Scholarpedia article on Bell's Theorem

[lugita15]

http://www.scholarpedia.org/article/Bell%27s_theorem#Some_controversy_regarding_the_EPR_argument

ttn, you make an interesting argument there that I can't seem to immediately refute:

"Here is the formulation of the "several axes" version of the EPR argument that does not involve counterfactuals: in order to explain (without violation of locality) the fact that the outcomes will be perfectly anti-correlated if the experimenters both measure spin along the z-axis, one has to assume that these outcomes are pre-determined. The same goes for measurements of spin along the x-axis. Even though, in each run of the experiment, either the z-axis or the x-axis is chosen along which to perform the measurements, the elements of physical reality that exist before the measurements cannot depend on choices that will be made later by the experimenters! This, indeed, doesn't follow from the assumption of locality itself but it does follow from the so-called "no conspiracy" assumption which states, roughly speaking, that the pair of particles prepared by the source does not "know" in advance what experiments are going to be performed on them later." (italics in original)

 

[DrChinese]

OK, we're on the same page. So what ttn needs to understand is that for a given photon pair, Bell's argument involves meaningfully discussing not only the two polarization attributes that are measured but also a third polarization attribute that is unmeasured but that could have been measured had the experimenter chosen to. In other words, the argument assumes counterfactual definiteness.

I think it is safe to say that ttn is comfortable with his position as is. On the other hand, ttn is unlikely to sway (with his argument) those who follow MWI or one of the other non-realistic (or non-deterministic) interpretations (since the vast majority of physicists are not Bohmian).

So my point to ttn remains: why make an argument that depends on wording ("simultaneous") that is soundly rejected? In other words: I reject his starting point that perfect correlations implies hidden variables*, a position I am quite comfortable with and involves no controversy. With my position, I can peacefully coexist with other interpretations, and await additional evidence to clarify matters. A position shared by most, and for which ttn has no lever to move any of us (since his assertion that he is right and we are wrong doesn't even make sense unless we all start from the same point).

Other than perhaps badgering, but I get than from the other side (local realists) just as well.

*In a time symmetric interpretation, there are no hidden variables but there are perfect correlations. Ditto for MWI. Ditto for the Copenhagen interpretation, because there is no possible greater specification of the system, and we live in a non-deterministic world. All of these interpretations reject realism. And all of these interpretations reject the idea that the current position of ALL distant particles in a system directly determines the outcomes of measurements here and now.

 

[harrylin]

http://www.scholarpedia.org/article/Bell%27s_theorem#Some_controversy_regarding_the_EPR_argument

I have read that argumentation and it looks sound to me - not that I'm sure that it gets rid of "counterfactuals", but simply that there is no need for the introduction of such additional complex new concepts.

 

[harrylin]

[..] All of these interpretations reject realism. And all of these interpretations reject the idea that the current position of ALL distant particles in a system directly determines the outcomes of measurements here and now.

OK, you're right of course that Bell's argument assumes realism, and in the Bertlman's socks paper he mentions that point - I guess that without realism (like in the movie Matrix) about anything is possible because nothing really happens.

 

[DrChinese]

ttn, you make an interesting argument there that I can't seem to immediately refute...

I have read that argumentation and it looks sound to me - not that I'm sure that it gets rid of "counterfactuals",...

The fact that the Bohmian view is contextual should be an immediate tipoff that there is something wrong with his argument. Contextual essentially being code for "non-realistic". So of course in the end, there are no simultaneous definite values for a, b and c which is my assertion. If there are no counterfactuals, there is no realism. Of course, the Bohmian view is that there is determinism. So again we are back to the meaning of words. The Bohmian view is non-local deterministic, i.e. there are non-local hidden variables. But it is not any more realistic than other interpretations.

To ttn, of course, this distinction is meaningless: he argues "against realism". But to you, you must decide if you accept the idea that at the time entanglement begins, the outcomes have been predetermined in the context of the inevitable future measurement settings and NO OTHERS (since Bohmian theories don't address the DrChinese challenge either). If that doesn't blatantly violate ttn's premise ("cannot depend on choices that will be made later by the experimenters") to you, then I would say his argument can be accepted. I see a contradiction, but hey, that's why my conclusion is different.

So the answer is: your viewpoint subtly colors your definitions. A slight change will make a difference. I, for example, would be likely to answer ttn's "Against Realism" with an argument we can call "Against Locality". By a suitable shift in definitions, we would be left concluding that locality is irrelevant to the matter; i.e. realism is not tenable by any theory agreeing with the predictions of QM. And you know what: Bohmian types would fall inside, not outside, my definition. For the reasons stated in the first paragraph.

 

[ttn]

ttn, you make an interesting argument there that I can't seem to immediately refute:

At least try to pretend that you're not surprised!

 

[ttn]

I think it is safe to say that ttn is comfortable with his position as is. On the other hand, ttn is unlikely to sway (with his argument) those who follow MWI or one of the other non-realistic (or non-deterministic) interpretations (since the vast majority of physicists are not Bohmian).

None of this has anything to do with being Bohmian. One of the biggest supporters of Bell's view of all this (which view I of course share) is GianCarlo Ghirardi, the principle proponent of the non-deterministic GRW version of QM. Also, I don't understand why you call MWI "non-realistic (or non-deterministic)". I would call it both realistic and deterministic -- if what you mean by "realistic" is just that it gives some definite account of micro-physical reality. (Of course, if you mean by "realistic" something about non-contextual hidden variables, then, OK, MWI isn't "realistic" in that sense... but maybe the point here is that you and others should stop using the word "realistic" without saying *exactly* what you mean.)

So my point to ttn remains: why make an argument that depends on wording ("simultaneous") that is soundly rejected? In other words: I reject his starting point that perfect correlations implies hidden variables*, a position I am quite comfortable with and involves no controversy. With my position, I can peacefully coexist with other interpretations, and await additional evidence to clarify matters. A position shared by most, and for which ttn has no lever to move any of us (since his assertion that he is right and we are wrong doesn't even make sense unless we all start from the same point).

After all these years, I don't hold out any hope of changing your position, that's true. But it is factually wrong to suggest that my attempts to change your mind are based on the mere "assertion that [I am] right". They have instead all along been based on trying to explain the *argument*, which you systematically fail to grasp. Once more for the record, it's not an argument "that perfect correlations implies hidden variables" -- it's rather an argument that perfect correlations *plus locality* implies hidden variables. That is, the only way to explain the perfect correlations locally is for each particle to carry pre-determined answers to all possible questions that can be asked of it.

I honestly have no clue what you have in mind with this word "simultaneous". I think you mean to be referring back to the actual EPR paper, where they talk about simultaneous values for non-commuting operators. But that's just an awkward way of saying that there are more real definite properties than QM can consistently attribute values to, i.e., that QM doesn't provide a complete description of the physical state. But who cares about QM. It plays no role whatever in the argument for the conclusion I wrote in the last sentence of the previous paragraph. Also, you remember that the EPR paper was written by Podolsky, and Einstein thought he botched it, right? So please don't think of its precise wording as somehow perfectly capturing the argument. Einstein didn't think it did, and neither do I.

*In a time symmetric interpretation, there are no hidden variables but there are perfect correlations.

As I explained back in the beginning of this thread, a time symmetric interpretation isn't local. It involves causal influences coming from outside the past light cone.

Ditto for MWI.

It's hardly that simple. Normally the phrase "perfect correlations" denotes the following: the single actual outcome on the right perfectly matches the single actual outcome on the left, for each particle pair. MWI denies that there *is* such a thing as "the single actual outcome on the right", and same on the left. So saying that "there are perfect correlations in MWI" involves, at least, changing the meaning of the terms involved. Probably the right thing to say is that, in an MWI-ish world, the inhabitants will be fooled into thinking that "perfect correlations" occur. That's not quite the same as saying that perfect correlations do actually occur.

Ditto for the Copenhagen interpretation, because there is no possible greater specification of the system, and we live in a non-deterministic world.

Copenhagen is also not a local theory.

All of these interpretations reject realism.

What do you mean by "realism"? Hidden variables? If that, then I would put the conclusion differently: there are a bunch of ("regular type") theories, some "realistic" and some not, and they're all nonlocal. (Oh and then there's this one very irregular type theory, MWI, where nothing is as it seems and it's not really clear what the heck to say.) So the upshot is clear: you can have "realism" or not, but what you can't do is explain the correlations in a local way (at least, not without playing MWI games).

And all of these interpretations reject the idea that the current position of ALL distant particles in a system directly determines the outcomes of measurements here and now.

Actually, as worded, Bohmian mechanics also rejects this idea. See, you really need to be more careful/precise/clear with what you mean by "realism".

 

[lugita15]

OK, you're right of course that Bell's argument assumes realism, and in the Bertlman's socks paper he mentions that point - I guess that without realism (like in the movie Matrix) about anything is possible because nothing really happens.

Denying realism, in this context, may not necessarily mean you believe the world is an illusion and nothing is real. Realism here just means that measurable attributes have well-defined values no matter what, even if they're not measured.

 

[lugita15]

I honestly have no clue what you have in mind with this word "simultaneous".

He means that e.g. three polarization attributes for a given photon pair are assumed to have simultaneously well-defined values, even though we only measure at most two of those polarization attributes in the experiment.

 

[lugita15]

To ttn, of course, this distinction is meaningless: he argues "against realism". But to you, you must decide if you accept the idea that at the time entanglement begins, the outcomes have been predetermined in the context of the inevitable future measurement settings and NO OTHERS (since Bohmian theories don't address the DrChinese challenge either).

Isn't what you're describing superdeterminism, i.e. a violation of the no-conspiracy condition? Are you saying that Bohmian mechanics is superdeterministic?

 

[harrylin]

Denying realism, in this context, may not necessarily mean you believe the world is an illusion and nothing is real. Realism here just means that measurable attributes have well-defined values no matter what, even when they're not measured.

Then it may be that "realism" isn't sufficiently well defined... for example we can measure an orange by pushing on it so that it bursts and we detect the range of drops on the floor. The drop pattern on the floor isn't "well-defined no matter what, even when it's not measured"; from which I would conclude that some attributes of an orange are "non-realistic". I don't see what such a concept has to do with Bell's analysis.

 

[lugita15]

Can anyone think of a refutation of the argument by ttn I quoted in post #316, which says that if the question of which elements of reality exist before measurement is determined by the measurement decisions of the experimenter, then this constitutes a violation of the no-conspiracy condition?

 

[lugita15]

Then it may be that "realism" isn't sufficiently well defined... for example we can measure an orange by pushing on it so that it bursts and we detect the range of drops on the floor. The drop pattern on the floor isn't "well-defined no matter what, even when it's not measured"; from which I would conclude that some attributes of an orange are "non-realistic". I don't see what such a concept has to do with Bell's analysis.

Would you not say that the question of what pattern of drops it produces if we pushed on it is a measurable attribute of the orange, and that this question has a well-defined answer even if we do NOT push on it?

 

[ttn]

The fact that the Bohmian view is contextual should be an immediate tipoff that there is something wrong with his argument. Contextual essentially being code for "non-realistic". So of course in the end, there are no simultaneous definite values for a, b and c which is my assertion.

First off, for the millionth time, a, b, and c are angles. They are axes along which one might contemplate measuring the spin/polarization of a particle. They aren't properties. So it doesn't even make any sense to talk about whether "there are simultaneous definite values for a, b, and c" or not. Presumably what you mean is whether there are simultaneous definite values for spin-along-a, spin-along-b, and spin-along-c. OK. You're right that, for Bohmian mechanics, spin is contextual. That means, basically, that Bohmian mechanics does not claim that spin-along-a, spin-along-b, and spin-along-c all exist with simultaneous definite values.

But what in the world do you think this has to do with Bell's argument? In the two-step version (as opposed to going directly from locality to CHSH) the argument runs like this:

step 1: locality + perfect correlations --> X

where X is "spin-along-a, spin-along-b, and spin-along-c all exist with simultaneous definite values that are simply revealed by whichever measurement actually gets made"

step 2: X --> Bell's inequality

overall conclusion (i.e., what you get by combining step 1 and step 2):

locality + perfect correlations --> Bell's inequality

We know from experiment that "perfect correlations" is true and "Bell's inequality" is false. It follows that "locality" is false.

Now you want to come along and say "Aha, but there's this one candidate theory, Bohmian Mechanics, which denies X -- so the argument falls apart." But what in the world are you thinking? Nothing falls apart. Theories can say X or deny X or dip X in chocolate and eat it, and none of it has any implications whatsoever for the argument just presented. You are just saying something that is a complete and total non-sequitur.

If there are no counterfactuals, there is no realism.

OK, so Bohm's theory isn't "realistic". So what? You think that somehow refutes Bell's argument?

Of course, the Bohmian view is that there is determinism. So again we are back to the meaning of words. The Bohmian view is non-local deterministic, i.e. there are non-local hidden variables. But it is not any more realistic than other interpretations.

OK, fine, yes, great, let's use the words that way. I agree, Bohm's theory is no more realistic than other interpretations. So what? You think that somehow refutes Bell's argument??

To ttn, of course, this distinction is meaningless: he argues "against realism". But to you, you must decide if you accept the idea that at the time entanglement begins, the outcomes have been predetermined in the context of the inevitable future measurement settings and NO OTHERS (since Bohmian theories don't address the DrChinese challenge either).

Hogwash. Maybe you have to decide that if you are trying to decide which theory to believe. But you simply do not have to decide that, or even confront the question at all, if you are just trying to follow Bell's proof that you can't explain the empirical data without nonlocality. Dr Chinese continues to fall back to this totally false idea that X (which stands for "realism" or "non-contextual hidden variables" or "simultaneous elements of reality" or whatever) is an *assumption* of the argument. But it's simply not. There is no such assumption. To quote Bell: to the limited extent to which it plays any role at all, it is *inferred* rather than *assumed*. And note clearly that if there is even the slightest bit of confusion or uncertainty about this, all you have to do is go and look at the "Bell's theorem without perfect correlations" section of our article (or any of several of Bell's papers) where the empirically refuted inequality is derived *straight* from locality, without the need even to ever *mention* any suspicious-sounding X.

So the answer is: your viewpoint subtly colors your definitions.

That's probably true. But more relevant here is the idea that missing an argument entirely, blatantly dumps buckets of paint on your definitions such that what you are talking about is entirely and fatally obscured.

By a suitable shift in definitions, we would be left concluding that locality is irrelevant to the matter; i.e. realism is not tenable by any theory agreeing with the predictions of QM.

Actually I agree. If you redefine "realism" to mean "causal influences on an event come exclusively from its past light cone" -- and redefine "locality" to mean whatever the heck anybody wants -- then indeed, Bell's theorem would refute realism and have nothing to do with locality. Is that a "suitable shift"?

And you know what: Bohmian types would fall inside, not outside, my definition. For the reasons stated in the first paragraph.

OK, so let me drop the sarcasm and ask you straight: how precisely do you propose to redefine words? I *think* your point in the first paragraph was supposed to be that, actually, Bohmian mechanics is not realistic (because it is contextual). OK, fine, I'm cool with that. But that's not going to show *anything* about locality. Bohmian mechanics will still be nonlocal, no matter how you define "realistic". So... how do you propose to redefine "local" such that Bohmian mechanics becomes a local theory?

And a more important question (since Bell's argument has nothing to do with Bohmian mechanics): are you suggesting that you can still derive a Bell inequality from (your) "locality"?

But the most important question of all: what the heck does any of this have to do with Bell's argument? Even supposing you could redefine "locality" (in some way such that Bohm's theory comes out as local) and still derive a Bell inequality from this redefined "locality", who cares? We're all busy being shocked by *Bell's argument*, which proves that his regular kind of locality is false! Do you think that somehow you playing this game (defining things a new way and trying to construct your own argument) refutes Bell's argument? At best, you could only hope to *distract* people from Bell's argument with this game. But if Bell's argument is sound -- and I don't exactly hear you pointing out a flaw in it -- then it's sound, end of discussion.

 

[ttn]

He means that e.g. three polarization attributes for a given photon pair are assumed to have simultaneously well-defined values, even though we only measure at most two of those polarization attributes in the experiment.

Actually we only measure at most *one* of them (on any single particle).

But, whatever. The point is, whatever exactly he means, he's *wrong* if he's saying it's an *assumption* in Bell's argument -- i.e., something that you could deny in order to escape the conclusion that nonlocality is required.

 

[DrChinese]

I honestly have no clue what you have in mind with this word "simultaneous". I think you mean to be referring back to the actual EPR paper, where they talk about simultaneous values for non-commuting operators. ... What do you mean by "realism"? See, you really need to be more careful/precise/clear with what you mean by "realism".

I think I have been quite clear on this point. The standard definition for realism in this context is that there exist simultaneous elements of reality for non-commuting (and commuting) observables, exactly following EPR and their definition for "elements of reality". Not sure how hard that is to grasp, I say the same thing every time and quote from the paper repeatedly. EPR says that it is unreasonable to require elements of reality to be simultaneously predictable. That included, we have a working definition of realism.

So to be clearer: If I can predict a with certainty, and I can predict b and c etc with certainty, and I assume the definition of realism a la EPR, then I would define a, b and c are real. That is, of course, subject to challenge. I don't know of any major interpretations in which definite simultaneous counterfactual real values can be assigned to these if they do not commute. In other words, realism is soundly rejected by all. This is in direct contradiction to Einstein's view that an electron has well defined spin, position etc at all times.

 

[ttn]

I think I have been quite clear on this point. The standard definition for realism in this context is that there exist simultaneous elements of reality for non-commuting (and commuting) observables, exactly following EPR and their definition for "elements of reality". Not sure how hard that is to grasp, I say the same thing every time and quote from the paper repeatedly.

OK.

So to be clearer: If I can predict a with certainty, and I can predict b and c etc with certainty, and I assume the definition of realism a la EPR, then I would define a, b and c are real. That is, of course, subject to challenge. I don't know of any major interpretations in which definite simultaneous counterfactual real values can be assigned to these if they do not commute. In other words, realism is soundly rejected by all. This is in direct contradiction to Einstein's view that an electron has well defined spin, position etc at all times.

First off, Einstein never said any such thing. What's expressed in the EPR paper (and I assume this is what you meant) is that (e.g.) one of the two electrons in the pair has both a well defined position and momentum. Or if we translate to the spin version of the argument, each electrons has simultaneous pre-existing non-contextual definite values for spin-along-a, spin-along-b, spin-along-c, etc. So let's pretend that Einstein actually claimed that.

The question is, why did he claim that? Your view seems to be "he just did, he just assumed it." But that is wrong. He claimed that because he had just presented a perfectly valid logical argument showing that this conclusion *followed* from locality (plus certain predictions of ordinary QM, namely, the perfect correlations). Of course, Einstein, inventor of relativity theory, had no reason to doubt that locality was true. So he actually believed the conclusion of this argument. But that he turned out to be wrong about *that* in no way invalidates the *argument*. It's still true that "the only way to explain the perfect correlations in a local theory is with pre-existing values [i.e., what you insist on calling "realism"]". Einstein was not wrong about *that*!

And this matters, because if it's still true that

1: locality --> X

and (I don't think anybody doubts)

2: X --> contradiction with experiment

then it's still true that

locality --> contradiction with experiment

even if we no longer accept 1 as a reason to believe X, indeed, whether we believe X or not.

It just doesn't matter. You keep coming back to this idea that you can somehow elude Bell's conclusion (that nonlocality is true) by disagreeing with Einstein. But it's a matter of elementary logic. You cannot elude Bell's conclusion merely by saying "Einstein was wrong to believe X". If you want to elude Bell's conclusion you have to actually find something wrong with Einstein's argument from locality to X. Whether X is true, and whether locality is true, are not the same thing as whether locality --> X.

 

[DrChinese]

1. First off, for the millionth time, a, b, and c are angles. They are axes along which one might contemplate measuring the spin/polarization of a particle. They aren't properties. So it doesn't even make any sense to talk about whether "there are simultaneous definite values for a, b, and c" or not. Presumably what you mean is whether there are simultaneous definite values for spin-along-a, spin-along-b, and spin-along-c. OK. You're right that, for Bohmian mechanics, spin is contextual. That means, basically, that Bohmian mechanics does not claim that spin-along-a, spin-along-b, and spin-along-c all exist with simultaneous definite values.

2. OK, so let me drop the sarcasm and ask you straight: how precisely do you propose to redefine words? I *think* your point in the first paragraph was supposed to be that, actually, Bohmian mechanics is not realistic (because it is contextual). OK, fine, I'm cool with that. But that's not going to show *anything* about locality. Bohmian mechanics will still be nonlocal, no matter how you define "realistic". So... how do you propose to redefine "local" such that Bohmian mechanics becomes a local theory?

1. Is it really hard for you to see that when I say a, I actually mean the element of reality which corresponds to certain prediction I am making at angle a? Do I really need to say those words? You say "spin-along-a" here, what difference does the notation make to our conclusions? I really think people can understand that a b and c are angles, there are outcomes of measurements at those angles, and there might be elements of reality associated with those outcomes. You use the same shortcuts when it is convenient to you.

2. Seriously, I never said otherwise. By all definitions, I would call dBB type theories non-local.

What I said was that IF I wanted to, I might alter the definitions of Bell such that "realism is not tenable by any theory agreeing with the predictions of QM"*. Rather than the usual conclusion that "local realism is not tenable by any theory agreeing with the predictions of QM". And in contrast to your conclusion: "locality is not tenable by any theory agreeing with the predictions of QM".

My point being that there really are no candidate realistic theories, because it is almost universally accepted that - as a result of Bell - there cannot be simultaneous values for non-commuting elements of reality. Exactly in accordance with garden variety QM.

*Please note that I am not making/advancing this argument, just showing that definitions matter to wording of one's conclusion. We don't need to debate whether my argument is a good one or not; the fact that I am not ready to push it is a simple nod that it isn't good enough to do anything useful for anyone at this point. Most essentially already hold this opinion in one form or another as it is.

 

[DrChinese]

It's still true that "the only way to explain the perfect correlations in a local theory is with pre-existing values [i.e., what you insist on calling "realism"]". Einstein was not wrong about *that*!

That's not what EPR said, and Einstein never said that. At least not that I am aware of. Perhaps you have a quote similar to that? (Not one where you deduce this, please.) EPR was arguing that a more complete specification of the system was possible, specifically that there were definite values for non-commuting observables. Their focus was more on demonstrating that reality was not observer dependent, rather than locality requires the existence of hidden variables.

But I see where you would be tempted to go from your statement above to your argument about locality. And in some ways, I think it is good since I certainly can't imagine a local mechanism without hidden variables which delivers perfect correlations (using your definition of locality). That does not change the fact that Bell is dependent on realism as an assumption. You should address the strongest arguments against your position. There is no justification, other than by assumption, for the use of a, b and c in the Bell argument unless there is experimental support for it. Which there is not.

 

[ttn]

1. Is it really hard for you to see that when I say a, I actually mean the element of reality which corresponds to certain prediction I am making at angle a? Do I really need to say those words? You say "spin-along-a" here, what difference does the notation make to our conclusions? I really think people can understand that a b and c are angles, there are outcomes of measurements at those angles, and there might be elements of reality associated with those outcomes. You use the same shortcuts when it is convenient to you.

I'm not trying to be anal about it. I'm not saying "nobody but me gets to take notational shortcuts!". But, try as I might, I have trouble following what you are saying much of the time. So maybe your notation/terminology is not as clear as you take it to be. For example, you always make a big fuss about how Bell introduced a *third* angle, c, in the a/b/c triplet. Whereas before people had only talked about 2. Or something like that. None of that makes any sense to me. You can only measure polarization along *one* angle (without radically altering the state), and according to QM a state can only possesses a definite value along one such angle at a time. Talking about there being definite values for *two* such angles thus already means one is endorsing some kind of "hidden variable" or "realism" or whatever. Yet you always say, no, 2 is fine, it's 3 that introduces some big new suspicious issue. So I am constantly feeling: either what he's saying is complete nonsense, or he's using terminology in a way I don't understand. I try whenever possible to give the benefit of the doubt and assume the latter. Hence my complaining about what seems like dubious/confusing terminology.

2. Seriously, I never said otherwise. By all definitions, I would call dBB type theories non-local.

What I said was that IF I wanted to, I might alter the definitions of Bell such that "realism is not tenable by any theory agreeing with the predictions of QM"*. Rather than the usual conclusion that "local realism is not tenable by any theory agreeing with the predictions of QM". And in contrast to your conclusion: "locality is not tenable by any theory agreeing with the predictions of QM".

I don't think you understood my complaint. I don't understand *at all* what you think you can do here. It's really simple. Either you redefine "realism" to mean "locality" -- which is obviously just a stupid pointless trick. Or you think you can redefine "locality" in a way that still preserves some semblance of the usual meaning of that concept, but which now allows local theories to be compatible with the predictions of QM. Well, I want to see it. Talk is cheap. Tell me your proposed formulation of "locality".

My point being that there really are no candidate realistic theories, because it is almost universally accepted that - as a result of Bell - there cannot be simultaneous values for non-commuting elements of reality. Exactly in accordance with garden variety QM.

But don't you see that this is totally irrelevant to bell's proof that locality is untenable? Suppose instead of "realism" we focus on the class of theories according to which all the particles are coated in a tasty layer of pink frosting. Call these the pinkistic theories. Now, it can be shown that

locality + pinkism --> contradiction with QM predictions and with experiment

It is also true, as I look around at various extant candidate theories, that none of them are pinkistic.

Does this mean that, instead of accepting Bell's conclusion that locality is untenable, I can *instead* deny pinkism? No, it does not mean this. Because it can *also* be shown that

locality --> contradiction with QM predictions and with experiment

That is, you don't *need* to assume pinkism to get the contradiction. You *only* need to assume locality. Pinkism was actually a *superfluous premise* in the first argument! So (until/unless you can show what's wrong with the proof that locality --> contradiction...) you have to accept that locality is untenable *whether or not you believe pinkism*. Pinkism, it turns out, is just a red herring. It has nothing whatsoever to do with Bell's theorem.

Of course, maybe there are *other* reasons -- perhaps even really strong reasons -- to deny pinkism. Maybe we can even all agree that pinkism is false. What of it? It has no implications for Bell's theorem.

Everything I've just said remains true if one substitutes "realism" for "pinkism". Note in particular that we can all agree that "realism" (as Dr C has defined it here) is false. This is known from the various no-hidden-variables theorems (Kochen-Specker, etc.) which show that there is no consistent way to assign pre-existing values to non-commuting observables and reproduce the quantum statistics. So *that* is why there are no extant "realist" theories. We know they can't exist (and make the right predictions)! But ... again ... what in the world does that have to do with Bell's proof of the untenability of locality?

Nothing!

*Please note that I am not making/advancing this argument, just showing that definitions matter to wording of one's conclusion. We don't need to debate whether my argument is a good one or not; the fact that I am not ready to push it is a simple nod that it isn't good enough to do anything useful for anyone at this point. Most essentially already hold this opinion in one form or another as it is.

Well, I guess I think we *do* need to debate whether your argument is good or not. You keep making it. You keep suggesting that there is some loophole here, some way of saying a bunch of words about realism or whatever that somehow amounts to there being, actually, a *choice* about whether we deny locality or instead deny something else. But there is no valid substance to any of what you're saying. You fail to grasp Bell's actual argument, bring up a bunch of distracting red herrings, and then say a bunch of vague words about how it all comes down to definitions/semantics. None of that constitutes a valid argument against what Bell has claimed to show.

 

[ttn]

EPR was arguing that a more complete specification of the system was possible, specifically that there were definite values for non-commuting observables.

Correct.

Their focus was more on demonstrating that reality was not observer dependent, rather than locality requires the existence of hidden variables.

Also correct, but misleading. Recall that Podolsky wrote the paper and Einstein said the main point was buried. The main point Einstein had in mind was: locality. So yes, it's true that "their focus was [not so much on] locality". But it *should* have been, according to Einstein at least.

See, e.g., my article "Einstein's boxes" (from AJP several years ago, or online here

http://arxiv.org/abs/quant-ph/0404016

) for the sorts of Einstein quotes you seek.

As to the EPR paper itself, the use of locality is hidden in the assumption that the "reality criterion" can be *applied* to the case at hand. That is, they say that an element of reality exists if we can predict in advance what the value will be for some property *without in any way disturbing the system*. Well, what grounds do we have for thinking that measuring some property on this particle over here, won't affect the physical state of that other particle over there? Locality.

It is indeed unfortunate that this wasn't spelled out more clearly in the paper. Einstein thought so too. Podolsky's text makes way too big a fuss over the "reality criterion", and way too *little* a fuss over the reason we expect it to *apply* to the case at hand.

But I see where you would be tempted to go from your statement above to your argument about locality. And in some ways, I think it is good since I certainly can't imagine a local mechanism without hidden variables which delivers perfect correlations (using your definition of locality).

OK, that's a good start. Actually it can be made more precise and rigorous -- that is, it is possible (indeed, surprisingly trivial) to show rigorously that the appropriate sort of "hidden variables" are *required* by locality + perfect correlations. So it's stronger than just "I can't think of a way to do it".

That does not change the fact that Bell is dependent on realism as an assumption.

How many millions of times have I explained, in detail, that this is not true? That realism is *not* an assumption of Bell, but instead something that gets *inferred* from locality?

You should address the strongest arguments against your position.

I'm trying. Let me know when you find one.

There is no justification, other than by assumption, for the use of a, b and c in the Bell argument unless there is experimental support for it. Which there is not.

So, your whole thing comes down to: if we can't measure it, it doesn't exist? I don't agree with that, on philosophical grounds. But that is irrelevant here. Because actually what you say is just plain false. There *is* a "justification ... for the use of a, b and c in the Bell argument". That justification is: the EPR argument, which *proves* -- savor that word -- that locality requires a, b, and c.

Really, given what you've said, here's how you should think about all this.

Step 1: (the EPR argument) locality --> a, b, and c

Step 2: (the Dr C argument) a, b, and c can't exist because we can't measure them all simultaneously

Conclusion: locality is false (because it implies something that we know is wrong).

My point is that you should be *quicker* than me or others to conclude that locality is wrong. You think you already know that one of the things it entails, is false! Of course, if you tried to present this as a proof for nonlocality, you'd have people like me disagreeing with step 2 of the argument! So I don't consider this 2-part argument as a valid proof of nonlocality! But my point is, if you accept "step 1" (as you seem to, at least sometimes) and you accept "step 2" (as you seem to), you should conclude that locality is false. Instead, you think that step 2 somehow "undoes" step 1. That is, you think denying "a, b and c" somehow constitutes a refutation of the EPR argument. Of course it does not. You don't refute an argument by simply denying the conclusion.

Or maybe you're using a special Dr C version of quantum logic or something.

 

[DrChinese]

For example, you always make a big fuss about how Bell introduced a *third* angle, c, in the a/b/c triplet. Whereas before people had only talked about 2. Or something like that. None of that makes any sense to me. You can only measure polarization along *one* angle (without radically altering the state), and according to QM a state can only possesses a definite value along one such angle at a time. Talking about there being definite values for *two* such angles thus already means one is endorsing some kind of "hidden variable" or "realism" or whatever. Yet you always say, no, 2 is fine, it's 3 that introduces some big new suspicious issue.

Before EPR: 1 (ONE: a) value is what QM says you can know with certainty of any non-commuting set (along with mixed variations A, B, C...). I hope this is obvious. Let's call that result_of_A_observation (or just a).

EPR says: I can know 2 (TWO: a, b) values: result_of_A_observation AND result_of_B_observation (which tells me something if I could otherwise predict with certainty). This requires a bit of an assumption though about reasonable definitions of reality (let's call that realism).

BELL says: If I extend the EPR assumption of realism to a simultaneous unmeasured/counterfactual 3rd value result_of_C_observation (THREE: a, b, c), then a contradiction arises.

I hope this is clear and I got my a, b, c's right.

 

[Gordon Watson]

1. Is it really hard for you to see that when I say a, I actually mean the element of reality which corresponds to certain prediction I am making at angle a? Do I really need to say those words? You say "spin-along-a" here, what difference does the notation make to our conclusions? I really think people can understand that a b and c are angles, there are outcomes of measurements at those angles, and there might be elements of reality associated with those outcomes. You use the same shortcuts when it is convenient to you.

2. Seriously, I never said otherwise. By all definitions, I would call dBB type theories non-local.

What I said was that IF I wanted to, I might alter the definitions of Bell such that "realism is not tenable by any theory agreeing with the predictions of QM"*. Rather than the usual conclusion that "local realism is not tenable by any theory agreeing with the predictions of QM". And in contrast to your conclusion: "locality is not tenable by any theory agreeing with the predictions of QM".

My point being that there really are no candidate realistic theories, because it is almost universally accepted that - as a result of Bell - there cannot be simultaneous values for non-commuting elements of reality. Exactly in accordance with garden variety QM.

*Please note that I am not making/advancing this argument, just showing that definitions matter to wording of one's conclusion. We don't need to debate whether my argument is a good one or not; the fact that I am not ready to push it is a simple nod that it isn't good enough to do anything useful for anyone at this point. Most essentially already hold this opinion in one form or another as it is.

With apologies: I'm away from my office and not yet able to efficiently join this debate.

BUT, to add my support to alternative views of Bell's theorem, to reinforce some thoughts in the quote above, and to encourage questioning of the "realism" assumed in Bell's Theorem:

I find no reason anywhere to abandon Einstein-locality!

 

[DrChinese]

Step 2: (the Dr C argument) a, b, and c can't exist because we can't measure them all simultaneously

Just want to be clear: this is not MY assertion at all. We must either follow the EPR program and consider this as a reasonable assumption, or not. Once we define Realism to include this assumption, we can proceed.

If a person objects, as EPR says: "Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

So obviously we must agree that this is an assumption or the EPR result falls, since that is exactly what they said. Ditto, the Bell result falls as well if this is not agreed to (notice they say two or more). And apparently, this is quite a source of debate so I say it is a rather important assumption, yes. EPR explicitly acknowledge it!

 

[ttn]

Before EPR: 1 (ONE: a) value is what QM says you can know with certainty of any non-commuting set (along with mixed variations A, B, C...). I hope this is obvious. Let's call that result_of_A_observation (or just a).

EPR says: I can know 2 (TWO: a, b) values: result_of_A_observation AND result_of_B_observation (which tells me something if I could otherwise predict with certainty). This requires a bit of an assumption though about reasonable definitions of reality (let's call that realism).

BELL says: If I extend the EPR assumption of realism to a simultaneous unmeasured/counterfactual 3rd value result_of_C_observation (THREE: a, b, c), then a contradiction arises.

I hope this is clear and I got my a, b, c's right.

That indeed clarifies what you're thinking, but basically I think you're barking up the wrong tree. The same exact reasoning that gives EPR's *second value*, gives as many values as you want. The only reason the EPR paper stresses the *two* values is that they (really, he, Podolsky) want(s) to show that more values exist than QM (in particular the so called eigenstate-eigenvalue link) can accommodate. To establish that, two values will suffice. Though it remains true that actually their argument establishes 3, 4, ... infinity.

As a further historical digression, note that even focusing on *two* values in this way is a kind of unfortunate and unnecessary. If the point is just to show that QM is incomplete, it'd be perfectly sufficient to show that even just a single *one* of these properties possesses a definite value. Remember we're talking about an entangled state here, so according to the usual QM eigen-eigen link, "particle 2" over there doesn't possesses a definite value for spin along *any* direction. (It's not an eigenstate of the particle 2 spin operator for *any* direction.) So really all EPR needed was to say, look: by measuring some arbitrary spin component on particle 1, we can discover "without in any way disturbing particle 2" the value of the corresponding spin component of particle 2. So that (one single) spin component of particle 2 must exist, even though ordinary QM says it doesn't have a definite value. So QM is incomplete.

See how all this one vs. two vs. three business is a red herring? What's important is just that locality + perfect correlations require that *all* the spin components of particle 2 must exist, that is, particle 2 must possesses a pre-determined value for spin along *any direction you like*. That is the right way to understand what the EPR argument actually shows. Then, if you want to merely argue that locality --> the incompleteness of ordinary QM, that's easy, you just need to mention a single direction. If you want to derive a Bell inequality you need to mention several. But whatever. There's no more or worse "realism" in 2 or 3 than in 1, and it's the same one argument that gets you 1, 2, 3, and as many more as you might happen to want.

 

[ttn]

Just want to be clear: this is not MY assertion at all. We must either follow the EPR program and consider this as a reasonable assumption, or not. Once we define Realism to include this assumption, we can proceed.

If a person objects, as EPR says: "Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

So obviously we must agree that this is an assumption or the EPR result falls, since that is exactly what they said. Ditto, the Bell result falls as well if this is not agreed to (notice they say two or more). And apparently, this is quite a source of debate so I say it is a rather important assumption, yes. EPR explicitly acknowledge it!

I don't think you (correctly) understand what is going on in this passage you quote from the EPR paper. Now, in your defense, it is admittedly cryptic. Einstein thought and said so too, and we should take that seriously. In other words, we should not take the text of the EPR paper too seriously. When Einstein says it flubs and obscures the argument, we should listen to him, and hence rely on *other texts* to try to understand *his* views.

That said, here is what I think is the correct way to understand the passage you quoted. Actually, the just-following part (which you have also recently quoted) is highly relevant, so let me include the whole passage uninterrupted:

...We are thus forced to conclude that the quantum-mechanical description of physical reality given by wave functions is not complete.

One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality *only when they can be simultaneously measured or predicted*. On this point of view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definitely of reality could be expected to permit this.

Now look carefully at the sentences after the one you had quoted. What are they about? They are about ... LOCALITY. He doesn't exactly say that, and it could surely have been made a bit clearer, but look at what he actually wrote. It amounts to this: "to say that whether P or Q of the distant particle exists depends on which of those quantities I *actually* measure over here, would mean that what's real over there depends on what I freely choose to do here." In other words: that would violate locality!

Thus, I think one should understand the infamous last sentence -- "No reasonable definition of reality could be expected to permit this" -- as a statement about locality. He is trying to express that *non-locality* should be considered unreasonable! But it would have been much clearer, and certainly better for the historical development on these issues, if he had just written: "locality requires this" instead of injecting simultaneously an assessment of how reasonable or unreasonable it is to consider rejecting locality. In any case, setting aside the question of whether or not one should believe locality (clearly E, P and R would all have believed it, but the point here is that that doesn't really matter) the idea being expressed is just what I keep saying: you'd have to deny locality (or the QM-predicted perfect correlations) to deny that (here) Q and P have pre-existing definite values.

 

[DrChinese]

That indeed clarifies what you're thinking, but basically I think you're barking up the wrong tree. The same exact reasoning that gives EPR's *second value*, gives as many values as you want. The only reason the EPR paper stresses the *two* values is that they (really, he, Podolsky) want(s) to show that more values exist than QM (in particular the so called eigenstate-eigenvalue link) can accommodate. To establish that, two values will suffice. Though it remains true that actually their argument establishes 3, 4, ... infinity.

Of course I agree. As you say (and I thought I said), 2 values are sufficient for EPR, and more are implied. But 2 was not sufficient for Bell though. He had to have at least 3, one of which is counterfactual. Hopefully, you don't question that.

Look, there is no need for you to spend time with me on this. I am not really arguing with you so much as laying out some of the counter-reasoning* to readers who are following this thread.

*Which is the standard view of most physicists.

 

[billschnieder]

I think there is some very unfortunate choice of words being used here is adding to the confusion. EPR never suggested that outcomes of measurements *exist* prior to measurement. Such a statement commits a modal fallacy and can be rejected outright. They said elements of reality corresponding to *definite* predictions *exist*, not that the outcomes themselves *exist* prior to the measurement. Therefore to say

perfect correlations require that *all* the spin components of particle 2 must exist

is unfortunate. Maybe what was meant here is that the spin components have *definite* values (cf. EPR's "predict with certainty"). Which does not mean the same thing as they *exist*.

This is particularly important when you start comparing experimental outcomes, which ALL *exist*, with inequalities involving predictions which although all simultaneously valid, can not simultaneously *exist*. Failure to understand this distinction is at the root of many unnecessary paradoxes.

 

[ttn]

Of course I agree. As you say (and I thought I said), 2 values are sufficient for EPR, and more are implied. But 2 was not sufficient for Bell though. He had to have at least 3, one of which is counterfactual. Hopefully, you don't question that.

Well, at least one of the 2 was already counterfactual. So I still fail to see why you think anything important was added by going from 2 to 3. (It's also relevant that there are Bell type inequalities with only 2 settings on each side. Again: 2, 3, whatever. There is no issue here. Barking up the wrong tree.)

Look, there is no need for you to spend time with me on this. I am not really arguing with you so much as laying out some of the counter-reasoning* to readers who are following this thread.

*Which is the standard view of most physicists.

But that is exactly why I consider it worth my time to spend time with you on this. You do such a perfect job of expressing "the standard view of most physicists", and I think a lot of good might come from the audience seeing Bell's unorthodox views (or at least my best attempt to channel them) pitted up against these standard views in open discussion.

 

[camboy]

Hi ttn,

Sorry to butt in. I'm thoroughly enjoying this debate, and I've digested your article and enjoyed it. For me you win on points (and possibly even a technical knockout - sorry Dr. C.).

But that is exactly why I consider it worth my time to spend time with you on this. You do such a perfect job of expressing "the standard view of most physicists", and I think a lot of good might come from the audience seeing Bell's unorthodox views (or at least my best attempt to channel them) pitted up against these standard views in open discussion.

I agree - this debate is well worth having.

Now if you go further along this path the ultimate pit of hell into which you can descend is to argue with the very wonderful Lubos Motl. His views on nonlocality are here, and - not unexpectedly - they're pretty scathing and in direct contradiction to the conclusions of your article. Now, of course it's not worth arguing with him on his blog site, since he mostly just deletes comments that disagree with him. Nevertheless, is there anything substantive in his arguments that you can see?

 

[ttn]

Hi ttn,

Sorry to butt in. I'm thoroughly enjoying this debate, and I've digested your article and enjoyed it.

Thanks, and thanks for saying so. It's nice to know that there are people watching and getting something out of this!

Now if you go further along this path the ultimate pit of hell into which you can descend is to argue with the very wonderful Lubos Motl. His views on nonlocality are here, and - not unexpectedly - they're pretty scathing and in direct contradiction to the conclusions of your article. Now, of course it's not worth arguing with him on his blog site, since he mostly just deletes comments that disagree with him. Nevertheless, is there anything substantive in his arguments that you can see?

I have read some of Motl's comments on (e.g.) Bohm's theory before. Hadn't seen this particular post though. I skimmed it, only reading carefully the part where he purports to explain "misconceptions about nonlocality in QM". Basically everything he says is standard stale white bread orthodoxy: a rather dilute mixture of vague anti-realism, positivism, the (erroneous) identification of causal influences with signaling, and vitriol exuded toward the whole issue. It's of course telling that he doesn't even mention Bell. I'm sure (despite the in-passing reference to Bertlmann's Socks) that he's never actually read that paper, or any of Bell's other papers. Otherwise, don't you think he'd try to actually say what's wrong with *Bell's argument* -- instead of just talking tediously about what he feels is the right way to understand orthodox QM?

Note also that the essence of his attempt to argue that (you know, contrary to what the crackpot morons like me think) orthodox QM is perfectly local, is actually just a proof that the marginal distribution of outcomes on one system is independent of what might (or might not) be measured on an entangled system. So if I were going to discuss the issue with him, perhaps I would start by asking whether he thinks that the de Broglie - Bohm pilot wave theory is also local since, of course, it is also true in that theory that the marginal on one side is independent of what's measured on the other side. Presumably he'd so "no, obviously it's not, you &*#@ing moron" and then we could start discussing how to define "locality" so it captures the idea of "causal influences only coming from the past light cone" rather than merely this constraint on the marginals (which is roughly equivalent to a prohibition on signaling) ... and then a miracle occurs ... and then he becomes convinced that, yes, actually, if you formulate a precise notion of "locality" in *that* sense, then yes, not only Bohmian mechanics but also ordinary QM is nonlocal and indeed it can be proved that *any* theory sharing QM's empirical predictions will have to be nonlocal.

Somewhat more seriously, though, if someone were going to try to engage him on this issue, I'd say just recommend that he read "Bertlmann's Socks" or "La Nouvelle Cuisine" and challenge him to explain what's wrong with Bell's argument.

 

[camboy]

Hi ttn,

Thanks for the response. I esssentially agree with you..

It's of course telling that he doesn't even mention Bell.

To be fair, I just searched for the word 'Bell' in the article, and it comes up with:

"Entanglement isn't any sign of a nonlocality. Bell's inequalities guaranteed that the experimentally observed correlations can't be explained by a local realist theory. But in a striking contrast with the popular scientific literature, the wrong assumption isn't locality; it's realism. Locality is just a property of relativistic and similar theories, whether they're quantum or classical. And indeed, it holds. The validity of locality was one of the key results of Einstein's special relativistic revolution of 1905, a revolution that can't be undone anymore."

"On the contrary, realism is an assumption behind all classical theories, whether they're relativistic or not. And it's been shown invalid in the 1920s because classical physics has been shown wrong. Only probabilities of actual measurements may be predicted by physics. This is what the quantum revolution of the mid 1920s is all about. The new picture of the world is "local, non-realist". Everyone who suggests that it's "nonlocal, realist" apparently misunderstands both major revolutions of the 20th century physics, quantum mechanics and relativity."

Hardly an in-depth discussion, but still..

Somewhat more seriously, though, if someone were going to try to engage him on this issue, I'd say just recommend that he read "Bertlmann's Socks" or "La Nouvelle Cuisine" and challenge him to explain what's wrong with Bell's argument.

I'm sure he will be utterly delighted if one of us were to suggest that, or to point out what is wrong with his arguments. Perhaps he might even buy you a present to show his gratitude? I love it when people buy me presents.

 

[ttn]

To be fair, I just searched for the word 'Bell' in the article, and it comes up with:

"Entanglement isn't any sign of a nonlocality. Bell's inequalities guaranteed that the experimentally observed correlations can't be explained by a local realist theory. But in a striking contrast with the popular scientific literature, the wrong assumption isn't locality; it's realism. Locality is just a property of relativistic and similar theories, whether they're quantum or classical. And indeed, it holds. The validity of locality was one of the key results of Einstein's special relativistic revolution of 1905, a revolution that can't be undone anymore."

"On the contrary, realism is an assumption behind all classical theories, whether they're relativistic or not. And it's been shown invalid in the 1920s because classical physics has been shown wrong. Only probabilities of actual measurements may be predicted by physics. This is what the quantum revolution of the mid 1920s is all about. The new picture of the world is "local, non-realist". Everyone who suggests that it's "nonlocal, realist" apparently misunderstands both major revolutions of the 20th century physics, quantum mechanics and relativity."

Hardly an in-depth discussion, but still..

Oh yeah, I stopped reading at the end of the "misconceptions..." section and didn't even notice that the next section was about QFT rather than one of the other weirdo thing he talks about.

So, Dr C will be pleased -- here's a real live regular physicist who thinks we get to choose whether to reject "locality" or "realism". But, IMHO, the opinion of this particular regular physicist is completely worthless since he has never actually looked into these issues but is instead just repeating what he read in textbooks written by others who had never actually looked into it...

I'm sure he will be utterly delighted if one of us were to suggest that, or to point out what is wrong with his arguments. Perhaps he might even buy you a present to show his gratitude? I love it when people buy me presents.

I seem to recall that several years ago another Bohm-fan (and PhysicsForums participant) posted some comments on one of Motl's blog posts, and his (Motl's) responses made me think it wasn't worth trying to discuss any of this stuff with him. I can deal with ignorance and I can deal with hostility, but the combination tends to be unfruitful and unpleasant to argue with.

 

[zonde]

As a further historical digression, note that even focusing on *two* values in this way is a kind of unfortunate and unnecessary. If the point is just to show that QM is incomplete, it'd be perfectly sufficient to show that even just a single *one* of these properties possesses a definite value. Remember we're talking about an entangled state here, so according to the usual QM eigen-eigen link, "particle 2" over there doesn't possesses a definite value for spin along *any* direction. (It's not an eigenstate of the particle 2 spin operator for *any* direction.) So really all EPR needed was to say, look: by measuring some arbitrary spin component on particle 1, we can discover "without in any way disturbing particle 2" the value of the corresponding spin component of particle 2. So that (one single) spin component of particle 2 must exist, even though ordinary QM says it doesn't have a definite value. So QM is incomplete.

Are you sure that EPR was arguing for particles possessing some definite properties?
It seems possible to me that EPR was attacking Heisenberg's "uncertainty as measurement disturbance" view. So in that case the goal of EPR argument would be to show incompatibility between QM formalism and hidden variables which it does.

If we would think that QM formalism is compatible with hidden variables then argument with one value would give nothing.

 

[harrylin]

Would you not say that the question of what pattern of drops it produces if we pushed on it is a measurable attribute of the orange, and that this question has a well-defined answer even if we do NOT push on it?

No I would not say that it is a measurable attribute of the orange: the exact way this measurement is done influences the outcome, and this may be not exactly reproducible so that it is a property of both the orange and the detection instrument.