I Nature Physics on quantum foundations

[vanhees71]

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.

This we can discuss as soon as we have this "final theory" ;-)).

 

[WernerQH]

The whole idea of fields came about because there are not actions at a distance, but through the medium of a field.

Yes, it is firmly based on our macroscopic intuitions. We tend to think of a solid as continuous, rather than a cloud of atoms.

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

I don't think of locality as important. It was @vanhees71's insistence on calling QFT "local" ("by construction"!) that irked me. I think it's misleading.

What are examples of non-local fields?

Think of magnetization in a solid. It is not the continuous field that you use in calculations, but rather a statistical characterization of isolated spins. The term "field" is just a convenient idealization.

 

[vanhees71]

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!?

The point indeed is that fields make interactions local. Who claimed otherwise?

 

[vanhees71]

Yes, it is firmly based on our macroscopic intuitions. We tend to think of a solid as continuous, rather than a cloud of atoms.

I don't think of locality as important. It was @vanhees71's insistence on calling QFT "local" ("by construction"!) that irked me. I think it's misleading.

For me to the contrary, it's misleading to claim that QFT is non-local. The entire standard model by construction is local.

Think of magnetization in a solid. It is not the continuous field that you use in calculations, but rather a statistical characterization of isolated spins. The term "field" is just a convenient idealization.

These are simplifying models. On a fundamental level all many-body physics can be described by local QFTs.

 

[martinbn]

The point indeed is that fields make interactions local. Who claimed otherwise?

Well, @WernerQH said, at least that is how I understood it, that any system that is extended in space has to be called non-local. And my comment was that this would mean that all fields are non-local.

 

[martinbn]

Yes, it is firmly based on our macroscopic intuitions. We tend to think of a solid as continuous, rather than a cloud of atoms.

I don't think of locality as important. It was @vanhees71's insistence on calling QFT "local" ("by construction"!) that irked me. I think it's misleading.

No, I didn't mean you. I had in mind every person who likes Bohmian mechanics. They very strongly insists on the use of the term non-local when it comes to QM.

Think of magnetization in a solid. It is not the continuous field that you use in calculations, but rather a statistical characterization of isolated spins. The term "field" is just a convenient idealization.

I don't understand this example. What is the field here, and why is it non-local?

 

[Demystifier]

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.

With such a way of thinking it is absolutely impossible to understand what's the purpose of concepts such as "beable" or "ontology". If you really want to understand what those words are supposed to mean, you must look at physics from an entirely different angle, an angle that seems to be totally foreign to you.

 

[vanhees71]

Of course, non-relativistic QM is non-local as is non-relativistic classical physics, because it works with "actions at a distance" as a paradigm. It's, of course, not by chance, that the discovery of local field theory for electromagnetism by Faraday and Maxwell, lead to relativity and the locality principle. The causal structure of relativistic spacetime makes it obviously hard to construct consistent non-local theories since there are none ;-).

 

[vanhees71]

With such a way of thinking it is absolutely impossible to understand what's the purpose of concepts such as "beable" or "ontology". If you really want to understand what those words are supposed to mean, you must look at physics from an entirely different angle, an angle that seems to be totally foreign to you.

Exactly, that's why I don't understand the purpose of introducing new words or insist on ontology.

 

[WernerQH]

I had in mind every person who likes Bohmian mechanics. They very strongly insists on the use of the term non-local when it comes to QM.

Of course, they have to. I can't make sense of Bohmian mechanics, and I especially dislike the features of the quantum potential.

I don't understand this example. What is the field here, and why is it non-local?

Have you looked at applications of QFT or statistical field theory in condensed matter physics? There you can have additional fields popping up when a phase transition occurs, and these fields are not fundamental, but merely a statistical characterization of an underlying discontinuous substrate.

 

[martinbn]

Of course, they have to. I can't make sense of Bohmian mechanics, and I especially dislike the features of the quantum potential.

:) Why do they have to? It only brings confusion.

Have you looked at applications of QFT or statistical field theory in condensed matter physics? There you can have additional fields popping up when a phase transition occurs, and these fields are not fundamental, but merely a statistical characterization of an underlying discontinuous substrate.

No, I haven't seen any of that. But these fields, it seems to me, do not correspond to any system, do they?

 

[Demystifier]

Exactly, that's why I don't understand the purpose of introducing new words or insist on ontology.

It's OK to not understand something and hence to ignore it. But it's not OK then to tell others that they should ignore it too. (For example I don't understand what's so great about Shakespeare, but I don't tell others they should ignore Shakespeare.)

 

[Lord Jestocost]

Regarding debates about quantum theory, a lot of disagreements might lie in a difference of antecedent beliefs, which are not directly addressed in the debate. This sort of disagreements might be partially resolved to my mind if it would be clear whether arguments “for or against” are ontically motivated, viz. emphasizing a viewpoint independent of observers or measurements, or whether arguments “for or against” are epistemically motivated, viz. focusing on what we could know and infer from observed phenomena.

 

[Demystifier]

(For example I don't understand what's so great about Shakespeare, but I don't tell others they should ignore Shakespeare.)

A fun off-topic. Shakespeare is #6 at the list of most academically influential people of the world: https://academicinfluence.com/people/william-shakespeare-1

The same algorithm gives you that #1 is Aristotle, that Einstein is #8 and that I am #1,530,540. You can also check out where are you on the list.

 

[PeterDonis]

I think he suggested that neuroscience can be relevant to quantum interpretations.

He said he was studying neuroscience in the hope of getting more information about the things he mentioned in the post I quoted from, which, at best, might be somewhat related to some QM interpretations. But that in itself is not enough to make neuroscience on topic in this forum. One would have to base discussion in this forum on something in the QM interpretations literature that actually tried to demonstrate how something in neuroscience was relevant.

 

[PeterDonis]

The observable in the SG experiment is the spin of the particle, not the flash on the photo plate

While this is what the simplest mathematical treatment of the SG experiment does, there are complications that this treatment sweeps under the rug.

What does an SG magnet actually do? What it does not do is "observe" the spin of the particle. The operation of the SG magnet, by itself, is a unitary, reversible operation and does not amount to a measurement of anything (to put it another way, it does not cause decoherence). All the SG magnet actually does is to entangle the spin and momentum degrees of freedom of the particles passing through it, so that one output beam corresponds to "up" spin and the other output beam corresponds to "down" spin.

That means that in order to make a measurement of "spin" using an SG magnet, one needs to add a detector that tells which output beam each particle came out in. The photo plate is the usual detector that is used. But that means that, in the actual experiment as it is actually run, the actual measurement--the thing that causes decoherence and makes the process irreversible--is the flash on the photo plate. That flash directly measures which output beam the particle came out in--i.e., its momentum. The fact that the SG magnet entangled momentum and spin then let's us deduce the spin from the measured momentum. Or, to put it another way, since momentum and spin are entangled, decohering the momentum degree of freedom also decoheres the spin degree of freedom and thereby amounts to a "measurement" of spin, but an indirect one.

 

[Morbert]

What does an SG magnet actually do? What it does not do is "observe" the spin of the particle. The operation of the SG magnet, by itself, is a unitary, reversible operation and does not amount to a measurement of anything (to put it another way, it does not cause decoherence). All the SG magnet actually does is to entangle the spin and momentum degrees of freedom of the particles passing through it, so that one output beam corresponds to "up" spin and the other output beam corresponds to "down" spin.

That means that in order to make a measurement of "spin" using an SG magnet, one needs to add a detector that tells which output beam each particle came out in. The photo plate is the usual detector that is used. But that means that, in the actual experiment as it is actually run, the actual measurement--the thing that causes decoherence and makes the process irreversible--is the flash on the photo plate. That flash directly measures which output beam the particle came out in--i.e., its momentum. The fact that the SG magnet entangled momentum and spin then let's us deduce the spin from the measured momentum. Or, to put it another way, since momentum and spin are entangled, decohering the momentum degree of freedom also decoheres the spin degree of freedom and thereby amounts to a "measurement" of spin, but an indirect one.

As I noted in my post in the previous thread in response to yours, the flash on the photo plate is an observable of the system being measured (the particle): it is a measurement of its momentum (by measuring which output beam of the SG magnet the particle came out in). The function of the SG magnet is to entangle the particle's momentum with its spin so that its measured momentum can be used to deduce its spin.

i) I understand the flash on the detector plate to be a classical, irreversible datum expressing the measurement outcome, such that even if we interpret the SGE as a direct measurement of the particle's momentum rather than spin, the flash is still not an observable of the particle. It is the classical record, expressed by the macroscopic apparatus, of the outcome of a measurement of an observable of the system being measured.

ii) I don't think this constitutes an indirect measurement as they are normally presented in literature (even though I understand the way in which you are using the term here). Detectors placed behind two slits in a double-slit experiment do not immediately respond to the particle passing through a slit, but instead to the particle's position after it has already passed through the slits. This position is correlated with which slit the particle has passed through, and so we say the detectors measure which slit the particle went through. I normally see "indirect measurement" reserved for data produced with non-interaction between the measured system and the measurement apparatus

 

[PeterDonis]

I understand the flash on the detector plate to be a classical, irreversible datum expressing the measurement outcome, such that even if we interpret the SGE as a direct measurement of the particle's momentum rather than spin, the flash is still not an observable of the particle.

If you insist, yes, you can draw a distinction between the momentum observable of the particle, and the flash as a "pointer reading" corresponding to a measurement of that observable. That doesn't change the substance of what I am saying.

I don't think this constitutes an indirect measurement as they are normally presented in literature

"Indirect" might not be the precisely correct technical term. My point is that the observable actually measured by the flash on the detector (however you want to interpret what is happening there--see above) is the particle's momentum, not its spin; any information gained about spin is gained by deduction, not direct measurement.

 

[Demystifier]

My point is that the observable actually measured by the flash on the detector (however you want to interpret what is happening there--see above) is the particle's momentum, not its spin

The momentum's direction cannot be inferred from position of the flash, if the two branches of the wave function are made parallel after their split by the magnet (for visualization, think of fork). What the position of the flash is really correlated with is the position of the particle at the time of detection, not its momentum. (And no, this fact has nothing to do with Bohmian interpretation.)

 

[gentzen]

I had in mind every person who likes Bohmian mechanics. They very strongly insists on the use of the term non-local when it comes to QM.

Of course, they have to. I can't make sense of Bohmian mechanics, and I especially dislike the features of the quantum potential.

To "dislike the features of the quantum potential" is actually a good idea. The interesting question is to explain why the version of Bohmian mechanics using the quantum potential is "bad". I didn't know about Antony Valentini (and his approach to BM) before

There are actually two schools of thought on that, the Valentini et al school that it rapidly approaches the equilibrium, and the Durr et al school that it is always in equilibrium. For a review see https://www.mdpi.com/1099-4300/20/6/422

After reading more of Valentini and his positions, my preliminary conclusion was that many of his positions feel misguided to me. But his approach does explain why using the quantum potential is "bad". Since even Bohm himself failed to notice this, I am a bit suspicious whether "my" Durr et al school successfully notices that the quantum potential is "bad" (in the sense that some crucial argument would fail to hold for the quantum potential version of BM).

 

[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?

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.

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

I didn't claim that your position would "contradict" information found in standard textbooks. What I said was that you claim your position would be generally accepted, even so they are not. Replying "Any textbook on quantum many-body theory will do." is exactly such a "it is generally accepted" claim.

For fun, I now checked whether
Wolfgang Nolting, "Grundkurs Theoretische Physik 7: Viel-Teilchen-Theorie"
has anything to say with respect to "The only theory that's really both microscopic as macroscopic is QT, and Born's rule applies to both microscopic and macroscopic systems." Of course, as expected, nothing at all was said about that topic.

What do you think contradicts the standard minimal interpretation?

Again here, the same issue. I didn't claim that your position would "contradict" the standard minimal interpretation. You make stronger statements than the minimal interpretation, and then claim that those would coincide with what is claimed by the minimal interpretation.

 

[vanhees71]

Which stronger claims do I make? You always claim, I'd made some claims beyond the "standard interpretation" but never concretely give an example.

The only additional piece in my argument is to take into account relativistic local QFT, which is not treated in detail in Ballentine's book, which adds the "locality by construction" to the discussion of correlations due to entanglement and the impossibility of "spooky actions at a distance" in Bell-test experiments. I think, also this is rather standard and not very controversial. For textbooks emphasizing the microcausality constraint, see Weinberg, QT of Fields and Duncan, The conceptual framework of QFT.

Nolting's book is pretty standard too, though very sloppy in the definition of "state" (vectors rather than stat. ops./rays), not clearly distinguishing "measurements" and "preparations". In addition, as most textbooks, he introduces the projection/collapse postulate, which goes beyond the minimal interpretation.

 

[Demystifier]

I am a bit suspicious whether "my" Durr et al school successfully notices that the quantum potential is "bad" (in the sense that some crucial argument would fail to hold for the quantum potential version of BM).

The Durr et al school also accepts that quantum potential is "bad". The following excerpt from the book "Bohmian Mechanics" by Durr and Teufel illustrates that very well.

 

[Demystifier]

impossibility of "spooky actions at a distance" in Bell-test experiments. I think, also this is rather standard and not very controversial.

If impossibility of "spooky actions at a distance" in Bell-test experiments is not controversial, then I don't know what is.

 

[martinbn]

If impossibility of "spooky actions at a distance" in Bell-test experiments is not controversial, then I don't know what is.

@vanhees71 means that "there are no signals that propagate from one to the other faster than light" is not contraversial.

 

[gentzen]

Which stronger claims do I make? You always claim, I'd made some claims beyond the "standard interpretation" but never concretely give an example.

I did provide concrete quotes. It is simply a bit of work to repeat those quotes, that is why I only did this for the first part with respect to "Any textbook on quantum many-body theory will do."

So on your request, I now also did this work for the second part "You make stronger statements than the minimal interpretation, and then claim that those would coincide with what is claimed by the minimal interpretation."

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.

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.

 

[vanhees71]

Again, for me these statements are pretty standard ;-).

 

[Demystifier]

Which stronger claims do I make? You always claim, I'd made some claims beyond the "standard interpretation" but never concretely give an example.

Here are some claims beyond the standard interpretation that you often claim:
1) There is no collapse.
2) Entanglement correlations are caused by local interactions.
3) Wave function only specifies an ensemble of similarly prepared systems, it doesn't specify an individual system.

Notes:
- 1) and 3) are claimed by the Ballentine interpretation, but not by the standard interpretation.
- In addition to 3) you also often say that
3') Wave function completely specifies an individual system,
which of course contradicts 3).
- 2) is not claimed by any published interpretation that I am aware of.

 

[martinbn]

Here are some claims beyond the standard interpretation that you often claim:
1) There is no collapse.
2) Entanglement correlations are caused by local interactions.
3) Wave function only specifies an ensemble of similarly prepared systems, it doesn't specify an individual system.

Notes:
- 1) and 3) are claimed by the Ballentine interpretation, but not by the standard interpretation.
- In addition to 3) you also often say that
3') Wave function completely specifies an individual system,
which of course contradicts 3).
- 2) is not claimed by any published interpretation that I am aware of.

By standard, he means standard minimal statistical interpretation.

About 2), why is that controversial? I thought that is just QM and interpretation independent?

 

[vanhees71]

Here are some claims beyond the standard interpretation that you often claim:
1) There is no collapse.

For me the minimal interpretation is the standard interpretation, but indeed the collapse in my opinion cannot be a general postulate, because what happens to the measured system depends on the specific measurement device.

2) Entanglement correlations are caused by local interactions.
3) Wave function only specifies an ensemble of similarly prepared systems, it doesn't specify an individual system.

Notes:
- 1) and 3) are claimed by the Ballentine interpretation, but not by the standard interpretation.
- In addition to 3) you also often say that
3') Wave function completely specifies an individual system,
which of course contradicts 3).
- 2) is not claimed by any published interpretation that I am aware of.

How can 3') contradict 3), when we are obviously preparing single systems to form the ensemble?

2) follows from local/microcausal QFT.

 

[Demystifier]

About 2), why is that controversial? I thought that is just QM and interpretation independent?

It is standard that entanglement is caused by local interactions, but it's not standard that resulting correlations are caused by local interactions. Entanglement is a property of the abstract state in the HiIlbert space, while correlation is a property of the concrete measurement outcomes. Creation of entanglement is clearly deterministic, while creation of outcomes is seemingly random. Some interpretations say that correlations are caused by non-local action at a distance, some that they are not caused at all, etc.

 

[vanhees71]

But entanglement implies the corresponding correlations. So if entanglement is caused by local interactions, then also the correlations, described by it, are caused by local interactions.

If one accepts standard micocausal relativistic QFT, there cannot be non-local action at a distance.

 

[martinbn]

It is standard that entanglement is caused by local interactions, but it's not standard that resulting correlations are caused by local interactions. Entanglement is a property of the abstract state in the HiIlbert space, while correlation is a property of the concrete measurement outcomes. Creation of entanglement is clearly deterministic, while creation of outcomes is seemingly random. Some interpretations say that correlations are caused by non-local action at a distance, some that they are not caused at all, etc.

How can there be a non-local action at a distance and no faster than light signaling at the same time?

 

[martinbn]

How can 3') contradict 3), when we are obviously preparing single systems to form the ensemble?

Because the state describes the ensemble not the single system.

 

[Demystifier]

For me the minimal interpretation is the standard interpretation,

Something called "standard" should be universal, not "for you".

How can 3') contradict 3), when we are obviously preparing single systems to form the ensemble?

If you don't accept Ballentine's argument that 3') contradict 3), there is no hope that you would accept mine.

2) follows from local/microcausal QFT.

Except that there is no reference claiming it, making the claim 2) nonstandard.

 

[vanhees71]

Then you imply that one member of the ensemble influences any other, but that can be excluded in modern experiments (e.g., only single diphotons in Bell-test experiments, building an ensemble of really independent single-system realizations).

 

[martinbn]

Then you imply that one member of the ensemble influences any other, ...

How is that implied?

 

[vanhees71]

Because then you say that the quantum state cannot be determined by preparing a single system, i.e., that the ensemble is defined by the probabilistic properties of a single-system preparation procedure. For me this contradicts the findings of many Bell-test experiments, where it is ensured that always single systems are measured.

 

[Demystifier]

How can there be a non-local action at a distance and no faster than light signaling at the same time?

Signal is transmitted by controlled action, while action in general does not need to be controlled. For example, non-local action at a distance may be transmitted by some hidden variables, and variables which are hidden obviously cannot be controlled. Another example is random collapse, which involves non-local action at a distance but cannot be controlled because it's random.

 

[martinbn]

Because then you say that the quantum state cannot be determined by preparing a single system, i.e., that the ensemble is defined by the probabilistic properties of a single-system preparation procedure. For me this contradicts the findings of many Bell-test experiments, where it is ensured that always single systems are measured.

It can but only after you know what state the procedure produces. How do you know that by just one preperation?

 

[Demystifier]

For me this contradicts the findings of many Bell-test experiments

"For me" is the key. It's your view, not standard view.

 

[martinbn]

Signal is transmitted by controlled action, while action in general does not need to be controlled. For example, non-local action at a distance may be transmitted by some hidden variables, and variables which are hidden obviously cannot be controlled. Another example is random collapse, which involves non-local action at a distance but cannot be controlled because it's random.

Ok, suppose we have the standard Alice and Bob scenario. We do it 1000 times, Alice makes a measurement on either the first 500 or on the last 500, the other 500 she does nothing. Bob's task is by measuring his particle to determine when Alice measured, in the first half of the trials or in the second. Can he do it?

 

[vanhees71]

It can but only after you know what state the procedure produces. How do you know that by just one preperation?

Of course, I can't know this, but I have to prepare ensembles and do statistics with them. To completely determine the state you need several different measurements on the ensembles. The ensembles are defined by prepratation procedures of single systems. That's why it is so important to clearly distinguish "states" and "measurements". A state is a preparation procedure (for single systems), allowing to form well-defined ensembles. The outcomes of measurements are random and the statistics of these outcomes is given by the corresponding statistical operator according to Born's rule.

 

[Demystifier]

Ok, suppose we have the standard Alice and Bob scenario. We do it 1000 times, Alice makes a measurement on either the first 500 or on the last 500, the other 500 she does nothing. Bob's task is by measuring his particle to determine when Alice measured, in the first half of the trials or in the second. Can he do it?

He can't do it. This demonstrates that Alice by performing measurement cannot send a signal to Bob. But it does not imply that measurement by Alice doesn't have any influence on Bob's results.

 

[vanhees71]

Ok, suppose we have the standard Alice and Bob scenario. We do it 1000 times, Alice makes a measurement on either the first 500 or on the last 500, the other 500 she does nothing. Bob's task is by measuring his particle to determine when Alice measured, in the first half of the trials or in the second. Can he do it?

No, all Bob will see is a stream of random results with statistics determined by the prepared state. The correlations due to entanglement can only be verified by exchanging information about A's and B's measurement results on each individual entangled system.

 

[vanhees71]

He can't do it. This demonstrates that Alice by performing measurement cannot send a signal to Bob. But it does not imply that measurement by Alice doesn't have any influence on Bob's results.

If the measurement events are space-like separated within relativistic microcausal QFT there cannot be any influence of one of the measurements on the other.

 

[Demystifier]

If the measurement events are space-like separated within relativistic microcausal QFT there cannot be any influence of one of the measurements on the other.

Yes, but this does not contradict my claim. Standard relativistic microcausal QFT is one possible model of reality consistent with existing experiments, but it's not the only possible model of reality consistent with existing experiments.

 

[martinbn]

He can't do it. This demonstrates that Alice by performing measurement cannot send a signal to Bob. But it does not imply that measurement by Alice doesn't have any influence on Bob's results.

What kind of influence can there be if you cannot find any!

(By the same logic you could claim that there is a little sprite that looks at Alice's results and changes Bob's accordingly.)

 

[martinbn]

Of course, I can't know this, but I have to prepare ensembles and do statistics with them. To completely determine the state you need several different measurements on the ensembles. The ensembles are defined by prepratation procedures of single systems. That's why it is so important to clearly distinguish "states" and "measurements". A state is a preparation procedure (for single systems), allowing to form well-defined ensembles. The outcomes of measurements are random and the statistics of these outcomes is given by the corresponding statistical operator according to Born's rule.

Yes, and that is why there is a difference between an ensemble and a single system representing the ensemble. The state describes the ensemble.

 

[Demystifier]

But entanglement implies the corresponding correlations.

No, entanglement + Born rule implies the corresponding correlations. The origin of Born rule is controversial, hence the origin of correlations is controversial.