I Understanding Bohmian Mechanics of Instrumentalists

  • #51
PeterDonis said:
No, it wouldn't. Causality could still determine the possible results of a particular stochastic "jump" (such as the result of a quantum measurement). Saying that the result is not determined is not at all the same as saying the result is totally disconnected from everything that has gone before.

But a QM measurement does not cause a "jump" anymore than when you try to find your lost keys and based on past events, you deduce the probability of finding it at certain place. Compared to when you look for your keys and you know where you put them.
 
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  • #52
user30 said:
a QM measurement does not cause a "jump"

In a stochastic underlying model, it might.
 
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  • #53
PeterDonis said:
In a stochastic underlying model, it might.

How does that same model account for QM correlations?
 
  • #54
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  • #56
PeterDonis said:
I said "a" model, not "the" model. Different models would do this in different ways. For example, the following two models are both stochastic, but are very different in how they arrive at standard QM in an appropriate approximation:

https://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory

https://en.wikipedia.org/wiki/Stochastic_quantum_mechanics
Just having a read of these now. In the GRW page it mentions "each particle of a system described by the multi-particle wave function ##|\psi \rangle## independently undergoes a spontaneous localisation process (or jump)"

Is a "jump" the same thing as the "collapse of the wave function"?
 
  • #57
Lynch101 said:
Do these models involve "jumps"?

In the sense of discontinuous changes in the quantum state, yes.
 
  • #58
Lynch101 said:
Is a "jump" the same thing as the "collapse of the wave function"?

In the GRW model, yes, the "spontaneous localization process" is the model's version of collapse of the wave function. So in this model, the collapse is a real physical process.
 
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  • #60
user30 said:
It does not say how those models account for action at a distance.

As far as I know, mathematically they do it the same way standard QM does; their predictions don't differ from standard QM in that respect. But I have not spent a lot of time digging into the details. You would have to do that to figure out in detail how they arrive at predictions for things like correlations in Bell-type experiments.

As a matter of interpretation, I don't know that these models resolve the open issues. But deterministic models don't either.
 
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  • #61
E
PeterDonis said:
As far as I know, mathematically they do it the same way standard QM does; their predictions don't differ from standard QM in that respect. But I have not spent a lot of time digging into the details. You would have to do that to figure out in detail how they arrive at predictions for things like correlations in Bell-type experiments.

As a matter of interpretation, I don't know that these models resolve the open issues. But deterministic models don't either.

The math isn't the question, it's the conceptual inconsistency.

It should also be noted

"The GRW theory differs from standard quantum mechanics for the dynamical principles according to which the wave function evolves[10][11]."

So perhaps not appropriate to discuss in this forum since it is not a main stream view.
 
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  • #62
user30 said:
The math isn't the question, it's the conceptual inconsistency.

What conceptual inconsistency?

user30 said:
it is not a main stream view

There isn't really a "mainstream view" as far as QM interpretation is concerned. That's why we have this forum separate from the plain QM one, to allow for the fact that discussions of QM interpretations are different in that respect.

That said, the GRW model is not really a QM interpretation, it's an alternative theory, since it does make different predictions from standard QM in some respects (for example, it predicts that there should be limits on how long, even in principle, quantum coherence can be maintained in a system, whereas standard QM says there is no limit to that in principle, the only limits are practical). But AFAIK it does not make different predictions about things like EPR correlations and violations of the Bell inequalities.
 
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  • #63
PeterDonis said:
What conceptual inconsistency?
There isn't really a "mainstream view" as far as QM interpretation is concerned.
According to the quote there is. Quantum jumps are exclusively in stochastic models, and not part of the standard view, and is thus an obscure model of quantum mechanics that is yet to gain traction.
 
  • #64
user30 said:
Quantum jumps are exclusively in stochastic models, and not part of the standard view

I get that, but I don't get how it's a conceptual inconsistency. The "standard view" is just the basic math of QM, which takes no position at all on whether the quantum state is "real" or whether the collapse that happens in the math when you make a measurement corresponds to a "real" process or not. So the "standard" view is consistent conceptually with a model like the GRW model.

It is true that the GRW model is not consistent with interpretations that do not treat the quantum state as "real", but that is a property of those particular interpretations of QM, not of QM itself.
 
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  • #65
PeterDonis said:
I get that, but I don't get how it's a conceptual inconsistency.

I was referring to quantum correlations only, since these indicate the very opposite of a stochastic world, and instead complete reliability and repeatability.
 
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  • #66
user30 said:
I was referring to quantum correlations only, since these indicate the very opposite of a stochastic world

No, they don't. They just indicate that whatever stochastic processes might be involved have to be able to produce such correlations.

user30 said:
and instead complete reliability and repeatability.

Quantum correlations indicate no such thing, since the results of the experiments involved are not completely reliable and repeatable. The correlations can be repeatably observed, within experimental error, but the individual results of the individual measurements are not repeatable. If Alice and Bob are each measuring one of a pair of entangled photons, and their measurement angles differ by, say, 30 degrees, the individual measurement results, and whether they are the same or opposite, are not repeatable. Only the correlations shown in the statistics of a large number of repetitions are.
 
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  • #67
PeterDonis said:
In the GRW model, yes, the "spontaneous localization process" is the model's version of collapse of the wave function. So in this model, the collapse is a real physical process.
Is it in anyway accurate to think that the wave function, in this case, represents a real physical thing, as in a field or a particle? I'm thinking of a particle or field spread out over some area but then spontaneously localising to a single location before it interacts with the measurement device.
 
  • #68
Lynch101 said:
Is it in anyway accurate to think that the wave function, in this case, represents a real physical thing, as in a field or a particle?

In the GRW model, yes.
 
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  • #69
PeterDonis said:
In the GRW model, yes.
Thanks Peter.

Would it then be fair to amend the previous statement and say: interpretations that are truly indeterminate/ stochastic AND not based in realism, would seem to require that the current state of the system be causally disconnected from its antecedent state. This would seem to necessitate the total absence of causality with events occurring without reference to a prior cause?
 
  • #70
Lynch101 said:
interpretations that are truly indeterminate/ stochastic AND not based in realism

Are there any?
 
  • #71
PeterDonis said:
Are there any?
Not all interpretations are realist are they?
 
  • #72
Lynch101 said:
Thanks Peter.

Would it then be fair to amend the previous statement and say: interpretations that are truly indeterminate/ stochastic AND not based in realism, would seem to require that the current state of the system be causally disconnected from its antecedent state. This would seem to necessitate the total absence of causality with events occurring without reference to a prior cause?

The Copenhagen interpretation does not think it's meaningful to speak of the antecedent stage since it does not have well defined properties. It simply bows out of the discussion. I suppose a good analogy would be your train of thought prior to wakening up. It's simply not relevant to define and speak of from the Copenhagen interpretations point of view.
 
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  • #73
Lynch101 said:
Not all interpretations are realist are they?

You said not realist and "truly indeterminate/stochastic". Are there any?

My point is that you should not be asking questions about hypothetical interpretations or general categories of interpretations that might or might not exist. You should be asking questions about specific interpretations that actually exist, and your questions should be based on what those actually existing interpretations actually say.
 
  • #74
PeterDonis said:
You said not realist and "truly indeterminate/stochastic". Are there any?

My point is that you should not be asking questions about hypothetical interpretations or general categories of interpretations that might or might not exist. You should be asking questions about specific interpretations that actually exist, and your questions should be based on what those actually existing interpretations actually say.
Ah, I see, my apologies.

It might have been better to ask if there are such non-realist interpretations and which one they are. Are you familiar with any such non-realist interpretations? I thought Copenhagen was an example of a non-realist interpretation, or at least a variation of it and I thought QFT could be interpreted as non-realist.

I know discussing the details of them would be off-topic in this thread. The general question about realist vs non-realist theories is, I would say, relevant to the discussion of Demystifier's paper, but probably at the boundaries. The details of non-realist interpretations might be too far in that direction though.
 
  • #75
@Demystifier

Another idea that caught my attention in your paper was this statement:
What we propose here is that the Earth (and everything else) is made of ether. No experiment so far ruled out that possibility, so such a neo-Lorentzian ether theory is a viable possibility.

It certainly sounds like a very interesting suggestion. I'm not even sure what the right questions to ask are to delve into this further. In a very crude manner I would interpret this as there being a Universal field, of some sort, or that the Universe is a single [to use an even more crude term] "thing" or substance. Would you be able to offer any clarifications on how to think about this.

Apologies, this is a very brute force attempt to delve into this idea. I'm not even sure how to phrase the questions, I'm just hopeful that simply starting a discussion on it will lead somewhere. Even if you might only be able to offer an explanation that would be over my head, it might give me a starting point to look into it further.
 
  • #76
Lynch101 said:
I thought Copenhagen was an example of a non-realist interpretation

Copenhagen as usually understood does not take the quantum state to be the actual real state of the system, yes.

However, Copenhagen as usually understood also does not take any position on whether the indeterminacy in the math of QM regarding measurement results (i.e., that the math only predicts probabilities) reflects a "true" indeterminacy in reality. So I don't think Copenhagen as usually understood qualifies as "truly indeterminate/stochastic".

Lynch101 said:
I thought QFT could be interpreted as non-realist

AFAIK QFT admits the same interpretations as QM in general, which would include both realist and non-realist ones.
 
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  • #77
PeterDonis said:
Copenhagen as usually understood does not take the quantum state to be the actual real state of the system, yes.

However, Copenhagen as usually understood also does not take any position on whether the indeterminacy in the math of QM regarding measurement results (i.e., that the math only predicts probabilities) reflects a "true" indeterminacy in reality. So I don't think Copenhagen as usually understood qualifies as "truly indeterminate/stochastic".
Ah I see, thank you for the explanation. I have read a few things which have gone from Copenhagen to a non-realist interpretation but I guess that should be viewed as appending an interpretation to Copenhagen. Is Copenhagen then synonymous with "Shut up and calcualte"?

PeterDonis said:
AFAIK QFT admits the same interpretations as QM in general, which would include both realist and non-realist ones.
Are there other non-realist interpretations, outside of a non-realist interpretation of QFT?
 
  • #78
Lynch101 said:
Is Copenhagen then synonymous with "Shut up and calcualte"?

Not quite. As far as I can tell, Copenhagen as it is usually understood adds to "shut up and calculate" the belief that there is in fact (not just as a matter of doing calculations to make predictions) nothing more there: there is no deeper "underlying reality" beneath the QM model.
 
  • #79
Lynch101 said:
Are there other non-realist interpretations, outside of a non-realist interpretation of QFT?

I'm not sure what you mean. QFT is a quantum theory and, as I said, has the same interpretations as QM in general.
 
  • #80
Lynch101 said:
Is Brownian motion not attributable to a fundamentally deterministic process though? With the apparent randomness being due to a lack of information on our part, but the underlying particle collisions being, themselves, deterministic?
That's true for the physical Brownian motion, but not for the mathematical Brownian motion. The latter, as a model for the former, is fundamentally non-deterministic.
 
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  • #81
Lynch101 said:
Even if you might only be able to offer an explanation that would be over my head, it might give me a starting point to look into it further.
As a first step, look at Ref. [49] in the paper.
 
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  • #82
Demystifier said:
That's true for the physical Brownian motion, but not for the mathematical Brownian motion. The latter, as a model for the former, is fundamentally non-deterministic.
Is the same true for the Nelson interpretation, you mentioned before, where it is the mathematical model that is non-deterministic while the physical system itself is deterministic?

Going on what @PeterDonis has said about the GRW interpretation, would it be fair to say that the GRW interpretation suggests both physical and mathematical non-determinism?
 
  • #83
Demystifier said:
As a first step, look at Ref. [49] in the paper.
Thanks, I've downloaded a copy of that and will check it out now.
 
  • #84
Lynch101 said:
Is the same true for the Nelson interpretation, you mentioned before, where it is the mathematical model that is non-deterministic while the physical system itself is deterministic?
No. The Nelson interpretation assumes that stochasticity is fundamental.

Lynch101 said:
Going on what @PeterDonis has said about the GRW interpretation, would it be fair to say that the GRW interpretation suggests both physical and mathematical non-determinism?
Yes.
 
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  • #85
So how does the probabilistic math of Markov Chain differ from the probabilistic math of partial differential equations in Quantum Mechanics?

The descriptions look very similar yet all partial differential equations are deterministic. What is the fundamental difference between the two?

https://en.m.wikipedia.org/wiki/Markov_chain
 
  • #88
Demystifier said:
As a first step, look at Ref. [49] in the paper.
Thanks for the direction on this Demystifier. Firstly, apologies if the questions that follow are of the sort that you don't even know where to begin to try and explain. I have been in discussions before where someone poses a question that just doesn't seem to make sense and you can be left scratching your head as to how to even begin addressing it. So, no offence taken if this post falls into that category.

I read the paper but didn't understand most of it. I've started studying some maths again, so far refreshing what I've learned in high-school, so it'll be a long time before I can understand the maths in the paper. Is it possible to get a very rough, general idea of what the paper says, do you think, without the mathematics? In my mind I'm trying to get a rough understanding of the consequences of what the paper says, as opposed to being able to determine the accuracy of what is being said, if that makes sense?

In truth, I probably didn't understand too much after the introduction even (or possibly even in the introduction).
To allow for gravitational Lorentz violation without abandoning the framework of general relativity (GR), the background tensor field(s) breaking the symmetry must be dynamical. Einsteinæther theory is of this type. In addition to the spacetime metric tensor field gab it involves a dynamical, unit timelike vector field u a .
.
I have a very limited understanding of fields, but I tend to think of them as analogous to a sheet of some sort stretched out through space, with a vector field having some sort of directionality to it at a given location - perhaps another analogy here might be like a putting green, the kind you see in golf video games, where arrows show the direction the ball is likely to break.

My understanding of the above would be that there some form of fundamental, deeper lying field - the background tensor field - which might break Lorentz invariance, which appears to be a necessity for Bohmian mechanics.

To say that the Earth and everything else is made of Ether, I am picturing some sort of fundamental field which gives rise to all matter. My intuitive thinking would be that this would be a single universal field, but I think you suggested that this might not be the case.

A separate, but related question: under the Bohmian interpretation, is the Universe continuous?
 
  • #89
Lynch101 said:
Is the same true for the Nelson interpretation, you mentioned before, where it is the mathematical model that is non-deterministic while the physical system itself is deterministic?
In general, every deterministic theory can be obtained from a more fundamental stochastic theory, and every stochastic theory from a more fundamental deterministic one.
 
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  • #90
@Demystifier
Another question just occurred to me: is there a "measurement problem" with Bohmian Mechanics?
 
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  • #92
@Demystifier Sabine Hossenfelder just put out a video about Bohmian Mechanics. Any thoughts?

 
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  • #95
How does a measurement occur in Bohmian Mechanics? I was thinking that the pilot wave guided the particle towards the detector and once it comes into contact with the detector a measurement is registered (allowing for the internal processes of the measurement apparatus). Is that accurate, or is it possible that a particle can come into contact with a detector but a measurement fail to be registered?
 
  • #96
Lynch101 said:
How does a measurement occur in Bohmian Mechanics? I was thinking that the pilot wave guided the particle towards the detector and once it comes into contact with the detector a measurement is registered (allowing for the internal processes of the measurement apparatus). Is that accurate, or is it possible that a particle can come into contact with a detector but a measurement fail to be registered?
It's not accurate because it's not clear what do you mean by "comes into contact". Do you mean that particles come into contact, or that wave functions come into contact? Does "coming into contact" mean they arrive at the same place and touch each other, or just that they influence each other? Have in mind that particles interact non-locally in BM, so particles don't need to arrive at the same place to influence each other.
 
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  • #97
Demystifier said:
It's not accurate because it's not clear what do you mean by "comes into contact". Do you mean that particles come into contact, or that wave functions come into contact? Does "coming into contact" mean they arrive at the same place and touch each other, or just that they influence each other? Have in mind that particles interact non-locally in BM, so particles don't need to arrive at the same place to influence each other.
Don't want to intrude on your conversation, but what does it mean for the wave functions to come into contact? Also what is meant by the particles influence each other?
 
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  • #98
martinbn said:
Don't want to intrude on your conversation, but what does it mean for the wave functions to come into contact? Also what is meant by the particles influence each other?
You think that BM is nonsense, so why bother?
 
  • #99
Demystifier said:
You think that BM is nonsense, so why bother?
Can you blame me? Every Bohmian mechanic gives me such answers.
 
  • #100
martinbn said:
Can you blame me? Every Bohmian mechanic gives me such answers.
I blame you for asking questions you are not really interested in. What do you mean by "such answers"?
 
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