A An argument against Bohmian mechanics?

[vanhees71]

The Stern-Gerlach experiment is the best example for something that can be understood by pure quantum-theoretical time evolution even analytically (under some simplifying assumptions). There's entanglement between the spin state and macroscopically defined position due to unitary time evolution.

 

[RockyMarciano]

This is completely meaningless.

A more accurate description would be that you just don't understand, then you have to ask what you don't understand in case you were interested in understanding ;)

 

[RockyMarciano]

No, that's not what "local" means according to the Bell terminology.No, determinism has not been ruled out by EPR-type experiments and the empirical success of local QFT.

I would expect from posters something more than just yes or no answers without further specific arguments to justify their position. Please define locality according to Bell's theorem, and why you think classical determinism has not been ruled out by EPR-type experiments and the empirical success of local QFT,

 

[stevendaryl]

Local according to the Bell terminology is equivalent to not allowing FTL signal transmission.

No, they are not equivalent according to Bell. Bell's notion of locality is stronger than simply saying that FTL communication is not possible.

Here's a counter-example. Suppose we have three subjects who are far, far away from each other: Alice, Bob and Carol. They perform the following experiment:

1. At an agreed-upon time, Alice picks a message either, "yes" or "no" and texts it to Bob using her magic cell phone.
2. Carol flips a coin, and gets either "heads" or "tails".
3. Bob checks his magic cell phone, where he either gets the message "yes" or "no".

When they do this experiment many times, they find the following pattern:

1. If Carol's coin flip resulted in "heads", then Bob's cell phone receives the message Alice sent.
2. If Carol's coin flip resulted in "tails", then Bob's cell phone receives the opposite of the message Alice sent.
3. Carol's coin produces "heads" or "tails" with equal likelihood.
   
4. This effect is undiminished by distance---so even if Alice, Bob and Carol are lightyears apart (so that it is impossible for any of them to communicate in the length of the time to perform one round of the experiment), the same pattern holds.

It's impossible for Alice to use her magic cell phone to communicate to Bob, because Bob's message always has a 50/50 chance of being "yes" or "no", regardless of Alice's message. But it's also impossible to implement this setup using only local interactions. So it violates Bell's notion of locality, but doesn't allow FTL communication.

 

[stevendaryl]

I would expect from posters something more than just yes or no answers without further specific arguments to justify their position. Please define locality according to Bell's theorem, and why you think classical determinism has not been ruled out by EPR-type experiments and the empirical success of local QFT,

Bell's notion of locality is not about FTL. It's about factorizability of probability. A local theory, according to Bell, has the property that for any measurement, the outcome of the measurement (whether deterministic or not) depends only on conditions at the location where the measurement took place. What this means is that whenever there is a correlation between distant measurements, that correlation must be mediated by local state variables (which may be hidden).

 

[martinbn]

I would expect from posters something more than just yes or no answers without further specific arguments to justify their position. Please define locality according to Bell's theorem, and why you think classical determinism has not been ruled out by EPR-type experiments and the empirical success of local QFT,

No, this is an A-level thread, posters need not explain basic notions.

 

[stevendaryl]

No, they are not equivalent according to Bell. Bell's notion of locality is stronger than simply saying that FTL communication is not possible.

Here's a counter-example.[deleted]

There is a much simpler counter-example. Suppose that Alice and Bob are far away from each other, and they are both flipping coins. It happens to be the case that Alice's result is always the same as Bob's result. That's a nonlocal correlation, even though it can't be used for communication between Alice and Bob.

 

[martinbn]

A conjecture can be thought of as a conditional solution. To take example from pure math, consider the status of Fermat's last theorem before the work of Weil. If the Taniyama-Shimura conjecture is true, then the problem of proving the Fermat's last theorem is essentially solved. Analogously, if the conjecture that particles have Bohmian trajectories is true, then the measurement problem of QM is essentially solved.

I understand your point, and I guess I'll take your word about BM being analogous. By the way it is Wiles, Weil is related in a different way. And at the time it wasn't known that it implies Fermat nor that it is related, that came a bit later.

 

[stevendaryl]

But there is no cut, as far as we know. If you are able to prepare macroscopic objects carefully enough and keep them sufficiently isolated from uncontrollable influence "from the environment" they show quantum behavior, and as soon as you cease this isolation you get quickly into classical behavior through the very efficient mechanism of decoherence, which in a way is "coarse graining" in the sense that many quasi random interactions through coupling to the environment average over many microscopic degrees of freedom. A nice example is Zeilinger's double slit experiment with buckyballs which can be seen as pretty large ("mesoscopic") systems. It's already enough not to cool them down too much to have almost classical behavior through the emission of "thermal photons".

Well, it seems to me that without a classical/quantum cut, then you're in the same boat as "many-worlds" when it comes to explaining why we only see one outcome when we perform a measurement. If we view the entire universe as a quantum system, and an electron is in a superposition of spin-up and spin-down, and we measure the spin, then the entire universe should be in a superposition of "measured spin-up" and "measured spin-down". Which is basically many-worlds.

 

[Demystifier]

I understand your point, and I guess I'll take your word about BM being analogous. By the way it is Wiles, Weil is related in a different way. And at the time it wasn't known that it implies Fermat nor that it is related, that came a bit later.

Thanks for the corrections. (I knew it was Wiles, but I often have problems with mixing up similar names. For instance, at one occasion I was explaining to someone the "Cartan diagonalization" as the proof that reals are not countable.) By the way, how do you like my analogy with non-standard analysis?

 

[Demystifier]

No, this is an A-level thread, posters need not explain basic notions.

Brilliant answer, I should use it more frequently!

 

[Demystifier]

I would expect from posters something more than just yes or no answers without further specific arguments to justify their position. Please define locality according to Bell's theorem, and why you think classical determinism has not been ruled out by EPR-type experiments and the empirical success of local QFT,

https://arxiv.org/abs/1303.2849

 

[stevendaryl]

Where is the minimal interpretation disproven, i.e., why is it wrong to say that there is no cut? Where is it proven that the classical behavior of macroscopic objects are due to dynamics that contradicts the standard quantum dynamics? Where is the measurement problem, i.e., is there an real-world experiment that cannot be described by minimally interpreted QT?

Well, that's why I said it was a technical problem, rather than a philosophical problem. You're not allowed to assume that a technical claim is true just because nobody has managed to prove it false. Whether the classical world, in which macroscopic objects have more-or-less definite positions and velocities, emerges from a minimal interpretation is a purely technical (mathematical) question, you can't decide it experimentally.

 

[stevendaryl]

The Stern-Gerlach experiment is the best example for something that can be understood by pure quantum-theoretical time evolution even analytically (under some simplifying assumptions). There's entanglement between the spin state and macroscopically defined position due to unitary time evolution.

Yes, but the part that is not described by unitary evolution is the transition from:

• The electron is in a superposition of being deflected to the left and being deflected to the right.
• The election was either deflected left or deflected right (with such-and-such probability).

 

[vanhees71]

Why should you wish for something like this? All you need to know are the probabilities to measure the electron at a certain place and then you also know it's spin state due to the entanglement provided by the SG apparatus (which is, in essence, just the inhomogeneous magnetic field). Then you put some detector (photoplate in the case of the original Frankfurt experiment) and check, whether the probabilities are predicted right. More isn't provided by QT here, and as far as we know, there's indeed no more to the phenomenon than that. You can of course ask what happens to the electron after it hit the photoplate, where it leaves a spot through the corresponding chemical reaction, but that's of course hard to describe. I'd say, it got simply absorbed and is not identifiable anymore for further investigations.

I think, it's just that we are so used to the classical worldview that we cannot emotionally accept that there's no more to know about the electron than the probabilistic content of the quantum formalism. Due to this classical prejudice it's hard to accept for us that the electron's properties are neve completely determined, and that this indeterminism is not just due to our ignorance due to some complexity which hinders us to gain complete knowledge about its state but that the complete knowledge about the quantum state doesn't imply determined values for all observables, but that's what QT is telling is with overwhelming persistence. Any experimental attempt to disprove this consequence of QT to our worldview was in vain, and QT always turned out to give the correct description. Also all theoretical attempts to "correct" for the supposed "shortcomings" of QT by modifying it are not very convincing. So you get a whole bunch of "interpretations", i.e., non-scientific but rather philsophical extensions, which however don't solve the apparent "problems" of QT, among them the measurement problem. From a scientist's point of view, however there are no principle problems, but QT is very successful in predicting what's observed, and that's what science is about. Let any speculations on "ontology" to philosophy, where it belongs to!

 

[RockyMarciano]

No, they are not equivalent according to Bell. Bell's notion of locality is stronger than simply saying that FTL communication is not possible.

Bell's notion of locality is not about FTL. It's about factorizability of probability. A local theory, according to Bell, has the property that for any measurement, the outcome of the measurement (whether deterministic or not) depends only on conditions at the location where the measurement took place. What this means is that whenever there is a correlation between distant measurements, that correlation must be mediated by local state variables (which may be hidden).

There is a much simpler counter-example. Suppose that Alice and Bob are far away from each other, and they are both flipping coins. It happens to be the case that Alice's result is always the same as Bob's result. That's a nonlocal correlation, even though it can't be used for communication between Alice and Bob.

https://arxiv.org/abs/1303.2849

There seems to be some miscommunication here. I am not referring to the "local realism" or "local hidden variables" whole concept, I made an effort in previous posts to analyze the usual splitting of "local realism" into a purely local part and a purely realistic part, I guess you missed it. When I refer to Bell's notion of local I mean the first part, not the whole concept of local realism that is opposed to the so called "nonlocal correlations" quantum behavior that I agree is broader.

 

[stevendaryl]

Why should you wish for something like this? All you need to know are the probabilities to measure the electron at a certain place.

Well, if everything (including whatever device I used to detect an electron at a particular location) is described by unitary evolution, then why should the measurement result in a unique answer, as opposed to the universe being put into a superposition of

1. Detecting an electron on the left, and
2. Detecting an electron on the right

My problem is that if measurement is nothing but a complicated interaction of the type that individual particles undergo, then I don't see how there is any room for an additional assumption about measurements (that they give a definite result with particular probabilities). It's as if Newton's laws states that individual particles obey the three laws of motion, and then you add another law saying that snowflakes have six-fold symmetry. Either such a law is redundant (it is derivable from the laws of motion applied to the particles making up the snowflakes), or else it implies that there is something going on besides Newton's laws of motion.

 

[RockyMarciano]

No, this is an A-level thread, posters need not explain basic notions.

Brilliant answer, I should use it more frequently!

Please see my last post above.
I'll just ignore the trolling fragrance of your last remarks.

 

[stevendaryl]

When I refer to Bell's notion of local I mean the first part, not the whole concept of local realism that is opposed to the so called "nonlocal correlations" quantum behavior that I agree is broader.

I know you said that, but it's wrong. Bell's notion of "local" is not the same thing as "no FTL communication".

 

[RockyMarciano]

I know you said that, but it's wrong. Bell's notion of "local" is not the same thing as "no FTL communication".

So you find that locality split from realism is the same as local realism? What's the point of splitting the notion then? You might be missing a later paper by Bell after his famous 1964 one.

 

[vanhees71]

First of all what's a "realstic part". Note that after all these years in this forum, I still haven't get a clear-cut definition, what's meant by "realistic" in this context. As far as I can see, it's usually synonymous with deterministic.

Further in QT are correlations across large distances of parts of a quantum system, e.g., of the polarization of polarization-entangled photons measured at far distant places. That's what's usually termed imprecisely as "nonlocal correlations". Einstein used the much better term "inseparability" (my translation for the German "Nichtseparabilität").

Finally locality in relativistic QFT refers to the Lagrangian being a polynomial of the fields and its 1st derivatives at the same space-time point. Together with microcausality, i.e., the commutability of local observables at space-like separation of their arguments, particularly the Hamiltonian density, this excludes any FTL communication and the validity of the linked-cluster principle, as detailed in Weinberg, Quantum Theory of Fields, vol. I.

 

[RockyMarciano]

Hmmm... I'm saying the same thing Dr. Chinese has been saying for years but when I say it the reaction is different:interesting. I'll try and look up some quotes.

 

[DrChinese]

Hmmm... I'm saying the same thing Dr. Chinese has been saying for years but when I say it the reaction is different:interesting. I'll try and look up some quotes.

My summary of Bell:

No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics.

So I would say that Bohmian Mechanics is not ruled out by Bell. There may be reasons to rule it out, but a Bell test wouldn't be enough by itself.

 

[RockyMarciano]

First of all what's a "realstic part". Note that after all these years in this forum, I still haven't get a clear-cut definition, what's meant by "realistic" in this context. As far as I can see, it's usually synonymous with deterministic.

Further in QT are correlations across large distances of parts of a quantum system, e.g., of the polarization of polarization-entangled photons measured at far distant places. That's what's usually termed imprecisely as "nonlocal correlations". Einstein used the much better term "inseparability" (my translation for the German "Nichtseparabilität").

Finally locality in relativistic QFT refers to the Lagrangian being a polynomial of the fields and its 1st derivatives at the same space-time point. Together with microcausality, i.e., the commutability of local observables at space-like separation of their arguments, particularly the Hamiltonian density, this excludes any FTL communication and the validity of the linked-cluster principle, as detailed in Weinberg, Quantum Theory of Fields, vol. I.

The realistic part is indeed classical determinism. I agree with the rest of your post. I'm not sure if it was addressed to me, in any case I'm not sure if you are agreeing or disagreeing and about what?

 

[RockyMarciano]

My summary of Bell:

No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics.

So I would say that Bohmian Mechanics is not ruled out by Bell. There may be reasons to rule it out, but a Bell test wouldn't be enough by itself.

Sure, I'm not saying it is Bell by itself. It is basically Bell plus the cluster decomposition theorem from QFT plus their experimental confirmation of course.

 

[vanhees71]

Well, if everything (including whatever device I used to detect an electron at a particular location) is described by unitary evolution, then why should the measurement result in a unique answer, as opposed to the universe being put into a superposition of

1. Detecting an electron on the left, and
2. Detecting an electron on the right

My problem is that if measurement is nothing but a complicated interaction of the type that individual particles undergo, then I don't see how there is any room for an additional assumption about measurements (that they give a definite result with particular probabilities). It's as if Newton's laws states that individual particles obey the three laws of motion, and then you add another law saying that snowflakes have six-fold symmetry. Either such a law is redundant (it is derivable from the laws of motion applied to the particles making up the snowflakes), or else it implies that there is something going on besides Newton's laws of motion.

I don't believe that the registration of the electron with the photoplate in my example obeys any different than QT laws of nature. Why should I assume such a thing? That I always find one electron, if I have one electron and not added another one somehow, is already in the assumptions of QT. It's nothing you can derive, i.e., it's not somehow derived from QT but put into it as a postulate. If you'd rather find two cloudy extended spots when a single electron hits the photoplate, then QT would be invalid or rather you'd probably had a chance to interpret Schrödinger's wave function as Schrödinger initially did himself, identifying the electron with his wave function (field) in the sense of a classical field. It was exactly the observation of single electrons at one spot rather than the smeared charge distribution according to the wave function (or more precisely $|\psi|^2$) that brought Born to his probabilistic interpretation of the wave function rather than to adopt Schrödingers classical-field interpretation.

That snowflakes have six-fold symmetry most probably cannot be derived from classical physics. I guess for this you need quantum theory. As far as I know the condensed-matter physicists now believe that the phase diagram of water is now understood completely from fist principles, but it's for sure not classical but quantum physics.

 

[vanhees71]

The realistic part is indeed classical determinism. I agree with the rest of your post. I'm not sure if it was addressed to me, in any case I'm not sure if you are agreeing or disagreeing and about what?

I don't know, whether I agree or disagree with what you've written in #166, because I find this philosophical talk always so imprecise that I cannot decide what's meant. E.g., why does one call something "realism" (which has very many meanings and is not clearly defined in a concise enough way to even discuss it from a scientific point of view) "determinism" (which for physicists has a clear meaning)? It was the great progress made by Bell's papers that it brought these unclear philosophical ideas to the precision at an degree where it became science to decide the question empirically whether the quantum probabilities are understandable by a local deterministic HV theory or not, and the high-precision experiments of the last 3 decades tell us that the answer is a clear no, and that QT is very accurately giving the right predictions. For me the case is closed and QT has won the game with a huge significance!

 

[RockyMarciano]

The only meaningful loophole is that there is faster than light communication between members of the pair. The result is that you either deny the existence of predetermined values at all possible measurement setting (there are no hidden variables); or you deny that the speed of light c is the limit for causal influences. Your choice between these 2 options...

This is from a different thread. The choice and the 2 options(locality and realism, defined here as: c as limit for causal influences, i.e. "no ftl allowed" and "existence of predetermined values at all possible measurements" respectively) I'm referring to are the ones referred to in the quote.
Is this clearer? Any further doubt what I mean by locality and by realism?

 

[RockyMarciano]

I don't know, whether I agree or disagree with what you've written in #166, because I find this philosophical talk always so imprecise that I cannot decide what's meant. E.g., why does one call something "realism" (which has very many meanings and is not clearly defined in a concise enough way to even discuss it from a scientific point of view) "determinism" (which for physicists has a clear meaning)? It was the great progress made by Bell's papers that it brought these unclear philosophical ideas to the precision at an degree where it became science to decide the question empirically whether the quantum probabilities are understandable by a local deterministic HV theory or not, and the high-precision experiments of the last 3 decades tell us that the answer is a clear no, and that QT is very accurately giving the right predictions. For me the case is closed and QT has won the game with a huge significance!

Glad we agree then.

 

[vanhees71]

This is from a different thread. The choice and the 2 options(locality and realism, defined here as: c as limit for causal influences, i.e. "no ftl allowed" and "existence of predetermined values at all possible measurements" respectively) I'm referring to are the ones referred to in the quote.
Is this clearer? Any further doubt what I mean by locality and by realism?

I'd say that also this is settled very convincingly by the recent quantum-optics experiments. All quantum optics is well described with QED, and QED is a local and microcausal relativistic QFT, having the impossibility of FTL communication built in from the very beginning. As long as there is no contradiction of any empirical fact with QED, I'm convinced that there is no FTL communication.