B Uncertain position and momentum -- A property of particles?

[Demystifier]

As long as you are not trying to avoid the confrontation with non-classical thinking involved in learning QM. It's a dangerous game to jump into alternative theories to avoid the difficulties of grasping a new subject. It's all right for the experts like @Demystifier but what exactly are you going to learn if your first step is to reject mainstream QM thinking?

I can tell by this post that you haven't yet even learned what the uncertainty principle actually is and what it isn't - that a particle is represented by a probabilistic wave function. Maybe you should have learned properly what QM is saying before deciding you need an alternative?

In a first study of QM, it's perfectly OK to teach only the mainstream and not to mention alternatives (such as Bohmian mechanics). However, one should not make statements which incorrectly suggest that alternatives are impossible. One should keep only those mainstream statements which are true even for the known alternatives. For instance, it is OK to say
1) Position and momentum cannot be simultaneously measured (with perfect accuracy).
2) Position and momentum cannot be both predicted (with perfect accuracy).
3) Quantum formalism does not contain states in which both position and momentum have definite values.
These statements are OK because they are also true in known alternatives. However, it is not OK to say
4) The particle does not have both position and momentum.
or even worst, that
5) Experiments prove that it is impossible for a particle to have both position and momentum.
It is better to stay agnostic than to claim something which isn't really proved. When a smart student asks a tricky question like "Is uncertainty intrinsic or due to imperfect measurements?", an acceptable mainstream answer is
6) We still don't have a full answer to that question.
or
7) It seems intrinsic according to the present knowledge, but we are still not yet completely sure.
Indeed, admitting that some interesting questions are still not answered by science may only increase the curiosity and research spirit of young students who, one day, might become serious researchers themselves.

 

[Prem1998]

In a first study of QM, it's perfectly OK to teach only the mainstream and not to mention alternatives (such as Bohmian mechanics). However, one should not make statements which incorrectly suggest that alternatives are impossible. One should keep only those mainstream statements which are true even for the known alternatives. For instance, it is OK to say
1) Position and momentum cannot be simultaneously measured (with perfect accuracy).
2) Position and momentum cannot be both predicted (with perfect accuracy).
3) Quantum formalism does not contain states in which both position and momentum have definite values.
These statements are OK because they are also true in known alternatives. However, it is not OK to say
4) The particle does not have both position and momentum.
or even worst, that
5) Experiments prove that it is impossible for a particle to have both position and momentum.
It is better to stay agnostic than to claim something which isn't really proved. When a smart student asks a tricky question like "Is uncertainty intrinsic or due to imperfect measurements?", an acceptable mainstream answer is
6) We still don't have a full answer to that question.
or
7) It seems intrinsic according to the present knowledge, but we are still not yet completely sure.
Indeed, admitting that some interesting questions are still not answered by science may only increase the curiosity and research spirit of young students who, one day, might become serious researchers themselves.

Great post. You're completely right. Even if they avoid teaching about facts, which are not proved, to students, I don't think that mainstream quantum mechanics would be much affected. They can still teach the facts which are proved. It will be less confusing for the students.
It was bothering me that every book that I read so far, they were making statements that uncertainties exist in particles and are not due to observations. Thanks for telling me that it's not a proved fact. Now that I know this, I can still learn mainstream quantum mechanics. But there will be less confusions. I'm not saying that I will directly jump into Bohmian mechanics. I definitely won't do that. But now, it will be less confusing for me to learn quantum mechanics.

 

[Demystifier]

Great post. You're completely right. Even if they avoid teaching about facts, which are not proved, to students, I don't think that mainstream quantum mechanics would be much affected. They can still teach the facts which are proved. It will be less confusing for the students.
It was bothering me that every book that I read so far, they were making statements that uncertainties exist in particles and are not due to observations. Thanks for telling me that it's not a proved fact. Now that I know this, I can still learn mainstream quantum mechanics. But there will be less confusions. I'm not saying that I will directly jump into Bohmian mechanics. I definitely won't do that. But now, it will be less confusing for me to learn quantum mechanics.

Great! To avoid other similar confusions and "myths" in QM, see also
https://arxiv.org/abs/quant-ph/0609163

 

[zonde]

Actually, most physicists interpret Bell's theorem differently. It is usually taken to be the best evidence we have against hidden variable theories such as Bohmian mechanics.

If it's indeed as you say it's quite unfortunate state of affairs because:

The reason is that Bell's theorem tells us that we would have to give up locality if we wanted to introduce hidden variables.

Bell's theorem does not say that you can violate Bell inequalities by giving up hidden variables while keeping locality. And if you consider alternative proofs of Bell inequalities you can see that you actually can't escape Bell inequalities by giving up hidden variables.

 

[Prem1998]

Great! To avoid other similar confusions and "myths" in QM, see also
https://arxiv.org/abs/quant-ph/0609163

Thanks. This website lists a number of myths which are just widely accepted unproved claims. I don't like that book authors write widely accepted unproved things as true facts when there are still people, and even scientists, believing in both theories.
This website also lists wave particle duality. Is it also just a widely accepted claim?

 

[atyy]

Actually, most physicists interpret Bell's theorem differently. It is usually taken to be the best evidence we have against hidden variable theories such as Bohmian mechanics. The reason is that Bell's theorem tells us that we would have to give up locality if we wanted to introduce hidden variables. But then, we would have to explain why non-locality cannot be used by humans for FTL communication and why there is no observed violation of special relativity. Every explanation would have to involve a huge conspiracy of the universe against us humans and thus, while hidden variables are not ruled out by experiments, they are essentially ruled out by Occam's razor.

That is not true. https://arxiv.org/abs/1208.4119 may seem to support what you say, but they do comment on Valentini's ideas favourably.

 

[rubi]

That is not true. https://arxiv.org/abs/1208.4119 may seem to support what you say, but they do comment on Valentini's ideas favourably.

Their exact comment is that Valentini's version is "less objectionable". However, a mechanism whose only purpose is to hide the conspiracy from humans, does still qualify as a conspiracy.

 

[Demystifier]

Actually, most physicists interpret Bell's theorem differently. It is usually taken to be the best evidence we have against hidden variable theories such as Bohmian mechanics. The reason is that Bell's theorem tells us that we would have to give up locality if we wanted to introduce hidden variables. But then, we would have to explain why non-locality cannot be used by humans for FTL communication and why there is no observed violation of special relativity. Every explanation would have to involve a huge conspiracy of the universe against us humans and thus, while hidden variables are not ruled out by experiments, they are essentially ruled out by Occam's razor.

No, Bell's theorem is not and cannot be evidence against hidden variable theories such as Bohmian mechanics. It is evidence against local hidden variables, which are unlike Bohmian mechanics. One may dislike non-local hidden variables for other reasons, but there is no way to use Bell's theorem as an argument against non-local hidden variables.

Every regularity may look like a conspiracy, until you learn the mechanism that can explain the regularity.
Life is a conspiracy, until you learn the theory of evolution.
Kepler laws are a conspiracy, until you learn the Newton law of gravity.
The radiation spectrum from hydrogen atom is a conspiracy, until you learn quantum mechanics.
The idea of hidden variables is a conspiracy, until you learn the laws of Bohmian mechanics.

 

[Demystifier]

That is not true. https://arxiv.org/abs/1208.4119 may seem to support what you say, but they do comment on Valentini's ideas favourably.

I find this paper highly misleading, because there is nothing conspiratorial about equilibrium (be it classical, quantum, or sub-quantum equilibrium). It is rather the absence of equilibrium (needed, e.g., for life at the classical level) that requires certain conspiracy.

 

[atyy]

Their exact comment is that Valentini's version is "less objectionable". However, a mechanism whose only purpose is to hide the conspiracy from humans, does still qualify as a conspiracy.

Essentially it's a fine tuning problem - would you consider attempts to solve fine tuning of the cosmological constant or the hierarchy problem to be conspiracy theories?

 

[rubi]

No, Bell's theorem is not and cannot be evidence against hidden variable theories such as Bohmian mechanics. It is evidence against local hidden variables, which are unlike Bohmian mechanics. One may dislike non-local hidden variables for other reasons, but there is no way to use Bell's theorem as an argument against non-local hidden variables.

Well, I did actually qualify my claim. I did say that there is no experimental evidence against hidden variables. My point is that from the point of view of Occam's razor, hidden variables should be considered (and are commonly considered) questionable, since the conspiracy involved is just too big.

Every regularity may look like a conspiracy, until you learn the mechanism that can explain the regularity.
Life is conspiracy, until you learn the theory of evolution.
Kepler laws are a conspiracy, until you learn the Newton law of gravity.
The radiation spectrum from hydrogen atom is a conspiracy, until you learn quantum mechanics.
The idea of hidden variables is a conspiracy, until you learn the laws of Bohmian mechanics.

I don't agree that the case of Bohmian mechanics is analogous to the former ones. The former ones all have testable consequences and advanced our understanding of science. We can test evolution by studying fossils. We can assure ourselves of Newton's laws by sending a sattelite to a comet and calculating its trajectory in advance. The exact same laws that lead to the hydrogen spectrum also explain the band structure of semiconductors. However, the only purpose of the equilibrium in Bohmian mechanics is to hide the conspiracy from humans.

Anyway, I don't really want to discuss interpretations again. I just find it kind of dishonest to advocate non-mainstream theories to beginners and not explain to them why they are usually disregarded by the mainstream, especially if you demand to respect from mainstream science as well (see your post #51).

 

[atyy]

Anyway, I don't really want to discuss interpretations again. I just find it kind of dishonest to advocate non-mainstream theories to beginners and not explain to them aware of why they are usually disregarded by the mainstream, especially if you demand to respect from mainstream science as well (see your post #51).

But your explanation for why they are disregarded (it is not even accepted that they are disregarded!) is wrong. In the standard interpretation, there is a Heisenberg cut. You invoke Occam's razor - but one form of Occam's razor is that there should be no Heisenberg cut since the laws of physics should apply to the whole universe.

 

[PeroK]

Indeed, admitting that some interesting questions are still not answered by science may only increase the curiosity and research spirit of young students who, one day, might become serious researchers themselves.

That's very different from where this thread started with, for example:

... So, uncertainties only exist when predicting the future, but the present existence of a particle must have definite position and momentum. Future is uncertain before it happens, but when it happens it must give an outcome.
Then, why is it said that microscopic particles posses uncertain positions and momentum?

I would suggest that the OP has avoided dealing with the awkward questions that QM poses by the get out clause "it's only one theory and not a proven fact".

I just find it kind of dishonest to advocate non-mainstream theories to beginners and not explain to them aware of why they are usually disregarded by the mainstream

Absolutely!

 

[rubi]

Essentially it's a fine tuning problem - would you consider attempts to solve fine tuning of the cosmological constant or the hierarchy problem to be conspiracy theories?

Depends on whether the solutions are just ad-hoc solutions that replace the fine-tuning by a mechanism whose only purpose is to fix the fine-tuning without any additional explanatory power apart from that or whether the solutions add something to our understanding of physics.

But your explanation for why they are disregarded (it is not even accepted that they are disregarded!) is wrong.

Well, as I argued, I don't consider it wrong, but maybe it's a matter of taste and different people are going to accept different explanations as reasonable. I certainly don't consider Valentini's solution an acceptable solution.

In the standard interpretation, there is a Heisenberg cut. You invoke Occam's razor - but one form of Occam's razor is that there should be no Heisenberg cut since the laws of physics should apply to the whole universe.

Yes, the Heisenberg cut is certainly a problem of the standard presentation of the theory and this is why I prefer presentations that incorporate modern insights, which don't suffer from this problem. For instance, consistent histories makes it pretty clear that no such cut is needed and it is just an intelligent reformulation of the standard Copenhagen point of view. (Hence, it is often referred to as "Copenhagen done right".)

 

[Prem1998]

Anyway, I don't really want to discuss interpretations again. I just find it kind of dishonest to advocate non-mainstream theories to beginners and not explain to them why they are usually disregarded by the mainstream, especially if you demand to respect from mainstream science as well (see your post #51).

As Demystefier said, admitting that some question are still not answered by science increases the curiosity of young students.
Now leave that aside. Just tell me why Bohmian mechanics isn't mainstream? Is it because it is difficult and complicated, so that it must not be true? And, what about the weird results of quantum mechanics that are simplified in Bohmian mechanics while still agreeing to the same experimental data?

 

[DrChinese]

Bell's theorem does not say that you can violate Bell inequalities by giving up hidden variables while keeping locality. And if you consider alternative proofs of Bell inequalities you can see that you actually can't escape Bell inequalities by giving up hidden variables.

I agree with rubi on this one. Bell clearly rules out either hidden variables or locality if certain QM predictions are correct. The only question is whether suitable interpretations lacking one or both can be formulated, which has nothing to do with Bell per se.

 

[DrChinese]

Just tell me why Bohmian mechanics isn't mainstream?

Beauty is in the eye of the beholder when it comes to interpretations. And many physicists don't consider choice of an interpretation important when there is no predictive difference. I am not sure there is any teaching advantage one way or the other. But I will pass along a couple of comments that may tend to answer your question.

1. Bohmian Mechanics has issues with relativity. I don't believe there is an accepted relativistic version at this time.
2. Bohmian Mechanics rejects the property of spin as being fundamental (on a par with position).
3. And the issue I have questions about: why does non-locality appear only with respect to entanglement?

I do consider Bohmian Mechanics to be a viable interpretation regardless of the above. Choice is really a personal opinion.

 

[Prem1998]

That's very different from where this thread started with, for example:I would suggest that the OP has avoided dealing with the awkward questions that QM poses by the get out clause "it's only one theory and not a proven fact".
Absolutely!

Thank you for reminding me about that. I don't think that anyone actually answered that. At the start of the thread, I was actually talking about some 'predicting the future' thing. It was about the wave function from which we can determine the probability of where the particle will be at a future instant 't'. But, unlike in the case of a die, which has uncertainties before it was thrown but it actually gives a definite outcome, then why, in this case of quantum mechanics, when the instant 't' happens, the outcome is also uncertain?

 

[vanhees71]

I don't like Bohmian mechanics, because it doesn't provide anything in addition to the minimal interpretation that's very convincing. It adds a kind of "trajectory picture" for non-relativistic particles, but the trajectories are not what's observed in particle experiments, and where they are observed (like, e.g., in terms of a trace in a cloud chamber) it's well understood within the standard minimal intepretation for decades (the first paper I know is is by N. Mott from 1929 or 1930).

 

[Prem1998]

Beauty is in the eye of the beholder when it comes to interpretations. And many physicists don't consider choice of an interpretation important when there is no predictive difference. I am not sure there is any teaching advantage one way or the other. But I will pass along a couple of comments that may tend to answer your question.

1. Bohmian Mechanics has issues with relativity. I don't believe there is an accepted relativistic version at this time.
2. Bohmian Mechanics rejects the property of spin as being fundamental (on a par with position).
3. And the issue I have questions about: why does non-locality appear only with respect to entanglement?

I do consider Bohmian Mechanics to be a viable interpretation regardless of the above. Choice is really a personal opinion.

Maybe you have some valid points against Bohimian mechanics. But what about the weird concepts that quantum mechanics introduces? Bohmian mechanics avoids them. It avoids the concept that particles don't posses definite position and momentum. Saying that particles don't have well defined position and momentum is as weird as you can get. I don't think that people even themselves understand what they're saying when they talk about such concepts of quantum mechanics. Even the inventors of quantum mechanics would have had a hard time to get something meaningful from statements like this.
So, yes, Beauty is in the eye of the beholder.

 

[Dale]

This discussion is closed due to being both philosophical and heated.