Graduate I think I just became a QBist ?

[strangerep]

I think I just became a "QBist" ?

Just finished a first reading of this paper:

C.A.Fuchs, N.D.Mermin, R.Schack,
"An Introduction to QBism with an Application to the Locality of Quantum Mechanics",
http://arxiv.org/abs/1311.5253

Abstract:

We give an introduction to the QBist interpretation of quantum mechanics. We note that it removes the paradoxes, conundra, and pseudo-problems that have plagued quantum foundations for the past nine decades. As an example, we show in detail how it eliminates “quantum nonlocality”.

Interesting that it has ideas that remind me of Rovelli's Relational QM and Relational EPR (which I find appealing), though Rovelli is not cited in the FMS paper.

I like it because (imho) it cuts through a lot of the widespread BS that wafts around QM.

(I mention the FMS paper here in BSTM, rather than the quantum forum, since it's a bit off the mainstream.)

 

[mitchell porter]

I don't have the patience to work out how "QBism" is different from the Copenhagen interpretation (in its original, epistemic form, which says that observables are what's real and that wavefunctions etc are just calculating devices), but if it helps QM make sense to you as a theory making probabilistic connections between states of the world that by classical standards are incompletely specified (e.g. because the definiteness of complementary observables is constrained by the uncertainty principle) - then good for you.

Just don't kid yourself that such epistemic, instrumental, operational... interpretations of QM, make sense as a final statement about the nature of reality. You have to beware of this because authors of epistemic interpretations of QM, right back to Bohr himself, are always inventing convoluted rationalizations as to why certain questions don't need to be answered, why it makes sense to say that reality is objectively indefinite, and so on.

Eventually, physics will have to return to objectivity to progress. Purely instrumental theories are necessarily "incomplete" (unfinished; not the full story about reality). The status of QM makes a lot more sense once you accept that it is incomplete as a theory of reality. Then you can accept it for what it is - something that works, but not an ultimate truth

 

[Salman2]

For balance, Chris Fields offer a critique of the QBism model presented by Fuchs based on the "measured" quantum reality of the wavefunction for the agent (the observer):

http://arxiv.org/pdf/1108.2024v2.pdf

At the end of the paper is this comment by Fields about QBism:

"QBism provides no physical distinction between observers and the systems they observe, treating all quantum systems as autonomous agents that respond to observations by updating beliefs and employ quantum mechanics as a “users’ manual” to guide behavior. However, it treats observation itself as a physical process in which an “observer” acts on a “system” with a POVM and the system” selects a POVM component as the “observer’s experience” in return. This requirement renders the assumption that systems be well-defined - i.e. have constant d-impossible to implement operationally. It similarly forces the consistent QBist to regard the environment as an effectively omniscient observer, threatening the fundamental assumption of subjective probabilities and forcing the conclusion that QBist observers cannot segment their environments into objectively separate systems."

 

[Naty1]

...a lot of the widespread BS that wafts around QM.

maybe that 'aroma' wafting around stems from old intuitions which turned out to be predictive, extremely useful, but nobody still really knows why they work...but suspect the theory is likely incomplete...but the old gaffers were rather insightful!


With QM, the Schrödinger equation describes the continuous time evolution of a system's wave function and is deterministic. However, the relationship between that wave function and the observable properties of the system appear to be non-deterministic….and fundamentally inconsistent with many of our 'classical' prejudices...


from Roger Penrose [the mathematical physicist] celebrating Stephen Hawking’s 60th birthday in 1993 at Cambridge England...

..Either we do physics on a large scale, in which case we use classical level physics; the equations of Newton, Maxwell or Einstein and these equations are deterministic, time symmetric and local. Or we may do quantum theory, if we are looking at small things; then we tend to use a different framework where time evolution is described... by what is called unitary evolution...which in one of the most familiar descriptions is the evolution according to the Schrödinger equation: deterministic, time symmetric and local. These are exactly the same words I used to describe classical physics.

However this is not the entire story... In addition we require what is called the "reduction of the state vector" or "collapse" of the wave function to describe the procedure that is adopted when an effect is magnified from the quantum to the classical level...quantum state reduction is non deterministic, time-asymmetric and non local...The way we do quantum mechanics is to adopt a strange procedure which always seems to work...the superposition of alternative probabilities involving w, z, complex numbers...an essential ingredient of the Schrödinger equation. When you magnify to the classical level you take the squared modulii (of w, z) and these do give you the alternative probabilities of the two alternatives to happen...it is a completely different process from the quantum (realm) where the complex numbers w and z remain as constants "just sitting there"..

If we were not so involved, it would make rather funny reading.

 

[atyy]

Does Bayes rule really imply Bayesian updating?

 

[marcus]

...

C.A.Fuchs, N.D.Mermin, R.Schack,
"An Introduction to QBism with an Application to the Locality of Quantum Mechanics",
http://arxiv.org/abs/1311.5253Interesting that it has ideas that remind me of Rovelli's Relational QM and Relational EPR (which I find appealing), though Rovelli is not cited in the FMS paper.

...
(I mention the FMS paper here in BSTM, rather than the quantum forum, since it's a bit off the mainstream.)

We had an earlier thread here in BTSM about the Smerlak-Rovelli paper that you clearly are familiar with, that deals with non-locality in what i think is essentially the same way!
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
(Submitted on 10 Apr 2006)
We study the EPR-type correlations from the perspective of the relational interpretation of quantum mechanics. We argue that these correlations do not entail any form of 'non-locality', when viewed in the context of this interpretation. The abandonment of strict Einstein realism implied by the relational stance permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality.
10 pages, published in Foundations of Physics 37:427-445,2007

Thanks for pointing us to the Fuchs-Mermin-Schack! It's very well written, could be the clearest exposition of this idea so far, plus the catchy new name.

The BTSM thread was
https://www.physicsforums.com/showthread.php?t=117286

 

[strangerep]

I see that DrDu has resurrected an old thread in the QM forum that was about the "Quantum Bayesian Interpretation of QM": https://www.physicsforums.com/showthread.php?t=692569

I looked at the FS paper which started that thread (note: no Mermin). I quickly became bored with the philosophical style that DrDu refers to as speculations about what Feynman meant. I, too, have no time for that sort of thing.

Probably, if I had put more time into the FS paper, and that old thread, I wouldn't have bothered to read the new FMS paper. But I like the FMS paper, from which I infer that N.D.Mermin must have had quite a large input into it.

Anyway, maybe I shouldn't call myself a "QBist" after all -- since the original FS paper makes me feel like I'm suffocating. Similarly for Fields' critique.

I don't have the patience to work out how "QBism" is different from the Copenhagen interpretation [...]

Er, it's not clear whether you actually read the FMS paper.(?) Most of your post #2 seems to be constructing a straw man to represent me. (I don't respond to straw men.)

We had an earlier thread here in BTSM about the Smerlak-Rovelli paper that you clearly are familiar with, that deals with non-locality in what i think is essentially the same way!

Yes, precisely. -- I had a feeling you'd recognize that.

[Fuchs-Mermin-Schack is] very well written, could be the clearest exposition of this idea so far, plus the catchy new name.

It's certainly clear -- I was able to read it easily in one sitting without getting bored, which is quite unusual (for me) with "interpretative" papers.

But when looked again at the older FS paper on QBism (i.e., in a nonrelativistic context), I got depressed again. That name clearly has a sour taste for some.

The value (imho) lies in the synergy that emerges when working with those ideas in the context of special relativity.

 

[atyy]

A quick look at Fuchs and Schack's "Quantum-Bayesian Coherence: The No-Nonsense Version" http://arxiv.org/abs/1301.3274 seems to suggest that Bayesian updating cannot be derived from Bayes rule, as they state it as an assumption.
"Assumption 2: Principle of Reciprocity: Posteriors from Maximal Ignorance Are Priors." (Eq 122)
"Resumption 1: Principle of Reciprocity: Posteriors from Maximal Ignorance Are Priors." (Eq 130)

This is why I think collapse (or Bayesian updating) must be postulated, or additional axioms introduced if it is not postulated.

I do like the idea of wave function collapse as Bayesian updating. It's intuitive (I've heard it informally many times before hearing of the QBist programme). Also, the Bohmian interpretation always seemed to me like Bayesian updating. So I googled, and found Howard Wiseman's "Grounding Bohmian Mechanics in Weak Values and Bayesianism" http://arxiv.org/abs/0706.2522 , which might be interesting to compare and contrast. Wiseman's remarks "As soon as an innocent observer were to open her eyes she would collapse her state of belief about x from P_prior(x; t) to a much sharper P(x; t), by observing the location of objects (from the pointer on a meter to the stars in the sky) relative to her. Note that this "collapse" is completely classical: it is just Bayesian updating of her beliefs about the positions of macroscopic objects."

 

[mitchell porter]

Er, it's not clear whether you actually read the FMS paper.(?)

I can hardly bring myself to read it, it is so obnoxiously vacuous. It is effectively solipsistic. All the mysteries of quantum mechanics are to be resolved by focusing on the successive experiences of a single agent, and by refusing to think about any reality beyond that.

The further I look into this paper, the worse it looks. For example, in the title they promise a demonstration that quantum mechanics is local. How does it work? Well, their method, as I mentioned, is to focus on the experience of a single agent. Nonlocality involves mysterious connections between two spatially separate locations; but a single agent is only ever in one place at a time, therefore nonlocality cannot arise - apparently other places just don't exist!

Or look at the end of page 6 and beginning of page 7. EPR's perplexity arose because they insisted on thinking that physical theories make statements about physical reality. If only they had understood instead that physical theories are about "firmly held beliefs", then we could have avoided all that unpleasant agitation about the implications of quantum mechanics for the nature of physical reality. Under the QBist interpretation, QM is simply not about physical reality, therefore it has no such implications! Problems solved!

Most of your post #2 seems to be constructing a straw man to represent me. (I don't respond to straw men.)

What I wrote is a protest against a certain type of quantum sophistry and a warning against falling for it, intended for anyone reading a thread like this.

On a second viewing, my judgement of this paper is even harsher. I take back what I said about how it might have some positive value.

If people want QM to make sense, all they have to say is that it gives you the probability for going from one physical situation to another, but that it doesn't tell you what happens in between. So it's incomplete. It's not the final theory of physics. That's all that has to be said.

(I put that in bold so that people who are looking for a quantum philosophy, a way to understand what QM means, have something to work with. Those sentences in bold - that is the attitude towards QM that I recommend.)

But too many people want to turn QM into a philosophical idol, a new kind of science, in which its incompleteness is mysteriously a virtue. For example, in this paper, the authors want to turn QM into a sort of solipsistic anti-theory, in which the answer to various questions is just: un-ask the question, we should only care about the experience of a single observer, nothing else matters.

I call this approach an anti-theory because a theory ventures to make statements about reality, and their whole approach is to eschew such statements on principle. They misunderstand the significance of the fact that everything comes to us through personal experience.

Unless one intends to be a solipsist, with no explanation at all of the regularities in your experience, then there is more to reality than just your private sensations. And traditionally, a physical theory consists of some hypotheses, right or wrong, about what's going on in that greater reality.

It's one of the distinctive oddities of quantum mechanics that (at least, according to Copenhagen) it does not present a hypothesis about what takes place between observable X taking the value x, and observable Y later taking the value y, it just presents a calculational procedure for obtaining the conditional probability Pr(Y=y|X=x).

According to QBism - if these authors have portrayed it correctly - then QM is most truly itself when the observables are private sensations (qualia) of some individual, and when every other part of reality is regarded with Copenhagenist disinterest. That is the formula they propose to use, in order to resolve all the conundra of quantum mechanics.

Technically one might object to this on the grounds that we just don't use QM in that way. In actual applications of QM, the observables are things like "the spin of the particle" or "the location of the particle"... not "my experience of the apparatus, after I cycled to the lab, unlocked the door, and sat down to check the readouts".

And I've already stated the philosophical objection - unless you're a solipsist, there is more to reality than just your private stream of experience, and an interpretation of QM needs to say something about the theory's implications for the world outside your skull. Unless you're a solipsist, exclusively focusing on private experience is just a way to evade questions, not a way to answer them.

strangerep, sorry if you experience my scorn for this outlook as a personal attack. You don't actually express your own ideas much, so I don't know your opinions. But the philosophy of this paper, when scrutinized in detail, is absurd and needs to be exposed as such. It is also pernicious to the discovery of truth, if people read a paper like this and come away thinking that various unanswered questions have been satisfactorily answered in it. That's why I say it has negative value, because it produces an illusion of conceptual progress.

P.S. As atyy has just posted, it absolutely makes sense to think of the use of QM as part of a process of Bayesian updating. My point is just that this perspective doesn't deal with an ontological problem like the nature of quantum nonlocality, and yet the headline claim of this "QBist" paper is precisely that this problem has been dissolved.

 

[Demystifier]

Unless one intends to be a solipsist, with no explanation at all of the regularities in your experience,

How about being solipsist WITH an explanation of the regularities of one's experiences?
http://lanl.arxiv.org/abs/1112.2034

 

[martinbn]

...from Roger Penrose [the mathematical physicist] celebrating Stephen Hawking’s 60th birthday in 1993 at Cambridge England...

Are you sure? Hawking was not 60 in 1993!

 

[marcus]

I can hardly bring myself to read it, it is so obnoxiously vacuous. It is effectively solipsistic...

I don't think so. Seems rather to parallel "Relational EPR" (Smer. Rov. 2006) which addresses the issue and is careful to explain that the view of reality presented is not solipsist. I think according to Aristotle reality consists of what we all see and can agree on.

So the longstanding Mediterranean idea is that reality is determined by a community of observers who communicate among themselves.

All Rel-EPR adds to that traditional picture, as I see it, is take note of the fact that each of the observers is subject to quantum mechanics (is a "quantum system") and their intercommunication is limited by the usual 1905 speed conventions.

IOW there is something REAL out there, and don't let it throw you into a tizzy if it is a complexly entangled superposition. Your perception/intervention extracts classical pictures and facts. I accept you, as a fellow observer, are real. You are a quantum system too. I don't know what you just experienced because you haven't told me---the news hasn't reached me yet. But I acknowledge realty is real and making an impression on you, affecting your quantum state. Each of us "classicalizes" the reality we interact with.

 

[atyy]

Matt Leifer has a classification of realist and non-realist interpretations in http://mattleifer.info/2011/11/20/can-the-quantum-state-be-interpreted-statistically/

"1. Wavefunctions are epistemic and there is some underlying ontic state. Quantum mechanics is the statistical theory of these ontic states in analogy with Liouville mechanics.
2. Wavefunctions are epistemic, but there is no deeper underlying reality.
3. Wavefunctions are ontic (there may also be additional ontic degrees of freedom, which is an important distinction but not relevant to the present discussion)"

"Options 1 and 3 share a conviction of scientific realism, which is the idea that there must be some description of what is going on in reality that is independent of our knowledge of it. Option 2 is broadly anti-realist, although there can be some subtleties here[2]."

His footnote [2] about whether the subtleties of option 2 being "anti-realist" is

"The subtlety is basically a person called Chris Fuchs. He is clearly in the option 2 camp, but claims to be a scientific realist. Whether he is successful at maintaining realism is a matter of debate."

Bolding is mine, not Matt Leifer's.

Leifer has additional comments on the QBist proposal which seem very congruent with mitchell porter's:

"I would classify the Copenhagen interpretation, as represented by Niels Bohr[3], under option 2. One of his famous quotes is:

There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature…[4]

and “what we can say” certainly seems to imply that we are talking about our knowledge of reality rather than reality itself. Various contemporary neo-Copenhagen approaches also fall under this option, e.g. the Quantum Bayesianism of Carlton Caves, Chris Fuchs and Ruediger Schack; Anton Zeilinger’s idea that quantum physics is only about information; and the view presently advocated by the philosopher Jeff Bub. These views are safe from refutation by the PBR theorem, although one may debate whether they are desirable on other grounds, e.g. the accusation of instrumentalism."

 

[marcus]

If people want QM to make sense, all they have to say is that it gives you the probability for going from one physical situation to another, but that it doesn't tell you what happens in between. So it's incomplete. It's not the final theory of physics. That's all that has to be said.

But too many people want to turn QM into a philosophical idol, a new kind of science, in which its incompleteness is mysteriously a virtue...

AFAiCS it is in no way "incomplete". What do you want to be "in between", a classical trajectory?

What happens in between observations/interactions is that the quantum system continues to evolve.

 

[strangerep]

You don't actually express your own ideas much, so I don't know your opinions.

Many years ago, I had plenty of opinions, and expressed them readily. Most of those opinions turned out to be wrong. I now regard opinions and beliefs as being of little worth.

So... my stance now could be summed up as: "evidence-based, plus Occam's razor", and "no, I don't want what they're smoking".

Evidence that the Earth is not flat is absurd according to a flat-Earth believer.

A suggestion that there is not one single over-arching "reality", but a multitude which nevertheless have interactions and hence partial correlations, thereby synthesizing an impression of a single reality, is probably absurd and pernicious to anyone who adheres to the loaded meaning of the word "reality" as "all that is".

I prefer to keep an open mind and follow the evidence.

(And now I wait for this thread to be terminated for being too philosophical...)

 

[marcus]

Today I followed the audio and slides of Tim Koslowski's ILQGS talk. The audio is faint part of the time (not speaking close enough to the microphone,…or a bad connection.) "What can we learn from Shape Dynamics?"
He seems to be saying no single over-arching spacetime.

This also seems to be the message from some work by Laurent Freidel and others called "relative locality".

In neither case do they bother to deny a single overarching reality. It is just that the mathematical representation, the model, allows the possibility that there is no one single spacetime---geometric discrepancies are somehow allowed.

According to Koslowski the conventional spacetime of General Relativity is recovered somehow, but not so precisely or completely as to prevent a test. I'll get the links to the slides PDF and to the audio, in case anyone is interested.
http://relativity.phys.lsu.edu/ilqgs/koslowski111213.pdf
http://relativity.phys.lsu.edu/ilqgs/koslowski111213.wav

An early "relative locality" paper: http://arxiv.org/abs/1101.0931

 

[MathematicalPhysicist]

I don't have the patience to work out how "QBism" is different from the Copenhagen interpretation (in its original, epistemic form, which says that observables are what's real and that wavefunctions etc are just calculating devices), but if it helps QM make sense to you as a theory making probabilistic connections between states of the world that by classical standards are incompletely specified (e.g. because the definiteness of complementary observables is constrained by the uncertainty principle) - then good for you.

Just don't kid yourself that such epistemic, instrumental, operational... interpretations of QM, make sense as a final statement about the nature of reality. You have to beware of this because authors of epistemic interpretations of QM, right back to Bohr himself, are always inventing convoluted rationalizations as to why certain questions don't need to be answered, why it makes sense to say that reality is objectively indefinite, and so on.

Eventually, physics will have to return to objectivity to progress. Purely instrumental theories are necessarily "incomplete" (unfinished; not the full story about reality). The status of QM makes a lot more sense once you accept that it is incomplete as a theory of reality. Then you can accept it for what it is - something that works, but not an ultimate truth

Ah, the elusive ultimate truth, is it even possible to attain, who knows.

 

[mitchell porter]

Seems rather to parallel "Relational EPR" (Smer. Rov. 2006) which addresses the issue and is careful to explain that the view of reality presented is not solipsist. I think according to Aristotle reality consists of what we all see and can agree on.

So the longstanding Mediterranean idea is that reality is determined by a community of observers who communicate among themselves.

The relational interpretation, as I understand it, is that states of quantum objects are supposed to be relative to observers, in a way somehow analogous to the relativity of motion. But consider Schrödinger's cat. Either the cat is alive, the cat is dead, or there's more than one cat. It makes no sense to say that there is only one cat, and it's alive relative to those who see it as alive, and it's dead relative to those who see it as dead.

As for reality, it just is what it is, regardless of whether we know the facts or accept them or guess them rightly or wrongly. Observers may share evidence or coordinate their guessing, but how can that have anything to do with e.g. whether physics is local? The Copenhagenist attempt to rationalize an epistemic, instrumental theory as the final word in physics ... perhaps I should call it the "old Copenhagen interpretation", since so many people now think Copenhagen means objective wavefunction collapse ... the old Copenhagen interpretation left a legacy of confused thinking which lives on in many newer interpretations.

I acknowledge reality is real and making an impression on you, affecting your quantum state. Each of us "classicalizes" the reality we interact with.

That sounds like "consciousness collapses the wavefunction", except that everyone is doing it in this version, not just the ghost of von Neumann.

AFAiCS it is in no way "incomplete". What do you want to be "in between", a classical trajectory?

What happens in between observations/interactions is that the quantum system continues to evolve.

How does this work? I'll use Penrose's paraphrase of quantum mechanics according to von Neumann. There is a unitary evolution U between observations, and a quantum jump R (R for reduction, as in state vector reduction) at the moment of observation. I assume that the relational interpretation is supposed to avoid the need for R, because the states of quantum objects are relativized to observers somehow? So the only process is unitary U, but when we look at objects, it's as if R has occurred? Sorry, I don't see how this makes sense. Again I refer to the cat. Are you really going to say that its quantum state is |dead>+|alive>, but that it is dead "relative to" a person who sees it as dead, etc? What would that even mean?

A suggestion that there is not one single over-arching "reality", but a multitude which nevertheless have interactions and hence partial correlations, thereby synthesizing an impression of a single reality, is probably absurd and pernicious to anyone who adheres to the loaded meaning of the word "reality" as "all that is".

I prefer to keep an open mind and follow the evidence.

So what are these multiple realities like? What sort of things are they? How would you describe one of these partial realities to me? I can't entertain a hypothesis if I don't know what it is.

 

[marcus]

Hi Mitchell, I didn't mean that "consciousness" was involved. What I said about interacting quantum subsystems (like "you") was meant more generally:

==quote==
I acknowledge reality is real and making an impression on you, affecting your quantum state. Each of us "classicalizes" the reality we interact with.
==endquote==

You don't have to be alive or conscious for your environs to make an impression on you. You could e.g. just be the electrode in a photocell.

I'm trying to articulate the viewpoint I get from "Relational EPR". Maybe I don't understand the Smerlak Rovelli paper correctly. If someone else can interpret it better that would be most welcome!

==quote Mitchell==
How does this work?
==endquote==

You are asking about the MATHEMATICAL REPRESENTATION of reality without continuous trajectories. I'm not an expert and I don't follow QM foundations research at all closely, but my sense is that this is work in progress. One way it might "work", I suspect, is this:

You represent reality by a C* algebra A and TIME as a one-parameter subgroup α_t on A.
A priori there is no Hilbert space, there is just the algebra of observables. A STATE is a positive functional ρ(a) defined on the algebra. The state can be used to construct the flow α_t.

So the flow of time depends on the state ρ. I'm simply speculating about how it might work out. There are continuous trajectories at the level of observables, in this picture, and there are transition probabilities I suppose, but in this picture there are no continuous trajectory *outcomes*. One cannot say what happened IN BETWEEN along the way

Maybe you or Strangerep or someone else can improve on this. Strangerep mentioned the Smerlak Rovelli paper "Relational EPR" which gives what I think is a fairly clear well-thought-out description of how reality could be. When you ask "how it works" I think you are asking about the possible ways to make a mathematical implementation. Clearly it does not have to "work" in an ENGLISH COMMON-LANGUAGE description and according to metaphors from everyday life. Math as an invented language for describing how reality works is evolving/adapting is more suitable and we just have to wait and see where it goes. I've suggested one possible direction.

 

[atyy]

The relational interpretation, as I understand it, is that states of quantum objects are supposed to be relative to observers, in a way somehow analogous to the relativity of motion. But consider Schrödinger's cat. Either the cat is alive, the cat is dead, or there's more than one cat. It makes no sense to say that there is only one cat, and it's alive relative to those who see it as alive, and it's dead relative to those who see it as dead.

As for reality, it just is what it is, regardless of whether we know the facts or accept them or guess them rightly or wrongly. Observers may share evidence or coordinate their guessing, but how can that have anything to do with e.g. whether physics is local? The Copenhagenist attempt to rationalize an epistemic, instrumental theory as the final word in physics ... perhaps I should call it the "old Copenhagen interpretation", since so many people now think Copenhagen means objective wavefunction collapse ... the old Copenhagen interpretation left a legacy of confused thinking which lives on in many newer interpretations.

I don't know if RQM works or solves any problems, but Smerlak and Rovelli http://arxiv.org/abs/quant-ph/0604064 do have interesting interpretational comments, and claim to disavow solipsism. "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 see an elephant. If not, something is wrong. ... So, what happens if A and B compare notes? Have they seen the same elephant? ... It is clear that everybody sees the same elephant. More precisely: everybody hears everybody else stating that they see the same elephant they see. This, after all, is a sound definition of objectivity."

This actually reminds me a bit of Zurek's http://arxiv.org/abs/0707.2832 "It is therefore not clear whether one is forced to attribute "reality" to all of the branches of the universal state vector. ... It can acquire objective existence only by "advertising itself" in the environment. This is obviously impossible for universal state vector - the Universe has no environment. Objective existence can be acquired (via quantum Darwinism) only by a relatively small fraction of all degrees of freedom within the quantum Universe ..."

Zurek seems to be working within some sort of relative state interpretation, so in his tentative view the wave function of the universe is the deep quantum reality. While the classical reality is only available for some states and observers. At least that's what he seems to be aiming for. In http://arxiv.org/abs/1205.3197 , he, Jess Riedel and Zwolak write "Whether an essentially unique quasi-classical realm [41, 42] can be identified from such principles is a deep, open question [43, 44] about the quantum-classical transition."

What is certainly not clear to me is whether RQM is really against what Smerlak and Rovelli call "Einstein's realism" that "there exists a physical reality independent of substantiation and perception.". Smerlak and Rovelli do write "RQM departs from such strict realism.". It is quite unclear to me why their view is incompatible with strict realism, except trivially. If by perception, they mean any interaction (they say "An atom interacting with another atom can be considered an observer") - then yes, things that don't interact at all can be considered not real. But even a strict realist would agree that it's not meaningful to talk about invisible fairies in the garden. So perhaps an error they make is that they take a realist view, but do not call it so.

Also, I don't know if Rovelli really favours anyone interpretation of QM. In his 2008 review http://relativity.livingreviews.org/Articles/lrr-2008-5/fulltext.html of LQG he writes "Loop quantum gravity is a standard quantum (field) theory. Pick your favorite interpretation of quantum mechanics, and use it for interpreting the quantum aspects of the theory. I will refer to two such interpretations below. When discussing the quantization of area and volume, I will use the relation between eigenvalues and outcomes of measurements performed with classical physical apparatuses; when discussing evolution, I will refer to the histories interpretation. The peculiar way of describing time evolution in a general relativistic theory may require some appropriate variants of standard interpretations, such as Hartle’s generalized quantum mechanics [140], or a suitable generalization of canonical quantum theory [261, 243, 245, 242]."

 

[atyy]

http://arxiv.org/abs/1302.4228
The Copenhagen Interpretation as an Emergent Phenomenon
Timothy J. Hollowood

Hollowood's paper seems to be inspired by Rovelli's RQM, but appears to have objective reality which is a wave function which undergoes unitary time evolution.

Question: Naively I think Bell's theorem says we have either locality or reality. But assuming Many-Worlds works, it seems to have reality, but it isn't clear to me that Many-Worlds is nonlocal. Is Many-Worlds nonlocal, or is it incorrect to think that Bell's theorem implies either locality or reality?

 

[strangerep]

So what are these multiple realities like? What sort of things are they?

I had in mind the ideas in the Smerlak-Rovelli relational EPR paper that others have already cited above.

 

[Paulibus]

Maybe you (Mitchell Porter) or Strangerep or someone else can improve on this. Strangerep mentioned the Smerlak Rovelli paper "Relational EPR" which gives what I think is a fairly clear well-thought-out description of how reality could be. When you ask "how it works" I think you are asking about the possible ways to make a mathematical implementation. Clearly it does not have to "work" in an ENGLISH COMMON-LANGUAGE description and according to metaphors from everyday life. Math as an invented language for describing how reality works is evolving/adapting is more suitable and we just have to wait and see where it goes.

I agree, and think the elephant in this particular room is that how physicists describe stuff is dictated by what physicists are; primates driven hard by evolution to try and describe how we experience a complex and often dangerous 'reality' : that which exists physically whether we are there to describe it or not. The invented language of mathematics can give physics descriptions a hugely advantageous quantitative, predictive and practical flavour, compared with much other hot air we generate so easily. But it seems to me a bit of a stretch to regard such wonderfully useful descriptions as 'eternal truth'. Or, for that matter, to so think of inventions like numbers, or equations like 1 + 1 = 2. Also, it seems natural, but sometimes wrong, to expect that domains of reality beyond our reach, like the territory covered by QFT, or the cosmos at large, are to be understood in terms of extrapolated descriptions of ordinary human experience.

I think that it would be appropriate for Fuchs, Mermin and Schack to be Stellenbosched, but nicely.

 

[DrDu]

I think that it would be appropriate for Fuchs, Mermin and Schack to be Stellenbosched, but nicely.

Please forgive me if this is slightly of topic, but here's always something to learn, at least for us poor non native speakers, in this forum...
http://en.wiktionary.org/wiki/stellenbosch

 

[Paulibus]

DrDu: An aside: if you look at the Fuchs et al. paper you will see that the Stellenbosching has in fact already been done!

 

[atyy]

DrDu: An aside: if you look at the Fuchs et al. paper you will see that the Stellenbosching has in fact already been done!

Question: Naively I think Bell's theorem says we have either locality or reality. But assuming Many-Worlds works, it seems to have reality, but it isn't clear to me that Many-Worlds is nonlocal. Is Many-Worlds nonlocal, or is it incorrect to think that Bell's theorem implies either locality or reality?

One explanation seems to be that Bell's theorem requires "counterfactual definiteness". However, it is hard for me to see what is counterfactual indefinite about Many-Worlds, since the wave function is real. A more approachable explanation to me seems to be that Bell's theorem assumes that each measurement has one outcome, whereas all outcomes happen in Many-Worlds.

I had in mind the ideas in the Smerlak-Rovelli relational EPR paper that others have already cited above.

Some thoughts about RQM. I basically don't understand it at all. Mainly, it seems to me not an interpretation. It feels like shut-up-and-calculate, which includes the well-known observation that the Heisenberg cut can be consistently placed in many different places.

One of RQM's motivations mentioned in http://arxiv.org/abs/quant-ph/9609002 is "As such, it bears a vague resemblance with Einstein’s discussion of special relativity, which is based on the critique of the notion of absolute simultaneity. The notion rejected here is the notion of absolute, or observer-independent, state of a system; equivalently, the notion of observer-independent values of physical quantities." However, that to me suggests the opposite lesson. While special relativity and general relativity did away with the notion of observer-independent simultaneity, it replaced it with a deeper reality - spacetime - which unifies the relations between all observers.

In a way, I feel like RQM is not about the existence or non-existence of reality. Rovelli's derivations in Eq 5-22 of http://arxiv.org/abs/quant-ph/9609002 feel much more like Lucien Hardy's Reconstructing Quantum Theory http://arxiv.org/abs/1303.1538 .

Another interesting read from Matt Leifer is his winning essay in this year's fqxi contest ""It from bit" and the quantum probability rule" http://arxiv.org/abs/1311.0857 . He talks about "it from bit" which I think is the spirit of RQM or Hardy's Reconstructing Quantum Theory, but also argues that it does not conflict with reality or "bit from it".

 

[S.Daedalus]

The relational interpretation, as I understand it, is that states of quantum objects are supposed to be relative to observers, in a way somehow analogous to the relativity of motion. But consider Schrödinger's cat. Either the cat is alive, the cat is dead, or there's more than one cat.

But this isn't even true classically. Consider the wheather: either it rains here, or it doesn't rain here (or there's more than one wheather here, whatever that may mean). But of course, neither of these propositions has any content, they're not capable of truth valuation. What's missing is their temporal index: it rains here now can only either be the case, or not the case. (In a sense, this means that 'there's more than one wheather here', as it may not have rained yesterday, but may rain tomorrow, but you can't really consider the wheater at two distinct points in time to coexist, since existence itself is time-indexed: things exist at one point in time, and not at another.)

So there's only one wheather, and relative to a certain point in time, it either rains or doesn't rain. This is the way in which I take statements about relative quantum states to be used, as well, mostly following Saunders. Relative to the vial being broken, the cat is dead, relative to it being whole, it is alive; relative to the cat being alive, the observer sees a living cat, etc. This isn't any more puzzling than the existence of tensed facts, I think; it just corresponds to adding a new indexical to the description of the world, additional to here, now, I and so on, which keeps track of the branching of the wave function, if you will. Our experiences are then not just a linear totally-ordered set, like beads on a chain corresponding to different moments, but only partially ordered.

If you insist on reifying what is indexed by our new branch-indexical, then you end up with something like the Many Worlds-interpretation, but I think this is where things get dicey: in a way, it's like insisting that different moments and the things existing within those moments exist in some sense alongside one another, which I don't think is consistent; rather, I would prefer understanding existence as being both time- and branch-indexed. So in the same sense that there is only one cat at different moments in time, there is only one cat at different moments and branches. This is also, I think, much closer to Everett's original relative states than the modern Many Worlds interpretation is.

Again I refer to the cat. Are you really going to say that its quantum state is |dead>+|alive>, but that it is dead "relative to" a person who sees it as dead, etc? What would that even mean?

From this perspective, it's the same as the difference between 'alive now' and 'dead later'. Picture a being that does not have any experience of time: it would be as confused by this change as we are by the components of a superposition. It could as validly ask what it would mean that the cat appeared dead to one observer (at one moment), and alive to another (at a different one). I think a useful concept in this regard is that of an 'observer moment', that is, any given 'slice' of experience of some observer. We consider observer moments to only be temporally indexed, but I don't think that additional problems arise---beyond a vague metaphysical uneasiness stemming from our old preconceptions---if we consider them indexed by time and branch. It seems to me that this produces a perfectly acceptable meaning for the cat being dead 'relative to' a particular observer (moment), and alive to another.

 

[mitchell porter]

S.Daedalus, the only way I can make sense of your remarks is to interpret them as saying "there is more than one cat". Again, either the vial with the poison gas breaks, or it does not break, or it both breaks and does not break, but in the last case there must be two worlds and hence two cats. But it's pretty clear that QBists, and "relational" thinkers, don't want to take the latter path, or else they would state their affiliation with Everett and many worlds. Even Zurek (mentioned by atyy) tries a similar dodge with his personal interpretation, the "existential interpretation".

You are asking about the MATHEMATICAL REPRESENTATION of reality without continuous trajectories.

I'm asking what is the physical or ontological hypothesis. What is the idea about reality being advanced here.

Since you aren't talking about extreme relativity of states any more (one and the same cat being dead to one observer and alive to another), I consider the main concept to now be, that there are objective quantum jumps of some sort. There are transitions between one eigenstate and another. That would be fine with me, that's an ontological hypothesis that I can take seriously. The main question is then whether anyone can come up with a reasonable exact proposal for what observables get to become real, and when and where this happens.

Consider something like the consistent histories formalism. Given a wavefunction of the universe and a decoherence functional, you can then specify a history of the universe with a set of projection operators, and obtain an apriori probability for this history using the decoherence functional. By classical standards, the history described by the projection operators has to be coarse-grained, in order for the decoherence functional to return an answer; but otherwise, the specification of the history is radically unconstrained... The challenge of constructing an ontology of objective quantum jumps, I consider very analogous to the challenge of finding some principle which singles out a particular class of coarse-grained projector-defined histories. The latter was a serious topic of research in the late 1980s - Gell-Mann hoped that some principle of maximality, requiring that the history is as fine-grained as possible, might single out a special class of quasiclassical histories - but by the early 1990s it seems this had been abandoned - Adrian Kent wrote about the problems at some length.

I don't want to talk too much about this line of thought, it's not what this thread is about. But this is where I think that sort of research has to go, if it wants to produce a meaningful ontological interpretation. Markopoulou and others wrote a number of papers in which the space-time history is described by a poset of quantum states or quantum operators - to me that seems ripe for ontological interpretation. That poset is a candidate for the mathematical description of an objective quantum space-time.

I would have no philosophical objections to that program at all; it would just be a matter of whether it made sense in the quantitative and other details. My argument in this thread has been against a number of philosophical hypotheses which are being used to frame or interpret various QM formalisms. Specifically, I'm against crypto-solipsist QBism, and against relative states without many worlds. The former covertly uses instrumentalist solipsism as a superficial cure-all for quantum foundations, and the latter I think is simply conceptually incoherent.

As I keep saying, I can make no sense of the claim that the cat is alive and dead at the same time, and adding that the aliveness and the deadness is relative changes nothing. The emphasis on observers observing the cat would seem to be saying that there is no cat at all, there are just observations of a dead cat and observations of a live cat...

That could be an important thought in making sense of strangerep's reference to a multitude of partially correlated realities. We're faced with the old debate between materialism and idealism. I mean metaphysical idealism, the belief that consciousness is fundamental and that there are no independent physical objects, there are only sensory experiences of various observers, coordinated as if there were an independent three-dimensional world.

I must say it's a strange turn for that idea to show up in the culture of physics. It more naturally belongs to the mindset of radical skepticism, that wonders if you're dreaming or in the Matrix; or perhaps in parapsychology. In fact, I'll go further and say that physical theory offers the weakest of reasons to be thinking in those directions. Chopra-esque "what-the-bleep" quantum idealism is just a backfire of Copenhagen positivism; it's the natural impulse to form some concept of reality, at work in the conceptual vacuum left by a phlegmatic instrumentalism which talks of measurements and observers as if they were fundamental. It's no wonder that the idealist philosophy, in which the consciousness of observers is ontologically fundamental, should find a foothold in the post-Copenhagen vacuum, but as we all know, the reality of laboratory quantum physics has nothing to do with this.

Marcus talked about 'interacting quantum subsystems', and one could try to make a non-idealist version of this 'multiple partial reality' interpretation, in which the 'parts' are not 'experiences of conscious observers', but just simple physical systems which 'experience' their environments in the form of, say, their physical boundary conditions. In fact I recognize that you can find something like this in the papers on relational interpretations. Any entity A that quantum-interacts with another entity B, is said to have a relative state with respect to B.

But as I've already argued, there are severe limits to how far you can go with state relativism, and without many worlds, and still make sense. Those limits are undoubtedly crossed long before you get to Schrödinger's dead-and-alive cat...

So in the end I return to my earlier remarks, about posets of quantum states, and particular coarse-grained histories, as viable paradigms. Extreme versions of state relativism don't make sense; extreme versions of the partial-reality idea require an unwarranted excursion into consciousness-first metaphysical idealism; and moderate approaches which avoid both these extremes, will end up looking a lot like what I've already called the viable paradigms, and won't have the conceptual exoticism of 'relational ontology' or 'no unified reality'. Those are the theses that I nail to this thread. :-)

 

[S.Daedalus]

S.Daedalus, the only way I can make sense of your remarks is to interpret them as saying "there is more than one cat". Again, either the vial with the poison gas breaks, or it does not break, or it both breaks and does not break, but in the last case there must be two worlds and hence two cats. But it's pretty clear that QBists, and "relational" thinkers, don't want to take the latter path, or else they would state their affiliation with Everett and many worlds. Even Zurek (mentioned by atyy) tries a similar dodge with his personal interpretation, the "existential interpretation". I'm asking what is the physical or ontological hypothesis. What is the idea about reality being advanced here.

What about the temporal case, do you think there's more than one cat, i.e. different cats at different points in time, as well? Or do you see some principled difference between a temporally extended cat, and one that's extended across different branches of the wave function?

 

[atyy]

What about the temporal case, do you think there's more than one cat, i.e. different cats at different points in time, as well? Or do you see some principled difference between a temporally extended cat, and one that's extended across different branches of the wave function?

Maybe ...

Each instant of time a new Universe
http://arxiv.org/abs/1305.1615

So in the end I return to my earlier remarks, about posets of quantum states, and particular coarse-grained histories, as viable paradigms. Extreme versions of state relativism don't make sense; extreme versions of the partial-reality idea require an unwarranted excursion into consciousness-first metaphysical idealism; and moderate approaches which avoid both these extremes, will end up looking a lot like what I've already called the viable paradigms, and won't have the conceptual exoticism of 'relational ontology' or 'no unified reality'. Those are the theses that I nail to this thread. :-)

Apparently a diet of worms does produce quantum reality :)

http://phys.org/news/2012-12-earthworms-quantum-dots.html