Everybody sees the same elephant (says Carlo Rovelli)

[vanesch]

I imagine Rovelli would consider his picture "multiple-observer" and "single world".

Well, "multiple-observer/single world" is of course what we all would like to see because it fits much better with our intuitive concept of how the world ought to be (how many times have people not said "how the world ought to be" ), but it is now clearly established that QM's predictions for a multiple observer/single world are such that if there is to be given an objective picture behind this, that this picture is necessarily non-local (that's Bell's result in a nutshell). It is in essence the approach of Bohmian mechanics, or of "standard" von Neuman quantum theory (with collapse upon observation), if one assigns some objective reality to the wavefunction (which represents your single world). All of these approaches violate Lorentz invariance in their inner workings (in other words, require non-local mechanisms to act upon the object that is supposed to represent the single world).

An exception to this are the approaches that consider quantum theory as just a technique to calculate probabilities of outcomes of experiments, but I'd classify that rather as "multiple observers / no world" because these approaches explicitly forbid you to think of any of the mathematical constructions to represent an objective reality.

Rovelli picture is multiple-observer in the sense that it ALLOWS for having one or more observers. You can have one, or you can have 100. You have as many hilbertspaces as you do observers. Objective reality arises from the AGREEMENT of these guys. I personally see no connection with "many worlds".

If you have "a hilbert space for each observer", then what goes wrong is the following, if one assumes that each hilbert space is an "independent generator for probabilities", and if one assumes a continuity in an observer's knowledge (that is, if at a point t1 on his world line, observer A measured result A+, then at a point t2>t1 on his world line, he will not suddenly conclude that back at t1 he measured finally A- when learning about another result).

Assume that the initial state is u |a+> |b-> + v |a->|b+> with u much smaller than v, and assume that A and B measure their particle's state "simultaneously" (in some frame), without contact.
This means, that in A's hilbert space, with an overwhelming probability, he's going to find A-, and in B's hilbert space, he's going to find B+. So indeed most of the time, after they meet, they will be in agreement.
But for the rare cases where A, in his hilbert space, finds A+, and registers this along his world line, there is no reason why B, in his *independent* hilbert space, will have to obtain the rare event B-. If B's hilbert space is independent of A's, then B will most of the time, project on the B+ state (independent of what A found).
In the same way, in those rare cases where B projects upon B- in his independent hilbert space, there's no reason why A should not project on A- most of the time.
The individual statistics are all right of course, but when they meet, something goes wrong, if there is continuity of their registered measurement results, AT LEAST IF THERE IS ONLY ONE "WORLD" (that is, if each observer has his own, single, outcome). Because the single B that is around cannot "forget" having seen B+ when he meets A having seen A+.

This is exactly the problem that a many world approach fixes: from A's PoV, there are TWO "B" observers, one of which he'll meet, and from B's PoV, there are TWO "A" observers, one of which he'll meet. And during the meeting, only those "versions" of observers can interact that are in agreement (that is, in the rare cases A has seen the A+ outcome, he'll only interact with the version of the B observer that has seen the B- outcome).

But these two meetings are not the SAME two observers: hence two different "worlds". This is what "goes wrong" with many worlds: the hard-to-swallow idea (however, entirely in agreement with subjective observation) that, when we "see an outcome" that this is just one of the several "me's" that sees an outcome, and that there all other possibilities are realized also, with "other me's". We'd intuitively like to have the case that there's only one "me" and that there's only one outcome.

However, if you restrict yourself to only one observer (say A) then all this is fine, because B is not really an observer, but just a quantum system as any other. As such, B has no "definite result" upon interacting with particle b, and B's state is simply a kind of copy of the state of particle b, which is waiting to be observed by A to take on a definite result - which is how I understand Rovelli: that, from A's PoV, B has not yet a definite result until it is observed by A. This is fine.

You cannot, however, require B NOT to have a definite result from A's PoV, and (1) B to HAVE a definite result from B's PoV, have these results being generated in (2) *independent* hilbert spaces, and hope that (3) the correlations upon meeting will come out all right. Something has to give.
If it is (1), well clearly we have only a single observer (B has no result) ;
If it is (2), well then there IS an "action at a distance" in some way in the inner workings to make the two hilbert spaces "dependent",
and if it is (3) then we're not reproducing the standard quantum theory correlations.

But if you see a connection with "many worlds" that is fine!

As I said, I see Rovelli's idea as the "single observer/single world" version of the "many observers/many worlds" view. I'm not sending out any critique, but I'm just saying that these ponderings are already around for at least 40 years, in several different forms and presentations.

 

[selfAdjoint]

if there is to be given an objective picture behind this, that this picture is necessarily non-local (that's Bell's result in a nutshell)

Well, this is the crux of the whole disagreement. Your understanding of "objective" is different from the relational one. Your view, call it QM with a priviledged observer, or QMPO says that before the PO "observes" the observed system it has no definite values in spacetime but after that priviledged act it does. RQM on the other hand says there are no priviledged observers and no priviledged values, but only interacting quantum systems. Bell and many since have conceiled this possibility from the reader by appeals to traditional classical images, and the tendency to talk about observers as people and give them names like Alice and Bob reinforces this misdirection.

If there is nothing but quantum interactions and those interaction are symmetric, changing the quantum state of both interacting systems, can there be any real objectivity? Smerlak and Rovelli say that thanks to QM the answer is yes!


SInce it is key to this discussion I have quoted the entire section 4.3 of the paper:

4.3. Consistency. Let us bring B back into the picture. It is far from the spirit of RQM to assume that each observer has a “solipsistic” picture of reality, disconnected from the picture of all the other observers. In fact, the very reason we can do science is because of the consistency we find in nature: if I see an elephant and I ask you what you see, I expect you to tell me that you too you see an elephant. If not, something is wrong. But, as claimed above, any such conversation about
elephants is ultimately an interaction between quantum systems. This fact may be irrelevant in everyday life, but disregarding it may give rises to subtle confusions, such as the one leading to the conclusion of nonlocal EPR influences. In the EPR situation, A and B can be considered two distinct observers, both making measurements on α and β. The comparison of the results of their measurements, we have argued, cannot be instantaneous, that is, it requires A and B to be in causal contact. More importantly, with respect to A, B is to be considered as a normal quantum system (and, of course, with respect to B, A is a normal
quantum system). So, what happens if A and B compare notes? Have they seen the same elephant?

It is one of the most remarkable features of quantum mechanics that indeed it automatically guarantees precisely the kind of consistency that we see in nature [5]. Let us illustrate this assuming that both A and B measure the spin in the same direction, say z, that is n = n′ = z.
Since B is a quantum system, there will be an observable S^n_{AB} corresponding to B’s answer (at time t1) to the question “which value of the spin have you measured?”. That is, S^n_{AB} is the observable describing the pointer variable in the detector B. Then consistency demands that:
(i) If A measures S^n_{AB} after having measured S^n_{A\beta} she will get

(4) S^n_{AB} = S^n_{A\beta}.

(ii) If a third observer C, who has the prior information that measurements have been performed by A and B, measures at a later time the two
pointer variables: S^n_{CA}and S^n_{CB} then

(5) S^n_{CB} = S^n_{CA}

But this follows from standard QM formalism, because an interaction between β and B that can be interpreted as a measurement is an interaction such that the state (1) and the initial state of α, β and B evolve into the state (relative to A)

(6) |\psi>^(A)_{\alpha+\beta+B} = \frac{1}{\sqrt{2}} (|\downarrow>_{\alpha} |\uparrow>_{\beta} |\uparrow>_B − |\uparrow>_{\alpha} |\downarrow>_{\beta} |\downarrow>_{B})

with obvious notation. Tracing out the state of α that plays no role here, we get the density matrix

(7)\rho^{(A)}_{\beta+B} = \frac{1}{2} (|\uparrow>_{\beta} |\uparrow>_B <\uparrow|_{\beta} <\uparrow|_B + |\downarrow>_{\beta} |\downarrow>_B <\downarrow|_{\beta} <\downarrow|_{B}) .

from which (4) follows immediately. Similarly, the state of the ensemble of the four systems α, β,A,B, relative to C evolves, after the two nteractions at time t0 into the state

(8)|\psi^{(C)}_{\alpha+\beta+A+B} = \frac{1}{\sqrt{2}}(|\downarrow>_{\alpha} |\uparrow>_{\beta} |\downarrow>_A |\uparrow>_B−|\uparrow>_{\alpha} |\downarrow>_{\beta} |\uparrow>_A |\downarrow>_{B})

again with obvious notation. Tracing out the state of α and β, we get the density matrix

(9) \rho^{(C)}_{A+B} = \frac{1}{2}( |\downarrow>_A |\uparrow>_B <\downarrow|_A <\uparrow|_B+|\downarrow>_A |\uparrow>_B <\downarrow|_A <\uparrow|_{B} ).

which gives (5) immediately. It is clear that everybody sees the same elephant. More precisely: everybody hears everybody else stating that they see the same elephant he sees. This, after all, is the best definition of objectivity.

Note that the particle states have been traced away in computing the density matrix. Only the observer states remain.

 

[marcus]

sA,
missing a minus sign in equation (5)
sometimes those things don't copy and paste right
worked pretty well though!

also in eqns. (6) and (8) the symbol "8722;" did not translate to a minus sign
(at least for me) but the reader can easily make that substitution

sA and vE, I will save a post by commenting on #104 here:
[EDIT]
Rovelli does not use the term "branch" and the term has no intuitive or widely understood meaning in the context of a discussion of the paper. Therefore we can test the logic of any comment on the paper by replacing every occurrence of the word "branch" by a neutral word like BLEXO.

Since (until defined) branch does not mean anything in connection with the paper it would have to be defined by the commentor in terms used in Rovelli's conceptual framework and shown to apply. one would need a definition of BLEXO and also to say what "same" BLEXO and "different" BLEXO mean.

In the Rovelli scheme all the observers are IN THE WORLD.
It is even meaningless to say they are IN THE "SAME" WORLD because there is just this world. There is not imagined to exist a different world so that one could meaningfully talk about "same" or "different".

The QM formalism as in Rovelli eqn (6) and (8) allows the individual observer to include different POSSIBILITIES in his or her state space but these do not correspond to REALITIES---possibilities in the Hilbertspace formalism are just a way of representing information and the lack thereof.
Let's now apply the test to see if this statement has anything to do with the Rovelli Smerlak paper:

"The consistency requirement that is worked out here in all detail, comes simply down to the MWI statement: observers in different blexo only learn about the answers of their peers in the SAME BLEXO."[/color]

I think if someone were to define for us (in another thread please) what a BLEXO is and what it means to be in a different Blexo and what it means to be in the same Blexo, that it might be quite interesting (for some people) to think about and discuss! But I regret to say that AFAICS talking about "branch" HAS NOTHING TO DO WITH ROVELLI'S PAPER.

He does not use the blexo concept. There is not even the "same" world: there is just the world

cordial thanks to all for the comments received so far on this paper!

 

[vanesch]

Well, this is the crux of the whole disagreement. Your understanding of "objective" is different from the relational one. Your view, call it QM with a priviledged observer, or QMPO says that before the PO "observes" the observed system it has no definite values in spacetime but after that priviledged act it does.

Well, I fail to see the "disagreement, Dick. In fact, I'm 100% "in sync" with what you quoted, and of course the essential point is when A "measures" the state of B with an observable that says "what did you, B, measure?"

The consistency requirement that is worked out here in all detail, comes simply down to the MWI statement: observers in different branches only learn about the answers of their peers in the SAME BRANCH.

In other words, if we end up with:

|alice+> |bob+> |stuffA> + |alice->|bob->|stuffB>

then this means that any such interaction when Alice asks bob what he measured, comes down to: alice+ will only hear from bob+, and alice- will only hear from bob- ; alice+ will never hear from bob-.

As such, indeed, "everybody" will see "the same elephant" ; that is, "alice+" will see the same elephant as bob+ (namely stuffA), and "alice-" will see the same elephant as bob-, namely stuffB.
It is implicitly assumed in MWI that this is the case of course, otherwise the view would be immediately falsified! The *entire idea* is indeed that though there is a "multiplication of worlds/branches/terms", nevertheless, each observer is "locked up" in its own term, ONLY being able to interact with its "peers" in the same branch, so that everything happens AS IF this branch was, to him, the only thing that existed, and the others "faded away".

It is my understanding that this is what is worked out in detail in the section you quoted: that observers in a branch only "see" their peers in the same branch, with "see" meaning: get their results through an interaction which is "what did you get as a result", and that they DON'T get any result of an observer in another branch. The wavefunctions in (6) and (8) are nothing else but what I write above here. That's really nothing
but the MWI version of the measurement situation, and that was what I tried to say here since the beginning: that Rovelli re-discovered MWI, or at least the aspect that each branch "only sees itself", and that there is a self-consistency in A measures system a, B measures system b, and then A measures B.

As to your comment, or I misunderstand you, or you misunderstand me :-)
In MWI, there is no "priviledged observer", because there are many copies of each observer, one in each branch. However, it was my understanding, maybe erroneously, that Rovelli takes it that there is only ONE version of each observer. Clearly that runs into troubles (and is in fact contradicted with his equations 6 and 8 from your quote): B cannot be "in a superposition wrt A" and have "a single and definite measurement" wrt itself (unless, and that's the entire MWI approach, B is dedoubled into two B's). So I took it that Rovelli saw everything from the PoV of observer A (which can then be thought of to be "single"), allowing B to be in superposition until A saw him (the kind of PO you are talking about). And from the PoV from B, we forget what we did for A, we now do things from the PoV of B, and now B is "single" and A is "in superposition".
Both viewpoints together of course correspond to A as well as B in "superposition" but agreement in the corresponding branches. Which is, to my understanding, nothing else but MWI...

 

[selfAdjoint]

Well, I fail to see the "disagreement, Dick. In fact, I'm 100% "in sync" with what you quoted, and of course the essential point is when A "measures" the state of B with an observable that says "what did you, B, measure?"

The consistency requirement that is worked out here in all detail, comes simply down to the MWI statement: observers in different branches only learn about the answers of their peers in the SAME BRANCH.

In other words, if we end up with:

|alice+> |bob+> |stuffA> + |alice->|bob->|stuffB>

then this means that any such interaction when Alice asks bob what he measured, comes down to: alice+ will only hear from bob+, and alice- will only hear from bob- ; alice+ will never hear from bob-.

As such, indeed, "everybody" will see "the same elephant" ; that is, "alice+" will see the same elephant as bob+ (namely stuffA), and "alice-" will see the same elephant as bob-, namely stuffB.
It is implicitly assumed in MWI that this is the case of course, otherwise the view would be immediately falsified! The *entire idea* is indeed that though there is a "multiplication of worlds/branches/terms", nevertheless, each observer is "locked up" in its own term, ONLY being able to interact with its "peers" in the same branch, so that everything happens AS IF this branch was, to him, the only thing that existed, and the others "faded away".

It is my understanding that this is what is worked out in detail in the section you quoted: that observers in a branch only "see" their peers in the same branch, with "see" meaning: get their results through an interaction which is "what did you get as a result", and that they DON'T get any result of an observer in another branch. The wavefunctions in (6) and (8) are nothing else but what I write above here. That's really nothing
but the MWI version of the measurement situation, and that was what I tried to say here since the beginning: that Rovelli re-discovered MWI, or at least the aspect that each branch "only sees itself", and that there is a self-consistency in A measures system a, B measures system b, and then A measures B.

As to your comment, or I misunderstand you, or you misunderstand me :-)
In MWI, there is no "priviledged observer", because there are many copies of each observer, one in each branch. However, it was my understanding, maybe erroneously, that Rovelli takes it that there is only ONE version of each observer. Clearly that runs into troubles (and is in fact contradicted with his equations 6 and 8 from your quote): B cannot be "in a superposition wrt A" and have "a single and definite measurement" wrt itself (unless, and that's the entire MWI approach, B is dedoubled into two B's). So I took it that Rovelli saw everything from the PoV of observer A (which can then be thought of to be "single"), allowing B to be in superposition until A saw him (the kind of PO you are talking about). And from the PoV from B, we forget what we did for A, we now do things from the PoV of B, and now B is "single" and A is "in superposition".
Both viewpoints together of course correspond to A as well as B in "superposition" but agreement in the corresponding branches. Which is, to my understanding, nothing else but MWI...

My use of the term priviliedged observer refers to the (originally Copenhagian) macroscopic observer who stands above and beyond the quantum world. Whether you have one or many of these chaps, they and the classical values they measure have really no place in a totally quantum view of the world.

S&M abandon this nonquantum fellow for a picture containing only interacting quantum systems ("the facts of the world are interactions", they say someplace in the paper). Some of these systems in a particular case function as "observation systems", i.e. they have pointer states. But critically these observer systems can be in superimposed states, entangled states, and so on, and they evolve quantally. This is how we can write the interacting state in terms of not only the evolving state of observed system (the "particle") but also including the observer state ("what the observational system records") in the state functions and density matrix calculations, which is the heart of their demonstration.

I recognize there are some problems to be dealt with in getting rid of the PO; notably if she goes, so does her watch, that external nonquantum clock that supplies the time parameter to QM. But people are working on that.

 

[vanesch]

S&M abandon this nonquantum fellow for a picture containing only interacting quantum systems ("the facts of the world are interactions", they say someplace in the paper). Some of these systems in a particular case function as "observation systems", i.e. they have pointer states. But critically these observer systems can be in superimposed states, entangled states, and so on, and they evolve quantally. This is how we can write the interacting state in terms of not only the evolving state of observed system (the "particle") but also including the observer state ("what the observational system records") in the state functions and density matrix calculations, which is the heart of their demonstration.

Uh, but that, to me, has always been the heart of any MWI view!

That an observation is a change in an observer state due to an interaction between the system and the observer ; now, if - as you rightly point out - these "observer degrees of freedom" - can be in superposed/entangled... states, then what becomes an "observer" now, with a specific 'observational record', is not the original degree of freedom (of which the quantum description, indeed, appears in superposition), but its single manifestation in ONE term. But that means that there are now "copies" of the observer (the different states, in the different terms=branches) around, each with different 'observational recordings' (a different one in each term).

So I fully agree with that. But I'm saying that this idea is the core idea of any MWI view. It is MWI's way to deal with the AND/OR problem: the fact that the observer is NOT the degree of freedom (= the device), but ONE of the different states in which this degree of freedom occurs (= one of the pointer states). An observer IS the pointer state, and not the apparatus, to put it rather bluntly. (and this leads me, in the case of human beings, to distinguish between the conscious observation of the pointer state, versus the entire bodystate)

THIS is, the way I understand it, the fundamental insight of an MWI view.

Now, I thought (maybe erroneously, I didn't read Rovelli's paper in all detail very thoroughly once I thought I recognized the main line), that Rovelli somehow thought he could get away with this "superposition of observer states" and hence solve the AND/OR problem by looking only at the measurement of A onto B and find out that everything is allright.

If Rovelli moreover recognizes this superposition of pointerstates, then I really don't see in what way his idea is different from the general idea of MWI. This is what I tried to say here from the beginning.

I recognize there are some problems to be dealt with in getting rid of the PO; notably if she goes, so does her watch, that external nonquantum clock that supplies the time parameter to QM. But people are working on that.

Yes, this is the famous "problem of time" which I understood was one of the principal difficulties in QG.
Note that in special relativity, as such, this is not necessarily an issue, because unitarity is conserved under a lorentz transformation, so you can "re-tell" the story along different timelike axes, the entire system is normally consistent.

 

[vanesch]

Rovelli does not use the term "branch" and the term has no intuitive or widely understood meaning in the context of a discussion of the paper.

But also in an MWI view, "branch" is a loosely used concept, which often introduces more confusion than anything else. As such, "branch" has no objective existence, but has only a kind of meaning WRT AN OBSERVER.

The problem MWI tries to address (one of the core problems in the measurement problem) is the AND/OR problem: How do you go from a superposition of pointer states to one (with a probability given by the Born rule). In Copenhagen/von Neumann, this is "solved" by the projection postulate, but this introduces fundamental difficulties (one is: WHY/WHEN does it happen - what sets an "observation" apart from a "physical interaction" ; the other is its non-local character).
MWI wants no such projection postulate, and is then confronted with the AND/OR problem. Its proposed solution is then purely "conceptual": instead of saying that the outcome is "in the apparatus" (with the different pointer states), say that the outcome "is in the pointer state".
That seems like a cheap trick of course, what have we gained ?
Well, FROM THE POINT OF VIEW OF AN OBSERVER (= a pointer state = a container of measurement record) we can say that HIS world did split in many branches: BEFORE the measurement, there was ONE pointer state (the pre-measurement state of the apparatus), so there was ONE observer associated with the apparatus, and AFTER the measurement there were as many DIFFERENT observers associated with the apparatus as there were used pointerstates. And "the" observer is just ONE of the many pointer states.
"Branches"(wrt the observer) are simply these different pointerstates that evolved out of one. Again, it is not an objective concept, it is observer-related.

It's because it is intuitively as if "the single observer" branched now into many (the single pre-measurement pointer state evolved into many) that we talk about "branches". But you do not even need to talk about branches in MWI. You simply say that observers are pointer states ; are "records of observation". While we usually think of observers as degrees of freedom (apparatus/bodies...).

The QM formalism as in Rovelli eqn (6) and (8) allows the individual observer to include different POSSIBILITIES in his or her state space but these do not correspond to REALITIES---possibilities in the Hilbertspace formalism are just a way of representing information and the lack thereof.

Well, up to a point, even in MWI, that's open to interpretation. Do you call a "reality" the record of an observer (= a pointer state), or do you call "reality" the entire unitary structure ?
One is inclined to say that the former is a "subjective reality" and the latter an "objective reality".

Let's now apply the test to see if this statement has anything to do with the Rovelli Smerlak paper:

"The consistency requirement that is worked out here in all detail, comes simply down to the MWI statement: observers in different blexo only learn about the answers of their peers in the SAME BLEXO."[/color]

I think if someone were to define for us (in another thread please) what a BLEXO is and what it means to be in a different Blexo and what it means to be in the same Blexo, that it might be quite interesting (for some people) to think about and discuss! But I regret to say that AFAICS talking about "branch" HAS NOTHING TO DO WITH ROVELLI'S PAPER.

Well, "branch or world or Blexo" for several observers is then simply the term in the wavefunction corresponding to their entirely entangled pointer states. It is the consistent set of records which "survive in interaction between different observers".

For instance, if we are in the state (where a and b are two measurement apparatus):
|a+>(|b+> + |b->) + |a->(|b+>-|b->)

we cannot yet talk about a "common world for a and b" ; however, we can talk about the two branches wrt a, namely the one with a+ (a pointer state, where clearly a has a definite record of observation, namely +) and another "branch" with a-.
We now say that there are TWO a-observers now (each with their branch).

After a and b interact (exchange information), then the above state cannot exist anymore. We have now:

|a++>|b++> + |a+->|b-+> + |a-+>|b+-> - |a-->|b-->

(because a now has ALSO the information of b, and vice versa).

So now there are FOUR a observers (= a-pointer states = a-branches = a observation records), and there are FOUR b observers with CONSISTENT records with their a observer.
We now have that the 4 a pointer states=observers correspond to the 4 b pointer states, and we COULD, if we wanted to, talk about FOUR DIFFERENT WORLDS.
However, for each of these a-observers, there is a SPECIFIC record, and it is IN AGREEMENT with the accessible record of his "associated" b-observer (that's the consistency requirement that SA pointed us to).

In how much we have to talk about "realities" or "possibilities" does not really matter, it is semantics. The straightforward interpretation is that the 4 different possibilities correspond to an "objective" reality, and each of the observers, with his record, his peers with which he's in interaction... lives in his "subjective reality". But these are just words.

It was my understanding that Rovelli simply never stated these things EXPLICITLY but somehow assumed them (by keeping observer B in a "superposition wrt A as long as A didn't measure B"), and just concentrated on "one term" from a single pointer state's point of view.

 

[selfAdjoint]

That an observation is a change in an observer state due to an interaction between the system and the observer ; now, if - as you rightly point out - these "observer degrees of freedom" - can be in superposed/entangled... states, then what becomes an "observer" now, with a specific 'observational record', is not the original degree of freedom (of which the quantum description, indeed, appears in superposition), but its single manifestation in ONE term. But that means that there are now "copies" of the observer (the different states, in the different terms=branches) around, each with different 'observational recordings' (a different one in each term).

But all that happens is that the observer pointer state goes into some value. Where is it demanded that they go into all possible values on different branches? The mathematics traces over all those values intermediately in calculating the density matrix, but all you have at the end of the day for a result is the reconciled value, determined by yet another interaction way down in the future light cones of all the experiment interactions, which the quantum analysis in the paper, and you say you agree with it, says will come out as it should.

Is there some reification (sorry, "ontological") issue I am missing?

 

[marcus]

Is there some reification (sorry, "ontological") issue I am missing?

Reification was the word that occurred to me as I was thinking about it this morning. The reification of terms in an algebraic expression. I believe there is no negative connotation that prevents using that word, which seems to be the correct one----certainly hope not anyway!

this discussion is helping me get a clearer understanding, thanks to you both

 

[vanesch]

But all that happens is that the observer pointer state goes into some value. Where is it demanded that they go into all possible values on different branches?

The point is simply that, once you take it that observers are (pointer)STATES and not DEGREES OF FREEDOM, well, then several pointerstates evolve out of one.
Example: the degree of freedom is the angle of the needle on an old voltmeter ; the states are the different quantum states that are taken up by this degree of freedom ; in an entangled situation, several of these states appear in the Schmidt decomposition H_needle x H_rest.
Usually one takes the "needle" (= degree of freedom) as "observer/measurement apparatus", but the idea of MWI is that it is the STATES which are the observers (= containers of measurement record).
Depending on the interactions with environment, the resulting states in the decomposition correspond to rather well-localized angles for the needle ; so now the initial observer (|needle = 0>) has evolved into, say, 3 new observers (|needle = 10 degrees> ; |needle = 25 degrees> ; |needle is 35 degrees>), each having a rather well-defined (but different) record of what was the "measurement result".

Now, from the PoV of one observer, you can or cannot consider that the others have an "existence". You can decide not to talk about the others - and that's what Rovelli does (except in the equations 6 and 8 for instance). You can now say that you "are" the observer "needle = 10 degrees" and decide not to mention the others.

Is there some reification (sorry, "ontological") issue I am missing?

Well, maybe *I* am the one who is missing something, but apart from this ontological issue as you say, which is not made explicit in Rovelli's paper (he never says what exists objectively, but only what exists from the PoV of an observer), I fail to see the difference in approach between MWI (where one DOES explicitly talk about an explicit ontology with several pointerstates for one observer degree of freedom, and hence different observers evolving out of one) and his - where he does the same, but from the PoV of one observer.

As I said, it is in the end, a matter of semantics what one calls "real". If you decide to call "real" only what is in your own "observation record" and you decide to call the other stuff "possibilities", why not. The thing that we then obtain, is that "reality" has only a meaning from the PoV from an observer, as a function of what he has in his specific observation record (is nothing else but his state, in fact). The problem is that we can now not talk anymore of any objective reality independent of an observer (and that's what happens with Rovelli: he cannot talk about B's reality from A's PoV as long as they didn't interact to exchange information).
In MWI, you CAN do so, on the condition of allowing for several observers (= pointer states) to exist simultaneously, associated with the same observer degree of freedom (= measurement apparatus). But you can just as well NOT decide to talk about that, restrict yourself only to "reality from the PoV of one observer", and then I fail to see what's different with Rovelli's PoV.

So, again, I'm not criticising Rovelli, I'm just pointing out that I don't see the fundamental difference between his PoV and the MWI PoV ; except for the emphasis, where MWI usually talks about the "objective reality" (with all those parallel observers), while Rovelli talks about "reality from the PoV of one observer".

 

[selfAdjoint]

The point is simply that, once you take it that observers are (pointer)STATES and not DEGREES OF FREEDOM, well, then several pointerstates evolve out of one.
Example: the degree of freedom is the angle of the needle on an old voltmeter ; the states are the different quantum states that are taken up by this degree of freedom ; in an entangled situation, several of these states appear in the Schmidt decomposition H_needle x H_rest.
Usually one takes the "needle" (= degree of freedom) as "observer/measurement apparatus", but the idea of MWI is that it is the STATES which are the observers (= containers of measurement record).
Depending on the interactions with environment, the resulting states in the decomposition correspond to rather well-localized angles for the needle ; so now the initial observer (|needle = 0>) has evolved into, say, 3 new observers (|needle = 10 degrees> ; |needle = 25 degrees> ; |needle is 35 degrees>), each having a rather well-defined (but different) record of what was the "measurement result".

Now, from the PoV of one observer, you can or cannot consider that the others have an "existence". You can decide not to talk about the others - and that's what Rovelli does (except in the equations 6 and 8 for instance). You can now say that you "are" the observer "needle = 10 degrees" and decide not to mention the others.

As I see it the multiple states are just potential possibilities, which may never come to pass, so it would be incorrect to identify them as observers. They rather constitute the available range of behaviors. Of course I don't like the word observer, since it brings in a lot of freight that relational QM is trying to get rid of. I would identify the observational quantum system as the needle, or rather of course whatever quantum system ultimately causes it to assume a particular position. This system has available states, as you define them, and as a result of interacting with whatever system is being observed it will assume one of those states, in exactly the same way an electron assumes one of its available energy states.

 

[vanesch]

As I see it the multiple states are just potential possibilities, which may never come to pass, so it would be incorrect to identify them as observers. They rather constitute the available range of behaviors. Of course I don't like the word observer, since it brings in a lot of freight that relational QM is trying to get rid of. I would identify the observational quantum system as the needle, or rather of course whatever quantum system ultimately causes it to assume a particular position. This system has available states, as you define them, and as a result of interacting with whatever system is being observed it will assume one of those states, in exactly the same way an electron assumes one of its available energy states.

What you set out is one of both:
1) the standard view (but then there's nothing relational about), where a measurement induces an objective record of the measurement, eventually for all to see - in other words, a genuine collapse of the state. Then there's nothing much to say about, it is not observer-related or anything, it is just as objective as in classical physics. There is now an objective projection.
The problem with this is of course a) that we now have to set apart "observations" from "physical interactions" and b) that we have non-locality as per Bell's theorem. But I cannot imagine that this is what Rovelli is talking about: it doesn't make sense then to talk about any "observation with relation to the observer" or that "B has no result until A saw it".

OR:

2) Yes, for a *particular* observer (but now we cannot identify an observer with the degree of freedom (the needle) but only with a state (the specific pointer state)), ONE result is realized, and all the others are possibilities, and this is something that is intimately related to *the
observer* (not the apparatus, because that one is entangled in many different pointerstates now, and there's no way to get it out of there without a projection - which we want to avoid if we're to avoid the difficulties a) and b) of point 1) ).
It was my understanding that Rovelli wanted to point out that we have to do quantum theory "from the point of view of an observer" while "fully applying quantum theory to it" (which means, to consider it also as a quantum system - this, to me, makes it clear that it gets *entangled* and that no *projection* is to be considered, otherwise he'd not write that we have to see the observer as a quantum system).

As such, the degree of freedom gets hopelessly entangled, with no way to pick out one state - but a STATE of course, by definition, is only one and is picked out. So Rovelli's article is to me only understandable if we equal observer with STATE and not with degree of freedom (apparatus), the latter one, being considered as a quantum system, not being able to pick out one particular pointer state. So, Rovelli says: you're an observer (= a pointer state), and you'll only see CONSISTENT other pointer states of other observers when you interact with them.
From YOUR PoV, the other states are mere possibilities, but they aren't "your state", hence you can forget about them. I entirely agree with that. But this is SEEN FROM YOUR POV. It is not an "objective" reality that is shared by all observers, because you're in the assymetric situation that when you "did your measurement" (meaning: when you assumed a certain pointer state) that your result is "clear" (it is the particular state you decided to call "yourself"), but that other poor guy over there, who did his measurement (interacted with his system), but didn't have any time yet to come and see you IS STILL TO BE SEEN IN A SUPERPOSITION. (otherwise it would be meaningless to talk about the measurement you're going to do upon him, if he's already in one single definite state: we're back to point 1) if we do that and there's nothing relational about the entire business)
So this viewpoint *only makes sense* when you specify a PoV of an observer = specific pointer state. Then YOU have your result, but poor Bob over there can measure all he wants, he's in a superposition until you measure him.

And from Bob's PoV, things are now opposite. Bob only has a definite result, and YOU are in a superposition until he measures you.

Both together is the MWI picture, where your degree of freedom as well as Bob's degree of freedom are in "superpositions", but combine in the correct way in different terms, so that the correct POINTER STATES get correctly together.

But then you cannot "get rid" of the "other possibilities", unless by not talking about them - which is what Rovelli does.

So yes, FROM THE POV OF AN OBSERVER (= pointer state), the "others" are mere possibilities that didn't get realized.

 

[marcus]

We should recall the closest rovelli comes to DEFINING objective reality:

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

As I said, it is in the end, a matter of semantics what one calls "real". If you decide to call "real" only what is in your own "observation record" and you decide to call the other stuff "possibilities", why not?

ClearlyRovelli does not do this. On the contrary his idea of reality has a collective, one might say even SOCIAL flavor. And BTW I can think of a reason "why not?" It would be solipsist. I don't like solipsism and prefer Rovelli definition of reality: what all the observers see and hear themselves agreeing about.

(note that Rovelli ALLOWS FOR THE FINITE SPEED OF SIGNALS in his definition, seeing and hearing is not instantaneous)

I also like that Rovelli DOES NOT CONFUSE REALITY WITH A vector in some Hilbertspace. A Hilbertspace is not in nature, it is an artificial human construct. Some alien might have invented something else to keep track of information. It is the logical notepad which we happen to use at this stage to record and sort out info. John von Neumann even said he did not believe in Hilbertspace "no more" (apparently at one time he did). For some people hilberspace is a kind of ONTOLOGICAL FETISH that defines for them what is real----they have to have it. But that is a peculiarity. I don't think it is normal. "Reality" was not invented in 1926.

The thing that we then obtain, is that "reality" has only a meaning from the PoV from an observer, as a function of what he has in his specific observation record (is nothing else but his state, in fact).

It sounds like you may suffer from this confusion of "reality" with a vector in some vector space. I hope you do not.

The problem is that we can now not talk anymore of any objective reality independent of an observer (and that's what happens with Rovelli: he cannot talk about B's reality from A's PoV as long as they didn't interact to exchange information).

No, you misrepresent Rovelli. His "definition of objectivity" is what all the observers see and hear themselves agreeing about. REALITY IS NOT ABOUT HILBERTSPACES.

HILBERTSPACE IS ABOUT INFORMATION THAT A PARTICULAR OBSERVER HAS ACCUMULATED. HILBERTSPACE is not about reality it IS ABOUT INFORMATION

restrict yourself only to "reality from the PoV of one observer", and then I fail to see what's different with Rovelli's PoV.

Rovelli never talks about "reality from the PoV of one observer" .
PLEASE, VANESCH WHAT PAGE AND WHAT LINE ARE YOU QUOTING? Where in heaven's name does he say "reality from the PoV of one observer"

MWI usually talks about the "objective reality" (with all those parallel observers), while Rovelli talks about "reality from the PoV of one observer".

Again you say this. I don't think he ever does. If you are going to use quote marks, then please give a page and line where he says "reality from the PoV of one observer". I think you misrepresent Rovelli quite severely---I hope it is just by accident.

I think Rovelli does indeed talk about the INFO that one system has about another system----I think quantum physics is about information one thing has about something else and it is a very good approach to information. the best we have invented so far.

But one should not naively equate quantum theory models with "reality"

That would be to make a fetish of the mathematical formalism. Like the person for whom it is not a sexy situation unless he has his sexual fetish, there is the danger of getting the habit that one cannot think something is objective reality unless one connects it with the ontological fetish.

Democritus, in a different but related situation, is supposed to have used the phrase "that which we all see and know".[/color][/size]

Rovelli (who has a Classical Mediterranean streak a mile wide) is actually very close to Democritus------his reality is sort of equal to Democritus PLUS allowing for finite speed of signals.

 

[vanesch]

We should recall the closest rovelli comes to DEFINING objective reality:

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

Absolutely. But this "objectivity" is then related to an observer (and is, up to a point, indeed solipsist).

Here are the quotes you ask for:
on page 1:
"From the relational perspective,
the apparent “quantum non-locality” is a mistaken
illusion caused by the error of disregarding the quantum
nature of all physical systems."

This means that we have to consider OBSERVERS AS QUANTUM SYSTEMS. Quantum systems are described by a vector in Hilbert space, and by unitary interactions, right ? So they end up ENTANGLED. There's no way out of this. Classical systems can have definite states, quantum systems end up entangled.

Another quote from p 1:
"Here we take this conceptual evolution to what appears
to us to be its necessary arriving point: the possibility that
EPR-type experiments disprove Einstein’s strong realism,
rather than locality."

So Rovelli takes it that we shouldn't look at an objectively reality, I take it.


A bit further, down on p1:
"The way out from the
confusion suggested by RQM consists in acknowledging
the fact that different observers can give different accounts
of the same sequence of events [5]. Notice, indeed, that
there is no operational definition of observer-independent
comparison (one is tempted to say “synchronization”) of
different observers’ information about a system: the information
of different observers can be compared only by
a physical exchange of information between the observers.
But since all systems are quantum systems, any exchange
of information is a physical interaction, and as such subject
to the laws (and in particular the uncertainties) of
quantum mechanics. The comparison of information is
itself a physical quantum process."

What else is this, but to state that observers have their OWN, INDIVIDUAL accounts (which can be different one from the other), and that the only way to find out about the "other" observer is by interacting with it.

And further, on p 2:
"In the context of the
EPR debate, realism is taken as the assumption that, in
Einstein’s words, ”there exists a physical reality independent
of substantiation and perception” [23]1. RQM departs
radically from such strict Einstein realism. In RQM,
physical reality is taken to be formed by the individual
quantum events (facts2) through which interacting systems
(objects3) affect one another.
Quantum events exist only in interactions4 and the reality
of each quantum event is only relative to the system
involved in the interaction. In particular, the reality of
the properties of any given system S is only relative to a
physical systems A that interacts with S and is affected
by these properties."

I interpret this as saying that "we shouldn't look at any objective reality (we shouldn't look at the entire wavefunction, say), we are only defining reality wrt an interacting system".

If that is not "solipsist" I don't know what is.

Rovelli does not do this. I can think of a reason "why not?" It would be solipsist. I don't like solipsism and prefer Rovelli definition of reality: what all the observers see and hear themselves agreeing about.

... from the PoV of an observer.

I also like that Rovelli DOES NOT CONFUSE REALITY WITH A vector in some Hilbertspace.

Of course: from the PoV of an observer, the vector in hilbert space is just a summary of his own state and knowledge (= Pointer state, remember), and a tool to find out what results he can expect from other observers, *through interaction*. This is entirely solipsist!

For some people hilberspace is a kind of ONTOLOGICAL FETISH that defines for them what is real----they have to have it. But that is a peculiarity. I don't think it is normal. "Reality" was not invented in 1926.

No, the Hilbert space is indeed the ontological construct that allows you to "objectify" the solipsist viewpoint of RQM: by puzzling all these different individual observer histories together into one common, objective construction. You are free to do so or not.
If you do so, then you've constructed an objective reality, and that's MWI. If you do not do so, then you limit yourself to a solipsist viewpoint, and that's RQM. RQM is, as I said before, the one-observer "subjective" reality view that goes with MWI, while the wavefunction is the objective reality viewpoint.

As of the quotes above, Rovelli *rejects* the objective reality viewpoint, and then the only thing that remains is the solipsist viewpoint.

It sounds like you may suffer from this confusion of "reality" with a vector in some vector space. I hope you do not.

It's not a matter of "confusion". It's a matter of words. The vector in hilbert space = "objective reality". It is shared by all observers. Rovelli clearly rejects the notion of "objective reality". That's THE VERY DEFINITION OF SOLIPSISM: that reality only exists from a subjective point of view.

No, you misrepresent Rovelli. His "definition of objectivity" is what all the observers see and hear themselves agreeing about. REALITY IS NOT ABOUT HILBERTSPACES.

No, read well his first page: his definition of "objectivity" is what an observer sees other observers agree upon (which is usually called subjectivity :-).

Again, I quote:
"Quantum events exist only in interactions and the reality
of each quantum event is only relative to the system
involved in the interaction. In particular, the reality of
the properties of any given system S is only relative to a
physical systems A that interacts with S and is affected
by these properties."

This means that the "reality" of the other observers is ALSO only relative to the concerned observer, and so is their mutual agreement. I cannot imagine what could be more solipsist.

Rovelli never talks about "reality from the PoV of one observer" .
PLEASE, VANESCH WHAT PAGE AND WHAT LINE ARE YOU QUOTING? Where in heaven's name does he say "reality from the PoV of one observer"

Read, for instance, on p4:
"In other words, in the sequence of events which is real
for A there is no definite quantum event regarding β at
time t0, and therefore no element of reality generated nonlocally
at time t0 in the location where B is."

If that is not "describing reality from the PoV of an observer (in casu A), then I don't know what is.

 

[marcus]

To put it briefly. It would help continue the discussion if we could agree that rovelli's idea of objective reality is by no stretch of the imagination SOLIPSIST

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

So let's dispense with that and not keep having it come up. the issue of what is real has never been finally resolved but there is a classical tradition which is social, like the city state, and he comes close to that. the Democritus idea "what we can all see and agree about" plus the finite speed of signals.

The classical idea is not fool-proof----it depends on reasonableness and good faith. There can always be stubborn imbeciles who insist that they see something else so that agreement is ultimately impossible. But it could be the most practical working definition. At all events rovelli says "This, after all, is the best definition of objectivity."

But here we should primarily be talking NOT about reality but about quantum mechanics-----how best to represent and sort out and process the information that an observer have about the world.
Each observer's hilbertspace is like that observers "personal digital assistant" which the observer uses to record and organize his knowledge and his uncertainty.

I would like if we could move ahead in this discussion. Can we all agree that the view of reality in this paper is collective and NOT SOLIPSIST?

 

[vanesch]

To put it briefly. It would help continue the discussion if we could agree that rovelli's idea of objective reality is by no stretch of the imagination SOLIPSIST

No, on the contrary. That's my entire point. His viewpoint IS solipsist.
He argues to reject objective reality. That's the very definition of solipsism.

 

[marcus]

Good. now the issue is clearly drawn. Let us try to make our posts very short. Rovelli says this:

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

I say this is a rather traditional (almost classical) idea of objective reality, and not solipsist at all! To arrive at eventual agreement I can imagine each person keeps his own individual Hilbert scratchpad but the objective reality is what they all eventually agree about. It is one. It is really there.

You say this:

No, on the contrary. That's my entire point. His viewpoint IS solipsist.
He argues to reject objective reality. That's the very definition of solipsism.[/color][/size]

Let us be very brief. So as not to bore each other with long involved argumentation. Do you agree with this clear statement of the disagreement?

 

[selfAdjoint]

Solipsism does not mean the rejection of someone else's hypothesis about the nature of reality. S&R have a definition of objectivity wherein all observers, those involved in the interaction and those not, agree on the results of (e.g.) an entaglement experiment, and they show that this definition is satisfied by their version of RQM. Patrick, you can only call such a position "solipsist" by distorting the meaning of the word out of all recognition.

 

[vanesch]

I will ask you a simple question, but first let's be clear about the words "objective" and "subjective", ok ?

What is objective does not need a reference to an observer, while what is subjective is with reference to an observer. Of course, an objective statement can involve an objective fact about an observer. I think we can be clear about that. The statement "Observer A sees a green light" can be both objective and subjective, in the following sense:
If everybody can agree conceptually that observer A sees a green light (whether one has that information or not, yet) at a certain event, then the statement is objective. There is an objective reality which says that observer A sees a green light at that event. If it turns out LATER that we have the information that two days ago, observer A SAW a green light, at that event then that is an objective fact, also back then.
However, if it is only CLEAR FOR OBSERVER A that he's seeing a green light, then this is SUBJECTIVE. One cannot say that it is the objective reality FROM THE POINT OF VIEW OF A. That is a contradiction in terms. If it is "from the point of view of A", then it is SUBJECTIVE.
So, one cannot say that two days later, when I learn about what observer A saw back then, that this NOW becomes an objective fact. It is, or it isn't, an objective fact at an event. Any needed reference to another, or same, observer, to conceptually think of the reality of the statement, renders it subjective.

This is trivial, and corresponds to the usual definitions of subjective and objective, but I wanted to make sure that we all are using the words with the same meaning.

Now, solipsism is the philosophical viewpoint that DENIES the existence of an OBJECTIVE REALITY. It means that all that is real, is subjective.

Mind you, there is a difference between solipsism, and the statement that only measurements "exist", as in the Copenhagen view. In the latter view, the quantum world has no genuine existence (subjective or objective), but all the classical observations are OBJECTIVE. That means that when Joe saw the pointer go to 5, then this is an objective fact, clear for everybody. If I learn 3 days later that Joe SAW the pointer go to 5 3 days ago, then this was an objective fact already 3 days ago, and not only when I learned about it. It's just that I didn't know the result, but that didn't undo the objective status of Joe's observation.

Can we agree up to this point, or are there already differences in opinion on the terms used ?

Assuming there is agreement, I can ask my simple question:

When A is doing his measurement on Alpha centauri on an entangled photon a, and B is doing his measurement on Earth on the twin of that photon, b, and we're talking in a reference frame in which both are at rest more or less, so that we can talk about an almost simultaneous measurement at both sides, DOES THERE EXIST, OR NOT, AN OBJECTIVE REALITY TO OUTCOME OF BOTH MEASUREMENTS AT THEIR MOMENT OF MEASUREMENT, or if you like, at these two events, according to Rovelli ?

The answer is a yes or a no.

To make it spicier, let's assume that this measurement is not done at each place by a single observer, but by a crew of 50 people. So on BOTH SIDES, they "mutually agree upon the outcome of the measurement on their side".

DOES, OR DOESN'T, Rovelli, in his scheme, assign an objective reality to the outcome of these measurements AT THESE TWO EVENTS ? In other words, does, or doesn't, the outcome of measurement exist ?

 

[vanesch]

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

I say this is a rather traditional (almost classical) idea of objective reality, and not solipsist at all! To arrive at eventual agreement I can imagine each person keeps his own individual Hilbert scratchpad but the objective reality is what they all eventually agree about. It is one. It is really there.

Don't forget to answer my question in my previous post :-)

But I can only say this: the quote, by Rovelli, is in a certain way self-contradictory, because what it really says is that one's subjective reality is self-consistent. In naive realism, one equals one's subjective reality with an objective reality.

Imagine two rooms, which are entirely separated, girls in the first, boys in the second. They don't know of each other's existence. In the first room, all the girls see girls, and agree that they only see girls. In the second room, of course, full of boys, they all agree that they only see boys.

What Rovelli essentially says, is that the girls agree amongst themselves about the statement "the room is full of girls", and that's then the objective reality.
But the boys, too, agree amongst themselves. So now, the objective reality is "the room is full of boys". Huh ?

Clearly, there can be TWO realities here, one about the girls, and one about the boys. As such, such a statement does not define any objective and unique reality ! Ok, but I used a cheap trick...
There is of course an objective reality, which is that there are girls in one room, and boys in another.

But it is even worse. In fact, imagine now a single room, where there are girls dressed in blue, and boys dressed in red. The girls dressed in blue have glasses that can only show people dressed in blue. The boys have glasses that can only show people dressed in red. Moreover, boys can only talk and listen to other boys, and girls the same.
The boys all agree amongst themselves that the room is full of boys, no girls. The girls again, all agree amongst themselves that the room is full of girls, no boys.
Again, ALL agree amongst themselves (at least those that can interact). So is it now an objective reality that "the room is full of boys, no girls" ; or that "the room is full of girls, no boys" ?

This is nevertheless what is demonstrated.

 

[marcus]

Please Patrick, let us try to make our posts very short. Rovelli says this:

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

I say this is a rather traditional (classical) idea of objective reality, and not solipsist. To arrive at eventual agreement I can imagine each person keeps his own individual Hilbert scratchpad but the objective reality is what they all eventually agree about when it is the intersection of their past lightcones. Reality is one. It is really there.

Personally I do not think one can legitimately MAKE ASSERTIONS ABOUT OBJECTIVE REALITY THAT IS NOT IN ONE'S PAST LIGHTCONE and I cannot picture a group of people agreeing and hearing each other agree about events not in the intersection their past lightcone. Whereof they cannot speak, thereof should they remain silent.

Rovelli affirms the existence of a single objective reality. And he defines it. I am happy with this definition, which is essentially a classical or traditional one. And so I affirm also the existence of one single objective reality.

You however disagree, and you say

No, on the contrary. That's my entire point. His viewpoint IS solipsist.
He argues to reject objective reality. That's the very definition of solipsism.[/color][/size]

And you also say

Now, solipsism is the philosophical viewpoint that DENIES the existence of an OBJECTIVE REALITY.[/color][/size]

I didnt see anywhere that Rovelli "denies" objective reality. I didnt see anywhere that he "argues to reject" objective reality.

I hear you talking about things that do not have operational meaning. they do not correspond to observation. You are talking about some instantaneous observation of things on Earth and centauri star. I urge you for the good of your soul to think of Niels Bohr and seal your lips from talking of un-observable stuff with no operational meaning! what man? Defy the Devil! Consider he's an enemy to mankind!

 

[vanesch]

Solipsism does not mean the rejection of someone else's hypothesis about the nature of reality.

No, solipsism is the denial of the existence of an objective reality independent of any "point of view". Now, this is exactly what Rovelli does on p4:
"In other words, in the sequence of events which is real
for A there is no definite quantum event regarding β at
time t0, and therefore no element of reality generated nonlocally
at time t0 in the location where B is."

It is what I thought was the essence of his viewpoint.
From the moment that you write "which is real for A" and "no element of reality [wrt A] generated at B" (while B is MEASURING at that very event), you're talking about subjective reality.
If "reality" is only "with respect to A", then I call that vision solipsist, because apparently no reality INDEPENDENT OF A is allowed for.

From B's PoV, of course, what was real for a becomes "no element of reality is generated" and what didn't have an element of reality for A now is of course real for B.

Reality is hence purely subjective in this business, and that's what I call solipsist.

 

[marcus]

exactly what Rovelli does on p4:
"In other words, in the sequence of events which is real
for A there is no definite quantum event regarding ? at
time t0, and therefore no element of reality generated nonlocally
at time t0 in the location where B is."
...

that is not called solipsism Patrick. That is called SPECIAL RELATIVITY

at time t0 the event at the other location is NOT IN HIS PAST LIGHTCONE

 

[vanesch]

I hear you talking about things that do not have operational meaning. they do not correspond to observation. You are talking about some instantaneous observation of things on Earth and centauri star.

Objective reality, not bound to an observer, is defined AT AN EVENT. So I ask you, does the measurement AT EVENT B has an objective meaning ? Does the measurement AT EVENT A have an objective meaning ?

Remember, for something to have an objective meaning, you're not allowed to make a reference to an observer, which IS WHAT YOU TRY TO DO.

You try to say that ONE AND THE SAME OBSERVER can of course not observe both events. True. But that's no problem, because objective reality is not supposed to be attached to an observer (it would be subjective at that point, proving my point).

Bohr didn't say that one and the same observer had to do all the observations, he only said that things become objectively real IF THEY ARE OBSERVED (eventually by different observers), because he considered a transition from the non-existing quantum world to the existing classical world, and observation is what rendered things classical. You cannot use Bohr's reasoning in an all-quantum setting, btw. The transition to classical (and hence real) was essential for him.

 

[vanesch]

that is not called solipsism Patrick. That is called SPECIAL RELATIVITY

at time t0 the event at the other location is NOT IN HIS PAST LIGHTCONE

So special relativity now says that events outside of my past light cone have no element of reality to them ? And this is of course an entirely OBJECTIVE and hence not-observer-related concept ? So WHOSE past light cone is going to say what is objectively real and not ?

EDIT: I was first thinking you are kindly trying to make a fool out of me in a very strange way, but I think that I see where the confusion comes from. You seem to equate:
"has objective reality" with "I can have information about"

I would think that both concepts are of course totally disjunct. Something can be objectively real, without me knowing about it of course.

 

[vanesch]

Personally I do not think one can legitimately MAKE ASSERTIONS ABOUT OBJECTIVE REALITY THAT IS NOT IN ONE'S PAST LIGHTCONE and I cannot picture a group of people agreeing and hearing each other agree about events not in the intersection their past lightcone. Whereof they cannot speak, thereof should they remain silent.

Objective reality is - as I tried to outline - INDEPENDENT OF ANY OBSERVER, by its very definition. An event is objectively real or not. This is not related to an observer, and hence not to any "past light cone". Something does not "become" objectively real. At an event, it is, or it isn't objectively real.

If you have no observer to relate to, you have no "past light cone".

If Betelgeuse goes supernova NOW (in our reference frame), then that is OBJECTIVELY REAL, even though there is no way of knowing that, and we will only find out that much later. But its status of objectively real supernova is undiscussable, and is attached to the event where it happened, totally independent of us, earthlings. THAT is objective reality. If you deny its existence, then you are - by definition - solipsist.

 

[vanesch]

I quit.

BTW, I think I'll stop this discussion.

The point is not that "I want to be right", or that "marcus wants to be right". I think I said all I have to say about what I think of Rovelli's viewpoint, I illustrated it amply, and now it is up to any eventually interested reader to take from that what he/she finds useful.

I think that for someone who sees my viewpoint, it has been clearly explained, and I think that someone who doesn't want to see it, there's no point in me trying to convince the person (that doesn't bring me any personal gain or anything).

 

[marcus]

OH NO PATRICK, YOU ARE QUITE WRONG ABOUT MY POSITION.

For me, and i think rovelli too, objective reality is one real thing. The classical definition is quite vague---what we all see and eventually agree about.

We can only agree on, and make statements about WHAT IS IN OUR COLLECTIVE PAST LIGHTCONE.

Reality is ONE and INDEPENDENT of any observer. But what statements we can meaningfully make about it depend on who we are and what we have seen.[/color]

I don't know what Rovelli would say, and don't want to put words in others. But for me objectivity is a traditional COLLECTIVE OR SOCIAL defined thing. Our world is what we all see and eventually can agree on.
(there is no supernatural or metaphysical referent).

So I accept the limitations imposed by Special Rel, signals traveling finite speed. I just refuse to talk about what hasnt been observed or couldn't be, as if it were real. EVENTUALLY when it is in our collective observation and we can compare notes, then we can make statements about it.

Like I say, that is not solipsism, that is just Special Rel.

Incidentally I think that one way to understand Rovelli's purpose in this paper is simply to MODERNIZE THE CLASSICAL (NON SOLIPS) view of objective reality TO TAKE ACCOUNT OF SPECIAL REL.

... for something to have an objective meaning, you're not allowed to make a reference to an observer, which IS WHAT YOU TRY TO DO...

Wrong Patrick. In elementary special rel one makes reference to an observer. In elementary quantum mechanics one makes reference to an observer who makes the observation corresponding to an observable.
In modern physics it does not need to detract from objective meaning to introduce the observer.

My objective reality is ONE and INDEPENDENT OF ALL OBSERVERS and the test of it is that observers can eventually come to agreement about what is in their collective past lightcone-----this is what Democritus "what we all see" means (the collective intersection of lightcones)

 

[vanesch]

I don't know what Rovelli would say, and don't want to put words in others. But for me objectivity is a traditional COLLECTIVE OR SOCIAL defined thing. Our world is what we all see and eventually can agree on.
(there is no supernatural or metaphysical referent).

I said I quit, and that's what I'll do, but this statement of yours makes it clear why we cannot come to an agreement. I have a totally different, and metaphysical view on objective reality. What you write is exactly what I call a subjective reality (and indeed the only one we have access to).

 

[vanesch]

To illustrate what I said:

Wrong Patrick. In elementary special rel one makes reference to an observer. In elementary quantum mechanics one makes reference to an observer who makes the observation corresponding to an observable.
In modern physics it does not need to detract from objective meaning to introduce the observer.

As an example:
In more advanced relativity, one introduces the manifold, in order to become independent again of the observer. *This* is what I call objective reality: the mathematical structure that has a metaphysical platonic existence, independent of any observer, which is just a substructure of it (in casu a world line).

 

[marcus]

This discussion has been helpful in crystalizing for me a view of objective reality that takes account of some 20th century physics AND philosophy. thanks to all who contributed so far

Basically we are progressing towards a clearer understanding of the key statement in rovelli's paper.

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

This is a traditional (classical) idea of objective reality which has been adjusted to taken account of finite signal speed. Yes reality is a SHARED consensus-type thing. It has a collective social foundation because it ultimately rests on LANGUAGE and on the communty of those with whom we TALK

There is Democritus phrase from a different but related context " that which we all see" and can agree on.

Connections to the 20th Century Cambridge School of "common language" philosophers===and to Wittgenstein both of the Tractatus and the Investigations. But mainly just to common sense. Objective reality can have no supernatural or metaphysical foundations. Ultimately it rests on the collective observation and agreement of those we talk with.

But notice how the finite signal speed is incorporated:
"... More precisely: everybody hears everybody else stating that they see the same elephant he sees."[/color][/size]

Before we can know of the consensus we must wait until the sound of their voices has reached us!

Reality is one, and objectively there. But the SHARED AGREEMENT which defines it takes some time to ASSEMBLE! So there is this element of DELAY before meaningful statements can be made ABOUT the reality.

So the DELAY is one thing I have learned to see in Rovelli's definition of objective reality.

Another thing I have learned is the danger of making a fetish of Hilbert space. Each person keeps his own individual Hilbert scratchpad like a "personal digital assistant" to record, sort and process the information he receives ABOUT reality. But this manmade device one should not say that it IS reality. It is just a device, like a cell-phone or ipod or laptop.

Because the quantum state vector in an individual's Hilbert has to embody uncertainty and lack of knowledge, it has to have terms representing different possibilities. But one shall not RE-IFY these terms and imagine that they have objective real existence. One's Hilbert is merely an appliance---not something in nature.

Another thing the discussion has helped with so far is to refresh my understanding of what Wittgenstein said: Whereof one cannot speak, thereof one must be silent.

It doesn't mean the NONEXISTENCE necessarily, the "whereof" could refer to stuff outside the past lightcone, or which has not been measured. A fair amount of 20th century physics has been about limitations on information----and always it has given increased importance to the OBSERVER.

I don't believe that 20th century developments should be said to reduce the sense of a SINGLE OBJECTIVE REALITY. They should be understood as having made our getting information about that objective reality OBEY CERTAIN RULES.

Rovelli affirms the existence of a single objective reality which you find out about subject to certain rules----one of which is that your Hilbert has YOUR NAME ON IT---and another of which is you don't know what the shared consensus is about something until it is in the shared past.

I hope to find out more about this paper and to have some more to say about it. Hopefully some basic issues of interpretations are now settled

 

[El Hombre Invisible]

I enjoyed reading this paper, and the original outlining RQM. There are strong similarities with MWI: both contain the idea of many separate, irreconcilable realities, one per world in MWI, and many in one world in RQM. Do you think MWI is now a redundant theory? Assuming they both yield the same predictions in all circumstances, RQM seems the most reasonably derived and the most simple. If not, what does MWI have that RQM doesn’t (since it’s clear what RQM has that MWI doesn’t).

I am happy to see that the distinction between quantum and classical systems has been jettisoned, and indeed the distinction between a conscious observer and a non-conscious one, mostly because I found such distinctions cop-outs. I am glad my prejudices aren’t too misguided… it bodes well for the future.

However, my knowledge of QM is still very incomplete and knowledge of information theory almost non-existent. There are a few things I didn’t understand, and a few things that I found unconvincing, undoubtedly because I didn’t understand them, or because the paper assumed a knowledge of other texts or papers.

Firstly, it is not absolutely clear to me what the nature of the kind of information referred to here is. When A measures S, A has information about S. S is affected by the measurement such that it is in a state of |1> or |2> relative to A. A cannot perform a self-measurement to tell if A has changed at all. In fact, without a second measurer, there is no concept that A has been affected by the measurement. A seems to remain, so long as S-A is a closed system, unaffected by its own measurement. How, then, can it hold information about S? To go from not having information to having information is a change, but if A remains unmeasured there is nothing for it to be a change relative to.

Secondly, though I support the idea that observer and observed systems are equivalent, and both are quantum systems that interact quantum mechanically, I’m not sure Rovelli mentions this point: If A measures S to determine q, A will have information about S as a result of that experiment. Since both systems are equivalent, then presumably S also holds information about A, otherwise you have a preferred observer. Both realities have been effected by one single measurement. The question then is: assuming S never measures anything, can anything be learned from S’s information about A, or any subsequent observer?

Thirdly, what is reality to a photon detector?

Fourthly, how is information conserved: within a system, or within a system in one reality?

Fifth: Rovelli claims to have derived QM from three postulates, namely (if I have understood) and vaguely: i) all of the information about a system can be known in a finite number of measurements; ii) then you can know something different, or perhaps the same thing, by doing more measurements; iii) the superposition principle (I won’t lie, I didn’t get his third postulate). I did not see, however, how he derived the correlation rules from these, that is: how the operator M takes form without further postulates. I assume this is already derivable from QM principles which are in turn, according to Rovelli, derivable from his postulates. Although if this were so, why entertain ideas such as ‘the collapse of the wavefunction’? Could someone outline or provide a link to a (non-technical as possible) description of how such rules are derived? It’s the one part I’m still uncomfortable with. It’s liable to lead to a heavy bout of existential angst.

On the EPR paper specifically, the basic argument seems to be: nothing in QM derived from Rovelli’s postulates will forbid you from predicting with certainty what measurement you will make in future relative to yourself. Sounds sensible to me.

Lastly, on the ‘objective/subjective’ issue, if you accept RQM and have a situation where all observers agree relative to observer A and call it ‘subjective’, what are you going to call it when observer A measures one thing, but measures some other observers to have measured something else? You’re out of words. Might as well redefine the word ‘objective’ the same way you’ve redefined everything else (such as facts, information, state and reality): i.e. when all observers agree relative to the pertinent observer. That sounds sensible to me too. Furthermore, everyone’s reality is already a subjective reality. No-one has insight into someone else’s reality. Rovelli didn’t make this up.

 

[selfAdjoint]

I will ask you a simple question, but first let's be clear about the words "objective" and "subjective", ok ?

I'm very willing

What is objective does not need a reference to an observer, while what is subjective is with reference to an observer. Of course, an objective statement can involve an objective fact about an observer. I think we can be clear about that. The statement "Observer A sees a green light" can be both objective and subjective, in the following sense:
If everybody can agree conceptually that observer A sees a green light (whether one has that information or not, yet) at a certain event, then the statement is objective. There is an objective reality which says that observer A sees a green light at that event. If it turns out LATER that we have the information that two days ago, observer A SAW a green light, at that event then that is an objective fact, also back then.
However, if it is only CLEAR FOR OBSERVER A that he's seeing a green light, then this is SUBJECTIVE. One cannot say that it is the objective reality FROM THE POINT OF VIEW OF A. That is a contradiction in terms. If it is "from the point of view of A", then it is SUBJECTIVE.

So, one cannot say that two days later, when I learn about what observer A saw back then, that this NOW becomes an objective fact. It is, or it isn't, an objective fact at an event. Any needed reference to another, or same, observer, to conceptually think of the reality of the statement, renders it subjective.

But smuggling nonlinear human behavior into this discussion of linear quantum systems is not legitimate. If there is a population of necessarily truthful observers who will all attest that your observer told them back then (necessarily speaking truth) that he saw the green light then that past event is just as objective now as it was when it happened.

Of course if all the quantum systems in the case go through further interactions and lose the record of their states at the time (if some terrorist drops a bomb on Fermilab and destroys all the experimenters and their data for example), then the event "might as well never have happened" whatever its ontological status was. To assert that this kind of historical contigency denies objectivism is bad philosophy.

This is trivial, and corresponds to the usual definitions of subjective and objective, but I wanted to make sure that we all are using the words with the same meaning.

On the contrary, I think you are using them in a private sense, and one that is vrey inappropriate to talking about quantum events.

Now, solipsism is the philosophical viewpoint that DENIES the existence of an OBJECTIVE REALITY. It means that all that is real, is subjective.

Mind you, there is a difference between solipsism, and the statement that only measurements "exist", as in the Copenhagen view. In the latter view, the quantum world has no genuine existence (subjective or objective), but all the classical observations are OBJECTIVE. That means that when Joe saw the pointer go to 5, then this is an objective fact, clear for everybody. If I learn 3 days later that Joe SAW the pointer go to 5 3 days ago, then this was an objective fact already 3 days ago, and not only when I learned about it. It's just that I didn't know the result, but that didn't undo the objective status of Joe's observation.

Can we agree up to this point, or are there already differences in opinion on the terms used ?

From http://en.wikipedia.org/wiki/Solipsism" wikipedia article on solipsism we get this definition:

Solipsism (Latin: solus, alone + ipse, self) is an extreme form of skepticism, saying that nothing exists beyond oneself and one's immediate experiences. More generally, it is the epistemological belief that one's self is the only thing that can be known with certainty and verified (sometimes called egoism). Solipsism is also commonly understood to encompass the metaphysical belief that only one's self exists, and that "existence" just means being a part of one's own mental states — all objects, people, etc, that one experiences are merely parts of one's own mind.

Which shows that your definition is not the one commonly used. The difference is that a solipsist doesn't believe anything exists outside himself. Your definition is closer to some forms of idealism. But Smerlak and Rovelli aren't proposing idealism, they are on the side of externally existing reality. In section 2.1 they say: "In RQM, physical reality is taken to be formed by the individual quantum events (facts) through which interacting systems (objects) affect one another. And they quote Wittgenstein's Tractatus: (1.1)"The world is a totality of facts, not things", (2.01)"An atomic fact is a combination of objects (entities, things). It is essential to a thing that it can be a constituent part of an atomic fact", and (2.01.2)"There is no object that we can imagine excluded from the possibility of combining with others.".

Their external world may be empty of existent Hilbert spaces and wave functions but it's chock full of interacting quantum systems. This is Rovelli, after all. For him spacetime itself is a collection of interacting quantum systems.

Assuming there is agreement, I can ask my simple question:

When A is doing his measurement on Alpha centauri on an entangled photon a, and B is doing his measurement on Earth on the twin of that photon, b, and we're talking in a reference frame in which both are at rest more or less, so that we can talk about an almost simultaneous measurement at both sides, DOES THERE EXIST, OR NOT, AN OBJECTIVE REALITY TO OUTCOME OF BOTH MEASUREMENTS AT THEIR MOMENT OF MEASUREMENT, or if you like, at these two events, according to Rovelli ?

The answer is a yes or a no.

Yes. (Recorded in the states of the tasked quantum systems)

To make it spicier, let's assume that this measurement is not done at each place by a single observer, but by a crew of 50 people. So on BOTH SIDES, they "mutually agree upon the outcome of the measurement on their side".

DOES, OR DOESN'T, Rovelli, in his scheme, assign an objective reality to the outcome of these measurements AT THESE TWO EVENTS ? In other words, does, or doesn't, the outcome of measurement exist ?

He does. So does his coauthor Matteo Smerlak (actually senior author of the paper, but I guess the Matthew principle applies). So do I. And when the 50 quantum systems that were tasked to interact with the proceedings on Earth and record their resulting states are brought together with the 50 that did the same function on Alpha Centauri, they will all agree on what the results were; there is no reason why the single observer C of section 4.3 could not be multilied a hundredfold.

 

[marcus]

Hello Hombre,
thanks for the careful reading and thoughtful questions. I will only try to respond to part---mainly at a personal level. I enjoyed these papers too.
You refer to two papers, i think

http://arxiv.org/abs/quant-ph/0604064
Relational EPR

and

http://arxiv.org/abs/quant-ph/9609002
Relational Quantum Mechanics

you may also have looked at the article on RQM in the Stanford Encyclopedia of Philosophy, mentioned here
https://www.physicsforums.com/showpost.php?p=962479&postcount=24

I guess the main message I got from was that it was a waste of time for people to worry about the alleged "non-locality" of QM based on EPR correlations, because it isnt.
I quoted the defintion in post #2

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

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

the other paper is more lengthy and goes into more topics. Like sketching the history of various interpretations of QM and explaining why RQM is different from them.

==============
I will try to respond a little. I see rovelli as in one sense putting observer and observed on the same footing because they are both quantum systems. There is no classical system observing a quantum one.

but I don't see that means that promotes every observed system to the status of an observer! Maybe i am missing something---dont claim special expertise or to thoroughly understand the papers. Object if you think I'm mistaken!---but for me something is only an observer if it has a Hilbertspace labeled with its name that is used to keep track of its experience (possibly of itself and/or the rest of the world, possibly including experiments it has performed)

I guess it could be an automaton. The main thing is it has an active Hilbert in which data occasionally gets recorded. that, for me, is the act of "observing"-----something changing the state in its Hilbert. And one of the main duties that observers (each equipped with a Hilbert) perform is to take part in Gedankenexperiments.

So you asked if an observer can record info about ITSELF. My own personal opinion is YES. It doesn't have to but it can. It can make an experimental measurement, and enter the result in its Hilbert PDA (personal digital assistant) and then ALSO make a diary entry 'today I made a measurement and observed q and entered that'. It probably doesn't have to, because the PDA probably automatically records changes, but I guess the observer could be keeping track of changes in himself that result from making observations of another system.

IMO as a quantum system he IS AFFECTED but it may not be observed because not everything in the world is always on camera being recorded in somebody's Hilbert. I see that as definitely optional.

You can see I am trying to respond to what you said here

Secondly, though I support the idea that observer and observed systems are equivalent, and both are quantum systems that interact quantum mechanically, I’m not sure Rovelli mentions this point: If A measures S to determine q, A will have information about S as a result of that experiment. [/color]

Some of the others will probably be jumping all over me for whatever I just said. the more philosophical the more something breeds dispute
Since both systems are equivalent, then presumably S also holds information about A, otherwise you have a preferred observer. [/color] According to me, they AREN'T equivalent unless they are both equipped with Hilberts and recording their observations. they are only MORALLY equivalent in the sense of both being quantum systems "in the eyes of Mother Nature". But a piece of equipment or an electron doesn't necessarily have to be an OBSERVER. imo imo imo hope nobody objects

Thirdly, what is reality to a photon detector?[/color]
In my experience they are not observers so it doesn't matter what their reality is BAH! MERE PEASANTS!

 

[vanesch]

I posted two long replies here, and then I decided to erase them, as I said that I was going to stop, because I found myself again explaining and making the same arguments.

Nevertheless, I keep one little piece:
I'd like to finish on a humoristic tone: I don't know if you know the comic "Calvin and Hobbes" of Bill Watterson.
At a certain point, the 6-year old boy Calvin asks his dad how come that old pictures are black and white. His dad tells him, that actually, before the 1930ies, the WORLD was black and white, and then slowly started taking on some color. So these black and white pictures are actually color pictures of the world, which was black and white.
When Calvin then asks how come that old *paintings* are colorful, his dad said: yes, but paintings are made by artists, not normal people. These artists were a bit crazy, and painted a black and white world in color...

 

[vanesch]

From http://en.wikipedia.org/wiki/Solipsism" wikipedia article on solipsism we get this definition:

Solipsism (Latin: solus, alone + ipse, self) is an extreme form of skepticism, saying that nothing exists beyond oneself and one's immediate experiences. More generally, it is the epistemological belief that one's self is the only thing that can be known with certainty and verified (sometimes called egoism). Solipsism is also commonly understood to encompass the metaphysical belief that only one's self exists, and that "existence" just means being a part of one's own mental states — all objects, people, etc, that one experiences are merely parts of one's own mind.

Which shows that your definition is not the one commonly used. The difference is that a solipsist doesn't believe anything exists outside himself.

To this, I want to react. My definition of solipsism "denial of objective reality" is in fact equivalent to the above definition. Indeed, if there is no objective reality, then there's no way I should assume that anything beyond my subjective reality exists ; especially, I should not assume that what I learn from "others" (of which their only reality for me is in my subjective reality now, given that they have no objective existence) learns me anything beyond my subjective reality.
It is only the assumption of objective reality which can make me assume that there might exist other subjective realities.

In Rovelli's paper, this is where there is an ambiguity when talking about "everybody agrees with everybody"... or do I see "everybody in my subjective reality" agree with "me and telling me that they agree with themselves" ?

In order for everybody to agree with everybody, this agreement must have an objective status.

So I think that I adhere to the standard definition of solipsism when I say that a solipsist denies the existence of an observer-independent, objective reality. Because there remains nothing else for him, than his own subjective reality in that case. And that's the standard definition of solipsism which has been cited.

As a few notes: an article on "objectivity/subjectivity"
http://www.iep.utm.edu/o/objectiv.htm

and the article you cite yourself on solipsism is very good
http://en.wikipedia.org/wiki/Solipsist

Interesting is also:
http://en.wikipedia.org/wiki/Metaphysical_solipsism

"Metaphysical solipsism is the variety of idealism which maintains that the individual self of the solipsistic philosopher is the whole of reality and that the external world and other persons are representations of that self having no independent existence (Wood, p. 295)."

 

[vanesch]

Relational QM is not a novel explanation of quantum mechanics

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

but more a way of adapting one's ideas of the world and of nature to reflect the lessons learned from ordinary usual QM. Instead of changing QM to fit one's ideas, one adjusts one's ideas to fit QM.

Thanks, Marcus, this link is to me far more enlightening than the original articles.

Especially, here we have, from your link:

Relational ideas underlie also the interpretations of quantum theory inspired by the work of Everett. Everett’ original work (Everett 1975) relies on the notion of "relative state" and has a marked relational tone (see quantum mechanics: Everett's relative-state formulation of). In the context of Everettian accounts, a state may be taken as relative either (more commonly) to a "world", or "branch", or (sometimes) to the state of another system (see for instance Saunders 1996, 1998). While the first variant (relationalism with respect to branches) is far from the relational views described here, the second variant (relationalism with respect to the state of a system) is closer.

However, it is different to say that something is relative to a system or that something is relative to a state of a system. Consider for instance the situation described in the example of Section 5: According to the relational interpretation, after the first measurement the quantity q has a given value and only one for O, while in Everettian terms the quantity q has a value for one state of O and a different value for another state of O, and the two are equally real. In Everett, there is an ontological multiplicity of realities, which is absent in the relational point of view, where physisical quantities are uniquely determined, once two systems are given.

The difference derives from a very general interpretational difference between Everettian accounts and the relational point of view. Everett (at least in its widespread version) takes the state Ψ as the basis of the ontology of quantum theory. The overall state Ψ includes different possible branches and different possible outcomes. On the other hand, the relational interpretation takes the quantum events q, that is, the actualizations of values of physical quantities, as the basic elements of reality (see Section 1.1 above) and such q's are assumed to be univocal. The relational view avoids the traditional difficulties in taking the q's as univocal simply by noticing that a q does not refer to a system, but rather to a pair of systems.

For a comparison between the relational interpretation and other current interpretations of quantum mechanics, see Rovelli 1996.

It is funny to see that "the basic elements of reality" q have no observer-independent ontological status.
This is what I was indeed finding incomprehensible in this interpretation, and what lead to my objections.

I have serious difficulties with statements such as: "q happened with respect to P", from P's PoV, but "q didn't happen wrt P" from Q's PoV, together with the claim that "q happens wrt to P" is an element of reality.

We are now saying things like:
"the photon went through slit A, not through slit B from slit A's PoV" and "the photon went though slit B, not through slit A from slit B's PoV" and "the photon went though slit A and B from the detector's PoV", and these 3 statements are now what constitutes "the elements of reality".

So let's say that if you can live with an "observer-dependent ontology" - whatever that may mean - then RQM is fine (nevertheless, if you have such a flexible mind as to be able to accept a concept such as "observer-dependent ontology" then I'd guess you'd not have any problem with Copenhagen either).

If you insist on having an observer-independent ontology (which, for me, is the only possibility for an ontology), then you can complete RQM into MWI.

But indeed, RQM is different from MWI. In RQM, there's no observer-independent ontology, apparently (even though words such as objective reality are used - of which I'd really say that this is an abuse, no ?).

I nevertheless continue to see MWI as the "ontological completion" of RQM, which avoids having to assign observer-dependent truth values to "elements of reality".

EDIT: to continue my "critical reading of RQM", of which I now claim even more that it is only concerned with a web of "self-consistent but mutually incompatible" subjective realities and an absence of objective reality (no matter how often that word is seeded in the text), consider the following situation:

A double-slit experiment, with a glansing mirror behind each slit, and a detector that will observe the interference pattern. I only need the glancing mirrors in order for something to be there, "slits" by themselves not being very material systems :-)

A photon, P is sent to the two slits (and hence, the two mirrors M1 and M2), to the detector D. Or let us consider 10000 photons, P1, P2,...P10000.

Now, correct me if I'm wrong, but if I understand RQM well, we can have several "quantum events" here:

When calculating the probability of hitting a mirror using the wavefunction of the photon P1, and considering M1 a measurement device, we can say that the quantum event relating P1 and M1, q1 is "M1 is hit by P1" (with probability 1/2). Let's say that from M1's PoV, this is true, so:

q1_1, between P1 and M1, is "M1 is hit by P1".
we'll have similar quantum events:
q1_2, between P2 and M1: is "M1 is NOT hit by P2"
...
We will have as such, ~ 5000 of these q1_i statements where M1 is hit, and ~5000 of these statements where M1 is not hit.

q2_1, between P1 and D is: "P1 hits at position x1"

again, we'll have about 10000 q2_i statements, and we can assume they build up an interference pattern in the different xi.

Now, clearly, the set of all these {q2_i} statements is incompatible with the set of all {q1_i} statements (we cannot say through which slit the photon went AND talk about the interference), so we cannot have a logically consistent web of statements {q} which includes both the quantum events q1 and q2, which would be the case if they did build up an objective reality.

However, we can talk about two DIFFERENT, and SUBJECTIVE realities, that is the one of the mirror M1, and the one of the detector D.

The point is of course that the mirror "keeps no record" of whether it was hit or not. One could replace, btw, the mirror by any system that interacts with the photon but keeps coherence intact (and can, as such, not entangle for good with the photon). But nowhere was said whether "quantum events" should have a lasting record or not: in fact, one should have a quantum theory in order to already say whether or not a record is kept (in other words: "keeping records" is not a primitive concept, but should be derived in a theory respecting itself).

I only used this example to show that you run into troubles if you consider the joint set of "quantum events" where there is a definite outcome "for a subsystem looking at a lower subsystem" while keeping at the same time the possibility of quantum interference of this subsystem as observed with a larger system: this joint set forms an incompatible set of statements, in other words, from a certain PoV (the subsystem) one has to assign the truth value "true" to a quantum event, and from another PoV (the larger system) one cannot assign this truth value to this same relational quantum event, for it destroys quantum interference.

So the only thing to do is to only consider ONE subset at a time, related to ONE subsystem at a time. This subset is then of course not a system-independent set of events, but only spans "the subjective reality" of the subsystem in question.

EDITbis:

As I understand it, quantum theory presents us with a measurement problem which can be tackled in several ways, each leading to a different interpretation. Nevertheless, each interpretation suffers from some problem, which the marketing department of said interpretation tries to burry under a load of words.

The first aspect of the measurement problem is that "not all systems are the same for the law" ; some are "observers" and others aren't.
A second aspect is the ontological status, what objective reality does the interpretation give us, up to what point
The third aspect is locality.
The fourth aspect is agreement with our intuition and daily observation

Copenhagen suffers from the first 3: the ontological world is "classical" and the quantum world has no existence, but "comes into existence" upon a measurement, which is an unexplained process, which generates "quantum noise" in an essentially classical view.
That's why we are urged, by Bohr, "not to talk about the position of an electron until it is measured". The position (which is here the considered QM degree of freedom) "has no ontological existence" until it is "measured".
Moreover, the classical results at spacelike intervals, to which of course ontological status is given, violate Bell's theorem, so the view is non-local in this respect.

von Neumann, who gives somewhat more ontological status to the quantum world, described by a wavefunction, tackles problem number 2. Nevertheless, 1 and 3 stand: 1: certain systems "apply process 1", and others "apply process 2". Also, the projection of the now more ontologically seen wavefunction is non-local.

Everett solves 1,2, and 3, but runs into troubles with 4. Indeed, all systems are now equal for the law (1), the ontology is clear (the wavefunction) (2), and locality can be preserved this way (3). However, Everett meets a big resistance because of the unintuitive character of the ontological status of "parallel worlds". As long as electrons suffer strange things, we don't care, but if it touches our human situation, we don't like this. Although Everett can succeed in showing that we "are deluded into thinking our daily experiences are really there", this product nevertheless doesn't sell too well. People find it too weird.

As I see it, RQM exchanges (4) for (2): in order to come "intuitively clean", it keeps (1) and (3), but has to sacrifice (2): there's no ontological status.

However, what could locality, or "all systems equal for the law" still mean without an ontological reality ? So let's invent the "observer-dependent objective reality"...
I have to say that as a marketing exploit, this is a great success.

And hey, it sells !

So I remain highly sceptical of having to resort to such concepts in order to "solve" problems.

Nevertheless, I thank the contributors to this thread here to help me understand the principal features of RQM.

 

[El Hombre Invisible]

Hello Hombre,
thanks for the careful reading and thoughtful questions.

Thanks to you for the interesting thread. This has fascinated me endlessly for three days.

but I don't see that means that promotes every observed system to the status of an observer! Maybe i am missing something---dont claim special expertise or to thoroughly understand the papers. Object if you think I'm mistaken!---but for me something is only an observer if it has a Hilbertspace labeled with its name that is used to keep track of its experience (possibly of itself and/or the rest of the world, possibly including experiments it has performed)

I guess it could be an automaton. The main thing is it has an active Hilbert in which data occasionally gets recorded.

Well, Rovelli says in his own abstract to the original RQM paper: "All systems are to be assumed equivalent, there is no observer-observed distinction." Of whether or not the observer can be an automaton, he says: "I do not make reference to conscious, animate, or computing, or in any other manner special system. I use the word "observer" in the sense in which it is conventionally used in Galilean relativity when we say that an object has a velocity "with respect to a certain observer"."

So you asked if an observer can record info about ITSELF. My own personal opinion is YES. It doesn't have to but it can.

I ask because of Rovelli's reference to Breuer's proof (of which I know nothing) that: "no system (quantum nor classical) can perform a complete self-measurement". In the case of simple, non-conscious systems, such as particles, I am wondering what kind of thing it can observe of itself in part.

My main gist to the question you responded to was that, if by being measured S has information about A, assuming observer/observed and sentient/non-sentient equivilence, how does this manifest itself? The information would be about observer O with respect to the system S. Presumably that information will probably not exist with respect to any other system. Presume S to be, say, a particle.

Some of the others will probably be jumping all over me for whatever I just said.

I'm just glad no-one jumped all over me. I have been out of my depth before.

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.

Anyway, I've thought about this all day, and I'm not so keen on it any more. I'm going to follow this with a huge post explaining why. Some of it might be right, some not. Wouldn't mind some pointers, but its a big post so no big deal if none are forthcoming.

Take care.

El Hombre

 

[El Hombre Invisible]

Big breath, and...

The more I reread this paper, the less I like it, that is: the less good I think it is. I was keen to like it because many of its assumptions and postulates seemed more reasonable than other interpretations. However, I don't think his argument makes much sense. Consider and correct me if I'm wrong...

Rovelli observes: "In quantum mechanics different observers may give different accounts of the same sequence of events."

He maintains that: "we have two descriptions of the physical sequence of events E... These are two distinct CORRECT descriptions of the same sequence of events."

All of this is prior to formulating his RQM theory. He is just describing a standard QM experiment.

Let us look at the sequence of events for both O and P. For O the story is simple: to start with he has not measured the system so does not know the value of q; then he interacts with it and measures a value of 1. For P the story is: to start with she does not know the value of P but knows that O doesn't know it either; afterwards she does not know what value O measured but knows it must be the same as what the value actually is.

These are two pretty DIFFERENT sequences of events. For O there's an element of measuring and deducing. For P there's nothing. Since she knows that q must be 1 or 2, and that whichever it is, that will be what O observes, she can make the same prediction at t_1 that she can at t_2. No new information is available to P at t_2. (You could argue that she at least knows at t_2 that O did measure something. At t_1, there was a possibility that O would simply drop dead without performing the experiment. I'll get on to that later. Suffice to say that it is still not possible for P to know that O did measure something unless she asks him, i.e. measures the S-O system. P might turn round and say that some fiend had stolen S.)

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.

This is supported by Rovelli himself in the way he describes a measurement. First he notes that all systems, observers and observed, are quantum systems. He then states that to be a measurement there must be a quantum mechanical interaction between observer and observed. He says that during this measurement, the observed system is 'affected' by it, and that after it the observer "has information" about the system. Of course, both the effect and the information are relative only to the observer.

So all measurements lead to a change in the system (to give a definite value such as q = 1), and a change in the observer's information, and this must be mediated by a physical interaction.

In Rovelli's words: "A quantum description of the state of a system S exists only if some system O (considered as an observer) is actually "describing" S, or, more precisely, has interacted with S."

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.

But more blindingly obvious a contradiction is that Rovelli assigns quantum descriptions to S before a measurement has taken place, and twice assigns ones for S-O before they are measured. If the above quote is true, then at t_1 a quantum description of S by O cannot exist, so cannot be |psi>. Likewise, P cannot say anything about S and O at t_1 or S-O at t_2 because she has not measured them.

The actual description by Rovelli of P's sequence of events reads: "assume that P does not perform any measurement on the S-O system during the t_1 - t_2 interval, but that she knows the initial states of both S and O".

How can P know these initial states? To describe these states, which are quantum descriptions, she must (according to Rovelli) have measured them. But if she measured S at t_1, it would have been affected by HER measurement and she would have observed the states as |1> or |2>, not |psi>. So if she DOESN'T measure S, she cannot give its state a quantum description; if she does, she gets a classical, definitive answer. By Rovelli's own logic, the experiment he uses as the basis for his main observation would not happen.

One more point about the kind of measurements Rovelli talks about. His measurements consist of 'asking a question' (not necessarily intentionally or even literally) of a system after which the maximum N bits of information are returned to exhaust the observer's possible knowledge about the system and the observer then holds that information. Rovelli makes use of the idea of a measuring hand, pointing to the observed state, that holds the information, not to be taken literally of course.

It then makes sense to ask what kind of questions can be asked. For instance, when measuring the value of q in the system S, observer O is surely asking: what is q? q can take two values: q = 1 and q = 2, and O will receive information that tells him that q is equal to one value (q = 1 for instance), This information will be stored on O's measuring hand- it will point to 'q is 1'.

What can P ask at t_3? She wants to know, perhaps, first of all: has O measured S? So P asks O - this is a physical interaction between P and S-O. Let us call x the property of the S-O system that P is measuring. It can take two values: x = 0 (O has NOT measured S) and x = 1 (O has measured S). By finding out whether the experiment has taken place, this is the property of the S-O system that P is measuring, just as q was the property of S that O was measuring. And just as in O's measurement, P will effect S-O in the interaction and get a state of, say, |Y> or |N>, that is she will receive and store information that x = 0 or x = 1. Let us assume x = 1.

P may then ask S-O what value of q was measured. Let q' be the property of S-O that P is measuring which has the same values as q. Same as before. We'll say q' = n.

So where does, for instance, the state at t_2 alpha|1>X|O1> + beta|2>X|O2> actually come from? Is this a state that P can measure of S-O? Can there be some property of S-O that P can measure such that the value of it might be alpha|1>X|O1> + beta|2>X|O2>? I don't think so. I think any measurement of S-O by P will be just like O's measurement of S - it will yield a single definite answer. If it doesn't, the correspondence principle doesn't hold. We should be able to observe systems in superpositions of states.

There is a measurement that P can make, however, that does yield alpha|1>X|O1> + beta|2>X|O2> as an answer. She can ask the question: "what are the probabilities that O has measured the various possible values of S?" She would not get this answer if she measured the S-O system, but she might get it if she measures a QM reference book.

 

[selfAdjoint]

Thanks to you for the interesting thread. This has fascinated me endlessly for three days.


Well, Rovelli says in his own abstract to the original RQM paper: "All systems are to be assumed equivalent, there is no observer-observed distinction." Of whether or not the observer can be an automaton, he says: "I do not make reference to conscious, animate, or computing, or in any other manner special system. I use the word "observer" in the sense in which it is conventionally used in Galilean relativity when we say that an object has a velocity "with respect to a certain observer"."


I ask because of Rovelli's reference to Breuer's proof (of which I know nothing) that: "no system (quantum nor classical) can perform a complete self-measurement". In the case of simple, non-conscious systems, such as particles, I am wondering what kind of thing it can observe of itself in part.

My main gist to the question you responded to was that, if by being measured S has information about A, assuming observer/observed and sentient/non-sentient equivilence, how does this manifest itself? The information would be about observer O with respect to the system S. Presumably that information will probably not exist with respect to any other system. Presume S to be, say, a particle.


I'm just glad no-one jumped all over me. I have been out of my depth before.

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.

Anyway, I've thought about this all day, and I'm not so keen on it any more. I'm going to follow this with a huge post explaining why. Some of it might be right, some not. Wouldn't mind some pointers, but its a big post so no big deal if none are forthcoming.

Take care.

El Hombre

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.

 

[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?)