Graduate Does the Bell theorem assume reality?

[bhobba]

Who do they think they are trying to convince. We're not real.

I think the concept of real is like the concept of time - its one of those things that's hard to pin down. Time is what a clock measures - real is the common-sense idea that what we experience comes from something external to us that actually exists. All these can be be challenged by philosophers, and often are circular, but I think in physics pretty much all physicists would accept you have to start somewhere and hold views similar to the above.

For what its worth I think Gell-Mann and Hartel are on the right track:
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

The above, while for a lay audience, contains the link to the actual paper.

Thanks
Bill

 

[Demystifier]

Bell's theorem doesn't prove reality is non-local, that's only one way out of the theorem. As I mentioned above retrocausal or acausal theories are another way out.

I agree about the retrocausal, but I'm not sure what do you mean by acausal. The Bell's theorem, especially some later versions of it, does not depend on the assumption of determinism. So I guess by acausal you mean something different from non-deterministic, but I am not sure what exactly would that be. Perhaps influences with a finite speed larger than c? That was ruled out by a theorem of Gisin.

Or perhaps by acausal you mean the idea that things just happen, without a cause? This, indeed, is very much Copenhagen in spirit. But it violates the Reichenbach common cause principle, which is at the root of all scientific explanations. So acausal in that sense is rather non-scientific in spirit, it is a form of mysticism. It's a perfectly legitimate position, of course, but one needs to say clearly what is at stake when one adopts that position.

Moreover, if we accept that acausality in this sense is a way to save locality, then Bohmian mechanics can also be interpreted as local. Particles just happen to move along those funny Bohmian trajectories, without a cause. Furthermore, in this sense, any quantitative physical theory can be interpreted as acausal. For instance, classical mechanics is acausal too; particles just happen to move along those classical trajectories, without a cause. And so on, and so on ... I don't think that such an acausal perspective helps to explain anything. I think it is worse than mysticism, it's a nonsense.

 

[akvadrako]

A mixed state can result from simple classical ignorance of a pure state source which may pump out, say, one of four pure states. It would be described by a mixed state due to the classical ignorance, but this has nothing to do with entanglement or multiple copies of the observer, i.e. even in Many Worlds in such a case there wouldn't be multiple copies. It's the difference between a proper and improper mixture.

I'm a bit surprised by this disagreement. Unitary QM is deterministic, so if only one of four states will occur it must be due to initial conditions. Those different initial conditions correspond to different copies of the observer. Those copies could exist on different branches due to previous interactions but SLU doesn't depend on an environment or interaction. It just requires that indistinguishable observers are created in different locations; those could be spacetime points, quantum branches or independent pre-quantum universes.

 

[stevendaryl]

(14a) and (14b) were introduced at post #95.

That post does not say what they are. It does introduce the character strings "14a" and "14b", but that doesn't help by itself.

 

[Stephen Tashi]

The realist claims that there is reality independent of the act of observation, and the results at A are independent of the nature of a measurement on B. Because every possible measurement result on A can be predicted in advance, together these *imply* that every possible measurement result (on A) pre-exists.

I don't understand what it means for something to "pre-exist". Doe the prediction of a possible measurement imply predicting a definite outcome for it? Or does such a realist only claim the existence of definite probabilities for various outcomes?

There is a common language notion of "real" that is identical to the common language notion of "actual". There is also a common language notion that statements involving hypothetical events can be "really" true. For example, if my local grocery store has tangelos today then it's stock of tangelos is "real" in the sense of being "actual". If I say "If Alice goes to my local grocery store, she will find tangelos for sale" then, by common notions, this is "really" true. And it is not "really" true that "if Alice goes to my local grocery store, she will not find tangelos for sale". However, from the standard mathematical point of view, a false premise "really" implies any conclusion. So "if Alice goes to my local grocery store today then 2+2 =5" is true when Alice does not go to the store. Likewise "If Alice goes to my local grocery store today then she will not find tangelos" is true when Alice does not go to the store

The writing of mathematical equation does not constitute a statement until words are supplied to interpret it. The use of symbols representing conditional probabilities does not, by itself, say anything definite about a notion of truth for "if...then..." type statements that is different from the standard mathematical notion of truth about them. It seems to me that the physical notions involving "counterfactuals" require establishing some context for equations that goes beyond the purely mathematical interpretation.

 

[stevendaryl]

I don't understand what it means for something to "pre-exist".

Well, suppose I take a pair of shoes and split them up, putting one in one box and sending it to Alice, and putting the other in another box and sending it to Bob. Alice and Bob don't see which shoe was sent to which person.

Alice would give the subjective probability of 50/50 for her finding a left shoe or a right shoe when she opens the box. She would give the same odds for Bob finding a left or right shoe. However, knowing how the boxes were produced, when she opens her box, she immediately knows which shoe Bob will find, even though he has not yet opened his box. So in that case, we would say that Bob's result, left or right, is pre-determined. Even though he hasn't looked, Alice knows what he will see.

The original EPR argument was that measurement of correlated particles must have a similar explanation.

 

[Stephen Tashi]

Well, suppose I take a pair of shoes and split them up, putting one in one box and sending it to Alice, and putting the other in another box and sending it to Bob. Alice and Bob don't see which shoe was sent to which person.

...

The original EPR argument was that measurement of correlated particles must have a similar explanation.

So, do you define "realism" to be the belief that such hidden variables really (i.e. actually) exist?

 

[DrChinese]

...

So, do you define "realism" to be the belief that such hidden variables really (i.e. actually) exist?

In the context of EPR, the idea of realism - much as stevendaryl says - was as follows: since a measurement result of Alice could be predicted with certainty (by prior reference to Bob), there must be an element of reality to whatever led to that result. They *assumed* objective realism as part of that conclusion, specifically that Alice's result did not depend on the choice of measurement on Bob.

In the context of Bell, that idea was expressed slightly differently. That there were probabilities of outcomes at 3 different pairs of measurement angle settings, and that they were independent on each other. The angle settings being AB, BC, and AC. This matches the EPR assumption, although it is a bit looser.

You could ALSO say that there were "real" hidden variables, that is your choice. Doesn't really change much if you do. The point is that Bell showed the EPR assumption to lead to a contradiction with QM. Unless of course there were FTL influences at work.

 

[DarMM]

I'm a bit surprised by this disagreement

So, in the case of a proper mixture we simply have classical uncertainty. There is no more need for multiple observers or SLU than with a coin in a box that could be head or tails, it's pure standard "I don't know" uncertainty from plain Kolmogorov probability.

In the Unruh case, no observer measures or becomes entangled with anything and yet a supposedly ontic object $\rho_\Omega$ is a pure state for a Minkowski observer, but a mixed state (of simple "I don't know if the coin is heads or tails" form) for another. So we have a supposedly ontic object having some purely epistemic content in an accelerating frame.

Unless you interpret even coin tosses or classical probability in general in a multiple physical copies of the observer sense.

 

[akvadrako]

Unless you interpret even coin tosses or classical probability in general in a multiple physical copies of the observer sense.

That depends. Is there only one result possible, but the observer just isn't smart enough to figure it out? That would seem to be epistemic uncertainty without multiple observers. Though the observer being just ignorant of a result doesn't seem to have much physical relevance.

On the other hand, if there are multiple outcomes compatible with the observer's state, then I would say worlds corresponding to each outcome exist and SLU applies. In regards to your example, I looked up the Unruh effect but didn't find much relevant to this aspect. However, I don't see how the details could matter. One can imagine 4 worlds, as viewed from outsiders, each containing a stationary observer with the same state and accelerating observers with different states.

 

[DarMM]

I agree about the retrocausal, but I'm not sure what do you mean by acausal

So to be clear:

Retrocausal is propogation into the past light cone.

Acausal describes physics where you take a 4D chunk of spacetime with some matter in it and declare the events that occur are those that satisfy a specific constraint given conditions on hypersurfaces at opposite ends. This basically occurs in Classical Mechanics in the least action formalism, however due to the resulting least action trajectory obeying the Euler-Lagrange equations, you can convert this to a 3+1D picture of what is going on, i.e. of a particle moving in response to a potential.

Acausal views of QM declare the set of events is a result of a constraint different to the least action principal, however one where the resulting set of events can't be understood in a 3+1D way, i.e. as initial conditions evolving in time under a PDE or something similar.

Reichenbach's common cause principle doesn't hold, because later events don't result from previous ones which are their causes, rather the set of events as a whole is selected by a constraint. However things still have a fairly clear scientific explanation.

 

[N88]

That post does not say what they are. It does introduce the character strings "14a" and "14b", but that doesn't help by itself.

PS: I might be missing your point? See [[inserts]] next:

Post # 95 says: "4. Now IF we number Bell's 1964 math from the bottom of p.197: [[starting with Bell's ]] (14), (14a), (14b), (14c), [[and finishing with Bell's]] (15): THEN Bell's realism enters between (14a) and (14b) via his use of his (1)."

That is, we faciltate discussion of Bell's key move --- (14a) to (14b) --- by properly identifying the place that it occurs: between Bell 1964:(14) and Bell 1964:(15). HTH

The point I seek to make is that Bell's inequality is a mathematical fact of limited validity.

1. It is algebraically false.

2. It is false under EPRB (yet Bell was seeking a more complete specification of EPRB).

3. So IF we can pinpoint where Bell's formulation departs from #1 and #2, which I regard as relevant boundary conditions, THEN we will understand the reality that Bell is working with.

4. Now IF we number Bell's 1964 math from the bottom of p.197: (14), (14a), (14b), (14c), (15): THEN Bell's realism enters between (14a) and (14b) via his use of his (1).

So the challenge for me is to understand the reality that he introduces via the relation ...

$B(b,\boldsymbol{\lambda})B(b,\boldsymbol{\lambda}) = 1. \qquad(1)$

... since this is what is used --- from Bell's (1) --- to go from (14a) to (14b).

And that challenge arises because it seems to me that Bell breaches his "same instance" boundary condition; see that last line on p.195. That is, from LHS (14a), I see two sets of same-instances: the set over $(a,b)$ and the set over $(a,c)$. So, whatever Bell's realism [which is the question], it allows him to introduce a third set of same-instances, that over $(b,c)$.

It therefore seems to me that Bell is using a very limited classical realism: almost as if he had a set of classical objects that he can non-destructively test repeatedly, or he can replicate identical sets of objects three times; though I am open to -- and would welcome -- other views.

Thus, from my point of view: neither nonlocality nor any weirdness gets its foot in the door: for [it seems to me], it all depends on how we interpret (1).

PS: I do not personally see that Bell's use of (1) arises from "EPR elements of physical reality." But I wonder if that is how Bell's use of his (1) is interpreted?

For me: "EPR elements of physical reality" correspond [tricky word] to beables [hidden variables] which I suspect Bell may have been seeking in his quest for a more complete specification of EPRB. However, toward answering the OP's question, how do we best interpret the reality that Bell introduces in (1) above?

Or, perhaps more clearly: the reality that Bell assumes it to be found in Bell's move from (14a) to (14b). HTH.

 

[stevendaryl]

PS: I might be missing your point? See [[inserts]] next:

My point is that I don't know what "14a" and "14b" are.

 

[N88]

"From a classical standpoint we would imagine that each particle emerges from the singlet state with, in effect, a set of pre-programmed instructions for what spin to exhibit at each possible angle of measurement, or at least what the probability of each result should be…….

From this assumption it follows that the instructions to one particle are just an inverted copy of the instructions to the coupled particle……..

Hence we can fully specify the instructions to both particles by simply specifying the instructions to one of the particles for measurement angles ranging from 0 to π………."

see: https://www.mathpages.com/home/kmath521/kmath521.htm

Thanks for this. Since the above assumption leads to an unphysical result, I prefer an alternative classicality. Let's call it "Einstein-causality" --- or [maybe better] "Einstein-classicality" after this from Bell:

Einstein argued that EPR correlations ‘could be made intelligible only by completing the quantum mechanical account in a classical way,' Bell (2004:86).

Therefore let Bell's λ denote a particle's total angular momentum, a term common to CM and QM, ..., ... . Then that particle's interaction with a polarizer is DETERMINED via [in classical terms] spin, torque and precession. So the interaction of its pairwise correlated twin is likewise DETERMINED similarly: and in a law-like fashion, the law being readily discerned.

Thus, under this "Einstein-causality" we have "correlation-at-a-distance" and the QM results delivered painlessly. More importantly: we avoid 'spooky-action', nonlocality, AAD, etc: which is what I presume QFT provides?

PS, in so far as the OP and I are interested in Bell and his assumed realism: regarding the "Einstein-causality" above, are you able to also comment on the above from the QFT point-of-view?

 

[N88]

My point is that I don't know what "14a" and "14b" are.

Please see the top two unnumbered relations on Bell (1964:198).

If you do not have it, Bell (1964) is readily and freely available online: http://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf

 

[stevendaryl]

Please see the top two unnumbered relations on Bell (1964:198).

If you do not have it, Bell (1964) is readily and freely available online: http://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf

Okay, I've looked at that paper several times, but there were no equations labeled 14a and 14b, so I assumed you meant another paper.

 

[DrChinese]

Okay, I've looked at that paper several times, but there were no equations labeled 14a and 14b, so I assumed you meant another paper.

He just meant the ones following 14, before 15. I think.

 

[N88]

He just meant the ones following 14, before 15. I think.

Yes, thanks. (14a), (14b), and (14c) are Bell's three UNNUMBERED math expressions [now numbered] ... following 14, before 15.

PS: DrChinese, I would welcome you POV on Bell's transformation of (14a) to (14b). Thank you again.

 

[stevendaryl]

Okay, I've looked at that paper several times, but there were no equations labeled 14a and 14b, so I assumed you meant another paper.

Bell is assuming that $A(\overrightarrow{a}, \lambda)$ is a function returning $\pm 1$, and the interpretation is that if a particle has the hypothesized hidden variable $\lambda$, and you measure the spin along direction $\overrightarrow{a}$ (or polarization), then you will get the result $A(\overrightarrow{a}, \lambda)$.

So $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$ can be written:

1. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$
$= A(\overrightarrow{a}, \lambda) (A(\overrightarrow{b}, \lambda) - A(\overrightarrow{c}, \lambda))$

At this point, we can use that $A(\overrightarrow{b}, \lambda) = \pm 1$, which implies that $A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) = 1$. So we can rewrite

$A(\overrightarrow{c}, \lambda)) = A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda)$

Since the first two factors multiplied together yield +1. So we can plug this in for $A(\overrightarrow{c}, \lambda))$ into the right-side of equation 1 to get:
2. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$
$= A(\overrightarrow{a}, \lambda) (A(\overrightarrow{b}, \lambda) - A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda))$
$= A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda)(1 - A(\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda))$

 

[stevendaryl]

Therefore let Bell's λ denote a particle's total angular momentum, a term common to CM and QM, ..., ... . Then that particle's interaction with a polarizer is DETERMINED via [in classical terms] spin, torque and precession. So the interaction of its pairwise correlated twin is likewise DETERMINED similarly: and in a law-like fashion, the law being readily discerned.

Thus, under this "Einstein-causality" we have "correlation-at-a-distance" and the QM results delivered painlessly. More importantly: we avoid 'spooky-action', nonlocality, AAD, etc: which is what I presume QFT provides?

I think that QFT is a red-herring. QFT explains (or fails to explain) EPR in the same way that QM does.

But I don't understand what you're saying here. Let's go through again why Einstein thought there was spooky action at a distance (or hidden variables):

• We produce a pair of correlated particles.
• Assume that Alice measures some property of her particle before Bob measures the corresponding property of his particle.
• For the properties that Bell was discussing, there are two possible results, which we can map to $\pm 1$.
• Immediately before Alice performs her measurement, she would give the subjective probability of Bob's two results as 50/50.
• Immediately after she performs her measurement, she knows with certainty what Bob's result will be (assuming he measures spin or polarization along the same axis that Alice did).

So Alice's subjective likelihood of Bob getting +1 changed instantaneously from 50% to 100% (or 0%, whichever it is). Einstein reasoned that there were two possible explanations for this sudden change:

1. Somehow Alice's measurement affected Bob's particle, even though it was far away. This would be "spooky action at a distance".
2. Alternatively, maybe Bob's measurement result was pre-determined to be whatever before Alice performed her measurement, and her measurement only informed her of this fact. This would be a hidden variable.

Bell's argument showed that interpretation 2 is not possible. So spooky action at a distance it is. Of course, you can argue that there are more than two possibilities, but of the two Einstein considered, spooky action at a distance seems to be the one that is not ruled out by experiment.

 

[DrChinese]

... So Alice's subjective likelihood of Bob getting +1 changed instantaneously from 50% to 100% (or 0%, whichever it is). Einstein reasoned that there were two possible explanations for this sudden change:

1. Somehow Alice's measurement affected Bob's particle, even though it was far away. This would be "spooky action at a distance".
2. Alternatively, maybe Bob's measurement result was pre-determined to be whatever before Alice performed her measurement, and her measurement only informed her of this fact. This would be a hidden variable.

Bell's argument showed that interpretation 2 is not possible. So spooky action at a distance it is.

I call it "quantum nonlocality" rather than "spooky action at a distance" for the simple reason that the "distance" is in spacetime, not space. "Spooky action at a distance" is often thought to be the same as "instantaneous action at a distance", which (IMHO) it is not. There are obvious limits that can be seen in any diagram showing quantum nonlocality (i.e. anywhere there is entanglement).

 

[N88]

Bell is assuming that $A(\overrightarrow{a}, \lambda)$ is a function returning $\pm 1$, and the interpretation is that if a particle has the hypothesized hidden variable $\lambda$, and you measure the spin along direction $\overrightarrow{a}$ (or polarization), then you will get the result $A(\overrightarrow{a}, \lambda)$.

So $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$ can be written:

1. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$
$= A(\overrightarrow{a}, \lambda) (A(\overrightarrow{b}, \lambda) - A(\overrightarrow{c}, \lambda))$

At this point, we can use that $A(\overrightarrow{b}, \lambda) = \pm 1$, which implies that $A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) = 1$. So we can rewrite

$A(\overrightarrow{c}, \lambda)) = A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda)$

Since the first two factors multiplied together yield +1. So we can plug this in for $A(\overrightarrow{c}, \lambda))$ into the right-side of equation 1 to get:
2. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$
$= A(\overrightarrow{a}, \lambda) (A(\overrightarrow{b}, \lambda) - A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda))$
$= A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda)(1 - A(\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda))$

But this is where I have the problem. Why I am seeking to undertstand the "reality" that Bell is using here.

Bell and you say: $A(\overrightarrow{b}, \lambda) = \pm 1$, which implies that $A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) = 1$

But, to me, this implication only holds if the pair of $A (\overrightarrow{b}, \lambda) = ±1$ come from the same instance. See last line on Bell 1964, p.195.

And we can see from

1. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$

that there are two sets of instances: one over the

$(\overrightarrow{a},\overrightarrow{b})$ settings and one over the $(\overrightarrow{a},\overrightarrow{c})$ settings.

So it seems to me that Bell is combining two independent non-correlated variables [because they come from different instances]: so each combination will = +1 or -1.

This why I am asking about the realism that Bell is postulating when he uses

$A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) = 1$.

Or, to put my problem another way: Bell seems to combine results from two different sets of instances [a no-no?] and creates a third set of instances: those over

$A(\overrightarrow{b}, \lambda) A(\overrightarrow{c}, \lambda)$.

Could this be the reason that QM disgrees with Bell's inequality? I do not see QM combining results from two differents instances:.

From the QM formula in Bell 1964:(3): σ₁ = -σ₂ . Is it not the case then, in QM, that the sigmas are pairwise-matched from the same instance?

And that they are therefore pairwise antiparallel via the pairwise conservation of total angular momentum in each instance?

 

[DarMM]

On the other hand, if there are multiple outcomes compatible with the observer's state, then I would say worlds corresponding to each outcome exist and SLU applies. In regards to your example, I looked up the Unruh effect but didn't find much relevant to this aspect. However, I don't see how the details could matter. One can imagine 4 worlds, as viewed from outsiders, each containing a stationary observer with the same state and accelerating observers with different states.

In the Unruh effect there is only one accelerating observer, not multiple copies, even when viewed by others.

The whole point is that this is a property of the states alone prior to measurement, so you can't invoke multiple copies of the observer.

Just posit one inertial observer and one accelerating observer, the Bogolyubov transformation on the field's modes induced by the coordinate transformations between their frames alone causes the transition from a pure state to a mixed state, with no entanglement with the observer/measurer.

Of course none of this is meant to be a killing argument, it's just suggestive of an epistemic view (like the no cloning theorem, teleportation, $\psi$ obeying things like diFinetti's theorem, etc) and in the original context of why I mentioned this, why QBists aren't obviously wrong to reject the reality of $\psi$.

Basically there are properties of $\psi$ that appear epistemic. One so far can give them a $\psi$-ontic reading, but that's not an argument against deciding to read them epistemically. You've said that nature could be such that the ontic stuff is structurally the same as epistemic knowledge of it. You might be right, but of course some people are going to look at those epistemic-like structures and read them purely epistemically. Until there is some sort of no-go theorem this is the end point and an objective analysis not favoring any interpretation based on one's own preferences or intutions can't proceed further.

 

[stevendaryl]

But this is where I have the problem. Why I am seeking to undertstand the "reality" that Bell is using here.

Any assumptions about reality were made prior to the manipulations here. From this point on, it's just mathematics.

Bell and you say: $A(\overrightarrow{b}, \lambda) = \pm 1$, which implies that $A(\overrightarrow{b}, \lambda) A (\overrightarrow{b}, \lambda) = 1$

But, to me, this implication only holds if the pair of $A (\overrightarrow{b}, \lambda) = ±1$ come from the same instance.

As I said, this is just mathematics. We're deriving a property of a function of two variables. If a number is either +1 or -1, then its square is 1.

It doesn't make sense to ask about whether it "comes from the same instance". It's a function.

See last line on Bell 1964, p.195.

And we can see from

1. $A(\overrightarrow{a}, \lambda) A(\overrightarrow{b}, \lambda) - A(\overrightarrow{a}, \lambda) A(\overrightarrow{c}, \lambda)$

that there are two sets of instances:

As I said, Bell is proving a fact about functions. At this point, "instances" don't come into play. I really don't understand what you're objection is. It might make sense to object to Bell's assumption that the outcome of a particle measurement is described by some unknown function $A(\overrightarrow{a}, \lambda)$, but if you grant that assumption, then everything after that point is just mathematics.

 

[zonde]

Acausal describes physics where you take a 4D chunk of spacetime with some matter in it and declare the events that occur are those that satisfy a specific constraint given conditions on hypersurfaces at opposite ends. This basically occurs in Classical Mechanics in the least action formalism, however due to the resulting least action trajectory obeying the Euler-Lagrange equations, you can convert this to a 3+1D picture of what is going on, i.e. of a particle moving in response to a potential.

Acausal views of QM declare the set of events is a result of a constraint different to the least action principal, however one where the resulting set of events can't be understood in a 3+1D way, i.e. as initial conditions evolving in time under a PDE or something similar.

Reichenbach's common cause principle doesn't hold, because later events don't result from previous ones which are their causes, rather the set of events as a whole is selected by a constraint. However things still have a fairly clear scientific explanation.

What are your considerations for your conclusion that acausal explanations are scientific?
Is your considerations along the lines that in classical mechanics acausal explanation can be converted into causal explanation which we consider scientific?

 

[DarMM]

What are your considerations for your conclusion that acausal explanations are scientific?
Is your considerations along the lines that in classical mechanics acausal explanation can be converted into causal explanation which we consider scientific?

If they can select out the statistics you see in experiments then you can confirm them like any other scientific theory.

 

[zonde]

If they can select out the statistics you see in experiments then you can confirm them like any other scientific theory.

We are speaking about interpretation, so this is not going to work as statistics are already known and confirmed by experiments.
Then say superdeterministic interpretation might give the same statistics, but you won't consider such explanation scientific, right? So there should be other considerations too that are more relevant for interpretations.

 

[akvadrako]

The whole point is that this is a property of the states alone prior to measurement, so you can't invoke multiple copies of the observer.

It's still a bit unclear to me how this mixed state forms, but I assume the way it works is that hidden (or ignored) initial conditions determine which of several outcomes occur. So the copies already exist before the start of the transformation, sitting in different worlds where those critical variables differ.

Until there is some sort of no-go theorem this is the end point and an objective analysis not favoring any interpretation based on one's own preferences or intutions can't proceed further.

We already have Bell's theorem, but QBism seems immune to no-go theorems. I wouldn't even say it's wrong though — they've just restricted it's domain of applicability to single-user experiences. If one is interested in how nature works and doesn't take a solipsistic view, QBism doesn't have anything to say. The interpretations which deal with a shared objective reality shed light on my experience by analyzing them through the eyes of others. This is something QBists refuse to do; their line of reasoning may eventually lead to some insights, but for now it doesn't provide an alternative.

 

[DrChinese]

Then say superdeterministic interpretation might give the same statistics...

There are no superdeterministic interpretations that give the quantum mechanical stats. People hypothesize that there could be such, and there have been a few toy models. That is far and away different from, say, Relational Blockworld. That acausal theory is explained in an entire book, and numerous peer-reviewed papers.

 

[DarMM]

We are speaking about interpretation, so this is not going to work as statistics are already known and confirmed by experiments.
Then say superdeterministic interpretation might give the same statistics, but you won't consider such explanation scientific, right? So there should be other considerations too that are more relevant for interpretations.

Well for different "interpretations" of the same theory then if they have the exact same predictions as each other there is no way to select one from the other experimentally period, doesn't matter if they are retrocausal, acausal, nonlocal, etc

My response was more for acausal theories in general where you'd confirm them like anything else, via checking their predicted statistics for experiments.

I have interpretations in quotes above, as many of the supposed interpretations of QM either have some scenarios where they make different predictions, or in fact have never been shown to obtain the predictions of QM, so whether they are actually interpretations can be strictly false or unknown currently.