Consistent Histories and Locality

[PeterDonis]

the probabilities for outcomes of measurements on 1&4 are independent of what is done on 2&3

One has to be very careful phrasing this. As you state it, it could be true or it could be false, depending on how your ambiguous wording is interpreted.

It is impossible to send signals to observers measuring photons 1 & 4 by choosing whether or not to allow a swap operation to take place on photons 2 & 3 at the BSM. In that sense your statement is true.

However, if you do two experiments, one in which the swap operation is done and one in which it is not, and you hand the two sets of data (the measurement results by run for all four photons) to someone, without telling them which set is the "swap" set and which is the "no swap" set, they can tell which is which by looking at the photon 1 & 4 correlations in each subset picked out by the four possible combinations of photon 2 & 3 results. In the "swap" set, there will photon 1 & 4 correlations in each subset, and those correlations can violate the Bell inequalities, whereas in the "no swap" set there will be no correlations even by subset. In that sense your statement is false.

 

[martinbn]

One has to be very careful phrasing this. As you state it, it could be true or it could be false, depending on how your ambiguous wording is interpreted.

It is impossible to send signals to observers measuring photons 1 & 4 by choosing whether or not to allow a swap operation to take place on photons 2 & 3 at the BSM. In that sense your statement is true.

However, if you do two experiments, one in which the swap operation is done and one in which it is not, and you hand the two sets of data (the measurement results by run for all four photons) to someone, without telling them which set is the "swap" set and which is the "no swap" set, they can tell which is which by looking at the photon 1 & 4 correlations in each subset picked out by the four possible combinations of photon 2 & 3 results. In the "swap" set, there will photon 1 & 4 correlations in each subset, and those correlations can violate the Bell inequalities, whereas in the "no swap" set there will be no correlations even by subset. In that sense your statement is false.

My point is that if you give them the two sets of the measurement result only of photons 1&4, then they cannot tell which is which.

 

[PeterDonis]

My point is that if you give them the two sets of the measurement result only of photons 1&4, then they cannot tell which is which.

Yes, but why would you do that? Photons 2 & 3 are part of the total system; photon 2 starts out entangled with photon 1, and photon 3 starts out entangled with photon 4, and those entanglements are stipulated in the preparation of the system. If you leave out those results, you're leaving out relevant information.

It is of course true that a reduced density matrix for a quantum system that traces over other systems with which that system might be entangled, will not show correlations with those other systems. That's a simple mathematical fact that is stated in pretty much every QM textbook. But that doesn't mean those correlations do not exist, or that they are not physically meaningful.

 

[martinbn]

Yes, but why would you do that? Photons 2 & 3 are part of the total system; photon 2 starts out entangled with photon 1, and photon 3 starts out entangled with photon 4, and those entanglements are stipulated in the preparation of the system. If you leave out those results, you're leaving out relevant information.

It is of course true that a reduced density matrix for a quantum system that traces over other systems with which that system might be entangled, will not show correlations with those other systems. That's a simple mathematical fact that is stated in pretty much every QM textbook. But that doesn't mean those correlations do not exist, or that they are not physically meaningful.

But the claim is that measurment on 2&3 affects 1&4. What is the meaning of that if it cannot be observed?

 

[PeterDonis]

the claim is that measurment on 2&3 affects 1&4. What is the meaning of that if it cannot be observed?

It can be observed: when you look at the presence or absence of correlations in the 1&4 subsets corresponding to the four possible combinations of 2&3 results, depending on whether the experimenter made a choice to have a swap take place, you are observing that the experimenter's choice of whether or not to make a swap at 2&3 affects 1&4. In any other branch of science, that would be a commonplace claim: experimenter makes an intervention and the presence vs. the absence of that intervention shows up in a predictable way in the data. Why it somehow becomes problematic when we're talking about QM and entangled systems is not clear to me.

 

[martinbn]

It can be observed: when you look at the presence or absence of correlations in the 1&4 subsets corresponding to the four possible combinations of 2&3 results, depending on whether the experimenter made a choice to have a swap take place, you are observing that the experimenter's choice of whether or not to make a swap at 2&3 affects 1&4. In any other branch of science, that would be a commonplace claim: experimenter makes an intervention and the presence vs. the absence of that intervention shows up in a predictable way in the data. Why it somehow becomes problematic when we're talking about QM and entangled systems is not clear to me.

I don't understand. If you don't make a BMS on 2&3 there is still going to be for subsets of the data for 1&4 with those correlations. We may not be able to tell which trials to look at but they are there. So I still don't understand why in one siruation they were caused by something romote and in the other not? Also in any branch of science if the probabilities of the outcomes do not change base on whether you do something over there or not suggests that the doing or not of something over there is not the cause of the outcoms.

 

[PeterDonis]

If you don't make a BMS on 2&3 there is still going to be for subsets of the data for 1&4 with those correlations.

No, there will not. That's the whole point. If no swap is done on 2&3, then the 1&4 subsets corresponding to each of the four possible combinations of 2&3 results (HH, HV, VH, VV) will not show any correlations, because photons 1&4 are not entangled if no swap is done. But if a swap is done on photons 2&3, then those four subsets of 1&4 results will show correlations, corresponding to the entangled Bell state that is indicated by each of the four 2&3 combinations.

These correlations do not allow signaling, since they are only detectable once the results are all collected and the subsets picked out. But they are a measurable difference between the swap and no swap cases.

 

[martinbn]

No, there will not. That's the whole point. If no swap is done on 2&3, then the 1&4 subsets corresponding to each of the four possible combinations of 2&3 results (HH, HV, VH, VV) will not show any correlations, because photons 1&4 are not entangled if no swap is done. But if a swap is done on photons 2&3, then those four subsets of 1&4 results will show correlations, corresponding to the entangled Bell state that is indicated by each of the four 2&3 combinations.

These correlations do not allow signaling, since they are only detectable once the results are all collected and the subsets picked out. But they are a measurable difference between the swap and no swap cases.

No, i am not saying that the subsets of 1&4 will correspond to the outcomes HH, HV, VH, and VV of the 2&3. I am saying that the data set of the 1&4 can be partitioned into such four subsets.

 

[PeterDonis]

i am not saying that the subsets of 1&4 will correspond to the outcomes HH, HV, VH, and VV of the 2&3.

This makes no sense. Of course you can pick any subsets out of the total 1&4 data set you like, if you don't care what they mean or don't mean. But the subsets that matter, physically, are the ones that correspond to the four possible combinations of 2&3 outcomes--because those are the ones that are predicted by the standard math of QM to show the Bell state correlations if the experimenter chooses to do a swap, but not if the experimenter doesn't. In other words, those are the relevant subsets for testing the theory against experiment.

I am saying that the data set of the 1&4 can be partitioned into such four subsets.

Of course it can. I'm saying the same thing. And I'm also saying that if you partition the 1&4 data that way, then you will see Bell state correlations in each subset if and only if the experimenter chose to do a swap.

If you dispute this, you are disputing both the standard math of QM and the experimental facts.

 

[DrChinese]

So I still don't understand why in one situation they were caused by something remote and in the other not? Also in any branch of science if the probabilities of the outcomes do not change base on whether you do something over there or not suggests that the doing or not of something over there is not the cause of the outcoms.

You have everything backwards. The 4 fold outcomes DO change depending on "whether you do something over there or not"! That's what the point of the experiment is!! To summarize (for the Nth time):

- 4 fold coincidences when experimenter selects SWAP=on (indistinguishable HH or VV for 2&3): Correlation high, as predicted by QM.
- 4 fold coincidences when experimenter selects SWAP=off: (distinguishable HH or VV for 2&3): Correlation negligible, as predicted by QM.

We are simply saying that there is a published experiment by a top team, and we are asking for a description of how an QM interpretation can explain THAT experiment without recourse to nonlocality. Because to the naked eye, the results appear* to clearly demonstrate non-signaling nonlocality.

---

What you are saying is that the 2 fold correlated outcomes don't appear to change because we don't know which bin to place the "subsets" in. So what? That's a different experiment and has nothing to do with entanglement swapping. You can test that with any two sources and get the same results. Imagine you are testing the Earth's gravitational acceleration, 32 ft/sec^2. You let an apple fall 16 feet, but you don't record or report the elapsed time duration. That is essentially what you are describing, half an experiment.


*Of course there are Interpretations that are explicitly nonlocal. Other Interpretations may have features that can explain the apparent nonlocality in some manner that retains locality. Those are the explanations I am requesting.

 

[Morbert]

This makes no sense. Of course you can pick any subsets out of the total 1&4 data set you like, if you don't care what they mean or don't mean. But the subsets that matter, physically, are the ones that correspond to the four possible combinations of 2&3 outcomes--because those are the ones that are predicted by the standard math of QM to show the Bell state correlations if the experimenter chooses to do a swap, but not if the experimenter doesn't. In other words, those are the relevant subsets for testing the theory against experiment.


Of course it can. I'm saying the same thing. And I'm also saying that if you partition the 1&4 data that way, then you will see Bell state correlations in each subset if and only if the experimenter chose to do a swap.

If you dispute this, you are disputing both the standard math of QM and the experimental facts.

The standard math of QM says that Bell-inequality-violating correlations between the appropriate measurements on 1 and 4 will themselves be correlated with outcomes of a BSM on 2 and 3. This is all the math of QM commits us to. It does not commit us to nonlocal influence unless we insist on a hidden variable theory that reproduces these correlations QM predicts.

 

[DrChinese]

The standard math of QM says that Bell-inequality-violating correlations between 1 and 4 will themselves be correlated with outcomes of a BSM on 2 and 3. This is all the math of QM commits us to. It does not commit us to nonlocal influence unless we insist on a hidden variable theory that reproduces these correlations QM predicts.

That is most certainly not true. Any of the following are consistent with Bell:

a) Denial of locality via nonlocal hidden variables interpretations. (Bohmian, etc.)
b) Denial of hidden variables via various "local" interpretations (MWI, Time symmetric, etc.)
c) Denial of both local and realism/hidden variables. (I would personally place standard QM in this bucket.)

We should probably be discussing type c) in order to be consistent with all of the experiments I am aware of. The Heisenberg Uncertainty Principle - and experiments supporting it - strongly imply there is no realism or hidden variables. Swapping experiments strongly imply Einsteinian locality and causality fails.

 

[PeterDonis]

The standard math of QM says that Bell-inequality-violating correlations between the appropriate measurements on 1 and 4 will themselves be correlated with outcomes of a BSM on 2 and 3. This is all the math of QM commits us to. It does not commit us to nonlocal influence

The words "nonlocal influence" do not appear anywhere in my post. I am simply trying to make sure we are all clear about what the standard math of QM and the experimental facts say, because the posts by @martinbn appear to me to indicate that that is not clear to everyone in this thread.

 

[martinbn]

This makes no sense. Of course you can pick any subsets out of the total 1&4 data set you like, if you don't care what they mean or don't mean. But the subsets that matter, physically, are the ones that correspond to the four possible combinations of 2&3 outcomes--because those are the ones that are predicted by the standard math of QM to show the Bell state correlations if the experimenter chooses to do a swap, but not if the experimenter doesn't. In other words, those are the relevant subsets for testing the theory against experiment.


Of course it can. I'm saying the same thing. And I'm also saying that if you partition the 1&4 data that way, then you will see Bell state correlations in each subset if and only if the experimenter chose to do a swap.

If you dispute this, you are disputing both the standard math of QM and the experimental facts.

I don't dispute any of this. May be I don't express myself well. What confuses me is that it seems that we say exactly the same thing except that you and @DrChinese add at the end "Therefore this proves that the measurements on 2&3 affect the outcomes for 1&4". I just don't see it.

 

[martinbn]

The words "nonlocal influence" do not appear anywhere in my post. I am simply trying to make sure we are all clear about what the standard math of QM and the experimental facts say, because the posts by @martinbn appear to me to indicate that that is not clear to everyone in this thread.

I am not questioning QM nor the experimental facts (which confirm QM, so in a theoretical discussion they are not really needed). I am questioning the conclusions @DrChinese makes from these experiments. He does use the words "nonlocal influence". And I was responding to his post that I quoted.