Is this Quantum non-locality set-up right?

  • #1
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Main Question or Discussion Point

I'm not sure if this set-up has been put forth already but I'm curious what people think of it.
It is about ascertaining the correlation of the polarization state of a pair of photons(A and B) by two observers considered inertial (x and y)that await at the same sufficient distance at opposite sides from an experimenter that sends two entangled photons in opposite directions. Each observer upon measuring the polarization state of each of the respective photons A and B automatically sends the result to the other observer electromagnetically. So for each observer x and y we consider two events timelike separated: measurement of one of the photons and reception of the info of the measurement of the other respective photon. Observers x and y can be considered spacelike separated regarding the physical process of correlation of this particular pair of photons they must verify and with respect to wich they act as observers. Each observer verifies this physical process with a pair of timelike events that is temporally reversed with respect to photons A and B, right?
Also these events observed by each observer are inside the future lightcone of the event of the experimenter sending the photons.
 

Answers and Replies

  • #2
Nugatory
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IEach observer verifies this physical process with a pair of timelike events that is temporally reversed with respect to photons A and B, right?
I don't understand this part. What do you mean by "temporally reversed"?

Experiments in which the two detection events are spacelike-separated have been done.
 
  • #3
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I don't understand this part. What do you mean by "temporally reversed"?

Experiments in which the two detection events are spacelike-separated have been done.
I mean for observer x the timelike separated events are measurement of polarization of photon A and looking at the measurement of photon B that has reached him from observer y . And for observer y the measurement of polarization of photon B and looking at the result from A sent by observer x. For the same correlation process ascertainment the events involving photons A and B are time reversed between x and y, aren't they?
 
  • #4
Nugatory
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For the same correlation process ascertainment the events involving photons A and B are time reversed between x and y, aren't they?
There are four events: x measures A, y measures B, x receives signal from y, and y receives signal from x. As long as the two observers are at rest relative to one another, they will agree about the ordering of all four events, so there's no "time reversal" going on. It is true that some observers moving rapidly relative to the experimental setup will consider that the measurement of A happened first while others will consider that the measurement of B happened first. This creates no contradictions, as the predicted correlation is the same either way.

But I'm still not sure what you're asking here. Experiments equivalent to the one you're describing have been done many times, so if you're asking
I'm not sure if this set-up has been put forth already
then the answer is "yes".
 
  • #5
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There are four events: x measures A, y measures B, x receives signal from y, and y receives signal from x. As long as the two observers are at rest relative to one another, they will agree about the ordering of all four events, so there's no "time reversal" going on. It is true that some observers moving rapidly relative to the experimental setup will consider that the measurement of A happened first while others will consider that the measurement of B happened first. This creates no contradictions, as the predicted correlation is the same either way.
No, no, you are apparently not understanding the set-up (my fault no doubt), the observers x and y are spacelike separated so we are considering the events measurement of A and measurement of B as simultaneous just as both receiving signals events. Therefore there is no question about measurement of A or of B being first. so we only need to be concerned with the ordering of the 2 events observed by x on one side with respect to those observed by y on the other location.
 
  • #6
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Therefore there is no question about measurement of A or of B being first. so we only need to be concerned with the ordering of the 2 events observed by x on one side with respect to those observed by y on the other location.
If they are at rest relative to one another they will agree about the ordering of all four events.
 
  • #7
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If they are at rest relative to one another they will agree about the ordering of all four events.
How could that be checked, if they are spacelike separated? There is a frame where two of the events are simultaneous with the other two. For the analysis of the correlation between the entangled pair of photons, that is the frame I'm considering.
 
  • #8
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And for the purpose of the set-up it is of little importance whether the observers can be made to agree about the ordering of the 4 events or not. I'm only interested in the frame of observer x on one hand and in the frame of observer y on the other with respect to the physical process results of the correlation of the pair of entangled photons. For their conclusion in each of their frames the other observer means nothing, it is just that to reach their results the ordering of the events regarding photons A and B is the opposite. For one observer is measurement of A and news of measurement of B and for the other measurement of B and news of measurement of A
 
  • #9
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How could that be checked, if they are spacelike separated?
You cannot measure that, simultaneity is something you calculate, not observe. You can verify that the events were spacelike separated, and you can let synchronized clocks run to check the times afterwards.

What you describe is an easy experiment that has been done long ago, but it is not sufficient to show nonlocality in your favorite nonlocal interpretation, because it does not rule out hidden variables unless you go to Bell-like measurements.
 
  • #10
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You cannot measure that, simultaneity is something you calculate, not observe. You can verify that the events were spacelike separated, and you can let synchronized clocks run to check the times afterwards.
Sure. See my post above(#8).
What you describe is an easy experiment that has been done long ago, but it is not sufficient to show nonlocality in your favorite nonlocal interpretation, because it does not rule out hidden variables unless you go to Bell-like measurements.
Well, it is not supposed to be described as a technically detailed perfect Aspect-like experiment. I was hoping that we could abstract the details to do that like many "in principle" examples of entanglement experiments whre it is assumed we can exclude local hidden variables.
 
  • #11
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So, now I'm reasonably convinced the setup is not flawed in any essential way, and even that the same or equivalent experiments have been already done in the past as commented by other posters, I'll go on and try to analyze it from a maybe novel angle.
We can assume that since x and y are at rest with respect to the earth they are at rest from one another without loss of generality for the purpose of the correlation of the entangled pair analysis.
We have the correlation analysis of an entangled pair of photons from the inertial frame of x that is done from the information about photons A and B in a certain timelike causal order and the correlation analysis of the same entangled pair from the inertial frame of y constructed from the information about photons A and B in a timelike causal order inverse from frame x(in y the correlation is built from info about photon B first and photon A second).
As any other inertial frames, x and y frames should be able to be transformed from one another with a proper orthochronous Lorentz transformation, but it seems that regarding the pair of photons correlation there isn't a transformation preserving the events(information on photon A and photon B) timelike order between frames x and y, is this correct?
 
  • #12
DrChinese
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What is your ultimate question about the setup? I'm not sure I picked that out. The temporal ordering doesn't really affect the statistical results, and there is no specific causal order that can be demonstrated to exist definitively.
 
  • #13
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What is your ultimate question about the setup? I'm not sure I picked that out. The temporal ordering doesn't really affect the statistical results
Absolutely, this is not the typical post about Bell violations trying to find some loophole or some local hidden variable(it there aren't), nor trying to dig up some ftl information sending possibility, it assumes no violation of causality(local microcausality as understood in QFT is preserved).
and there is no specific causal order that can be demonstrated to exist definitively.
You are right again.I'm not trying to demonstrate this either

My intention and my questions go in the direction of showing in formal terms how QM collapse appears in QFT. The backdrop is the claim by Penrose(expressed among other places in "The new emperor's mind and "The road to reality") and others that EPR effects are morally in conflict with the equivalence of inertial frames(principle of relativity).
 
  • #14
entropy1
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My intention and my questions go in the direction of showing in formal terms how QM collapse appears in QFT. The backdrop is the claim by Penrose(expressed among other places in "The new emperor's mind and "The road to reality") and others that EPR effects are morally in conflict with the equivalence of inertial frames(principle of relativity).
I'm not sure what you mean to ask. Are you wondering which of the two measurements gets to collapse first?
 
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  • #15
morrobay
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How could that be checked, if they are spacelike separated? There is a frame where two of the events are simultaneous with the other two. For the analysis of the correlation between the entangled pair of photons, that is the frame I'm considering.
They can be space like separated in the same frame : That is if the time between photon A measurement (first) and photon B measurement (second) is less than the time for a light signal from A to B.
Correct ?

Edit: Why dont you draw a spacetime diagram to convey your question ?
 
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  • #16
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I'm not sure what you mean to ask. Are you wondering which of the two measurements gets to collapse first?
No, that is frame dependent in this setup so it is not a meaningful question.
I'm jus showing the result(inverse order of timelike correlation events for each observer) that when considering these observer's frames in regards to how they describe the same entanglement experiment correlations, their Lorentz transformations doesn't seem to respect the connection to the identity, it formally appears as a Lorentz violation as understood in QFT. How should this be interpreted? Can it be trivialized so that it doesn't mean anything? How exactly?
 
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  • #17
entropy1
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their Lorentz transformations doesn't seem to respect the connection to the identity, it formally appears as a Lorentz violation as understood in QFT.
Maybe it would help if you have a source or an example...
 
  • #18
Nugatory
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Iit formally appears as a Lorentz violation as understood in QFT.
There's no violation of Lorentz invariance here. Observer x performs a measurement on A, sends a message to observer y, and receives a message from y, in that order. Observer y performs a measurement on B, sends a message to observer x, and receives a message from x, in that order. These are the facts, and they remain invariant under Lorentz transformations.

Entanglement is a complete red herring here.
 
  • #19
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There's no violation of Lorentz invariance here. Observer x performs a measurement on A, sends a message to observer y, and receives a message from y, in that order. Observer y performs a measurement on B, sends a message to observer x, and receives a message from x, in that order. These are the facts, and they remain invariant under Lorentz transformations.

Entanglement is a complete red herring here.
I'm obviously only considering the events that are correlated for each observer in a way that canot be explained by any local hidden variable theory and refer to the same quantum object, the entangled pair. Any other events have no bearing here, and it is only the statistical correlations belonging to the same quantum process that is relevant here so without entanglement experiment there is nothing to consider here. Meaning there is no correlated causal(local microcausal in QFT terms) process to analyze from two inertial frames then.
 
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  • #20
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Maybe it would help if you have a source or an example...
To maintain the time-order preserving Lorentz invariance you need timelike connected events referring to the same process to have the same temporal order. In the experiment the quantum correlations are obtained in the two inertial frames in different temporal order with respect to the information about polarizations of photons A and B .
 
  • #21
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To maintain the time-order preserving Lorentz invariance you need timelike connected events referring to the same process to have the same temporal order. In the experiment the quantum correlations are obtained in the two inertial frames in different temporal order with respect to the information about polarizations of photons A and B
If that were what Lorentz invariance meant, it would violated every time that my wife and I look out different windows of our house at the same time and then discover that our observations of the weather conditions are correlated.

As it is clear that this thread is based on a misunderstanding, it can be closed.
 

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