Everybody sees the same elephant (says Carlo Rovelli)

[hossi]

Hi Dimitri,

well, the LQG people like to claim their approach is much older, not newer. I can toally relate to your not-liking of LQG, it's definately 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



B. (gotta go home get some sleep :zzz:)

 

[Kalimaa23]

...essentially: anything is better than nuclear physics and solid state

Amen!

 

[Kea]

To come back to the Elephant, although an extravagance perhaps, consider this quote from 1943:

The only possible alternative is simply to keep to the immediate experience that consciousness is a singular of which the plural is unknown; that there is only one thing and that, what seems to be a plurality, is merely a series of different aspects of this one thing, produced by a deception (the Indian Maya); the same illusion is produced in a gallery of mirrors, and in the same way Gaurisankar and Mt Everest turned out to be the same peak seen from different valleys.

There are, of course, elaborate ghost-stories fixed in our minds to hamper our acceptance of such simple recognition.

Erwin Schroedinger, What is life? 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

 

[marcus]

nice quote from Schrödinger

this Kea koan not bad either

many worlds but one elephant

 

[marcus]

Ten Commandments Of The Church Of The Smaller Hilbert Space

found this on the web, Matt Liefer, a guy at Perimeter, had it on his blog
http://mattleifer.wordpress.com/2006/04/13/the-church-of-the-smaller-hilbert-space/

http://www.perimeterinstitute.ca/personal/mleifer/humor/commandments.pdf

in quoting I dropped some technical notation---the 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 space-time 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 CP-map 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 CP-map 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 CP-map ..., his POVM elements..., his update CP-maps..., 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===

 

[marcus]

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 men---what to do if you see an elephant, and so on.

A and B are well-educated 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 doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't 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.

 

[marcus]

I guess the point is that even if A knows B to be admirable in every respect---very 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, don't go collapsing it based on some suppostion about somewhere you are not in causal contact with, some spacelike separated place where you can't 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/2006/05/nonlocality.html

 

[vanesch]

A and B are well-educated 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 doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't 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.

Yes, this is a perfectly all right view from a "solipsist" viewpoint: there is only ONE observer in this world, in casu "A". We never talk about the experience "B" lives, we only talk about what A OBSERVES from "B"s state.

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 "one-observer" version of the many worlds view (where many observers are considered in parallel).

 

[vanesch]

A and B are well-educated 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 doesn't collapse the wavefunction in her state space that codes the outcome of the distant measurement because she doesn't 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.

Yes, this is a perfectly all right view from a "solipsist" viewpoint: there is only ONE observer in this world, in casu "A". We never talk about the experience "B" lives, we only talk about what A OBSERVES from "B"s state.

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 "one-observer" version of the many worlds view (where many observers are considered in parallel).

 

[marcus]

**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 two-way link occurs when the two are in the same place----linkage (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 "multiple-observer" and "single world". 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".

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

 

[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.