativistic observers would see different time orders. The only way they can be compared is to be brought together down in the intersection of A's and B's light cones.

{Added} There is nothing in QM proper which says "the wave function extends from A to B." After all, "The wave function of a system" is just a metaphor for a set of coded algebraic relationships which enables any observer who interacts with the system to calculate the probabilities of what she will see. The only evidence that might support the nonlocal extent of a quantum state is the experimental correlations. But Smerlak and Rovelli here show (and you agreed earlier in this thread that their derivation was correct) that under the realtional, local, non-objectivist hypothesis the same correlations will arise.

 

[marcus]

I think what "Everybody sees the same elephant" actually means in the paper is that when and if everybody gets together afterward to compare notes, they will agree on what they saw. But as long as they stay spacelike the simply have no relationship at all, and what each of them saw is only defined for that individual, and strictly undefined for the others. This is all that their math really supports.

If this is what MWI boils down to, that spacelike related individuals live "in different worlds", and it is acknowledged that those worlds can in principle be eventually brought into timelike relation and unified, then I see nothing mystical in it.

I agree and would like to interject that it is a common human experience that there is one objective world

AND that people believe different things about it and SAY different things

BUT that eventually, when a bunch of us are able to get in causal contact so that we have roughly the same lightcone we usually manage to iron out our differences!

and the ability to reconcile differences by people of good faith and a shared language IS WHAT DEFINES objective reality for me

which is kind of a "common language philosophy" definition of what is objective real---the meaning is the use and all that---what else does real "really" mean?

 

[vanesch]

RQM does not say this, for it would require the abandonment of the basic relational position to talk about "STILL IS" between spacelike related measurements. That is your objectivist way of thinking, what Smerlak and Rovelli say requires a privileged classical observer.

Forget the "still is" ; I wasn't implying any temporal relationship in fact. Objectivism is not related to a classical observer. One can objectively say (or not) that a system is in a superposition (which is of course not a classical statement, but can be an objective statement). Objectivism is related to assigning truth or falsity to statements without this truth value depending on a subject.
The thing that is related to a classical observer is not objectivity, but the existence of a single measurement result - because a quantum entity has automatically the superposition principle applied to it, and hence the co-existence of measurement results.
Classicism is the denial of superposition (hence of the denial of the parallel existence of several measurement outcomes).

At the end of section 3 they say:

"Of course the price to pay for this slution to the puzzle (i.e the relational solution of the EPR puzzle) is that the sequence of events which is real for B is different from the one which is real for A."

This is a flat denial of objectivism.

Yes, so they should stop talking about "objective realities": they only talk about subjective realities - subjective realities which ARE NOT LINKED by any underlying objective reality.

It is not made very clear however, that there is this full denial of an objective reality (not only of some underlying quantum world, but of the objective reality of *measurement results*). The impression is given that there IS some objectivity to "measurement results", as long as all observers agree upon it. But as we see, they cannot agree upon them when it matters, namely when they are spacelike separated.

A sees a system in superposition and measures it getting a result which is real for A. B also sees a system in superposition and measures it, getting a result which is real for B. These facts cannot be combined or put in time order because the measurements were spacelike related. Different relativistic observers would see different time orders. The only way they can be compared is to be brought together down in the intersection of A's and B's light cones.

No, that's the point. They cannot be compared, because there's no objective backbone. A sees a system in superposition and measures (affecting A's subjective reality) an outcome of his local system and then, later, measures the "other observer B" which is also in superposition in A's subjective reality, and then obtains AN OUTCOME within A's subjective reality for "B's outcome". So A now has two outcomes in his reality: its local one, and the result of its outcome on the other observer. It turns out that THESE are in consistent agreement.

However, in the other observer's reality, B, B has an outcome of his local measurement (which affects B's subjective reality) and then measures the "other observer A" which is in a superposition in B's subjective reality, hence obtaining an outcome within B's subjective reality for "A's outcome".

And here's the crux: THERE IS NO REASON TO ASSUME why "A's outcome"
in B's subjective reality has anything to do with "A's outcome in A's subjective reality". It would, if there were an underlying objective reality which linked both. But this is denied.

So we have here an "A-reality" where A obtained an outcome on his local system (say, pink elephant), and then an outcome on his measurement of B's outcome (say, pink elephant). These are in agreement.

We also have a "B-reality" where B obtained an outcome on his local system (say, blue elephant), and then an outcome of his measurement of A's outcome (say, blue elephant). These are also in agreement.

But these "realities" have nothing to do one with another, because they have been "generated" independently, from superpositions of quantum states into specific realisations using the Born rule.

There's no way in which both can be "synchronized after the fact" of course, because that would imply that the "blue elephant" seen by one observer would suddenly change in a pink one, or vice versa, and I assume that we can take it that if B, from B's viewpoint, saw "pink elephant", that this doesn't, in his reality, change into "blue elephant" upon his meeting with A ; also that from A's viewpoint, if A saw "blue elephant" that this is not going to change into "pink elephant" upon meeting B.

So if there is no mechanism to synchronize the two subjective worlds upon their respective initial measurement for their respective "owners", then there's no way to synchronize this later, when they meet. So the only way to conceive this situation is that the TWO realities have independent existence.

This is exactly MWI, except that MWI writes out these different "subjective realities" as a sum, in different branches, and calls the sum an "objective reality".

{Added} There is nothing in QM proper which says "the wave function extends from A to B." After all, "The wave function of a system" is just a metaphor for a set of coded algebraic relationships which enables any observer who interacts with the system to calculate the probabilities of what she will see. The only evidence that might support the nonlocal extent of a quantum state is the experimental correlations. But Smerlak and Rovelli here show (and you agreed earlier in this thread that their derivation was correct) that under the realtional, local, non-objectivist hypothesis the same correlations will arise.

I don't agree with this, because IN ORDER TO HAVE THIS DERIVATION, we have already to assume the quantum state of the combined system in the first place (1), otherwise there is no way to deduce the correct correlations. But, moreover, in order to have this derivation, it is ALSO necessary to consider the superposition of the two possible outcomes for the two observers OR to consider an overall projection affecting both observer states upon one measurement. There is no "completely factorised" derivation possible: that would namely be equivalent to Bell's theorem !

 

[selfAdjoint]

Patrick, I don't see how you can say this:

One can objectively say (or not) that a system is in a superposition (which is of course not a classical statement, but can be an objective statement). Objectivism is related to assigning truth or falsity to statements without this truth value depending on a subject.
The thing that is related to a classical observer is not objectivity, but the existence of a single measurement result - because a quantum entity has automatically the superposition principle applied to it, and hence the co-existence of measurement results.
Classicism is the denial of superposition (hence of the denial of the parallel existence of several measurement outcomes).

without believing one of two things:

1) The superposed (or entangled) state has a location in the real world, or
2) The particular states (elements of the eigen set) that a particular observer can see have an "objective" reality before being measured, or at least that when the eigenset happens to be reduced to a singleton set, that unique member has a reality before being measured.

Let us simplify the situation that Smerlak & Rovelli describe: two particles, alpha and beta, quantum eigenstates for each restricted to up or down, and prepared to be entangled so that the only eigen states available to the combined system of alpha and beta are "alpha up and beta down" or "alpha down and beta up". The probabilities for the first measurer to reduce the combined system to one or the other of these calculates to 50%. But this measurement does not produce a real value at the other location until a measurement happens there. What changes in the reduction is the probabilities for getting a result in that remote measurement.

Observer A always interacts with particle alpha. Spacelike separated observer B always interacts with particle B. A and B have clocks that they synchronized in the past when they were together so they will, when they come together again in the future agree on what times things happeded, and we can assume none of this is problematical.

Suppose that by prior agreement B measures particle beta at 10 AM by her clock, and that A measures particle alpha at 10:02 AM by his clock. Before 10 AM the probability that B will see up is 50% and her probability for down is 50% and likewise the probability that A will see up is 50% and the probability that he will see down is also 50%.

At 10 AM according to the agreement, B measures particle beta, in doing so she
1) gets a particular value (let is say up)
2) changes A's probabilities for seeing down to 100%, and for seeing up to 0%. Of course A knows from the agreement that his probability has been reduced to a delta, but he doesn't know which value has the delta and which has the zero. At 10:02 he measures and finds down. He infers (but cannot yet conclude) that B did the measurement and found up.

This is ALL THAT HAPPENS. The state is not a thing, but just a way of getting the probabilities to see the real value from the situational data. For RQM, it ain't real until is measured ("Quantum events exist only in interactions").

When they get back together A and B will agree on what happened and that what both of them saw was consistent with alpha and beta being entangled.

 

[vanesch]

What changes in the reduction is the probabilities for getting a result in that remote measurement.

But Dick, that's the entire point of Bell's theorem ! That the probabilities of the OTHER guy/gal's observation CHANGE as a function of the result of the former one, and that this cannot be explained by any prior correlation.
The entire point of S/R to 'deny reality' to the other measurement is to avoid this conclusion.

Observer A always interacts with particle alpha. Spacelike separated observer B always interacts with particle B. A and B have clocks that they synchronized in the past when they were together so they will, when they come together again in the future agree on what times things happeded, and we can assume none of this is problematical.

Suppose that by prior agreement B measures particle beta at 10 AM by her clock, and that A measures particle alpha at 10:02 AM by his clock. Before 10 AM the probability that B will see up is 50% and her probability for down is 50% and likewise the probability that A will see up is 50% and the probability that he will see down is also 50%.

At 10 AM according to the agreement, B measures particle beta, in doing so she
1) gets a particular value (let is say up)
2) changes A's probabilities for seeing down to 100%, and for seeing up to 0%. Of course A knows from the agreement that his probability has been reduced to a delta, but he doesn't know which value has the delta and which has the zero. At 10:02 he measures and finds down. He infers (but cannot yet conclude) that B did the measurement and found up.

This is ALL THAT HAPPENS. The state is not a thing, but just a way of getting the probabilities to see the real value from the situational data. For RQM, it ain't real until is measured ("Quantum events exist only in interactions").

That was already the case in Copenhagen: the state is not real, only the outcomes of measurements. But to avoid Bell, that's not good enough: if B *really* obtains a result at 10:00 and A *really* obtains a result at 10:02, then that's ALL THAT IS NEEDED for Bell's theorem to apply, and to show that these results cannot be obtained by any a priori correlations in hypothetical different underlying states ; in other words, whatever "generates" the probabilities has a non-local character in order for this change in probability at A (to go from 50-50 to 100-0) to occur. There is hence a kind of nonlocal influence of B's result on A's probabilities of outcome, and they are NOT of the kind of simple a priori correlation - in which case one could assign the change in probability 50-50 -> 100-0 as a simple update of Bayesian probabilities as a function of extra knowledge. Bell's theorem tells us that the probabilities generated by QM CANNOT be seen this way.

So R/S have to go further than Copenhagen in their denial of reality: not only (as with Copenhagen) they deny the reality of a quantum state between measurements, but they ALSO deny reality of measurements other than the one performed by a certain observer. In your story, A denies the reality of B's outcome at 10:00, and tells himself that B DIDN'T HAVE AN OUTCOME until he meets him (at, say, 11:15), and that hence B's outcome only has a meaning from 11:15 onward, at which point, B's outcome is NOT (of course!) generated at a spacelike separated point, and hence that the causal influences inferred by Bell are not a problem, as they are now not spacelike connected events anymore and hence causal links are no problem.
In "A's reality", only two things happened: at 10:02, A did his local measurement, and at 11:15, A measured B and hence B's measurement became "real". Nothing happened at 10:00, and hence there was no measurement result at a spacelike interval at 10:00.
This is in fact, exactly also the way MWI avoids Bell, but here, it is clear why "B didn't have an outcome" at 10:00: there were two B's, one with each outcome, and it is only until 11:15 that A interacts/entangles with B (and hence has to choose a branch). So in MWI, the "B's outcome has no reality for A" is entirely clear.

S/R ressort to the same "explanation" in fact, but because they want to hide the entirely subjective character of their explanation (which would make their view just as weird and unattractive as MWI, or even worse), they talk about some redefinition of the word "objective" (which means in fact subjective).

When they get back together A and B will agree on what happened and that what both of them saw was consistent with alpha and beta being entangled.

Yes, but the only way for this to happen, is that the probabilities for A, changed magically when B did his measurement. S/R provide no more explanation for this than Copenhagen. It is exactly the fact that A's probabilities "have to change" according to the outcomes at B which is pointed out by Bell's theorem.

If we applied strictly the postulates of S/R, and "assigned no reality" to B's outcome for A, and applied "standard QM" for A, then A would DRAW INDEPENDENTLY his measurement result. A would NOT have his probabilities change, but A would change the probabilities of B's "telling him" at 11:15.

10:00: B gets "up" (but this only has a meaning for B, and no reality for A)

10:02: for A, the quantum description is still the superposition, but assuming that he knows that B measured, and B is a quantum system, A's description of the system is still a superposition AND NOT, as you write:
"Of course A knows from the agreement that his probability has been reduced to a delta, but he doesn't know which value has the delta and which has the zero." because this is the COPENHAGEN view on things.
If B is a genuine quantum system which interacts (according to a unitary operator) with an entangled system, the final state description is an ENTANGLED SUPERPOSITION of the two.
We do not have to assign any "reality" to this state description: it is just the correct quantummechanical formal calculation of the state, when A applies his quantummechanical calculation, and doesn't give B any special properties such as being an observer.
So, according to A, knowing that B's "measurement" has no reality at this point, applies Born's rule INDEPENDENTLY, for the first time, at 10:02, and finds, say: "up". He now also knows that when he will measure B's state at 11:15, that B will say "down".

Eh, but in B's reality, he saw "up" ? No problem, this is in B's reality, not in A's.

 

[selfAdjoint]

So R/S have to go further than Copenhagen in their denial of reality: not only (as with Copenhagen) they deny the reality of a quantum state between measurements, but they ALSO deny reality of measurements other than the one performed by a certain observer. In your story, A denies the reality of B's outcome at 10:00, and tells himself that B DIDN'T HAVE AN OUTCOME until he meets him (at, say, 11:15), and that hence B's outcome only has a meaning from 11:15 onward, at which point, B's outcome is NOT (of course!) generated at a spacelike separated point, and hence that the causal influences inferred by Bell are not a problem, as they are now not spacelike connected events anymore and hence causal links are no problem.

I disagree that A tells himself any such stupid thing. What he surely says to himself is "I have no way to be sure what happened with B, so I don't really know if my result (alpha down) was the result of her going first, as we planned or if by some chance she was delayed and I went first." There is no denial that a measurement took place, just a lack of information (i.e, relative to A, there is no chain of quantum interactions, real facts, connecting to B's condition, because B is spacelike related to A). This ignorance is not subjectiveness, because A has no doubt that B exists, it's just that they're out of touch.

In "A's reality", only two things happened: at 10:02, A did his local measurement, and at 11:15, A measured B and hence B's measurement became "real". Nothing happened at 10:00, and hence there was no measurement result at a spacelike interval at 10:00.

Yes, this is A's sequence, and B has a precisely similar structure. It's misleading to talk about this in terms of conscious observers, because RQM doesn't posit any causal relationship from consciousness to quantum interactions. Although this usage actually helps my case, since your position that A denies the reality of B's experience contradicts our common experience of not knowing what's going on with a friend who's out of tuoch, yet these consciouness images disguise the basic RQM structure. Actually the only reality that A experiences is the final quantum interactions that bring his sensations to his consciousness. Even Dr. Johnson's example of reality, kicking a stone, is only indirectly reported to our awareness. RQM isn't about that; it's about quantum interactions.

This is in fact, exactly also the way MWI avoids Bell, but here, it is clear why "B didn't have an outcome" at 10:00: there were two B's, one with each outcome, and it is only until 11:15 that A interacts/entangles with B (and hence has to choose a branch). So in MWI, the "B's outcome has no reality for A" is entirely clear.

But speaking at the level of quantum interactions, what is the reality of these branches for systems not directly involved? Say beta decay of two spacelike nuclei inside the sun; do we all have many branches of that to concern us?

 

[El Hombre Invisible]

As I said, if that's Rovelli's point, then I really fail to see the difference with MWI.

Not much as far as I can see. If you define 'world' as one persons subjective reality, it looks pretty similar. There's a huge philosophical difference between the Universe splitting up every time a system is measured and just saying 'reality is subjective', but not a huge difference when it comes to doing experiments. RQM looks faultier to me, but that could be ironed out.

Well, entanglement is part of the unitary part of quantum theory, so as long as a system is considered a quantum system (and it's understood that the entire unitary machinery is then valid for the system at hand), this is automatically part of it.

Yeah, I read up on it myself. I still couldn't find out whether entanglement is derived from more basic QM laws, such as the Schrodinger equation, or whether by adding the concept of entanglement other postulates were introduced into unitary QM. I think I'll give up trying to figure that one out for a year or two.

 

[vanesch]

But speaking at the level of quantum interactions, what is the reality of these branches for systems not directly involved? Say beta decay of two spacelike nuclei inside the sun; do we all have many branches of that to concern us?

As I said many times (mostly on the quantum physics forum), "branches" in MWI are an observer-dependent concept. They are the different, coarse-grained and stable terms in the wavefunction WHEN LOOKED UPON from a specific observer-degrees-of-freedom/rest-of-the-universe split (namely the Schmidt decomposition when we write the hilbert space "of the universe" in the special product H_mybody x H_rest_of_universe). And of course this is entirely dependent on the choosen split, and hence an observer-dependent concept.

In other words, a beta decay in the sun DOESN'T AFFECT my branches as long as this doesn't imply any entanglement with my body.

Often, the erroneous argument against MWI is used that the "universe splits in branches each time there's a quantum event on Andromeda" but that's doubly wrong: first of all, there are no "branches" in the overall wavefunction without any specification of what is an "observer body" ; and second, if it is, say, my body, this bodystate will NOT entangle with this quantum event over there and hence will NOT give rise to splitting branches.

Branches, seen this way, are very, very, very close to the concept of RQM of "reality according to an observer".

Just for your information, 3 days ago I received my copy of "3 roads to QG" by Smolin. I didn't read everything, but the beginning is actually nothing else but an exposition of RQM - which, of course, disappointed me :shy:

After reading a bit about it, and thinking over it, I'd say that what doesn't fit in this "altered logic" with non-absolute truths to events, but nevertheless common to everybody, is the following.

It is my impression that the kind of reasoning used, would even prove locality to hold if a genuine FTL telephone existed.

Let us do the following gedanken experiment. We're in the year 2150, and we know already for some time that relativity is false. We've discovered the "speedon" field, which goes much much faster than light, and there's a company in Santa Barbara which has commercialized a faster-than-light telephone based upon speedon communication.
It really works, and you can pick up your FTL phone and talk to your cousin who decided to live on Titan, without any delays. Great stuff.

But of course, speedon fields are not directly perceivable by humans: you cannot feel, hear, or see them. You need an apparatus. Also, humans are still restricted to less-than-lightspeed travelling. Only speedons can go much faster, not rockets or anything.

And let's apply some RQM logic to the following situation:
Your cousin Bob can decide or not to call you on the FTL phone. However, him being outside of your past light cone, whatever he decides is "not part of your reality". So your cousin Bob throws up a coin, and if it is face, he'll call you, otherwise he won't.
You will look at your FTL phone and see if it rings or not.
The 5th of january, your cousin flips his coin, 'measures' it, and finds out whether it is head or tails. Say it is heads, so he calls you.
You see your phone ring, also the 5th of januari. But of course, this is not in your cousin's past light cone, so this measurement of yours is not in his reality.
In fact, he can flip his coin 1000 times, and when he finds heads, he calls you, when he finds tails, he doesn't. You, on your side, write down when your phone rings, and when it doesn't.
Next, he takes a rocket, and comes to earth. A week later, the 12th of january, he meets you and shows you the result of his coin flipping. It was heads, so he called you. You tell him that indeed, you observed the phone ring. You are in agreement on the measurement. You compare the entire list, and lo and behold, each time he had "heads", you heard your FTL phone ring. And otherwise, not.
But all that is no non-local issue, because of course, now, you observing the phone ring is now in your cousin's past light cone.

So nature is local. Even though you can buy a working FTL phone in Santa Barbara.

If you DECIDE to say that all that is real, is only in your past light cone, then OF COURSE you will never detect anything non-local. You simply DEFINED it not to be real.

This is exactly the same reasoning as the one used in RQM to deny a problem with Bell, and to say that there's no issue in "the probability for Bob to observe a pink elephant changed from 50-50 to 100-0". The thing in QM which doesn't allow us to build an FTL phone is simply that it is not the *decision* of the measurement at Alice, but the *outcome* at Alice which does so, and adjusts the probability changes at Bob in such a way that the local statistics come out right - but this aspect is never used by RQM in the argument to deny the issue of non-locality in this change of probabilities over space-like distances. The only argument used is that it is "just not real, because outside of the past lightcone" and "at the moment we can check the correlations and agree upon the two events at spacelike intervals, both events are within the past lightcone of those doing the comparison".
Both aspects are still true in my story with the cousin on Titan and the FTL phone.

 

[vanesch]

I disagree that A tells himself any such stupid thing. What he surely says to himself is "I have no way to be sure what happened with B, so I don't really know if my result (alpha down) was the result of her going first, as we planned or if by some chance she was delayed and I went first." There is no denial that a measurement took place, just a lack of information (i.e, relative to A, there is no chain of quantum interactions, real facts, connecting to B's condition, because B is spacelike related to A). This ignorance is not subjectiveness, because A has no doubt that B exists, it's just that they're out of touch.

This goes wrong with Bell. Because it is sufficient to say that B HAD an outcome, even if you don't know which one, to be able to assign (Bayesian) probabilities to them. And then, any further subsequent quantum interference is impossible.
If you make the list of:
"B could have measured under 45 degrees, and found up"
"B could have measured under 45 degrees and found down"
"B could have measured under 22 degrees and found up"
...

and you assign a probability to each, then you know that this will NOT work: some probabilities have to be negative to be in agreement with the QM predictions.

So the only thing to say is that the above statements ARE NOT PART of a "probability space", which means, you cannot state that B did a measurement.

 

[vanesch]

Yeah, I read up on it myself. I still couldn't find out whether entanglement is derived from more basic QM laws, such as the Schrodinger equation, or whether by adding the concept of entanglement other postulates were introduced into unitary QM.

Well, I think that the superposition principle, applied to a composite system, automatically leads to entanglement.
The superposition principle applied to a single point in space leads to a complex wave or scalar field (an amplitude attached to each possible configuration of the point, which is each point in space ; so a mapping from the points in space into the complex numbers).
The superposition principle applied to a "point with different discrete states" leads to a spinor or vector field.
The superposition principle applied to A PAIR OF POINTS will give you a complex amplitude for EACH POSSIBLE PAIR of points, or a mapping from space x space into the complex numbers. This is nothing else but the tensor product of the mappings space -> C times space -> C, but is nothing else but the functions of two points into C.
Now, when this function f(a,b) cannot be written as f1(a) x f2(b), then we say that the state is entangled.
Most functions f(a,b) cannot be written as f1(a) x f2(b).

 

[selfAdjoint]

This goes wrong with Bell. Because it is sufficient to say that B HAD an outcome, even if you don't know which one, to be able to assign (Bayesian) probabilities to them. And then, any further subsequent quantum interference is impossible.
If you make the list of:
"B could have measured under 45 degrees, and found up"
"B could have measured under 45 degrees and found down"
"B could have measured under 22 degrees and found up"
...

and you assign a probability to each, then you know that this will NOT work: some probabilities have to be negative to be in agreement with the QM predictions.

So the only thing to say is that the above statements ARE NOT PART of a "probability space", which means, you cannot state that B did a measurement.

No way! A doesn't have this controlled knowledge about B. B could have been called away; a cosmic could have struck the equipment and fouled up the data, anything else might have happened. A and B are spacelike related and just because they both have the entangled particles from the intersection of their past light cones doens't mean they can overcome the realities of noncommunication. So A can't draw up a meaningful exhaustive set of alternatives for B, and therefore he can't compute genuine probabilities for B.

B's reality is NOT present for A. A doesn't deny that B has one (although, you know, B could have died...) but it means nothing to him as an experimenter. And if we take this down to the non conscious interaction leve, there is no extension of A's interaction to B's location.

Patrick, RQM is not just trivially stupid, any more than MWI is.

 

[vanesch]

No way! A doesn't have this controlled knowledge about B. B could have been called away; a cosmic could have struck the equipment and fouled up the data, anything else might have happened. A and B are spacelike related and just because they both have the entangled particles from the intersection of their past light cones doens't mean they can overcome the realities of noncommunication.

The conclusion doesn't change when you add "B didn't perform a measurement as anticipated". The "others" catchall. This doesn't solve the issue, you know, it makes it WORSE. You now have an extra positive term to add to a sum which was already too big.

So A can't draw up a meaningful exhaustive set of alternatives for B, and therefore he can't compute genuine probabilities for B.

But nevertheless, later, he can find them out. After having found out for the 10000th time (upon meeting with B, and separating again) that B usually DOES measure the outcomes, usually isn't hit by a comet hitting his lab, usually does what they agreed about, the Bayesian probability of "for one reason or another, B didn't perform the experiment as anticipate" is very small. One could assume that if the experiments are often enough repeated, that one starts to see a pattern in them, and that all these catastrophic events such as B falling in a stellar black hole hitting his lab are just excuses which are highly improbable, or at least smaller, say, than 5%. And, as I said, this doesn't solve the issue, it makes the Bell inequalities even more severe.

B's reality is NOT present for A. A doesn't deny that B has one (although, you know, B could have died...) but it means nothing to him as an experimenter. And if we take this down to the non conscious interaction leve, there is no extension of A's interaction to B's location.

Yes, that's what I said. If you take this logic to the end, you can even derive that a FTL phone is working according to local physics. The "cousin calling" means nothing to him, his phone is ringing locally, he just writes down locally when it rings, and when the cousin arrives, and they compare, well, they just see that there's a correlation between the cousin deciding to ring, and your phone ringing. No problem. No non-locality.

Patrick, RQM is not just trivially stupid, any more than MWI is.

RQM is not stupid if it confines itself to "subjective realities" which are entirely separated, and attached individually to an observer. But then it is - using the term maybe a bit loosely - solipsist.

RQM *is* trivially stupid if it wants to make these "subjective realities" coincide as "objective reality" when "observers meet", because they never meet, or there is a logical flaw, because, as I said, the used logic can then be used to demonstrate the locality of the physics of a FTL telephone.

The point always being the same:

if, in observer A's "reality", the quantum description tells him that he will see with 50-50% probability a pink or a blue elephant tail, and that in observer B's "reality", the quantum description tells him that he will see with 50-50% probability a pink or a blue elephant head,
and if A makes an observation, this CHANGES B's probabilities from 50-50, into 100-0, then there are only two explanations possible: or "A's observation influenced somehow what happened at B" (which is a non-local interaction), OR there was a common reality to both and the 50-50 probability were probabilities of ignorance of a pre-existing underlying reality. But Bell's theorem tells us that there are more complicated situations so that this last case cannot be the explanation.
The verbal exercise of saying that "A's result has no meaning for B" as long as they don't meet makes us then even prove the locality of a genuine FTL telephone. So this doesn't fly.

Then, what remains ? Well, that observer A has 1) seen a result at his side (pink tail), and 2) has seen a result when B came to see him (pink head) and 3) that these two are in agreement to A (both pink).
The above is the content of "A's reality". This is what A thinks is "objectively real" because he sees B agree with him.

Observer B has 1) seen a result at his side (blue head) and 2) has seen a result when A came to see him (blue tail) and 3) that these two are in agreement for B (both blue).
The above is the content of "B's reality". It is what B thinks is objectively real because he sees A agree with him.

But there's no way for them to be in sync without some verbal hocus pokus which jumps fro and back of "objective/subjective" and "real / not real", and "meaningful/not meaningful" but the point is:

if both have to objectively agree upon having seen a pink elephant, there's no escaping of the probability, upon their measurement, in their own reality, to BE INFLUENCED by the outcome of the other, and we know that this cannot be explained by an a priori correlation of which there was ignorance (= Bell's theorem). If such an influence is talked away, then we can even talk away in the same way any non-locality in a genuine FTL telephone.

So, there are only two ways out: or there IS such an influence and it is hence non-local, or we are only talking about a reality subjective to A (and if you want to, another one subjective to B, which is entirely different and not compatible with the first).

What is sneaked in without any justification in the expositions of RQM, is that the result of B, seen by A when meeting B, and the result of B, seen by B, are the same, and the proof used (equations (6) and (8) we've been talking about0 only prove the former, not the latter.

 

[selfAdjoint]

But nevertheless, later, he can find them out. After having found out for the 10000th time (upon meeting with B, and separating again) that B usually DOES measure the outcomes, usually isn't hit by a comet hitting his lab, usually does what they agreed about, the Bayesian probability of "for one reason or another, B didn't perform the experiment as anticipate" is very small. One could assume that if the experiments are often enough repeated, that one starts to see a pattern in them, and that all these catastrophic events such as B falling in a stellar black hole hitting his lab are just excuses which are highly improbable, or at least smaller, say, than 5%. And, as I said, this doesn't solve the issue, it makes the Bell inequalities even more severe.

This is Laplace's famous argument for the Bayesin probability that the Sun will come up tomorrow; it has been roundly derided by nearly every serious student of probability since! Are you now introducing it into your analysis of QM?

And I do want to object again: you keep casting the discussion in these social terms, perhaps because you want to have consciousness built into QM, but what I want to talk about is what RQM, and the paper under discussion says. According to my reading, interaction B is a real fact of the universe, which is maybe relevant to interaction A, but the outcomes (measured observables) and their amplitudes and probabilities are NOT. The only outcomes and probabilities that are relative to interaction A are A's own.

So all of this B woulda-coulda analysis just doesn't arise in the context posited by S&R.

 

[vanesch]

And I do want to object again: you keep casting the discussion in these social terms, perhaps because you want to have consciousness built into QM, but what I want to talk about is what RQM, and the paper under discussion says. According to my reading, interaction B is a real fact of the universe, which is maybe relevant to interaction A, but the outcomes (measured observables) and their amplitudes and probabilities are NOT. The only outcomes and probabilities that are relative to interaction A are A's own.

So all of this B woulda-coulda analysis just doesn't arise in the context posited by S&R.

Let's repeat my viewpoint here. I'm not criticising RQM per se, it depends upon what one makes of it. I'm criticising the statement that RQM gives a non-solipsist viewpoint that resolves the preferred basis problem and the non-locality problem. It doesn't any of these things. The only thing it takes away is an objective description of the world, and hence has to rely on non-physical subjective worlds containing "information" which are not coded in any physical state.

There are indeed two faces to the measurement problem: the first one is the different treatment between "observer" and "observed" (some aspect of it is the preferred basis problem) and the second one is the non-local correlations that seem to be generated by it.

These are the issues that any view on QM has to address.
In the different families of interpretation, these issues are treated in different ways.

1) in Copenhagen, the first issue is dealt with, by making a distinction between macroscopic, and hence classical objects versus quantum objects. Classical objects have no preferred basis problem because we SAY what are the preferred states: the classical states. The second issue is dealt with by introducing a genuinly non-local process, which is the projection. So, in Copenhagen, QM is nor universal, nor local.

2) there are people who say that QM is just a way of calculating probabilities of outcomes of observation, not an attempt of a description of nature. As such, the concepts of "observer" and "observation" remain outside of a physical description and are based, in the end, upon entirely intuitive conceptions. It is left totally open how to resolve the preferred basis problem: a measurement apparatus that is supposed to measure "position" simply measures "position", because the salesman told you so. There's no detailled physical analysis possible of why this apparatus measures position and not momentum. The only analysis possible is by analysing how the apparatus WOULD work if things were entirely classical. And they don't solve any issue with non-locality as per Bell. One can hardly call such a view "complete", it is just an operational definition of a calculational algorithm, and the algorithm indicates that non-local things go on. One has to introduce by hand what hermitean operator (preferred basis) corresponds to what apparatus, inspired by what the apparatus could do if all things were classical. But as the algorithm has not the pretention to be the description of any phenomenon, it's left to our guessings what is the source of this non-locality (or even of what's nature like, or what our observations are supposed to mean).

3) in MWi-like views, QM is said to be universally applicable, and to be a description of nature. It could pretend to completeness this way, if the issue of what exactly is an observer is dealt with. The concept of observer is now redefined into a set of specific states associated to a subjective experience. It has the merit of being able to explain the *observation* of apparently non-local correlations due to a totally local process. MWI as such, explains why QM acts as an algorithm to produce probabilities of observation from the point of view of an observer which is picked randomly to be one of the states according to the hilbert norm of the term in which it occurs. The price to pay is that there are now multiple potential "observer-subjective realities" and one objective reality.
I noted that Rovelli (as many others btw) think that a "many minds" version is a kind of Copenhagen version, because "minds" are classically-like states - but this is missing the point: minds are not physical objects (minds are not brains, but specific brain-states to which a subjective - non-physical experience is associated, introducing hence a dualist vision).

When looking at RQM, none of the above issues is actually specifically handled.

RQM starts with some postulates concerning "information" an "observer" might possess. However, this begs the question: can an "observer" (being a quantum object) possesses specific information as a bit stream ? Possessing information means: being in one state of several (of a finite number of possibilities), and hence certainly NOT in a superposition or anything of the kind.
As such, the "information" of an observer associated with an object has nothing to do with any "state" of the object, because if it were, the information would be CODED in the physical state of the object, something which is forbidden. As such, the observer is a kind of subjective world ASSOCIATED with a physical object, in exactly the same way as in MWI.
We have to see an "observer" OA attached to a physical object as the information possessed by that physical object A. However, this information is of course NOT coded in the state of the object, because that would mean that the object has an objective state which it is supposed not to have. So this information is present only in a kind of subjective world attached to an object but certainly not in the object itself and there is no physical description of what aspect this information/memory/... might take. All that has been done is the setup of the information that this subjective world has as a result of interactions of the object with the rest of the world (but which, I repeat, is NOT stored in the object itself as a physical state).

If now ANOTHER object B, with ANOTHER observer OB attached to it, interacts with object A, then at most, observer OB gets information about the object A, but not of any "observer" attached to it (given that the information is not coded in a physical state of any object).

Now, my point is: this subjective world OB which possesses information, when asking things about OBJECT A, will in principle not get any answers from the OBSERVER OA, but only from the object A itself according to an independent probability generation (using the formalism of quantum theory). As such, there's no reason to assume that there is any synchronisation between any "information" OA might have, and any answers OB might obtain when asking them to OBJECT A.

This is what I meant: OA might have seen a blue tail and then ask object B what head it saw, and get an answer "blue head", while OB might have seen a pink head, and when asking object A what tail it saw, get the answer "pink tail".

As such, there is in the subjective (information) world of OA, of course OA's information itself and a kind of illusion of what "an" OB saw. In the same way, in the totally distinct subjective world OB, there is of course the information of OB, and the illusion of what "an" OA saw. And these two have a priori nothing to do with one another.
And this is exactly the description MWI gives, except that MWI DOES give a physically objective description of the information possessing unit: namely the specific quantum state of object A or object B, which is entangled with the observer state we are considering, while this is denied to exist in RQM: there are just these subjective, intuitive "information" worlds associated with each object that might serve as an observer, and OF WHICH THE INFORMATION IS NOT CODED IN ANY OBJECTIVE STATE (and as such, NOT accessible to another physical object through a physical interaction).

So the fallacy in this entire construction is to say that there is some kind of objectivity (even though systems don't have objective states, and hence don't have objective memories, and hence don't have objective information) to the "agreement between observers" because in each individual subjective world there seems to be agreement with its own illusions, and the object of this agreement is different in each subjective world.

The only way to have these two subjective worlds agree, is by making their information physically available in an objective state. But this then misses the entire point of RQM.

Now, you accuse me of dragging in "consciousness" in the RQM discussion, but there's no way out: if there is no objective state to be attached to an object, which might correspond to the information possessed in the whole discussion, by said object, then there is no objective physical container for that information, and we are hence FORCED to consider a subjective world that goes with it (call it a consciousness if you want) and where this information "lives", so to say, as there's no room for it in any objective state.

And from the moment that this information IS objective and physical, so that it CAN be asked by another observer to enter its information world, then the entire issue of non-locality rears its ugly head again, because then the answers HERE depend on what objective information was obtained THERE in response to what question.

Really, there's no way around: locality, the predicted probabilities by QM, and Bell's theorem are ONLY compatible when there are multiple sets of results, a potentially different one for each observer. Of course, if there is only one observer, there's no issue. But when there are 2 observers, any talking away of the issue also talks away the non-locality of a FTL telephone.
What RQM describes is ONE such set. As such, it is entirely correct, but needs intuitive concepts for which it doesn't give a physical description, such as "information possessed by an observer" and "the questions asked by the observer to the system" (this last thing is another way of introducing a preferred basis, as in Copenhagen).
From the PoV of one such set, the "rest of the world" can indeed be described as a quantum state to which the Born rule is applied when asking the questions.

MWI gives you exactly all that, PLUS an "objective description of reality", PLUS a description of the state of an observer, plus a way to investigate in an objective physical process that happens during "measurement" (something that doesn't make sense in RQM: an observer just "gets answers to its questions, and how this answer comes about is left in the middle).

So, again, I fail to see what issue RQM resolves, apart from the points that were already dealt with from a single-observer viewpoint in MWI, with which I can identify it, if we leave out all the "objective" talk.

In fact, I would even see RQM as a precursor to MWI. Indeed, let us follow the following reasoning: we take as a first viewpoint, that there is a subjective experience (a la Descartes that seems to correspond to "information" such an experience has about a hypothetical objective world.
And the subjective experience wonders if it can make sense out of any objectively existing reality.
All what is empirical is of course subjective, as in the end it is an experience by the subjective experience. Those empirical observations by the subject make it conclude to the fact that it can "know" stuff (first Rovelli postulate), a kind of complementarity (~ second Rovelli postulate) and the superposition principle (third Rovelli postulate). From this, the subjective experience can deduce an algorithm (= formalism of quantum theory) to calculate probabilities of his future answers to empirical questions, all still within its subjective existence of course.

This is where RQM stops. It explains simply the algorithm the subjective experience has of the probabilities of its future answers to questions. But the subjective experience doesn't know whether this means anything, beyond its own experience.

But we could go further:
The subjective experience wonders if there can be a logical construction which might describe an objective reality from which its subjective experiences are drawn, and then wonders if the "algorithm" could not somehow correspond to some objective world out there.
And lo and behold, it can ! It is sufficient to call a certain set of experiences corresponding to certain answers, his preferred states, of which he experiences (of course) one, and say that there's an objective world corresponding to a unitary structure. If he then draws this conclusion to the end, he arrives at MWI.
There still remains something of the irreducible character of subjective experience in this model, in that the subjective experience now has to identify itself with specific states in the overall wavefunction and declare the probability of being one of the several possible to be the hilbert norm, but this is way better than not assuming even the existence of anything outside its own subjective existence, no ?

 

[selfAdjoint]

Let's repeat my viewpoint here. I'm not criticising RQM per se, it depends upon what one makes of it. I'm criticising the statement that RQM gives a non-solipsist viewpoint that resolves the preferred basis problem and the non-locality problem. It doesn't any of these things. The only thing it takes away is an objective description of the world, and hence has to rely on non-physical subjective worlds containing "information" which are not coded in any physical state.

We have been around the barn on the solipsism issue. I have quoted you from Smerlak and Rovelli's paper that they assert interactions are real. Real means not just in someone's consciousness (solipsism) or in some hypothetical universal consciouness (idealism) but out there in the world. You keep ignoring these facts and reasserting solipsism. I suppose you will just continue to do so, but I have to say frankly that I don't think you stubborn refusal to acknowledge the facts of the paper you are discussing does you any credit.

As to the basis problem, it's true that S&R don't discuss it, but Paul Merriam in his paper I keep touting, quant-ph/0506228, claims to solve it in the context of RQM. In my own mind I always apply his thought when I'm thinking about RQM; thus when two syst4em interact, the result relative to one are different from the results relative to the other, and this is at least partly because each has to experience the other's dimension, L, M, and T, as transformed in order to keep the experience based on the correct vaue of Planck's constant.

And as to non-locality, I still don't see where locality fails in RQM. If you don't talk about conscious observers and "learning" I don't see any basis for non-locality: spacelike related systems can't interact; what happens in one interaction can't affect another that is spacelike to it. Both of them can result in interactions down in the intersection of their future light cones, however.

There are indeed two faces to the measurement problem: the first one is the different treatment between "observer" and "observed" (some aspect of it is the preferred basis problem) and the second one is the non-local correlations that seem to be generated by it.

My own thought is that two interacting systems are on the same footing; To call one of them "observer" and the other "observed" is to introduce concerns that do not inhere in the problem. WHat I would like to see discussed in how RQM+Merriam fails to solve the problems, if it does.

These are the issues that any view on QM has to address.
In the different families of interpretation, these issues are treated in different ways.

1) in Copenhagen, the first issue is dealt with, by making a distinction between macroscopic, and hence classical objects versus quantum objects. Classical objects have no preferred basis problem because we SAY what are the preferred states: the classical states. The second issue is dealt with by introducing a genuinly non-local process, which is the projection. So, in Copenhagen, QM is nor universal, nor local.

2) there are people who say that QM is just a way of calculating probabilities of outcomes of observation, not an attempt of a description of nature. As such, the concepts of "observer" and "observation" remain outside of a physical description and are based, in the end, upon entirely intuitive conceptions. It is left totally open how to resolve the preferred basis problem: a measurement apparatus that is supposed to measure "position" simply measures "position", because the salesman told you so. There's no detailled physical analysis possible of why this apparatus measures position and not momentum. The only analysis possible is by analysing how the apparatus WOULD work if things were entirely classical. And they don't solve any issue with non-locality as per Bell. One can hardly call such a view "complete", it is just an operational definition of a calculational algorithm, and the algorithm indicates that non-local things go on. One has to introduce by hand what hermitean operator (preferred basis) corresponds to what apparatus, inspired by what the apparatus could do if all things were classical. But as the algorithm has not the pretention to be the description of any phenomenon, it's left to our guessings what is the source of this non-locality (or even of what's nature like, or what our observations are supposed to mean).

3) in MWi-like views, QM is said to be universally applicable, and to be a description of nature. It could pretend to completeness this way, if the issue of what exactly is an observer is dealt with. The concept of observer is now redefined into a set of specific states associated to a subjective experience. It has the merit of being able to explain the *observation* of apparently non-local correlations due to a totally local process. MWI as such, explains why QM acts as an algorithm to produce probabilities of observation from the point of view of an observer which is picked randomly to be one of the states according to the hilbert norm of the term in which it occurs. The price to pay is that there are now multiple potential "observer-subjective realities" and one objective reality.
I noted that Rovelli (as many others btw) think that a "many minds" version is a kind of Copenhagen version, because "minds" are classically-like states - but this is missing the point: minds are not physical objects (minds are not brains, but specific brain-states to which a subjective - non-physical experience is associated, introducing hence a dualist vision).

When looking at RQM, none of the above issues is actually specifically handled.

Yes, because it is about (at least I believe it is about) systems rather than observers, and so doesn't rest on "entirely intuitive conceptions".

RQM starts with some postulates concerning "information" an "observer" might possess. However, this begs the question: can an "observer" (being a quantum object) possesses specific information as a bit stream ? Possessing information means: being in one state of several (of a finite number of possibilities), and hence certainly NOT in a superposition or anything of the kind.

The states of each of the interacting systems were in superposition before the interaction, and both superpositions were reduced as a result of the interaction. Is that problematic?

I have an appointment, and will have to defer reacting to the rest of your very clear and comprehensive post. But I will, unless the conversation has moved on while I'm away from the computer. Catch you later!

 

[vanesch]

We have been around the barn on the solipsism issue. I have quoted you from Smerlak and Rovelli's paper that they assert interactions are real. Real means not just in someone's consciousness (solipsism) or in some hypothetical universal consciouness (idealism) but out there in the world. You keep ignoring these facts and reasserting solipsism. I suppose you will just continue to do so, but I have to say frankly that I don't think you stubborn refusal to acknowledge the facts of the paper you are discussing does you any credit.

I stubbornly refuse to acknowledge this, because it is contradictory to part of the exposition of RQM. It is true that R&S never say this explicitly (and rather try to argue the contrary, namely that 'shared observations are real'), but I consider that as verbal exercise by the marketing department in order to sell the view for more than what it is. True, all this is my interpretation of it, but it is because I try to read the logic behind Rovelli's paper.

Most of the time, Rovelli argues from the point of view of a single observer. His entire construction, from his three postulates, about the probability rules of things the observer observes, are from a single-observer viewpoint. He derives that a single observer will derive something like a quantum theory for the probabilities of the answers he'll observe to questions he'll ask. Fine.
Now, where does this observer STORE all his answers ? In his "physical state" (as would be in any classical theory) ? Rovelli EXPLICITLY tells us that the "state of a system" (and hence also of an observer) is an ENTIRELY OBSERVER-DEPENDENT thing, and that there is NO SUCH THING AS THE PHYSICAL STATE OF A SYSTEM.

So, this "observer" stores his history of answers in "his own physical state WITH RESPECT TO HIMSELF", but certainly not in an objective physical state, the existence of which is explicitly forbidden.

As such, THIS INFORMATION IS TOTALLY INVISIBLE FOR ANY OTHER OBSERVER. For if it were, both would SHARE the same state description of the first observer, and it would be the TRUE state of the first observer.

So THIS INFORMATION IS NOWHERE PHYSICALLY PRESENT as an objective quantity: it is explicitly forbidden by Rovelli to do that. As such, there is no hope for the second observer, which has ANOTHER STATE DESCRIPTION of the former observer, which DOES NOT CONTAIN THIS INFORMATION, to obtain as answers to his questions about it, the same answers as those that the first one got.

As such, the answers that the first observer has, in his "information base", are only exclusively for him and there is no physical trace for them for a potential second observer. This second observer will get OTHER answers, which are entirely in agreement with OTHER data this second observer might have from other questions elsewhere, BUT THERE IS NO POTENTIAL FOR AGREEMENT with the former observer, because each of them lives in their own little subjective world of answers to questions, of which the data are NOT stored in any objective physical state (and hence not accessible by another physical entity).

As such, the entire story of RQM makes perfect sense to me, but I call that "solipsist", because each observer lives entirely in his own locked-up world, and his communication with eventual other observers does not pertain to THEIR eventual subjective world.
What I find in the limit intellectually unfair, is the sustained CONFUSION which is held up, between "the second observer OBSERVES the first observer to agree with him" (only in this second subjective world) - which I think is true in the RQM view ; and "the second observer AGREES with the first one", where suddenly an access is made, so that the second observer DOES have access to the first observer's "information state" although this is on the other hand strictly forbidden.

It is from this confusion that the "definition of objective reality" as "that what all observers agree upon" is then derived, but it is entirely based upon the fallacy that they have (although it is forbidden) access to the "information states" of each other, while these information states are normally only "relative to themselves", and NOT objective physical states relative to others.

This verbal extension of RQM, which tries to avoid the "solipsist" character of RQM, and which tries to make the reader believe that everything is finally all right and objective on an "everyday level" is what I'm objecting against. You understand why: it is exactly this kind of "everybody in his own little subjective world" viewpoint which makes MWI unattractive, and I claim that, as far as RQM makes sense, it has the same (if not worse) difficulty. I say, "if not worse" because at least, MWI ALSO gives an objective state description from which the "illusions" can be derived, while RQM does not even do that.

As to the basis problem, it's true that S&R don't discuss it, but Paul Merriam in his paper I keep touting, quant-ph/0506228

I'll have a look at it. Hope you won't send me from paper to paper

And as to non-locality, I still don't see where locality fails in RQM. If you don't talk about conscious observers and "learning" I don't see any basis for non-locality: spacelike related systems can't interact; what happens in one interaction can't affect another that is spacelike to it. Both of them can result in interactions down in the intersection of their future light cones, however.

Ok, but then, as I pointed out, Faster-than-light telephones are local too, in this vision of "locality", because it can only be verified that what the FTL telephone did at event A, was in sync with what was decided at B, when a and b come together in C. As long as a and b themselves cannot travel faster than light, A and B will be in C's past lightcone, and hence the OBSERVATION of the correlation between B's decision, and A's observation will only happen when A and B are in the past lightcone of C.


Now, maybe I'm misreading RQM, and, after all, there is some form of "externality" to the "answers" a certain observer has (although it is then not present in his state, but a little bit it should, ... don't get it).
Now, IF the answers an observer has ARE available to others (though they are NOT coded in their quantum description of the other observer), then they HAVE some objective existence to them, and then they ARE subject to Bell's results.

If that's the case, the entire "avoidance reaction" of non-locality is in vain, because the answers B got at the other place DID exist before A met B. In that case, RQM is not better off than Copenhagen, concerning non-locality.

My own thought is that two interacting systems are on the same footing; To call one of them "observer" and the other "observed" is to introduce concerns that do not inhere in the problem. WHat I would like to see discussed in how RQM+Merriam fails to solve the problems, if it does.

I'll have a look at it.

 

[selfAdjoint]

I stubbornly refuse to acknowledge this, because it is contradictory to part of the exposition of RQM. It is true that R&S never say this explicitly (and rather try to argue the contrary, namely that 'shared observations are real'), but I consider that as verbal exercise by the marketing department in order to sell the view for more than what it is. True, all this is my interpretation of it, but it is because I try to read the logic behind Rovelli's paper.

I have to admit that you and I are looking at the "elephant" of RQM and seeing different objects! I will try to reply to your points on solipsism in this post.

Most of the time, Rovelli argues from the point of view of a single observer. His entire construction, from his three postulates, about the probability rules of things the observer observes, are from a single-observer viewpoint. He derives that a single observer will derive something like a quantum theory for the probabilities of the answers he'll observe to questions he'll ask. Fine.
Now, where does this observer STORE all his answers ? In his "physical state" (as would be in any classical theory) ? Rovelli EXPLICITLY tells us that the "state of a system" (and hence also of an observer) is an ENTIRELY OBSERVER-DEPENDENT thing, and that there is NO SUCH THING AS THE PHYSICAL STATE OF A SYSTEM.

I am dubious about this description of $$\psi$$ myself, because I have trouble translating it into nonconscious processes. Particularly is don't like the reference to Bayesian probability and Jaynes, for the same reasons. I would rather have had him say "for any two interacting systems A & B, and any observable that is the subject of the interactions, there is a density matrix, derivable from the states of A relative to B and the states of B relative to A, and the algebra of building the density matrix is such that we can represent it by actions on the complex square root of its entries". Or something like that. The point being that each component system of the interaction can only interact on the basis of what it experiences in the other system, that is on the states of the other systrem relative to it. And if you take this seriiously you should be able to build the quantum algebra from it.

But of course this is my construction, and not what the authors said., so I will give you that it is possible to read their words and derive solipsism.

So, this "observer" stores his history of answers in "his own physical state WITH RESPECT TO HIMSELF", but certainly not in an objective physical state, the existence of which is explicitly forbidden.

As such, THIS INFORMATION IS TOTALLY INVISIBLE FOR ANY OTHER OBSERVER. For if it were, both would SHARE the same state description of the first observer, and it would be the TRUE state of the first observer.

The states are only real in the interaction. And each component system reacts to the states of the other, but really the interaction is the reality. If this is so, then in order to have something to bring to the next interaction with another system, each system must be interacting with itself. Conceivably these could be virtual interactions, like the electron continuously emitting and reabsorbing virtual photons.

So THIS INFORMATION IS NOWHERE PHYSICALLY PRESENT as an objective quantity: it is explicitly forbidden by Rovelli to do that. As such, there is no hope for the second observer, which has ANOTHER STATE DESCRIPTION of the former observer, which DOES NOT CONTAIN THIS INFORMATION, to obtain as answers to his questions about it, the same answers as those that the first one got.

Well of course I deprecate logic based on "questions about". There are no relevant "questions" except in the context of an interaction. And within that context (although the authors aren't too explicit about this), the relational information seems coherent to me.

As such, the answers that the first observer has, in his "information base", are only exclusively for him and there is no physical trace for them for a potential second observer. This second observer will get OTHER answers, which are entirely in agreement with OTHER data this second observer might have from other questions elsewhere, BUT THERE IS NO POTENTIAL FOR AGREEMENT with the former observer, because each of them lives in their own little subjective world of answers to questions, of which the data are NOT stored in any objective physical state (and hence not accessible by another physical entity).

As such, the entire story of RQM makes perfect sense to me, but I call that "solipsist", because each observer lives entirely in his own locked-up world, and his communication with eventual other observers does not pertain to THEIR eventual subjective world.
What I find in the limit intellectually unfair, is the sustained CONFUSION which is held up, between "the second observer OBSERVES the first observer to agree with him" (only in this second subjective world) - which I think is true in the RQM view ; and "the second observer AGREES with the first one", where suddenly an access is made, so that the second observer DOES have access to the first observer's "information state" although this is on the other hand strictly forbidden.

It seems there are two levels here that may be confused. On the one hand we have A and B as interacting systems as I said above. On the other we have A and B as spacelike separated system interacting with separate components of an entangled system. Use curly braces to denote an interaction, then $${A,\alpha}$$ bears no relation, i.e. does not in any way influence, $${B,\beta}$$. Comparison of notes will have to wait for later. When that later interaction {A,B} happens, it will be found that the states agree.

It is from this confusion that the "definition of objective reality" as "that what all observers agree upon" is then derived, but it is entirely based upon the fallacy that they have (although it is forbidden) access to the "information states" of each other, while these information states are normally only "relative to themselves", and NOT objective physical states relative to others.

I think that the demonstration in section 4.32 that a third system C would get the same results as A and B agree on is a valid basis for consensual objectivism. I don't think of $$psi$$ as requiring, or supporting, real states in the universe. When you try to postualte that you get a differenct "branch of reality" for each superimposed state, which I think grossly weakens the definition of reality.

Here's an example from algebra: a cubic equation can have three real roots (the so-called irreducible case) and we can imagine some such equation describing some real trifold property of classical reality. But the solution of the equation requires us to represent those three real numbers as the differences of complementary complex numbers. The case is very similar to how the eigenvalues of Hermitian operators come out real. Now what is the status of those complex numbers? I certainly don't think they express any physical reality by themselve, but when they are combined according to the solution algorithm they do. Does the universe split into three parts to accommodate them?

This verbal extension of RQM, which tries to avoid the "solipsist" character of RQM, and which tries to make the reader believe that everything is finally all right and objective on an "everyday level" is what I'm objecting against. You understand why: it is exactly this kind of "everybody in his own little subjective world" viewpoint which makes MWI unattractive, and I claim that, as far as RQM makes sense, it has the same (if not worse) difficulty. I say, "if not worse" because at least, MWI ALSO gives an objective state description from which the "illusions" can be derived, while RQM does not even do that.

I think the three postulates, and the discussion in section 4.2, do hang together and generate a coherent theory. Much of the discussion elsewhere in the paper strikes me as a rhetorical attempt to persuade, with quotations from famous thinkers and all. You read this closely and find it incoherent, I just blow by it and concentrate on the firm analysis.

 

[vanesch]

I have to admit that you and I are looking at the "elephant" of RQM and seeing different objects! I will try to reply to your points on solipsism in this post.

To summarize our differences: Rovelli (and you) accept the statement that "everybody sees the same elephant", while I think that what has been demonstrated is only:
"everybody sees everybody agree on the same elephant"

The difference is of course subtle, but in the last case, there's no reason that everybody really agrees upon the same elephant, but only has a consistent picture of its interactions with others, and is led to believe that they saw the same elephant. As such, everybody can see a different elephant, and different "others", and nevertheless be convinced, each one in his own world, that everybody agrees with him.

This is what is REALLY demonstrated (by those equations (6) and (8)). This is the internal consistency (which was already noted by von Neumann) of observation.

But I'm not basing myself on any specific Hilbert formulation of the issue to derive this, I base this only on the rejection of the notion of objective state of an observer. Only an objective state of an observer could contain the necessary information about what he really observed, and be accessible through interaction to another observer, and this objective state is denied to exist (not only because it would wreck havoc to the claim that, for another observer, the former observer is a quantum object, and hence DID NOT pick a particular outcome, but also, to avoid the problem with Bell's inequalities).

So if the state a|o1>|s1> + b|o2>|s2> from P's PoV and |s1> from O's PoV are BOTH correct descriptions then one can say two possible things:
these are only descriptions of OUR KNOWLEDGE. Indeed, P doesn't know yet what O did, so is still in "suspension".
Or we can say that S (and O) simply do not HAVE an objective state.

It was my understanding that R&S go for the latter, and they have to, because the former would only be a description of KNOWLEDGE, not of physical state, and would not stop us from ASSUMING that there is an objective physical state. This assumption would then wrek havoc for as well the "all quantum" view, as for Bell. Also the claim to completeness goes in this direction.

Now, is a |o1>|s1> + b |o2> |s2> is the best we can do, from P's PoV, not only to describe any knowledge, but of what could potentially be the physical state, then *there is no way for P to pick |o1>|s1> for sure*.
This happens only with probability |a|^2. There's no physical mechanism by which S can "inform" P about its observation of o1, because, from P's PoV, both possibilities are still present and they are NOT about ignorance, they are really physically present.

That's why I don't see how one can claim that P will obtain |o1>|s1> for sure, if S has picked |s1>.

What has been demonstrated, however (equations 6 and 8 you quoted way back in this thread), is that P will observe only o1 together with s1, OR o2 together with s2. So P will find "agreement" between his observation of O and of S, and hence deduce that "S agrees with him". But he doesn't know what S observed, really, because this information is not "physically present" BY DEFINITION. If it were physically present, the STATE for P would NOT be a |o1>|s1> + b |o2> |s2>, but rather |o1> |s1> with (Bayesian) probability |a|^2 and |o2>|s2> with (Bayesian) probability |b|^2, where the Bayesian probabilities do not descibe an objective state, but only P's knowledge.
The objective state would of course be |o1>|s1>, but P would simply be ignorant of it.
And we're back to Copenhagen.

Well of course I deprecate logic based on "questions about". There are no relevant "questions" except in the context of an interaction. And within that context (although the authors aren't too explicit about this), the relational information seems coherent to me.

I was only referring to Rovelli's "vector of answers to binary questions" on which he builds a quantum theory as describing the correct probabilities for the next answer. So the only thing that exists here, are "answers to binary questions".

It seems there are two levels here that may be confused. On the one hand we have A and B as interacting systems as I said above. On the other we have A and B as spacelike separated system interacting with separate components of an entangled system. Use curly braces to denote an interaction, then {A,\alpha} bears no relation, i.e. does not in any way influence, {B,\beta}. Comparison of notes will have to wait for later. When that later interaction {A,B} happens, it will be found that the states agree.

Yes, but that's "by sheer magic" if postulated, because IF indeed {A, \alpha} is independent of {B,\beta} there's no reason for them to agree afterwards.
So, or they don't agree (but aren't aware of it), or they agree, and then the question is: given that {A,\alpha} and {B,\beta} didn't influence each other, how come ? The question has then not been solved at all.

You could say:
From A's PoV, only {A,\alpha} and {A,B} happen. They agree. This is found by the quantum formalism (where B is treated as a quantum system, which did NOT get an explicit answer back then, but just entangled).

From B's PoV, only {B,\beta} and {A,B} happen. They also agree. This is also found by the quantum formalism, where this time, A is treated as a quantum system which did NOT get an explicit answer back then, but just entangled.

However, there's no way to make both magically equal without saying that {A,\alpha} REALLY resulted in one result or the other, and {B,\beta} REALLY resulted in one result or the other, and ALREADY AGREED BACK THEN. And then we're back home, with EPR correlations and all that.

I think that the demonstration in section 4.32 that a third system C would get the same results as A and B agree on is a valid basis for consensual objectivism.

Well, no. That's the point: it is only shown that C will OBSERVE A and B as having the same results.
A might observe C and B to agree also upon the same results (although they differ from the first set), and B might observe A and C to agree also upon the same result (which might still differ from the first and second set).

That is what is really demonstrated.

Assuming that they are equal is an extra assumption, which, IMO, goes against the statement that the observation result of B is not an objective result, coded into its state, for everybody.

That's the difference between "everybody observes the same elephant", and "everybody observes the others as observing the same elephant".

I think the three postulates, and the discussion in section 4.2, do hang together and generate a coherent theory. Much of the discussion elsewhere in the paper strikes me as a rhetorical attempt to persuade, with quotations from famous thinkers and all. You read this closely and find it incoherent, I just blow by it and concentrate on the firm analysis.

So do I. I think the derivation of the quantum formalism from the 3 postulates is nice, and coherent. But, as I said, this only pertains to the "answers" vector of a single observer, and nowhere the link is really made to the "answers vector" of another observer.
So it looks like me to be a theory about what an observer can know about the world.

 

[marcus]

...So it looks like me to be a theory about what an observer can know about the world.

Hi vanesch, there is a problem with word order in the last sentence of your post which makes it difficult to understand. Could be a simple typing error. Is this what you meant to say?:

...So it looks to me to be a theory about what an observer can know about the world.

If that is the desired meaning, then slightly more idiomatic ways to say it would be:

...So it seems to me to be a theory about what an observer can know about the world.

or

...So it looks to me like a theory about what an observer can know about the world.

(two common idioms are "looks like" and "seems to be")

 

[vanesch]

Hi vanesch, there is a problem with word order in the last sentence of your post which makes it difficult to understand. Could be a simple typing error. Is this what you meant to say?:

Yes, sorry. Some of my posts are typed while I'm reading simultaneously about dinosaurs for the kid and discussing with my wife

BTW, I just finished almost "Three roads to QG", and it occurred to me:
isn't the "solipsist" version of how I read RQM, just an extreme version of the holographic principle ?

 

[marcus]

Yes, sorry. Some of my posts are typed while I'm reading simultaneously about dinosaurs for the kid and discussing with my wife

BTW, I just finished almost "Three roads to QG", and it occurred to me:
isn't the "solipsist" version of how I read RQM, just an extreme version of the holographic principle ?

I too must sometimes quickly finish typing a post while I am discussing something with my wife.

about your BTW question:
you have to get selfAdjoint to comment on that.

My tentative opinion is like this: the holographic principle seems to be about INFORMATION and maybe also about boundaries or horizons.

the idea of a boundary or horizon is connected to the idea of an observer. you sort of define an observer, in some contexts, by putting a plastic bag around the system and say that the observer is somewhere outside that plastic bag. In a strange way, in some contexts but probabably not in others, the observer IS the plastic bag, and what she or he observes is the information that reaches the plastic bag and maybe comes through it.

so I have to answer I DONT KNOW if what you are asking about is the same, but I do understand you that the concern is about information (as I gather also is the central focus in quantum mechanics whether Carlian or not)

 

[vanesch]

In a strange way, in some contexts but probabably not in others, the observer IS the plastic bag, and what she or he observes is the information that reaches the plastic bag and maybe comes through it.

What I meant was the following. SA and you seem not to agree with me, but I think that the RQM formulation is entirely "observer" centered, and is about information an observer has, to himself, which is not available to eventual other observers.

Now, the (strong) holographic principle, as I understand it, comes down to saying that whatever we can know about anything, is entirely determined by the closed surface around us ; or even (strong version), that that is in fact the only thing that really is, and the "volume behind it" is just an illusion created by the information flowing in from the surface ; with suitable reformulation, all physics is the physics of the surface around us, observers.

Now, consider each "observer" in its own "plastic bag", thinking there's an entire universe behind it, with other observers walking around in it and all that, but he looks just at the physics of his plastic bag ; nevertheless, he'll see his "peer observers" on his plastic bag agree with him.

And so each observer lives in his own plastic bag, with his own "knowledge" and information flow coming off it, thinking there's an entire universe behind it.

This is then nothing but the "subjective worlds" I talked about - though now they become indeed "individually objective" (but surely with some form of solipsism to it).