# I Question about an entanglement paper

#### Buzz Bloom

Gold Member
There is no universal clock because there is no frame-invariant definition of what "instant" means. That is an essential part of relativity regardless of if you have FTL or not.
Hi Dale:

I am not sure I am understanding this discussion, but I would like to ask a question.

Assumptions:
Imagine a train of carriages, each with a technologically identical clock. Imagine this train is moving at a constant speed on a circular track. Assume the track itself is stationary. Imagine in the exact center of the track is another same kind of clock. The center clock periodically broadcasts a signal identifying the current time on the center's clock. When each carriage clock receives the broadcast, it makes an immediate acknowledgement response. The center clock checks that all of the acknowledgement returned signals arrive simultaneously, and also notes the time it took for the round trip of signals. The center clock includes with the time signal a message (1) confirming all of the round-trip times of the carriage returned signals are identical, and (2) that the message also communicates the time(on the center clock) the signal was sent, and (3) the amount of time the round trip signal took. Each of the carriage clocks for itself confirms for each received message, that the time on the carriage clock is exactly equal to the time given in message from the center clock plus one-half of he round trip message travel time.

Question:
Is it correct, or is it not-correct, to say that all of he clocks involved are synchronized?

Regards,
Buzz

#### Dale

Mentor
Is it correct, or is it not-correct, to say that all of he clocks involved are synchronized?
In which frame?

#### Buzz Bloom

Gold Member
In which frame?
Hi Dale:

I apologize for posting my question in this thread. I thought (senior moment) I was in the thread
I also just checked, and that thread was closed to further posts, so maybe I am fortunate to have posted in this thread after all, since you were kind enough to respond.

I am puzzled by the concept of "frame", but if I am required to specify a frame, it would be the frame of the stationary central clock.

Regards,
Buzz

#### PeterDonis

Mentor
Is it correct, or is it not-correct, to say that all of he clocks involved are synchronized?
It depends on what you mean by "synchronized"--or, to put it another way, it depends on your choice of simultaneity convention, i.e., your choice of which events happen at the same time. There is no physical fact of the matter about that; it's a choice we humans make for convenience in modeling.

#### Dale

Mentor
I am puzzled by the concept of "frame", but if I am required to specify a frame, it would be the frame of the stationary central clock.
In that frame all of the carriage clocks would be synchronized. They would not remain synchronized with the central clock except momentarily.

#### DanielMB

I am re-reading paper(s) by Huw Price and Ken Wharton that present an interesting explanation of entanglement. I will see if I can paste a link, otherwise one can find them on arXiv: one is for example, “Disentangling the Quantum World”. The idea is that there could be retro-casual communication between the particles, an idea first suggested by Costa de Beauregard in the late 1940s (a student of de Broglie!). Basically, a quantum particle could “know” in advance what measurement will be made in its future.

They claim that explanation is compatible with the prohibition of faster-than-light communication in Special Relativity. My question is very specifically limited to that claim. Wouldn’t such a retro-casual explanation require super-luminal retro-communication? If a particle such as a photon knows in advance what type of measurement it will eventually encounter, then that seems to mean that information had to travel back in time faster-than-light. Or else that information had to start the trip even before the photon came into existence? It does not seem to me that their explanation is compatible with Special Relativity.
I've found this : https://arxiv.org/abs/1508.01140

#### kurt101

In whatever this interpretation is that you are discussing, do the results of spacelike separated measurements depend on the order in which they are made? If they don't, the interpretation does not violate relativity (more precisely, it violates QFT). If they do, it does.
Is that a typo? Did you mean to say (more precisely, it does not violate QFT)? If not, I am very confused.

I have said no such thing. You are not reading carefully. See above.
In response to my statement "measuring one entangled particle instantly affects the state of the other entangled particle" you said "is simply wrong". How is my statement not a description of Bell's non-local action? Is it because I added the word "instantly"? Because that is the only difference I can see between the two.

The reason I use the term "instant" is because in experiments like the Aspect's 1983 EPR experiment, the measurement device is modified right before the entangled photon is measured, yet the non-local action is still present in the statistical results. I realize this is with entangled photons, not entangled particles, but that is the experiment that I have in my head when I am using the term instant and invoke terms like universal clock.

Know what? That the results of spacelike separated measurements cannot depend on the order in which they are made? That is a requirement of QFT, which has extensive experimental confirmation.
I know that the order of measurement does not affect the statistical result of QFT or the actual experiments. In fact that is what I was trying to convey earlier. However this is not the same as saying that the order of a measurement affects the result for a single pair of entangled photons. So when you say it is a requirement of QFT, are you referring to the statistical result or the result of a single pair?

#### PeterDonis

Mentor
Is that a typo? Did you mean to say (more precisely, it does not violate QFT)?
Oops, yes, it was a typo, I meant "does not violate QFT". I have fixed the original post.

How is my statement not a description of Bell's non-local action? Is it because I added the word "instantly"?
No, it's because your statement requires each individual entangled particle to have a state before measurement, and that is false. I have repeatedly explained why.

when you say it is a requirement of QFT, are you referring to the statistical result or the result of a single pair?
Both. QFT requires field operators at spacelike separated events to commute. That means single pairs must commute, and it also implies that the statistical results will be independent of ordering.

#### kurt101

Also, if the order mattered you could send an FTL message using entanglement. Any proposed scheme for this is clearly testable.
I think the order matters to the interpretation because it preserves causality, but unless you can some how predict the state of the entangled particle before measuring it, I don't see how you could use this knowledge to send a message FTL.

#### PeterDonis

Mentor
I think the order matters to the interpretation because it preserves causality
Not in QFT; in QFT the ordering of spacelike separated measurements is irrelevant to causality. That's what the condition that spacelike separated measurements must commute means.

#### kurt101

Oops, yes, it was a typo, I meant "does not violate QFT". I have fixed the original post.
The interpretation of non-local action that I am using as applied to the Alain Aspect 1983 experiment is that measuring one of the entangled photons affects the other. For this interpretation, statistically the order does not matter and for all practical purposes you would not be able to detect this order. To the best of my knowledge QFT applies to the practical things we can actually measure. So based on the test you outlined, I would not rule out this non-local interpretation of the Aspect experiment as a possibility.

Both. QFT requires field operators at spacelike separated events to commute. That means single pairs must commute, and it also implies that the statistical results will be independent of ordering.
Single pairs must commute for the math to work out, but I am thinking that this is different than the actual mechanism having to commute. If the actual mechanism does not commute, but the state it acts upon is random, you would get the same statistical result.

#### PeterDonis

Mentor
Single pairs must commute for the math to work out, but I am thinking that this is different than the actual mechanism having to commute.
In QFT, the field operators are the "actual mechanism".

#### PeterDonis

Mentor
The interpretation of non-local action that I am using
Are you trying to describe the interpretation used in the paper described in the OP? Or is this an interpretation that appears in some other reference? Or is it just something you made up?

#### PAllen

I think the order matters to the interpretation because it preserves causality, but unless you can some how predict the state of the entangled particle before measuring it, I don't see how you could use this knowledge to send a message FTL.
If the order matters, you have causality outside of the light cone. It is precisely the commuting property that guarantees there is no ftl causality, because you can’t distinguish whether A caused B or B caused A.

#### Dale

Mentor
I think the order matters
The order can’t matter because the order is not the same in all frames.

#### Buzz Bloom

Gold Member
They would not remain synchronized with the central clock except momentarily.
Hi Dale:

I think I may be understanding this. Each carriage experiences a radial acceleration away from the center. This acceleration changes the relative time rate in each carriage as, compared with the time rate at the center. Is this correct? Is it also correct that the the rate of time is the same in all of the carriages? If I am correct about these guesses, I would appreiate seeing an equation that compares the rate of time differences between the carriages and the center point in terems of the accelleation and/or the radius and the tangential velocity. I tried (and failed) to find this by an online search, was probably chose the search keys badly.

Regards,
Buzz

#### Ibix

This acceleration changes the relative time rate in each carriage as, compared with the time rate at the center. Is this correct?
No. It's just that the circling clocks are moving at constant speed $v$ compared to the central clock, so have the usual time dilation factor of $1/\sqrt{1-v^2/c^2}$. GPS satellite clocks tick at a very slightly modified rate to account for a (slightly more complex because of gravity) version of this.

#### kurt101

Are you trying to describe the interpretation used in the paper described in the OP? Or is this an interpretation that appears in some other reference? Or is it just something you made up?
No, nothing from the paper, but what I thought Bell meant by non-local action as applied to an actual experiment.

#### PeterDonis

Mentor
what I thought Bell meant by non-local action as applied to an actual experiment.
Please give a reference for whatever Bell paper you are basing this on.

#### kurt101

The order can’t matter because the order is not the same in all frames.
If the order matters, you have causality outside of the light cone. It is precisely the commuting property that guarantees there is no ftl causality, because you can’t distinguish whether A caused B or B caused A.
The order matters to the *interpretation* of non-local action; the interpretation is not something we can directly measure, but only something we can infer from statistics and the experiments.

With the non-local action interpretation there is no way to directly distinguish whether A caused B or B caused A and so your comment about causality outside of the light cone does not apply.

Take the EPR experiment done by Alain Aspect in 1983. Call the entangled photons A and B. You can only measure either A or B first. Say you decide to always measure A first. At the end of the experiment, after many iterations, you still get the correlation that A's polarizer had an effect on B. You get that correlation even if you change the orientation of A's polarizer so it changes faster than any signal that could be transmitted at the speed of light. You can also say that B's polarizer had an effect on A, but this would violate causality, because you measured B after you measured A. You might say there were some observers that did not see it that way, and to that comment I would say steer B back around to where A was measured by using gravity, so that there is no question to any observer that B was measured second.

So if you are in the non-local action camp and subscribe to causality, based on this experiment it would be logical to conclude that there is a definite order even though you can not directly observe it.

#### PeterDonis

Mentor
The order matters to the *interpretation* of non-local action

With the non-local action interpretation there is no way to directly distinguish whether A caused B or B caused A
"No way to directly distinguish whether A caused B or B caused A" sounds like "order doesn't matter".

You can only measure either A or B first.
No, you set up the measurements to be spacelike separated so there is no frame-independent fact of the matter about which one you measure first.

Say you decide to always measure A first.
As the experiment was set up, there was no attempt to control the exact measurement times in this way. The only thing that was controlled was that the measurements were spacelike separated (meaning that the time windows for the measurements had to be small compared to the light travel time over the path lengths involved).

So I assume you are proposing an alternate version of the experiment, where the ordering in some particular chosen frame (the "lab" frame) is controlled explicitly?

f you are in the non-local action camp and subscribe to causality, based on this experiment it would be logical to conclude that there is a definite order even though you can not directly observe it.
I don't see how, since you could just as easily do the experiment enforcing the opposite order, and the results would be indistinguishable. In other words, experimentally the order doesn't matter. And relativity explains why: because the order is not invariant, and only invariants can affect the actual physics.

#### kurt101

Please give a reference for whatever Bell paper you are basing this on.
Bell's paper Bertlemann's socks: https://hal.archives-ouvertes.fr/jpa-00220688/document
The third explanation:
John Bell said:
Thirdly, it may be that we have to admit that causal influences
- do go faster than light. The role of Lorentz invariance in the completed theory would then be very problematic. An "ether" would be the cheapest solution /22/. But the unobservability of this ether would be disturbing. So would the impossibility of "messages' faster than light,
which follows from ordinary relativistic quantum mechanics in so far as
it is unambiguous and adequate for procedures we can actually perform.
The exact elucidation of concepts like 'message' and 'we' would be a
formidable challenge.

#### kurt101

So I assume you are proposing an alternate version of the experiment, where the ordering in some particular chosen frame (the "lab" frame) is controlled explicitly?
Yes, it is a slightly modified version of the experiment.

I don't see how, since you could just as easily do the experiment enforcing the opposite order, and the results would be indistinguishable. In other words, experimentally the order doesn't matter. And relativity explains why: because the order is not invariant, and only invariants can affect the actual physics.
Exactly, the results would be indistinguishable! This is what I have been saying all along and is why I call it an interpretation.

#### Dale

Mentor
The order matters to the *interpretation* of non-local action
I am OK with that. The interpretation can change from frame to frame.

the interpretation is not something we can directly measure, but only something we can infer from statistics and the experiments
Not really. Interpretations are simply assumed for convenience, aesthetics, philosophical, or personal reasons. I am not really interested in interpretations.

#### PeterDonis

Mentor
the results would be indistinguishable! This is what I have been saying all along and is why I call it an interpretation
I don't see the point of an interpretation that tries to distinguish indistinguishable results.

Bell's paper Bertlemann's socks: https://hal.archives-ouvertes.fr/jpa-00220688/document
The third explanation:
This explanation does not say that spacelike separated events that are causally connected have to have a definite order. It just says that spacelike separated events can be causally connected. Note that the word "order" appears nowhere in Bell's description of this interpretation.

Bell does propose an "ether" (which would seem to suggest some sort of preferred frame and therefore a preferred simultaneity convention) as one possible way of implementing this solution, but it's not the only possible one (and Bell does not claim it is--he only says ether would be the "cheapest" solution). Another would simply be to develop a concept of "causality" that did not require one to distinguish between the "cause" and "effect", at least not in the sense that one has to come before the other. Which is still a challenge, but as Bell says, any explanation you pick is going to be a challenge.

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