GR/StatMech/QM foundations, epistemic views only please

[marcus]

Hi Atyy, Rep,
interesting questions! I've been busy with things on the ground here and just got around to perusing the 2014 Edge essays. I thought the gist of Gefter's essay http://www.edge.org/response-detail/25513 was here:

Finally, the universe's retirement might offer some guidance as physicists push forward with the program of quantum gravity. For instance, if each observer has his or her own universe, then each observer has his or her own Hilbert space, his or her own cosmic horizon and his or her own version of holography, in which case what we need from a theory of quantum gravity is a set of consistency conditions that can relate what different observers can operationally measure.

Adjusting our intuitions and adapting to the strange truths uncovered by physics is never easy. But we may just have to come around to the notion that there's my universe, and there's your universe—but there's no such thing as the universe.​

You recall the earlier references to the idea of reconciling the accounts that different observers/narrators give. To take an analogy, 1905 specialrel gives us a way to reconcile the different stories told by observers moving in relation to each other. So there can be a deeper consistency between accounts that disagree at some level of detail. What she calls "consistency conditions" (allowing reconciliation) are in this case just the rules of Minkowski spacetime.

I think Gefter is saying we need a substantial advance (she calls QG) giving consistency criteria and reconciliation procedures to accommodate different observer's "universes". English doesn't have quite the right words. Maybe "narratives"? No one official account, rather many different narratives whose apparent disagreements can be explained (by her imagined new theory that she calls QG).

Another place where English may not be quite adequate yet is what Atyy asked about.
I think in RQM you cannot make a fixed distinction either classical or quantum because each observer considers himself a classical subject and all the other observers to be quantum objects, part of the universe that he is trying to understand and explain. Atyy, you seemed to be asking about a FIXED distinction, "either or". I think an observer isn't fixed to be one or the other.

It is not hard to see why (when a single official "universe" narrative is discarded) the EPR "proof" of non-locality breaks down. Rovelli and Mermin do not have to prove locality. The simply need to point out that the "proof" of non-locality depends on a questionable assumption of a single official account of reality, and reject that assumption.

 

[marcus]

Some 2014 Edge links:
Gefter http://www.edge.org/response-detail/25513 (retire *the* universe, i.e. a unique account of reality)
Zeilinger quantums is too about real stuff! http://www.edge.org/response-detail/25548
Rovelli retire geometry-as-the-description-of-physical-space http://www.edge.org/response-detail/25345
Giddings retire spacetime-as-fundamental http://www.edge.org/response-detail/25477

Einstein discovered that the Newtonian space described by geometry is in fact a field like the electromagnetic field, and fields are nicely continuous and smooth only if measured at large scales. In reality, they are quantum entities that are discrete and fluctuating. Therefore the physical space in which we are immersed is in reality a quantum dynamical entity, which shares very little with what we call "geometry". It is a pullulating process of finite interacting quanta. We can still use expressions like "quantum geometry" to describe it, but reality is that a quantum geometry is not much of a geometry anymore. --Rovelli​

The apparent need to retire classical spacetime as a fundamental concept is profound, and confronts the reality that a clear successor is not yet in sight. Different approaches to the underlying quantum framework exist; some show promise but none yet clearly resolve our decades-old conundrums in black holes and cosmology. The emergence of such a successor is likely to be a key element in the next major revolution in physics.--Giddings​

 

[atyy]

Another place where English may not be quite adequate yet is what Atyy asked about.
I think in RQM you cannot make a fixed distinction either classical or quantum because each observer considers himself a classical subject and all the other observers to be quantum objects, part of the universe that he is trying to understand and explain. Atyy, you seemed to be asking about a FIXED distinction, "either or". I think an observer isn't fixed to be one or the other.

It is not hard to see why (when a single official "universe" narrative is discarded) the EPR "proof" of non-locality breaks down. Rovelli and Mermin do not have to prove locality. The simply need to point out that the "proof" of non-locality depends on a questionable assumption of a single official account of reality, and reject that assumption.

But in other words, there is no locality either, since nonlocality and locality involve satisfaction or violation of Bell inequalities, and the claim is that some classical variable in the inequality does not exist, means the inequality cannot be formed. If the inequality does not exist, then neither locality nor nonlocality make sense. So there is no locality in this interpretation either.

 

[marcus]

Reminder: before invoking a theorem, check that the assumptions are satisfied.

 

[marcus]

In the debate at this year's FQXI conference (Jan 5-10, Puerto Rico) Raphael Bousso argued for Loop Quantum Gravity and skillfully attacked String Theory. His debate adversary, Carlo Rovelli, stubbornly defended String Theory and criticized LQG with zeal and gusto.


The topic of the 2014 conference was The Physics of Information
http://staging.fqxi.org/conference/home/2014
http://fqxi.org/conference/home/2014 (alternate link)

 

[DennisN]

Some 2014 Edge links:
Gefter http://www.edge.org/response-detail/25513 (retire *the* universe, i.e. a unique account of reality)
Zeilinger quantums is too about real stuff! http://www.edge.org/response-detail/25548
Rovelli retire geometry-as-the-description-of-physical-space http://www.edge.org/response-detail/25345
Giddings retire spacetime-as-fundamental http://www.edge.org/response-detail/25477

Some comments from me on the interesting essays:

Gefter on "The Universe":

In light of the universe's retirement, this all looks slightly less miraculous. After all, superpositions are really superpositions of reference frames. In any single reference frame, an animal's vitals are well defined. Cats are only alive and dead when you try to piece together multiple frames under the false assumption that they're all part of the same universe. (my bolding)

I don't get this at all. Is this the Many-Worlds Interpretation in disguise? If so, why didn't she just say MWI? And I would be very interested to hear what the frame of reference of a photon looks like... Nah, I don't think the Universe is close to retirement just yet. We need more observational/experimental data that support multi-hypotheses, I think .

Anton Zeilinger on "Reality":
I liked his essay.

But you don't have to go so far. It is enough to assume that quantum mechanics just describes probabilities of possible measurement results. Then making an observation turns potentiality into actuality and, in our case, the position of the particle becomes a quantity one can talk reasonably about. But, whether it has a well-defined position or not, the buckyball very well exists. It is real in the double-slit experiment, even when it is impossible to assign its position a well-defined value. (my bolding)

It's hard to argue against that.

Rovelli on "Geometry": I don't know enough about it to comment on it.

Giddings on "Spacetime":

The problem with fundamental spacetime is even more strongly hinted at from multiple developing perspectives. Notable among these hints is the physics of black holes, where it appears that evolution that respects quantum principles must violate the classical spacetime dictum that information does not propagate faster than the speed of light. (my bolding)

I don't understand/know anything about this. Anyone know what he is referring to?

In the debate at this year's FQXI conference (Jan 5-10, Puerto Rico) Raphael Bousso argued for Loop Quantum Gravity and skillfully attacked String Theory. His debate adversary, Carlo Rovelli, stubbornly defended String Theory and criticized LQG with zeal and gusto.

Very funny, thanks!

 

[strangerep]

In light of the universe's retirement, this all looks slightly less miraculous. After all, superpositions are really superpositions of reference frames.In any single reference frame, an animal's vitals are well defined. Cats are only alive and dead when you try to piece together multiple frames under the false assumption that they're all part of the same universe. (my bolding)

I don't get this at all. Is this the Many-Worlds Interpretation in disguise? If so, why didn't she just say MWI? And I would be very interested to hear what the frame of reference of a photon looks like... [...]

I see I'm not the only one who perceives that particular paragraph of Gefter's as poop on the carpet.

 

[atyy]

@strangerep, you have to consider that some people may read your comments while having dinner

 

[DennisN]

I see I'm not the only one who perceives that particular paragraph of Gefter's as poop on the carpet.

Our noses seem to have detected that something was not right on that carpet...

Regarding my previous post #36:

"

The problem with fundamental spacetime is even more strongly hinted at from multiple developing perspectives. Notable among these hints is the physics of black holes, where it appears that evolution that respects quantum principles must violate the classical spacetime dictum that information does not propagate faster than the speed of light. (my bolding)

I don't understand/know anything about this. Anyone know what he is referring to?"

Clarification:

I suspect it has something to do with the recent black hole research/debates considering firewalls, complementarity and unitarity, but I'm not aware of what Giddings exactly meant... nevermind, I won't hi-jack the thread with it - if anyone knows, please PM me .

 

[RUTA]

I should clarify the term epistemic by generalizing the blue highlight in post #2.

" A central issue is whether any mathematical model describes reality (the ontic view) or an agent's knowledge of reality (the epistemic view)."

The original highlighted quote from Mermin merely applied to "a quantum state" and I'm extending it to include classical states and physical models in general.

This discussion is a good idea, marcus. By exploring what we believe physics represents, we can consider changes to the way we understand the formalism. This can lead to entirely new approaches to physics.

1. The above quote can be understood as a false dichotomy. How can you argue that our models don't represent what we know (at least what we think we know, based on our experiments and observations)? So, that "any mathematical model describes ... an agent's knowledge of reality" holds regardless of whether or not you define reality ontologically by that which is represented by (or tacitly included in) your models. You may of course always assume reality contains more than is represented by your models.

2. Ontology can't be done without making epistemological assumptions and vice-versa.

Physics is a game of reconciling disparate perceptions. All I have to go on concerning your perceptions is what you tell me you perceive, and you telling me what you perceive is a subset of my perceptions, so I start playing the game by verifying what you tell me. For example, if you tell me you saw a pink elephant in my dining room, I will have to go into the dining room and see a pink elephant or I won't have a perception consistent with yours to reconcile. Another example, I see a cereal box between us and we tell each other what we see on our side of the box. I can then rotate the box 180 deg and verify what you reported. You then tell me that you see what I reported to you. I can then construct a model of you, me and a two-sided box between us. Most people reify this model calling it "reality" and assume equal ontic status of self, other, and box.

Where QBism/CBism deviates from business as usual is to acknowledge that the nature of the game is in fact the reconciliation of perceptions (knowledge). And, to question whether or not this reconciliation requires a unique model, e.g., self, other, and box, or a unique spacetime manifold. Thus, there is perhaps no unique model possible in the reconciliation even though there is a mathematical formalism that links the disparate perceptions. It sounds like some here assume that there exists a unique, underlying reality responsible for the disparate set of perceptions. What does this gain you by way of theory construction? If you succeed in constructing a model of this unique, underlying reality, you will have violated a central premise of QBism/CBism as I understand it. Am I missing something?

 

[marcus]

Excellent summary RUTA! I appreciate this both because it helps me personally to see the issues and because it contributes substantially to thread. I will excerpt:

... By exploring what we believe physics represents, we can consider changes to the way we understand the formalism. This can lead to entirely new approaches to physics.

==quote==
Physics is a game of reconciling disparate perceptions. ...
Where QBism/CBism deviates from business as usual is to acknowledge that the nature of the game is in fact the reconciliation of perceptions (knowledge). And, to question whether or not this reconciliation requires a unique model, e.g., self, other, and box, or a unique spacetime manifold. Thus, there is perhaps no unique model possible in the reconciliation even though there is a mathematical formalism that links the disparate perceptions. It sounds like some here assume that there exists a unique, underlying reality responsible for the disparate set of perceptions. What does this gain you by way of theory construction? If you succeed in constructing a model of this unique, underlying reality, you will have violated a central premise of QBism/CBism as I understand it. Am I missing something?
==endquote==

I think this misses nothing, but I would put the epistemic viewpoint a bit more softly. Yes, go right ahead and try to find a "unique underlying reality" which somehow gives rise to all these disparate perceptions. Do try to construct a math model of this "U.U.R."! Just don't be too upset if you cannot find it. We have not been promised by Nature that a single official account of what is really happening can be obtained.
Maybe it is, as you said, a game of reconciling disparate accounts without there being a unique correct one.

So it seems that someone like Mermin is saying (reinforced with all his quotes from Bohr and Schroedinger and others) that we should simply regard things like QM (and generel, specialrel, thermodynamics) and mathematical tools that an individual can use to organize his/her experience. Then all the paradoxes go away. As long as each observer has his/her own hilbert space, what's the problem?

 

[strangerep]

So it seems that someone like Mermin is saying (reinforced with all his quotes from Bohr and Schroedinger and others) that we should simply regard things like QM (and generel, specialrel, thermodynamics) and mathematical tools that an individual can use to organize his/her experience. Then all the paradoxes go away. As long as each observer has his/her own hilbert space, what's the problem?

ISTM, the tricky bit starts when one confronts the questions of how to model interactions in detail. After two (initially uncorrelated) systems have interacted, they are typically correlated (to some extent) with each other. Although one might start with (say) a tensor product Hilbert space, the interaction term must mix up the component spaces to establish the correlation, and... we're back to the notion of a single larger Hilbert space. But maybe that's ok, since Hilbert spaces are abstract.

In Gielen+Wise observer space, I see some ideas reminiscent of the decades-old attempts at many-time relativity, which now seems regarded as something of a fringe subject. Trump & Schieve wrote a book on that (which I feel lukewarm towards), but I don't detect much recent progress.

 

[marcus]

Just to clarify one point in what I said here,
==quote==
So it seems that someone like Mermin is saying (reinforced with all his quotes from Bohr and Schroedinger and others) that we should simply regard things like QM (and generel, speciarel, thermodynamics) as mathematical tools that an individual can use to organize his/her experience. Then all the paradoxes go away. As long as each observer has his/her own hilbert space, what's the problem?
==endquote==
I did not mean that each observer has a personal hilbert describing his OWN STATE, he has a hilbert to represent his knowledge and experience of the world around him including other observers (which he views as quantum entities). So when you have two observers A and B, there is no possibility to form the tensor product (then a transcendent being would be looking down at both of them). You simply have H_A = what classic A knows about quantum B and the rest of the world, and H_B = what classic B knows about quantum A and the rest of the world.

Each hilbert H_O is associated with a specified observer and is used by that observer to organize and understand that person's experience of the entire rest of the world, and calculate odds, keep records, make predictions, etc.
So it would not make sense, as I see it, to tensor two hilbert spaces together. If they are separate versions of qM that separate agents are using as conceptual/math devices to understand the world. Hence I don't grasp the import of this part of your post, or the context.

... Although one might start with (say) a tensor product Hilbert space, the interaction term must mix up the component spaces to establish the correlation, and... we're back to the notion of a single larger Hilbert space. ...

What am I missing about your intent?

 

[marcus]

Since we've turned a page, I'll bring forward a kind of thematic post as a reminder of what's involved in epistemic approaches such as QB or RQM…
====quote post#27====
... a reminder about the basic reason epistemic approaches like "QB" and "RQM" avoid various puzzles and pitfalls is that although there is a basic reality that all observers observe and although they can RECONCILE differences in the accounts of it different observers arrive at, they do have different accounts.
There is no one single official story.

To take a trivial illustration: Alice considers herself an "observer" and Bob as a quantum system, so her account of reality is obviously different from that of Bob, who considers himself the observer and Allce part of the quantum world he is trying to understand. Both use Quantum Mechanics to organize their experience and inevitably their stories differ, but yet can be reconciled.
Special rel gives us something analogous where observers can disagree about the order in which events occurred, and yet the disagreements can be explained by the observers' relative motion.

So there is a clear and serious cost. I think we all here realize this. Just for extra clarity I quote some excerpts from Mermin's recent writings and from the 2006 paper "Relational EPR".

==google "mermin now arxiv" ==
In a Physics Today Commentary, and more carefully, extensively, and convincingly with Chris Fuchs and Ruediger Schack, I argued that stubborn longstanding problems in the interpretation of quantum mechanics fade away if one takes literally Niels Bohr’s dictum that the purpose of science is not to reveal “the real essence of the phenomena” but to find “relations between the manifold aspects of our experience.” Here I note that the view of science as a tool that each of us can use to organize our own personal experience, called QBism by Fuchs and Schack, clarifies more than just quantum foundational problems. Recognizing that science is about the subject (the user of science) and not just about the object (the world external to that user) can eliminate well entrenched confusion in classical physics too.
==endquote==​

==google "relational EPR" ==
The relational approach claims that a number of confusing puzzles raised by Quantum Mechanics (QM) result from the unjustified use of the notion of objective, absolute, ‘state’ of a physical system, or from the notion of absolute, real, ‘event’.
The way out from the confusion suggested by RQM consists in acknowledging that different observers can give different accounts of the actuality of the same physical property [6]. This fact implies that the occurrence of an event is not something absolutely real or not, but it is only real in relation to a specific observer. Notice that, in this context, an observer can be any physical system.

Thus, the central idea of RQM is to apply Bohr and Heisenberg’s key intuition that “no phenomenon is a phenomenon until it is an observed phenomenon” to each observer independently. This description of physical reality, though fundamentally fragmented, is assumed in RQM to be the best possible one, i.e. to be complete [6]:

“Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the world”.
==endquote==​

My comment: Note that "complete" here means best possible. The completest description we can hope for,
admittedly fragmented into versions whose differences are, however, explainable. It means giving up on the hope for one single official account as seen from transcendent perspective.
====endquote====

 

[strangerep]

What am I missing about your intent?

Evidently it was I who was missing something about your intent.

[...]
I did not mean that each observer has a personal hilbert describing his OWN STATE, he has a hilbert to represent his knowledge and experience of the world around him including other observers (which he views as quantum entities). So when you have two observers A and B, there is no possibility to form the tensor product (then a transcendent being would be looking down at both of them). You simply have H_A = what classic A knows about quantum B and the rest of the world, and H_B = what classic B knows about quantum A and the rest of the world.

Each hilbert H_O is associated with a specified observer and is used by that observer to organize and understand that person's experience of the entire rest of the world, and calculate odds, keep records, make predictions, etc.
[...]

The basic question still remains though: how to express the establishment of consistent correlations across these Hilbert space? And how to model interactions between the observers within such a framework?

Edit: Since we're discussing some of Mermin's views, one should probably (re-)read his older paper: What is quantum mechanics trying to tell us?, especially the "SSC Theorem: Subsystem correlations determine the state".

(But not you, Atty -- I'm sure this paper would just annoy you. )

 

[marcus]

...
The basic question still remains though: how to express the establishment of consistent correlations across these Hilbert space? And how to model interactions between the observers within such a framework?

Heh, heh well I'm not an official spokesperson for RQM but let me just tell you my immediate reaction. In the RQM picture AFAICS there ARE no interactions between observers, as such. No need to model.

This view is taken in the original paper "Relational EPR": A considers herself observer and B a quantum system. The interaction between A and B is modeled in her H_A version of QM.
It is not the "interaction between two observers".
It is the interaction between observer A and part of the quantum world, namely B and his instruments.

She can radio to B and ask what spin he observed and he can report, this all takes time and this ensures no instant "action at distance".

I expect you read "relational EPR" so this is familiar.

Conversely B considers himself an observer and A as a quantum system. So he sees the B interaction with A NOT as interaction between two observers but between himself B the observer, and a quantum system A.

there is no need to model the interaction because it is ALREADY MODELED by each person individually

I think I already sketched this earlier in post#27 so I am just repeating in more detail. It's how EPR is resolved in the 2006 Smerlak Rovelli paper, and how locality is confirmed.

...
There is no one single official story.

To take a trivial illustration: Alice considers herself an "observer" and Bob as a quantum system, so her account of reality is obviously different from that of Bob, who considers himself the observer and Allce part of the quantum world he is trying to understand. Both use Quantum Mechanics to organize their experience and inevitably their stories differ, but yet can be reconciled.
Special rel gives us something analogous where observers can disagree about the order in which events occurred, and yet the disagreements can be explained by the observers' relative motion.
...

 

[strangerep]

(I edited my previous post while you were typing.)

Heh, heh well I'm not an official spokesperson for RQM but let me just tell you my immediate reaction. In the RQM picture AFAICS there ARE no interactions between observers, as such. No need to model.

Ah, but I meant in the generalized sense: correlations must somehow be established between them. I wasn't thinking "action at a distance".

 

[marcus]

... correlations must somehow be established between them. I wasn't thinking "action at a distance".

I probably was editing too while YOU were writing, or editing It is a liability of lively discussions. Maybe I will let the dust settle and come back in the morning. What time zone are you in? I am in PST (pacific)

Not sure what you mean by correlations must be established… In whose quantum theory? Is there a third observer? Someone who sees both A and B as quantum systems?

Hopefully RUTA will help clarify. He's a prof at a university in PA and has a research interest in this kind of thing. But it is nearly 11 PM here so it will be approaching 2AM in PA, won't hear from him until tomorrow I guess.

 

[Paulibus]

May I ask whether Qbism is all about multiple observers who concoct multiple descriptions of
quantum systems, differing among themselves because the systems observed include other observers with their differing perceptions? Is why Qbism is called 'subjective’?

Or if instead Qbism is a description of future 'reality' that involves personal guesses, rendered
quantitative and respectable by a calculated ‘probability’, making this the reason for calling Qbism
'subjective’ --- since ‘probability’ is a word that links the known past and present to the
unknown and mysterious future?

I suspect the latter, because a ‘probable’ event is something which may be expected to happen in
the future, rather than linking multiple past or current perceptions.

 

[atyy]

I suspect it has something to do with the recent black hole research/debates considering firewalls, complementarity and unitarity, but I'm not aware of what Giddings exactly meant... nevermind, I won't hi-jack the thread with it - if anyone knows, please PM me .

I'm not sure if the inability to signal classical information faster than light is exactly the same as the notion of locality used in Hawking's derivation of thermal radiation from a black hole. However, the derivation does depend on a notion of locality, and one hypothesis is that locality breaks down in order to preserve quantum mechanics.

Giddings sketches how locality is used starting from Eq 4.1 of http://arxiv.org/abs/1105.2036.
Mathur gives a lot more detail on the use of locality in getting Hawking's result http://arxiv.org/abs/0909.1038.

May I ask whether Qbism is all about multiple observers who concoct multiple descriptions of
quantum systems, differing among themselves because the systems observed include other observers with their differing perceptions? Is why Qbism is called 'subjective’?

Or if instead Qbism is a description of future 'reality' that involves personal guesses, rendered
quantitative and respectable by a calculated ‘probability’, making this the reason for calling Qbism
'subjective’ --- since ‘probability’ is a word that links the known past and present to the
unknown and mysterious future?

I suspect the latter, because a ‘probable’ event is something which may be expected to happen in
the future, rather than linking multiple past or current perceptions.

Yes, that's part of the reason, as they say in http://arxiv.org/abs/1301.3274 . Also, there are various interpretations of probability including frequentist, objective Bayesian, and subjective Bayesian. A key theorem in classical subjective Bayesian inference is the beautiful de Finetti representation theorem. Hudson and Moody proved a quantum version, and Caves, Fuchs and Schack gave a later, different proof of it that makes the parallel to the classical de Finetti representation theorem more obvious http://arxiv.org/abs/quant-ph/0104088 .

 

[marcus]

May I ask whether Qbism is all about multiple observers …

Or if instead Qbism is a description of future 'reality' that involves personal guesses, rendered
quantitative and respectable by a calculated ‘probability’, making this the reason for calling Qbism 'subjective’ ...

I think the latter. I merely use the two-observer situation as a trivial example of how different views MUST differ. Each is subjective from that observer's standpoint.
But in applying this conception of QM it is not essential to have more than one observer. In a bizarre universe in which there were ONLY ONE, that observer could still be making subjective judgements of probabilities based on the information accumulating from his/her experiences.

What is essential to the concept is to have AT LEAST one observer. However as we all know there are lots of us so the discussion quickly moves to considering that additional complication.

Here's a nice quote from the conclusions of the 1301 paper that Atyy just linked:
==quote page 27, 28 of http://arxiv.org/abs/1301.3274 ==
Of course, as a single-user theory, quantum mechanics is available to any agent to guide and better prepare him for his own encounters with the world. And although quantum mechanics has nothing to say about another agent’s personal experiences, agents can communicate and use the information gained from each other to update their probability assignments.

In the spirit of the Paulian Idea, however, querying another agent means taking an action on him. Whenever “I” encounter a quantum system, and take an action upon it, it catalyzes a consequence in my experience that my experience could not have foreseen. Similarly, by a Copernican-style principle, I should assume the same for “you”: Whenever you encounter a quantum system, taking an action upon it, it catalyzes a consequence in your experience.

By one category of thought, we are agents, but by another category of thought we are physical systems. And when we take actions upon each other, the category distinctions are symmetrical. Like with the Rubin vase, the best the eye can do is flit back and forth between the two formulations.

The previous paragraph should have made clear that viewing quantum mechanics as a single user theory does not mean there is only one user. QBism does not lead to solipsism. Any charge of solipsism is further refuted by two points central to the Paulian Idea. (Fuchs, 2002b).
One is the conceptual split of the world into two parts—one an agent and the other an external quantum system—that gets the discussion of quantum measurement off the ground in the first place. If such a split were not needed for making sense of the question of actions (actions upon what? in what? with respect to what?), it would not have been made. Imagining a quantum mea- surement without an autonomous quantum system participating in the process would be as paradoxical as the Zen koan of the sound of a single hand clapping.
The second point is that once the agent chooses an action {E_i}, the particular consequence E_k of it is beyond his control. That is to say, the particular outcome of a quantum measurement is not a product of his desires, whims, or fancies—this is the very reason he uses the calculus of probabilities in the first place: they quantify his uncertainty (Lindley, 2006), an uncertainty that, try as he might, he cannot get around. So, implicit in this whole picture—this whole Paulian Idea—is an “external world . . . made of something,” just as Martin Gardner calls for…
==endquote==

 

[atyy]

The way I like to think about QBism is that it is "Copenhagen" in the style of the "orthodox" or "naive textbook" interpretation given in Landau and Lifshitz, except that where L&L had a classical measurement apparatus, QBism asks can we have a classical rational agent, where rational is defined analogously to that in subjective Bayesian inference. As long as one does not go on and say, and there is no deeper reality (what can such a thing mean, since reality is just something in our model, which is only our useful approximation to the more mystical notion of reality), I think one could even say QBism can be derived from Bohmian mechanics, which is certainly a proposal for a deeper reality. Why? Because Bohmian mechanics does use probability, but is agnostic as to whether that probability is frequentist, objective Bayesian or subjective Bayesian.

Here is an approach to combining Bohmian mechanics and subjective Bayesian inference:

http://arxiv.org/abs/0706.2522
Grounding Bohmian Mechanics in Weak Values and Bayesianism
H. M. Wiseman

"I believe the most fruitful viewpoint to adopt, in BM as elsewhere, is the subjective or Bayesian interpretation of probabilities. This approach is perhaps best summed up by the slogan "probability is not real" [36]."

"[36] B. de Finetti, Theory of Probability (Wiley, New York, 1974–5)."

QBism does not solve the problem of locality (because it wasn't a problem to start), and it does not solve the measurement problem, because there is still the fuzzy cut between classical and quantum. The cut between classical agent and quantum world is fundamental in QBism, but we can still ask exactly which neurons of an agent are in his agent self, and which neurons are part of the quantum world.

 

[strangerep]

QBism [...] does not solve the measurement problem, because there is still the fuzzy cut between classical and quantum. The cut between classical agent and quantum world is fundamental in QBism, but we can still ask exactly which neurons of an agent are in his agent self, and which neurons are part of the quantum world.

I don't see the need for any such cut at all. Classical systems are just quantum systems in a certain limit.

 

[atyy]

I don't see the need for any such cut at all. Classical systems are just quantum systems in a certain limit.

The cut is fundamental in Qbism. The agent is a primitive notion, just like the classical apparatus is a primitive notion in the orthodox interpretation (Landau & Lifshitz) and other interpretations with a fundamental collapse postulate (or Bayesian updating). It is true that in all interpretations, classical systems are a limit of quantum mechanics, but in the orthodox interpretation, the cut is also necessary to collapse the wave function.

Here is a QBist exposition in which the agent and the cut between the agent and the external world is a primitive notion http://arxiv.org/abs/1301.3274 (p27): "Primitive notions: a) the agent, b) things external to the agent, or, more commonly, “systems,” c) the agent’s actions on the systems, and d) the consequences of those actions for her experience."

For why I say Bayesian updating is a sort of collapse, an example is found in Bohmian mechanics with the Bayesian interpretation of probability http://arxiv.org/abs/0706.2522 (p10): "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. The guiding function |ψ(t)> of course does not collapse."

Although both QBiism and Bohmian mechanics with subjective probability have cuts. The difference is that the observer in QBism cannot consider himself quantum, but the external world is. If the QBist agent includes himself in the wave function, experiments will not yield definite outcomes for him (in QBism, the wave function applies to single systems). In QBism, it is not clear if there is any wave function if there is no observer. In Bohmian mechanics, the observer can consider himself made of the same stuff as the external world. Furthermore, Bohmian mechanics can be conceived to run whether or not there is an observer in the universe.

Incidentally, the collapse of the wave function can also be rigourously derived from quantum mechanics as a limit. This was done by Klaus Hepp in http://dx.doi.org/10.5169/seals-114381. John Bell argued that this mathematical limit was correct, but not of physical significance for solving the measurement problem http://dx.doi.org/10.5169/seals-114661. (I found out about Bell's paper in Allahverdyan et al's http://arxiv.org/abs/1107.2138)

 

[marcus]

Mermin talks about where the dividing line is, it doesn't seem all that important to me---could be put where the individual observer says.
I suppose another possibility could be to imagine that each agent A has a Laptop programmed to do quantum mechanics, with the essential information about that agent's hilbert space H_A stored in its memory.
When the agent gets information he/she enters it through the keyboard and it takes mathematical form. The quantum state is updated. Maybe the classic observer is defined to be some portion of the Laptop.

IOW whatever file system registers the new information and gets updated, THAT we could consider the observer. To be precise, the mathematical structure represented in the database. And any other isomorphic to it.

And then the wording of some of what Mermin says would have to be changed a bit, but I'm not sure it would matter.

What makes the human image so compelling, I think, is that the observer can be imagined as a professional bettor, to whom probabilities have a clear subjective meaning. If the bookie thinks the odds of something happening are 80% what that means is that he will pay anything UP TO $0.80 for a bet that pays $1.00 if the event occurs. And he will be willing to sell such a bet (agree to pay $1.00 if it occurs) for any offer in excess of $0.80.

I suppose a successful professional bookie could be constructed as an automaton. Maybe that is what physics PhDs do these days when they go to work in Finance. But none of that matters. What I want to do here is focus on the information that is registered at the observer in some form, and that is processed according to a quantum mechanical model, leading to the state at that observer being updated.

 

[atyy]

Mermin talks about where the dividing line is, it doesn't seem all that important to me---could be put where the individual observer says.
I suppose another possibility could be to imagine that each agent A has a Laptop programmed to do quantum mechanics, with the essential information about that agent's hilbert space H_A stored in its memory.
When the agent gets information he/she enters it through the keyboard and it takes mathematical form. The quantum state is updated. Maybe the classic observer is defined to be some portion of the Laptop.

IOW whatever file system registers the new information and gets updated, THAT we could consider the observer. To be precise, the mathematical structure represented in the database. And any other isomorphic to it.

And then the wording of some of what Mermin says would have to be changed a bit, but I'm not sure it would matter.

What makes the human image so compelling, I think, is that the observer can be imagined as a professional bettor, to whom probabilities have a clear subjective meaning. If the bookie thinks the odds of something happening are 80% what that means is that he will pay anything UP TO $0.80 for a bet that pays $1.00 if the event occurs. And he will be willing to sell such a bet (agree to pay $1.00 if it occurs) for any offer in excess of $0.80.

I suppose a successful professional bookie could be constructed as an automaton. Maybe that is what physics PhDs do these days when they go to work in Finance. But none of that matters. What I want to do here is focus on the information that is registered at the observer in some form, and that is processed according to a quantum mechanical model, leading to the state at that observer being updated.

The Bayesian rational observer can certainly be a classical computer (it's a standard concept in machine learning). But there is an external world described by the wave function. And the dividing line between the classical computer and its external world is primitive in QBism, and fuzzy when we have to implement it. As with the orthodox interpretation, there hasn't been a problem in practice. But it is a problem in principle.

 

[RUTA]

Hopefully RUTA will help clarify. He's a prof at a university in PA and has a research interest in this kind of thing. But it is nearly 11 PM here so it will be approaching 2AM in PA, won't hear from him until tomorrow I guess.

I wasn't notified about posts on this thread today. I just now ck'd and was surprised to see all this activity.

Anyway, I was hoping you guys would explain QBism to me, haha. What little I know of it is consistent with what marcus posted today.

I need to understand how we can have a unique classical reality that is supposed to result from quantum events when each person's collection of quantum events (housed in their view of the unique classical spacetime) is different. Even the inconsistency of simultaneity and spatial and temporal measurements of SR is accommodated by a single structure, i.e., M4. So, we have this unique spacetime structure of classical physics that is constructed from all observers' input of classical events. Then each observer conducts QM experiments and analyses using that classical structure (that's how you get your operators, eigenvectors, and eigenvalues for spin measurement, for example). Then, when it's time to record and report your QM measurements, the single unique spacetime structure of classical physics disappears and it's every man for himself. Yet, this unique spacetime structure is supposed to be nothing but a multitude of quantum events. I haven't wrapped my head around that yet.

 

[atyy]

@RUTA: that part of QBism, especially as represented in the FMS article, makes no sense to me either.

 

[marcus]

...I need to understand how we can have a unique classical reality that is supposed to result ...

Happily enough it turns out that (as I understand epistemic approaches such as QB or RQM) we are relieved from having to understand that, because a single unique classical reality is not assumed.

There is no one unique official mathematical description of reality. Various constructs in classical physics and quantum mechanics are used by the physicist to organize his experience, give an account of it, calculate probabilities, make predictions. But they are viewed as conceptual tools, and not equated with reality.

 

[atyy]

Happily enough it turns out that (as I understand epistemic approaches such as QB or RQM) we are relieved from having to understand that, because a single unique classical reality is not assumed.

There is no one unique official mathematical description of reality. Various constructs in classical physics and quantum mechanics are used by the physicist to organize his experience, give an account of it, calculate probabilities, make predictions. But they are viewed as conceptual tools, and not equated with reality.

There is a classical spacetime which is part of one observer's classical reality (since apparently "locality" is real). Then there are also multiple classical observers. Why are they not part of anyone observer's classical reality even though there is a real classical spacetime on which the wave function describing other observers (who are classical to themselves) are not? Also, in what space is "classical observer A" identified with "quantum observer A"? If the classical and quantum versions of each observer are not identified, there isn't really a problem. But they do seem to want each observer to be either classical or quantum, depending on context.