EPR paradox revisited, again. hehehe

[jackle]

This may be deemed irrelevant but we can construct a thought experiment starting off with schrodinger's cat in a box and when we open the box (observer) we act accordingly. Therefore, I don't think we can show that the observer's behaviour is always pre-determined in quantum experiments, regardless of clear instructions and how well they are followed.

 

[jackle]

... i recommend the philosophy forum. Please stop posting in this thread.

There are monitors to do this. It isn't your job.

 

[vanesch]

reply to vanesch post 25:



By quantum state what do you mean, do you mean the state often referred to as a superposition of states, or the state of spin up (also quantum) or both?

With quantum state, I mean: the element in Hilbert space (or better, the ray) describing the quantum state of the degrees of freedom of the system. What do _you_ understand by quantum state ?

 

[vanesch]

Alfred, in your post #8, you say:

Then they carry out the 2000 measurements and compare their results. Now if Alice is not really affecting Bob's measurement and if therefore Bob still has a fifty fifty chance of measuring either up or down every time, then you would expect that Bob would NOT have the exact opposite results to Alice. But he will! So it seems quite clear that Alice's measurement events do really affect Bob's measurement events, before any signal would have had time to travel between them.

and I take it that you conclude this only FROM THE OUTCOMES, right ? No matter what theory behind it. If people just showed you these _outcomes_ (not even knowing anything about quantum theory ; imagine that this experiment were performed in 1867) you would conclude that Alice's measurement affects Bob's outcomes, or not ?

All right, and then you've had examples (with dice, and with colored balls) where you would get EXACTLY THE SAME KINDS OF OUTCOMES. Is, in this case, Alice's measurement affecting Bob's outcome in this case too ?

I take it that in the last case, it is clear that there is no such thing as a non-local influence from Alice looking at a red ball, and (hence) Bob also looking at a red ball, right ? Then how can you conclude from EXACTLY THE SAME SET OF OUTCOMES in the first case that this indicates any influence of Alice's meassurement upon Bob's (purely based upon the experimental results, and NOT based upon any theory that is supposed to be behind it) ?

 

[alfredblase]

reply to vanesch post 39:

What I am saying is that someone who believes QM to be an adequate description of reality is violating the speed of light limit under certain experimental circumstances, and that ttn's dice argument does not prove otherwise. Since you admitted the argument you made was essentially equivalent, then your argument is inadequate also. That is all. I have still to be presented with proof that QM does not violate the speed of light limit in EPR type experiments.

 

[vanesch]

What I am saying is that someone who believes QM to be an adequate description of reality is violating the speed of light limit under certain experimental circumstances

And for someone who does NOT believe that QM describes accurately reality, but has the results of your post #8 ?

You see, it is not clear to what you are referring. Let's imagine for a moment that QM gives about correct predictions of experimental outcome, but that's all. And let's now forget the formalism of QM, and suppose that someone only had access to the RESULTS of the experiment you suggested in your post #8. Would he conclude, purely from the RESULTS, that no matter what description, there must be a violation of the speed of light limit or not ?
And what would be your argument to come to that conclusion *purely from the outcomes of experiment* and NOT from the formalism of QM ?

 

[alfredblase]

reply to vanesch post 41:

And let's now forget the formalism of QM, and suppose that someone only had access to the RESULTS of the experiment you suggested in your post #8. Would he conclude, purely from the RESULTS, that no matter what description, there must be a violation of the speed of light limit or not ?
And what would be your argument to come to that conclusion *purely from the outcomes of experiment* and NOT from the formalism of QM ?

Ok. Bob and Alice adhere to the speed of light limit. They used to believe QM was an adequate description of reality. However since QM clearly violates the speed of light limit they discard QM and (in answer to your question) they begin to look for an adequate description of reality that does not violate the speed of light limit.

 

[jackle]

They taught me about Bob and Alice at Primary school.

Bob and Alice adhere anything they like. They never believed QM was an adequate description of reality. They are systematically rude to people who disagree. They ask questions on public forums and then tell specific respondants to keep quiet. They discard one of the best tested physical theories ever created on the basis of flawed reasoning and then begin to look for a better description of reality but fail dismally.

I'm awaiting more abuse.

 

[alfredblase]

whatever jackle. if you insist on posting at least stop posting nonsense. (admittedly though i did find your last post quite amusing) by the way, shouldn't yoube going to bed? you've got primary school tomorrow...

 

[Doc Al]

Rather than trade insults, why not answer vanesch's question in post #41?

 

[alfredblase]

hehehe, i have answered it =)

I answered it in post 42 :)

i'm actually impatiently awaiting a reply from him xD

 

[Doc Al]

hehehe, i have answered it =)

I answered it in post 42 :)

No you didn't. Reread the post and the question. (he he he)

 

[alfredblase]

No you didn't. Reread the post and the question. (he he he)

Ok, since you believe I misunderstand his "question" (in fact there are 3 question marks in his post, indicating three questions) will you rephrase his post and state the _question_ (singular) which you think I have not answered please?

 

[leandros_p]

:
The behaviours of observers are determined before any experiment!

There is nothing wrong with communitcating before an experiment takes place!

Well, you wrote in post #8

:They agree before hand that they will always measure in the x direction, and they agree before the experiment which particular direction this will be. Also they synchronise their measurements,... Further they synchronise their measurements so that Alice will always be the first one to make the measurement.

Are these just preparations of measurement ?

No sir, they are not.

Alice has to follow Bob's direction AT THE TIME OF ACTUALL MEASUREMENT

Alice has to synchronise her measurements with Bob's timing AT THE TIME OF ACTUALL MEASUREMENT

Alice has to make the measurement before Bob's measurement AT THE TIME OF ACTUALL MEASUREMENT

I asked before: "how is she going to achieve this synchronization with Bob without exchanging messages" and you answered: "they will agree on these directives before the experiment".

The issue is that if Alice is using a common "clock" with Bob (she has to do that in order to comply with the predetermined agreed directives of synchronization), and if she knows the direction that Bob is measuring, then she is geting, from the experiment, the same "information" that is observed by Bob at the same time. You are right on this, but you fail to realize the meaning of this "information".

This information is produced or affected neither by Alice, nor by Bob. It is just a common observation of a phenomenon, that carries no other physical information other than being observed at once by both observers. It's like looking at the sky seeing the same positions of the stars with another observer at once.

A common observation may be used for synchronization between two observers, but in this case the common observation is useless, because Alice and Bob are already synchronized by a common clock, or else they could not accomplish the experiment in the way you force then in your post #8. They are already using a common "clock" AT THE TIME OF ACTUALL MEASUREMENT. You are forcing them to be synchronized and they know it already.

So the observations that you describe in post #8 contain no usefull information for both observers. I mean, the observations do not provide information for them other that of information they already know: that they are synchronised because they had agreed to be synchronized.

Leandros

 

[ttn]

reply to ttn post 31:

Finally we can agree. However by agreeing with me on this point you also agree that your dice "proof" is inadequate as in that post you argued that: ...

I think you've mistaken me for someone else...

According to QM Bob goes from a state of less definate reality into a state of more definate reality.

I think what you mean is: the state of Bob's particle changes (when Alice makes her measurement). It goes from something "fuzzy" in regard to the value of a certain attribute, to something "definite" in regard to that attribute. That's all true. And I think we agree that that involves a kind of nonlocality (a kind which can be made precise using a precise def'n of locality such as Bell Locality).

QM offers a description of Alice's and Bob's reality. Everytime Bob makes a measurement he is aquiring more and more statistical evidence that the QM description of reality is no longer what he expected it to be.

That doesn't quite make sense to me, unless you're just speaking loosely. What the *theory* says is that every time Bob makes a measurement he affects the state of the world -- in particular, the state of his *and Alice's* particles. And since Alice's particle is far away, that "affecting" is a case of nonlocal causation. But if Alice and Bob know QM, this is all precisely what they'd "expect." So the point isn't really about their expectations at all. It's a point about how the theory works, what the theory says. And what the theory says is that there is a kind of nonlocal causation.

But as I've tried to always remind us, that leaves open the question of whether or not the theory is *right* -- i.e., whether that nonlocality is a real fact of nature, or merely an embarrassing feature of that particular theory.

Bobs results when compared to Alice are inconsistent with the OQM description of reality.

No, the OQM explanation of certain real processes is not local. Or maybe you meant: "Bob's results ... are inconsistent with" the description that would be given by a theory that was just like OQM in all respects except that, say, the causal effects of measurements (the "collapse of the wf") occurred along the future light cone of the measurement in question (or something like that).

So since the proofs/explanations that have been given so far are inadequate, I restate my original appeal: I ask for a clear proof that OQM does not violate the speed of light limit.

If you're talking about the underlying causal structure of the theory, then OQM violates locality, and there can be no such proof. If you're talking about whether the theory permits humans to send messages faster than light, then the proof is trivial and pretty well known. Your phrase "the speed of light limit" is just ambiguous -- it's not clear what you're asking for.

 

[ttn]

Ok. Bob and Alice adhere to the speed of light limit. They used to believe QM was an adequate description of reality. However since QM clearly violates the speed of light limit they discard QM and (in answer to your question) they begin to look for an adequate description of reality that does not violate the speed of light limit.

Now you've got it. That's exactly what Einstein thought. This is exactly the EPR argument (or what the EPR argument was supposed to be, before Podolsky botched the writing of the paper...). Orthodox QM provides a nonlocal explanation of these correlations. If you want to have a local theory, you have to look elsewhere -- specifically, you have to posit a certain kind of hidden variables that determine the outcomes on each side (independent of what's going on on the other side). That is just exactly the point of EPR.

...which means you're now ready for the *second* part of the proof that nature is nonlocal: Bell's Theorem. Bell's Theorem proves that the kind of theory just posited in the last paragraph (posited, remember, on pain of violating locality!) cannot reproduce the full slew of QM predictions. Thus, *no* theory whatsoever (respecting a certain locality condition, Bell Locality) can be consistent with the full slew of QM predictions. And since we know from experiment that those predictions are *correct*, it follows that nature ain't Bell Local.

 

[Sherlock]

... I restate my original appeal: I ask for a clear proof that OQM does not violate the speed of light limit.

There can't be a proof that OQM doesn't violate locality for the same reason that there can't be a proof that OQM does violate locality. OQM isn't a causal theory, it's a probabilistic one. There are correlations, that's all. The underlying physical reasons for the correlations are a mystery.

The assumption that the clearly deterministic or causal components of the mathematical structure of quantum theory are necessarily a 1-1 mapping of an underlying quantum world is just that --- an assumption. It's not an assumption that's part of OQM. OQM is the probabilistic interpretation.

Given the assumption that formal qm is a 1-1 mapping of an underlying quantum world, then maybe it's possible to infer that quantum theory (and, hence, underlying reality --- since it's being assumed that the theory is a description of underlying reality) is or isn't nonlocal. But, given that assumption, you're not dealing with OQM --- and, given that assumption, you might, as well, infer the existence of the alternative realities of MWI.

So, the proof you seek depends on what is assumed about the relationship of the formalism of quantum theory to an underlying quantum world. Since we have no sensory apprehension of this underlying reality, the wisest course seems to me to make no assumption about this relationship given the extant data. This is what OQM does, at least wrt its pedagogical presentation. It treats this consideration as essentially meaningless --- even though it's probably a good bet that at least some of the physicists who apply and develop the theory DO have their own ideas and intuitions about such a relationship.

Wrt OQM then, Alice and Bob are not affecting each other. The rate of detection, the probability of detection, at one end is not altered by events at the other end.

Make an assumption about the relationship of the theory to an underlying reality and it's a different story. The catch is that there's no definitive way to ascertain whether or not such an assumption is true.

 

[vanesch]

Ok, since you believe I misunderstand his "question" (in fact there are 3 question marks in his post, indicating three questions) will you rephrase his post and state the _question_ (singular) which you think I have not answered please?

The question I asked (and which Doc Al clearly understood) is: do you conclude a faster-than-light emission FROM THE DATA or do you conclude it from the formalism and its interpretation ?

I had the impression, in your post #8, that you concluded it FROM THE DATA.

And as your data (for the example that you presented in post #8) are identical with data that you can obtain with a set of colored balls, I was wondering how you could do so.

There's only one question mark here, so there should be no ambiguity as to what the question is

 

[alfredblase]

Leandros,

if anyone else can make sense of your objections and put them forward in a clear manner I shall try and answer them. I'm sorry but I do not have a clue what you are talking about.

 

[alfredblase]

I think what you mean is: the state of Bob's particle changes (when Alice makes her measurement). It goes from something "fuzzy" in regard to the value of a certain attribute, to something "definite" in regard to that attribute. That's all true.

we agree on this then.

And I think we agree that that involves a kind of nonlocality (a kind which can be made precise using a precise def'n of locality such as Bell Locality).

From here on you mention locality many times, (something I haven't mentioned at all). Please give the definition of locality given by Bell Locality.

Thanks.

 

[alfredblase]

sherlock you also refer to locality as being the violated principle. Is your definition of locality that given by Bell Locality or do you use another one? If another please give this definition.

Thanks.

 

[ttn]

From here on you mention locality many times, (something I haven't mentioned at all). Please give the definition of locality given by Bell Locality.

Thanks.

Bell discusses this in detail in several essays, including "La Nouvelle Cuisine" and "The Theory of Local Beables." Both essays appear in the 2nd edition of "Speakable and Unspeakable". If you don't have easy access to either of those, there's a nice discussion in section 2 of this paper

http://www.arxiv.org/abs/quant-ph/0601205

which is basically a rigorous version of the EPR argument formulated in terms of Bell Locality.

 

[alfredblase]

The question I asked (and which Doc Al clearly understood) is: do you conclude a faster-than-light emission FROM THE DATA or do you conclude it from the formalism and its interpretation?

The answer is that I never conclude a faster than light emission.

But I think you also mean to test my understanding of what is in question...

I suspect you also mean to ask me if I think there is a difference between an experiment involving classical objects such as pairs of oppositely coloured balls, and one involving objects that exhibit QM properties such as pairs of electrons with opposite spin angular momentum, right?

QM tells us that spin is a non-commuting observable. This tells us that until we measure the spin of one of the entangled electrons, in a particular direction, we are violating local realism. Meaning that we are violating the assumption that all properties are defined in reality as being something in particular (up or down in this case), even if a particular properety has never been measured.

Coloured balls on the other hand... well there are some who would argue that... well anyway we need not concern ourselves with balls because there is no QM wavefunction defining the colour of billiard sized balls, and so since we are performing this experiment to test the validity of QM we need not discuss balls.

Thanks.

 

[alfredblase]

The following is Bell’s description of the locality con-
dition, along with his accompanying figure:

“A theory will be said to be locally causal
[i.e., what we are calling Bell Local] if the
probabilities attached to values of local be-
ables in a space-time region 1 are unaltered
by specification of values of local beables in
a space-like separated region 2, when what
happens in the backward light cone of 1 is al-
ready sufficiently specified, for example by a
full specification of local beables in a space-
time region 3.”

(from that paper ttn linked in post 57, oh and I didn't include the figure)

what is a beable?

oh and concerning my experiment, what do you consider region 1 to be? region 2? and region 3?

what is considered a "full specification"?

 

[ttn]

what on Earth is a beable?

It's a kind of semi-joke of Bell's. Standard quantum mechanics talks a lot about observ-ables. Bell thought that any real theory ought, in addition, to specify that which is *real* -- not just what can be observed, but what *is*. Hence "be-able" as an alternative to "observ-able". As has been pointed out many times, this neologism is unfortunate in a way, since it implies that "beables" are only potentially real, whereas Bell actually meant to refer not to a mere potential but to what is really actually real.

An example he gives somewhere: in Maxwellian electrodynamics, the E and B fields (and functionals of them) are beables, while the potentials V and A are not. That is why nobody thinks locality is violated when, in Coulomb gauge, the potential V at some point changes instantaneously when a distant charge is moved. By contrast, if the fields E or B were to change instantaneously, that *would* be a problematic kind of nonlocality since those fields are supposed to represent "beables".

Does that clarify? The point is basically: every theory has to say that something or other is real; it has to be *about* *something*. And then Bell Locality is the requirement that those real somethings shouldn't be causally affected by stuff outside of the past light cone.

One way of reading orthodox QM is to take wave functions as beables. If we do that, the theory violates Bell Locality. On the other hand, if we don't do that -- and hence say that the theory has no beables at all, then the question of the theory's locality becomes meaningless because, really, it isn't even a theory unless it asserts *something* about the way the world works.

 

[alfredblase]

thanks ttn, I also added more questions afterwards, sorry I tend to post and keep retouching them until I'm happy.

 

[leandros_p]

Leandros,

if anyone else can make sense of your objections and put them forward in a clear manner I shall try and answer them. I'm sorry but I do not have a clue what you are talking about.

OK.

Thank you for trying to understand my post.

Leandros

 

[ttn]

oh and concerning my experiment, what do you consider region 1 to be? region 2? and region 3?

1 and 2 are the past light cones of the two measurement events.

what is considered a "full specification"?

That's a very important question. The answer is: how the heck should anybody know, a priori? But the point is a *theory* provides some definite candidate for what a "full specification" is supposed to consist of. For example, if we're talking about Maxwell electrodynamics, a full specification means: the positions and velocities of all charged particles plus the E and B field configurations. To posit a theory is to posit some definite proposal for what a full specification of beables would mean. And once you do that, you can ask: does this theory respect Bell Locality? This is why the criterion of Bell Locality applies primarily to theories. If all you have are, say, the results of some experiments, there is no way to answer the question "Was Bell Locality respected?" But, using the 2-part argument that Bell gave, it is, in the end, possible to conclude that no Bell Local theory can agree with what we know about the correlations from experiment. So therefore nature doesn't respect Bell Locality (which of course leaves open exactly what the right theory is -- we know the right theory will not be Bell Local, but that's about it).

 

[alfredblase]

you havn't specified region 3 :P =)

 

[ttn]

you havn't specified region 3 :P =)

Look at the other figure in the paper. The slice across spacetime (after the particles are emitted, but before the measurements are made) plays the role of "region 3" for *both* detection events. So the point is, you need (i.e., "lambda" is) a complete specification of the state of the particles on this slice, i.e., at this time. According to OQM that is just the wave function. According to some kind of hidden variable theory, it would be something else or something more.

I have class in 5 seconds, so to be continued later if you're still confused...

 

[alfredblase]

reply to ttn post 60:

An example he gives somewhere: in Maxwellian electrodynamics, the E and B fields (and functionals of them) are beables, while the potentials V and A are not. That is why nobody thinks locality is violated when, in Coulomb gauge, the potential V at some point changes instantaneously when a distant charge is moved. By contrast, if the fields E or B were to change instantaneously, that *would* be a problematic kind of nonlocality since those fields are supposed to represent "beables".

ok so a beable is a description of a variable that does not need a field from a source different to its own in order to be defined. do you agree?

 

[vanesch]

QM tells us that spin is a non-commuting observable. This tells us that until we measure the spin of one of the entangled electrons, in a particular direction, we are violating local realism.

Ok, so you were NOT talking, finally, about the experimental results (which were identical for the balls and the electrons). You are talking about the inner gears and wheels of the FORMALISM of quantum theory, and you are looking at how the inner wheels and gears of the formalism run, and from their appearance, you conclude about a non-locality.

But then you have to know that how exactly these inner wheels and gears are seen to correspond to the real world, is exactly the interpretation of the theory!

Now, I tell you, that in the MWI interpretational frame, there is NO communication (as a FTL interaction) between the different parts of the photon pair upon measurement.

I also agree that in the "standard" (von Neuman - Copenhagen) view, there IS a clear FTL mechanism, which is the projection postulate, which is applied when the measurement is performed. However, as the wavefunction is not seen as representing something physical in this picture, it is not really clear if this means that there is an FTL ACTION in nature. I would like to point out that the MWI view does away with this projection postulate as a physical operation.

Finally, Bohm's theory (which is empirically equivalent to QM) gives you a clear, explicit FTL interaction, by the quantum potential.

Meaning that we are violating the assumption that all properties are defined in reality as being something in particular (up or down in this case), even if a particular properety has never been measured.

But these are statements which only make sense within the Copenhagen interpretation of QM, and NOT in the MWI view.

well anyway we need not concern ourselves with balls because there is no QM wavefunction defining the colour of billiard sized balls, and so since we are performing this experiment to test the validity of QM we need not discuss balls.

Again, in the MWI view, billiard size balls have just as well a wavefunction description as electrons. In the Copenhagen view, on the other hand, the macroscopic world does NOT have a quantum description.

So, you see, when looking at the inner gears and wheels of the quantum formalism, it depends on HOW you look upon it. All you've been arguing was by implicitly taking on the Copenhagen view. There, of course, you cannot "prove" that the machinery is not doing anything FTL, because the basic operation of projection after measurement is an FTL operation!
And in MWI, on the other hand, there is NEVER an FTL operation. Nevertheless, they both share the same unitary quantum theory.

 

[ttn]

ok so a beable is a description of a variable that does not need a field from a source different to its own in order to be defined. do you agree?

The beables of a theory are those variables (in it) which we are supposed to take as descriptions of really-existing entities. Or, for short: the beables are whatever some theory says is *real*.

For Newtonian mechanics, massive particles and the forces they exert on each other are the "beables." For Maxwell's theory, the electric and magnetic fields are the "beables." For Bohmian quantum theory, the wave function and the particle positions are the "beables." For orthodox QM, the wave function is the only "beable." Maybe that helps?

If you can manage to get ahold of a copy of Bell's book ("Speakable and Unspeakable") you really really really really should. He is a breath of fresh air, and if you have any interest in quantum physics at all, there is literally no other author which it is so mandatory to have read! :!) In honor of Valentine's Day I'll state it here publicly: I love Bell!

 

[alfredblase]

hmmm so all wavefunctions in QM are beables?

 

[alfredblase]

replyto vanesch post 67:

and from their appearance, you conclude about a non-locality.

I have at no point talked about locality in any of my arguments. What is your definition of locality please?