I Determinism, realism, hidden variables

[Paul Colby]

Yes that theory(!) assumes that Alice's measurement collapses the wavefunction non-locally.

Yes, and I may owe the people here an apology. My interpretation of QM is quite "standard" at least from a 1960's view. I happen to hold the view that QM is quite strange, however, is quite consistent and doesn't need any help from additional interpretation. Reading the discussions here convince me even more of this position.

 

[Derek Potter]

You bring much to what is being said that really hasn't been said by me. The EPR measurement (FTL propagation problem as you put it) assumes a causal connection that just isn't there.

I cannot see how you can deny a causal connection when Bob's probabilities depend on Alice's basis and value.

Viewing QM measurement as some form of random interaction is a flawed concept unsupported by experiment.

Well we agree on something then.

 

[Derek Potter]

Yes, and I may owe the people here an apology. My interpretation of QM is quite "standard" at least from a 1960's view. I happen to hold the view that QM is quite strange, however, is quite consistent and doesn't need any help from additional interpretation. Reading the discussions here convince me even more of this position.

Depends what you mean by "need". If you want to know what happens when you fire electrons through a double slit you can "shut up and calculate" - the formalism will give you everything you need. If you want to make sense of what's going on you need interpretation. That's why Bell's contribution is so profound. It places severe contraints on how you can interpret QM.

 

[Paul Colby]

If you want to make sense of what's going on you need interpretation.

Well, nature doesn't have to make sense in the way you define sense. There are other examples in physics where nature doesn't make sense. People have over the years redefined what makes sense means for these cases. People did it with both relativities and with electromagnetic fields requiring a medium for propagation. You could say, well these make sense to me, and I would accept your assessment without question. Even to this day there are people who dispute the relativities and those that still flog the ether concept. What these people miss is they need to view the observed rules of nature as fundamental and move on. So, have you thought about why relativity makes sense to you but QM doesn't? The non-local nature of some QM states may seem spooky but I think this is just a refusal to accept the vector nature of QM state and what measurements mean operationally.

 

[Derek Potter]

So, have you thought about why relativity makes sense to you but QM doesn't? The non-local nature of some QM states may seem spooky but I think this is just a refusal to accept the vector nature of QM state and what measurements mean operationally.

I have never said QM doesn't make sense to me. Not for several years anyway and certainly not here. What I say is that non-locality does not make sense. Ergo, you may deduce, I believe QM is local. Not classical but local.

 

[Paul Colby]

I have never said QM doesn't make sense to me. Not for several years anyway and certainly not here. What I say is that non-locality does not make sense. Ergo, yopu may deduce, I believe QM is local. Not classical but local.

We,agree on even more. So, electrons are fermions, photons bosons etc. How much of this seeming non-locality is really due to shabby treatment of the problem. After all we are assuming distinguishable particles in all that's written here.

 

[Derek Potter]

We,agree on even more. So, electrons are fermions, photons bosons etc. How much of this seeming non-locality is really due to shabby treatment of the problem. After all we are assuming distinguishable particles in all that's written here.

Well one of the particles is a detector called Alice with a big red label on it and it lives in the West wing of the building and the other is Bob with a big blue label and it lives in the East wing. That should be distinguishable enough.

 

[Paul Colby]

Well one of the particles is a detector called Alice with a big red label on it and it lives in the West wing of the building and the other is Bob with a big blue label and it lives in the East wing. That should be distinguishable enough.

Okay, what ever. Alice and Bob hear clicks and these are associated by time of flight coincidence. There are many details kind of glossed over that I usually find helpful in understanding what is actually observed. Bottom line is they are looking at excitations of field modes in highly correlated states. Does this help? Probably not. However, field theory is manifestly local as far as I understand. I'm guessing how this locality gets lost is likely in the skipped details.

 

[Derek Potter]

Okay, what ever. Alice and Bob hear clicks and these are associated by time of flight coincidence. There are many details kind of glossed over that I usually find helpful in understanding what is actually observed. Bottom line is they are looking at excitations of field modes in highly correlated states. Does this help? Probably not. However, field theory is manifestly local as far as I understand. I'm guessing how this locality gets lost is likely in the skipped details.

Strange. I am reliably informed that QFT is notoriously non-local.

In any case, as I have said before, the *observed* violations of Bell's Inequality do not depend on quantum theory. (Bell's Theorem says that the BI will be violated under QM, the BI itself is classical statistics of observed events.)

If QFT could explain the correlations locally, the field excitations would be local hidden variables so one of the other BI criteria would have to be wrong - causality, or reality. For the probability of an interaction at Bob to depend on what Alice does but not be caused by it would be odd to say the least, but perhaps no odder than Bob making reliable observations when there is nothing there to observe.

 

[Demystifier]

However, field theory is manifestly local as far as I understand.

Strange. I am reliably informed that QFT is notoriously non-local.

There are different kinds of locality, and people should distinguish them. The two most important kinds are signal locality and Bell locality. QFT obeys signal locality, but not Bell locality. In other words, you are both right and both wrong. Or more correctly, you are both vague unless you specify what kind of locality you have in mind.

 

[Derek Potter]

There are different kinds of locality, and people should distinguish them. The two most important kinds are signal locality and Bell locality. QFT obeys signal locality, but not Bell locality. In other words, you are both right and both wrong. Or more correctly, you are both vague unless you specify what kind of locality you have in mind.

We're talking exclusively about Bell locality. Signalling doesn't come into it - not least because the violations of the BI that are observed as well as predicted can't be used for signalling.

 

[Demystifier]

We're talking exclusively about Bell locality. Signalling doesn't come into it - not least because the violations of the BI that are observed as well as predicted can't be used for signalling.

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

 

[Paul Colby]

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

Agreed, I was confused.

 

[Derek Potter]

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

Whereas it simply wouldn't occur to others that anyone would confuse the two :) [edit - sorry Paul, just seen your comment. Ho ho.]

To be honest I've never seen the no-signalling theorem as asserting a kind of locality. It seems to me to be something that can be cited as a caveat to non-locality along the lines of "entanglements are non-local but you cannot signal with them". The in-ability to influence remote events at will doesn't suggest anything about where the variables are located. The other way round would make sense. If there were a theory which did allow signalling, then it would be non-local. Or conversely, a local theory would not allow signalling. Here, though, we have a non-local theory that does not allow signalling so I don't see why it should be called local at all.

 

[Paul Colby]

Depends what you mean by "need". If you want to know what happens when you fire electrons through a double slit you can "shut up and calculate" - the formalism will give you everything you need. If you want to make sense of what's going on you need interpretation.

For photons the interference is evident prior to quantization, same for electrons. This doesn't counter the shock an awe of Bell non-locality but the interference pattern is a result of classical waves. The wave amplitudes are then quantized and people dance gleefully about the particle picture. When viewed as a quantum filed slit interference doesn't leave me reeling.

 

[Paul Colby]

To be honest I've never seen the no-signalling theorem as asserting a kind of locality.

I'm still trying to parse your last post. Isn't it true that signaling locality follows from local quantum field theory? Like the first paragraph Weinberg's field theory book states that QFT is just an application of quantum theory. If so the Bell non-locality is baked into QFT from the get go. This leaves us with a consistent story, no?

 

[Demystifier]

Isn't it true that signaling locality follows from local quantum field theory?

Yes.

If so the Bell non-locality is baked into QFT from the get go. This leaves us with a consistent story, no?

I don't understand what are you saying here. Can you rephrase it?

 

[Paul Colby]

By consistent I mean we have a theory that matches observed phenomena. All things happen via the standard model (LQFT) therefore all experiments are signal local. Bell non-locality is an inescapable consequence of QM which is the founding cornerstone of LQFT. Well, you say, Bell non-local experiments are evidence for non-local cause. Bell non-locality is not an interaction in the same sense as passing a signal. It's not a non-local interaction in this sense. It's more like precognition.

 

[Derek Potter]

I'm still trying to parse your last post. Isn't it true that signaling locality follows from local quantum field theory? Like the first paragraph Weinberg's field theory book states that QFT is just an application of quantum theory. If so the Bell non-locality is baked into QFT from the get go. This leaves us with a consistent story, no?

Yes - a story about signal non-locality. We are not talking about signal locality since there is no dispute about it. It's quite irrelevant that QFT explains signal locality - with signal locality :). Violations of Bell's inequality require Bell non-locality. QFT is incorrigibly non-local in the Bell sense. I want a consistent story about Bell non-locality. Under the Bell criteria of reality, causality and locality, no such story exists.

 

[Demystifier]

By consistent I mean we have a theory that matches observed phenomena. All things happen via the standard model (LQFT) therefore all experiments are signal local. Bell non-locality is an inescapable consequence of QM which is the founding cornerstone of LQFT.

Agreed.

Well, you say, Bell non-local experiments are evidence for non-local cause. Bell non-locality is not an interaction in the same sense as passing a signal. It's not a non-local interaction in this sense. It's more like precognition.

Well, Bell non-locality, as such, is rather agnostic on the existence of non-local interaction. Both precognition-like mechanisms and interaction-like mechanisms can be compatible with Bell non-locality. To say something more on the cause of non-local correlation, the Bell theorem is not enough. One needs an explicit model. The best known model is Bohmian mechanics, which leads to yet another notion of non-locality - Bohmian non-locality. Bohmian non-locality is very similar to Bell non-locality (the former is an explicit realization of the latter), but is not exactly the same. In Bohmian mechanics there is some kind of non-local interaction, often described as the quantum potential.

 

[Paul Colby]

I want a consistent story about Bell non-locality. Under the Bell criteria of reality, causality and locality, no such story exists.

The following 3 statements appear consistent to me,

1) All things are signal local
2) Bell non-locality is fundamental aspect of QM and not a non-local interaction (signal)
3) Alice's ability to predict the outcome of Bob's experiment based on her observations doesn't imply non-local "cause" since what happens in Bob's experiment could/would happened even if Alice had not made her observation.

If an experiment refuting 3 is known then I would concede there is something "missing" from the story.

 

[Demystifier]

2) Bell locality is fundamental aspect of QM

Obviously, you meant Bell non-locality.

Otherwise, I agree with the post. But I would like to add that such a precognition-like interpretation of Bell non-locality seems very similar to superdeterminism. What do you think about superdeterminism?

 

[Paul Colby]

What do you think about superdeterminism?

If such theories provide observable effect not currently covered then I'd have to go with them. If they are simply constructed to ease peoples minds I really don't see it being useful (Okay, the construction of such a theory would be technically interesting to say the least). My question and the only reason for annoying people in this thread is to understand the motivation behind wanting such a theory. Are such concerns real or justified? As I've said multiple times there are facts known about nature that are not intuitive and are very much simply dictated by the facts. Is QM significantly different in this regard? Like so many things it's likely just personal choice.

 

[Derek Potter]

The following 3 statements appear consistent to me,

1) All things are signal local
2) Bell non-locality is fundamental aspect of QM and not a non-local interaction (signal)
3) Alice's ability to predict the outcome of Bob's experiment based on her observations doesn't imply non-local "cause" since what happens in Bob's experiment could/would happened even if Alice had not made her observation.

If an experiment refuting 3 is known then I would concede there is something "missing" from the story.

Well a refutation of 3 would violate no-signalling. However the fact that the story is incomplete does not hinge on doing something known to be impossible. Certainly Bob cannot tell whether Alice has made the measurement. But just saying that two probability distributions are the same is not a realist story. You need to account for the correlations, not Bob's inability to tell what Alice is up to.

 

[Paul Colby]

However the fact that the story is incomplete does not hinge on doing something known to be impossible.

The claim is that Alice's knowledge of the statistical outcome of any potential Bob measurement is somehow impossible. Well, it's a fact, as we all agree, so by impossible is meant; impossible to understand within a given world view. This is enough for me to chuck said world view and form a new one in which things make sense. I was trained as an experimentalist so maybe I can be forgiven. I certainly can't provide a classical picture of the correlations and suspect one is impossible.

That said I still think there are things of interest I've learned. In thinking about this it's critical to remember that the inevitable interaction between measurement device and the system being measured is very much not the source of Bell non-locality at least within the current theoretical framework. The EPR style experiments show this in spades. Bob's measurements are local on his part of the system, yet, because Alice gained knowledge of Bob's particle state before he measured it, she has knowledge of the statistical outcome of Bob's measurement. If people claim that the randomness of quantum measurement is somehow a forceful collapse or random disturbance cause by the measurement device, I would claim this clearly shows them to be wrong.

 

[Derek Potter]

The claim is that Alice's knowledge of the statistical outcome of any potential Bob measurement is somehow impossible. Well, it's a fact, as we all agree, so by impossible is meant; impossible to understand within a given world view. This is enough for me to chuck said world view and form a new one in which things make sense. I was trained as an experimentalist so maybe I can be forgiven. I certainly can't provide a classical picture of the correlations and suspect one is impossible.

It's impossible under the Bell criteria: reality, locality, causality, definiteness.

That said I still think there are things of interest I've learned. In thinking about this it's critical to remember that the inevitable interaction between measurement device and the system being measured is very much not the source of Bell non-locality at least within the current theoretical framework. The EPR style experiments show this in spades. Bob's measurements are local on his part of the system, yet, because Alice gained knowledge of Bob's particle state before he measured it, she has knowledge of the statistical outcome of Bob's measurement. If people claim that the randomness of quantum measurement is somehow a forceful collapse or random disturbance cause by the measurement device, I would claim this clearly shows them to be wrong.

Absolutely. Alice cannot cause anything to happen to Bob's particle, his detectors or himself. That's the entire point. Her choice changes the joint observed probabilities: it does not change the physical state (whether quantum or anything else) at Bob. Since local causal reality is sacrosanct to realists it must be definiteness that has to go. EPR is easily explained, locally, causally, realistically as long as one accepts that observations are not definite - Alice and her electron together are in a state which is entangled with Bob and his electron, and remain so indefinitely, or at least until Charles collates the observations..

Ho hum, I'm sure that will draw some heated denials but as Sherlock Holmes says "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".

 

[Paul Colby]

Since local causal reality is sacrosanct to realists it must be definiteness that has to go.

So, I'm left wondering why is realism so sacrosanct? Sounds like people beating the aether drum. I guess I'll have to put up with the "unrealistic" label.

 

[Derek Potter]

So, I'm left wondering why is realism so sacrosanct? Sounds like people beating the aether drum. I guess I'll have to put up with the "unrealistic" label.

You won't be alone. But don't forget, the Bell criteria apply to any proposed explanation of BI violation, which is a real phenomenon. So anti-realism is to say that there is no explanation possible whether quantum or not. Now that is a world-view, I'll grant you. And therefore frowned upon in PF.

(Yes I know each term here needs to be defined carefully but I think we've settled on the meanings by now.)

 

[Paul Colby]

So anti-realism is to say that there is no explanation possible whether quantum or not.

I guess I view the QM as an explanation. Is this disallowed?

 

[Derek Potter]

I guess I view the QM as an explanation. Is this disallowed?

I'm getting tired of writing out "realist, local, causal, definite explanation of Bell inequality violation" and variations of the same. I propose the convention that all such be designated "Z".

We have just spent several days establishing that Z is impossible. If Z is impossible then to view QM as Z is disallowed by logic. The fact that QM asserts non-locality does not make it Z, it means that it is non-Z.

[sorry, edited a bit since you read it]