Quantum measurement of a Strontium ion

[vanhees71]

A measurement does not reveal a preexisting value except the state was such that the measured observable takes a predetermined value (for pure states, if it's represented by an eigenstate). An ideal measurement leads to a result with the probability given by the state the system is prepared in when measured.

So I think we agree. What you formulated in your final sentence is, by the way, Schrödinger's point of view, who was for my taste much more to the point than the Copenhagen gang (particularly Heisenberg), and he was the one who has (besides of course Einstein) seen the true "revolutionary" content (entanglement and inseparability) much more clearly than the "philosophers in Copenhagen" though Schrödinger himself was also much inclined to the philosophical side in his later years, but I think he separated it better from his physics than the Copenhagians.

 

[PeterDonis]

if you consider the meausurment device as external (which makes of course sense), then there's no unitary time evolution for the Ag atom alone

You missed this statement of mine:

Remember that in the Stern-Gerlach measurement, the magnet is not the detector. The detector is the screen that the Ag atoms hit after they have exited the magnetic field. You can treat the atom plus magnet system as closed and undergoing unitary time evolution.

Nobody is arguing that collapse (whether it's viewed as apparent or real) occurs when the Ag atom passes through the magnet. So there is no reason not to consider the magnet as part of the "system" along with the Ag atom, and analyzing the atom + magnet as undergoing unitary time evolution.

For a closed system, AFAIK there's no clear argument or experimental fact that indicates that the usual dynamical rules of QT were incomplete and some fundamental quantum-classical cut is necessesary.

Yes, but with the proper definition of "closed system", any quantum experiment stops being closed as soon as a result is observed (in the case of the SG measurement, this is when the Ag atom hits the detector and makes a bright spot). So the claim you are making here, while true, is much, much less broad than you seem to think it is.

 

[vanhees71]

I think we have just a semantical misunderstanding. Let me try in a different way to make clear what I mean.

On a fundamental level you have quantum theory as the comprehensive theory of physics, including unitary time evolution (of course I neglect gravity here, which is not yet described quantum-theoretically in a satisfactory way). On a fundamental level the SGE from the preparation of the Ag atoms (through letting them out of an oven as a beam) to the detection on the photoplate everything is in principle described by unitary time evolution.

This you can of course never describe in full analytical detail since it would include a huge macroscopic system (the oven with the silver vapor in it, the aperture(s) to shape the beam, the magnet, and finally the photoplate). It is also not necessary to describe all this setup in microscopic detail, and that's why you use effective macroscopic descriptions of the relevant macroscopic observables. In other words you "project on" the relevant information (in the sense of many-body theory), leading to a massive coarse graining. This of course leads to a discription of a huge part of the setup in terms of macroscopic observables and their classical behavior. This classical behavior is emergent in this sense but, at least from a phenomenologist's point of view, complete compatible with the underlying microscopic unitary dynamics. That's why I think there's neither a need for a fundamental quantum-classical cut nor is there any empirical evidence for its existence.

The idea of an instantaneous collapse is of a different caliber: It's contradicting the very foundations of the theory in its form as a relativistic microcausal QFT to begin with. At the same time the collapse proponents claim it's just an interpretational element of this very theory. So, while a "quantum-classical cut" is not principally ruled out based on the theory, the collapse hypothesis is a contradictio in adjecto. An as some of the very recent experimental investigations we discuss here, it seems as if it can now be experimentally demonstrated not to occur at all: There are no instantaneous quantum jumps but just quantum-theoretical time evolution, which can be very rapid on a macroscopic observational scale but are still continuous and smooth at a finer time resolution, and this can even be demonstrated experimentally, including the possibility for "preventing" a "just to appear" quantum jump in the process.

 

[PeterDonis]

On a fundamental level the SGE from the preparation of the Ag atoms (through letting them out of an oven as a beam) to the detection on the photoplate everything is in principle described by unitary time evolution.

With correct definitions of the two boundaries (the end of preparation and the start of detection), yes. But you immediately make those boundaries too broad:

This you can of course never describe in full analytical detail since it would include a huge macroscopic system (the oven with the silver vapor in it, the aperture(s) to shape the beam, the magnet, and finally the photoplate).

You are misstating this. The correct statement is that the oven and the photoplate are not described by unitary time evolution in our current quantum theory. (See further comments below.) That means we do not know whether unitary time evolution actually applies to the oven and the photoplate. The claim that it does is not a well-tested conclusion of quantum theory. It is an extrapolation of the theory to a domain in which nobody knows how to test it empirically, and in which the straightforward extrapolation, applying unitary evolution to everything, gives answers that seem obviously contrary to observation (i.e., the MWI). This is the reason why there are multiple interpretations of QM and the question of which, if any, of them are correct remains unresolved.

It is also not necessary to describe all this setup in microscopic detail

Not if you just use standard QM, because, as above, standard QM does not describe the oven and the photoplate using unitary time evolution. It just declares by fiat that the oven produces Ag atoms in a particular state, and that the photoplate gives probabilities for showing a bright spot in different places when an Ag atom hits it. Neither of those things are unitary time evolution.

This classical behavior is emergent in this sense but, at least from a phenomenologist's point of view, complete compatible with the underlying microscopic unitary dynamics.

Only if you accept the MWI, since the MWI is what you get when you apply unitary dynamics to everything.

The idea of an instantaneous collapse is of a different caliber: It's contradicting the very foundations of the theory in its form as a relativistic microcausal QFT to begin with.

The simplest version of the collapse interpretation certainly seems that way, yes. That's another reason why there are multiple interpretations of QM and the question of which, if any, of them are correct remains unresolved. Your personal preference is against collapse; that's fine. But it's still your personal preference: it's not an established theoretical conclusion.

as some of the very recent experimental investigations we discuss here, it seems as if it can now be experimentally demonstrated not to occur at all: There are no instantaneous quantum jumps but just quantum-theoretical time evolution

I've already responded to this: these experiments don't show that there is no collapse, because collapse interpretations don't put the collapse where the experimenters are saying they don't observe collapse. These experiments are basically equivalent to demonstrating that the Ag atom + magnet in the SG experiment undergo unitary evolution, and then claiming that shows there's no collapse--when in fact collapse interpretations put the collapse where the Ag atom hits the photoplate.

 

[Demystifier]

The correlations are there because of the preparation in the entangled state at the very beginning of the experiment

I have two problems with this statement.

First, it is not how Ballentine interprets it (nor any other peer reviewed paper AFAIK) , so it's not a part of the minimal statistical ensemble interpretation. It's your own interpretation.

Second, the Bell theorem explicitly rules out such an interpretation. Sure, the Bell theorem rests on some assumptions, so we would like to understand which of those assumptions do you deny. Here is how I see it. When one talks about preparation, Bell assumes that it does make sense to ask - preparation of what? Then Bell assumes that this thing which is prepared can be analysed mathematically and calls it $\lambda$. And from this (and from some additional assumptions that you wouldn't deny) he derives that those $\lambda$ must obey some nonlocal laws. So to avoid the Bell theorem, you deny that the question "Preparation of what?" makes sense to begin with. In other words, you avoid the Bell theorem by refusing to talk explicitly and mathematically about the object which is prepared. In your interpretation, preparation as an action makes sense, but the object which is prepared doesn't.

It's like using the following defense in the court: Yes your honor, I did kill, I don't deny it. But nobody found the murdered body, so it doesn't make sense to say that I killed someone. Therefore I commited no crime, for there is nothing wrong in the act of killing if that act is not applied to any concrete human.

 

[vanhees71]

If you prepare a system (say a two-photon Fock state via parametric down conversion) in an entangled state, it's in this entangled state, isn't it, and the correlations are thus there? What's there left to be interpreted? I'm also not aware, where this view contradicts any textbook formulation, let alone Ballentines, of quantum theory.

 

[Demystifier]

If you prepare a system (say a two-photon Fock state via parametric down conversion) in an entangled state, it's in this entangled state, isn't it,

Yes it is.

and the correlations are thus there?

No they aren't.

What's there left to be interpreted?

Correlations only make sense if one talks about objects which are correlated. Those objects are the things which are left to be interpreted.

Entanglement is in the state in the Hilbert space. Correlation is in the physical things described by this state.

I'm also not aware, where this view contradicts any textbook formulation, let alone Ballentines, of quantum theory.

Ballentine's book, page 607-608:
"Many assumptions, other than locality, that seem to be implicit in Bell’s original argument have been identified, but in every case it has been possible to deduce a contradiction of quantum mechanics without that assumption."

Peres's book, page 173:
"In summary, there is no escape from nonlocality."

 

[PeterDonis]

"In summary, there is no escape from nonlocality."

To be clear, "nonlocality" here means that Bell's "locality" assumption is violated; that assumption is that the function describing the joint probabilities for the two spacelike separated measurements factorizes, i.e., schematically, $P(A, B|a, b, \lambda) = P(A|a, \lambda) P(B|b, \lambda)$, i.e., the probability of a given result for each measurement only depends on the setting of that measuring device (and the hidden variables $\lambda$). The QM probability for an entangled state obviously violates this assumption since it depends on the angle between the two measuring devices (for the case of spin measurements, which is the case Bell treated in his original paper).

However, this definition of "locality" is not the same as the definition of "locality" in QFT. In QFT, "locality" means that spacelike separated measurements commute, i.e., the probabilities are independent of the order in which the measurements are done. The QM probability satisfies this property, so QM is "local" in the QFT sense even thought it is "nonlocal" in the Bell's theorem sense.

 

[vanhees71]

Why aren't the correlations there?

If you have a two-spin state like the singlet state $|1/2,-1/2 \rangle-|-1/2,1/2 \rangle$, then though neither of the single spins has a determined value (but it's described by the mixed state $\hat{\rho}_1=\hat{\rho}_2=\hat{1}/2$) you have a 100% correlation: Whenever you measure $\sigma_z=1/2$ on particle 1, you'll measure $\sigma_z=-1/2$ on particle 2 or vice versa.

As long as you ensure that nothing disturbes the state, the entanglement is preserved, and you can measure the single-particle spins at arbitrary far distance. It's done by local interactions with the measurement device (according to standard relativistic QFT, which has by construction only local interactions, because it obeys the microcausality principle, i.e., the Hamilton density commutes with any local observable at space-like distance of the arguments), but still you find the correlations. There's no spooky action at a distance, since the measurement events determining the outcome of the spin measurements can be spacelike separated. In such a case the microcausality condition rules out an causal effect of one of the single-particle measurements on the other.

So of course there's some kind of "non-locality" here, but it's not non-local interactions but correlations between parts of the entangled system measured at (possible very large) spatial distances. I'd rather use Einstein's word "inseparability" of entangled quantum systems than the somewhat ambiguous word "non-locality".

It's also clear that within the standard interpretation, what the observers measure on the single particles is simply that its unpolarized. As long as you don't polarize the particles before measuring them (and with that of course also destroy the before prepared entangled state) that finding is independent of whatever is measured at the other particle (again at least as long as the measurement events are at spacelike distances).

 

[Demystifier]

The QM probability satisfies this property, so QM is "local" in the QFT sense even thought it is "nonlocal" in the Bell's theorem sense.

I agree. But ontological theories (such as Bohmian mechanics) are also "local" in the QFT sense, so it doesn't make sense to criticize such theories for being incompatible with QFT locality.

 

[PeterDonis]

ontological theories (such as Bohmian mechanics) are also "local" in the QFT sense

Yes, "QM" here includes any interpretation of QM, since all of them make the same predictions, and therefore all of them satisfy the property that spacelike separated measurements commute for the case under discussion.

 

[Demystifier]

Why aren't the correlations there?

I already said, because correlations are in the observed objects, not in the Hilbert space.

If you have a two-spin state like the singlet state $|1/2,-1/2 \rangle-|-1/2,1/2 \rangle$, then ... you have a 100% correlation:

No, as long as you only talk about the state in the Hilbert space, you cannot talk about correlation at all.

Whenever you measure $\sigma_z=1/2$ on particle 1, you'll measure $\sigma_z=-1/2$ on particle 2 or vice versa.

Yes, "measure" is the key word. To paraphrase Peres, measurement doesn't happen in the Hilbert space, it happens in the laboratory.

As long as you ensure that nothing disturbes the state, the entanglement is preserved,

But measurement does disturb the state.

 

[vanhees71]

I already said, because correlations are in the observed objects, not in the Hilbert space.

Sure, but the quantum state is formulated in Hilbert space. It describes the probabilities for the outcome of measurements, and the correlations described by entangled states are measurable, and they are in the objects.

No, as long as you only talk about the state in the Hilbert space, you cannot talk about correlation at all.Yes, "measure" is the key word. To paraphrase Peres, measurement doesn't happen in the Hilbert space, it happens in the laboratory.

Sure, but were do I claim something else? The polarization of the single photons in a typical Bell experiment with photon pairs are measured in the lab, and the correlations are revealed by such measurements when comparing the outcome of these measurements for each photon pair (for which you need an adequate measurement protocol to enable this comparison for the outcome of local single-photon measurements belonging to each pair; usually that's done by sufficiently precise "time stamps" of the single-photon measurement events.

But measurement does disturb the state.

Of course, when doing a measurement on at least one of a single photon you usually destroy the entanglement and another measurement after that won't show the correlations described by the original entangled state but something different, depending on how you changed the state by the first measurement.

I don't see any contradiction between what you and Peres say with my very simple point of view. It just takes the quantum state with its solely probabilistic meaning seriously. Indeed the formalism is a formalism not the real world, but that's true for any physical theory. In Newtonian mechanics the Earth in its Orbit around the Sun is also not a triple of real numbers used to describe the Earth's position in terms of some coordinates.

 

[Demystifier]

It just takes the quantum state with its solely probabilistic meaning seriously. Indeed the formalism is a formalism not the real world, but that's true for any physical theory.

I agree that the formalism is not the real world. But at least the real world should be represented by a formalism. The problem is that the quantum state does not represent the real world (except in the collapse interpretations and many world interpretations). The quantum state represents the probability, but probability is not the real world. The real world is the thing which we observe in a single measurement, and probability does not represent a single measurement. A click in the detector is not represented by a number $p\in[0,1]$. So we need some additional formalism and some additional variables that represent a single measurement. Bell calls such additional variables $\lambda$ and shows that they must obey some nonlocal laws in the sense of action at a distance, even if those laws are probabilistic.

But you refuse to even think about an additional variable $\lambda$. So the problem is not that you take the probabilistic meaning of the quantum state seriously. The problem is that you refuse to take anything else seriously. The probability $p$ is a probability of some event mathematically represented by some variable $E$, so we really have $p(E)$. The Bell theorem asks: OK, if we have $p(E)$, then what does it tell us about $E$? The minimal interpretation, by contrast, talks about $p(E)$, but refuses to talk mathematically about $E$. The minimal interpretation talks about events informally, but refuses to talk about them mathematically. That's why the minimal interpretation is incomplete and that's why (your version of) the minimal interpretation cannot see the implications of the Bell theorem.

 

[vanhees71]

The quantum state represents the real world in the sense that it predicts probabilities for the outcome of measurements, given the preparation of the measured system. One should not forget the instrumental definition of the quantum state as a preparation procedure (or an equivalence class of preparation procedures) for ensembles enabling one to check the probabilistic predictions of the theory.

In general QT doesn't tell you much about the outcome of a single measurement, but only about the statistical properties of ensembles of equally (in a stochastic sense!) prepared systems. Whether or not this is a "complete" description of the world is a philosophical debate going basically on since Born's probability-interpretation footnote. I think from a physical point of view you can never prove whether any theory is "complete". As far as we know today, QT is complete in the sense that it describes all observations right it can describe. As it seems the room for any deterministic (then necessarily nonlocal!) theory seems to be very small given all the stringent Bell-test experiments we have for some decades now.

As long as there's no nonlocal deterministic theory which describes as much as QT, I don't consider it very interesting to think about it. For me it's enough to see that local deterministic theories in Bell's sense are ruled out, while Q(F)T, which deals with local interactions only and is causal within the relativistic spacetime description and at the same time describes the observed inseparability and long-distance correlations in the sense stated in my previous posting.

I don't believe in scholastic approaches. The strong belief even in "physics beyond the standard model" hasn't lead to success yet. I think we need clear hints of observations to see even the standard model fall yet. So I've not much hopes that one can come up with a possible successor of QT on an even more fundamental level by pure thought.

 

[pinball1970]

The quantum state represents the real world in the sense that it predicts probabilities for the outcome of measurements, given the preparation of the measured system. One should not forget the instrumental definition of the quantum state as a preparation procedure (or an equivalence class of preparation procedures) for ensembles enabling one to check the probabilistic predictions of the theory.

In general QT doesn't tell you much about the outcome of a single measurement, but only about the statistical properties of ensembles of equally (in a stochastic sense!) prepared systems. Whether or not this is a "complete" description of the world is a philosophical debate going basically on since Born's probability-interpretation footnote. I think from a physical point of view you can never prove whether any theory is "complete". As far as we know today, QT is complete in the sense that it describes all observations right it can describe. As it seems the room for any deterministic (then necessarily nonlocal!) theory seems to be very small given all the stringent Bell-test experiments we have for some decades now.

As long as there's no nonlocal deterministic theory which describes as much as QT, I don't consider it very interesting to think about it. For me it's enough to see that local deterministic theories in Bell's sense are ruled out, while Q(F)T, which deals with local interactions only and is causal within the relativistic spacetime description and at the same time describes the observed inseparability and long-distance correlations in the sense stated in my previous posting.

I don't believe in scholastic approaches. The strong belief even in "physics beyond the standard model" hasn't lead to success yet. I think we need clear hints of observations to see even the standard model fall yet. So I've not much hopes that one can come up with a possible successor of QT on an even more fundamental level by pure thought.

I am reading the comments as they come through and will continue to do so, I do not want you thinking I have posted this then ran.

Obviously my understanding of the discussion is limited as the points are technical / subtle but there are recurring themes that I can investigate further.

Thanks

 

[Lord Jestocost]

...but only about the statistical properties of ensembles of equally (in a stochastic sense!) prepared systems.

One has to talk about a collective of identically prepared systems. Nothing differentiates the members of the collective from each other in a stochastic sense. Don't draw naively the wrong conclusion that the post-measurement situation mirrors the pre-measurement situation; in case you understand - within the framework of QM - a collective of identically prepard systems in the pre-measurement situation as a statistical collective, you touch down in the hidden variable camp.

 

[vanhees71]

I don't understand. What's the difference between "collective" and "ensemble"? Aren't these synonyms in this context. I also don't understand what you mean by "you touch down in the hidden variable camp". To the controrary I follow the minimal interpretation and say that there's nothing else than these probabilities, because all observables that are not determined are really not determined, i.e., there are no hidden variables that would determine them.

 

[Lord Jestocost]

Ensemble is a collection of large number of systems which are macroscopically identical but microscopically different.

 

[vanhees71]

Ok, that's a very strict definition. Usually it's also used in quantum-theory textbooks concerning microscopic systems like a single particle.

 

[Lord Jestocost]

I think we have a comletely different view regarding the terms "probabilistic" and "statistical". My point of view becomes accessible from the following quotes.

David Mermin in "What Is Quantum Mechanics Trying to Tell Us?"

“The view that probabilistic theories are about ensembles implicitly assumes that probability is about ignorance….”V. A. Fock in “ON THE INTERPRETATION OF QUANTUM MECHANICS”, Czech J Phys (1957) 7: 643

“The deeper reason for the circumstance that the wave function cannot correspond to any statistical collective [AKA “ensemble”, LJ] lies in the fact that the concept of the wave function belongs to the potentially possible (to experiments not yet performed), while the concept of the statistical collective belongs to the accomplished (to the results of experiments already carried out).”

 

[vanhees71]

I do not understand this statement by Mermin. An ensemble is in the usual understanding within QT equally prepared independent realizations of an observational/experimental setup. E.g., at the LHC they provide beams of protons colliding head on at about 14 TeV center-of-momentum energy. This can of course be more quantitatively specified by giving the corresponding particle distributions within the bunches, etc.

Probabilities are for me theoretically calculated predictions for the frequency with which I get a given outcome of an measurement on an ensemble (I use the frequentist interpretation of probabilities since this is how it's used in real-world experiments; one can argue about other interpretations like Bayesian, but that obscures the discussion usually even further).

Statistics are measured and quantified frequencies for outcomes of measurements on an ensemble prepared in the lab in the above sense. They can be formally tested against the predicted probabilities in the sense above, which includes a detailed error analysis and a determination of the statistical significance.

The statement by Fock is interesting. As I said, so far I understand under ensemble what he calls a collective.

It's of course clear that you can perform only one measurement at each individual realization of the ensemble, i.e., in general you cannot measure sharply incompatible observables (except in a weak sense, which then needs the use of more complicated (and usually also more realistic) descriptions of the measurement in terms of POVMs; remember the discussion we had about this aspect some time ago).

 

[Demystifier]

As it seems the room for any deterministic (then necessarily nonlocal!) theory seems to be very small given all the stringent Bell-test experiments we have for some decades now.

It doesn't make sense. The Bell test experiments are not an evidence against nonlocal deterministic theories at all. Just the opposite, they are evidence for nonlocal (either deterministic or stochastic) theories. The only assumption in the Bell theorem (that the minimal interpretation may deny) is some weak assumption of realism, essentially saying that measurement outcomes exist even when nobody observes them. Sure, it's a philosophical assumption, one cannot prove by the scientific method that measurement outcomes exist when nobody observed them. But this is the assumption that you actually accept. You just don't accept the logical consequences of the assumption that you accept.

 

[vanhees71]

Which logical consequences do you think I don't accept. For me QT is a satisfactory description of the known empirical facts (except gravity of course, and that's the only point where it is incomplete). I don't need deterministic (or "realistic" though I don't know what's meant by this word anymore, because philosophers have blurred its meaning to an extent making the word useless because it expresses everything and nothing) theories, because we have QT.

Of course, I don't need a human observer that measurement outcomes exist. Storing the result in a computer, as is the modern way of storing experimental outcomes, is enough to fix the outcomes.

My quoted sentence is unfortunate gibberish indeed. What I wanted to say is that (a) if there's a deterministic theory as successful in explaining all observed facts (except gravity) must be nonlocal (in the sense that QFT explicitly is not) and (b) that this makes possibilities for a consistent relativistic deterministic model very narrow since then you have a real tension between nonlocal interactions and relativistic causality (which local, i.e., microcausal, QFT by construction cannot have).

 

[Demystifier]

Which logical consequences do you think I don't accept.

The Bell theorem, namely that any theory (satisfying some weak assumptions of realism) consistent with QM must be nonlocal. For the assumptions of the Bell theorem see http://de.arxiv.org/abs/1501.04168

My quoted sentence is unfortunate gibberish indeed. What I wanted to say is that (a) if there's a deterministic theory as successful in explaining all observed facts (except gravity) must be nonlocal (in the sense that QFT explicitly is not)

That's a gibberish again. In the sense in which QFT is local, deterministic theories such as Bohmian mechanics are local too. For instance, field operators commute at spacelike distances in Bohmian mechanics just as they do in standard QFT. Bohmian mechanics is nonlocal in a different sense.

and (b) that this makes possibilities for a consistent relativistic deterministic model very narrow since then you have a real tension between nonlocal interactions and relativistic causality (which local, i.e., microcausal, QFT by construction cannot have).

It's not narrow at all. It's in fact quite easy to construct nonlocal Bohmian-like theories that make the same measurable predictions as standard QFT. See my lecture 5 in https://www.physicsforums.com/threads/reading-materials-on-quantum-foundations.963543/post-6299524

 

[vanhees71]

Yes, and my point is that QM is fulfilling everything a good theory needs. I don't need "realism" of whatever kind, if QM describes the observational facts! That's what I understand as "realistic" in the natural sciences.

Concerning Bohmian mechanics, I'm not convinced yet that it works in the relativistic context. That may well be due to my ignorance though.

 

[Demystifier]

Yes, and my point is that QM is fulfilling everything a good theory needs. I don't need "realism" of whatever kind, if QM describes the observational facts! That's what I understand as "realistic" in the natural sciences.

If it was true that you don't need "realism", then the Bell theorem would indeed be irrelevant to you. But I can prove that you actually need realism (even though you say that you don't).

Proof 1: You believe that the Moon is there even when nobody observes it. (You usually say that you believe it due to the conservation laws, but that's nonsense because the conservation only proves that something must be there, not that this something must have all the detailed characteristics of the Moon. For instance, the Moon has a spherical shape, but there is no conservation law that guarantees a conservation of the shape.) Therefore, you believe in reality that cannot be directly tested by measurements. Q.E.D.

Proof 2: You often emphasize that the Bell theorem is important because it replaces vague philosophy with a measurable prediction. But the theorem itself depends on a philosophical assumption, namely that a certain kind of reality exists. Therefore you need Bell theorem (because it makes a measurable prediction) and Bell theorem needs realism, from which it follows that you need realism. Q.E.D.

Concerning Bohmian mechanics, I'm not convinced yet that it works in the relativistic context. That may well be due to my ignorance though.

Fair enough! But you can reduce your ignorance without spending much time on it by reading the lecture I mentioned.

 

[vanhees71]

No, I just draw the conclusion that the Bell tests empirically show that the world is not described right by a local realistic theory. Thus I've two choices: to give up realism or locality. Since there's no nonlocal theory consistent with relativity I rather give up realism and simply stay with relativistic local QFT, which works very well as the Standard Model of elementary particles.

 

[Lord Jestocost]

...I rather give up realism...

Now you're talking!

 

[Morbert]

Proof 1: You believe that the Moon is there even when nobody observes it. (You usually say that you believe it due to the conservation laws, but that's nonsense because the conservation only proves that something must be there, not that this something must have all the detailed characteristics of the Moon. For instance, the Moon has a spherical shape, but there is no conservation law that guarantees a conservation of the shape.) Therefore, you believe in reality that cannot be directly tested by measurements. Q.E.D.

A possible reply: Antirealism in QM is not a rejection of reality itself. Instead it's a rejection of the claim that what QM offers is an ontology. We can believe that the moon is real, and also believe that QM is only concerned with the likelihood that we will see it when we look up.