I Nature Physics on quantum foundations

[vanhees71]

What about the "ontology" of background spacetime, that you need to formulate QM in the first place? And the "ontology" of classical pointer records?

/Fredrik

Spacetime models (Galilei-Newtonian, Minowskian, and GR spacetimes) are descriptions about (local) observable events, among them "classical pointer records", no more, no less.

 

[Lynch101]

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.

Is an "observable", therefore, also be a "beable" prior to its being observed or prior to its interaction with a measurement device?

 

[vanhees71]

Let me guess (although I still think that "beable" is a very poorly defined buzz-word): An observable is a beable only, if the system is prepared in a state, where this observable takes a determined value, i.e., it's represented by the projector to the corresponding eigen-value subspace of the corresponding self-adjoint operator, $\text{Eig}(\hat{O},o)$, where $o$ is the determined value of the observable $O$, which is reprsented by the self-adjoint operator $\hat{O}$.

 

[Demystifier]

Ontology then is not covered by natural sciences, because natural sciences are about what's observed in nature. That doesn't imply that things don't exist, when not observed.

Is there scientific evidence that things exist when not observed?

 

[Demystifier]

Is an "observable", therefore, also be a "beable" prior to its being observed or prior to its interaction with a measurement device?

Beables, by definition, exist prior to both observation and interaction with a measurement device. But you seem to imply that any observable is also a beable, which is wrong.

 

[vanhees71]

If there's nothing, nothing will be observed, i.e., observing something is scientific evidence that there is this something. I think this is a pretty empty philosophical discussion once more. What should this be good for?

 

[CoolMint]

Is your philosophic conclusion above (that it's "a road to nowhere") itself a road to nowhere?

No, as I don't promote any particular interpretation. I am only able to recognize the presence of this underlying 'reality' for its observable effects. Interpretations generally shy away from explaining how the magic of single outcome happens even though it obviously does happen. Use the Schoredinger equation, get the probabilities, interpret the outcome. There is still no deep explanation why, so interpretations are more like descriptions with some added unobservale details.

 

[Demystifier]

Let me guess (although I still think that "beable" is a very poorly defined buzz-word): An observable is a beable only, if the system is prepared in a state, where this observable takes a determined value, i.e., it's represented by the projector to the corresponding eigen-value subspace of the corresponding self-adjoint operator, $\text{Eig}(\hat{O},o)$, where $o$ is the determined value of the observable $O$, which is reprsented by the self-adjoint operator $\hat{O}$.

You are trying to define beable by using only the language of standard QM, which is impossible.

 

[Demystifier]

No, as I don't promote any particular interpretation.

So in general it's valuable to use philosophy in science, but it's not valuable to use philosophy in interpretations of QM. Is that what you are saying?

 

[CoolMint]

So in general it's valuable to use philosophy in science, but it's not valuable to use philosophy in interpretations of QM. Is that what you are saying?

I think it is quite safe to assume that this underlying reality does exist between measurements.

Is there scientific evidence that things exist when not observed?

 

[Demystifier]

I think it is quite safe to assume that this underlying reality does exist between measurements.

But it's meaningless, or at least unscientific, to say anything more specific about this reality. Is it what you are saying?

 

[Demystifier]

If there's nothing, nothing will be observed, i.e., observing something is scientific evidence that there is this something. I think this is a pretty empty philosophical discussion once more. What should this be good for?

It's good for proving that you are a quantum computer, as defined in post #191.

 

[Lynch101]

Beables, by definition, exist prior to both observation and interaction with a measurement device. But you seem to imply that any observable is also a beable, which is wrong.

Might it be more accurate to say that it is a beable which interacts with the measurement device to give an observable (without implying all beables will give rise to observables)?

 

[martinbn]

Because it's not scientific, there is no experiment showing that things exist when we don't observe them.

Ontology is by definition things that exist even when we don't observe them, and you always say that ontology is not scientific.

There are two different "exist" that people use, and often they are not carefull in their language, which causes confusion. (Why? I don't know. Sometimes I think they do it on purpose.) One is the actual ontological sense of exist. The other is the existence of values of observables. Fro example QM says that it is not always meaningful to say that an electron has spin "up" in a given direction unless measured. But QM doesn't say that the electron doesn't exist unless measured.

 

[Demystifier]

Might it be more accurate to say that it is a beable which interacts with the measurement device to give an observable (without implying all beables will give rise to observables)?

No. First, it's not clear what does "it" refer to. Second, beable is not an operator, so it's not clear how a non-operator may "give" an operator (observable).

 

[Lord Jestocost]

I think it is quite safe to assume that this underlying reality does exist between measurements.

What "underlying reality" doesn’t “exist” between measurements? At least, one should denote the character of the “underlying reality” one has in mind.

 

[Demystifier]

But QM doesn't say that the electron doesn't exist unless measured.

Here you mean "exist" in the ontological sense, right? Can this ontological existence of electron be described mathematically?

 

[CoolMint]

But it's meaningless, or at least unscientific, to say anything more specific about this reality. Is it what you are saying?

We have one notable detail about the underlying reality. Mathematics plays a substantial role in it. Hopes are we can get more out of it in this route.
It gets more and more abstract though. Both string theory and LQG say that space and time and the dimensions are emergent, so we are on scientific terms when discussing emergence.

 

[Demystifier]

Mathematics plays a substantial role in it. Hopes are we can get more out of it in this route.

So if we say something about reality in a mathematical form, then we have better chances to produce concrete results, would you agree? Well, Bohmian formulation of QM expresses possible reality in a mathematical form, just sayin'.

 

[Lynch101]

No. First, it's not clear what does "it" refer to. Second, beable is not an operator, so it's not clear how a non-operator may "give" an operator (observable).

I'm thinking the observable [in a Stern-Gerlach experiment] is the flash on the photo plate

Would a beable be whatever "it" is, that interacts with the photo plate, prior to that interaction.

To try and clarify further: if we consider a Stern-Gerlach set-up without the photo plates, so there is no flash on the photo plate, there is no observation but there would still be beables.

Am I in the right ball park?

 

[martinbn]

Here you mean "exist" in the ontological sense, right? Can this ontological existence of electron be described mathematically?

The point I was making was that one should be clear when one talks about ontology/exist. Some people, this includes you, are not clear at all to the point that it seems intentional.

 

[CoolMint]

So if we say something about reality in a mathematical form, then we have better chances to produce concrete results, would you agree? Well, Bohmian formulation of QM expresses possible reality in a mathematical form, just sayin'.

We can certainly describe this underlying reality even better. The multidimensional Hilbert space with its degrees of freedom and other similar mathematical tools are probing deeper than any other physical(material) tool. Maybe we can build a model of what underlies everything. Of all the observations. For now, it seems the mathematical model of Hilbert space is what exists between measurements. As an accurate model at least(not as a physical object).

Quite intriguing topic, btw.

 

[Morbert]

I'm thinking the observable [in a Stern-Gerlach experiment] is the flash on the photo plate

Would a beable be whatever "it" is, that interacts with the photo plate, prior to that interaction.

To try and clarify further: if we consider a Stern-Gerlach set-up without the photo plates, so there is no flash on the photo plate, there is no observation but there would still be beables.

Am I in the right ball park?

The observable in the SG experiment is the spin of the particle, not the flash on the photo plate. More generally, an observable of a system is constructed from a spectral decomposition of the state space of the measured system, not the state space of the system performing the measurement. The flash of the photo plate would be better described as a pointer.

 

[Demystifier]

I'm thinking the observable [in a Stern-Gerlach experiment] is the flash on the photo plate

Would a beable be whatever "it" is, that interacts with the photo plate, prior to that interaction.

To try and clarify further: if we consider a Stern-Gerlach set-up without the photo plates, so there is no flash on the photo plate, there is no observation but there would still be beables.

Am I in the right ball park?

No, that's not "observable". The thing you have in mind would better be called perceptible, a notion I introduced in the paper linked in my signature.

 

[Lynch101]

The observable in the SG experiment is the spin of the particle, not the flash on the photo plate. More generally, an observable of a system is constructed from a spectral decomposition of the state space of the measured system, not the state space of the system performing the measurement. The flash of the photo plate would be better described as a pointer.

Thanks for that clarification.

 

[Lynch101]

No, that's not "observable". The thing you have in mind would better be called perceptible, a notion I introduced in the paper linked in my signature.

Thanks. I read your paper a long while ago, I'll give it another read

 

[physika]

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.

...but never square

 

[vanhees71]

You are trying to define beable by using only the language of standard QM, which is impossible.

Then it's useless. If I want to provide a language/interpretation for standard QM, it must be within standard QM.

 

[Demystifier]

Then it's useless. If I want to provide a language/interpretation for standard QM, it must be within standard QM.

Yes, but the thesis behind the notion of beable is that the standard QM is conceptually incomplete, so new concepts are needed to develop non-standard QM.

One of the reasons it's conceptually incomplete is the fact that it cannot explain emergence of macroscopic laws from the microscopic ones, because standard QM already contains a macroscopic law as a part of its unexplained fundamental axioms. By that I mean the Born rule, which in standard QM is valid only in the measurement context, which is a macroscopic notion.

 

[physika]

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.

Again, it cannot be related to standard formulation of quantum theory, but it is related to a non-standard formulation.

Objective collapse models, for example.

 

[CoolMint]

What "underlying reality" doesn’t “exist” between measurements? At least, one should denote the character of the “underlying reality” one has in mind.

I said that reality may or may not exist between measurements. A logical deduction would require that any under-lying reality(if it exists) exists even between measurements.

The character of the underlying reality the substrate of all being given what we know about this reality could be many things, incl my own brain.
Who says that you experience reality as it is? I can only speak about the reality from within the experience. It could be that one day an integrated discipline between fundamental physics and neurology will give us a picture of nature without the bias of the(useful) mental constructs.
You will likely say that I am my brain and this is so. Until you pass out. Then you are not in the reality, but your brain is. This separation is hard to fathom and takes time.

*Useful for survival and not much else

PS. I am studying neuroscience with the hope to get some additional details about these questions

 

[PeterDonis]

I am studying neuroscience with the hope to get some additional details about these questions

Please note that this forum is about quantum interpretations, not neuroscience.

 

[Demystifier]

Please note that this forum is about quantum interpretations, not neuroscience.

I think he suggested that neuroscience can be relevant to quantum interpretations. It's not even a weird position, given that many interpretations claim that subjective observations play a fundamental role in quantum interpretations.

 

[vanhees71]

Yes, but the thesis behind the notion of beable is that the standard QM is conceptually incomplete, so new concepts are needed to develop non-standard QM.

That's what I always guessed, i.e., that Bell was thinking that Einstein was right and that QT is incomplete. Also Clauser, as I read in one of the many newspaper features about the new Nobel laureate, was hoping to disprove QT. The more convincing are all these experiments: I.e., it disproved the expectation of some of the protagonists (both theoretical and experimental). So, indeed, the buzz-word "beable", which still doesn't seem to be well-defined anyway, is just a buzz-word, and nothing else. For me the trick with Bell's papers on the foundations of quantum theory is to ignore most of the words between the formulae and fill in the words by yourself. Together with all the experiments deciding in favor for QT and against "local realism" I get more and more convinced about the formalism with the minimal interpretation: There are neither hidden variables nor determinism concerning the values of observables. All there is to specify about a concrete physical system in the lab is the quantum state (represented by the statistical operator with the meaning to describe the result of a specific preparation procedure), and the meaning are probabilities for the outcome of specifically given measurement procedures, i.e., quantitative observations on the system with a given measurement device.

Together with locality (i.e., microcausality) of relativistic QFT this implies that indeed in entangled states the observables of specific parts of a system, that can be observed at arbitrarily far-distant places with space-like separated measurement events and even space-like separted choices of measurement protocols, do not take predetermined values, i.e., the outcome of their meausurement is probabilistic, and the statistical properties are given by the quantum state the measured system has been prepared in, and this implies that despite the indeteriminism of the observable there are the very strong correlations between observables, as described by entangled quantum states. These correlations are there due to the preparation in the entangled state, and (within local relativistic QFTs) no faster-than-light spooky action at a distance due to the choice of a measurement setup and observations on the values of the measured observables is needed to explain these correlations.

One of the reasons it's conceptually incomplete is the fact that it cannot explain emergence of macroscopic laws from the microscopic ones, because standard QM already contains a macroscopic law as a part of its unexplained fundamental axioms. By that I mean the Born rule, which in standard QM is valid only in the measurement context, which is a macroscopic notion.

It can. Standard quantum-many body theory explains the "emergence of a classical world" via "coarse graining", defining the "relevant macroscopic observables".

 

[Demystifier]

Standard quantum-many body theory explains the "emergence of a classical world" via "coarse graining", defining the "relevant macroscopic observables".

I didn't say that macroscopic laws cannot be explained from standard quantum laws. They can. I said that macroscopic laws cannot be explained from purely miscroscopic laws. The standard quantum laws are not purely miscroscopic, since the standard Born rule is not purely microscopic.

See also https://www.physicsforums.com/threads/is-quantum-theory-a-microscopic-theory.974961/

 

[vanhees71]

The only theory that's really both microscopic as macroscopic is QT, and Born's rule applies to both microscopic and macroscopic systems.

 

[physika]

The editors of high impact journal Nature Physics explain why the field of quantum foundations is important for physics.
https://www.nature.com/articles/s41567-022-01766-x

in a similar vein

https://physicsworld.com/a/thirty-years-of-against-measurement/...

 

[Demystifier]

The only theory that's really both microscopic as macroscopic is QT, and Born's rule applies to both microscopic and macroscopic systems.

The Born's rule applies only in the measurement context, which requires a measuring apparatus. How can measuring apparatus be microscopic?

 

[vanhees71]

Quantum theory is about the statistics of the outcome of measurements. What else should it be about? I didn't claim that a measuring apparatus is microscopic. Nevertheless QT describes both microscopic objects, which can be measured and macroscopic objects, including measurement apparati. There's no "Heisenberg cut", and the same quantum-physical laws work for measurement devices as for any other (macroscopic) piece of matter.

 

[WernerQH]

Together with locality (i.e., microcausality) of relativistic QFT this implies that indeed in entangled states the observables of specific parts of a system, that can be observed at arbitrarily far-distant places with space-like separated measurement events and even space-like separted choices of measurement protocols, do not take predetermined values [...]

I find that a rather peculiar notion of locality. I wouldn't call a theory local if it deals with such strange systems, having parts "at arbitrarily far-distant places", as one system. Yes, "local realism" has been ruled out, but I'd prefer to throw out locality rather than realism. You are still far away from a truly minimal interpretation: QFT is a machinery for calculating correlation functions (of events). I think talk about "entangled particles" just carries too much metaphysical baggage; we should be content to have a theory that let's us calculate the correlations between events. (It is not necessary to call them "state preparation" and "measurement".)

 

[vanhees71]

You may prefer what you wish. The most successful theory, the Standard Model of elementary particle physics, by construction is local, i.e., it excludes the possibility for causal connections between space-like separated events (through the so-called micro-causality constraint imposed on all local observables). The metaphysical baggage is stripped of, when you simply take the probabilistic meaning of the quantum state as well as the microcausality principle seriously.

How else you want to observe the violation of Bell's inequality and thus demonstrate the corresponding correlations than by careful "state preparation" and "measurements" on the so "prepared systems", I don't know.

 

[martinbn]

I find that a rather peculiar notion of locality. I wouldn't call a theory local if it deals with such strange systems, having parts "at arbitrarily far-distant places", as one system. ...

Then any field theory, classical or not, in your view wouldn't be local.

 

[gentzen]

The only theory that's really both microscopic as macroscopic is QT, and Born's rule applies to both microscopic and macroscopic systems.

Do you have a reference for this?

All there is to specify about a concrete physical system in the lab is the quantum state (represented by the statistical operator with the meaning to describe the result of a specific preparation procedure), and the meaning are probabilities for the outcome of specifically given measurement procedures, i.e., quantitative observations on the system with a given measurement device.

Together with locality (i.e., microcausality) of relativistic QFT this implies that ... These correlations are there due to the preparation in the entangled state, and (within local relativistic QFTs) no faster-than-light spooky action at a distance due to the choice of a measurement setup and observations on the values of the measured observables is needed to explain these correlations.

Again here, the problem for me is not so much that what you write is wrong, but that you basically don't care whether there are references backing up your point, or how your interpretation fits with existing interpretations.

I know that you claim that you are just following Ballentine's minimal statistical interpretation, but I don't buy this. You claim more than he does. Those claims themselves are probably defendable, but not the claim that they coincide with Ballentine's position.

 

[vanhees71]

Do you have a reference for this?

Any textbook on quantum many-body theory will do. It uses Born's rule (together with all the other mathematical formalism of Q(F)T) all the time to describe macroscopic systems.

Again here, the problem for me is not so much that what you write is wrong, but that you basically don't care whether there are references backing up your point, or how your interpretation fits with existing interpretations.

My interpretation is simply the minimal statistical interpretation. So it's far from being "my interpretation" to beging with.

I know that you claim that you are just following Ballentine's minimal statistical interpretation, but I don't buy this. You claim more than he does. Those claims themselves are probably defendable, but not the claim that they coincide with Ballentine's position.

What more do I claim? Of course, I argue with relativistic local/microcausal QFT when it comes to the claim of "spooky actions at a distance", and Ballentine is pretty sparse on the interpretation of relativistic QFT in his textbook as well as the RMP article introducing the minimal statistical interpretation, but for me it's pretty obvious that one has to combine the microcausality principle with the minimal interpretation to get a consistent picture about the puzzling features of entangled states in connection with the question, whether there are faster-than-light causal signal propagations necessary to explain the observations, and the pretty obvious result is that this is not the case. I don't think that this is a very original point of view of mine.

 

[gentzen]

Any textbook on quantum many-body theory will do.

No, it will not. This is exactly my point that you claim that your position would be generally accepted, even so it is not and would need to be defended.

My interpretation is simply the minimal statistical interpretation. So it's far from being "my interpretation" to beging with

Again, it is precisely this claim (now made explicit) that I object to.

What more do I claim?

For example:

I didn't change my mind. The state represents a "preparation procedure" for a single system, implying probabilities for the outcomes of measurements via Born's rule, and as such the state refers to an ensemble of equally prepared systems since there's no other way to test the probabilistic predictions than by making repeated measurements on such equally prepared ensembles.

 

[WernerQH]

How else you want to observe the violation of Bell's inequality and thus demonstrate the corresponding correlations than by careful "state preparation" and "measurements" on the so "prepared systems", I don't know.

The scope of quantum theory is much broader than experiments in the laboratory. But please continue to phrase it in funny (anthropocentric) language, if you think there is no better way. :-)

 

[WernerQH]

Then any field theory, classical or not, in your view wouldn't be local.

Lagrangians that depend only on fields and their derivatives enjoy special status and carry the special epithet "local". I don't think of them as fundamental, but rather approximate, Ginzburg-Landau type, or effective theories. Locality is a useful guide for constructing theoretical models, but I doubt it will have significance in the "final theory" of everything.

 

[martinbn]

Then any field theory, classical or not, in your view wouldn't be local.

Just to point out, that I find this strange. The whole idea of fields came about because there are not actions at a distance, but through the medium of a field. To call all field theories non-local seems backwards or at the very least confusing because of already existing terminology.

Why is this strong need for some people to use the word non-local!?

 

[martinbn]

Lagrangians that depend only on fields and their derivatives enjoy special status and carry the special epithet "local". I don't think of them as fundamental, but rather approximate, Ginzburg-Landau type, or effective theories. Locality is a useful guide for constructing theoretical models, but I doubt it will have significance in the "final theory" of everything.

What are examples of non-local fields?

 

[vanhees71]

No, it will not. This is exactly my point that you claim that your position would be generally accepted, even so it is not and would need to be defended.

What of my position contradicts what can be found in any quantum many-body textbook?

Again, it is precisely this claim (now made explicit) that I object to.

What do you think contradicts the standard minimal interpretation?