GR/StatMech/QM foundations, epistemic views only please

[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.

 

[marcus]

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...

I think you put it well earlier. In fact (although I don't think it's known at least at present) I personally suspect that it is NOT possible to have a unique model, a unique mathematical description.

IOW, whatever successful math devices people come up with (whether QM, or GR, or thermodynamics, or an unnamed conceptual tool still to be invented) will turn out to have a subjective part---will turn out to depend on the perspective of the observer. Just a suspicion.

 

[atyy]

I think you put it well earlier. In fact (although I don't think it's known at least at present) I personally suspect that it is NOT possible to have a unique model, a unique mathematical description.

IOW, whatever successful math devices people come up with (whether QM, or GR, or thermodynamics, or an unnamed conceptual tool still to be invented) will turn out to have a subjective part---will turn out to depend on the perspective of the observer. Just a suspicion.

That's far too general a reply. No one would disagree with that. The problem in QBism is if I am an agent, I have my classical world and my quantum world, and I am the only classical agent. There doesn't seem to me to be any other classical agents. Yet QBism seems to have multiple classical agents, so in whose model of reality do these agents exist? They seem to exist in FMS's god-like view, even though to me FMS are just quantum things and have no classical experiences.

 

[marcus]

... 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...

Not sure what you mean by "quantum observer A". I don't think I used the phrase "quantum observer" and can't recall reading it in the QB and RQM papers.

With B as the observer, A is not an observer but is just a quantum subsystem, part of the world that B (the observer) is using various theories, models, tools to understand.

Certainly A as a quantum chunk of the quantum world that B is observing is not equated to A the observer, in the disjoint context where A is the observer and B is part of the quantum world that A is observing.

Or so it seems to me, anyway. If you find anything to the contrary either in "Relational EPR" or the recent Mermin papers please let me know!

So maybe, as you say, "there isn't really a problem."

 

[atyy]

Not sure what you mean by "quantum observer A". I don't think I used the phrase "quantum observer" and can't recall reading it in the QB and RQM papers.

With B as the observer, A is not an observer but is just a quantum subsystem, part of the world that B (the observer) is using various theories, models, tools to understand.

Certainly A as a quantum chunk of the quantum world that B is observing is not equated to A the observer, in the disjoint context where A is the observer and B is part of the quantum world that A is observing.

Or so it seems to me, anyway. If you find anything to the contrary either in "Relational EPR" or the recent Mermin papers please let me know!

So maybe, as you say, "there isn't really a problem."

Yes, but the basic question is in which space are there multiple agents?

And the problem remains A is a quantum subsystem and classical observer - how and in which space is that identification made?

 

[marcus]

...Yet QBism seems to have multiple classical agents, so in whose model of reality do these agents exist?...

"Multiple classical agents"? I did not get that impression. Why do you say it "seems" to have them?

Notice QB is not an ontology. It is an way of regarding QM in a single-user context. In every implementation of QM, there is a specific observer, a physicist who is being aided by QM to understand and organize his experience.
AFAIK there is no concrete application or implementation of QM in which there are multiple observers, that I have ever seen referred to in QB. It sounds contrary to the spirit and intent of QB.
Of course I'm hardly and expert You've read more of it than I have, I think. Some of these links are really nice.

 

[atyy]

"Multiple classical agents"? I did not get that impression. Why do you say it "seems" to have them?

Notice QB is not an ontology. It is an way of regarding QM in a single-user context. In every implementation of QM, there is a specific observer, a physicist who is being aided by QM to understand and organize his experience.
AFAIK there is no concrete application or implementation of QM in which there are multiple observers, that I have ever seen referred to in QB. It sounds contrary to the spirit and intent of QB.
Of course I'm hardly and expert You've read more of it than I have, I think. Some of these links are really nice.

FMS say "There is a vestigial remnant in QBism of the Copenhagen classical domain, but the vestige of this “classical domain” varies from one agent to another and is limited to that agent’s directly perceived personal experience." Or http://arxiv.org/abs/1301.3274 say things like "two agents looking at the same data". So they do wish to say that there is more than one agent. They do identify the classical domain with an "objective (“classical”) domain". So my question is in which space do multiple agents exist? In my mind, as a QBist agent, there is only my objective experience - I cannot buy the claim that multiple agents exist. If I am to buy the claim that mutiple agents exist, I must operate on a different level than a QBist agent, the reality of FMS.

 

[strangerep]

The [classical/quantum] 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.

Well, then perhaps I was reading more into the FMS paper than they intended. I had in mind a collection of systems (of any kind).

So... I think I just ceased to be a QBist. It all starting to seem like a great volume of emptiness, clothed in sophistry.

I retreat to shut-up-and-calculate, since that produces all the results that are actually useful, with no need for a classical/quantum cut.

 

[Paulibus]

I now join Strangerep in his/her retreat to shut-up-and-calculate. Feynman is persuasive, as usual, and many of the fine words in this thread butter no parsnips!

Uneasily, I can't agree with Marcus' statements (e.g. post # 59) along the lines of "There is no one unique official mathematical description of reality". Instead, I maintain that the very purpose of physics is to devise such storied descriptions, quantified with the invented language of mathematics, which cleverly includes the statistical concept of quantified probability. This convenient feature let's physicists tell a unique story that incorporates a single-user's uncertain personal expectations of future happenings. This is how varying outcomes can be reconciled iff they come to pass.

Such happenings are quite unpredictable, sometimes grossly (e.g. horse racing) sometimes in detail (e.g. double-slit with individual electrons). The devil is in the future detail.

The real mystery is not the story (we anthropoid primates are uniquely adept at telling adaptable stories), but in the universal scale of 'the detail', manifested by the unexplained size of h. Perhaps the scale of 'the detail' in Nature is set by the Devil Herself?

 

[Haelfix]

I must admit as to being completely mystified by what you gentlemen are talking about in this thread. There are about 5 different completely different concepts, about twenty preprints with concepts ranging from the interpretations of quantum mechanics, to speculative proposals about modifying stat mech and quantum mechanics, to black hole physics, to ads/cft and into philosophy of science. THe edge magazine interview questions are discussing very different specific physics scenarios that really don't have anything to do with one another.

 

[marcus]

Hi Haelfix,
going back to the OP, here's what I intend the thread to discuss. There's a bunch of interesting current research including some by prominent mainstream authors having to do with taking an EPISTEMIC view of mathematical models. I.e. that they don't describe reality in any simple naive sense, they are tools by which the individual scientist understands, organizes experience, predicts--they represent knowledge of reality specific to the agent, not a uniquely correct version of reality itself. In fact according to this view one does not expect a unique account of reality, or a single correct version of the model.

If you have seen interesting recent research papers on foundations/interpretation of [GR/StatMech/QM], please share your links and thoughts. Argument along ontic versus epistemic lines is not approved--it is frequently a waste of time. So if you please use a separate thread if your views are ontic.

For a simple explanation of the difference, google "Mermin pirsa". You get a 45 minute video lecture http://pirsa.org/09090077/ "Confusing Ontic and Epistemic Causes Trouble in Classical Physics Too"
And the summary which you can read immediately without watching the talk says:
" A central issue is whether quantum states describe reality (the ontic view) or an agent's knowledge of reality (the epistemic view)."

The title of that video talk is a reminder that "QM interpretation" is only part of a tangle of foundational puzzles involving GR, Statistical Mechanics, Thermodynamics as well. In other words foundational confusion can cause trouble in CLASSICAL branches as well as quantum. I would like to know what other people have learned about this nexus of problems.

If you want a paper that is easy to read and quickly covers the material of that video, google "Mermin problem of the now". http://arxiv.org/abs/1312.7825 This defines a conceptual problem common to a lot of physics, notably (but not exclusively) classical. You might be interested in how Mermin resolves it.
That is a December 2013 paper---we're especially interested in recent work in this thread.

I want to mention some foundations connections between GR, StatMech, Thermodynamics, but will make a separate post of that so this one doesn't get too long.

If you google "introduction QBism" the top hit will be this November 2013 paper by Fuchs Mermin Schack http://arxiv.org/abs/1311.5253
An Introduction to QBism with an Application to the Locality of Quantum Mechanics
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 "non locality".
11 pages.

I don't think that's an empty claim and it signals a kind of change in the weather around quantum foundations and interpretation. Basically they say "let's put the agent (the subject, the physicist) into the picture instead of pretending that there's only the objective real world, and let's acknowledge that agents can communicate about their common reality." There is a kind of common sense realism here, I find.

This paper is one of two which for me personally characterize an epistemic view of QM. Bear in mind that there is more to this than merely Quantum Mechanics. There are significant epistemic developments in GR, StatMech, Thermodynamcs and in the interconnections among these fields. But just looking at QM for the moment, the OTHER paper personally significant for me is what you get when you google "relational EPR"

If you google "relational EPR" the top hit will be this April 2006 paper by Smerlak and Rovelli:
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
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
==excerpt==
... 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.
But, as claimed above, any such conversation about elephants is ultimately an interaction between quantum systems. This fact may be irrelevant in everyday life, but disregarding it may give rise to subtle confusions, such as the one leading to the conclusion of non-local EPR influences.
In the EPR situation, A and B can be considered two distinct observers, both making measurements on α and β. The comparison of the results of their measurements, we have argued, cannot be instantaneous, that is, it requires A and B to be in causal contact. More importantly, with respect to A, B is to be considered as a normal quantum system (and, of course, with respect to B, A is a normal quantum system). So, what happens if A and B compare notes? Have they seen the same elephant?
It is one of the most remarkable features of quantum mechanics that indeed it automatically guarantees precisely the kind of consistency that we see in nature [6]…
==endquote==

Both these papers are so thematically similar that I continue to find it odd that the November 2013 one does not cite the April 2006 one as a reference! In any case both have helped to form my own views and thinking about this topic.

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====

 

[RUTA]

Uneasily, I can't agree with Marcus' statements (e.g. post # 59) along the lines of "There is no one unique official mathematical description of reality". Instead, I maintain that the very purpose of physics is to devise such storied descriptions, quantified with the invented language of mathematics, which cleverly includes the statistical concept of quantified probability.

Certainly physics has (so far) sought a unique structure that accounts for the disparate observations of all observers. In fact, the symmetries of the unique structure that we base so much of our physics upon are the result of precisely the demand for this accommodation. And, as I said earlier, QM uses the unique structure from classical physics to apply its formalism. Without such a common reference structure, I don't see how you can verify anyone else's experimental results. So, again, I'm left to wonder how the QBist sees the common set of worldtubes in M4 of classical physics resulting from quantum events when the quantum events themselves defy placement in such a unique structure.

 

[marcus]

Maybe I should explain and qualify what I said in post #59. To put it in context, this was in response to your post #57 where I'll highlight points where I thought you were referring to a picture of reality built on a unique classical 4D spacetime manifold. Indeed I can't foresee future research and that might turn out to be the only way to build one! I don't want to rule out some future development. Plus, the current epistemic venture which I see theorists embarked on may be destined to fail! Let me try to lay out the issue with some quotes:

...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.

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.

Certainly physics has (so far) sought a unique structure that accounts for the disparate observations of all observers. In fact, the symmetries of the unique structure that we base so much of our physics upon are the result of precisely the demand for this accommodation. And, as I said earlier, QM uses the unique structure from classical physics to apply its formalism. Without such a common reference structure, I don't see how you can verify anyone else's experimental results...

RUTA, I don't see anything to disagree with in the last quote! Indeed physics HAS sought such. And I think the unique (classical) structure you were referring to as the basis for QM was, to put it explicitly, MINKOWSKI spacetime M4. Indeed QM is based thereon.

I think you realize that in this thread I am not advocating anyone particular epistemic approach. I am not favoring the QB view specifically. There are a bunch of epistemic gambits that I see in play. What they all seem to me to do is to give up the notion of a unique correct 4D manifold with events entered into it, and everybody using the same identical picture.

And this seems to me to be the essence of your objection "Without such a common reference structure, I don't see how you can verify anyone else's experimental results." This is admittedly a very serious complaint and I want to try responding to it with an example.

You most likely have read the Smerlak Rovelli 2006 paper "Relational EPR". Wouldn't you say that the two observers A and B, in that example are, in a sense, confirming each other's experimental results? And yet there is no violation of LOCALITY. Because each of them gives a different account of reality. Because each considers the other to NOT HAVE MADE A MEASUREMENT until the news arrives to the one who is the observer.

You see that in a limited way they are both using the same MINKOWSKI spacetime as a "reference structure". In the particular example it is not necessary to consider spacetime curvature, the world is flat. So they are both using M4, but they are recording events in it slightly differently. So their accounts of reality differ. However these accounts can be reconciled in that each experimenter can understand where the other is coming from, and why he or she said something different. The difference can be understood on either side WITHOUT the construction of a single mathematical model of reality that somehow contains both accounts.

That's how I'd put it, anyway. Do you get some sense of what I'm trying to say?

 

[marcus]

RUTA, I have to say that your participation in thread has been a great help to me in clarifying my own thinking about the epistemic trend I see currently taking shape among some theory researchers.
A large part of the epistemic take on things could, I think, be captured in this short bit of advice:

Sure, use whatever type of global mathematical model of the universe works for you and your colleagues, but don't confuse description with reality.

There is no evidence that there IS a single correct account of events (at least we have not found one yet ). On balance I would say the weight of evidence is to the contrary.

 

[RUTA]

RUTA, I don't see anything to disagree with in the last quote! Indeed physics HAS sought such. And I think the unique (classical) structure you were referring to as the basis for QM was, to put it explicitly, MINKOWSKI spacetime M4. Indeed QM is based thereon.

I think you realize that in this thread I am not advocating anyone particular epistemic approach. I am not favoring the QB view specifically. There are a bunch of epistemic gambits that I see in play. What they all seem to me to do is to give up the notion of a unique correct 4D manifold with events entered into it, and everybody using the same identical picture.

And this seems to me to be the essence of your objection "Without such a common reference structure, I don't see how you can verify anyone else's experimental results." This is admittedly a very serious complaint and I want to try responding to it with an example.

You most likely have read the Smerlak Rovelli 2006 paper "Relational EPR". Wouldn't you say that the two observers A and B, in that example are, in a sense, confirming each other's experimental results? And yet there is no violation of LOCALITY. Because each of them gives a different account of reality. Because each considers the other to NOT HAVE MADE A MEASUREMENT until the news arrives to the one who is the observer.

You see that in a limited way they are both using the same MINKOWSKI spacetime as a "reference structure". In the particular example it is not necessary to consider spacetime curvature, the world is flat. So they are both using M4, but they are recording events in it slightly differently. So their accounts of reality differ. However these accounts can be reconciled in that each experimenter can understand where the other is coming from, and why he or she said something different. The difference can be understood on either side WITHOUT the construction of a single mathematical model of reality that somehow contains both accounts.

That's how I'd put it, anyway. Do you get some sense of what I'm trying to say?

Thanks for engaging me, marcus. This dialogue is helping me wrap my head around QBism. I don't have the impression you're advocating for it, just trying to understand epistemic approaches to physics in general. As I said earlier, I believe this is useful as we struggle at the current impasse in trying to reconcile the Standard Model of particle physics and GR. I'm very glad you started this thread.

Let me consider your example using, say, spin measurements. Suppose you describe a Stern-Gerlach device and report your outcomes. I can replicate this device and verify your results. In that example, we don't need a common M4, just duplicate (but not common) devices. But, when we have two observers measuring the spins of two entangled particles, it seems the results do need to be placed in a common M4 with common equipment. Otherwise, what do the correlations in the spin measurements mean? The correlations come from spin operators, eigenvectors and eigenvalues created wrt global orientations of the magnets. And it's the correlations that establish "non-locality." So, I don't see how we can have our correlations and "no common classical reality" at the same time.

 

[atyy]

But could it be that this controversial point of "no common reality" is tangential to QBism, and not representative of epistemic approaches to QM in general? And the Mermin solo paper about the problem of "now" is maybe even more tangential - it was not signed by Caves, Fuchs or Schack. Take a look at the closing rhetoric of the most recent QBist review without Mermin http://arxiv.org/abs/1301.3274. It is, I think, far less controversial.

After all, there is a quantum de Finetti theorem - that's uncontroversial mathematics - and the relationship to the classical de Finetti theorem was made much clearer by the rederivation of Caves, Fuchs and Schack http://arxiv.org/abs/quant-ph/0104088. I don't think anyone can deny that the classical subjective Bayesian interpretation of probability built on the foundations laid by de Finetti is coherent and beautiful - even if one doesn't use it in practice. An example of how uncontroversial the subjective Bayesian approach is in standard statistics is provided by Michael Jordan's http://mlg.eng.cam.ac.uk/mlss09/mlss_slides/Jordan_1.pdf,in which the subjective Bayesian approach is described as a "fairly unassailable framework in principle, but there are serious problems in practice".

And Leifer and Spekkens who do consider themselves advocating an epistemic view, citing QBism as close in spirit, do not accept this one controversial point. http://arxiv.org/abs/1107.5849 "In this sense, our approach is more closely aligned in its philosophical starting point with quantum Bayesianism, the view developed by Caves, Fuchs and Schack" and "Unlike the quantum Bayesians, however, we are not committed to the notion that the beliefs represented by quantum states concern the outcomes of future experiments. Rather, the picture we have in mind is of the quantum state for a region representing beliefs about the physical state of the region, even though we do not yet have a model to propose for the underlying physical states."

In fact, an epistemic view of quantum states was also advocated by Einstein http://arxiv.org/abs/0706.2661, who was also aware that some version of it which strongly denied a common underlying reality might make quantum mechanics seem local (see the quote on p9 of http://arxiv.org/abs/quant-ph/0509061).

 

[marcus]

Part of the reason for the interest in GR/Thermo/QM foundations and interpretation, besides the resolution of paradox in several areas, is current moves toward unification at a foundations level. I mentioned the latter theme in post#2 of this thread as part of defining the topic.

... In 1995 Jacobson showed a fundamental connection between GR and thermodynamics
Most people have seen this paper, I think, but if you have not then you are invited to google "Jacobson GR thermodynamics arxiv" and as the first hit you will get
Thermodynamics of Spacetime: The Einstein Equation of State which DERIVES the equation of GR from the fundamental relation connecting heat, entropy, and temperature. What is the heat of geometry? Can geometry be understood as a cloud of "molecules of geometry" that wiggle and jitter and recombine in various ways and thereby have geometric temperature and entropy?
If you google that "Jacobson GR thermodynamics arxiv" I suggested the second hit will be
Non-equilibrium Thermodynamics of Spacetime
A four page 2006 paper by Jacobson and two co-authors.
More could be said, but I don't want to make this post too long. It seems clear that the dynamics of changing geometry is not a separate subject from Thermodynamics. GR and Thermodynamics are two classical theories which are related at fundamental level in a way we do not yet understand...
... Can you add some notices of recent research addressing foundational problems of GR/Thermo/StatMech/QM, perhaps suggesting connections among them, or a possibility of our getting a better understanding of one or several of them?

We just saw a major step forward taken, on precisely this front. This paper was posted yesterday.
http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(Submitted on 21 Jan 2014)
A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation. This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity. We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.
12 pages, 1 figure

The result appears to validate the spinfoam quantization of the gravitational field because it gives rise to the required thermodynamic relations without supposing additional degrees of freedom. The needed entropy, temperature etc are already there in the spinfoam quanta of geometric field. Occam. This is a foundations approach, you could say, to showing LQG has "the right classical limit."

==quote 1401.5262 page 1==
[Jacobson's result] is a beautiful piece of theoretical physics. But its interpretation is not clear. A common understanding [1–3] takes the result as evidence that Einstein’s equations have a statistical origin and should be interpreted as equations of state for unknown underlying degrees of freedom, with the metric being a macroscopic “coarse-grained” variable. In this paper we show that a different interpretation is possible.
The alternative interpretation, which we develop mostly following [4, 5], is based on the fact that the gravitational field has quantum properties. The microscopic degrees of freedom are those of the quantum gravitational field and the Einstein equations express only the classical limit of the dynamics. The entropy across the horizon measures the entanglement between adjacent spacetime regions. Its finiteness is evidence for the quantization of the gravitational field: this is analogous to the fact that the finiteness of the black-body electromagnetic entropy is evidence for the quantization of the electromagnetic field.
We show that the Jacobson result is consistent with this simpler and tighter scenario. The finiteness and the universality of the entanglement entropy across space-time regions indicates ultraviolet quantum discreteness, as it did for Planck and Einstein at the beginning of the XX century.
==endquote==
References [4,5] are to QG entanglement entropy papers by Bianchi and by Bianchi Myers.

 

[marcus]

Berlin just posted an interesting question in the thread specifically about the above mentioned CHRR paper. I hope others besides me will respond. There are several ways to answer IMO.

I emailed prof. Rovelli a few months ago asking his opinion on the '95 Jacobson paper. He replied by saying that he did not believe Einsteins GR is an equation of state. Excellent to see the CHRR paper so shortly afterwards! The paper (more than little above my pay grade!), gives me the impression that all of Jacobsons starting points (like the Unruh temperature etc, equation 1-3 in the paper) are explained from the bottom up. However, these assumptions are replaced by other ones which "follows only from the quantum properties of gravity". Later in the paper it is stated that the framework is built on the foundations of LQG with its spin networks. Are these foundations somehow more 'fundamental' (the paper calls this a "simpler and tighter scenario", which does not seem like a scientific remark) than Jacobsons? Are these spin networks not the same as the underlying degrees of freedom Jacobson mentions, or are these fundamantally different? May be semantics, but important to judge progress.

berlin

One way to answer is to say that
1. All they are doing is deriving the GR equation and the Thermo results in the LQG context. Maybe there are other quantizations of GR that would work. Maybe LQG is not the right one! But if you assume LQG then at least you get GR and those results without putting in additional degrees of freedom.

2. Regarding "Are these spin networks not the same as the underlying degrees of freedom Jacobson mentions, or are these…?" I think that is, as Berlin suggests partly SEMANTIC.
Personally I would say no the spin networks/foams are not additional DoF, they describe quantum states and transition amplitudes of the quantum gravitational field. But I think there is some latitude as to what one can call "additional degrees of freedom" which would make it partly a semantic question.

Let's see what is relevant in the preceding quotes:

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
... This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which... follows only from the quantum properties of gravity....==quote 1401.5262 page 1==
... A common understanding [1–3] takes the result as evidence that Einstein’s equations have a statistical origin and should be interpreted as equations of state for unknown underlying degrees of freedom, with the metric being a macroscopic “coarse-grained” variable…
...The microscopic degrees of freedom are those of the quantum gravitational field and the Einstein equations express only the classical limit of the dynamics. …
==endquote==

What I see here is that for many people "EQUATION OF STATE" connotes something with a STATISTICAL origin, based on underlying entities whose behavior is quite unlike the macroscopic phenomenon described by the EoS.
They are challenging this way of thinking about GR equation by saying no the equation is not the statistical consequence of unknown possibly exotic DoF which are additional to the picture. It is just the classical limit of what we already have been studying, namely the quantum gravitational field. Nothing "unknown" or "additional" here. So there is a subtle difference.

 

[marcus]

...Let me consider your example using, say, spin measurements. Suppose you describe a Stern-Gerlach device and report your outcomes. I can replicate this device and verify your results. In that example, we don't need a common M4, just duplicate (but not common) devices. But, when we have two observers measuring the spins of two entangled particles, it seems the results do need to be placed in a common M4 with common equipment. Otherwise, what do the correlations in the spin measurements mean? The correlations come from spin operators, eigenvectors and eigenvalues created wrt global orientations of the magnets. And it's the correlations that establish "non-locality." So, I don't see how we can have our correlations and "no common classical reality" at the same time.

Hi RUTA, I am belatedly getting around to replying. Got distracted by the appearance of the "Spacetime thermodynamics without hidden degrees of freedom" paper. I'm curious to know your reaction to that one!

I think in the "Relational EPR" type situation you refer to they definitely do need a at least a rough sketch of their common geometry. Some handle on the distance between A and B, and on orientations of the apparatus.

I'm sure you'd agree that they don't need a detailed model of the universe that somehow embodies both A and B's perspective and individual experience, a "common reality" in the fullest sense. But they definitely need a rough common map of the geometric layout.

I think that is is provided by the fact that A has a detailed QUANTUM model of the universe containing B and B's location. And B has the corresponding picture, which contains A. They can discuss their relative location, and say the directions of various stars etc. They can have communicated and arrived at some consistent sketch of the general layout, before they make their measurements.

Does this address the question you had in mind?

 

[marcus]

Incidental intelligence:
A string/QFT researcher, A.J.M. Medved
http://inspirehep.net/author/profile/A.J.M.Medved.1
http://arxiv.org/find/grp_physics/1/au:+Medved_A/0/1/0/all/0/1
has just weighed in with his OWN epistemic account of QM which he says is quite similar to ROVELLI'S. The paper, which Medved coauthored with his student K.Bryan, cites the Smerlak Rovelli "Relational EPR"

==Medved and Bryan, page 9==
1.5 Similar stances
There are some approaches in the literature which are similar to that of the current paper but with different motivations.
One approach is that applied by Mermin in his so-called Ithaca interpretation of quantum mechanics [32, 33]. This viewpoint places its conceptual emphasis on the correlations between the constituent subsystems of the total quantum system. What Mermin shows is that these correlations are entirely captured by the system’s density matrix and can be revealed by suitable tracing procedures... Our stance is similar because,as seen later, applying observer complementarity is tantamount to tracing over the inaccessible variables of the density matrix.

Another such approach is that of “relational” quantum mechanics, as first presented by Rovelli [35]. This interpretation is founded on the idea of describing reality strictly in terms of relations between (quantum) observers. This is philosophically similar to but operationally distinct from observer complementarity. Indeed, Rovelli and Smerlak’s resolution of the EPR paradox [35] resembles the current presentation; nonetheless, our motivation will be focused on adhering to the requirements of observer complementarity without resorting to additional assumptions and inputs from outside the realm of standard quantum mechanics.

Another common link between our treatment and Rovelli’s is with regard to the concept of a “super-observer”. By assigning an element of reality to Alice’s prediction of what Bob measures (or vice versa), EPR requires a hypothetical observer that can “see” the outcome of the prediction even if the implicated measurement never actually happens. Essentially, the predicted value must exist for some hypothetical observer who has access to all information that is held in the Universe. This element of the argument is elaborated on later in Section 4.
==endquote==
http://arxiv.org/abs/1401.5988
Quelling the concerns of EPR and Bell
K.L.H. Bryan, A.J.M. Medved
(Submitted on 23 Jan 2014)
We begin with a review of the famous thought experiment that was proposed by Einstein, Podolsky and Rosen (EPR) and mathematically formulated by Bell; the outcomes of which challenge the completeness of quantum mechanics and the locality of Nature. We then suggest a reinterpretation of the EPR experiment that utilizes observer complementarity; a concept from quantum gravity which allows spatially separated observers to have their own, independent reference frames. The resulting picture provides a self-consistent resolution of the situation that does not jeopardize causality nor unitarity, nor does it resort to ''spooky'' (non-local) interactions. Our conclusion is that EPR and Bell rely on an overly strong definition of locality that is in conflict with fundamental physics.
25 pages

Alan Joseph Michael Medved (PhD Manitoba 2000, adv. Kunstatter) has over 1700 citations to 70-some papers. He joined the faculty at Rhodes U. in 2011
http://www.ru.ac.za/physicsandelectronics/staff/academicstaff/drajmmedved/
Kate Bryan is an honors student at Rhodes.

 

[RUTA]

I think in the "Relational EPR" type situation you refer to they definitely do need a at least a rough sketch of their common geometry. Some handle on the distance between A and B, and on orientations of the apparatus.

I'm sure you'd agree that they don't need a detailed model of the universe that somehow embodies both A and B's perspective and individual experience, a "common reality" in the fullest sense. But they definitely need a rough common map of the geometric layout.

I think that is is provided by the fact that A has a detailed QUANTUM model of the universe containing B and B's location. And B has the corresponding picture, which contains A. They can discuss their relative location, and say the directions of various stars etc. They can have communicated and arrived at some consistent sketch of the general layout, before they make their measurements.

Does this address the question you had in mind?

Thanks, marcus, this is exactly what I was asking. We need this common frame of reference (the spatial configuration of equipment) to use the formalism of QM. After B and A exchange measurement outcomes (assume they have synchronized watches), they can finish a common M4 depiction of the entire experiment. Then they check correlations and confirm violation of Bell's inequality, so they have "non-locality" (aka "quantum weirdness") per the common M4 frame of reference. How does QBism avoid "non-locality" in this case?

 

[marcus]

... How does QBism avoid "non-locality" in this case?

Alice considers Bob, together with his apparatus, laboratory, and entire kit-and-kabboodle, to be a quantum object, which she gets response messages from, and thereby observes.

She gets the message from Bob only AFTER she has checked the spin on her own particle. So it didn't cause anything. No weird action-at-a-distance.

There is no common reality in which Alice and Bob are BOTH observers. Each has hizzer own account of reality in which heeshee is the one true observer and the rest of the world is quantum.
So each of them gives hizzer own account of the sequence of events. Live and let live.
Note the politically correct gender-neutral pronouns.

 

[atyy]

Alice considers Bob, together with his apparatus, laboratory, and entire kit-and-kabboodle, to be a quantum object, which she gets response messages from, and thereby observes.

She gets the message from Bob only AFTER she has checked the spin on her own particle. So it didn't cause anything. No weird action-at-a-distance.

There is no common reality in which Alice and Bob are BOTH observers. Each has hizzer own account of reality in which heeshee is the one true observer and the rest of the world is quantum.
So each of them gives hizzer own account of the sequence of events. Live and let live.
Note the politically correct gender-neutral pronouns.

http://arxiv.org/abs/1301.3274

""There are potentially as many states for a given quantum system as there are agents"

"But outside the textbook it is not difficult to come up with examples where two agents looking at the same data"

"By one category of thought, we are agents ..."

So multiple agents can exist in QBism. However, a QBist agent should consider only himself to be an agent. Therefore Fuchs and Schack when they refer to agents, are themselves either inconsistent as QBist agents, or they acknowledge a different level of reality in which in makes sense to refer to more than one agent in the same sentence.

 

[marcus]

... they acknowledge a different level of reality in which in makes sense to refer to more than one agent in the same sentence.

I would hope so! QBism and RQM (e.g. Relational EPR) are both interpretation of quantum mechanics.
What is QM good for? How do we use it? Is it epistemic or ontic? I would hope that if I met Drs Fuchs or Schack in person they would not be studying me as a quantum system, scientifically so to speak. We would be meeting socially, as people, not as scientific subject and object. So, on a different "plane" if you picture it that way. I would say in a different mode.

QM is a formal system. I guess we all agree about that. Thinking about foundations/interpretation of formal systems can inspire creative research---like what we are now seeing re: Spacetime Thermodynamics Without Hidden Degrees of Freedom.

BTW I think you would ask the same question and I would give the same answer regarding that new paper by A.J.M. Medved. He has a name for it "observer complementarity". I hope you take a look---I'd be interested in your reaction. It could be there is little or no essential difference between Medved, QBism, and Relational EPR. Just semantics. Quibble, in effect. But you may see some distinction between them that I've missed.

 

[atyy]

I would hope so! QBism and RQM (e.g. Relational EPR) are both interpretation of quantum mechanics.
What is QM good for? How do we use it? Is it epistemic or ontic? I would hope that if I met Drs Fuchs or Schack in person they would not be studying me as a quantum system, scientifically so to speak. We would be meeting socially, as people, not as scientific subject and object. So, on a different "plane" if you picture it that way. I would say in a different mode.

QM is a formal system. I guess we all agree about that. Thinking about foundations/interpretation of formal systems can inspire creative research---like what we are now seeing re: Spacetime Thermodynamics Without Hidden Degrees of Freedom.

BTW I think you would ask the same question and I would give the same answer regarding that new paper by A.J.M. Medved. He has a name for it "observer complementarity". I hope you take a look---I'd be interested in your reaction. It could be there is little or no essential difference between Medved, QBism, and Relational EPR. Just semantics. Quibble, in effect. But you may see some distinction between them that I've missed.

But the problem is if the social mode is in fact also a scientific mode, since their sentence is in a scientific paper. So even scientifically, it seems we need the social mode.

 

[marcus]

... it seems we need the social mode.

Perhaps you are right. QM is a formal system. If David Mermin or Matt Smerlak is talking to me about "how shall we think about this formal system, how shall we interpret it? how shall we put it on a common foundation with dynamic geometry of GR? listen to my ideas"
then they are talking to me person to person NOT in the context of a formal system. Or so it seems.

It does seem, as you say, we need to recognize an informal mode. I would shudder to hear about someone inventing a formal mathematical structure for person-to-person communication, whether it was classical or quantum.

But let's get back on topic, namely QM (and other e.g. Thermo&GR) foundations and interpretations.

Did you see anything new in that Medved paper? I gather he is Canadian (PhD Manitoba in 2000) and has taken a faculty job in South Africa. It seems like there is a wee bit of an epistemic bandwagon. He brings in AdS/CFT as I recall but somehow arrives at similar conclusions to QBism and to RQM

 

[atyy]

Perhaps you are right. QM is a formal system. If David Mermin or Matt Smerlak is talking to me about "how shall we think about this formal system, how shall we interpret it? how shall we put it on a common foundation with dynamic geometry of GR? listen to my ideas"
then they are talking to me person to person NOT in the context of a formal system. Or so it seems.

It does seem, as you say, we need to recognize an informal mode. I would shudder to hear about someone inventing a formal mathematical structure for person-to-person communication, whether it was classical or quantum.

But let's get back on topic, namely QM (and other e.g. Thermo&GR) foundations and interpretations.

The way I think about it, the social mode is related to QM foundations. Since the social mode is a scientific mode in which we talk about QM, an interpretation of QM that gets rid of nonlocality by saying it's a single-user theory, doesn't get rid of it in the social/scientific mode which we use to talk about the interpretation. Only if we can talk about QBism without the social mode, ie. entirely within QBism itself (ie. I am a QBist agent all the time), can there be no nonlocality, in some sense.

But in the larger picture, I do think this is very tangential to QBism, and there's plenty of nice ideas in it.

Did you see anything new in that Medved paper? I gather he is Canadian (PhD Manitoba in 2000) and has taken a faculty job in South Africa. It seems like there is a wee bit of an epistemic bandwagon. He brings in AdS/CFT as I recall but somehow arrives at similar conclusions to QBism and to RQM

I haven't read the Medved paper yet. I also don't know if RQM is like QBism in that QBism has a clear classical quantum cut, and an agent must be complex enough to carry out Bayesian inference. But in RQM, they say the cut can be anywhere, and even an atom interacting with another atom can be an observer.

 

[RUTA]

Alice considers Bob, together with his apparatus, laboratory, and entire kit-and-kabboodle, to be a quantum object, which she gets response messages from, and thereby observes.

She gets the message from Bob only AFTER she has checked the spin on her own particle. So it didn't cause anything. No weird action-at-a-distance.

The "non-locality" is only noticed after Bob and Alice have created their common M4 depiction of the entire experiment, i.e., the spatial configuration of equipment with detector clicks located in M4. They have synchronized clocks and a common spatial configuration of equipment, so this is easy to construct. And, by "it didn't cause anything" I assume you mean the collapse of the wave function is purely epistemic.

There is no common reality in which Alice and Bob are BOTH observers. Each has hizzer own account of reality in which heeshee is the one true observer and the rest of the world is quantum.

So each of them gives hizzer own account of the sequence of events. Live and let live.
Note the politically correct gender-neutral pronouns.

But, the two M4 depictions of the entire experiment that they create are equal and clearly show space-like separated correlations that violate Bell's inequality. The transmission of measurement outcomes between them can be included, but it is irrelevant.

 

[marcus]

...after Bob and Alice have created their common M4 depiction of the entire experiment, ...

I don't suppose they ever get around to constructing a common M4 depiction.

I tried to make clear that they could agree on some distances and directions at the beginning just to get started. So as I said, a rough M4 sketch. But neither Bob nor Alice considers M4 sacred, or the framework of a shared REALITY. Maybe your Bob and Alice do but mine don't.

But, the two M4 depictions of the entire experiment that they create are equal

Nah. Bob and Alice never bother to "create an M4 depiction of the entire experiment" after the fact.
Why should they, since they don't believe it corresponds in a detailed way to reality. Each of them is a "quantum bayesian" and each of them is devoted to using quantum mechanics to, as Mermin says, organize the features of their experience.

 

[marcus]

Since we've turned several pages since the last thematic post defining the topic I'll bring forward post #44

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".

==googling "mermin now arxiv" gets http://arxiv.org/abs/1312.7825 ==
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==​

==googling "relational EPR" gets http://arxiv.org/abs/quant-ph/0604064 ==
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====

 

[marcus]

RUTA, as I said earlier I really liked the way you put it in this post. I think this puts the epistemic view in a nutshell. It applies to more than just QBism/CBism.

...
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, ...
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...

To me the reconciliation does not have to occur in the context of a formal structure (like the "unique spacetime manifold" you offer as example..)
The reconciliation basically amounts to our understanding WHY my (quantum) model of reality differs in some subtle details from yours.
As you suggest "perhaps no unique model is possible".

Indeed this is what I suspect. It seems that forcing A and B at gunpoint to work out a single unique mathematical model of reality leads to "non-locality" or "spooky action at a distance" happening faster than light can travel.

There seems to be a trend towards the epistemic approach (recognizing the subject, the agent or observer as a distinguishing feature of the model) and not just in QM, but also in GR and GR-related areas. Maybe it is another stage in the decline of the "absolute" in physics.

 

[atyy]

But haven't I already shown that QBism is inconsistent on this point? In QBism, "I am an agent" makes sense, but " We are agents" does not. Yet Fuchs and Schack write "We are agents".

Also, there is no nonlocality problem in the first place, because relativity forbids classical information traveling faster than light. But the nonlocality in quantum mechanics does not lead to classical information traveling faster than light.

So the FMS article is basically wrong. I don't believe this is a matter of interpretation, it is objectively wrong. It is internally inconsistent solving a non-existent problem. I do believe there are viable interpretations in which it can be argued, persuasively, even if I don't agree fully, that if I consider myself classical and everything else is quantum then there is locality. But such an interpretation cannot say "We are agents" without being inconsistent.

 

[RUTA]

RUTA, as I said earlier I really liked the way you put it in this post. I think this puts the epistemic view in a nutshell. It applies to more than just QBism/CBism.


To me the reconciliation does not have to occur in the context of a formal structure (like the "unique spacetime manifold" you offer as example..)
The reconciliation basically amounts to our understanding WHY my (quantum) model of reality differs in some subtle details from yours.
As you suggest "perhaps no unique model is possible".

Indeed this is what I suspect. It seems that forcing A and B at gunpoint to work out a single unique mathematical model of reality leads to "non-locality" or "spooky action at a distance" happening faster than light can travel.

There seems to be a trend towards the epistemic approach (recognizing the subject, the agent or observer as a distinguishing feature of the model) and not just in QM, but also in GR and GR-related areas. Maybe it is another stage in the decline of the "absolute" in physics.

Thanks for your many patient replies, marcus. I think you're right, this is as much as we can glean from this epistemic approach. My complaint/confusion is ontological -- a demand for "absolute" physics, as you say. And, that's exactly what this epistemic approach is telling us to discard. What replaces it? Who knows, but it's not incumbent upon them to say -- at least not at this point.

 

[RUTA]

Also, there is no nonlocality problem in the first place, because relativity forbids classical information traveling faster than light. But the nonlocality in quantum mechanics does not lead to classical information traveling faster than light.

These two facts contribute to the problem, they don't solve it. With these restrictions, how do you explain correlations between space-like separated outcomes that violate Bell's inequality? That's the mystery referred to as "non-locality."

 

[atyy]

These two facts contribute to the problem, they don't solve it. With these restrictions, how do you explain correlations between space-like separated outcomes that violate Bell's inequality? That's the mystery referred to as "non-locality."

But given that there's no logical inconsistency, and good match to experiment, why does it require an explanation?

 

[RUTA]

But given that there's no logical inconsistency, and good match to experiment, why does it require an explanation?

The calculations of physics work and if that's all you care about (a perspective called "instrumentalism"), then no further explanation is needed. But, it's a big problem for those who want to know the nature of reality (ontology) that allows those calculations to work.

 

[atyy]

The calculations of physics work and if that's all you care about (a perspective called "instrumentalism"), then no further explanation is needed. But, it's a big problem for those who want to know the nature of reality (ontology) that allows those calculations to work.

That's an unusual definition. A generalized form of de Broglie-Bohm theory is usually considered to provide a classical ontology but it is nonlocal.

The ontology/instrumentalist divide is not due to nonlocality, but to the measurement problem.

 

[RUTA]

That's an unusual definition. A generalized form of de Broglie-Bohm theory is usually considered to provide a classical ontology but it is nonlocal.

The ontology/instrumentalist divide is not due to nonlocality, but to the measurement problem.

Instrumentalism means that you take your theory to be a computational device that does not necessarily provide any ontological insight. So, an instrumentalist attitude can be applied to any issue in QM.

A nonlocal (superluminal signaling) resolution to the EPR-Bell conundrum is considered problematic by most physicists.

 

[atyy]

Instrumentalism means that you take your theory to be a computational device that does not necessarily provide any ontological insight. So, an instrumentalist attitude can be applied to any issue in QM.

Yes, but that definition holds even for classical physics. For example, why is Newtonian gravity nonlocal? It's generally not considered a problem.

A nonlocal (superluminal signaling) resolution to the EPR-Bell conundrum is considered problematic by most physicists.

But that would be true of Newtonian gravity too.

 

[RUTA]

Yes, but that definition holds even for classical physics. For example, why is Newtonian gravity nonlocal? It's generally not considered a problem.

Actually, it is considered a problem that is resolved by GR.

 

[atyy]

Actually, it is considered a problem that is resolved by GR.

But if you take that to be a problem, then one can always have instrumentalist problems. For example, in GR, what enforces minimal coupling?

Sure we can explain that by taking GR to be quantum spin 2, and using energy conservation.

But then, if GR is quantum spin 2, it is not renormalizable, which means we effectively have a cut off.

Resolutions may be a fundamental lack of Lorentz invariance, in which case locality (in the sense of spacelike operators commuting) is destroyed again. Or maybe we have something like AdS/CFT, which is also nonlocal, and a picture in which spacetime fundamentally does not exist, so we cannot even define locality.

So yes this is a problem if one considers Newtonian gravity problematic.

But it is reasonable not to consider this a fundamental problem.

The fundamental problem in orthodox QM is that one starts by divding the universe into classical and quantum realms. The classical realm includes the measurement apparatus in textbook QM (say Landau and Lifshitz) or the rational agent (in QBism). But if presumably the apparatus or QBist agent is also physical, then shouldn't there be one set of laws covering the quantum and classical realms? The measurement problem is the fundamental problem in QM, not the problem of locality.