# Non-locality: (FTL) 'influence' at all?

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## Main Question or Discussion Point

I have a basic question about the concept of non-locality in QM. I'll explain as I understand it (correct me if I'm missing the essence):

The so called 'non-local' property of two entangled photons (for instance), suggests that the measurement of one of the pair 'communicates' with the other, possibly FTL. However, since in the case of photons that are space-like separated, in accordance with special relativity, cannot communicate FTL, there arises a paradox.

However, is there even a causal relation between the two photons if they are space-like separated, and can we even speak of 'influences' (FTL or not)? Are we just seeing FTL because we are not equiped to see the nature of the correlation between the particles? I know this may appear too philosophical, apologies for that. I don't mean to. It is just that this problem makes more sense to me if I'm taking a informational approach (which I don't discuss here).

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There is no way to send superluminal signals because only a fraction of the pairs display the correlations. One can show that the net probability for Alice to measure a particular result is independent of what measurement Bob makes and vice versa.

Are we just seeing FTL because we are not equiped to see the nature of the correlation between the particles?
Sure but any correlation must be non-local (where "locality" is intended in the standard sense: forward-causal, only one world...), so we're back where we started.

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Sure but any correlation must be non-local (where "locality" is intended in the standard sense: forward-causal, only one world...), so we're back where we started.
But the correlation only becomes apparent when the measurements are brought together in proximity (locality)? Without comparison (of the measurement results) there is no factual correlation established? How 'real' is the correlation if not observed?

But the correlation only becomes apparent when the measurements are brought together in proximity (locality)?
Yes.

Without comparison (of the measurement results) there is no factual correlation established?
There is in the theory: you expect the correlations to be there and when you do the comparison via classical signals you see them. So the theory is correct.

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There is in the theory: you expect the correlations to be there and when you do the comparison via classical signals you see them. So the theory is correct.
Does the theory have to take in account the measurement setup (perhaps experimenters included)? It seems to me these are always involved...

NOTE: I am not refering to consciousness collapse! (but rather decoherence involved with the measurement)

Does the theory have to take in account the measurement setup (maybe experimenters included)? It seems to me these are always involved...

NOTE: I am not refering to consciousness collapse! (but rather decoherence involved with the measurement)
In the Bell's inequalities no... It's just a simple property of entangled pairs. Of course actual tests are much more complicated and must rule out loopholes concerning the setup etc.

Gold Member
In the Bell's inequalities no.
Actually, I don't mean local properties of the photons themselves. I rather mean the locality aspect that seems to be necessary in the informational approach of the concept of correlation. To put it bluntly: can it be that the decoherence (in the measurement apparatus and all that occurs after the measurement readout) has to be (informational) consistent with the properties of the particles measured? That is, the particles only have a probability to have a specific value of their property until the information of both values is brought together? I could digress, but I don't think that is allowed. I hope my question is at least a little clear.

UPDATE: A different question would be: if Alice and Bob read their respective measurement results while they are still space-like separated, would they be in superposition of different possible values until they compared their results?

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The measurements locally record a definite outcome in, say, a memory unit for each particle, in principle faster than the experimental regions can become light-like separated (I think this was actually the case in some experiments). In short, it's all laid out on paper "long" before the comparison is made. Not sure if this is what you wanted to know.

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The measurements locally record a definite outcome in, say, a memory unit for each particle, in principle faster than the experimental regions can become light-like separated (I think this was actually the case in some experiments). In short, it's all laid out on paper "long" before the comparison is made. Not sure if this is what you wanted to know.
I think that is what I mean. To be sure: if Alice and Bob read their respective measurement results while they are still space-like separated, would they be in superposition of different possible values until they compared their results, or would they be in a definite state?

We're talking about real experiments so the only scenario where, say, the actual hard drives are in a superposition with different outcomes is Many Worlds.

entropy1
Nugatory
Mentor
That is, the particles only have a probability to have a specific value of their property until the information of both values is brought together?
That's basically a variant of Schrodinger's cat. We have a cat at each detector, and the detector is wired to release a lethal cloud of cyanide gas if the particle it detects is spin-up. We send one member of a spin-entangled pair to each detector - and now both cats are in a superposition of alive and dead until we get together and compare results. However, we never observe such superpositions, so that can't be what's going on.

entropy1
Gold Member
However, we never observe such superpositions, so that can't be what's going on.
Can we actually measure superpositions in practice in other cases?

Can we actually measure superpositions in practice in other cases?
Yes through interference phenomena.

entropy1
Gold Member
Yes through interference phenomena.
I am not sure if I understand that. Do you have an example?

I am not sure if I understand that. Do you have an example?
Double-slit or Mach-Zender interference can be predicted by placing the objects in path-superpositions.

This is a general slit calculation http://arxiv.org/abs/quant-ph/0703126

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I am not sure if I understand that. Do you have an example?
The buckyball is a fun example. But there's a precise way to tell what is interfering and what isn't: the race has been to get the biggest objects possible to show interference. A lay article: http://www.nature.com/news/2011/110405/full/news.2011.210.html