I New experiments supporting Bohmian mechanics?

[Denis]

I mean the velocity of the Bohmian particle, as defined by the guiding equation. So, no, it has nothing to do with p/m measurements.

 

[vanhees71]

Is it an observable quantity? If not, why is it a problem? Is it the same issue as in the WKB approximation at the "classical turning point"? If so than it's no issue at all, because the classical approximation breaks down there.

 

[Denis]

Is it an observable quantity? If not, why is it a problem?

For positivists like you it is not a problem. For those who think that a physical theory has to describe reality, it is a problem if something supposed to describe something real becomes, somewhere, infinite. Even if the infinity itself is only at a harmless place where the probability to appear is zero.

And my point was that the solution of this problem - even if only a problem for those interested in reality instead of observables only - will probably lead to a different sub-quantum theory with equations and solutions which differ from QT, even in its observable effects, so that, after this, you can come back and find out if that modified theory is better than QT.

With your self-restriction not to care about non-observable problems you restrict yourself from participation in the creation of such sub-quantum theories, which solve problems of different interpretations. Ignorance of problems means, first of all, refusal to participate in their solution. Your choice.

 

[berkeman]

If your reaction to the suggestion that you might be acting unreasonably is to threaten the use of mod powers, you might wish to ponder whether you should be wielding them at all. Little conflict of interest there.

Please rest assured that all the Mentors are watching your posts in this thread. It may not be Peter that gives that warning...

 

[LeandroMdO]

Here is a deal. I will make explicit correct calculation in Bohmian mechanics, provided that you first present a correct calculation of measurable time correlations in standard QM. In that way I will know at which level you understand measurable time correlations in standard QM, so that I can adjust my calculation to your level of understanding. And be careful, because the naive correlation function $$\langle\psi| x(t_1)x(t_2) |\psi\rangle$$ is not measurable.

I have no doubt that you could show how to calculate a different correlation function and have the results agree between BM and QM. But I'm not interested in other correlation functions: I'm interested in $$\langle\psi| x(t_1)x(t_2) |\psi\rangle$$. It's not like it's some esoteric thing, after all, $$\langle 0| T x(0)x(t_1) |0\rangle$$ for instance is just the two-point function for a 0+1-dimensional field theory, a quantity that can certainly be probed.

You told me that the A. Neumaier's calculation for the expectation value of this observable in BM was incorrect, and that if the calculation is done correctly the discrepancy disappears. That's certainly a possibility I'm willing to entertain. But now you tell me that not all observables are "measurable". I don't know what that word means. When you say "$$\langle\psi| x(t_1)x(t_2) |\psi\rangle$$ is not measurable", do you mean that you can't think of a way to measure it, or do you have a demonstration that it cannot be measured? Can you show me how to classify observables between "measurable" and "not measurable" ones, and then show that all of those in the first group agree between BM and QM?

Saying "you're asking a bad question" might be a perfectly fine response, but only with some more substantiation. Right now what I have is an indication that there is a class of observables whose expectation values disagree between BM and QM. I don't much care what that observable is because once a single one is found, even if it is "not measurable", the door is open for others, some of which might well be straightforward to measure.

Please rest assured that all the Mentors are watching your posts in this thread. It may not be Peter that gives that warning...

Of course. And I am watching you. You are entitled to think this irrelevant, but I question the seriousness of a physics forum that censors based on personal feelings and in-group bias. My impression was that this is a serious forum, but please do correct me if that impression was mistaken.

 

[berkeman]

My impression was that this is a serious forum, but please do correct me if that impression was mistaken.

No correction needed.

 

[PeterDonis]

You are entitled to think this irrelevant, but I question the seriousness of a physics forum that censors based on personal feelings and in-group bias.

Just to be clear: my objection was that you did not provide a specific reference (link to a specific post) when asked. It had nothing to do with "personal feelings" or "in-group bias". A request for a specific reference is in accordance with the PF rules, and the proper response is to provide one, not to say "it really isn't so hard" while refusing to specify what exactly you were referring to.

 

[Demystifier]

When you say "$$\langle\psi| x(t_1)x(t_2) |\psi\rangle$$ is not measurable", do you mean that you can't think of a way to measure it, or do you have a demonstration that it cannot be measured?

You can measure $x(t_1)$, and after that you can measure $x(t_2)$, and finally you can multiply the two values obtained by those two measurements. But this procedure is not the same as measurement of $x(t_1)x(t_2)$. In fact, the operator $x(t_1)x(t_2)$ is not even a hermitian operator, so it is not an observable. Only hermitian operators are observables, and only observables can be measured.

 

[Demystifier]

Well, I don't know, what sense it makes to discuss about things you can never check.

How about discussing things which can be checked in principle, but not in practice? E.g. string theory at the Planck scale, the nature of black-hole interior, or a macro system in which the entropy decreases? Does that make sense to you?

 

[LeandroMdO]

You can measure $x(t_1)$, and after that you can measure $x(t_2)$, and finally you can multiply the two values obtained by those two measurements. But this procedure is not the same as measurement of $x(t_1)x(t_2)$.

Indeed, but nobody suggested that one measure x(t_1) and then x(t_2). That is obviously inequivalent. However, there doesn't appear to be any fundamental difficulty in measuring or probing the correlation between x(t_1) and x(t_2) while leaving the positions themselves unmeasured. It's a matter of being creative in designing the proper experiment, unless it can be demonstrated that this is impossible. I don't think it's at all obvious that it should be impossible to measure a correlation between x(t_1) and x(t_2) without measuring either individually.

In fact, the operator $x(t_1)x(t_2)$ is not even a hermitian operator, so it is not an observable. Only hermitian operators are observables, and only observables can be measured.

Right, you have to use an appropriate symmetrization. x(t_1)x(t_2) + x(t_2)x(t_1) is Hermitian, and its naive expectation value in BM differs from that of QM. So the lack of Hermiticity is not the source of the discrepancy.

 

[vanhees71]

For positivists like you it is not a problem. For those who think that a physical theory has to describe reality, it is a problem if something supposed to describe something real becomes, somewhere, infinite. Even if the infinity itself is only at a harmless place where the probability to appear is zero.

And my point was that the solution of this problem - even if only a problem for those interested in reality instead of observables only - will probably lead to a different sub-quantum theory with equations and solutions which differ from QT, even in its observable effects, so that, after this, you can come back and find out if that modified theory is better than QT.

With your self-restriction not to care about non-observable problems you restrict yourself from participation in the creation of such sub-quantum theories, which solve problems of different interpretations. Ignorance of problems means, first of all, refusal to participate in their solution. Your choice.

I'm not a positivist, but I'm a realist. It's almost a nuissance to discuss about "realism", because this is a notion that the philosophers have loaded with so much unsubstantiated meaning that nobody knows, what is discussed anymore. So what's "realistic" for a scientist? It's defined as reproducible objective and quantitative observations of phenomena in nature, no more no less.

Physics is an interplay between experiment and theory, and this interplay has lead to the discovery of quantum theory, which today is the most realistic theory we have about objectively observable phenomena in nature, and this theory tells us, taken away all the metaphysical additions of various interpretations that are supposed to solve some socalled problems with this discovery, that quantities are objectively indetermined, depending on the state a system is in. Given the Bell experiments of the recent decades together with the overwhelming success of local relativistic QFT, my conclusion is that the most realistic theory, which is QT in its minimal interpretation, in indeterministic. At the same time it describes all objective quantitative observations with an astonishing accuracy, and this makes it realistic in the sense of science.

I don't know, what you refer to when you talk about infinities. If you mean the infinities of perturbative relativistic QFT, then it's also a problem that is completely solved by modern renormalization theory with the physical interpretation of the abstract formalism provided by K. Wilson, Kadanoff et al. That physical theories have limitations in their validity is part of them being "realistic" and not some shortcoming! A bit overexaggerated one might say that finding out the limitations of validity of our theories is the very goal of ongoing scientific research, because this leads to its progress!

Of course, I don't believe that the current status of physical theory is the final answer. As long as there is no consistent formulation of gravity that must be wrong, but I don't believe that any progress can be made without new empirical facts guiding us into the right direction of theory building. To solve philosophical fake problems has never lead to any progress in the natural sciences. It was the great breakthrough of modern natural science to get rid of this "scholastic" idea. To solve a vague "problem of realism" or the socalled "measurement problem" hasn't furthered physical theory building, except by triggering a vigorous research program to test QT, and so far no limit of validity has been found. Although it's likely that one day we'll need a new even better theory, but we won't find it by speculations about philosophical problems but only by the very methodology of modern natural sciences, which is to test QT empirically with ever higher accuracy (implying of course that the same accuracy must be reached by theory in applying QT to the concrete description of these measurements).

 

[Demystifier]

Indeed, but nobody suggested that one measure x(t_1) and then x(t_2). That is obviously inequivalent. However, there doesn't appear to be any fundamental difficulty in measuring or probing the correlation between x(t_1) and x(t_2) while leaving the positions themselves unmeasured. It's a matter of being creative in designing the proper experiment, unless it can be demonstrated that this is impossible. I don't think it's at all obvious that it should be impossible to measure a correlation between x(t_1) and x(t_2) without measuring either individually.
Right, you have to use an appropriate symmetrization. x(t_1)x(t_2) + x(t_2)x(t_1) is Hermitian, and its naive expectation value in BM differs from that of QM. So the lack of Hermiticity is not the source of the discrepancy.

I still have no idea how one could measure any of the two things above in practice. But if you tell me how, I will tell you how standard and Bohmian QM make the same measurable predictions in that case.

 

[vanhees71]

How about discussing things which can be checked in principle, but not in practice? E.g. string theory at the Planck scale, the nature of black-hole interior, or a macro system in which the entropy decreases? Does that make sense to you?

Hm, is there any prediction of string theory that is observable in principle? Then it's of course well worth studying, because who knows what will be possible to test in practice in the future. 50 years ago nobody had believed that one can ever test Bell's ideas in practice, but it's almost common routine in the AMO labs today.

 

[Demystifier]

I'm not a positivist, but I'm a realist. It's almost a nuissance to discuss about "realism", because this is a notion that the philosophers have loaded with so much unsubstantiated meaning that nobody knows, what is discussed anymore. So what's "realistic" for a scientist? It's defined as reproducible objective and quantitative observations of phenomena in nature, no more no less.

Any philosopher would say that you are not a realist but a positivist. However, since you are not a philosopher but a scientist, who uses a scientific and not a philosophic language, you have a right to say that you are a realist and not a positivist.

The only problem is that you like to discuss philosophic questions with using scientific and not philosophic way of thinking. This is like discussing art with using scientific and not artistic way of thinking. If you try to explain the value of Mona Lisa by using a scientific way of thinking, it does not make much sense neither to artists nor to scientists.

 

[Demystifier]

Hm, is there any prediction of string theory that is observable in principle?

Yes, there are plenty of such predictions which should be visible at the Planck scale. Extra dimensions, particles with masses of the order of Planck mass, supersymmetry, ... These are generic predictions which do not depend on unknown details of string theory.

 

[vanhees71]

Hm, but then there should be a separate philosophy forum, which separates off the philosophical from the scientific parts of QT. There are Snow's two cultures getting separated into two unrelated universes the more both fields progress, and that's good. For me using philosophical methodology to QT is at least as non-sensical as the value of Mona Lisa by science only.

 

[zonde]

but I don't believe that any progress can be made without new empirical facts guiding us into the right direction of theory building.

Certainly

To solve a vague "problem of realism" or the socalled "measurement problem" hasn't furthered physical theory building, except by triggering a vigorous research program to test QT, and so far no limit of validity has been found. Although it's likely that one day we'll need a new even better theory, but we won't find it by speculations about philosophical problems but only by the very methodology of modern natural sciences, which is to test QT empirically with ever higher accuracy (implying of course that the same accuracy must be reached by theory in applying QT to the concrete description of these measurements).

To find a possible disagreement between the theory and experiment you need a test theory i.e. some sort of speculation where the theory might fail.
But the speculations have to be built on sound scientific basis and majority of speculations in QT (interpretations) are not of that type. BM is one example that is scientifically sound even if it currently does not propose possible limit of QT.

 

[atyy]

I'm not a positivist, but I'm a realist. It's almost a nuissance to discuss about "realism", because this is a notion that the philosophers have loaded with so much unsubstantiated meaning that nobody knows, what is discussed anymore. So what's "realistic" for a scientist? It's defined as reproducible objective and quantitative observations of phenomena in nature, no more no less.

Physics is an interplay between experiment and theory, and this interplay has lead to the discovery of quantum theory, which today is the most realistic theory we have about objectively observable phenomena in nature, and this theory tells us, taken away all the metaphysical additions of various interpretations that are supposed to solve some socalled problems with this discovery, that quantities are objectively indetermined, depending on the state a system is in. Given the Bell experiments of the recent decades together with the overwhelming success of local relativistic QFT, my conclusion is that the most realistic theory, which is QT in its minimal interpretation, in indeterministic. At the same time it describes all objective quantitative observations with an astonishing accuracy, and this makes it realistic in the sense of science.

I don't know, what you refer to when you talk about infinities. If you mean the infinities of perturbative relativistic QFT, then it's also a problem that is completely solved by modern renormalization theory with the physical interpretation of the abstract formalism provided by K. Wilson, Kadanoff et al. That physical theories have limitations in their validity is part of them being "realistic" and not some shortcoming! A bit overexaggerated one might say that finding out the limitations of validity of our theories is the very goal of ongoing scientific research, because this leads to its progress!

Of course, I don't believe that the current status of physical theory is the final answer. As long as there is no consistent formulation of gravity that must be wrong, but I don't believe that any progress can be made without new empirical facts guiding us into the right direction of theory building. To solve philosophical fake problems has never lead to any progress in the natural sciences. It was the great breakthrough of modern natural science to get rid of this "scholastic" idea. To solve a vague "problem of realism" or the socalled "measurement problem" hasn't furthered physical theory building, except by triggering a vigorous research program to test QT, and so far no limit of validity has been found. Although it's likely that one day we'll need a new even better theory, but we won't find it by speculations about philosophical problems but only by the very methodology of modern natural sciences, which is to test QT empirically with ever higher accuracy (implying of course that the same accuracy must be reached by theory in applying QT to the concrete description of these measurements).

Wilson only solved fake philosophical problems. QED was successful before Wilson.

 

[Denis]

I'm not a positivist, but I'm a realist. It's almost a nuissance to discuss about "realism", because this is a notion that the philosophers have loaded with so much unsubstantiated meaning that nobody knows, what is discussed anymore. So what's "realistic" for a scientist?

Very simple, the scientist uses the definition of realism used in the relevant exact mathematical proofs. In this case, the most relevant proof is that of Bell's inequality, and the notion of realism used in this proof is sufficiently clear and precise.

And you obviously reject it. Given that you don't reject Einstein causality, and that realism and Einstein causality give Bell's inequality. Note also that what is used in the proof is also a very weak form of realism, namely the EPR criterion of reality. So, you have to reject even the EPR criterion of reality. So, no, you are not a realist, and naming yourself a realist is simply misleading.

It's defined as reproducible objective and quantitative observations of phenomena in nature, no more no less.

No. My dreams are also observations, phenomena, and some of them are repeatable too.

Physics is an interplay between experiment and theory, and this interplay has lead to the discovery of quantum theory, which today is the most realistic theory we have about objectively observable phenomena in nature, and this theory tells us, taken away all the metaphysical additions of various interpretations that are supposed to solve some socalled problems with this discovery, that quantities are objectively indetermined, depending on the state a system is in.

No, QT is not a realistic theory, there are some interpretations which are realistic, others are epistemic.

Given the Bell experiments of the recent decades together with the overwhelming success of local relativistic QFT, my conclusion is that the most realistic theory, which is QT in its minimal interpretation, in indeterministic.

Which is objectively false, because an explicit example of a deterministic realistic interpretation exists.

At the same time it describes all objective quantitative observations with an astonishing accuracy, and this makes it realistic in the sense of science.

No, this makes it phenomenological. A realistic theory should define a model of what really exists and what does not. QT in the minimal interpretation is not doing such a thing.

I don't know, what you refer to when you talk about infinities.

Take a look at the dBB formula for the velocity of a particle, by the guiding equation, and take a look at the limit of |v| at a zero of the wave function.

If you mean the infinities of perturbative relativistic QFT, then it's also a problem that is completely solved by modern renormalization theory with the physical interpretation of the abstract formalism provided by K. Wilson, Kadanoff et al.

Depends on what you mean by "completely solved". Of course, for people which follow common sense, QFT is completely satisfactory as an effective field theory, which, below some critical distance, will become invalid. They never expected that humans will be able to find out more than some large distance approximations. If you have in mind the problems of those who think Lorentz covariance is some fundamental truth or so, then it solves nothing. Because a theory which, below some critical length, will be replaced by a completely different unknown one, gives Lorentz covariance at best as some approximation. And theorems like Haag's theorem strongly suggest that there is no fundamental relativistic QFT. Nonrenormalizability of GR even more.

To solve a vague "problem of realism" or the socalled "measurement problem" hasn't furthered physical theory building, except by triggering a vigorous research program to test QT, and so far no limit of validity has been found.

Whatever, it was the consideration of such "philosophical" problems of QT which has given essentially the only interesting fundamental result - the violation of Bell's inequality. Following you, we would have found not even this result.

 

[vanhees71]

The EPR paper is very vague in stating what their criterion of reality is. The most famous sentence of this enigmatic paper (only Bohr's answer with the same title is more enigmatic ;-))

"If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity then there exists an element of physical reality to this quantity."

just says that a physical quantity only represents "an element of physical reality" if the system is prepared such that this quantity has a determined value. This is possible for any observable within quantum theory. The point, however, is that there's no state for which all observables have "an element of physical reality". So what? Does that make QT "unrealistic"? Of course not, because this prediction of QT, i.e., that observables may be undetermined, is well observed too. So where is the problem with "reality" here and why this should imply an incompleteness of QT, is enigmatic to me. The rest of the paper doesn't "disprove" QT but only QT with an additional "collapse assumption", and this we've discussed endlessly in this forum already!

 

[Denis]

Wilson only solved fake philosophical problems. QED was successful before Wilson.

No. Wilson has shown that QFT is not a basket of horrible manipulations with throwing away infinitely large things, but a meaningful theory, if understood correctly. It also allowed to make sense of quantum gravity as an effective field theory. I would say this is much more than solving "fake philosophical problems".

The EPR paper is very vague in stating what their criterion of reality is. ... [It] just says that a physical quantity only represents "an element of physical reality" if the system is prepared such that this quantity has a determined value. This is possible for any observable within quantum theory. The point, however, is that there's no state for which all observables have "an element of physical reality". So what?

What "so what"? The question is simple: Do you explicitly reject it? If "just" says, indeed. Are you ready to accept that, "just" in this particular case, there really exists this element of physical reality?

The point of a criterion which "just" says something about a very specific case is quite clear: In "just" this particular case the situation is so clear that if you refuse to accept, nonetheless, that there exists this "element of physical reality", then you clearly and obviously enough reject realism.

But if you accept this criterion - that means, nothing but that "just" in this case there exists that "element of physical reality" - then you can start to prove Bell's theorem. All you need there is to accept also Einstein causality. And in some realistic version of it, which is not "one cannot send signals FTL", but one which allows to claim that Bob's decision what to measure is not "in any way disturbing" the measurement made by Alice. Then you have Bell's theorem, which is falsified by observation and incompatible with QT, and after this you have the possibility to rethink what was wrong: The acceptance of the EPR criterion, or the thesis that Bob's decision what to measure does not disturb the measurement made by Alice.

Does that make QT "unrealistic"? Of course not, because

Indeed, because we know that there are realistic interpretations of QT. There is no problem with reality. There is only a problem with reality for those who are ready to reject realism - even in the extremely weak form of the EPR criterion of reality - to save Einstein causality.

 

[stevendaryl]

I'm not a positivist, but I'm a realist. It's almost a nuissance to discuss about "realism", because this is a notion that the philosophers have loaded with so much unsubstantiated meaning that nobody knows, what is discussed anymore. So what's "realistic" for a scientist? It's defined as reproducible objective and quantitative observations of phenomena in nature, no more no less.

I would not consider that "realism". I would consider it the opposite of realism, actually. Realism is the assumption that are observations are due to phenomena that exists independently of our observations.

 

[vanhees71]

Ok, again we are at the point, where I have to ask what you define as "realistic". For me QT is perfectly realistic, because it describes with high accuracy all and many even very precisely measured observational facts. That's enough for me to say that QT is a realistic (at the present time the most realistic) theory we have. Concerning EPR, my "so what?" was referring to the fact that according to EPR in all states a system can be in there are observables that don't take determined values and thus according to EPR do not reflect "an element of reality". This makes me ask "so what?", because it only shows that the definition of "reality" is an empty phrase, because we observe the very predictions of QT concerning the indeterminacy of observables for given state preparations and so it's realistic, contradicting the very assertion made that it is not realistic. The rest of the paper only shows that the collapse assumption leads at least to serious logical problems, but since the collapse assumption is not part of the standard QT formulation and it's never really needed to relate this formalism to real-world observations, you just drop it, and QT has no more problems (at least as far as the EPR paper is concerned).

 

[stevendaryl]

So what? Does that make QT "unrealistic"?

I would say yes.

 

[vanhees71]

I would not consider that "realism". I would consider it the opposite of realism, actually. Realism is the assumption that are observations are due to phenomena that exists independently of our observations.

I never denied that phenomena exist independently of our observations. How did you come to the assumption, I did imply this? Of course "the moon is there if nobody looks at it". Why shouldn't it? In QT there's nothing to imply that it might not be there, only because nobody looks at it.

 

[vanhees71]

I would say yes.

Why? The indeterminism of the spin-$x$ component of a particle prepared to have determined spin-$z$ component should be clearly observed (I guess with ultracold neutrons to a high accuracy; perhaps one can google it). Why does that (correct) prediction of QT make QT "unrealistic". To the contrary the observed facts agree with QT. So is reality itself unrealistic because of that?

 

[stevendaryl]

I never denied that phenomena exist independently of our observations.

Your definition of "realism" made no mention of it. You talked about reproducible observations.

 

[stevendaryl]

Why? The indeterminism of the spin-$x$ component of a particle prepared to have determined spin-$z$ component should be clearly observed (I guess with ultracold neutrons to a high accuracy; perhaps one can google it). Why does that (correct) prediction of QT make QT "unrealistic". To the contrary the observed facts agree with QT. So is reality itself unrealistic because of that?

A realistic model would say: The universe behaves in such-and-such a way. A theory that only describes our observations of the universe is not a realistic model.

 

[stevendaryl]

I never denied that phenomena exist independently of our observations. How did you come to the assumption, I did imply this? Of course "the moon is there if nobody looks at it". Why shouldn't it? In QT there's nothing to imply that it might not be there, only because nobody looks at it.

Does an electron have a spin component in the z-direction if nobody measures it (and nobody prepares it that way)?

 

[vanhees71]

Then physics as a whole is unrealistic, because it's about what we objectively observe, but what's realistic then? I think we become totally off-topic again. That's predetermined starting with "philosophy" and leaving the safe grounds of the "hard sciences"!

 

[stevendaryl]

Then physics as a whole is unrealistic,

No, it's not. Newtonian physics is realistic in my sense. So is Bohmian mechanics. So is General Relativity.

I think we become totally off-topic again.

We can drop it, but I wish you wouldn't use the word "realistic" to mean the opposite of what other people mean by it.

 

[vanhees71]

Does an electron have a spin component in the z-direction if nobody measures it (and nobody prepares it that way)?

One can't answer this. If I've not observed the electron's spin-z component, I can only make a guess. Making this guess as objective as possible, i.e., minimizing the prejudice we get, using the usual Shannon-Jaynes definition of entropy as a measure for the missing information, we should assume that the electron's spin state is given by $\hat{\rho}=1/2 \hat{1}$.

To verify this, you have to observe (measure) the spin-z component of equally prepared electrons (i.e., of randomly picked electrons from some source in this case) and make a statistical analysis. Then you can say with which signififance your "educated guess" is correct. If the source in fact sends polarized electrons to you, you were wrong and you'd change your description according to what you measured, but if you forbid me to observe it, it's just useless to make any attempt of a description to begin with.

I still don't get, what you are after!

 

[atyy]

I never denied that phenomena exist independently of our observations. How did you come to the assumption, I did imply this? Of course "the moon is there if nobody looks at it". Why shouldn't it? In QT there's nothing to imply that it might not be there, only because nobody looks at it.

By "there", you mean position. So quantum particles have positions when no one is looking. You are a secret Bohmian.

 

[vanhees71]

No, it's not. Newtonian physics is realistic in my sense. So is Bohmian mechanics. So is General Relativity.
We can drop it, but I wish you wouldn't use the word "realistic" to mean the opposite of what other people mean by it.

How can Bohmian mechanics be "realistic" if QT is not. Stripping off BM of unobservable metaphysical elements, you end at QT!

Newtonian physics is only realistic in a quite approximate sense. We know where it limitations are, i.e., where it doesn't describe reality accurately anymore. It's still not clear to me, what you define as "realistic".

 

[vanhees71]

By "there", you mean position. So quantum particles have positions when no one is looking. You are a secret Bohmian.

Of course have particles positions as long as you don't consider massless particles with spin $\geq 1$. According to standard QT the positions can be, however, objectively indetermined (e.g., if the particles are prepared to have pretty well determined momentum as in a particle accelerator). What this has to do with BM, I don't understand. I always thought that the Bohmian trajectories are considered as unobservable.

 

[atyy]

Any philosopher would say that you are not a realist but a positivist. However, since you are not a philosopher but a scientist, who uses a scientific and not a philosophic language, you have a right to say that you are a realist and not a positivist.

The only problem is that you like to discuss philosophic questions with using scientific and not philosophic way of thinking. This is like discussing art with using scientific and not artistic way of thinking. If you try to explain the value of Mona Lisa by using a scientific way of thinking, it does not make much sense neither to artists nor to scientists.

Reality is just a tool to predict measurement outcomes:)

 

[Denis]

How can Bohmian mechanics be "realistic" if QT is not. Stripping off BM of unobservable metaphysical elements, you end at QT!

What about a realistic theory having to contain a clear and certain description of what really exists? This is the standard definition - a realistic theory has an ontology, a description what really exists. Remove this description, and the resulting theory remains compatible with realism, but is not a realistic theory. For such theories there is a name, phenomenological theories.

Newtonian physics is only realistic in a quite approximate sense. We know where it limitations are, i.e., where it doesn't describe reality accurately anymore.

Sorry, but a realistic theory is a theory which makes claims about reality. These claims may be false - but this does not make the theory non-realistic. Newtonian physics is a realistic theory, completely realistic, because it has a well-defined ontology. Its predictions are only approximately true.

 

[stevendaryl]

One can't answer this. If I've not observed the electron's spin-z component, I can only make a guess. Making this guess as objective as possible, i.e., minimizing the prejudice we get, using the usual Shannon-Jaynes definition of entropy as a measure for the missing information, we should assume that the electron's spin state is given by $\hat{\rho}=1/2 \hat{1}$.

To verify this, you have to observe (measure) the spin-z component of equally prepared electrons (i.e., of randomly picked electrons from some source in this case) and make a statistical analysis. Then you can say with which signififance your "educated guess" is correct. If the source in fact sends polarized electrons to you, you were wrong and you'd change your description according to what you measured, but if you forbid me to observe it, it's just useless to make any attempt of a description to begin with.

I still don't get, what you are after!

A realistic theory in my opinion would be a description of how the world works in the absence of observations, measurements, observers, etc. Newtonian physics is such a description. So is Bohmian mechanics. So is General Relativity. That doesn't mean that a realistic theory cannot account for observations. To account for observations, you need additional assumptions that our observations correspond to certain phenomena in the realistic theory. If it is an attempt to be complete, then in principle it should be possible to describe the observers themselves in terms of the realistic theory.

Quantum mechanics with a rule such as the Born rule is simply not a realistic theory. A realistic theory would have no need for a postulate saying "When we measure a physical variable, we get an eigenvalue of the corresponding operator". If an observer is a physical system, described by the same physical laws as the system being observed, then the act of observation should itself be describable in the same theory. There should be no need for an additional assumption about what happens when an observer performs a measurement. There isn't in Bohmian mechanics. There isn't in Newtonian mechanics. There isn't in General Relativity.

 

[atyy]

Of course have particles positions as long as you don't consider massless particles with spin $\geq 1$. According to standard QT the positions can be, however, objectively indetermined (e.g., if the particles are prepared to have pretty well determined momentum as in a particle accelerator). What this has to do with BM, I don't understand. I always thought that the Bohmian trajectories are considered as unobservable.

The there-ness of the moon is unobservable when you are not looking at it.

 

[stevendaryl]

How can Bohmian mechanics be "realistic" if QT is not.

Because Bohmian mechanics is (right or wrong) a description of how the universe works, while QT in the "minimal" interpretation is simply a description of how observations work.

Saying "If I do X, I'll see Y" is not a realistic theory. A realistic theory would allow you to derive that fact (if it's true).

 

[Denis]

I always thought that the Bohmian trajectories are considered as unobservable.

No, all what we observe are Bohmian trajectories. The Schrödinger equation gives us the superposition of the dead and the living cat. The Bohmian trajectory is either the dead cat, or the living cat - it has a well-defined position. What we observe is either the dead cat or the living cat - in any way it is not their superposition. So, what we really observe is the Bohmian trajectory of the large things.

 

[stevendaryl]

Newtonian physics is only realistic in a quite approximate sense. We know where it limitations are, i.e., where it doesn't describe reality accurately anymore. It's still not clear to me, what you define as "realistic".

I think you are confusing two completely separate issues: (1) What type of theory is it? (2) How accurate is it?

Newtonian physics is a realistic theory in that it attempts to describe the way the world works, independently of our observations and measurements. Whether it succeeds (whether it is accurate) is a separate issue.

 

[stevendaryl]

I'm guessing that this thread will be shut down soon. It's way too philosophical.

 

[Demystifier]

I always thought that the Bohmian trajectories are considered as unobservable.

Only in practice, not in principle. As I explain in
https://arxiv.org/abs/1703.08341
non-observability of Bohmian trajectories is analogous to non-observability of dark matter.

 

[Demystifier]

I'm guessing that this thread will be shut down soon. It's way too philosophical.

You just made a measurable prediction, so it's not philosophy.

 

[vanhees71]

No, all what we observe are Bohmian trajectories. The Schrödinger equation gives us the superposition of the dead and the living cat. The Bohmian trajectory is either the dead cat, or the living cat - it has a well-defined position. What we observe is either the dead cat or the living cat - in any way it is not their superposition. So, what we really observe is the Bohmian trajectory of the large things.

Well, that's a BM-biased point of view. Here is my minimal-QT biased view:

(a) states of a cat "dead" and "alive" are macroscopic quantities and from a quantum-mechanical point of view a very coarse-grained description, for which by application of the corresponding effective description from the underlying microscopic dynamics (which is impossible to do in full detail and also impossible to observe in that detail). Such quantities behave, according to this description, classical (at least FAPP).

(b) we observe superpositions always "only" in the sense of Born's rule, i.e., we always measure accurate values on a single system of an ensemble corresponding to the used basis to define the superposition of the state (you must always give a basis to which the superposition refers; otherwise it doesn't make sense to say a vector representing a pure state is described by a superposition). That it is in a proper superposition wrt. to this basis implies that this value is indetermined and you'll measure with the corresponding probability, i.e., a relative frequency of the outcome of the measurement on an ensemble, the corresponding values. QT says not more about this observable and according to the minimal interpretation there's not more possible to be known.

Of course, to determine whether a system is really in a pure state described by this superposition, or in another mixed state with the same probabilities this simple measurement is insufficient. A complete state determination is pretty complicated. The most simple way to achieve it theoretically is to measure a complete set of compatible observables. This implies that "measuring" here must necessarily refer to measuring each single observable of this complete set separately at sufficiently large equally prepared ensembles.

 

[Denis]

Well, that's a BM-biased point of view.

What else do you expect if the question was if the Bohmian trajectories are observable or not? This is a question about Bohmian mechanics. A "minimal interpretation" view of the question if Bohmian trajectories are observable is clearly meaningless, because they don't even exist in the minimal interpretation.

 

[vanhees71]

Only in practice, not in principle. As I explain in
https://arxiv.org/abs/1703.08341
non-observability of Bohmian trajectories is analogous to non-observability of dark matter.

Hm, I'll read this in detail later this evening. I'd put me in the Copenhagen camp in a "superposition or mixture" between 1. and 3. in the list in Sect. 2.1 of your paper. Concerning Qbism, I've only the quibble that at least some "Qbists" want to reinterpret probability (independent of its reference to QT or not) as if it could be applied to single events rather than an ensemble. This doesn't make any sense to me, and I think it's an abuse of Bayes's ideas to do so. Of course, we adapt our probability description when getting more detailed information about it, but it's still probabilities we use then, and this only means that it describes the expected relative frequency when measuring an ensemble. The only reference ot Bayes is that of course conditional probabilities change when the condition changes. That's almost trivial, isn't it, i.e., it's just saying that in general $P(A|B) \neq P(A|C)$, which is obvious already by notation.

On the other hand I like information-theoretical foundations of statistical physics, particularly the information-theoretical foundation of the notion of entropy. I believe it's (quite directly) empirically confirmed to be the correct picture by the experiments with "quantum Maxwell demons" (PRL 115, 260602 (2015)), but that's again another topic.

 

[atyy]

A complete state determination is pretty complicated. The most simple way to achieve it theoretically is to measure a complete set of compatible observables.

I don't think the observables can be compatible. For example, https://arxiv.org/abs/quant-ph/0006006v1 gives the Pauli matrices for state determination on a spin 1/2 system.

 

[Blue Scallop]

↑ What do you mean by "Demystifier BM"? Do you mean the idea outlined in https://arxiv.org/abs/1703.08341 Sec. 4.3? If you do, then fundamental ontology are non-relativistic particles, from which relativistic fields emerge as effective description, from which relativistic particles (which are really quasiparticles) emerge as excitations of those fields.

In Sec 4.3, you wrote: "Such a condensed-matter style of thinking suggests an approach to a Bohmian theory of everything (ToE). Suppose that all relativistic particles of the Standard Model (photons, electrons, quarks, gluons, Higgs, etc.) are really quasi-particles. If so, perhaps the truly fundamental (as yet unknown) particles are described by non-relativistic QM. If so, then non-relativistic Bohmian mechanics is a natural ToE. In such a theory, Bohmian trajectories exist only for those truly fundamental particles."

Demystifier. Do you have other papers that expound on these truly fundamental particles that are described by non-relativistic QM? Who are the other authors who expound on this? If you don't.. what do you think are these non relativistic truly fundamental particles? What shall be their characteristics and are they described by any gauge symmetry such as U(1)xSU(2)xSU(3)?.

Also Relativistic particles = did you mean near light speed so sensitive to SR or particles that can create/annihilate as per our QFT or what?
Non relativistic particles = did you mean low speed particles or particles that can't create/annihilate as per our QFT or what?

Thank you !