Does Locality in Quantum Mechanics Exclude Retrocausality?

[entropy1]

Does the definition of locality in the QM sense include the prohibition of retrocausality?

 

[Vanadium 50]

Given that this is B-level, I have to ask if you know what those words mean. For example, how do you express locality mathematically? And if you don't, how can we provide a B-level answer?

 

[entropy1]

Given that this is B-level, I have to ask if you know what those words mean. For example, how do you express locality mathematically? And if you don't, how can we provide a B-level answer?

Can you give a yes/no answer?

 

[Vanadium 50]

No I can't because I can't figure out what you actually mean by your question.

 

[entropy1]

I looked up 'quantum mechanics locality', and did a few similar searches, but I can't find a definition of QM locality, so I don't know how to describe or formulate it. I mean the locality as it is ment by the Bell-inequality. If you need an I-thread I will ask to make this thread one.

 

[Nugatory]

Does the definition of locality in the QM sense include the prohibition of retrocausality?

In any proper journal article the authors will define what they mean by words of this sort, precisely because there is no single universally understood definition. Unless and until you do that, the answer to this question is going to be some variant of "It depends".

 

[entropy1]

Ok, suppose an event E lies in the same lightcone as an event C, that occurs earlier in time. Then C can causally affect E, as long as E lies in the same lightcone as C. Suppose that we call this "C locally causes E". Now my question is: if we consider retrocausality to be theoretically possible, then, if E would retrocause an event R that lies in the same lightcone as E, but earlier in time, would this be called "local"?

Sorry if I'm a bit fuzzy in my formulation.

 

[Demystifier]

Does the definition of locality in the QM sense include the prohibition of retrocausality?

The proof of the Bell theorem contains an assumption that there is no retrocausality. The transactional interpretation of QM assumes that it is precisely retrocausality that resolves the associated quantum puzzles.

 

[PeterDonis]

Sorry if I'm a bit fuzzy in my formulation.

You apparently haven't grasped the fact that it is the fuzziness that makes it impossible to give a definite answer to your question. That's not going to change no matter how many times you rephrase it.

 

[entropy1]

You apparently haven't grasped the fact that it is the fuzziness that makes it impossible to give a definite answer to your question. That's not going to change no matter how many times you rephrase it.

Well, maybe if I put it this way:
Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context? It is often explained as: "Spooky action at a distance", but I don't know what is ment by that.

 

[Demystifier]

Well, maybe if I put it this way:
Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context? It is often explained as: "Spooky action at a distance", but I don't know what is ment by that.

For that purpose you need to read an actual proof of the Bell theorem, because such a proof contains a precise formal definition of locality.

 

[PeterDonis]

Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context?

Have you read Bell's paper? He gives an explicit definition of his concept of "locality".

 

[entropy1]

Ok, I will check that out.

 

[entropy1]

I can only find this definition:

that the result of a measurement on one system be unaffected by operations on a distant system with which it has interacted in the past

Is that what you mean? Or need I do the math?

 

[PeterDonis]

I can only find this definition

Then you obviously aren't looking at Bell's actual paper. Please do so. Our own @DrChinese has provided links to key papers on this topic:

http://www.drchinese.com/David/EPR_Bell_Aspect.htm

 

[entropy1]

@PeterDonis I used this text, which is searchable. I think it is the same one. I searched for "local" and got four matches.

 

[Demystifier]

@PeterDonis I used this text, which is searchable. I think it is the same one. I searched for "local" and got four matches.

In this paper, the locality assumption is "the vital assumption" after Eq. (1).

 

[entropy1]

Ok. I tried to read through this paper at least two times in the past, but I find the math pretty complicated. I will give it another try.

 

[Demystifier]

For a simpler math try the Hardy proof of nonlocality, simplified in my http://lanl.arxiv.org/abs/quant-ph/0609163 page 17.

 

[stevendaryl]

It seems that Bell's separability condition is logically something beyond locality. Without even mentioning locality, there is an assumption along the lines of:

If $A$ is correlated with $B$, then it must be the case that one of the following is true:

1. $A$ influences $B$
2. $B$ influences $A$
3. There is some common cause $C$ that influences both.

If you make this assumption (@Demystifier probably knows the philosophical term for it), then it follows that quantum mechanics has FTL influences. Conversely (or contrapositively, maybe), if you assume that there are no FTL influences, then it follows that quantum mechanics violates the above principle/assumption.

So what is the status of that assumption? It doesn't seem logically necessary, but it seems that an assumption along those lines is behind every scientific investigation. The fact that two things are correlated is considered grounds to investigate would could cause the correlation.

An example: If you find two radios, you turn them on, and you find out that they are playing the same sequence of songs and news announcements, then you assume that either they are playing a recording, and the two have copies of the same recording, or else there is a radio station broadcasting to both of them. You assume there is a common cause for the correlation.

But there is nothing logically inconsistent about assuming that the radios are just emitting random noise, and one of the laws of the universe is that the random noise produced on one radio is always the same as that produced on the other. Entanglement in quantum mechanics without FTL influences seems a lot like this possibility. You have distant particles, and certain measurements on them are correlated, but there is no common cause to the measurement outcomes.

 

[Demystifier]

If $A$ is correlated with $B$, then it must be the case that one of the following is true:

1. $A$ influences $B$
2. $B$ influences $A$
3. There is some common cause $C$ that influences both.

If you make this assumption (@Demystifier probably knows the philosophical term for it),

It's called the Reichenbach's common cause principle.

 

[Demystifier]

But there is nothing logically inconsistent about assuming that the radios are just emitting random noise, and one of the laws of the universe is that the random noise produced on one radio is always the same as that produced on the other.

But if there was such a law, I don't see how can see such a law be considered a local law.

For that reason, some defenders of local interpretation of QM go a step further, by denying the existence of correlation before the observation of correlation. For instance, if Alice measures spin of one particle in New York and, at the same time, Bob measures spin of the other particle in London, there is no correlation until someone (say Charlie) looks at both measurement results.

 

[stevendaryl]

For that treason, some defenders of local interpretation of QM go a step further, by denying the existence of correlation before the observation of correlation. For instance, if Alice measures spin of one particle in New york and, at the same time, Bob measures spin of the other particle in London, there is no correlation until someone (say Charlie) looks at both measurement results.

What is the punishment for treason against the scientific method?

 

[stevendaryl]

But if there was such a law, I don't see how can see such a law be considered a local law.

Yes, I agree. But it's not logically necessary to posit a FTL mechanism for the correlations. You could just say that that's the way the universe works.

 

[martinbn]

Yes, I agree. But it's not logically necessary to posit a FTL mechanism for the correlations. You could just say that that's the way the universe works.

To add to this, I think that if you propose a FTL mechanism, you need to show, at least in principle, a scenario where there can be FTL transmission.

 

[stevendaryl]

To add to this, I think that if you propose a FTL mechanism, you need to show, at least in principle, a scenario where there can be FTL transmission.

I don't see why that's necessary, either. If you have some variables which have FTL influences, I don't see that it's necessary that those variables be settable (which is what it would take to use them for transmission of signals).

 

[martinbn]

I don't see why that's necessary, either. If you have some variables which have FTL influences, I don't see that it's necessary that those variables be settable (which is what it would take to use them for transmission of signals).

It is not logically necessary, I agree. But if you propose FTL you are proposing a lot, which goes against what is known so far, and I think that it is scientifically necessary to set up an experiment, even if it is a thought experiment.

 

[Demystifier]

Yes, I agree. But it's not logically necessary to posit a FTL mechanism for the correlations. You could just say that that's the way the universe works.

Fine but then, by the same token, one can say that Bohmian particle trajectories are what they are, without a FTL mechanism. My point is that Bohmian mechanics is not more non-local than such a more conventional interpretation.

 

[martinbn]

Fine but then, by the same token, one can say that Bohmian particle trajectories are what they are, without a FTL mechanism. My point is that Bohmian mechanics is not more non-local than such a more conventional interpretation.

True, but is it more realistic? The particles have trajectories, but what about spin? My understanding is that in BM spin is not real i.e. no definite values at all times.

 

[Demystifier]

True, but is it more realistic? The particles have trajectories, but what about spin? My understanding is that in BM spin is not real i.e. no definite values at all times.

Yes, but there is a good physical reason to think of positions as more real than spins. Spin is measured by the Stern-Gerlach apparatus, and if you think a bit how it works, you will see that the only thing that is directly observed by this apparatus is a position of something.

Anyway, it is not essential that everything observed must be real. For instance beauty is not real, it is only in the eyes of the beholder. What is essential for a non-solipsistic view of the world is that something is real, and that this something can explain all the observations.