Nature Physics on quantum foundations

[Peter Morgan]

I keep Einstein's advice about theorists: "Don't listen to their words. Look at their deeds." An empty phrase like "beable" doesn't help to understand what Bell wants to say. Looking at his math, defining what a "local realistic theory" is, is sufficient to understand the "hard content" of his work on EPR.

I have an appendix about beables in my paper in JPhysA 2006, "Bell inequalities for random fields", https://arxiv.org/abs/cond-mat/0403692 (DOI there).

I don't want to claim this is definitive, particularly because I was parenthetically facetious at the end of the second paragraph, but I think it's still pretty close to how I feel about the question. As far as I've ever seen, Bell's use of beables is in practice always associated with probabilities. I'd be glad of pointers to other stuff.

 

[vanhees71]

So I'm not alone in my question, what beables actually are meant to be. Bell's examples don't tell me, what he means this word to mean, particularly not in the obviously intended use in the connection with QT. You express my question very precisely at the end of the quoted appendix:

It is not quite clear what we should take the common feature
of these examples to be, except perhaps the odd behaviour (the electromagnetic potential is
guilty only of ‘funny behaviour’), which is the signal for mathematics to be taken to be only
a convenience instead of real.

 

[vanhees71]

Another thought: As you also mention in your appendix, Bell also says "the wave function" were not a beable. Is this because only its modulus squared is one (or in the abstract Dirac formalism, it's not the "state ket" that represents a pure state but only the corresponding statistical operator, i.e., the projector $|\psi \rangle \langle \psi|$ or equivalently the "unit ray") or is it, because it doesn't describe anything referring to a single actual quantum system but only a probability (distribution), which refers only to an ensemble of (equally prepared) systems? On the other hand Bell seems to be a Bohmian, and @dextercioby mentioned above that the Bohmian trajectories are beables though they are not observables, and as far as I understand the Bohmian standpoint the Bohmian trajectories refer to a single system. In this sense it may be a beable, but if it isn't observable, what's then the status of a "beable" as a physical property of a system? I can claim a lot of things to be a "beable", but if I can't observe it, it's not subject to the scientific method of testing it's meaning by observation.

 

[Fra]

I don't understand, what the difference between observables in the usual sense of the word and "beables" should be.

If we take Bells ideas as the definition of beable, then it becomes hard to guess of course. When interpreting them in the light of today perhaps he turns in his grave, who knows. I hope he is forgiving.

But I have within my own understanding always entertained the a notion of what relates to the agents state; which in turn can be understood as a result of it's own interactions/measurements on it's environment. I have then realized that this is strongly related to the notion of beable, but I have a very different context. Demystifiers solipsist HV, resonates fine with this.

From Bell's example from classical electrodynamics that the electromagnetic field is a "beable" within this theory but the electromagnetic potential is not, I can only conclude that "beables" are synonymous to "observables", i.e., a quantity within the theory which represents observables in the sense that this quantity is uniquely determined by the physical situation that is described. Gauge-dependent quantities in a gauge theory, i.e., a theory where some elements (here the electromagnetic four-potential) are not uniquely determined by the physical situation described, cannot represent observables.

In my view, the definition of what are beables is observer dependent. So I don't think Alices beables are not beables of Bob. Yet one can entertain the idea that the beables are their respective "locally encoded facts", they "just are" as I think Bell puts it. But that does not (I think) necessarily mean they are inexplicable! I consider them to be a result of a series of "local", agent-perspective measurements, that does NOT necessarily store their results in classical (public to the agent community) pointer variables. They are stored only in the agents state. (thus solipsist HV). One can think of they as definitely REAL (ontological), but this ontology is not inferrable to other agents, in the way can one copy classical information. This is my take on this. So since a few years, I found this "similarly" between my thinking which is at the almost opposite camp of hidden variable theories, to actually be compatible with this.

Observables OTOH, are defined by measurements attached in the classical domain, where agents can agree and share information, but this information (ie the whole equivalence class and it's structure and symmetries) are I think invisble to the agent itself.

Beable could then related to "intrinsic" measurements (by an agent), and observables can be related to "extrinsic" measurments (say by the classical collective of agents). I think that value of the beable, is that the interaction of beables has the potential to explain the emergence of observables.

/Fredrik

 

[Peter Morgan]

Another thought: As you also mention in your appendix, Bell also says "the wave function" were not a beable. Is this because only its modulus squared is one (or in the abstract Dirac formalism, it's not the "state ket" that represents a pure state but only the corresponding statistical operator, i.e., the projector $|\psi \rangle \langle \psi|$ or equivalently the "unit ray") or is it, because it doesn't describe anything referring to a single actual quantum system but only a probability (distribution), which refers only to an ensemble of (equally prepared) systems? On the other hand Bell seems to be a Bohmian, and @dextercioby mentioned above that the Bohmian trajectories are beables though they are not observables, and as far as I understand the Bohmian standpoint the Bohmian trajectories refer to a single system. In this sense it may be a beable, but if it isn't observable, what's then the status of a "beable" as a physical property of a system? I can claim a lot of things to be a "beable", but if I can't observe it, it's not subject to the scientific method of testing it's meaning by observation.

I'm pretty prejudiced about this, I'm afraid. I'm completely focused on how Bell uses beables in the argument in his article "The theory of local beables", using probabilities, and in the various articles where he rediscusses that argument in the light of Shimony's and others' introduction of what we would today call superdeterminism. As he used beables in those arguments, there is always a probability measure, which makes his usage effectively a classical equivalent of a quantum field, which we might call random-variable-valued distributions, or, as I do, using a pre-existing name in the mathematics literature, a random field.
Although Bell discusses Bohmian trajectories in many places because it's clearly a mathematically reasonable approach, my understanding is that he was enough of a field theorist in his CERN-influenced heart of hearts that he wanted a field theoretic approach and was dissatisfied with what could be done with Bohm's approach applied to quantum fields? For Bohmian trajectories, the particle positions in configuration space seem to me to be perfectly good beables, but they seem never to have been entirely palatable to Bell? Anyway, I have always been somewhat dissatisfied with configuration space as a theater for beables even though it is mathematically reasonable.

 

[Demystifier]

I can claim a lot of things to be a "beable", but if I can't observe it, it's not subject to the scientific method of testing it's meaning by observation.

You are right, it's meaning cannot directly be tested by observation. Beable is a tool for thinking. It is natural for a human mind to think that physical "things" exist even when we don't observe them, and "beable" is a concept referring to exactly such things. It is nevertheless "scientific", in the sense that at least some scientists find it useful in thinking about science. For example, I like to think that the Moon has a round shape even when it isn't observed, so for me the shape of the Moon is a beable. Perhaps you, on the other hand, prefer to think that the Moon has no shape when it's not observed (the shape is not a conserved Noether charge), so for you the shape of the Moon is not a beable.

 

[Demystifier]

I have always been somewhat dissatisfied with configuration space as a theater for beables

Why?

 

[vanhees71]

In my view, the definition of what are beables is observer dependent. So I don't think Alices beables are not beables of Bob. Yet one can entertain the idea that the beables are their respective "locally encoded facts", they "just are" as I think Bell puts it. But that does not (I think) necessarily mean they are inexplicable! I consider them to be a result of a series of "local", agent-perspective measurements, that does NOT necessarily store their results in classical (public to the agent community) pointer variables. They are stored only in the agents state. (thus solipsist HV). One can think of they as definitely REAL (ontological), but this ontology is not inferrable to other agents, in the way can one copy classical information. This is my take on this. So since a few years, I found this "similarly" between my thinking which is at the almost opposite camp of hidden variable theories, to actually be compatible with this.

But isn't this the perfectly opposite interpretation of "beable" to what Bell intended when introducing this word? He insisted on defining the theory without reference to "observers" and "measurements". Of course, I agree with you that this doesn't make sense within minimally interpreted QT, because there is hinges on the state (i.e., the applied preparation procedure in an experiment) whether an observable takes a defined value or not, and the question is, whether a "beable" must be some quantity which takes determined values. Then this would indeed imply that a "beable" can only be an observable which takes a determined value, and thus this would be state dependent, i.e., dependent on the preparation procedure for the system to be measured.

On the other hand it could also be that a "beable" is simply synomymous with "observable". Then it refers to the measurement procedure, and of course one can measure any observable, independent of the state the measured system is prepared in. But then the "beable" is a quantity which does not necessarily take a determined value, i.e., it makes only sense to talk about the probability for the outcome of a measurement of this "beable", but this again contradicts Bell's declared aim that "beables" should be defined as something independent of measurements.

That's the dilemma I'm in in my inability to understand what Bell means with his nice word play, introducing the notion of "beables".

Observables OTOH, are defined by measurements attached in the classical domain, where agents can agree and share information, but this information (ie the whole equivalence class and it's structure and symmetries) are I think invisble to the agent itself.

For me there's no distinction between "classical" and "quantum" domains. The classical behavior of macroscopic objects is due to an effective coarse-grained description of collective observables. E.g., a "classical point particle" never is an elementary particle like an electron but a "macrocopic body", and the position and momentum is something like the center-of-mass position and momentum.

Beable could then related to "intrinsic" measurements (by an agent), and observables can be related to "extrinsic" measurments (say by the classical collective of agents). I think that value of the beable, is that the interaction of beables has the potential to explain the emergence of observables.

/Fredrik

What is an "intrinsic" vs. an "extrinsic" measurement?

 

[Peter Morgan]

Why?

— have I "always been somewhat dissatisfied with configuration space as a theater for beables"?
It's just "somewhat". It's not a deal-breaker for deBB, which, given QM, is perfectly good mathematics, but it's enough that I feel motivated to look for alternatives.
I'm unhappy with nonrelativistic QM in its common axiomatic or textbook constructions because from modern experiments I think we have records of where and when events happened, not of particles and their positions and momenta (with the latter perhaps requiring continuous trajectories for us to construct them), and that reason also applies to deBB. The possibility of transforming the wave function into deBB's configuration space formalism derives from QM's insufficiently considered leap from the properties of events to the properties of particles.
Axiomatic QFT, algebraic QM, and Bell's account of classical physics in "The theory of local beables" associate measurements/beables with regions of space-time, not with specific particles, which I think better aligns with what we might call the raw signal and event data out of experimental apparatus. If we can with certainty derive the existence of particles from the events we record, then fine, but otherwise we should hesitate to put particles (though I won't be telling anyone here something they don't know if I say that usually the word used is "system") into the axioms. For FAAPP* work, however, I'm entirely happy to put tables and chairs and bulk components of an experimental apparatus into the axioms of whatever mathematics we're using.
*That's "For Almost All Practical Purposes", because there are always special cases.

 

[Peter Morgan]

For me there's no distinction between "classical" and "quantum" domains. The classical behavior of macroscopic objects is due to an effective coarse-grained description of collective observables. E.g., a "classical point particle" never is an elementary particle like an electron but a "macrocopic body", and the position and momentum is something like the center-of-mass position and momentum.

Unless I've misunderstood your meaning here, saying that 'there's no distinction between "classical" and "quantum" domains' has you needing, I think, something like my work to justify it. I suppose that for most physicists your lack of distinction must ring false. The traditional no-go theorems —Gleason, Kochen-Specker, and Bell— tell us fairly decisively, I think, that ordinary definitions of classical physics are not able to model experimental apparatus and analysis that can be modeled by quantum physics.
That turns EPR's 1935 question, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" on its head: the answer to "Can Classical-Mechanical Description of Physical Reality Be Considered Complete?" is "No".
I say explicitly that we have to "model experimental apparatus, results, and analysis" because Copenhagen, I think rightly, insists that that the experimental apparatus and results must be communicated classically, which in modern times requires Megabytes or Terabytes of interpersonally and institutionally sharable information, but the analysis of the results often requires us to consider results that are contextual or incompatible (or insert your preferred word here), which ordinary classical mechanics is not able to do in a systematic way. QM models that analysis by using noncommutativity, which ordinary definitions of classical physics are not allowed to use. Here I'll leave it to my recent published papers to fill in how I see how the story develops from there. If you don't like my papers —which you might not because most people say they find them a difficult read— then please publish something better (which, to be clear, I very much want someone to do but if that has to be me in a few years time, so be it), so everyone will nod along when you say 'there's no distinction between "classical" and "quantum" domains'.

 

[vanhees71]

I'm much less ambitious. My point is that the classical behavior of macroscopic objects can be understood in terms of quantum many-body theory, i.e., classicality is an emergent phenomenon.

 

[Demystifier]

I'm much less ambitious. My point is that the classical behavior of macroscopic objects can be understood in terms of quantum many-body theory, i.e., classicality is an emergent phenomenon.

Yes, but emergent from what? An obvious tentative answer is - from microscopic quantum laws. But one of those laws is supposed to be the law that the probability of a measurement outcome is given by the Born rule. The problem with this law is that it refers to a measurement, which is a macroscopic notion, not a microscopic one. Hence it seems that classicality cannot emerge from purely microscopic laws, simply because we start from the quantum theory not formulated as a purely microscopic theory.

A way out of this problem is to reformulate the Born rule such that it does not refer to measurement. Instead of talking about probability of an observed outcome, we should talk about probability of being in certain state. This is why we need a notion of beable, as distinct from observable. A beable is a variable with which we can associate a probability without specifying the measurement context. Various contextuality theorems (Kochen-Specker etc.) show that it is not possible to have beables for all observables at once. This is why Bohmian-like theories postulate beables only for some of the observables (and not for the others), typically only for particle positions (and not for their momenta).

In essence, this is why Bohmian formulation of quantum theory can explain classicality as emerging from purely microscopic laws, while standard formulation of QM, by referring to macroscopic measurements in one of its most fundamental laws, cannot explain classicality as emerging from purely microscopic laws.

As a byproduct, the argument above contains a very simple explanation of the notion of beable. To repeat, a beable is a variable with which we can associate a probability without specifying the measurement context.

 

[vanhees71]

But the probabilities we calculate with Born's rule are not probabilities for "being in [a] certain state" but the probabilities for the outcome of measurements.

Your definition of "beable" is empty, because you don't specify for what you associate a probability. It cannot be related to quantum theory, because the probabilities of quantum theory are only specified by the measurement context.

 

[Demystifier]

But the probabilities we calculate with Born's rule are not probabilities for "being in [a] certain state" but the probabilities for the outcome of measurements.

In the standard formulation of QM that's true. But it's legitimate to consider a non-standard formulation, if it can lead to some advantages over the standard one.

Your definition of "beable" is empty, because you don't specify for what you associate a probability. It cannot be related to quantum theory, because the probabilities of quantum theory are only specified by the measurement context.

Again, it cannot be related to standard formulation of quantum theory, but it is related to a non-standard formulation. More specifically, when the non-standard formulation is the Bohmian one, then it is shown in the literature that the standard QM probabilities of measurement outcomes are emergent from the non-standard probabilities of beables. In this sense, standard QM emerges from the Bohmian one. This, of course, doesn't prove that the Bohmian formulation is "true", but it does demonstrate that standard QM can emerge from something in which more fundamental probabilities can be defined without a measurement context. I think this is a great theoretical discovery by Bohm that such a more fundamental theory is at least possible.

 

[vanhees71]

These are still empty words. You don't say probabilities of what and you don't specify, how in Bohmian quantum mechanics probabilities are "derived".

 

[martinbn]

These are still empty words. You don't say probabilities of what and you don't specify, how in Bohmian quantum mechanics probabilities are "derived".

He said it is in the literature.

 

[vanhees71]

That's also a nice trick. You just claim "it's in the literature" and don't quote anything concrete. Sometimes people are even more "clever" and quote some literature, where the claimed issue is however not contained ;-).

 

[Demystifier]

You just claim "it's in the literature" and don't quote anything concrete.

See e.g. the reference in my signature.

 

[vanhees71]

But there you also assume the Born rule for position- (configuration-) space-wave functions. That from this it follows also for other observables, is derived in any QM textbooks, which start from wave mechanics without referring to BM at all. I still don't see, where this defines "beables". Your "perceptibles" make much more sense to me, btw.

 

[Lord Jestocost]

But isn't this the perfectly opposite interpretation of "beable" to what Bell intended when introducing this word? He insisted on defining the theory without reference to "observers" and "measurements".

Indeed, that was it in the end: to design an "objective", physical theory in the sense of classical phyics.

 

[Fra]

But isn't this the perfectly opposite interpretation of "beable" to what Bell intended when introducing this word? He insisted on defining the theory without reference to "observers" and "measurements".

Yes, probably. But discussing the actual original old ideas to restore classical mechanics is not interesting at all to me. I think most of us know it just does not work. Those things are indeed the opposite of my own thinking.

What I find entertaining here is to try to reinterpret the concepts in a way that might work a bit like the devils advocate! Just saying Bell was just wrong, is making it too easy.

After all, I suspect some of the original motivations of Einstein and others, was no just to restore "classical mechanics" and "determinism", but to find a novel intuitive understanding of the causal mechanisms in QM, just like we at least thought we have about some of classical mechanics. To understand how the bell pair can in fact end up corrleated and still violate the bell inequality is a challenge. We know nature has found a solution, and we can describe it but the current theory provides no proper explanation on a causal level in between the parts. Note that such an explanation does not necessarily mean that we must restored determinism, at least it does not for me.

Of course, I agree with you that this doesn't make sense within minimally interpreted QT, because there is hinges on the state (i.e., the applied preparation procedure in an experiment) whether an observable takes a defined value or not, and the question is, whether a "beable" must be some quantity which takes determined values. Then this would indeed imply that a "beable" can only be an observable which takes a determined value, and thus this would be state dependent, i.e., dependent on the preparation procedure for the system to be measured.

On the other hand it could also be that a "beable" is simply synomymous with "observable". Then it refers to the measurement procedure, and of course one can measure any observable, independent of the state the measured system is prepared in. But then the "beable" is a quantity which does not necessarily take a determined value, i.e., it makes only sense to talk about the probability for the outcome of a measurement of this "beable", but this again contradicts Bell's declared aim that "beables" should be defined as something independent of measurements.

My creative interpretation is that, if you by independent of "measurements", implicitly refers to "measurements" as per quantum mechanicsm and the statistical ensemble etc. Then, I think this makes sense for a beable.

For me there's no distinction between "classical" and "quantum" domains. The classical behavior of macroscopic objects is due to an effective coarse-grained description of collective observables. E.g., a "classical point particle" never is an elementary particle like an electron but a "macrocopic body", and the position and momentum is something like the center-of-mass position and momentum.

What is an "intrinsic" vs. an "extrinsic" measurement?

By extrinsic measurement I refer to what we consider a measurement in QM. The laboratory setup doing both preparations, detections and signalprocessing are solid and dominant and has unlimited informaiton processing capacity relative to the normalyl small subsystem we consider "the system" we study. This approximation also implies that the records and signal processing system is ot affected to deformed by the backreaction of the system. One can literally "control" the system and make it jump by choosing the right preparation. The observing context can simply consume the backreaction. I think quantum mechanics and QFT essentially dscribes these type of measurements. But they are idealisations.

An Intrinsic measurement follows the same principles of soundness, it employes detectors and counters and can postprocess the data. But this whole process is existing in a submissive state in a much larger environment it can not control. This corresponds more to cosmological observations, where the environment can not be predicted or "repeated", the mesurements themselves are similar to an evolutionary process where the observers itself is the life form. We do not have yet a theory of such intrinsic measurement, based on the same higher stnadard as QM. So this "inside observer" makes measurement in a less controlled way. Any spontaneous interaction is a "measurement", and the inside observer can form rational expectations of it's own future based on interaction history. The big difference is that if one tries to construct a "measurment theory" (ie replacement for QM) in this way, there are several constraints. For example you really can NOT encode arbitrary amounts of information, and you can not consider arbitrary scramling times for data etc. All this information processing, must be accounted for in time, as actual processes. There is also obviously in this view no background spacetime map. It literally has to be constructed by the agent itself.

Anyway, I think the beable is a potential candidate for such an inside measurement. It actually has the similar "standard" you would want from a rational measurement, but it's constrained in it's information capacity, and i think this limitation must be real. But it does NOT makes use of the kind of "observers" and "measurements" that conventional QM defines. So in this sense, it complies to Bells words?

I hope you see the difference between the "inside observer", and modelling/describing the measurement device in terms of QM as well? The latter method avoids the constraints, and allows for a description that is larger and more detailed that what can be encoded in the agent. Information in excess, that can't fit. Forcing this, is I think also not surprisingly a reason for the various divergences when one tries to extrapolate things down to the minimal scale or highest energy.

/Fredrik

 

[Fra]

But the probabilities we calculate with Born's rule are not probabilities for "being in [a] certain state" but the probabilities for the outcome of measurements.
...
Your definition of "beable" is empty, because you don't specify for what you associate a probability

It seems to me the probability for the counters future microstate (ie agents/observers microstate) is the same thing as the outcome of "future observations". But with the difference that you have no strict control over the repeats. Random/uncontrolled preparations?

And observing agent, trying to "survive" has to have good expectations of the future. It's like a hard core experiment, but without control or preparation. And the agents is it's own counter. And unlike a normal experiment, where you compare frequencies from statistics with the expectations from the preparation. The beable probability would be more like a "guide" for the agents action in the game of life.

A theory with descriptive probabilities are corroborated by comparing experiments with frequencies from data.

A theory of guiding probabilities, should I think be corroborated not by frequencies of "unobservable beables", but from the implications of theim - ie, the agents actions should be in compliance with the guidind probabilities. So it's an indirect corroboration, and i think it should be - in principe - possible.

/Fredrik

 

[Demystifier]

But there you also assume the Born rule for position- (configuration-) space-wave functions. That from this it follows also for other observables, is derived in any QM textbooks, which start from wave mechanics without referring to BM at all.

You are right that Born rule for other observables is derived from Born rule for positions without referring to BM, but I am not aware of any general QM textbook that actually derives it. Can you name at least one such textbook?

I still don't see, where this defines "beables". Your "perceptibles" make much more sense to me, btw.

I'm glad that my notion of "perceptible" makes sense to you. Since I have not defined it precisely, it demonstrates that a precise definition is not always necessary for understanding. But it depends on personality. For some people, like you, the notion of perceptible may be more intuitive than that of beable, while for other people it my be the other way around.

If you really want to see a precise mathematical definition of something like a "beable", see the definition of ontic in the context of the PBR theorem. If you want explicit references, I can give you some.

 

[vanhees71]

After all, I suspect some of the original motivations of Einstein and others, was no just to restore "classical mechanics" and "determinism", but to find a novel intuitive understanding of the causal mechanisms in QM, just like we at least thought we have about some of classical mechanics. To understand how the bell pair can in fact end up corrleated and still violate the bell inequality is a challenge. We know nature has found a solution, and we can describe it but the current theory provides no proper explanation on a causal level in between the parts. Note that such an explanation does not necessarily mean that we must restored determinism, at least it does not for me.

This is also an argument, I don't understand. We have QT, that precisely does all that. It explains the correlations, the violation of Bell's inequality, and (in it's realization as relativistic microcausal QFT) respects the causality structure of special relativity. It also provides a "causal explanation" of the correlations: It's just the preparation of, e.g., two photons in an entangled state by, e.g., parametric downconversion. So what do you think is lacking? What do you need in addition to standard Q(F)T to be satisfied with the accurate description of Nature?

My creative interpretation is that, if you by independent of "measurements", implicitly refers to "measurements" as per quantum mechanicsm and the statistical ensemble etc. Then, I think this makes sense for a beable.

My point is that all theoretical physics does is to describe in a more or less abstract mathematical way what we observe in Nature in some situation. There's no difference between classical and quantum physics except that the scientific method of precise, quantitative observation and mathematical model and theory building has revealed that observables can never all take determined values in any possible state of the system under investigation, and that this was hard to accept for the physicists 100 years ago is understandable, because it's indeed not easy to give up successful concepts. On the other hand, it was pretty clear that classical physics could not explain, how given the atomistic structure of matter (which was under heavy debate at the time too!) matter can be stable and how charged particles in bound states forming atoms, molecules, and condensed matter lead to discrete emission and absorption spectra of electromagnetic radiation/light. The answer of the scientific method to all these questions was the development of modern Q(F)T in the mid 1920ies.

By extrinsic measurement I refer to what we consider a measurement in QM. The laboratory setup doing both preparations, detections and signalprocessing are solid and dominant and has unlimited informaiton processing capacity relative to the normalyl small subsystem we consider "the system" we study. This approximation also implies that the records and signal processing system is ot affected to deformed by the backreaction of the system. One can literally "control" the system and make it jump by choosing the right preparation. The observing context can simply consume the backreaction. I think quantum mechanics and QFT essentially dscribes these type of measurements. But they are idealisations.

Of course they are idealizations, and these work, because a more or less "coarse-grained description" of the behavior of macroscopic apparati are sufficient. In this sense it depends on the accuracy you look at a macroscopic system, whether it's "behaving classically or quantum theoretically". E.g., there are these experiments with very large molecules to find the boundary between classical and quantum behavior, and indeed you can handle amazingly large molecules with such precision (particularly cooling them down to very law temperatures) that you can demonstrate quantum behavior on them.

An Intrinsic measurement follows the same principles of soundness, it employes detectors and counters and can postprocess the data. But this whole process is existing in a submissive state in a much larger environment it can not control. This corresponds more to cosmological observations, where the environment can not be predicted or "repeated", the mesurements themselves are similar to an evolutionary process where the observers itself is the life form. We do not have yet a theory of such intrinsic measurement, based on the same higher stnadard as QM. So this "inside observer" makes measurement in a less controlled way. Any spontaneous interaction is a "measurement", and the inside observer can form rational expectations of it's own future based on interaction history. The big difference is that if one tries to construct a "measurment theory" (ie replacement for QM) in this way, there are several constraints. For example you really can NOT encode arbitrary amounts of information, and you can not consider arbitrary scramling times for data etc. All this information processing, must be accounted for in time, as actual processes. There is also obviously in this view no background spacetime map. It literally has to be constructed by the agent itself.

Sure, but all this simply works in practice, as demonstrated by the precision of the predictions of QT in comparison with experiments, and indeed what can be observed today has developed with an amazing speed. For Freedman, Clauser, Aspect, et al who did the first investigations with entangled quantum systems of various kinds, it was cutting-edge technology to be able to just prepare such entangled states and make them "stable" enough to do experiments with them. Today we have amazing technologies to prepare and handle such states. It's at a stage of development, where it becomes an engineering science, i.e., it is so well understood that it's used to develop applications for everyday life. I think quantum cryptography is already in a stage, where it is in principle applicable for everyday work, and really convincing quantum computers won't take too long to be developed given the huge economical effort put into the corresponding engineering research and development.

Anyway, I think the beable is a potential candidate for such an inside measurement. It actually has the similar "standard" you would want from a rational measurement, but it's constrained in it's information capacity, and i think this limitation must be real. But it does NOT makes use of the kind of "observers" and "measurements" that conventional QM defines. So in this sense, it complies to Bells words?

This I don't understand.

I hope you see the difference between the "inside observer", and modelling/describing the measurement device in terms of QM as well? The latter method avoids the constraints, and allows for a description that is larger and more detailed that what can be encoded in the agent. Information in excess, that can't fit. Forcing this, is I think also not surprisingly a reason for the various divergences when one tries to extrapolate things down to the minimal scale or highest energy.

/Fredrik

But most modern measurement devices use QT to model and construct it. All semiconductor technology is obviously based on QT!

 

[Demystifier]

In this sense it may be a beable, but if it isn't observable, what's then the status of a "beable" as a physical property of a system?

Ah, now I see what confuses you. Nobody said that a beable cannot be observed. It can. But it is not "observable" in a technical sense, namely in the sense that beable is not described by a self-adjoint operator on the Hilbert space.

In this technical sense, all classical measurable variables (particle positions, electromagnetic fields) are not "observables", simply because classical theory does not deal with operators in the Hilbert space. Bohmian-like theories propose that, in addition to abstract self-adjoint operators in the Hilbert space, we also need less abstract "classical-like" variables, called "beables", that are not represented by operators in the Hilbert space.

 

[vanhees71]

Ok, so in the Stern-Gerlach experiment is a "beable" then a spot of a silver atom on the photo plate?

 

[Demystifier]

Ok, so in the Stern-Gerlach experiment is a "beable" then a spot of a silver atom on the photo plate?

Yes, it's a beable. (In addition, it's also a perceptible.)

 

[vanhees71]

Ok, then why is Bell giving the example that the electromagnetic field were a beable within classical field theory? In analogy to my example with the spot of a particle on a photo plate shouldn't then also only the directly recognizable actions of this field be beables? But then again, this contradicts the aim given by Bell to avoid all reference to measurements/observations, and indeed it would be a bit too narrow to only call things beables that can be directly observed with our human senses.

 

[Demystifier]

Ok, then why is Bell giving the example that the electromagnetic field were a beable within classical field theory? In analogy to my example with the spot of a particle on a photo plate shouldn't then also only the directly recognizable actions of this field be beables? But then again, this contradicts the aim given by Bell to avoid all reference to measurements/observations, and indeed it would be a bit too narrow to only call things beables that can be directly observed with our human senses.

That's because Bell is primarily interested in fundamental beables. The spot on the photo plate is not fundamental. The EM field is.

 

[Fra]

That's because Bell is primarily interested in fundamental beables. The spot on the photo plate is not fundamental. The EM field is.

Do you have a reference to where Bell defines "fundamental" in this context? What if I associate fundanental beable to say the smallest information unit encoded in a given solipsist view which is also the "primary" or raw distinguishable events from a given solipsist/agent view, ie elementary counters whcih can't be described in smaller units? (Ie from which more, complex beables are built by recoding or assembly)

It would be how i would think of it, and the question is if that might be in compliance with bells definitions?

/Fredrik

 

[Lord Jestocost]

What means in this respect “fundamental”? The notion was constructed by Bell’s mind.
"Fundamental" in an ontological sense or in an epistemological sense?

 

[Fra]

"Fundamental" in an ontological sense or in an epistemological sense?

At least from my perspective, if you require any epistemological results to be encoded in thw agents own state. This sort of relates epistemology and ontology. I wonder if this wasö bells intention as well or not?

/Fredrik

 

[Demystifier]

"Fundamental" in an ontological sense or in an epistemological sense?

Ontological. That Bell considers ontology more fundamental than epistemology is particularly clear from his article "Against measurement". I recommend reading this article to everybody who want to understand how Bell thinks.

@Fra I believe this also answers your question.

 

[vanhees71]

I think that's what makes Bell hard to understand for me. I don't understand, why you are after "ontology" when doing physics. As a natural science it's about what can be objectively and reproducibly be observed, and theoretical physics thus looks for generally valid laws from the patterns we observe.

 

[WernerQH]

I don't understand, why you are after "ontology" when doing physics. As a natural science it's about what can be objectively and reproducibly be observed, and theoretical physics thus looks for generally valid laws from the patterns we observe.

This makes you sound like a positivist. Surely you believe in atoms? Or in photons?
Bell (and, of course, this year's Nobel laureates) ruled out the idea of photons always having a definite state of polarization that they carry from the source to the detectors. (Unless they take part in conspiracy or superluminal communication.) What we think physics is about has an enormous effect on the direction of research.