Is Bell's Theorem a Valid Solution to the Locality Versus Nonlocality Issue?

[SpectraCat]

Hidden in the highlighted phrase is a modal fallacy. A prediction MUST always be conditioned on the assumptions, ie it can not be true apart from its conditioning assumptions. For example "If Bob and Alice measure the two photons at angles b and a, they will obtain x, and y" and "If Bob and Alice measure the two photons at angles c and d, they will obtain r, and s" These two statements can both be true at the same time because they both contain their conditioning statements built in. However, this does not mean "x, y, r, and s" must simultaneously exist. Which ones exist, will depend on which of the conditioning statements were actually realized based on which experiment has already been performed. Say Alice and Bob have measure the two photons at angles a and b. At that instance, "x and y" have independent truth values because it is a fact that Alice and Bob have measured at b and a. However, the other statement now becomes a counterfactual statement. "Had Bob and Alice measured the two photons at angles c and d, they would have obtained r, and s". This statement is still true, but "r and s" do not have independent truth values from the conditioning statements. In fact they can never have, because the two photons have already been measured and destroyed in the process.

Bell and his proponents insist that realism must mean "x, y, r and s" all have simultaneous reality independent of any conditioning statements. This is an unreasonable expectation and points to a naive understanding of simple modal logic. You can have a local realistic theory with hidden variables governing photons and still be limited by the fact that Bob and Alice can not repeat their measurement on the same two photons already measured and destroyed. You can even have non-locality with spooky action at a distance and still "x, y, r and s" will not have simultaneous reality for the same simple logical reasons.

Insisting that such a straw-man is the meaning of "realism", effectively renders impossible any experiment that could ever test it, no experimenter can ever recover their photons, restore them to their pristine condition and re-measure them.

That all seems like interpretation to me ... what experimental evidence can you offer that the world actually behaves the way you claim? The experimental evidence shows that coincident measurement statistics for entangled photons violate Bell inequalities (or CHSH inequalities, which I believe are even weaker than Bell inequalities in terms of the assumptions upon which they are based). The experiments do not assume anything a priori about which values will be measured ... can you explain the results in a local realistic fashion?

 

[Rap]

That all seems like interpretation to me ... what experimental evidence can you offer that the world actually behaves the way you claim? The experimental evidence shows that coincident measurement statistics for entangled photons violate Bell inequalities (or CHSH inequalities, which I believe are even weaker than Bell inequalities in terms of the assumptions upon which they are based). The experiments do not assume anything a priori about which values will be measured ... can you explain the results in a local realistic fashion?

If I may answer for billschnieder - The first paragraph is not a description of how the world behaves, it is pure logic, noting that CFD is an additional assumption needed to yield the paradox. The only part where he describes how the world works is to note that it impossible to test for the given description of reality. So, it seems to me, you are asking for experimental evidence that there can be no experimental evidence for the given description of reality.

 

[Rap]

Hidden in the highlighted phrase is a modal fallacy. A prediction MUST always be conditioned on the assumptions, ie it can not be true apart from its conditioning assumptions. For example "If Bob and Alice measure the two photons at angles b and a, they will obtain x, and y" and "If Bob and Alice measure the two photons at angles c and d, they will obtain r, and s" These two statements can both be true at the same time because they both contain their conditioning statements built in. However, this does not mean "x, y, r, and s" must simultaneously exist. Which ones exist, will depend on which of the conditioning statements were actually realized based on which experiment has already been performed. Say Alice and Bob have measure the two photons at angles a and b. At that instance, "x and y" have independent truth values because it is a fact that Alice and Bob have measured at b and a. However, the other statement now becomes a counterfactual statement. "Had Bob and Alice measured the two photons at angles c and d, they would have obtained r, and s". This statement is still true, but "r and s" do not have independent truth values from the conditioning statements. In fact they can never have, because the two photons have already been measured and destroyed in the process.

I agree

 

[JesseM]

If I may answer for billschnieder - The first paragraph is not a description of how the world behaves, it is pure logic, noting that CFD is an additional assumption needed to yield the paradox.

But it's not an assumption, it's derived from the basic assumption of a local realist model, along with the no-conspiracy condition.

 

[Rap]

The following highlights the modal error mentioned in my previous post. If you can see the error in the following argument, you will immediately see the logical error being made by Bell proponents:

A photon A is heading toward Alice's detector on a distant galaxy. They will interact tomorrow to produce an outcome of +1 or -1. But the 'laws' of the excluded middle (no third truth-value) and of noncontradiction (not both truth-values), mandate that one of the propositions "Alice's will get +1", "Alice's will get -1", is true (always has been and ever will be) and the other is false (always has been and ever will be). Suppose 'Alice's will get +1' is true today. Then whatever Alice does (or fails to do) before the photon hist her detector will make no difference: the outcome is already settled. Similarly if 'Alice's will get +1' is false today, no matter what Alice does (or fails to do), it will make no difference: the outcome is already settled. Thus, if propositions bear their truth-values timelessly (or unchangingly and eternally), then planning, or as Aristotle put it 'taking care', is illusory in its efficacy. The future will be what it will be, irrespective of our planning, intentions, etc. Free-will is an illusion."

Hint: admit the validity of CFD

I object to the language - "Then whatever Alice does (or fails to do)" implies she has a choice, while the idea that CFD denies free will contradicts this.

I think "free will" may be a classical concept, with more and more limited applicability as you go to the quantum realm. I say "may be" because I cannot prove it. Thus, I think accepting CFD may be a classical prejudice. When you say "Hint: admit the validity of CFD"... why?

 

[Rap]

But it's not an assumption, it's derived from the basic assumption of a local realist model, along with the no-conspiracy condition.

I think you might be right. I had blinders on, when billschnieder said "If Bob and Alice measure the two photons at angles b and a, they will obtain x, and y" and "If Bob and Alice measure the two photons at angles c and d, they will obtain r, and s". I took that to mean the particular case where a=b and they will obtain x and y=!x (equal and opposite spins), and c=d and they will obtain r and s=!r. This is the only case that is experimentally true, and then the statement is just logic and the acceptance of an experimental truth. His conclusion is still valid, I think.

 

[harrylin]

I said it was sufficient as a definition when I quoted it. I also said that there is no element of reality without that. I meant that per the definition in use. Perhaps you have a better definition.

Strictly speaking, it is certainly possible there is an element of reality WITHOUT us being able to predict it in advance. For example, I had to pay when my son wrecked the car even though I could not predict the amount in advance with certainty. And believe me, that was very real to my pocketbook.

So if you take the contranegative (also being true), you get: IF you cannot predict in advance with certainty, THEN there is no element of reality. But what can you do with this statement? I don't think too much, because you cannot prove the antecedent.

So my point is: Bell used the well accepted EPR definition. That definition is one which is easy to follow, and because it is sufficient it is enough for our examples. I.e. for entangled pairs. The only issue to Bell would be if you could prove convince folks that this was not a sufficient condition. That would be a tough hurdle. Keep in mind that was a cornerstone of EPR.

The issue here is that roughly speaking, Bell tried to prove the inverse of what EPR tried to prove, and that EPR stressed that the inverse of their condition is not true - if I state that an apple is a fruit (so that always apple=>fruit), it does not imply that a fruit is necessarily an apple (NOT fruit=>apple).

However, so far I have not found out if it matters for Bell's Theorem that EPR's condition of predictability is not a necessary condition for reality. I'm just aware that any subtle difference of interpretation about this topic can have great consequences.

I'll be grateful if someone can clarify this to me.

 

[harrylin]

of course you are Right.
Reality is what exist, the state of things as they actually exist. "No Strings Attached".

with CFD or without CFD.

.

You misquoted me: I said if EPR's view was correctly interpreted. However, it appears that they meant with "reality" quite the same as you and me.

 

[harrylin]

A-> B

implies

~B -> ~A

Too bad I reversed it.

OK [edit: I first misread] - using my earlier illustration:

<Apple> => <Fruit> is true;

and

<NOT Fruit> => <NOT Apple> is also true. So we agree now

Harald

 

[JesseM]

The issue here is that roughly speaking, Bell tried to prove the inverse of what EPR tried to prove, and that EPR stressed that the inverse of their condition is not true - if I state that an apple is a fruit (so that always apple=>fruit), it does not imply that a fruit is necessarily an apple (NOT fruit=>apple).

No, Bell did not try to prove the inverse, i.e. he never tried to prove that NOT (values predictable in advance)=>NOT (predetermined values prior to measurement)

 

[billschnieder]

I object to the language - "Then whatever Alice does (or fails to do)" implies she has a choice, while the idea that CFD denies free will contradicts this.

I think "free will" may be a classical concept, with more and more limited applicability as you go to the quantum realm. I say "may be" because I cannot prove it. Thus, I think accepting CFD may be a classical prejudice. When you say "Hint: admit the validity of CFD"... why?

The point is that the argument appears to suggest that everything that will happen in the future is already settled and nothing anyone does or does not do can change it. Do you agree or do you disagree with this and if so why? There is a modal error in such an argument, which I will present in moment after others have had a chance to think about the issue. It will reveal a subtle error being made by Bell proponents over and over.

To give a further hint, consider the following:
a) Statement made on Monday: "Bill will eat do-nuts for breakfast on Tuesday"
b) Statement made on Wednesday: "Bill ate do-nuts for breakfast on Tuesday"
Bill did in fact eat do-nuts for breakfast on Tuesday. So let us rewind time back to Monday, is statement (a) True? Of course both statements are true. Does it mean Bill had no choice or control over what Bill did on Tuesday simply because on Monday it was already true that he will eat do-nuts on Tuesday?

 

[Rap]

The point is that the argument appears to suggest that everything that will happen in the future is already settled and nothing anyone does or does not do can change it. Do you agree or do you disagree with this and if so why? There is a modal error in such an argument, which I will present in moment after others have had a chance to think about the issue. It will reveal a subtle error being made by Bell proponents over and over.

To give a further hint, consider the following:
a) Statement made on Monday: "Bill will eat do-nuts for breakfast on Tuesday"
b) Statement made on Wednesday: "Bill ate do-nuts for breakfast on Tuesday"
Bill did in fact eat do-nuts for breakfast on Tuesday. So let us rewind time back to Monday, is statement (a) True? Of course both statements are true. Does it mean Bill had no choice or control over what Bill did on Tuesday simply because on Monday it was already true that he will eat do-nuts on Tuesday?

I had hoped that I had made it clear that I am not an advocate for rejecting CFD, nor do I advocate against it. I just want to understand the implications of rejecting it, which I do not, at present.

Regarding Bill and the do-nuts. Of course both statements are true, but this is not an illustration of CFD. An illustration would be "If Bill buys do-nuts monday night, he will eat do-nuts for breakfast on Tuesday and if he does not, he will not" According to our model of this phenomenon, this will always happen. We observe a thousand times, Bill buys do-nuts monday night, and every time, he eats do-nuts for breakfast on Tuesday. A thousand times, Bill does not and he does not. Our model has been correct. Now suppose one monday night, Bill does not buy do-nuts, and does not eat do-nuts for breakfast on Tuesday. What is the truth value of the statement "if he had bought do-nuts, he would have eaten do-nuts for breakfast on Tuesday"? This is not the same as saying that if he does, he will. It is saying something definite (he would have eaten do-nuts) about something which is counter-factual (he in fact did not buy do-nuts). On the other hand, to say that if he does, he will, is not counterfactual.

If we reject CFD, then it is improper to even speak about what would have happened. Rejecting CFD, quantum mechanics then only presumes to speak about the future and the factual past, not about a hypothetical past. I don't so much think that rejection of CFD denies free will as it renders the concept improper, not a proper subject for scientific inquiry, an untestable concept, much like the concept of simultaneous position and momentum. And, much like simultaneous position and momentum, in the limit of classical physics, CFD gains meaning. I think that rejecting CFD may be the solution to Bell's paradox, but I don't know. I can't wrap my mind around the concept yet, just like once upon a time I couldn't wrap my mind around not knowing position and momentum simultaneously. Until I learned to identify and reject what amounts to a classical prejudice.

I mean, consider this counter-factual situation: suppose I prepare a system and measure the position of a particle and after many repeated preparations and measurements on many particles, I get the same answer. If, after one of those preparations, I had instead measured the momentum, would the position of the particle at that time been the same as what I have in fact measured in the past? The answer is to reject CFD in this case - the answer to the question is not yes or no, the answer is that the question is improper. If I accept CFD and say yes, it will have the same position, then, knowing the position and momentum will allow me to calculate exactly where it will be measured to be at some time in the future, and when I look for it, chances are it will not be there. A paradox - which may be resolved by rejecting CFD in this case (or assuming hidden variables).

I'm not saying this is a perfect analogy to the Bell paradox, it just ... resonates in my mind.

 

[SpectraCat]

The point is that the argument appears to suggest that everything that will happen in the future is already settled and nothing anyone does or does not do can change it. Do you agree or do you disagree with this and if so why? There is a modal error in such an argument, which I will present in moment after others have had a chance to think about the issue. It will reveal a subtle error being made by Bell proponents over and over.

To give a further hint, consider the following:
a) Statement made on Monday: "Bill will eat do-nuts for breakfast on Tuesday"
b) Statement made on Wednesday: "Bill ate do-nuts for breakfast on Tuesday"
Bill did in fact eat do-nuts for breakfast on Tuesday. So let us rewind time back to Monday, is statement (a) True? Of course both statements are true. Does it mean Bill had no choice or control over what Bill did on Tuesday simply because on Monday it was already true that he will eat do-nuts on Tuesday?

First of all, it seems like statement a) is only "true" if you assume complete determinism of the universe. I don't think everyone is willing to grant that ...

What if I add the following statement on Monday:

a') Courtney will assassinate Bill on Monday evening?

Now it sure seems like only one statement, a or a', can be "true", and we can't know which one until after the events have passed. Therefore it seems that one cannot make judgments about the truth of a (or a') until the event has already occurred. In a deterministic universe, I suppose one of the statements could be said to have been "true all along" ... but you cannot know that until after the fact, so it just seems like interprative post-rationalization to me. There is no way to use such scenarios to empirically deduce whether or not the universe is deterministic.

 

[Rap]

Now it sure seems like only one statement, a or a', can be "true", and we can't know which one until after the events have passed. Therefore it seems that one cannot make judgments about the truth of a (or a') until the event has already occurred. In a deterministic universe, I suppose one of the statements could be said to have been "true all along" ... but you cannot know that until after the fact, so it just seems like interprative post-rationalization to me. There is no way to use such scenarios to empirically deduce whether or not the universe is deterministic.

I'm taking the statement "Bill will eat do-nuts on Tuesday for breakfast" in the same sense that if Alice and Bob align their detectors and make measurements, "Alice and Bob will measure equal and opposite spins".

 

[SpectraCat]

I'm taking the statement "Bill will eat do-nuts on Tuesday for breakfast" in the same sense that if Alice and Bob align their detectors and make measurements, "Alice and Bob will measure equal and opposite spins".

But it's not the same. The corresponding statement about Bill that could (perhaps) be correlated the the Alic and Bob example is, if Bill is alive on Tuesday, and if he is awake in the morning, and if he is hungry, and if donuts are the only available food, (there are lots more qualifiers needed, but I guess you get the idea) then Bill will eat donuts for breakfast on Tuesday.

The Alice & Bob statement is based on a theory of physics that has been extensively tested and has never been found to be false. Making that statement is equivalent to saying, "Quantum mechanics is expected to still be valid when Alice and Bob make their measurements".

 

[billschnieder]

Most scientists do not accept that there is a value to unmeasured particle observables. They reject CFD. That is mainline QM. There are the various interpretations such as MWI, BM, Copenhagen, etc. which all make the same predictions.

Again you demonstrate that you do not understand what CFD means.


a) If I look at the moon, I will see it.
b) Had I looked at the moon, I would have seen it.

(a) is a True statement. In this case, it is implicit that the possibility of either looking at the moon or not looking at the moon still exists. (b) is a counter-factually definite statement. Statement (b) will be valid even if it is impossible for me to look at the moon now. Accepting (b) as a valid statement does not mean:

c) "Seeing the moon" exists prior to me looking at the moon.

Your description above of CFD is similar to statement (c) which demonstrates a lack of basic understanding of philosophy and logic.

The result which Alice will get when she tilts her device to angle b, could not possibly exist before Alice actually makes a measurement! To suggest that any realist has ever made such a claim is naive at best. I have pointed out this error to you multiple times but you continue to insist on making it so in case it is still not clear let me elaborate:


Let us denote observable "what Alice observes when she tilts her device to angle a" as A, and "what Alice observes when she tilts her device to angle b" as B and "what Alice observes when she tilts her device to angle c" as C.

For a single photon, All three observables A, B, C are possible, however if Alice never measures anything, none of them exist as actual observables. It is easy for Bellists to state without substantiation that realism implies the three observables must exist prior to measurement but I'm holding your feet to the fire to not only use terms A, B, C but spell out descriptively what A, B, and C mean. Let us take A as defined above,

A: "what Alice observes when she tilts her device to angle a"

Why would anyone with more than a single brain cell expect Alice to observe anything without performing a measurement. In other words, why would you expect me to see the moon without looking at it. Therefore although A,B,C are all "possible", only the one which Alice actually performs becomes actual. The others remain counter-factual definite.

By mixing "possibilities" with "actualities", you obtain paradoxes. So before you jump to deny CFD make sure you understand what it means. QM can also make predictions about experiments that are never performed and in some cases can no longer be performed. Yet the QM prediction tells us what we would have actually obtained had we performed the measurements. Why is that not CFD? You can not deny CFD without denying logic.

On the the other hand if the definition of CFD used by Bell proponents, is the idea that A actually exists prior to Alice making her measurement, or rather that "what Alice observes when she tilts her device to angle b" actually exists before Alice actually tilts her device and makes the measurement, such a ridiculous idea is non-classical, illogical and nonsensical. So attributing this ridiculous idea to "realists" or "classical systems" is naive at best.

 

[SpectraCat]

Again you demonstrate that you do not understand what CFD means.


a) If I look at the moon, I will see it.
b) Had I looked at the moon, I would have seen it.

(a) is a True statement. In this case, it is implicit that the possibility of either looking at the moon or not looking at the moon still exists. (b) is a counter-factually definite statement. Statement (b) will be valid even if it is impossible for me to look at the moon now. Accepting (b) as a valid statement does not mean:

c) "Seeing the moon" exists prior to me looking at the moon.

Your description above of CFD is similar to statement (c) which demonstrates a lack of basic understanding of philosophy and logic.

The result which Alice will get when she tilts her device to angle b, could not possibly exist before Alice actually makes a measurement! To suggest that any realist has ever made such a claim is naive at best. I have pointed out this error to you multiple times but you continue to insist on making it so in case it is still not clear let me elaborate:


Let us denote observable "what Alice observes when she tilts her device to angle a" as A, and "what Alice observes when she tilts her device to angle b" as B and "what Alice observes when she tilts her device to angle c" as C.

For a single photon, All three observables A, B, C are possible, however if Alice never measures anything, none of them exist as actual observables. It is easy for Bellists to state without substantiation that realism implies the three observables must exist prior to measurement but I'm holding your feet to the fire to not only use terms A, B, C but spell out descriptively what A, B, and C mean. Let us take A as defined above,

A: "what Alice observes when she tilts her device to angle a"

Why would anyone with more than a single brain cell expect Alice to observe anything without performing a measurement. In other words, why would you expect me to see the moon without looking at it. Therefore although A,B,C are all "possible", only the one which Alice actually performs becomes actual. The others remain counter-factual definite.

By mixing "possibilities" with "actualities", you obtain paradoxes. So before you jump to deny CFD make sure you understand what it means. QM can also make predictions about experiments that are never performed and in some cases can no longer be performed. Yet the QM prediction tells us what we would have actually obtained had we performed the measurements. Why is that not CFD? You can not deny CFD without denying logic.

On the the other hand if the definition of CFD used by Bell proponents, is the idea that A actually exists prior to Alice making her measurement, or rather that "what Alice observes when she tilts her device to angle b" actually exists before Alice actually tilts her device and makes the measurement, such a ridiculous idea is non-classical, illogical and nonsensical. So attributing this ridiculous idea to "realists" or "classical systems" is naive at best.

Thank you for that description .. I think I finally understand at least one of the points you are trying to make. I am not sure whether or not I agree yet, but that post was quite helpful. I also think that what I laid out in my previous reply to you is basically saying the same thing .. namely that events (i.e. measurements) obtain objective reality only after they occurred. Similarly, the CFD-based statements about the other possibilities which were NOT observed, only become valid after the measurement as well .. is that what you are saying?

 

[DrChinese]

Again you demonstrate that you do not understand what CFD means.


a) If I look at the moon, I will see it.
b) Had I looked at the moon, I would have seen it.

(a) is a True statement. In this case, it is implicit that the possibility of either looking at the moon or not looking at the moon still exists. (b) is a counter-factually definite statement. Statement (b) will be valid even if it is impossible for me to look at the moon now. Accepting (b) as a valid statement does not mean:

c) "Seeing the moon" exists prior to me looking at the moon.

Your description above of CFD is similar to statement (c) which demonstrates a lack of basic understanding of philosophy and logic.

The result which Alice will get when she tilts her device to angle b, could not possibly exist before Alice actually makes a measurement! To suggest that any realist has ever made such a claim is naive at best. I have pointed out this error to you multiple times but you continue to insist on making it so in case it is still not clear let me elaborate:


Let us denote observable "what Alice observes when she tilts her device to angle a" as A, and "what Alice observes when she tilts her device to angle b" as B and "what Alice observes when she tilts her device to angle c" as C.

For a single photon, All three observables A, B, C are possible, however if Alice never measures anything, none of them exist as actual observables. It is easy for Bellists to state without substantiation that realism implies the three observables must exist prior to measurement but I'm holding your feet to the fire to not only use terms A, B, C but spell out descriptively what A, B, and C mean. Let us take A as defined above,

A: "what Alice observes when she tilts her device to angle a"

Why would anyone with more than a single brain cell expect Alice to observe anything without performing a measurement. In other words, why would you expect me to see the moon without looking at it. Therefore although A,B,C are all "possible", only the one which Alice actually performs becomes actual. The others remain counter-factual definite.

By mixing "possibilities" with "actualities", you obtain paradoxes. So before you jump to deny CFD make sure you understand what it means. QM can also make predictions about experiments that are never performed and in some cases can no longer be performed. Yet the QM prediction tells us what we would have actually obtained had we performed the measurements. Why is that not CFD? You can not deny CFD without denying logic.

On the the other hand if the definition of CFD used by Bell proponents, is the idea that A actually exists prior to Alice making her measurement, or rather that "what Alice observes when she tilts her device to angle b" actually exists before Alice actually tilts her device and makes the measurement, such a ridiculous idea is non-classical, illogical and nonsensical. So attributing this ridiculous idea to "realists" or "classical systems" is naive at best.

I repeat: Most scientists do not accept that there is a value to unmeasured particle observables.

Not sure what the rest of this is all about. I am not debating the definition of CD, nor am I asserting that this is important to Bell. Bell says that using the EPR definition of realism, there can be no local realism. Not that hard really.

There are those who accept that local realism is ruled out and still believe in CD. Guess what, that is not inconsistent (because of nonlocality). But I would still say most scientists accept my statement above.

And by the way, QM does NOT make predictions about counterfactual setups. But if you try to (assuming some form of counterfactual reasoning), you can get a paradox. So I wouldn't recommend that.

 

[DrChinese]

Let us denote observable "what Alice observes when she tilts her device to angle a" as A, and "what Alice observes when she tilts her device to angle b" as B and "what Alice observes when she tilts her device to angle c" as C.

For a single photon, All three observables A, B, C are possible, however if Alice never measures anything, none of them exist as actual observables. ... Why would anyone with more than a single brain cell expect Alice to observe anything without performing a measurement.

Answer: because I can predict the results with certainty to anyone of those. That is what those single brain cell authors EPR believed anyway.

 

[DrChinese]

For a single photon, All three observables A, B, C are possible, however if Alice never measures anything, none of them exist as actual observables. It is easy for Bellists to state without substantiation that realism implies the three observables must exist prior to measurement but I'm holding your feet to the fire to not only use terms A, B, C but spell out descriptively what A, B, and C mean.

As I have told you any number of times previously, it is up to the realist to define realism! I am using the one from EPR, as Bell did. But if you want to give some improved definition that is useful, go for it! The only reason us poor Bellists use this definition is to hold the realist to something more firm than jelly.

So using your A, B and C: I would say that realism is having a definite value for these simultaneously.

 

[SpectraCat]

QM can also make predictions about experiments that are never performed and in some cases can no longer be performed. Yet the QM prediction tells us what we would have actually obtained had we performed the measurements. Why is that not CFD?

It is not CFD because CFD requires that ALL properties of a system simultaneously possesses reality and thus have values that can be predicted with certainty prior to measurement. QM specifically tells us that not all properties of a system can be simultaneously known to arbitrary accuracy. It tells us that you can construct a sequence of measurements that is fundamentally unpredictable .. for example the triple Stern-Gerlach experiment. It is not unpredictable because of random collisions, or chaos theory .. it is unpredictable because you can only know one vector component of the spin at a time .. the others exist in superposition states with no definite value. Thus standard QM does not satisfy CFD.

You can not deny CFD without denying logic.

Who said QM was supposed to be logical?

 

[DrChinese]

Oh, and Bill: if you don't want to give a useful definition of realism that has some bite to it... perhaps you should come over to the other side and visit us Bellists.

 

[Rap]

You can not deny CFD without denying logic.

I question that statement. In #102 of this thread, I think I stated clearly what CFD is. Maybe #102 was too long to read, if so I apologize.

Can you give an example of a logical contradiction that occurs when denying CFD, as I defined it in #102?

 

[harrylin]

No, Bell did not try to prove the inverse, i.e. he never tried to prove that NOT (values predictable in advance)=>NOT (predetermined values prior to measurement)

As I understood it, they believed that processes are "local" and they tried to prove that if something can be predicted, there must be an element of reality that corresponds to that prediction. Note that "roughly" is - again roughly speaking! - the inverse of a mathematical statement.

 

[billschnieder]

Similarly, the CFD-based statements about the other possibilities which were NOT observed, only become valid after the measurement as well .. is that what you are saying?

NO! Let us be very careful with our choice of words here.

The statements are valid and True. But they are not actual. If you insist that they are not valid, then the predictions of QM are not valid either, and no prediction can ever be valid until an experiment is performed. You have to carefully distinguish in your mind between validity and actuality.

QM can make simultaneous predictions about position and monentum:

"If the position is measured x will be obtained"
"If the momentum is measured p will be obtained"

Will you say both predictions are valid? Sure. But once you have measured the position, it becomes impossible to measure the momentum. Does that mean the momentum prediction was not valid? Of course not. It becomes a counterfactual statement. Had the momentum been measured instead, we would have obtained p.

 

[billschnieder]

Not sure what the rest of this is all about. I am not debating the definition of CD, nor am I asserting that this is important to Bell. Bell says that using the EPR definition of realism, there can be no local realism. Not that hard really.

But you and other Bellists continue to perform smoke and mirrors by suggesting that "realism" means observables must exist prior to measurement. And I am pointing out to you that such a ridiculous idea not only does not make any sense, but no realist has ever claimed as such ever.

And before you respond, affirming such a definition, remember what you would be saying:

that "I can see the moon without looking at it", "The result which Alice obtains by measuring a photon at angle a must exist as an actual result prior to Alice actually making the measurement". And you want to attribute such a ridiculous idea to realists? That is not being serious.

 

[billschnieder]

Oh, and Bill: if you don't want to give a useful definition of realism that has some bite to it... perhaps you should come over to the other side and visit us Bellists.

Realism means particles have objective properties at all times, whether or not they are measured or not, whether they are measurable or not.

According to realists, an observable is an outcome of a measurement and can either be a direct revealing of the underlying property, or a result of multiple influences such as all the objective properties of all the entities and variables involved at the moment of measurement.

All of the above is consistent with realism. Realists make no claim whether direct observation of particle properties in quantum scale particles are possible.

 

[yoda jedi]

it appears that they meant with "reality" quite the same as you and me.

not for me.

it is not necessary to have a defined value (CFD) to be real..

 

[Rap]

QM can make simultaneous predictions about position and monentum:

"If the position is measured x will be obtained"
"If the momentum is measured p will be obtained"

QM would never make such a statement, at least for the same point in time. It would imply that the wavefunction was simultaneously in an eigenstate of both position and momentum, which is impossible, since the two operators do not commute.

 

[SpectraCat]

QM would never make such a statement, at least for the same point in time. It would imply that the wavefunction was simultaneously in an eigenstate of both position and momentum, which is impossible, since the two operators do not commute.

I tried to point that out already .. maybe your explanation will make more sense to him.