A skeptic's view on Bohmian Mechanics

[RockyMarciano]

Here's the way that I understood Einstein's notion of realism. He thought of the world as a succession of physical states, where the current state determines the possible future states. Note that there is no assumption of determinism here, because this notion of realism is (to me) consistent with nondeterminism. Nondeterminism would just mean that there are many possible future states consistent with the current state.

If the current state determines uniquely the possible future states it is not compatible with nondeterminism as you define it. If it doesn't I don't think is the notion Einstein had of realism. Unless what you mean by indeterminism (that there are many possible future states compatible with the current state) in the sense of classical probabilities where the future state of a dice after tossing as determined by the actual state is compatible with 6 different results. This is not nondeterminism, this is lack of knowledge about the currrent state which is diferent and perfectly deterministic. Could you specify?

The point of Einstein's "elements of reality" is this: Suppose the world is in some state $S_1$ where you know with certainty the value of some future measurement; you know that measurement $M$ will result in $r$. Then if we make the non-solipsistic assumption that this prediction is revealing something about the world (as opposed to just being a prediction about future states of your own brain), then it seems that what we can conclude is that a future state $S_2$ is not possible if in state $S_2$ measurement $M$ has a different result than $r$. In that case, the "element of reality" can be defined explicitly:

For any state $S$, let $F(M,S)$ be the set of possible results of measurement $M$ in some possible future of $S$. The fact that $r \in F(M,S)$ is a fact about state $S$. It's an "element of reality".

Again that set is simply an assessment about our ignorance, like in classical deterministic probabilities. Violations of BI show how misleading that assessment can be if based in classical reasoning and probabilities.

So to me, Einstein's criterion for "elements of reality" just follows from the assumption that there exists a physical state, and the non-solipsistic view that the predictions of physics reveal something about the world.

So what does it mean to reject Einstein's realism? It seems to me that it means either solipsism, or it means rejecting the idea that there are physical states.

It clearly rejects states in their classical conception.

 

[RockyMarciano]

What does it mean for reality to be classical or not?

I'm referring to EPR realism, Demystifier implied in his word reality, that reality meant EPR realism.

 

[A. Neumaier]

Here's the way that I understood Einstein's notion of realism. He thought of the world as a succession of physical states, where the current state determines the possible future states.

I don't think this could ever have been Einstein's thoughts. Knowing relativity, it is clear that there is no objective physical meaning to a succession of physical states since different observers have different notions of succession. He thought of time as an obnoxious illusion. There would only be one timeless state - which would mean a Heisenberg picture of the universe.

 

[A. Neumaier]

Locality in QFT is defined as spacelike separated observables commute. Bohmian mechanics is local in that sense.

No. Bohmian mechanics is nonrelativistic and non-field, and its dynamics has not even a notion of spacelike separation that would be well-defined.

 

[RockyMarciano]

Locality in QFT is defined as spacelike separated observables commute. Bohmian mechanics is local in that sense.

The operators in QFT are very different mathematically from the operators in NRQM, so this is not possible.

J.S. Bell's Concept of Local Causality

This is a definition that simply mixes locality with realism making them indistinguishable while at the same time claiming it has nothing to do with realism and erroneously endorsing it to Bell. It is terribly confused.

 

[RockyMarciano]

Measurable or measured? Spin is measurable, but is it real before it is actually measured?

Furthermore, suppose that we have entangled EPR pair of particles with spin. Suppose that spin of one particle is measured by Alice, and spin of another particle is measured by Bob. Suppose that Alice and Bob do not compare the results of their measurements. In this case, are both spins real?

If by real you mean EPR real, they are not. They are physical measurements, if a physical measureent hasn't been performed yet it doesn't exist in that sense.

If yes, are the results of those measurements correlated?If yes again, is the correlation real?

Again.These are empty question without specifying the correlation, for instant the classical correlation is not real, as shown in experiments violating the BI.

 

[stevendaryl]

I don't think this could ever have been Einstein's thoughts. Knowing relativity, it is clear that there is no objective physical meaning to a succession of physical states since different observers have different notions of succession. He thought of time as an obnoxious illusion. There would only be one timeless state - which would mean a Heisenberg picture of the universe.

I mentioned relativity in my post. Spacetime can be sliced into spatial slices. There is no unique way to do that, but for any way of doing it, the laws of physics governing how one slice evolves into future slices applies.

I agree that viewing spacetime as an unchanging whole is one way to think of relativity---that's the "Block Universe" idea. But was that the way Einstein thought of it?

 

[DrChinese]

1.2 Making sense of local non-reality

- One interpretation of Bell theorem: local non-reality
- Physics is local, but there is no reality.

- Does it mean that nothing really exists?
- That would be a nonsense!

Non-reality is simply the same as saying that non-commuting observables do not simultaneously have well defined values. Then you ask: by what possible mechanism - or by dropping what classical assumption - can you make sense of an entangled system with spatial extent (constituent components are separated)?

Examples would be dropping the requirement that A causes B when A occurs first. I.e. deny causality. You can deny determinism and hidden variables, i.e. drop the requirement that hidden variables now determine outcomes later. In virtually any entanglement setup of 2 or more particles, the time sequencing does not appear to in any way affect the statistical outcome. Neither does causal ordering in many cases.

So my point is that non-realism has nothing to do with saying "nothing exists". It only has to do with saying: "no initial configuration determines the observed outcome." We live in a subjective universe, and how we choose to observe actively shapes the reality we see.

I accept c as a constraint on propagation of action, and on selecting what is part of a quantum context (which need not follow the usual temporal ordering requirements). So I would deny that the current state of any distant (non-local) quantum objects have any effect whatsoever on an entangled system, or is responsible for supplying the hidden variables that determine outcomes of quantum observations.

 

[RockyMarciano]

So my point is that non-realism has nothing to do with saying "nothing exists". It only has to do with saying: "no initial configuration determines the observed outcome." We live in a subjective universe, and how we choose to observe actively shapes the reality we see.

Watch out because even with the warning nothing to do with saying "nothing exists", some people might take the part about living in a subjective universe in the sense of claiming the mind or consciousness is constructing what we see, or the need of a personal observer and nonsense like "the moon is not there when you are not looking". Non-realism doesn't make a distinction between interactions that are measurements and the rest of interactions. All interactions either consciously observed or not shape the physics we ultimately get to observe.

 

[stevendaryl]

Examples would be dropping the requirement that A causes B when A occurs first. I.e. deny causality. You can deny determinism and hidden variables, i.e. drop the requirement that hidden variables now determine outcomes later. In virtually any entanglement setup of 2 or more particles, the time sequencing does not appear to in any way affect the statistical outcome. Neither does causal ordering in many cases.

But in an EPR-type situation, we can choose a time-slicing so that Alice measures spin-up along direction $\hat{a}$ a fraction of a second before Bob measures the spin of the corresponding particle along that axis. We know what result Bob will get ahead of time. So Bob's result in the future is determined by the state of the world right now (according to the time slicing). Bob's result is determined as soon as Alice gets her result. The facts about one time slice determine (some of) the facts about a later time slice.

 

[RockyMarciano]

But in an EPR-type situation, we can choose a time-slicing so that Alice measures spin-up along direction $\hat{a}$ a fraction of a second before Bob measures the spin of the corresponding particle along that axis. We know what result Bob will get ahead of time. So Bob's result in the future is determined by the state of the world right now (according to the time slicing). Bob's result is determined as soon as Alice gets her result. The facts about one time slice determine (some of) the facts about a later time slice.

We can choose any time-slicing we want just by exploiting the relativity of spacelike simultaneity, included those slicings that coincide with classical determinism. Non-realism in the mathematically specific QFT achieves it gets rid of any notion of spacelike simultaneity , not only absolute but relative, and this use of simultaneity is not possible anymore.

 

[A. Neumaier]

I agree that viewing spacetime as an unchanging whole is one way to think of relativity---that's the "Block Universe" idea. But was that the way Einstein thought of it?

"People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion." - Albert Einstein
This is from http://www.Alberteinsteinsite.com/quotes/einsteinquotes.html. I remember reading something very similar in one of his writings but cannot retrieve it. Perhaps someone else has the original reference.

 

[stevendaryl]

We can choose any time-slicing we want just by exploiting the relativity of spacelike simultaneity, included those slicings that coincide with classical determinism. Non-realism in the mathematically specific QFT achieves it gets rid of any notion of spacelike simultaneity , not only absolute but relative, and this use of simultaneity is not possible anymore.

I don't see how this changes my conclusion, which is that in a time slicing in which Alice's measurement takes place before Bob's, Bob's result is determined by the state at the slice on which Alice makes her measurement.

 

[A. Neumaier]

But in an EPR-type situation

Let me remind everyone that this is a thread about Bohmian mechanics and related foundational or philosophical issues, and not about EPR or Bell. If you want to continue the discussion of the latter, please do it in a new thread linking to here, and not in this thread!

 

[stevendaryl]

Let me remind everyone that this is a thread about Bohmian mechanics and related foundational or philosophical issues, and not about EPR or Bell. If you want to continue the discussion of the latter, please do it in a new thread linking to here, and not in this thread!

Fair enough.

 

[RockyMarciano]

I don't see how this changes my conclusion, which is that in a time slicing in which Alice's measurement takes place before Bob's, Bob's result is determined by the state at the slice on which Alice makes her measurement.

And you can have another time slicing that says exactly the opposite, so what? That is just different ways to order spacelike separated operators, in QFT they commute, so the different time-slicings are useless. I know this probably won't change your conclusion but I think there isn't much more can I do than offer you the math of QFT.

 

[RockyMarciano]

epr

Let me remind everyone that this is a thread about Bohmian mechanics and related foundational or philosophical issues, and not about EPR or Bell.

You don't think BM and Bell-EPR are closely related? Apparently Bell was basically motivated to construct his theorem by Bohm's theory.

 

[stevendaryl]

And you can have another time slicing that says exactly the opposite, so what?

So, for any choice of time slicing, the future results of measurements are determined by past results of distant measurements.

 

[RockyMarciano]

So, for any choice of time slicing, the future results of measurements are determined by past results of distant measurements.

Right, that is realism, non-realism what I wrote above, they are incompatible. So?

 

[DrChinese]

But in an EPR-type situation, we can choose a time-slicing so that Alice measures spin-up along direction $\hat{a}$ a fraction of a second before Bob measures the spin of the corresponding particle along that axis. We know what result Bob will get ahead of time. So Bob's result in the future is determined by the state of the world right now (according to the time slicing). Bob's result is determined as soon as Alice gets her result. The facts about one time slice determine (some of) the facts about a later time slice.

Not really. You can just as easily say Bob's result determines Alice's. But if you INSIST that Alice is determining Bob's results:

That all occurs in a LOCAL area of spacetime. I.e. places where there is a clear path traced by c (at the limit). The source and Alice are in a light cone, and the source and Bob are in a light cone. What is happening 1 LY away "now" has no influence at all. This is in direct opposition to the Bohmian view, in which things everywhere have a bit of influence here and now.

 

[stevendaryl]

Right, that is realism, non-realism what I wrote above, they are incompatible. So?

I can't really extract a meaning from your statement.

 

[A. Neumaier]

You don't think BM and Bell-EPR are closely related?

Only if everything in the foundations is deemed closely related.

Your discussion sheds light neither on BM nor on the possible mutual misunderstandings displayed in the link in post 1, hence doesn't contribute constructively to the thread. It is ok to have one or two posts clarifying some intermediate arguments from another post in the thread, but not a continuing discussion of these unless they are directly related.

 

[RockyMarciano]

I can't really extract a meaning from your statement.

Well, non-realism denies realism, is this clearer?

 

[stevendaryl]

Not really. You can just as easily say Bob's result determines Alice's.

In either time-slicing, the future result is determined once you know the past results.

 

[RockyMarciano]

Only if everything in the foundations is deemed closely related.

Your discussion sheds light neither on BM nor on the possible mutual misunderstandings displayed in the link in post 1, hence doesn't contribute constructively to the thread. It is ok to have one or two posts clarifying some intermediate arguments from another post in the thread, but not a continuing discussion of these unless they are directly related.

Ok, this is certainly not directly related to the challenge in the OP, but then the thread has been off-topic for many more posts that has been on-topic.

 

[stevendaryl]

In compliance with Professor Neumaier's request, I will not discuss EPR in this thread.

 

[A. Neumaier]

then the thread has been off-topic for many more posts that has been on-topic.

Yes. The point where it becomes intolerable is subject to an uncertainty relation, but at some point it is a macroscopically definite observable.

 

[DrChinese]

In either time-slicing, the future result is determined once you know the past results.

I am not contesting that point. I am saying things outside of the time cone have no impact. That is directly the opposite of predictions by non-local interpretations, which say a time cone is not a limitation. I say there aren't any situations in which entanglement appears and effects occur outside of a time cone (or in some cases, 2 or more connected time cones). The time cone, of course, traced by c and no value higher.

 

[A. Neumaier]

That is directly the opposite of predictions by non-local interpretations, which say a time cone is not a limitation.

I'd be interested to know what happens in a relativistic version of Bohmian mechanics. Do the microscopic particle positions respect Einstein causality, or is the latter considered to be a statistical effect?

 

[PeterDonis]

I'd be interested to know what happens in a relativistic version of Bohmian mechanics.

Is there one?

 

[DrChinese]

Is there one?

I don't think so. On the other hand, demystifier is on record saying "... it cannot be said that there exists a well-defined relativistic QM." There's always an out!

http://xxx.lanl.gov/pdf/quant-ph/0609163v2

 

[zonde]

No. Bohmian mechanics is nonrelativistic and non-field, and its dynamics has not even a notion of spacelike separation that would be well-defined.

Are you saying that Bohmian mechanics allow FTL communication (that spacelike separated observables do not commute)?

 

[ShayanJ]

Are you saying that Bohmian mechanics allow FTL communication (that spacelike separated observables do not commute)?

Bohmian mechanics is like quantum mechanics, defined on the absolute space and time of Newton. There is no notion of space-like separated events in Newton's view!

 

[zonde]

This is a definition that simply mixes locality with realism making them indistinguishable while at the same time claiming it has nothing to do with realism and erroneously endorsing it to Bell. It is terribly confused.

Ok, then here are some of Bell's own words:
"It is important to note that to the limited degree that determinism plays a role in the EPR argument, it is not assumed but inferred. What is held sacred is the principle of “local causality” or “no action at a distance”. Of course, mere correlation between distant events does not itself imply action at a distance, but only correlation between the signals reaching the two places. These signals, in the idealized example of Bohm, must be sufficient to determine whether the particles would go up or down. For any residual undeterminism could only spoil the perfect correlation. It is remarkably difficult to get this point across, that determinism is not a presupposition of the analysis."

And please try not to use "realism" in QM context unless you mean "not solipsism" because it's very confusing what you mean with it. (is it determinism here? or causality? or particles having spin at all times?)

 

[zonde]

Bohmian mechanics is like quantum mechanics, defined on the absolute space and time of Newton. There is no notion of space-like separated events in Newton's view!

Hmm, we can say that there can be no photon or any massive particle that has trajectory trough both events if they are spacelike separated by definition.