# Understanding Bohmian Mechanics of Instrumentalists

• I
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.

Lynch101
PeterDonis
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a QM measurement does not cause a "jump"
In a stochastic underlying model, it might.

Lynch101
In a stochastic underlying model, it might.
How does that same model account for QM correlations?

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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"?

PeterDonis
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Do these models involve "jumps"?
In the sense of discontinuous changes in the quantum state, yes.

PeterDonis
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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.

Lynch101
PeterDonis
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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.

Lynch101
E
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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PeterDonis
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The math isn't the question, it's the conceptual inconsistency.
What conceptual inconsistency?

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.

Lynch101 and mattt
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.

PeterDonis
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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.

Lynch101 and mattt
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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PeterDonis
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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.

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.

Lynch101 and mattt
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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.

PeterDonis
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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.

Lynch101
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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?

PeterDonis
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interpretations that are truly indeterminate/ stochastic AND not based in realism
Are there any?

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Are there any?
Not all interpretations are realist are they?

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.

Lynch101
PeterDonis
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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.

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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.

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@Demystifier