I CFD - Counterfactual Definiteness

[Simon Phoenix]

Not questioning if QFT makes this prediction, rather for an explanation of how it accounts for the inequality violations while maintaining locality

I'm not at all sure that the violation of the mathematical inequality has anything to do with locality (or non-locality) in QM.

First off, I think it's important to be clear about what is meant by 'locality' because this has different meanings in different contexts. What I mean by 'locality' in the context of the Bell inequality, and in an intuitive sense, is the following : the results of experiments 'here' are not affected by the settings of devices 'there'. [Or if they are, any such influence cannot travel faster than the speed of light]

Now let's take the standard Bell set-up in which we have some source of entangled particles, one of which goes to Alice and the other to Bob. When Alice and Bob choose, at random, from 3 possible measurement settings (say 0, 60 and 120 degrees) and collect data over many runs - they will see a violation of the Bell inequality when they compare their results. Assuming all the usual caveats about ideal experiments etc.

Now suppose Alice has the source of entangled particles in her lab. She measures her particle, but prevents its partner from ever getting to Bob. Instead she uses her result to prepare a new particle in the opposite spin state and in the same basis as indicated by her measurement result. She sends this new particle off to Bob.

Bob measures as normal, not realising Alice has made this switch. If Alice and Bob meet to compare results Bob will not be able to tell whether he has really received the partner of an entangled pair or some new particle prepared by Alice.

The upshot of this is that Alice can simply prepare particles at random in the up/down eigenstates for the 3 measurement settings (without using entangled particles at all) and send them to Bob. Using the data from her state preparations she can 'fool' Bob that they have been working with entangled pairs.

In other words the mathematical inequality can be violated by single particles - the correlation tested here is, of course, that between Alice's state preparation and Bob's measurements.

This latter experiment tells us nothing about locality or non-locality - since it is explicitly a local experiment. In principle for this set up we could construct a local hidden-variable theory. I haven't constructed such a theory but I feel it wouldn't look very natural or 'classical'.

The point is that the actual violation of the mathematical inequality has precious little to do with the locality or non-locality of the set-up.

By having the measurements of Alice and Bob spacelike separated this allows us to rule out local hidden variable theories of nature. This 'extra' correlation is there in QM whether or not we test it in a non-local setting - and we don't even need entangled particles to see it in a local setting. It was Bell's genius to figure out how this quantum correlation could be tested in a way that ruled out a whole class of theories by explicitly considering a non-local setting.

 

[Heinera]

Maybe a misunderstanding : Not questioning if QFT makes this prediction, rather for an explanation of how it accounts for the inequality violations while maintaining locality. Maybe this should be a new thread

QFT accounts for this the same way that QM does. Even in QFT, there is a state that "collapses" upon observation (OK, let's not get into a discussion of what "collapse" actually means.) So, even in QFT, an observation by Alice (or Bob) will collapse the combined state of Alice and Bob. Tongue in cheek, QFT is only local until someone makes an observation and causes a collapse.

 

[Zafa Pi]

I am thinking about a single run in a typical Bell-type scenario:

A joint-measurement of an (appropriately prepared) entangled property is performed in spacetime regions A and B at spacelike separation.

Regarding the above, in conjunction with Quantum Theory, would you say the following statement is true, false, or inapplicable? Why?

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

I am particularly interested in replies which assume a single (non-branching) universe.
Nonetheless, replies from all perspectives are valued.

In your 1st bold statement A and B are both making measurements and getting pairs of 0s and 1s, like (0,0), (1,0), (0,0), (1,1), ...
For the usual set up the values that A gets are like independent coin flips (same for B). If A chooses a particular setting a and B chooses b (where a and b do not differ by 45degrees) their results are not independent (in the sense of non-independent random variables). The mystery of entanglement.

In your 2nd bold statement it seems that A is making measurements and getting data where as B is not, but merely choosing a setting. Now you are asking if A's data is independent of B's setting. How is that different than asking if her data is independent of whether B is standing or not? What does it mean? A sees a fair sequence of 0s and 1s what ever is B's setting, and B sees nothing.

Interesting that DrChinese in post #2 believes that what A gets is not independent of B's setting, and Simon Phoenix in post #8 thinks they are independent, and bhobba likes both.
So when some one tells me what it means that my fair coin flips are independent of whether Bob is standing or not then I'll get to have an opinion too.

 

[Simon Phoenix]

Now you are asking if A's data is independent of B's setting. How is that different than asking if her data is independent of whether B is standing or not? What does it mean?

Well, think about what data Alice and Bob are collecting. They do thousands of runs. If they choose (independently and at random) from 3 possible detector settings (say 0,60 and 120 degrees) then after the experiment Alice and Bob can compile the data. It will look something like :
run 1 : Alice A = 1, a = 0, Bob B = 1, b = 60
run 2 : Alice A = -1, a = 120, Bob B = 1, b = 0
run 3 : Alice A = 1 a = 0, Bob B = -1, b = 0
run 4 : Alice A = 1, a = 60, Bob B = -1, b = 120

and so on and so on. Here A stands for Alice's 'result' and a stands for Alice's 'setting' - with B and b being Bob's equivalent quantities.

From this data the joint distribution P(A,B) can be experimentally estimated. Alice and Bob might also notice that whenever they happen to choose the same setting there is a perfect correlation and a weaker (non-zero) correlation whenever they choose different settings. So they're really looking here at subsets of the data - the results given particular settings. In other words they're looking at P(A,B | a,b).

They can also determine the quantities P(A | a,b) and P(B | a,b) which are the marginal distributions. Nothing special here - just analysing the measured data. So the question is whether P(A | a,b) is a function of both a and b or just a function of a? Does Alice's result probability also depend on the setting Bob has chosen?

Theoretically we might want to make the 'locality' assumption which is to state that Alice's result probability is independent of some remote setting of Bob's. Or to state that P(A | a,b) = P( A | a).

Bell goes further - he hypothesizes that there is some 'cause' for the correlation in terms of variables that we don't know about, or don't control - and that if we only did know the values of these variables we'd be able to explain the correlation. So he assumes that what we really have is a distribution of the form
P(A,B | a,b,h) where h is a symbol that stands for this collection of 'hidden' variables - which could be just one variable, a whole collection of them, or functions etc - the actual details are irrelevant.

By 'explain' we mean that we can write P(A,B | a,b,h) = P(A | a,b,h) P(B | a,b,h)

The locality assumption means that we can reduce this further to P(A,B | a,b,h) = P(A | a, h) P(B | b,h)

Bell showed that IF we make this hidden variable assumption then the data has to satisfy an inequality. [and in the proof there's also an assumption that it's meaningful to talk about the statistics of results if we'd measured things using a different angle - the counterfactual assumption]

The amazing thing, well it's amazing to me anyway, is that Bell has reduced the entire question to simply counting 'pings and dings' and reading 'angles'. Breath taking

If you really want to understand this I strongly recommend the Bertlmann's socks paper linked to by Bhobba

There is - its different to classical correlations such a bell mentions in his seminal paper with Bertlmanns socks:
https://cds.cern.ch/record/142461/files/198009299.pdf

Bell explains it with far greater clarity and insight than I could ever achieve.

 

[bhobba]

Bell explains it with far greater clarity and insight than I could ever achieve.

That he does.

But what can you say about one of the greatest physicists ever? All you can really say is he was a future Nobel Laureate before his untimely death:
http://www.americanscientist.org/bookshelf/pub/john-bell-across-space-and-time
http://www.irishphilosophy.com/2014/11/04/john-stewart-bell/

What is the key quality that elevates the greatest from the merely great? Its best illustrated in a story. One of the greatest mathematicians ever, likely in the top 10 of all time, a man Feynman readily acknowledged as above even him, just as much a magician as Feynman was, was the truly great polymath, not just a mathematician, but something much much greater, a polymath, John Von-Neumann. His mathematical insight was simply beyond compare, technically above even many of his great contemporaries, and well above Einstein who, while a competent mathematician was not even close to the class of Von-Neumann. Even Von-Neumann reportedly admitted Einstein was greater, as well as his contemporaries like Wigner. The reason is physical insight. Von-Neumann, along with Feynman, had that in spades. They could both see to the heart of a problem with frightening ease. But against this Einstein was greater again - he was simply unsurpassed. And this is the key to making progress - not the frightening ease with the substance behind the equations possessed by Feynman and Landau but few others, not the sublime mathematical competence of Von-Neumann - all very important of course - but the ability to see to the heart of the problem.

This Bell had, just as assuredly as Einstein did. This is what elevated him above the rest and assured him of a future Nobel.

Thanks
Bill
.

 

[Zafa Pi]

Well, think about what data Alice and Bob are collecting.

It is clear to me that you you didn't understand what I was asking.
Assume for the moment that I am familiar with the various Bell theorems and inequalities as well as Bertlmann's socks (because I am).
The OP is asking whether "The 'state of affairs' relevant to the outcome at A is independent of the setting at B." is true, false. or inapplicable.
In post #8 you wrote, "The results obtained by Alice are statistically independent of the setting of Bob,". In post #2 DrChinese believes the opposite.
You will notice that Bob is not collecting data, but merely choosing a setting, as I pointed out. The results obtained by Alice are +1 or -1 with probability 1/2.
Admittedly the data collected by both Alice and Bob are correlated, i.e. not the result of independent (1,-1) valued random variables. But how does one tell if her results are independent of just Bob's setting? What does it mean?

 

[Simon Phoenix]

But how does one tell if her results are independent of just Bob's setting? What does it mean?

I don't really understand what your problem is here.

If Alice does measurements and Bob just twiddles with his equipment (so to speak) but does no measurements then the measurement data we have is simply Alice's. Conceivably Alice's results might depend on Bob's setting as well as her own - but this can easily be checked from the data. In fact it's going to be a pretty dull experiment since nothing in this experiment is correlated at all :-)

You say you're familiar with the theory - so presumably you understand the locality condition that's imposed when Bell writes
P(A,B | a,b,h) = P(A | a,b,h) P(B | a, b,h) = P(A | a,h) P(B | b,h)
So what's your problem here? This is nothing more than an explicit recognition that Alice's results are assumed not to be conditioned upon Bob's setting (and vice versa). I don't see what your issue is with results 'here' being statistically independent of settings 'there'.

What am I missing?

Also, if you read post 2 carefully - I'm pretty sure Dr Chinese does not imply that Alice's results (alone) are dependent on Bob's setting. I think he's talking about the joint results of both Alice and Bob which are, of course, dependent on the relative angle of the settings.

[edit : piece removed that I need to think about a bit more and explain better :-) ]

 

[Eye_in_the_Sky]

You will notice that Bob is not collecting data, but merely choosing a setting, as I pointed out.

I will attempt to clarify.

Go back to the statement:

A joint-measurement of an (appropriately prepared) entangled property is performed in spacetime regions A and B at spacelike separation.

This is a scenario in which both Alice and Bob collect data. It is a "joint-measurement".

Next:

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

This is a comment about that very scenario.

By symmetry (in virtue of the 'structure' of spacetime), whatever value (i.e. true, false, or inapplicable) you assign to it, you are obliged to assign the same value to its vice versa:

The 'state of affairs' relevant to the outcome at B is independent of the setting at A.

Here are both statements in more compact form:

Bob's setting is IRRELEVANT to Alice's outcome.

Alice's setting is IRRELEVANT to Bob's outcome.
__________________

Now, you might ask, "How is the term IRRELEVANT to be understood/applied in our context?"
____

One aspect is as follows:

separable & mutually non-influencing → IRRELEVANCE .

The expansion of the terms in this expression is as follows:

separable: the joint-state of Alice's (measuring) instrument and Bob's (measuring) instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

So, in expanded form, the shorthand expression will read as:

IF

the joint-state of their instruments, in spacetime, is separable

AND

their instruments are mutually non-influencing

THEN

each one's setting is IRRELEVANT to the outcome of the other .

Now, go back to the original expression:

separable & mutually non-influencing → IRRELEVANCE .

This is a property of spacetime.
____

There is another aspect of how IRRELEVANCE is to be understood/applied.

This aspect is in connection with any consistent theory (purported to be about the phenomenon under consideration), as follows:

If Bob's setting is IRRELEVANT to Alice's outcome, and if the theory in question is CORRECT in this regard, then:

No contradiction can arise in the theory by supposing that Alice's outcome for (the hypothetical setting) b₂ would have been the same as that for (the actual setting) b₁.

This is a property that any consistent theory of the phenomenon is expected to have.
__________________

 

[Eye_in_the_Sky]

i) If Bob's setting had been b₂ instead of b₁, each of Alice and Bob would have obtained a definite outcome.

That statement is true, depending on what you mean by 'definite outcome'.

Simon, you have understood the statement exactly as I have meant it.

The statement is true by hypothesis. The hypothesis is that of an idealized experiment in which:

No matter what each one's setting happens to be, each one will obtain any of two possible outcomes, "YES" or "NO".

_____________________
_____________________

As Scotty never said to Kirk "It's correlation, Jim, but not as we know it"

- OR -

As Spock never did while saying, nor said:

Raising an open Hand, "Five by five," uttered the Vulcan.

_____________________
_____________________

 

[RUTA]

You may find Mermin's characterization of this experiment helpful https://www.physicsforums.com/insights/quantum-liar-experiment-instantiation-mermin-device/. [You can ignore the Quantum Liar aspect.]

 

[DrChinese]

Also, if you read post 2 carefully - I'm pretty sure Dr Chinese does not imply that Alice's results (alone) are dependent on Bob's setting. I think he's talking about the joint results of both Alice and Bob which are, of course, dependent on the relative angle of the settings.

You are right, of course. I said: "Both settings (A and B) are inputs to something (I don't know what or where or when). Therefore the outcomes at A and B reflect in some manner the mutual relationship of both settings. Therefore they are not independent."

We must consider the entire context. So I don't see how we can separate Alice and Bob's measurement choices from the rest of the experiment.

 

[Eye_in_the_Sky]

Does anyone have a comment on this part?

separable & mutually non-influencing → IRRELEVANCE

So, in expanded form, the shorthand expression will read as:

IF

the joint-state of their instruments, in spacetime, is separable

AND

their instruments are mutually non-influencing

THEN

each one's setting is IRRELEVANT to the outcome of the other .

Now, go back to the original expression:

separable & mutually non-influencing → IRRELEVANCE .

This is a property of spacetime.

True or false?

 

[DrChinese]

...the joint-state of their instruments, in spacetime, is separable...

It's all a part of the full context. So they would not be separable as you described.

The Bohmian has the easier time discussing this particular point as nothing is isolated by space - everything is affected by everything else. In other interpretations, there are somewhat more complex explanations, as both space and time (spacetime) are factors.

 

[morrobay]

Does anyone have a comment on this part?

separable & mutually non-influencing → IRRELEVANCE

True or false?

What about separable + mutually non-influencing = locality.
And from Bells Bertlmanns Socks , p. 4'
Einstein had no difficulty accepting that affairs in different places could be correlated.
What he could not accept was that an intervention at one place could influence, immediately affairs at the other "
And to address your OP : A joint measurement on entangled ( non separable) particles at A and B
"The state of affairs relevant to outcome at A is not independent of setting at B.

 

[Eye_in_the_Sky]

What about separable + mutually non-influencing = locality.
And from Bells Bertlmanns Socks , p. 4'
Einstein had no difficulty accepting that affairs in different places could be correlated.
What he could not accept was that an intervention at one place could influence, immediately affairs at the other "
And to address your OP : A joint measurement on entangled ( non separable) particles at A and B
"The state of affairs relevant to outcome at A is not independent of setting at B.

Both of us consider the statement below to be false:

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

(And so by symmetry, the statement with A and B transposed is also false.)

It seems to me that by "independent" we both 'mean' the same thing.

Now, let's bring in what you quote from Bertlmann's Socks:

Einstein had no difficulty accepting that affairs in different places could be correlated. What he could not accept was that an intervention at one place could influence, immediately, affairs at the other.

It is Bell himself who underlines the word "influence" in that second sentence. Yet, I disagree with the use of that word there. As I see it, 'locality' (in our context) has two aspects to it:

1) state 'separability' (of Alice and Bob's instruments) ;

2) influences 'cannot be superluminal' .

Thus, the correct diction for expressing the (alleged) 'nonlocal' nature of the intervention must have in it a notion of both influence and state.

I will rewrite 1) and 2) as:

1) separability ;

2) local causality .

Now, the quote from Bertlmann's Socks was brought by you in connection with a suggestion you made at the opening of your post:

What about separable + mutually non-influencing = locality.

I'd rather write it like this:

separability & local causality = locality .

Of course, in our case (due to 'spacelike separation') it is true that 'local causality' would imply 'mutually non-influencing'. So,

spacelike separation & locality → separable & mutually non-influencing ,

and

separable & mutually non-influencing → IRRELEVANCE .

Here again are the meanings of the terms I am using:

separable: the joint-state of Alice's (measuring) instrument and Bob's (measuring) instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

_________

The next part in the argument I would like to make is this:

There is another aspect of how IRRELEVANCE is to be understood/applied.

This aspect is in connection with any consistent theory (purported to be about the phenomenon under consideration), as follows:

If Bob's setting is IRRELEVANT to Alice's outcome, and if the theory in question is CORRECT in this regard, then:

No contradiction can arise in the theory by supposing that Alice's outcome for (the hypothetical setting) b₂ would have been the same as that for (the actual setting) b₁.

This is a property that any consistent theory of the phenomenon is expected to have.

... Morrobay, can you see where I am going with this? ... Does ANYONE (reading this) see where I am going?

Well (if I am not wrong), the above considerations lead to the conclusion that ONE of the following statements is TRUE:

B) Quantum Theory is inconsistent.

C) Bob's setting is IRRELEVANT to Alice's outcome, but Quantum Theory is INCORRECT in this regard.

A) Bob's setting is NOT IRRELEVANT to Alice's outcome.

I [am forced to] choose A), and I am thinking about this:

separable & mutually non-influencing → IRRELEVANCE .

 

[Simon Phoenix]

Both of us consider the statement below to be false:

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

OK - without getting too bogged down with the precise meaning of 'state of affairs' let us assume that this statement is false.

In other words, it is assumed there is some 'state of affairs' that is 'relevant' to the outcome at A that can be changed by a change of setting at B.

But what does 'relevant' mean? It can only mean that there is some measurable consequence - if there were no such consequence (no measureable change in the actual pings or dings) then in what sense would one actually describe it as 'relevant' to an outcome?

So let's assume there is some source S which emits these entangled particles and one goes off to A and one goes off to B. In the rest frame of S we will assume that changes of settings and subsequent measurements of A and B are simultaneous (which can easily be arranged with appropriate synchronisation). We will also consider A and B to be spacelike separated.

Let's suppose that 2 observers Clive and Doris are moving with respect to S such that Clive judges the changes of settings and measurements at A to occur before those at B. In Doris' frame she judges that the changes of settings and measurements at B occur before those at A.

The results, or the outcomes, the actual pings and dings on the detectors, are clearly independent of inertial frame.

Clive's perspective, based on our assumption that there IS some change relevant to the outcome, is that A has influenced the outcome at B. Doris would say the opposite. Given that the choices of measurement setting are made independently how does the frame-independence of the results make any kind of sense under the assumption that changes of settings influence 'states of affairs' relevant to an outcome?

Is it changes at A that are influencing the outcome at B, or changes at B that are influencing the outcome at A?

How can we then ascribe some influence on the frame-independent results to changes of either setting?

 

[morrobay]

Both of us consider the statement below to be false:

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

Not so sure this statement is false now : At spacelike separation the only thing that is happening instantaneously in EPR - Bell experiments when the setting at A or B is changed is the relative angle: 1/2 (sin θ/2)² and outcome correlations.

 

[Eye_in_the_Sky]

In my opinion, there is no way to make sense of the notion of one thing influencing another without indulging in counter-factual reasoning. You want to say that flipping a light switch caused the light to come on, but how can you distinguish that from mere correlation?

As Scotty never said to Kirk "It's correlation, Jim, but not as we know it"

_________

The meaning of "cause" here (in my way of thinking) necessarily involves the counter-factual consideration: "If I (counter to fact) had not flipped the switch, the light would not have come on".

In an unaired and never filmed Star Trek episode, for which a script was never written, Spock said:

In other words, the 'principle of causality' is rendered inapplicable if the joint-state of the "light switch" and the "light bulb" is 'nonseparable'.

Raising an open Hand, "Five by five," uttered the Vulcan.

 

[Eye_in_the_Sky]

Explanation in RBW is adynamical, so dynamical talk about "influences" wouldn't enter the explanans.

So, 'causality' as a principle has no place in RBW. Correct?

 

[RUTA]

So, 'causality' as a principle has no place in RBW. Correct?

Well, concerning quantum phenomena, that's what we would say, yes. Retrocausal advocates argue otherwise. They have a so-called “interventionist” account of causality. See the Insight https://www.physicsforums.com/insights/retrocausality/

 

[Zafa Pi]

Within QM theory if each of Alice and Bob are measuring one of a pair of entangled photons (from say √½(|00⟩ + |11⟩)) then each are observing a ±1 valued random variable with prob 1 = prob-1 = ½, irrespective of their observables (settings). These two r.v.s can range from independent to completely correlated depending on A and B's settings. That is all I understand.
If you think it is reasonable to ask whether Bob's setting has an affect on what Alice observes please propose a coherent method of how one could tell. For example, if A and B both choose the same setting then they will both observe the same value (1 or -1). Does that mean B's setting affects what Alice sees (and vise versa)?
A definition of "affect" here would be helpful. Does Alice like what she sees depending on Bob's setting?

Locality and CFD together are (is?) sufficient to prove a Bell inequality, which in turn can be contradicted by experiment. I'm not sure what more you're after. ("you" refers to anybody on this thread)

 

[morrobay]

1. Predictions for spin 1/2 pairs with Aa and Bb settings in xy plane for P++ = P-- is 1/2 (sin θ/2)²
With given settings above in setup change Aa to Aa' orientation and calculate prediction with a' - b = θ'
2. Now do experiment with Aa' setting and given Bb setting above
3. If experimental results for P++ and P-- match prediction for settings Aa' and Bb . θ', then there was no influence at Bb when setting change was made at Aa to Aa'

 

[Eye_in_the_Sky]

Well, concerning quantum phenomena, that's what we would say, yes. Retrocausal advocates argue otherwise. They have a so-called “interventionist” account of causality. See the Insight https://www.physicsforums.com/insights/retrocausality/

Thank you, for that, RUTA. I think I will have some more questions to ask you about RBW and the adynamical approach. ... But, in the mean time, I have a different sort of question on my mind and I am wondering what your opinion is.

Do you consider the statement below to be axiomatically true (where "the theory in question" can be any consistent theory 'about' the phenomenon)?

If Bob's setting is IRRELEVANT to Alice's outcome, and if the theory in question is CORRECT in this regard, then:

No contradiction can arise in the theory by supposing that Alice's outcome for (the hypothetical setting) b₂ would have been the same as that for (the actual setting) b₁.

 

[RUTA]

Thank you, for that, RUTA. I think I will have some more questions to ask you about RBW and the adynamical approach. ... But, in the mean time, I have a different sort of question on my mind and I am wondering what your opinion is.

Do you consider the statement below to be axiomatically true (where "the theory in question" can be any consistent theory 'about' the phenomenon)?

If Bob's setting is IRRELEVANT to Alice's outcome, and if the theory in question is CORRECT in this regard, then:

No contradiction can arise in the theory by supposing that Alice's outcome for (the hypothetical setting) b₂ would have been the same as that for (the actual setting) b₁.

I agree with Dr. Chinese (who is an expert in this area) -- Bob's setting is relevant to Alice's outcome in the sense that the Bell-inequality violating correlations obtain.

 

[Eye_in_the_Sky]

I agree with Dr. Chinese (who is an expert in this area) -- Bob's setting is relevant to Alice's outcome in the sense that the Bell-inequality violating correlations obtain.

Okay. Nobody knows what I'm talking about with that. So, I'll just explain what I'm trying to accomplish with it.

I am trying to resolve the dispute below.
_________

All Bell shows is if you want to have properties when not observed you need non local influences.

I agree with it (except for the usage of "All" and "is" in the sentence).

Do you agree that the following statement is also true?

[All] Bell shows [is] if you want the joint-state of Alice's ((macroscopic) measuring) instrument and Bob's ((macroscopic) measuring) instrument, in spacetime, to be separable then you need non-local influences.

No.

Its just a creelation. Thats it, that's all.

_________

SUMMARY:

Bhobba contends that the quantum correlations are compatible with both of these conditions taken together:

1) The joint state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable.

2) Their measuring instruments are mutually non-influencing.

I am saying that (at least) one of these conditions needs to be relinquished.
________________

Can anyone resolve this dispute? If not, can you make some helpful remarks about it?

 

[Eye_in_the_Sky]

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.

OK - without getting too bogged down with the precise meaning of 'state of affairs' let us assume that this statement is false.

In other words, it is assumed there is some 'state of affairs' that is 'relevant' to the outcome at A that can be changed by a change of setting at B.

But what does 'relevant' mean? It can only mean that there is some measurable consequence - if there were no such consequence (no measureable change in the actual pings or dings) then in what sense would one actually describe it as 'relevant' to an outcome?

Consider spin-½ singlet.

Suppose Bob measures S_x and obtains the result q, where q is one of "UP" or "DOWN". Then the 'state of affairs' relevant to Alice's outcome is such that

if Alice measures S_x she cannot obtain q.

On the other hand, if Bob had measured S_y instead, then (no matter what the outcome of Bob) the 'state of affairs' relevant to Alice's outcome would have been such that

if Alice measures S_x she can obtain q.
_______

Simon – and anyone else who would like to comment – in your eyes, does the above example demonstrate 'relevance'?

 

[N88]

Okay. Nobody knows what I'm talking about with that. So, I'll just explain what I'm trying to accomplish with it.

I am trying to resolve the dispute below.
__________________

SUMMARY:

Bhobba contends that the quantum correlations are compatible with both of these conditions taken together:

1) The joint state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable.

2) Their measuring instruments are mutually non-influencing.

I am saying that (at least) one of these conditions needs to be relinquished.
________________

Can anyone resolve this dispute? If not, can you make some helpful remarks about it?

HTH; it's all about correlations:

Under EPRB (Bell 1964), let a and b be the orientations of the principal axes of Alice's device and Bob's device in 3-space, respectively. Then the orientation of their respective output channels is ±a and ±b, and the corresponding results are A^± and B^±.

1. Under Einstein-locality, their devices are mutually non-influencing: the state of Alice's device D(a) is independent of Bob's device D(b).

2. BUT their devices are correlated by the function: C[D(a), D(b)] = cos(a,b). When (a,b) = 0, C = 1 and the devices are parallel; when (a,b) = π/2, C= 0 and the devices are orthogonal; when (a,b) = π, C = -1 and the devices are antiparallel; etc. So their output channels are also correlated.

3. Now, let each particle pair be anti-correlated via the pairwise conservation of total angular momentum; ie, the λ heading toward Alice is the opposite of the λ heading toward Bob.

4. No surprise then that the correlation-based expectation <AB> should equal - a.b.

 

[N88]

The 'state of affairs' relevant to the outcome at A is independent of the setting at B.Consider spin-½ singlet.

Suppose Bob measures S_x and obtains the result q, where q is one of "UP" or "DOWN". Then the 'state of affairs' relevant to Alice's outcome is such that

if Alice measures S_x she cannot obtain q.

On the other hand, if Bob had measured S_y instead, then (no matter what the outcome of Bob) the 'state of affairs' relevant to Alice's outcome would have been such that

if Alice measures S_x she can obtain q.
_______

Simon – and anyone else who would like to comment – in your eyes, does the above example demonstrate 'relevance'?

Caution required: Alice cannot ever obtain S_x = q from the particle that paired with the particle from which Bob obtained S_x = q. The probability that she obtains q in the second example (Alice measures S_x; Bob measures S_y on a new particle-pair and gets ±q) is well-known.

 

[RUTA]

Okay. Nobody knows what I'm talking about with that. So, I'll just explain what I'm trying to accomplish with it.

I am trying to resolve the dispute below.

SUMMARY:

Bhobba contends that the quantum correlations are compatible with both of these conditions taken together:

1) The joint state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable.

2) Their measuring instruments are mutually non-influencing.

I am saying that (at least) one of these conditions needs to be relinquished.
________________

Can anyone resolve this dispute? If not, can you make some helpful remarks about it?

Both 1 and 2 are correct because the measuring devices obey classical (non-quantum) physics. To understand what's happening in that classical context with the outcomes of the quantum experiment, you need an interpretation of QM.

 

[Eye_in_the_Sky]

Both 1 and 2 are correct because the measuring devices obey classical (non-quantum) physics. To understand what's happening in that classical context with the outcomes of the quantum experiment, you need an interpretation of QM.

If you say that both 1 and 2 together are compatible with the quantum correlations, then you must also be saying that the following implication is NOT a property of spacetime:

separable & mutually non-influencing → IRRELEVANCE .

Am I right about that, RUTA? By your reckoning this is NOT a property of spacetime.
________

Here are the meanings of the terms I am using:

separable: the joint-state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

 

[RUTA]

If you say that both 1 and 2 together are compatible with the quantum correlations, then you must also be saying that the following implication is NOT a property of spacetime:

separable & mutually non-influencing → IRRELEVANCE .

Am I right about that, RUTA? By your reckoning this is NOT a property of spacetime.
________

Here are the meanings of the terms I am using:

separable: the joint-state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

You're trying to conflate outcomes in the measuring devices with whatever quantum system is responsible for those outcomes. Essentially at that point you're trying to make the measuring devices quantum in nature.

 

[Eye_in_the_Sky]

You're trying to conflate outcomes in the measuring devices with whatever quantum system is responsible for those outcomes. Essentially at that point you're trying to make the measuring devices quantum in nature.

You didn't answer my question.

The IMPLEMENTATION of a setting and the REGISTRATION of an outcome:
- each is an EVENT in spacetime
- each is part of the STATE-description of the instrument

So please, answer my question.

"YES" or "NO"? Is the implication below a property of spacetime?

separable & mutually non-influencing → IRRELEVANCE
________

Here, again, are the meanings of the terms I am using:

separable: the joint-state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

 

[RUTA]

You didn't answer my question.

The IMPLEMENTATION of a setting and the REGISTRATION of an outcome:
- each is an EVENT in spacetime
- each is part of the STATE-description of the instrument

So please, answer my question.

"YES" or "NO"? Is the implication below a property of spacetime?

separable & mutually non-influencing → IRRELEVANCE
________

Here, again, are the meanings of the terms I am using:

separable: the joint-state of Alice's measuring instrument and Bob's measuring instrument, in spacetime, is separable

mutually non-influencing: each one's instrument is uninfluenced by that of the other

IRRELEVANCE: each one's setting is IRRELEVANT to the other's outcome

You keep talking about the measuring devices as if they're in a quantum (unobserved) state. In that case, you need an interpretation of QM to answer questions about their status in spacetime.

 

[Eye_in_the_Sky]

You keep talking about the measuring devices as if they're in a quantum (unobserved) state. In that case, you need an interpretation of QM to answer questions about their status in spacetime.

Hi, RUTA. I was digging through my old notes on my computer and came upon something which might prove helpful. It's a post of yours from some six years ago, and here's the part I'm quoting:

In today’s terminology we would say that the spacetime picture of relativity adheres to the following principles (Howard, 1997, pp 124-125):

Separability principle: any two systems A and B, regardless of the history of their interactions, separated by a non-null spatiotemporal interval have their own independent real states such that the joint state is completely determined by the independent states.

Locality principle: any two space-like separated systems A and B are such that the separate real state of A let us say, cannot be influenced by events in the neighborhood of B.

It is now generally believed that Einstein-Podolsky-Rosen (EPR) correlations, i.e., correlated space-like separated experimental outcomes which violate Bell’s inequality, force us to abandon either the separability or locality principle.

Here is the link:
https://www.physicsforums.com/threa...t-and.369328/page-28#post-2753865#post2753865
_______

From what you are saying here and now in this thread, it sounds like the belief you mentioned back then is no longer accepted. Even stronger than that, it sounds like you are saying the belief is demonstrably false. If so, what is the demonstration?

 

[Simon Phoenix]

Simon – and anyone else who would like to comment – in your eyes, does the above example demonstrate 'relevance'?

Well in this reasoning you're assuming a definite temporal order for the measurement events of Alice and Bob. But it's kind of the point of Bell experiments that the measurement events are spacelike separated (if they were not we would not able to rule out local hidden variable theories).

If the measurement events are spacelike separated then there exist frames of reference for which the events occur in the reverse order.

So is it Bob influencing Alice's 'state of affairs', or vice versa?

 

[entropy1]

I'm probably dead wrong, but this is my take on it for now:

Suppose Bob has setting b1, and Alice gets a series of outcomes S. Now suppose that if Bob had had setting b2, Alice would have got the same series of outcomes S. Then, since the correlation has changed (due to the changed settings of Bob), and Alice's outcomes are the same, Bob's outcomes must have changed. And this goes vice versa for Alice. So, this would mean that Alice's outcomes would only depend on her settings, and similarly for Bob. But then there would be no correlation dependent on the relative (!) parameters (it would be local). So, I suppose then that Bob's setting does influence the outcomes of Alice (and vice-versa). It just happens in a way that it is not noticed (locally)!

*hiding under a stone*

 

[zonde]

If the measurement events are spacelike separated then there exist frames of reference for which the events occur in the reverse order.

So is it Bob influencing Alice's 'state of affairs', or vice versa?

In Relativity simultaneity is only convention. There are no physical consequences for simultaneity because there are no FTL phenomena. If you speculate about FTL phenomena there are physical consequences for simultaneity and it can't be just convention. So it's outside of domain of applicability for Relativity.

 

[RUTA]

Hi, RUTA. I was digging through my old notes on my computer and came upon something which might prove helpful. It's a post of yours from some six years ago, and here's the part I'm quoting:

Here is the link:
https://www.physicsforums.com/threa...t-and.369328/page-28#post-2753865#post2753865
_______

From what you are saying here and now in this thread, it sounds like the belief you mentioned back then is no longer accepted. Even stronger than that, it sounds like you are saying the belief is demonstrably false. If so, what is the demonstration?

The measuring devices satisfy both principles, but the quantum systems responsible for the Bell-inequality violations do not.

 

[Eye_in_the_Sky]

You're trying to conflate outcomes in the measuring devices with whatever quantum system is responsible for those outcomes. Essentially at that point you're trying to make the measuring devices quantum in nature.

Can we, at least, agree on this much?

If one says there is no violation of 'causal locality', then one is forced to say that the PAIR of spacetime events – i.e. the IMPLEMENTATION of Bob's setting and the REGISTRATION of Alice's outcome – is 'nonseparably connected'.

 

[Eye_in_the_Sky]

Within QM theory if each of Alice and Bob are measuring one of a pair of entangled photons (from say √½(|00⟩ + |11⟩)) then each are observing a ±1 valued random variable with prob 1 = prob-1 = ½, irrespective of their observables (settings). These two r.v.s can range from independent to completely correlated depending on A and B's settings. That is all I understand.

Excellent! Me too.

All of the rest of the matter for me is mired by the poor diction and the fuzzy and entangled concepts with which we-all have fallen into using in our discussions of this topic.

A definition of "affect" here would be helpful.

If you think it is reasonable to ask whether Bob's setting has an affect ...

I have paused the statement at the word "affect"; it is a verb. The correct word is "effect"; it is a noun. I will define them both as follows:

effect: a 'state of affairs' that is brought about by a 'cause'

affect: to act, in a manner of 'causation', so as to produce an 'effect'

I will also define three more words:

influence: the agency through which an 'effect' is established

causation: the relationship between 'cause' and 'effect'

causality: the notion of 'causation'

 

[RUTA]

Can we, at least, agree on this much?

If one says there is no violation of 'causal locality', then one is forced to say that the PAIR of spacetime events – i.e. the IMPLEMENTATION of Bob's setting and the REGISTRATION of Alice's outcome – is 'nonseparably connected'.

Again, without specifying a particular QM interpretation, I would say the two outcomes are nonseparable if they are locally causal.

 

[Eye_in_the_Sky]

Again, without specifying a particular QM interpretation, I would say the two outcomes are nonseparable if they are locally causal.

Okay. I will recast my claim as follows:

A joint-measurement of an (appropriately prepared) entangled property is performed in spacetime regions A and B at spacelike separation.

If one says there is no violation of 'causal locality', then one is forced to say:

The 'state of affairs' in spacetime region A and the 'state of affairs' in spacetime region B are together in a condition of 'nonseparability'.

 

[RUTA]

Okay. I will recast my claim as follows:

A joint-measurement of an (appropriately prepared) entangled property is performed in spacetime regions A and B at spacelike separation.

If one says there is no violation of 'causal locality', then one is forced to say:

The 'state of affairs' in spacetime region A and the 'state of affairs' in spacetime region B are together in a condition of 'nonseparability'.

If by "state of affairs" you're referring to the device settings and experimental outcomes, then yes.

 

[Simon Phoenix]

In Relativity simultaneity is only convention

How so?

Let's imagine the classic train scenario. Alice is placed in the centre of a train carriage. Unfortunately for our hapless heroine, some dastardly criminal has strapped an explosive device to her. There is a light source at each end of the carriage. If the light from these sources reaches her at the same time as judged by the photodetectors on the explosive device - then it's goodbye Alice.

Bob, sitting on the embankment, watches the train go past and sees flashes from the end of the carriage that are simultaneous (by his reckoning) just as Alice passes him.

So does our heroine survive, or is she blown to bits?

I suspect Alice and Bob are going to view 'simultaneity' as something a little more serious than merely 'convention'

So it's outside of domain of applicability for Relativity

Again, how so?

The measurements of Alice and Bob are merely 2 events in spacetime. It doesn't matter one jot whether these events are measurements on entangled particles or measurements of the colour of the eyes on two fluffy bunnies. The events don't even have to be measurements of any kind - just two points in spacetime where something could happen, in fact.

What you seem to be saying is that relativity is not applicable for all possible events at these spacetime locations.

If there is a spacelike interval separating these two events, then it is possible that there are different orderings for the events in different frames.

So how can we say which event 'influences' the other when we talk of entanglement?

 

[zonde]

If the light from these sources reaches her at the same time as judged by the photodetectors on the explosive device - then it's goodbye Alice.

Bob, sitting on the embankment, watches the train go past and sees flashes from the end of the carriage that are simultaneous (by his reckoning) just as Alice passes him.

So does our heroine survive, or is she blown to bits?

I suspect Alice and Bob are going to view 'simultaneity' as something a little more serious than merely 'convention'

We can use word "simultaneously" in two different ways. We can describe single spacetime event (light pulses arrive at the same time at some place) or we can describe two distant spacetime events (light pulses are emitted at the same time from separate sources). If we describe single spacetime event then of course it's physical fact and has nothing to do with any convention.

Again, how so?

The measurements of Alice and Bob are merely 2 events in spacetime. It doesn't matter one jot whether these events are measurements on entangled particles or measurements of the colour of the eyes on two fluffy bunnies. The events don't even have to be measurements of any kind - just two points in spacetime where something could happen, in fact.

What you seem to be saying is that relativity is not applicable for all possible events at these spacetime locations.

Points in spacetime diagram by themselves represent physical facts and it has little to do with what I'm saying.

If there is a spacelike interval separating these two events, then it is possible that there are different orderings for the events in different frames.

Inconsistent orderings of spacetime events in different reference frames is a feature of relativity. It's fine (and very convenient) as long as there is no FTL phenomena.

 

[Simon Phoenix]

We can use word "simultaneously" in two different ways

We can use the word "simultaneously" however we like, but there is only one meaning in physics really. Two events are said to be simultaneous in a given inertial reference frame if they have the same time coordinate in that frame.

Simultaneity is relative, if this is what you mean by a matter of 'convention' then I agree. I would not personally describe it as a 'convention' though.

Some authors have even suggested that in fact one could even go so far as to suggest that all of special relativity is really a study of the relativity of simultaneity - not sure I'd fully agree with that statement, but I can see where they're coming from, so to speak.

Inconsistent orderings of spacetime events in different reference frames is a feature of relativity. It's fine (and very convenient) as long as there is no FTL phenomena.

So let me try to understand what you're saying.

Experiment 1 : our Alice and Bob make spacelike separated measurements on two particles. These particles are just prepared in random states with no correlation or entanglement whatsoever

Experiment 2 : same as above but now with entangled particles

Are you suggesting, somehow, that special relativity is applicable in the first experiment (so we're entitled to say that the order of measurement can differ in differing frames), but not in the second because we're now making measurements of entangled particles?

Or are you suggesting that entanglement (and maybe the Bohm view of things) invalidates special relativity?

 

[zonde]

So let me try to understand what you're saying.

Experiment 1 : our Alice and Bob make spacelike separated measurements on two particles. These particles are just prepared in random states with no correlation or entanglement whatsoever

Experiment 2 : same as above but now with entangled particles

Are you suggesting, somehow, that special relativity is applicable in the first experiment (so we're entitled to say that the order of measurement can differ in differing frames), but not in the second because we're now making measurements of entangled particles?

You might say so. Basically relativity is applicable in both cases as long as we describe our observations phenomenologically and do not speculate about possible physical models behind entanglement.

Or are you suggesting that entanglement (and maybe the Bohm view of things) invalidates special relativity?

Sort of yes, but I would rather say that entanglement phenomena (violation of Bell inequalities) indicates that domain of applicability of relativity is limited.

 

[Eye_in_the_Sky]

Suppose Bob has setting b1, and Alice gets a series of outcomes S. Now suppose that if Bob had had setting b2, Alice would have got the same series of outcomes S. Then, since the correlation has changed (due to the changed settings of Bob), and Alice's outcomes are the same, Bob's outcomes must have changed. And this goes vice versa for Alice. So, this would mean that Alice's outcomes would only depend on her settings, and similarly for Bob. But then there would be no correlation dependent on the relative (!) parameters (it would be local). So, I suppose then that Bob's setting does influence the outcomes of Alice (and vice-versa). It just happens in a way that it is not noticed (locally)!

Hi, entropy1. Thanks for contributing to this thread.

There is a difficulty with the above argument. The argument, as it stands, would apply equally well to a 'classical' correlation experiment. ... But, even there, there can be "correlation dependence" on the "relative parameters".

So, how can we clarify the matter? One way, is to do a step-by-step deconstruction of the full Bell argument in this context. Another way, is to choose a different entanglement scenario altogether, a much SIMPLER one, and pose our queries upon that background instead.

"Ah," you might ask, "there is a SIMPLER entanglement scenario I can consider?"

The answer is YES. And thus, I have started a new thread entitled:

"Bell made Simple - HARDY".

 

[Eye_in_the_Sky]

If one says there is no violation of 'causal locality', then one is forced to say:

The 'state of affairs' in spacetime region A and the 'state of affairs' in spacetime region B are together in a condition of 'nonseparability'.

If by "state of affairs" you're referring to the device settings and experimental outcomes, then yes.

Absolutely, the 'state of affairs' would include those.
________

You keep talking about the measuring devices as if they're in a quantum (unobserved) state. In that case, you need an interpretation of QM to answer questions about their status in spacetime.

Consider an inertial frame of reference in which the pair of outcomes occurs simultaneously, and let t_o be the time of occurrence in that frame.

Suppose there is no violation of 'causal locality', and suppose further that the joint-state of their instruments is 'separable' both before and after t_o.

But at t_o:

Each one's outcome is 'nonseparable' from the setting of the other; therefore, the joint-state of their instruments is 'nonseparable'.

... Is that wrong to say?

 

[RUTA]

Absolutely, the 'state of affairs' would include those.
________

Consider an inertial frame of reference in which the pair of outcomes occurs simultaneously, and let t_o be the time of occurrence in that frame.

Suppose there is no violation of 'causal locality', and suppose further that the joint-state of their instruments is 'separable' both before and after t_o.

But at t_o:

Each one's outcome is 'nonseparable' from the setting of the other; therefore, the joint-state of their instruments is 'nonseparable'.

... Is that wrong to say?

Now you're talking about the "state of the instruments," being "nonseparable" so I assume you're talking about the instruments in terms of being in a quantum state. I don't know what else you mean. If so, you need an interpretation of QM to discuss the situation ontologically because you have to deal with the measurement problem.