Register to reply 
Everybody sees the same elephant (says Carlo Rovelli) 
Share this thread: 
#91
Apr2006, 08:34 PM

P: 198

Hi Dimitri,
well, the LQG people like to claim their approach is much older, not newer. I can toally relate to your notliking of LQG, it's definitly not appealing. Anyway, what I wanted to say... ...what did I want to say... ...sorry, long day... ...essentially: anything is better than nuclear physics and solid state Best, B. (gotta go home get some sleep ) 


#92
Apr2106, 05:39 AM

P: 307




#93
Apr2106, 10:19 PM

P: 859

To come back to the Elephant, although an extravagance perhaps, consider this quote from 1943:
This is taken from a set of lectures given by Schroedinger in Dublin, which became very well known and were subsequently read by Watson and Crick and others hot on the trail of the mystery of genes. many worlds but one elephant 


#94
Apr2106, 11:25 PM

Astronomy
Sci Advisor
PF Gold
P: 23,108

nice quote from Schrödinger
this Kea koan not bad either many worlds but one elephant 


#95
May106, 11:18 AM

Astronomy
Sci Advisor
PF Gold
P: 23,108

found this on the web, Matt Liefer, a guy at Perimeter, had it on his blog
http://mattleifer.wordpress.com/2006...hilbertspace/ http://www.perimeterinstitute.ca/per...mmandments.pdf in quoting I dropped some technical notationthe link has Liefer's original =========quote======== TEN COMMANDMENTS OF THE CHURCH OF THE SMALLER HILBERT SPACE With apologies to Charlie Bennett and, of course, God. 1. I am ρ , your state, who brought you out of wavefunction realism, the place of orthodox dogmatism. 2. Do not have any other states except Me. Do not represent states by false purifications, conceived as ontological states of the Platonic forms above, of reality below, or of the spacetime foam underlying reality. Do not bow down to such states or worship them. I am ρ your state, a state that demands exclusive belief. 3. Do not announce your state ρ in vain. ρ will not allow the one who announces it in vain to go unpunished by a Dutch bookie. 4. Remember the CPmap E to keep dynamics meaningful. You can work things out using six different Kraus decompositions or Steinspring dilations and do all your tasks. But the CPmap E is an equivalence class to ρ your state. Do not do anything that attaches meaning to the arbitrary tools you choose to work with. This includes your Hilbert Space basis..., your purifications... 5. Honor your forefathers by using the Hilbert space algorithm they handed down to you to calculate your expected utilities. You will then live long on the land that ρ your state describes your beliefs about. 6. Do not commit murder, since there is no other “branch of the wavefunction” in which your victim will survive. 7. Do not adulterate the Schrödinger equation by adding nonlinear terms designed to cause collapse. 8. Do not steal from classical physics by insisting that particle position or field configuration variables must evolve deterministically. 9. Do not testify as a false witness to the existence of histories of events that do not appear in the empirical records. 10. Do not be envious of your neighbor’s state σ. Do not be envious of your neighbor’s dynamical CPmap ..., his POVM elements..., his update CPmaps..., his Kraus operators..., his donkey, or anything else that is your neighbor’s, for they only describe his beliefs (except for the donkey), which naturally differ from yours. ===endquote=== 


#96
May2406, 07:15 PM

Astronomy
Sci Advisor
PF Gold
P: 23,108

Bee has introduced another animal into the picture, this time a dog.
A number of people were discussing this at her blog and I posted twice already and still have a bit more to say, so I will say it here. In Bee's story there are two experimenters A and B and they agree to take their entangled electrons to separate places and on a certain day each perform an measurement. Let's make them women for a change. They agree to point the machines to the East and read the spin. Each person has a hilbertspace and each person has a wavefuntion or state in that space which describes what they have learned about life so far, and about the universe, like what to expect if you go out with Italian menwhat to do if you see an elephant, and so on. A and B are welleducated so they expect that one of them will get +1 and one get 1. Everything is crystal clear to them. Off they go to their respective stations, which are quite far apart. the day arrives and A does her measurement and she gets 1, so she applies the appropriate projection operator and collapses part of her wavefunction to show the new information she has about HER electron. She also has in her wavefunction or state vector some experience of how RELIABLE the other experimenter, B, is. And how often B's location is hit by hurricanes. In the hypothetical situation that B is TOTALLY reliable, always remembers to do what she is supposed to, TOTALLY competent, always gets her lab machines to work perfectly, and NEVER hit by hurricanes, then of course B would be expected to be reading +1 right now, because the spins on a given axis add up to one. But that would not be realistic. So A does not commit herself right away, she doesnt collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesnt have that information yet. Before she does that she will at least telephone, or maybe even go and check out the other station, where B is. Before recording any information about B's electron, she has to get in CAUSAL CONTACT. SO THE COLLAPSE OF A's wavefunction is LOCAL. Somebody had to get in somebody else's lightcone, or even go over and stand next to them at the same spot, for it to happen. 


#97
May2406, 07:28 PM

Astronomy
Sci Advisor
PF Gold
P: 23,108

I guess the point is that even if A knows B to be admirable in every respectvery reliable and competent in the lab etc.maybe B is just then having an argument with her boyfriend, or maybe she has a hangover, or there has been a supernova explosion, or it just simply isn't her day. This happens sometimes. So you can't be sure.
the advice to A is, DONT COLLAPSE YOUR WAVEFUNCTION UNTIL YOU SEE THE WHITES OF THEIR EYES and you have definite information, dont go collapsing it based on some suppostion about somewhere you are not in causal contact with, some spacelike separated place where you cant have any idea what is happening there. so now what about the DOG??? Well Sabine ups the narrative stakes by having B take along a dog to her station, and if her reading comes out +1 then B should SHOOT THE DOG. And conversely if the measurment comes out 1 then she should not shoot the dog. There is a picture of the dog, which is an unpleasant overweight bulldog which it would be tempting to shoot regardless how the experiment turned out. I will get a link so you can go read further discussions of this http://backreaction.blogspot.com/ http://backreaction.blogspot.com/200...nlocality.html 


#98
May3006, 03:57 PM

Emeritus
Sci Advisor
PF Gold
P: 6,236

As such, one can indeed see quantum theory as the theory that explains A's experience: first A sees her local result (local collapse of A's state), then A encounters B (local collapse of A's experience of B's state)... So collapse occurs when A becomes, say, consciously aware of something. This is all fine and well. The problem arrives when we want the theory to describe at the same time also what B experiences, from its viewpoint. Now, you can of course say that we should now apply the formalism on B's side, but there's a problem. When A became aware of her result, and locally collapsed HER wavefunction, what can we say on B's side, from B's point of view ? If you have A's wavefunction collapse from B's side too, then we are in contradiction with what we tried to establish, namely only "local collapse upon local becoming aware of the result". But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world. From A's PoV: State before A measures: a+>(u b> + v b+>) + a> (x b+> + y b>) A measures and "experiences" the result, it is +: > collapse (for A) into A+>(u b> + v b+>) B might measure far away, but A doesn't know yet... A meets B, and "measures" B's state, result of B is +: > collapse (for A) into A+>  B+> fine. From B's PoV: State before A measures: a+>(u b> + v b+>) + a> (x b+> + y b>) A measures, but this doesn't change anything to B yet. B measures now, finds  > Collapse on B's side: (u a+> + y a>) B> B meets A, finds : > Collapse on B's side: A>B> All this is nice and well, except that from A's PoV, A and B got + and +, while from B's PoV, they both got  and  This can be solved in a MWI scenario, by replacing collapse by "branch is consciously observed by" Take # the "is conscious state by A" tag, and * the "is conscious state by B" tag, then we just have the state: Before measurement, all states are still part of the "relative state" of A and of B: a+#*>(u b#*> + v b+#*>) + a#*> (x b+#*> + y b#*>) A does a measurement, and found +, only changes the accessible states of A ; we remove what is NOT anymore part of "A's conscious world" (is projected out by the "collapse according to A": a+#*>(u b#*> + v b+#*>) + a*> (x b+*> + y b*>) B does a measurement and found : a+#*>(u b#*> + v b+#>) + a*> (x b+> + y b*>) Now, A and B meet. First, A "measures" B (that is, A learns about the "B" state) A learns that B saw +, so this gives: a+#*>(u b*> + v b+#>) + a*> (x b+> + y b*>) About at the same time, B measures A, that is, B learns that "A saw ": a+#>(u b> + v b+#>) + a*> (x b+> + y b*>) What now emerges is that, in "A's mental world", symbolized by #, the state of the first particle is seen to be a+ and the state of the b particle is seen to be b+. In "B's mental world", symbolized by *, the state of particle a is , and the one of b is  too. So both "relative views" intermixed is simply a many worlds view where the "objective" wavefunction didn't collapse, but where "the awareness of a state" narrowed down its scope as a function of what it was made aware off, to the piece of the overall wavefunction that corresponds to its measurement results. As such, the Rovelli flavor of the relative state view is the "oneobserver" version of the many worlds view (where many observers are considered in parallel). 


#99
May3006, 03:58 PM

Emeritus
Sci Advisor
PF Gold
P: 6,236

As such, one can indeed see quantum theory as the theory that explains A's experience: first A sees her local result (local collapse of A's state), then A encounters B (local collapse of A's experience of B's state)... So collapse occurs when A becomes, say, consciously aware of something. This is all fine and well. The problem arrives when we want the theory to describe at the same time also what B experiences, from its viewpoint. Now, you can of course say that we should now apply the formalism on B's side, but there's a problem. When A became aware of her result, and locally collapsed HER wavefunction, what can we say on B's side, from B's point of view ? If you have A's wavefunction collapse from B's side too, then we are in contradiction with what we tried to establish, namely only "local collapse upon local becoming aware of the result". But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world. From A's PoV: State before A measures: a+>(u b> + v b+>) + a> (x b+> + y b>) A measures and "experiences" the result, it is +: > collapse (for A) into A+>(u b> + v b+>) B might measure far away, but A doesn't know yet... A meets B, and "measures" B's state, result of B is +: > collapse (for A) into A+>  B+> fine. From B's PoV: State before A measures: a+>(u b> + v b+>) + a> (x b+> + y b>) A measures, but this doesn't change anything to B yet. B measures now, finds  > Collapse on B's side: (u a+> + y a>) B> B meets A, finds : > Collapse on B's side: A>B> All this is nice and well, except that from A's PoV, A and B got + and +, while from B's PoV, they both got  and  This can be solved in a MWI scenario, by replacing collapse by "branch is consciously observed by" Take # the "is conscious state by A" tag, and * the "is conscious state by B" tag, then we just have the state: Before measurement, all states are still part of the "relative state" of A and of B: a+#*>(u b#*> + v b+#*>) + a#*> (x b+#*> + y b#*>) A does a measurement, and found +, only changes the accessible states of A ; we remove what is NOT anymore part of "A's conscious world" (is projected out by the "collapse according to A": a+#*>(u b#*> + v b+#*>) + a*> (x b+*> + y b*>) B does a measurement and found : a+#*>(u b#*> + v b+#>) + a*> (x b+> + y b*>) Now, A and B meet. First, A "measures" B (that is, A learns about the "B" state) A learns that B saw +, so this gives: a+#*>(u b*> + v b+#>) + a*> (x b+> + y b*>) About at the same time, B measures A, that is, B learns that "A saw ": a+#>(u b> + v b+#>) + a*> (x b+> + y b*>) What now emerges is that, in "A's mental world", symbolized by #, the state of the first particle is seen to be a+ and the state of the b particle is seen to be b+. In "B's mental world", symbolized by *, the state of particle a is , and the one of b is  too. So both "relative views" intermixed is simply a many worlds view where the "objective" wavefunction didn't collapse, but where "the awareness of a state" narrowed down its scope as a function of what it was made aware off, to the piece of the overall wavefunction that corresponds to its measurement results. As such, the Rovelli flavor of the relative state view is the "oneobserver" version of the many worlds view (where many observers are considered in parallel). 


#100
May3006, 04:34 PM

Astronomy
Sci Advisor
PF Gold
P: 23,108

**But if A's wavefunction DIDN'T collapse from B's point of view, then WE'VE LOST THE POTENTIAL LINK WITH A's experienced world.
** I think we didnt lose the possible linkage because a twoway link occurs when the two are in the same placelinkage (one way or two way)follows causal contact there is more on this at Bee's blog. To my considerable pleasure and surprise she indicates agreement to some extent. I don't know how far her agreement goes, so you'd best consult the blog or her directly. Im happy for you to totally disagree! I just can't take the time to discuss this too much. I imagine Rovelli would consider his picture "multipleobserver" and "single world". Rovelli picture is multipleobserver in the sence 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". But if you see a connection with "many worlds" that is fine! 


#101
May3106, 03:27 AM

Emeritus
Sci Advisor
PF Gold
P: 6,236

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. 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 hardtoswallow 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 realised 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. 


#102
Jun306, 07:05 PM

Emeritus
PF Gold
P: 8,147

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: 


#103
Jun406, 10:18 AM

Astronomy
Sci Advisor
PF Gold
P: 23,108

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 REALITIESpossibilities 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." 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! 


#104
Jun406, 10:32 AM

Emeritus
Sci Advisor
PF Gold
P: 6,236

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 rediscovered MWI, or at least the aspect that each branch "only sees itself", and that there is a selfconsistency 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... 


#105
Jun406, 01:07 PM

Emeritus
PF Gold
P: 8,147

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. 


#106
Jun406, 02:00 PM

Emeritus
Sci Advisor
PF Gold
P: 6,236

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. Note that in special relativity, as such, this is not necessarily an issue, because unitarity is conserved under a lorentz transformation, so you can "retell" the story along different timelike axes, the entire system is normally consistent. 


#107
Jun406, 02:32 PM

Emeritus
Sci Advisor
PF Gold
P: 6,236

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 nonlocal 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 premeasurement 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 observerrelated. It's because it is intuitively as if "the single observer" branched now into many (the single premeasurement 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...). One is inclined to say that the former is a "subjective reality" and the latter an "objective reality". 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 aobservers 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 (= apointer states = abranches = 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 aobservers, there is a SPECIFIC record, and it is IN AGREEMENT with the accessible record of his "associated" bobserver (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. 


#108
Jun406, 05:32 PM

Emeritus
PF Gold
P: 8,147

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


Register to reply 
Related Discussions  
Monte Carlo and Inverse Monte Carlo  Good Sources for Self Study?  General Physics  1  
Can your elephant do this?  General Discussion  2  
Zero points for the elephant  General Discussion  1  
The Carlo Rovelli book on loop quantum gravity  Beyond the Standard Model  25  
Quantum Gravity by Carlo Rovelli, to be published by Cambridge UP  General Physics  0 