GR/StatMech/QM foundations, epistemic views only please

[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.

 

[marcus]

...The fundamental problem in orthodox QM is that one starts by diving 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.

Hi Atyy, don't you think this is getting a bit off topic? I think the aim in this thread is to get a better understanding of these epistemic approaches on their own terms.

BTW I don't recall any passage by Mermin where he says the observer/agent is "classical".
I don't recall the statement of any dichotomy such as you suggest, dividing the world into classical+quantum "realms". You get that dichotomy in historical earlier discussions earlier interpretations like Copenhagen.
In CB, by contrast, agent is neither classical nor quantum. It is something the agent itself is not trying to model.

I don't want this thread to descend to the level of philosophical opinions.I want us to LEARN more about some of these epistemic developments not just in QM but also Thermo & GR on their own terms! So how about we PROVISIONALLY adopt the following uncritical attitude? See'f this'd be temporarily acceptable:
Let's imagine that the "agent" is not trying to understand his own understanding, or include his own knowledge in the physical world he is measuring and modeling. So there is not supposed to be "one set of laws, covering" everything (including even the abstract information and decisions and curiosity that infest the abstract mind of the agent).

I realize this is a bit ridiculous. But let's table the question of what could possibly constitute the non-physical identity of the subject that is trying to understand the physical object.

Let's also see if this is acceptable to RUTA.

RUTA, again I really like the patient clarity with which you have been summarizing the epistemic viewpoint, and thoughtfully weighing it in your estimation. I also sympathize with the ontological hunger you clearly express. The feeling that something is missing, when one is told not to hope for a comprehensive absolute physical model of everything. Or not to count on there being one, anyway. Maybe one should always keep hoping. You actually said this better, I think, a few posts back.

 

[atyy]

Hi Atyy, don't you think this is getting a bit off topic? I think the aim in this thread is to get a better understanding of these epistemic approaches on their own terms.

No, because the measurement problem is the fundamental problem, and collapse of the wave function is part of that problem. One general feature of epistemic views, not particular to QBism, is that the collapse of the wave function as a natural interpretation as a form of Bayesian inference. I do not challenge the coherence and beauty of subjective Bayesian thought.

BTW I don't recall any passage by Mermin where he says the observer/agent is "classical".
I don't recall the statement of any dichotomy such as you suggest, dividing the world into classical+quantum "realms". You get that dichotomy in earlier discussions earlier interpretations.
In CB, by contrast, agent is something the agent is not trying to model.

http://arxiv.org/abs/1311.5253 "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."

I don't want this thread to descend to the level of philosophical opinions.I want us to LEARN more about some of these epistemic developments not just in QM but also Thermo & GR on their own terms! So how about we PROVISIONALLY adopt the following uncritical attitude? See'f this'd be temporarily acceptable:
Let's imagine that the "agent" is not trying to understand his own understanding, or include his own knowledge in the physical world he is measuring and modeling. So there is not supposed to be "one set of laws, covering" everything (including even the abstract information and decisions and curiosity that infest the abstract mind of the agent).

It is not personal opinion if a logical contradiction is demonstrated. I do believe there can be objective discussion of QM interpretation. We judge on whether the predictions of QM are reproduced, and whether the interpretation is internally consistent. Where personal opinion would enter is if given two internally consistent interpretations of QM, one chooses one over the other. My claim is that QBism in saying "We are agents" is inconsistent, since there should be only "I am an agent". Thus "We are agents" as a QBist statement, implies a different level of QBist reality from the QBist reality in which only "I am an agent" makes sense. There is nothing inconsistent with two levels of reality within QBism. But then locality can only be established in one plane of QBism, not both.

To make it clear, I do not challenge that it is reasonable and consistent for a QBist agent not to include himself in the wave function - in fact, I am fond of such a view, which is why I consider this settled and not up for discussion, unless someone wants to talk about it. I do challenge any claim that a theory in which there is no classical/quantum cut cannot exist. And I do assert that "I am an agent", "We are agents", and no nonlocality at all is inconsistent.

 

[RUTA]

But if you take that to be a problem, then one can always have instrumentalist problems.

Again, an instrumentalist solution is to ignore any ontological inference (problematic or not) suggested by an otherwise successful theory. The problem with Newtonian gravity being nonlocal isn't ontological, it's a theoretical problem for those who believe all theories of physics should be mutually consistent. In that case, SR suggests all theories should be local and Newtonian gravity is not. A new theory of gravity, GR, then resolves the theoretical conflict.

 

[RUTA]

atyy's last point might relate to what I don't understand about QBism. The problem I pointed out was that Alice and Bob see "nonlocality" when they construct the M4 depiction of the experiment after exchanging measurement outcomes. That problem exists in the realm "we are agents." If Bob, say, rather sticks to "I am an agent" and records only what he observes (to include his observation of Alice's results sent to him in null or time-like fashion), and he doesn't bother to put her results in an M4 depiction to give credence to her as an agent, then he has no ontological basis for nonlocality. Am I on the right track?

 

[atyy]

Again, an instrumentalist solution is to ignore any ontological inference (problematic or not) suggested by an otherwise successful theory. The problem with Newtonian gravity being nonlocal isn't ontological, it's a theoretical problem for those who believe all theories of physics should be mutually consistent. In that case, SR suggests all theories should be local and Newtonian gravity is not. A new theory of gravity, GR, then resolves the theoretical conflict.

But then can you consistently say why the nonlocality in GR is not an ontological problem while that in quantum mechanics or de Broglie-Bohm theory is?