A skeptic's view on Bohmian Mechanics

[Demystifier]

I had the impression that you equated non-realism in the Bell context wih denying external reality.

Any comments on the extended version of my post #57?

 

[RockyMarciano]

I would put Bell in the same camp as Einstein in this regard. And I don't agree that there is a consensus that Einstein was wrong. He was certainly wrong about local hidden variables--tests of Bell's inequality show that there are no such variables. As to whether the lack of local hidden variables implies nonrealism or nonlocality, there isn't a consensus.

I guess I'm one of the lucky ones that has already reached a conclusion in the choice between nonrealism and nonlocality just by discarding any theory that contemplates FTL communication. It was not that hard, I'd say most physicists agree that predictive acausal nonlocality(that is nonlocality not just as a non-falsifiable ontology allowing "FTL influences" which adds nothing to a theory) is not compatible with science, which is causal(or at the very least microcausal like QFT shows) by definition.

 

[RockyMarciano]

As one possible meaning, let me copy-paste from my presentation at a conference:

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!

Here is what it should really mean:

- Physics is not a theory of everything.
- Something of course exists, but that’s not the subject of physics.
- Physics is not about reality of nature,
it is only about what we can say about nature.
- In physics we should only talk about measurable stuff.
- It’s important to talk also about non-measurable stuff,
but just because it’s important is not a reason to call it physics.

Bell theorem ⇒ reality is non-local
- logically correct, but that is not physics

QM ⇒ signal locality
- that is measurable, so that is physics

In short, “local non-reality” should mean:
- Reality is non-local.
- Physics is about the measurable, which is local.

- In that form, local non-reality does not necessarily
need to be accepted, but at least can be reasonably debated.

I dislike the use of the word "reality" here because even if an effort is made in your reasoning to separate non-realism from solipsism it still assumes that reality is classical, and I also dislike the distinction between reality and physics that is added by you, as if reality wasn't precisely what is measurable in principle.
Non-realism, that I prefer to call non-classicality in the vein of the Werner text quoted by DrChinese can be reasonably debated just by defining reality as "measurable stuff", that is all is needed in QFT for instance.

 

[stevendaryl]

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.

(Relativity would generalize this, in the sense that given the history of the world, there are infinitely many ways to "slice" that history into a succession of states. Any laws of physics saying which future states follow from which current states would have to work for all possible ways of slicing.)

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".

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.

 

[stevendaryl]

I dislike the use of the word "reality" here because even if an effort is made in your reasoning to separate non-realism from solipsism it still assumes that reality is classical

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

 

[ShayanJ]

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.

But from your description, it seems to me that rejecting Einstein's realism means accepting contextuality which isn't discussed as frequently as realism or locality, although it seems to be the more important concept.

EDIT: It may seem that accepting that there is no state is the same as accepting contextuality. But I don't think so. Contextuality can mean that the state should always take into account the observer. Its even compatible with the existence of an underlying non-contextual theory.

 

[zonde]

Bohmian mechanics cannot be local in the way QFT is as BM is indistingushable from NRQM, so that is already showing Norsen assertions are at odds whith what most people know.

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

You could try and define what is the definition of locality(separated from classicality) that Bell assumed.

J.S. Bell's Concept of Local Causality

 

[stevendaryl]

But from your description, it seems to me that rejecting Einstein's realism means accepting contextuality which isn't discussed as frequently as realism or locality, although it seems to be the more important concept.

I don't see how contextuality changes anything. According to Wikipedia (https://en.wikipedia.org/wiki/Quantum_contextuality), contextuality means

...the measurement result of a quantum observable depends on the specific experimental setup being used to measure that observable

I don't see how that affects the discussion of realism. In my description, nowhere did I assume noncontextuality.

As I understand it, noncontextuality would allow us to say: "The particle has spin up along axis $\hat{a}$", while contextuality would only allow us to say: "With such-and-such setup for measuring spin along axis $\hat{a}$, I got spin-up". That distinction is probably important, but I don't see how it affects the discussion of realism. Instead of the element of reality being the spin of the particle, the element of reality would be the future result of a measurement of the spin using such-and-such setup. It makes the elements of reality more complicated, but conceptually, I don't understand what difference it makes.

 

[ShayanJ]

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∈F(M,S) is a fact about state S. It's an "element of reality".

This notion of reality seems too fundamental for physics to be abandoned and its general enough to include QM too, even in the Copenhagen interpretation. So I think this is not Bell's notion of reality because he was trying to compare the classical laws of physics with the quantum laws of physics.(I mean laws that have the same familiar nature as the classical intuitive laws that we know, vs. the unintuitive laws with the same nature as QM.)

 

[Demystifier]

defining reality as "measurable stuff"

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

 

[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.