I Question about an entanglement paper

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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.
 
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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?
I am sure they are aware of that, so what was their justification in the paper?

It might be good to link to the specific paper and identify the paragraph in question.
 
I don’t see where they explicitly address the speed that would be required for their retro-causal communication. How about I cut and paste a few paragraphs from one of their papers? 1510.06712.pdf (5 Nov 2015)

A live alternative to quantum spooks

Bottom page 1, top page 2:

Hanson’s experiment does, as claimed, make a convincing case for closing
some of the best-known loopholes in the case for action-at-a-distance.
However, it is much further from settling the case against Einstein than
these responses suggest. There’s a large and promising loophole that simply
doesn’t show up on most commentators’ radar – or most physicists’ radar,
for that matter.3 When it gets noticed at all, it tends to get confused for
something else. So the fact that there’s still a viable alternative to actionat-
a-distance – arguably, a much more attractive alternative, and certainly
one that is untouched by the results from Hanson and his team – remains a
well-hidden secret.
This invisible loophole falls into a well-known category. The argument
for action-at-a-distance assumes that quantum particles don’t know what
measurements they are going to encounter in the future. A little more
technically, it assumes that the state of a particle before a measurement
is independent of the particular setting chosen for that measurement (the
choice whether to measure position or momentum, say). This sounds innocuous
enough. How could the particle know about that, before it reaches
the measurement device? But innocuous or not, it is crucial. Without this
independence assumption, the argument for action-at-a-distance just doesn’t
go through.
The invisible loophole rejects this independence assumption, but it is
confused for and obscured by another proposal for doing the same thing.
This better-known cousin is a well-recognised but deservedly unpopular little
loophole called superdeterminism. To see how superdeterminism proposes to
reject the independence assumption – and how there’s a much more attractive
way of doing the same thing – let’s take a detour via medieval theology.
(Superdeterminism has ancient ancestors.)

Bottom page 5:

As Costa de Beauregard also pointed out, an attractive feature of his
proposal is that because both arms of the zigzag lie in or on the lightcones,
it is immediately congenial to special relativity, in a way in which direct
action-at-a-distance is not. This also distinguishes retrocausality from other
explanations that hope to fill the gap between entangled particles, perhaps
with continuous processes travelling faster than light.

Top page 6:

Many commentators think of the tension between action-at-a-distance
and special relativity as the source of some of deepest puzzles about quantum
theory. As David Albert and Rivka Galchen put it, in a piece in Scien-
tific American: “Quantum mechanics has upended many an intuition, but
none deeper than [locality]. And this particular upending carries with it a
threat, as yet unresolved, to special relativity—a foundation stone of our
21st-century physics.”8 But if we keep our eyes on the fact the zigzag retrocausal
proposal avoids this tension completely, it is easy to see that it isn’t
just another form of spooky action-at-a-distance.
 

DarMM

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The retrocausal signals are confined to the past light cone of the emission event, thus they never traverse spacelike distances.
 
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because both arms of the zigzag lie in or on the lightcones,
it is immediately congenial to special relativity, in a way in which direct
action-at-a-distance is not. This also distinguishes retrocausality from other
explanations that hope to fill the gap between entangled particles, perhaps
with continuous processes travelling faster than light.
So it seems like they are still forbidding any causal connection outside of the light cone, but they are allowing effects to precede causes as long as they are still within the light cone. It is an unusual approach, but it does seem to respect the causal structure of SR. Sort of.
 
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they are allowing effects to precede causes as long as they are still within the light cone. It is an unusual approach, but it does seem to respect the causal structure of SR. Sort of.
There is actually no issue with SR as long as the "effect" and the "cause" commute--in other words, that what happens at each of those events does not depend on the order in which they occur. Since this is true for any pair of measurements on entangled particles in QM, there's no problem with having such a pair of measurements be timelike separated and having the one you call the "effect" precede the one you call the "cause".
 

DarMM

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So it seems like they are still forbidding any causal connection outside of the light cone, but they are allowing effects to precede causes as long as they are still within the light cone. It is an unusual approach, but it does seem to respect the causal structure of SR. Sort of.
Yeah retrocausality is one of the five classes of mechanical explanations of entanglement permitted by Bell's theorem. They're all pretty odd.
 
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Yeah retrocausality is one of the five classes of mechanical explanations of entanglement permitted by Bell's theorem. They're all pretty odd.
What are the other four?
 
Actually, I don’t think I understand reverse time light cones. I guess it has to be like a movie going exactly in reverse, one frame at a time, back down the world lines to the apex of the forward light cone from the emission event. Does the retro-signal have to get to the emission event before the emission? Yes, since “we” cannot actually go back in time, the detection event boundary conditions from each branch are “already in” the other’s branch, and that is super-luminal. Or if the detection boundary condition message does not start until the first detection event, and then travels back to the emission and forward along the other branch, that is also super-luminal.

It seems that retro-time boundary condition information movement requires super-luminal movement regardless of how one imagines it happens. The only way I can see where it might not is if both of the final detection boundary conditions start their journeys long before even the emission event. Spooky^2.

But wait: there is no proper time lapse for a photon? Maybe that plays a role?
 

DarMM

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What are the other four?
They are:
  1. Multiple Worlds

  2. Acausal. There is simply no dynamic account of what occurs. The 4D history as a whole obeys a certain constraint, but that constraint results in a set of events that can't be broken down in a 3+1D way, i.e. as initial events/conditions on a 3D surface which then evolves under a PDE or similar

  3. Nonlocal interaction, i.e. objects affecting each other at spacelike distances. Ultimately this would mean Relativity is wrong

  4. Spacetime is highly topologically nontrivial, e.g. there are microscopic wormholes everywhere. This essentially invalidates treating any small system as if Special Relativity were true, you must always use General Relativity for objects beneath a certain size due to the topologically nontrivial nature of spacetime
Note that some don't think any of the above or retrocausality is correct and would say entanglement does not have a mathematical explanation.
 
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Nonlocal interaction, i.e. objects affecting each other at spacelike distances. Ultimately this would mean Relativity is wrong
Can you explain why non-local interaction means relativity is wrong? I have seen this said by various moderators and I don't understand the conflict with relativity. For example in another thread PeterDonis said:
#1 is incorrect because of relativity: if the two spin measurements are spacelike separated, their time ordering is not invariant, so there is no invariant fact of which one was first. The only invariant is that the measurement results are independent of which one is first. Which means there is no valid way to interpret either one as triggering a change in the other.
If the measurement of one entangled particle instantly affects (in a universal clock sense) the other entangled particle, how would we even know it broke a rule of relativity since the states of each particle are effectively hidden until the measurement?
 
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Nonlocal interaction, i.e. objects affecting each other at spacelike distances. Ultimately this would mean Relativity is wrong
That's not quite right. Relativity (more precisely, quantum field theory, i.e., quantum mechanics + relativity) does not say that there cannot be any interaction between spacelike separated events. It only says that any such interaction must commute, i.e., what happens at the events cannot depend on the order in which they happen.
 
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There is no universal clock. That's what relativity says.
Relativity applies when events are described relative to the speed of light, but this interpretation requires instant action (faster than light). How else can you describe that measuring one entangled particle instantly affects the state of the other entangled particle without invoking the concept of a universal clock?


Relativity (more precisely, quantum field theory, i.e., quantum mechanics + relativity) does not say that there cannot be any interaction between spacelike separated events. It only says that any such interaction must commute, i.e., what happens at the events cannot depend on the order in which they happen.
I agree that this interpretation violates the statement "interaction must commute, i.e., what happens at the events cannot depend on the order in which they happen". Assuming this is an accurate law of relativity then I am forced to say that DarMM is correct in saying "
Ultimately this would mean Relativity is wrong". That is all fine, but this violation of relativity does not affect the math for normal events that we calculate or measure; and so there is no good reason to discount this interpretation on the basis that it violates relativity.

And ultimately what I am really trying to understand is why would anyone discount this interpretation? And so far the answer because it violates relativity just seems like a cop-out to me.
 
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Relativity applies when events are described relative to the speed of light
As far as we know relativity always applies. Sometimes other non-relativistic theories are good approximations to relativity, but relativity always applies.
 
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I'm sorry, but that does not make any sense.
The special theory of relativity is based on the idea that the speed of light is constant in all reference frames and based on this you get proper time. If you are discussing events that are caused by faster than the speed of light action, then this theory and this definition of time can not apply. That is what I am trying to convey in this sentence.
 

PAllen

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The special theory of relativity is based on the idea that the speed of light is constant in all reference frames and based on this you get proper time. If you are discussing events that are caused by faster than the speed of light action, then this theory and this definition of time can not apply. That is what I am trying to convey in this sentence.
This is not true. There are several models extending conventional SR to include FTL phenomena that maintain frame variance of light speed and the same definition of proper time.
 
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This is not true. There are several models extending conventional SR to include FTL phenomena that maintain frame variance of light speed and the same definition of proper time.
By FTL, I mean instant, which is why I used the term universal clock. How you measure one entangled photon impacts the other entangled photon instantly. I am trying to understand how the theory of relativity discredits this interpretation.
 

PAllen

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By FTL, I mean instant, which is why I used the term universal clock. How you measure one entangled photon impacts the other entangled photon instantly. I am trying to understand how the theory of relativity discredits this interpretation.
‘Instant’ raises no separate issues, because any ftl influence is instant or retrocausal for some observers. Note that instant is really not meaningful. An entanglement situation will look retrocausal in a different frame. None of this need affect invariance of c or or the definition of proper time. Just look at something as simple as tachyons, a simple extension of conventional SR. A tachyon message trivially may be instant or retrocausal depending on the frame viewing it.
 
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Relativity applies when events are described relative to the speed of light
No, relativity applies period. More precisely, quantum field theory (QM + relativity) applies period. There is no way to avoid relativity by not "describing events relative to the speed of light".

How else can you describe that measuring one entangled particle instantly affects the state of the other entangled particle without invoking the concept of a universal clock?
By realizing that the statement "measuring one entangled particle instantly affects the state of the other entangled particle" is simply wrong. Neither particle even has a state before the measurement. Only the entangled two-particle system does.

this violation of relativity does not affect the math for normal events that we calculate or measure
Huh? Why do you think that? Quantum field theory has extensive experimental confirmation, so there are lots of events that have been calculated and measured that confirm relativity in this regime.

Assuming this is an accurate law of relativity
It's an accurate law of quantum field theory, yes.

so far the answer because it violates relativity just seems like a cop-out to me.
It shouldn't. See above.
 
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No, relativity applies period. More precisely, quantum field theory (QM + relativity) applies period. There is no way to avoid relativity by not "describing events relative to the speed of light".
I accept and agree that all events must be described relative to the speed of light. In the interpretation that I am discussing, the entanglement collapse happens instantly compared to the speed of light. Given that this instant collapse event can't actually be observed, only inferred, I don't see how it can violate relativity.

By realizing that the statement "measuring one entangled particle instantly affects the state of the other entangled particle" is simply wrong. Neither particle even has a state before the measurement. Only the entangled two-particle system does.
I don't see how the description of entanglement having shared versus individual states are effectively different. If I had to write a program to model the instance behavior of entanglement collapse, how I describe the states prior to measurement does not matter; what matters is the output to the program, i.e. the behavior at measurement.

Maybe you are not being clear about what you are really trying to say and what you are really trying to say is there is currently no instance model that can reproduce the statistics that QFT can. That I can readily accept as criticism to Bell's non-local action idea, but if this is what you are really trying to say than why not just say that? Why say the non-local action that Bell describes is wrong unless you know for certain that it can't be a possibility?

Huh? Why do you think that? Quantum field theory has extensive experimental confirmation, so there are lots of events that have been calculated and measured that confirm relativity in this regime.
I am not disputing QFT in anyway that I am aware of. I am trying to understand why Bell's non-local action idea is in conflict with QFT. You are the one who is saying Bell's non-local action possibility is wrong. I am taking your word of "wrong" to mean that Bell's non-local action is not a valid possibility for the universe. If this is truly what you are saying, than I would like to understand why.

PeterDonis said:
The only invariant is that the measurement results are independent of which one is first. Which means there is no valid way to interpret either one as triggering a change in the other.
How can you even know this? You can't replay history and try to measure the system both ways to see if you get the same or differing results.
 
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I accept and agree that all events must be described relative to the speed of light.
That's not what I said. In fact I don't even understand what you mean by "described relative to the speed of light".

In the interpretation that I am discussing, the entanglement collapse happens instantly compared to the speed of light.
Which doesn't even make sense. See above.

Given that this instant collapse event can't actually be observed, only inferred, I don't see how it can violate relativity.
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 [Edit: does not violate] QFT). If they do, it does.

Notice that I posed that question without ever saying anything about "collapse", much less "instantaneous relative to the speed of light".

Why say the non-local action that Bell describes is wrong
I have said no such thing. You are not reading carefully. See above.

How can you even know this?
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.
 
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PAllen

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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.
Also, if the order mattered you could send an FTL message using entanglement. Any proposed scheme for this is clearly testable.
 
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By FTL, I mean instant, which is why I used the term universal clock.
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.

In the interpretation that I am discussing, the entanglement collapse happens instantly compared to the speed of light.
You need to use quantum field theory, not non-relativistic quantum mechanics.
 

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