A new interpretation of Quantum Mechanics

[martinbn]

None of this fits the requirements I presented. So you don't get the $10, but as consolation prize: I'll buy you that Lone Star anytime you are in Dallas.

And I agree that Chris keeping a spreadsheet can be considered classical.

I don't understand! You wanted photons 1 and 4, which are initially unrelated in any way (arbitrary states and arbitrary far way in space and time from each other) to be entangled or not depending on what Chris does. My example does exactly that! If you disagree you need to tell me why.

But that is not what Chris' role is. He is independently choosing to entangle photons 1 & 4 by doing something to photons 2 & 3. And when Chris chooses to entangle, the final stats should show the perfect correlations (or anti-correlations); and when Chris chooses not to entangle, there can be no correlation. Alice, Bob and Chris are sufficiently distant that their results will be independent. They all send their independent results to some other party for summarizing. We can call that person Dave. Dave buys the beer, by the way.

There is no 2&3 in the task for the bet. I think I stratified all your requirements. Here they are:

• a. The photons (or whatever classical objects you prefer) detected by Alice and Bob never exist/interact in a common light cone. Let's call these objects 1 and 4 to match my experimental references.
• b. 1 and 4 cannot be entangled or otherwise made identical in their initial states, because the decision to entangle them (or not) will be made in a remote (FTL distant) place by Chris. So Alice, Bob and Chris are spacelike separated at the time that 1 and 4 become entangled - or correlated, or whatever you care to call it. They are also all spacelike separated when Alice and Bob perform their chosen measurements.
• c. Alice and Bob can choose to measure either i) on any same basis (in which case we must see perfect correlation); or ii) on different bases (a la CHSH, and violating a Bell inequality). I'll be impressed if you can do this for even just case i).
• d. Chris can choose to entangle - or not - the 1 and 4 objects. The observed Alice/Bob correlations must change along with this choice. No correlation if Chris chooses not to classically correlate.

Why do you not accept my example?

PS: I will replay to the rest of your post in a minute.

 

[martinbn]

So @martinbn: You said you don't agree with what I say about this type of remote entanglement swapping. What specifically do you not agree with? I am describing actual experiments, there's not much to imagine. As @PeterDonis says, different interpretations tend to take different views as to whether Chris' decision is the "cause" of the swap. So I get that. Also, some disagree that anything nonlocal was transmitted, even a quantum state. Nonetheless, Chris's decision is undeniably a part of the overall experimental context in which the final quantum state of 1 & 4 is different than the initial quantum states of 1 & 4. Which were always nonlocal to each other.

This was addressed by others in the other threads. But here is what I disagree with:

1. You insists on the statement that 1&4 have an entangled state at the end. But since there isn't any notion of them existing at the same time to say that they (the system consisting of the two) have any state (entangled or not) is erroneous. And this is interpretation independent.

2. You always neglect the fact that the results that show the correlations happen only on a subset of the trials. In statistical terms you could only claim that a sub ensemble is entangled. And this would be interpretation dependent. But you claim that every single pair 1&4 are entangled.

3. You always point out that the order of the measurements is irrelevant. So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!? If I am looking at the paper with the results, will they magically change before my eyes?

 

[PeterDonis]

There is no 2&3 in the task for the bet.

They're in the papers that were referenced in the post where the bet was originally proposed.

 

[PeterDonis]

you claim that every single pair 1&4 are entangled.

No, he doesn't. He is only claiming that the 1 & 4 pairs in runs of the experiment in which Chris performs the "entanglement swap" operation on photons 2 & 3 are entangled. The experimental data for each run contains an indication of whether or not the "entanglement swap" operation was performed, so it contains sufficient data to post-select the subensemble of runs in which 1 & 4 are entangled.

The interpretation difference comes in when you state what all that means. In the interpretation @DrChinese is using, where the wave function describes each individual set of photons in each individual run, the runs in which 1 & 4 are entangled have those individual photons being entangled, and the "entanglement swap" operation done in those runs is a physical process that physically swaps the entanglement from the pairs 1&2, 3&4 to the pairs 1&4, 2&3.

In an ensemble interpretation, which is what you appear to be implicitly using, the wave function only describes an abstract ensemble of systems that are all prepared by the same process. Post-selecting a subensemble by picking out only the runs whose data contains the "entanglement swap" indicator amounts to including that indicator being present in your definition of the preparation process. On this interpretation, the wave function says nothing about individual photons or combinations of photons in individual runs; it only allows you to make predictions about the statistics of the ensemble, which should be approximated reasonably well by the statistics from your actual experimental data if the number of runs is sufficiently large. In particular, this interpretation makes no claim that the "entanglement swap" operation has to do something physical to each individual set of photons in each individual run; it only claims that that operation, being part of the preparation process, affects the statistics of the ensemble you end up selecting.

 

[PeterDonis]

the order of the measurements is irrelevant.

Yes. See further comments below.

So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!?

Obviously this kind of interpretation requires any "change" involved to not respect the usual rules of causality. Either your description of which way the causality goes, i.e., which event is the "cause" and which is the "effect", has to change depending on the order (so if 1 & 4 are measured first, then it is their measurements that cause whatever happens at 2 & 3 to happen), or you have to expand your definition of "causality" to allow events to be causally connected even if those events commute, i.e., what happens at them is independent of the order in which they occur, so that it is impossible to pick out which is the "cause" and which is the "effect".

 

[PAllen]

Yes. See further comments below.


Obviously this kind of interpretation requires any "change" involved to not respect the usual rules of causality. Either your description of which way the causality goes, i.e., which event is the "cause" and which is the "effect", has to change depending on the order (so if 1 & 4 are measured first, then it is their measurements that cause whatever happens at 2 & 3 to happen), or you have to expand your definition of "causality" to allow events to be causally connected even if those events commute, i.e., what happens at them is independent of the order in which they occur, so that it is impossible to pick out which is the "cause" and which is the "effect".

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

 

[DrChinese]

I don't understand! You wanted photons 1 and 4, which are initially unrelated in any way (arbitrary states and arbitrary far way in space and time from each other) to be entangled or not depending on what Chris does. My example does exactly that! If you disagree you need to tell me why.

There is no 2&3 in the task for the bet. .

Sorry, I thought you were following the remote entanglement swapping experiment I referenced:

High-fidelity entanglement swapping with fully independent sources
"Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations."

See Figure 1, which shows the labeling of the photons by number. Alice sees 1, Bob sees 4, Chris chooses to execute (or not) the remote swap by overlapping 2 & 3 and detecting a Bell state (which also requires a 4-fold coincidence). In this particular setup, I assigned the names Alice/Bob/Chris just for clarity/discussion purposes. Also in this particular setup, the Alice and Bob locations are far enough separated that photons 1 & 4 do not share any backward light cone. However, as I read the specs, the Chris location is not specifically distant from Alice & Bob. (Other similar experiments I have referenced make this clear.)

So Chris can choose to execute the entanglement of 1 & 4 together by choosing (or not) to allow photons 2 & 3 to overlap. That is Chris's role. Alice, Bob, and Chris can be arbitrarily far apart, the order of their detections can be made as desired, and they can be made as close to simultaneous as desired - without changing the observed results. Those being: observation of HOM dip, and violation of CHSH inequalities. Note that violation of CHSH (in agreement with the predictions of QM) always implies that perfect correlations will be observed as well.

So yeah, you gotta have Chris doing something to 2 & 3 to get entanglement between 1 & 4. And Chris is too far away for a classical signal to arrive at Alice or Bob's stations before 1 and 4 arrive. So the results occur in mutually remote (in terms of light speed distance) locations.

 

[PeterDonis]

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

In this case the measurements commute regardless of whether they are timelike, spacelike, or null separated. Their commutation is unrelated to the QFT condition that spacelike separated measurements must always commute.

That, in itself, already makes it clear that whatever kind of "connection" there is between the measurement events, it can't be an ordinary causal connection of the kind we are familiar with.

 

[PAllen]

In this case the measurements commute regardless of whether they are timelike, spacelike, or null separated. Their commutation is unrelated to the QFT condition that spacelike separated measurements must always commute.

That, in itself, already makes it clear that whatever kind of "connection" there is between the measurement events, it can't be an ordinary causal connection of the kind we are familiar with.

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

 

[DrChinese]

This was addressed by others in the other threads. But here is what I disagree with:

1. You insists on the statement that 1&4 have an entangled state at the end. But since there isn't any notion of them existing at the same time to say that they (the system consisting of the two) have any state (entangled or not) is erroneous. And this is interpretation independent.

2. You always neglect the fact that the results that show the correlations happen only on a subset of the trials. In statistical terms you could only claim that a sub ensemble is entangled. And this would be interpretation dependent. But you claim that every single pair 1&4 are entangled.

3. You always point out that the order of the measurements is irrelevant. So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!? If I am looking at the paper with the results, will they magically change before my eyes?

1. There is no requirement that 1 & 4 ever co-exist, but in the references I am supplying they do. They just don't co-exist in a common backward light cone because they were never close to each other. Upon creation, they head out in opposite directions. As far as I know, there is no interpretation that disputes this point.

2. There is no subensemble that is neglected. All cases in which a 4 fold coincidence occurs are considered.

3. My "claim" on this is quite simple. This experiment has already been performed (although by the same team, it is a different paper per below). It is 100% consistent with the predictions of QM, which again AFAIK no interpretation disputes. Different interpretations do explain it differently though. So explain it as you will, but denying experimental results is never a good look. The bet is to attempt to explain these results using a classical explanation with local realism. In fact, even an acceptable classical local explanation would be enough for me to lose the bet.

Delayed-choice gedanken experiments and their realizations (2016)

See particularly section 5.A. starting on page 22. Page 23: "The diagram of the temporal order of the relevant events is shown in Fig. 33. For each successful run (a 4-fold coincidence count), both Victor’s [Victor is my "Chris"] measurement event and his choice were in the time-like future of Alice’s and Bob’s measurements."

 

[DrChinese]

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

This is more fully explained in my post #80, but this version has been explicitly performed years ago. Time ordering (with a * for a specific single particle exception per @PeterDonis) doesn't change the observable results. (Note that I didn't say it doesn't change the results, I just say that there is no observable change.)

Delayed-choice gedanken experiments and their realizations (2016)

See particularly section 5.A. starting on page 22. Page 23: "The diagram of the temporal order of the relevant events is shown in Fig. 33. For each successful run (a 4-fold coincidence count), both Victor’s [Victor is my "Chris"] measurement event and his choice were in the time-like future of Alice’s and Bob’s measurements."

 

[PeterDonis]

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

This wouldn't change anything. Once a measurement is made, communicating the result to others doesn't change anything further.

 

[PAllen]

This wouldn't change anything. Once a measurement is made, communicating the result to others doesn't change anything further

I am confused here. Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

 

[DrChinese]

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

Adding experimenter 5 changes nothing.

I am confused here. Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

I think what you are missing is that the 1 & 4 pairs are randomly thrown into 1 of 4 Bell states in very nearly equal measure. 2 of these can be identified, because they result in 4 fold coincidences*. The two identifiable states are ψ+ and ψ-. One yields correlation, the other yields anti-correlation. So until you see the results from Chris (a/k/a Victor), you won't be able to confirm the type of entanglement of 1 & 4.


*In the 2 that cannot be identified, there are only 3 fold coincidences. That happens because the 2 & 3 photons end up in the same detector so close together in time that only a single detection is registered. Current technology for photon avalanche detectors requires too large a time interval between trigger events to yield 2 detections in such cases. This 3 fold coincidence happens 50% of the time, while the other 50% result in 4 fold coincidences.

 

[PeterDonis]

Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

The "decision" to have 2 & 3 interact is not fully under the experimenter's control. The 2 & 3 photons have to arrive at the beam splitter that could execute the entanglement swap within a certain time window. If they don't, the experimenter cannot make 2 & 3 interact no matter what.

The QM prediction is then that, in any case where 1 & 4's measurement results indicate no entanglement, the 2 & 3 photons will not arrive within the required time window and will be unable to interact no matter what the experimenter does.

I believe this alternative has actually been realized in at least one of the experiments described in the papers @DrChinese has referenced, and the QM prediction is confirmed.

Of course it is hard to imagine how this would work; welcome to how counterintuitive QM is.

 

[DrChinese]

1. The "decision" to have 2 & 3 interact is not fully under the experimenter's control. The 2 & 3 photons have to arrive at the beam splitter that could execute the entanglement swap within a certain time window. If they don't, the experimenter cannot make 2 & 3 interact no matter what.

2. The QM prediction is then that, in any case where 1 & 4's measurement results indicate no entanglement, the 2 & 3 photons will not arrive within the required time window and will be unable to interact no matter what the experimenter does.

3. I believe this alternative has actually been realized in at least one of the experiments described in the papers @DrChinese has referenced, and the QM prediction is confirmed.

Of course it is hard to imagine how this would work; welcome to how counterintuitive QM is.

Just clarifying a couple of things, everything you say is correct but might not be intuitively obvious to some.

1. The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled. This is accomplished by making the 2 & 3 photons distinguishable. This is accomplished by delaying the 2 photon but not the 3 photon (or vice versa).

And of course there are cases in which the 2 & 3 photons naturally do NOT arrive within the same time window, because they are created at random times by independent sources. (I will explain this more in a separate post for those interested.)


2. If you were looking only at 1 & 4 coincidence outcomes at the same angles (perfect correlation is ψ+, perfect anti-correlation is ψ-): You could rule out one - but not the other - of whether there could be entanglement or not. If you got HH> for example, you certainly cannot have ψ- as a result for photons 2 & 3. If you instead got VH>, you certainly cannot have ψ+ as a result for photons 2 & 3.


3. Yes, the QM prediction is always confirmed! At least so far.

At least one specific experiment the decision to interact (swap) or not was specifically randomized and analyzed. The results showed no correlation when the entangled swap was prevented via an intentionally introduced time delay.

 

[PAllen]

Just clarifying a couple of things, everything you say is correct but might not be intuitively obvious to some.

1. The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled. This is accomplished by making the 2 & 3 photons distinguishable. This is accomplished by delaying the 2 photon but not the 3 photon (or vice versa).

And of course there are cases in which the 2 & 3 photons naturally do NOT arrive within the same time window, because they are created at random times by independent sources. (I will explain this more in a separate post for those interested.)


2. If you were looking only at 1 & 4 coincidence outcomes at the same angles (perfect correlation is ψ+, perfect anti-correlation is ψ-): You could rule out one - but not the other - of whether there could be entanglement or not. If you got HH> for example, you certainly cannot have ψ- as a result for photons 2 & 3. If you instead got VH>, you certainly cannot have ψ+ as a result for photons 2 & 3.


3. Yes, the QM prediction is always confirmed! At least so far.

At least one specific experiment the decision to interact (swap) or not was specifically randomized and analyzed. The results showed no correlation when the entangled swap was prevented via an intentionally introduced time delay.

So if you arrange the experiment so 2 and 3 never get close to each other until the causal future of measurements of both 1 and 4, then no action on 2 and 3 will make 1 and 4 entangled?

 

[PeterDonis]

The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled.

Yes, agreed. He just can't make them entangled with 100% certainly, because he does not control whether or not photons 2 & 3 arrive at the BSM device within the required time window.

Also, a scenario in which, whenever Chris is informed that the 1 & 4 measurement results do not show entanglement, he explicitly prevents the swap from occurring, would not, I assume, be the kind of thing we want to discuss. I would assume we want to discuss the case where Chris never explicitly takes action to prevent the swap, so that the only factors involved are the arrival times of photons 2 & 3 at the BSM and whatever other "natural" factors contribute to whether or not a swap occurs.

 

[PeterDonis]

So if you arrange the experiment so 2 and 3 never get close to each other until the causal future of measurements of both 1 and 4, then no action on 2 and 3 will make 1 and 4 entangled?

No. A swap happening at the BSM when 2 & 3 arrive can be in the causal future of the 1 & 4 measurements. It's just that, in any case where the swap does happen at 2 & 3, it must also have been the case that the 1 & 4 measurement results were consistent with entanglement.

 

[PAllen]

No. A swap happening at the BSM when 2 & 3 arrive can be in the causal future of the 1 & 4 measurements. It's just that, in any case where the swap does happen at 2 & 3, it must also have been the case that the 1 & 4 measurement results were consistent with entanglement.

But this would mean that 1 and 4 can communicate their results to where and when 2 and 3 arrive (that's what causal future means), at which point someone at 2&3 future interaction point is free to perform an action supposedly inconsistent with this result.

 

[DrChinese]

For those of you interested in some of the more specific ideas behind the general Entanglement Swapping protocol, I will run through a rough example.

1. Keep in mind that the time/distance traveled can be relatively lengthened or shortened by adding fiber to one part of the overall setup or the other.

a) So the independent sources I and II can be kilometers apart.
b) Alice (observing photon 1 from PDC source I) and Bob (observing photon 4 from PDC source II) can be kilometers apart.
c) Each of those can be kilometers away from their source.
d) And Chris can be located such that Chris' decision to enable entanglement (via the swap mechanism, called a Bell State Measurement or BSM) can occur before or after the detectors of Alice and Bob click. Let's keep it simple, and have the BSM/swap positioned to occur before Alice and Bob see their clicks.


2. What needs to be synchronized?

a) The laser sources driving their respective PDC crystals must be phase locked.
b) The difference in the photon travel time from source I for photon 1 as compared to the photon travel time from source II for photon 4 should be known. This is to match an Alice click with a Bob click, indicating 2 of the 4 fold coincidence clicks we are looking for. That time difference is used to compensate for relative travel distance. The 1 & 4 photons don't need to actually arrive at their respective detectors at exactly the same time if this difference is known.
c) The difference in the photon travel time from source I for photon 1 as compared to the photon travel time from source I for photon 2 should be known. This allows for 3 fold coincidence detection.
d) The difference in the clicks of Chris's detectors (there are 4) must be known. If these do not occur within a sufficiently small time window, there can be no swap.


3. A single PDC source might randomly emit entangled pairs at the rate of perhaps 100,000 per second, or about 1 every 10,000 nanoseconds. We need the 2 & 3 photons - coming at random times from the two phase locked sources I and II to arrive within a narrow time window, such that the arrival time of its initial entangled partner (1 or 4) would not give us a clue as to which is which (they must be indistinguishable). Let's pretend that window is 5 nanoseconds. That means that on the average, sources I and II emit pairs that will ultimately overlap (at random intervals) within our desired time window of about 1 in 2,000 of each of those 100,000. That would be about 5,000 per second (this number is not particularly accurate, we are just for this example).

a) We would expect 2 fold coincidences (for 1 & 2, or 3 & 4) of about 200,000 per second, as most of the time the source I and source II pairs would not fire close enough together to be within the 5 nanosecond window.
b) We would expect 3 or 4 fold coincidences - for 1 & (2 or 3) & 4 - of about 5,000 per second, whenever the the source I and source II pairs do fire close enough together to be within the 5 nanosecond window.
c) We would expect 4 fold coincidences (for 1 & 2 & 3 & 4) of about 2,500 per second, whenever the the source I and source II pairs do fire close enough together to be within the 5 nanosecond window... AND the Bell state is identifiable. This occurs half of the time of the b) group above.


4. Keep in mind that ALL of the identifiable c) cases are considered in our results. Half of the b) cases are not used because the Bell state cannot be identified. That does NOT mean those cases were not entangled - all 5,000 per second are entangled. But half of those are φ+ or φ- entangled, and we can't distinguish between those - because both the 2 & 3 photons end up in the same detector (yielding only 1 click for the two photons). Only the ψ+ and ψ- can be identified (because the 2 & 3 photons end up in different detectors, yielding 2 clicks).

It is essential that the clicks from the 2 & 3 photons cannot provide any information as to which click is photon 2, versus which click is photon 3. That is why close arrival time is needed. And when I say close arrival time - as indicated by detector clicks: I really mean that photon 2 and photon 3 travel through the beam splitter portion of the BSM swapping mechanism close enough in time that they are allowed to interact. Because if photon 3 is delayed sufficiently such that there is no close overlap, photons 2 & 3 become distinguishable. That would be evident by one click alone arriving late, identifying photon 3. Then no swap will occur.

I hope these details will help some readers understand exactly what these experiments are demonstrating. Everything presented is orthodox QM.

 

[DrChinese]

So if you arrange the experiment so 2 and 3 never get close to each other until the causal future of measurements of both 1 and 4, then no action on 2 and 3 will make 1 and 4 entangled?

If Chris allows the 2 & 3 swap to occur, 1 & 4 will be entangled. This is true regardless of when Chris executes the swap. Distance does not matter, and timing of Chris' decision - future relative to 1 & 4 or past relative to the 1 & 4 measurements - does not matter.

So you call it the causal future, but I might simply call it the future. I would also say that the entire context of the experimental setup should be considered, and that there is no classical causality to demonstrate or consider. However, that statement is interpretation dependent.

 

[DrChinese]

But this would mean that 1 and 4 can communicate their results to where and when 2 and 3 arrive (that's what causal future means), at which point someone at 2&3 future interaction point is free to perform an action supposedly inconsistent with this result.

They cannot communicate anything to the future (assuming you interpret it that way) other than a random bit of information. The outcomes Alice and Bob see now can be consistent with 2 of 4 Bell states, only one of which can be identified. In other words: Yes, I can see from the time stamps of the Alice and Bob detections whether or not the 2 & 3 photons would have overlapped in Chris's beam splitter. And I can predict what will happen IF they are allowed to overlap. But there is no opportunity here to send a message to Chris in the future - except by a classical signal.

 

[PAllen]

They cannot communicate anything to the future (assuming you interpret it that way) other than a random bit of information. The outcomes Alice and Bob see now can be consistent with 2 of 4 Bell states, only one of which can be identified. In other words: Yes, I can see from the time stamps of the Alice and Bob detections whether or not the 2 & 3 photons would have overlapped in Chris's beam splitter. And I can predict what will happen IF they are allowed to overlap. But there is no opportunity here to send a message to Chris in the future - except by a classical signal.

I think we are not understanding each other. It is claimed that photon 2 and 3 can arrive somewhere such that an action can be taken to make them interact or not, and that this arrival event is in the causal future of measurements at 1 and 4. Causal future means people at 1 and 4 can send a classical signal to experimenter where 2 and 3 arrive. This signal can contain clock readings and measurement results. Experimenter at 2 and 3 is then free to perform actions that should be inconsistent with those measurements.

 

[DrChinese]

Also, a scenario in which, whenever Chris is informed that the 1 & 4 measurement results do not show entanglement, he explicitly prevents the swap from occurring, would not, I assume, be the kind of thing we want to discuss. I would assume we want to discuss the case where Chris never explicitly takes action to prevent the swap, ...

Agreed, except for this point. There can be no correlation whatsoever between 1 & 4 when Chris chooses to prevent the swap. And that is what is observed in this situation: no swap, no correlation.

Keep in mind that when the 2 & 3 photons do NOT overlap, they still exhibit any prior/previously determined attributes they might have had. If, for example, you follow a realistic type interpretation. If you did, the problem you have is that you are saying that the overlap does NOT "cause" (or otherwise contribute) any physical change in photons 1 & 4. In other words: If you believe the 2 & 3 overlap is simply revealing information that pre-existed, then 2 & 3 indistinguishability is not really a requirement for 1 & 4 entanglement, is it? That means the 2 & 3 clicks simply reveal a pre-existing Bell state.

Again, all of this is interpretation to varying extent. But the entire purpose of this thread is (as far as I am concerned): If someone is selling a new interpretation, does it pass the sniff test? My sniffing, of course, being these newer* state of the art experiments.


*Newer meaning: last 25 years... LOL.

 

[DrChinese]

I think we are not understanding each other. It is claimed that photon 2 and 3 can arrive somewhere such that an action can be taken to make them interact or not, and that this arrival event is in the causal future of measurements at 1 and 4. Causal future means people at 1 and 4 can send a classical signal to experimenter where 2 and 3 arrive. This signal can contain clock readings and measurement results. Experimenter at 2 and 3 is then free to perform actions that should be inconsistent with those measurements.

That's simply not true. These experiments are not ideas: they have been performed. There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

The problem is the word "causal". Yes, perhaps we are changing the future - or the future is changing the past. I certainly don't know anything more than what the experiments (which match theory) tell us. But all that anyone ever sees as a result is a random outcome. So not much to get out of random bits, which is why the word "causal" is a problem. If I can't make a specific outcome occur, how do you attach the word "cause" to an action? Or send a signal anywhere?

 

[PAllen]

That's simply not true. These experiments are not ideas: they have been performed. There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

Nonsense. All signals travel in space and time in the future direction. What you can't do is send a signal to anything other than the causal future. Claiming the arrival event of 2 and 3 is in the causal future of 1 and 4 MEANS that 1 and 4 measurement results can be sent via a classical signal to this event. If they cannot, then the 2 and 3 arrival point is at spacelike separation not timelike, and is NOT in the causal future of 1 and 4.

The problem is the word "causal". Yes, perhaps we are changing the future - or the future is changing the past. I certainly don't know anything more than what the experiments (which match theory) tell us. But all that anyone ever sees as a result is a random outcome. So not much to get out of random bits, which is why the word "causal" is a problem. If I can't make a specific outcome occur, how do you attach the word "cause" to an action?

Causal in SR/GR has a rigid, fixed meaning everyone accepts.

 

[PeterDonis]

this would mean that 1 and 4 can communicate their results to where and when 2 and 3 arrive (that's what causal future means), at which point someone at 2&3 future interaction point is free to perform an action supposedly inconsistent with this result.

No, they can't, because there is no such action that anyone is free to perform.

There is no single 1 & 4 measurement result that requires entanglement (entanglement can only be shown by appropriate statistics on a sufficient number of runs to confirm the required correlations), so it is impossible for the experimenter at 2 & 3 to freely choose to prevent entanglement and be inconsistent with the 1 & 4 results. The experimenter will simply find that the subensemble of runs for which they freely chose to prevent 1 & 4 entanglement will in fact show no 1 & 4 entanglement.

For the case where a single 1 & 4 measurement result rules out entanglement (i.e., a combination of results that cannot occur in an entangled state), the experimenter cannot freely choose to force the entanglement swap to occur at 2 & 3, because they do not control all of the necessary factors. They can only freely choose to not do anything to prevent the entanglemetn swap. They cannot make it occur. And, again, they will find that the subensemble of runs for which the 1 & 4 measurement results rule out entanglement, will also lack the indicator at the 2 & 3 operation that would indicate that a swap occurred--i.e., even though the experimenter did not prevent the swap from occurring, it nevertheless did not occur.

 

[lodbrok]

There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

We are so out of topic at this point but aren't all signals sent to the future by definition in general relativity?

 

[DrChinese]

We are so out of topic at this point but aren't all signals sent to the future by definition in general relativity?

Sure, GR, QM, all signals go from past to future alone (at a speed not in excess of c). No one is saying otherwise AFAIK.

What I am saying is: A decision by Chris can entangle - or not - 2 distant photons. That decision can be made at any time (or distance) relative to the measurements of those photons by Alice and Bob. The photons they observe need not have ever existed in a common backward light cone. This much is pretty much standard and supported by experiment.

What is interpretation dependent is whether Chris' decision is the cause of the entanglement in those cases where it is run in the Delayed Choice version. In that case, some might say Chris' future decision caused the entanglement of the previously observed photons. Were that true, you might interpret that as retrocausality.

Of course, there are dozens of Delayed Choice experiments that all have this similar feature. What makes this unique is that the entangled photons were never local to each other, and need never have been local to Chris.

 

[DrChinese]

1. Nonsense. All signals travel in space and time in the future direction. What you can't do is send a signal to anything other than the causal future.

2. Claiming the arrival event of 2 and 3 is in the causal future of 1 and 4 MEANS that 1 and 4 measurement results can be sent via a classical signal to this event. If they cannot, then the 2 and 3 arrival point is at spacelike separation not timelike, and is NOT in the causal future of 1 and 4.

1. Yes, all signals travel in the future direction at speeds not to exceed c. I didn't make that sufficiently clear, simply because I am not saying any signal can be sent FTL (and I have never implied otherwise).

2. No it does not. There is absolutely no requirement that Chris be close (enough for a classical signal) to Alice and/or Bob when the swap occurs. That's what I keep trying to tell you. This experiment has already been performed. Alice, Bob and Chris can be arbitrarily far apart, and the ordering of the events can be arbitrary without any observable difference. This is exactly as predicted by QM.

Chris can execute the swap in the future of Alice and Bob, while also being far enough away that no signals can be exchanged between any of the 3 before their observations are completed. At the same time, the photons of Alice and Bob need not have ever co-existed in a common light cone.

 

[PeterDonis]

A decision by Chris can entangle - or not - 2 distant photons.

As I have already noted, though, Chris does not have control over all of the relevant factors involved, so his freedom to make a "decision" is limited. Chris can choose to prevent the entanglement swap with certainty, by delaying one of the photons; but Chris cannot choose to make the entanglement swap happen with certainty, because Chris cannot guarantee that, if he does not delay either of the photons, they will both arrive within the required time window and cause a swap. The best Chris can do, if he wants a swap to happen, is to not choose to prevent it.

 

[DrChinese]

As I have already noted, though, Chris does not have control over all of the relevant factors involved, so his freedom to make a "decision" is limited. Chris can choose to prevent the entanglement swap with certainty, by delaying one of the photons; but Chris cannot choose to make the entanglement swap happen with certainty, because Chris cannot guarantee that, if he does not delay either of the photons, they will both arrive within the required time window and cause a swap. The best Chris can do, if he wants a swap to happen, is to not choose to prevent it.

Certainly this is true. Chris could just turn off his station too. So we agree.

But there is a nuance here I’m trying to make. With a specific delay, Chris can insure there is no swap but also could affirmatively say a swap would have occurred without that delay. IFF the timing aligned otherwise. I.e. 3 fold coincidence with the 4th running just behind.

There is another odd twist. Adding the delay does not actually reduce the total number of swaps! It changes the line-up of timings such that differently timed pairs match up. For a swap to occur, there must be overlap in Chris’ beam splitter. Even if there is extra fiber added, the number of overlapping 2 & 3 photons stays the same, on average. In such case, the 4 fold timing would look a bit different, but a swap would occur.

This is easiest to see if you imagine that a “normal” 4 fold detection has all 4 photons traveling the same distance (within experimental accuracy). All 4 time stamps would be within the same coincidence window without adjustment. The 1 &2 photons are therefore created nearly simultaneously as the 3 & 4 photons. But each source creates pairs randomly and independently. The swap is strictly based on the indistinguishable timing of the overlap of 2 & 3, nothing else.

 

[PeterDonis]

With a specific delay, Chris can insure there is no swap but also could affirmatively say a swap would have occurred without that delay.

Is that true? As I understand it, even if there is no artificially imposed delay, it is still possible that the photons do not arrive within the required time window to cause a swap; that is not under the experimenter's control. So there is no way to affirmatively say that a swap must occur if there is no artificially imposed delay.

 

[PeterDonis]

Adding the delay does not actually reduce the total number of swaps! It changes the line-up of timings such that differently timed pairs match up.

Wouldn't this only be true for some very precisely chosen values of the delay timing?