Is QM Inherently Non-local in EPR and Bell Discussions?

[Careful]

Why not in the future lightcone of B also? The precise relationship between the motions of A and B subsequent to their interaction remains until one or the other, or both, are subjected to external influences (such as B interacting with C). The motions of B and C subsequent to their interaction will be, in part, due to B's prior interaction with A.
It's just a convenient way to talk about it. Individual detections are particles.

This should be only so for that part of B which interacted causally with A (which is de facto in the future lightcone of A), not the part that did not interact with A at all. The real problem is that reduction of the state (in B) is an instantaneous non - local (causal) process and that is why the influence of A through B will travel to C.

 

[vanesch]

I do not dispute that QM is successful in calculating the spectrum of the H and He atom (you cannot predict above He) and explaining the Lamb shift.

?? I think that QM has rather more successes on its name than just H and He ! I am even convinced that the "classical field" approaches (the coupled Dirac-Maxwell fields + eventually some noise) have serious problems with higher than He configurations. At best these theories give the same predictions as the Hartree-Fock method with the self-consistent potential, but it is well-known in quantum chemistry that this gives a good approximation, but sometimes not good enough and one needs to add things like "configuration interaction" to get closer to experimental values.

However, I am convinced that these successes have perfect (albeit more difficult and subtle) classical explanations. A good step in this direction is the theory of stochastic electrodynamics which reproduces a good bunch of these so called exclusive results from second quantization in a firstly quantized framework (such as the Casimir effect, and the H atom I believe). Barut and Dowling have done quite some work on this issue ...

Yes, that's fascinating work, I agree. The problem is, however, with most of these approaches, that they tackle ONE SPECIFIC aspect of quantum predictions, and that we can then vaguely hope that they will, one day, be as successfull as standard quantum machinery in all the rest.
It is just not reasonable to accept the quantum machinery for about all the predictions it makes, *except* for those very few predictions that kill off your original belief of how nature ought to be.
The reason why this work is 1) fascinating and 2) probably misguided can be found by "reductio ad absurdum". Indeed, if these classical theories were correct, their computations would be gazillion times simpler than quantum computations. Non-linear partial differential equations in 3 dimensions are, computationally, peanuts as compared to, say the Feynman path integral in QFT, and can be attacked much much easier with finite-element methods than QFT. It would reduce quantum chemistry, and even nuclear physics, to something computationally just as easy as weatherforcasting. Not that weatherforecasting is so simple, but it is doable, while QFT calculations only start to be tractable with lattice techiques. So if it were possible to do so, it would have been done already since a long time.

My intention is not at all to dispose of standard QM, I am perfectly aware of the fact that it provides an effective way of calculating the statistics of experiments with microscopic objects when applied on the *correct* problems with considerable thought. However, I also gives the wrong answers in some cases and it does not provide any insight into the dynamics of a single particle.

I would like to know in what specific cases quantum theory comes up with the wrong experimental predictions which have been falsified by experiment.

I want to obtain *insight* into the microworld which I believe obeys the same laws as the macroworld (that is GR and electromagnetism), therefore entanglement is a crucial issue and one should not take it lightly.

In a way, I *also* adhere to a belief: it is that there are a few fundamental principles on which the entire formalism of physical theory has to be constructed.

Concerning your remark about the extrapolation of succes, I can only say that people have been looking for over 50 years for a perpetuum mobile; I hope one is not going to look 100 years for entanglement. By the way, Newton theory was also correct for 300 years.

If you want my guess, I don't think quantum mechanics in its present form will still be around (except as a useful approximation) 300 years from now - or it will, but then because of lack of progress (for instance, lack of experimental input on quantum gravity phenomena). But as of now, it is still the best thing we have - and it has to be admitted that it is vastly more successful in vastly different fields than anything that tries to rival with it. At best you get *identical* predictions in certain areas. Entanglement is a very standard part of the quantum formalism, and *is* confirmed by many experiments in the sense that these calculations DO correspond to predictions that are verified: see further.

Concerning Hartree, you have to include a classical radiation field determined by the probability current of the particles. In that way, you obtain a QM where each particle has its own wave function and where interactions propagate via a classical maxwell field determined by the sum over all probability currents times the appropriate charges (I think Barut called this the self field approach to QED, it's non-linear of course).

I expected that this is what you meant but wasn't sure. It is indeed the self-consistent field method used in quantum chemistry. A good approximation, but with known deviations from experiment, which is improved upon by configuration interaction techniques which are nothing else but "entanglements" of the different electrons. Even in the H2 molecule, this is experimentally visible (although small). More successes can be found with the H20 molecule, especially the angle between the two bonds, which for the H-F selfconsistent field method gives us 106.1 degrees, while the CI technique gives 104.9 degrees (experiment being 104.5 degrees). Took this from "Modern quantum chemistry" by Szabo and Ostlund.
There are many many examples like this. The problem with the CI technique is of course the huge system of equations that it generates - hence my proof by contradiction of a classical theory doing the same thing: if it worked, it would be done since long.

 

[Careful]

Hi Vanesh,

It is late for me, so I shall treat some part of your comments and shall be back tomorrow for more...
I think your most substantial argument is that the Hartree approximation although it is good is known to deviate slightly from experimental outcome. This is a fact. However, I did not say that Hartree is the full theory, neither did I claim that Fock corrections is what one should be looking for:
(a) your computability argument is incorrect, good computer experiments concerning, say the classical three body problem, have only very recently been performed and obtaining a thorough understanding of it is still on the way. The same comment applies to GR where the post Newtonian approximation is often known not to be adequate and obtaining the full solution (to the nonlinear equations) is a notoriously difficult problem (and picking out the right finite element method can take a considerable amount of time, even for the trained mathematician).
(b) I did not say I accept all predictions of QM except those which kill my belief : I accept all predictions which are confirmed by experiment but this does not imply that QM is the only way to get to these results.
(c) I am fully aware that these aternative approaches are in some sense behind QFT. The reason for this is is easy to think of: just compare the amount of money which is put into both research branches.
(d) I know it is the attitude of most researchers to conclude from Hartree is not equal to, but very close to, experiment implies that entanglement vindicates again. However, here I disagree : could we simply not have forgotten something? Why are these predictions so close while we totally ignore all non-local correlations? One could think now of adding other interaction terms between different wave packages, as you mention, in order fit accurately to the results but this is patchwork. I will come back to this tomorrow.

 

[Sherlock]

You seem to say that (a) QM gives the right predictions ...

It seems to be the most accurate across a wide range of experimental applications. Saying that QM's predictions are "right" is sort of an iffy statement, isn't it? After all, there are limits (due to error and due to fundamental constraints specified by QM itself) to what can be experimentally determined. QM predicts values that experimental runs will approach, and, afaik (I'm just learning), it's calculations agree with experiment.

... and (b) spacetime is real

Space and time are conventions.

... and all processes (which are real by assumption) have speed smaller or equal than light.

Yes, I assume that Nature obeys the principle of locality.

(b) logically implies that the wave function of (a) must be real which implies that processes exist which go faster than with the speed of light.

Space time is a convention. So is the wave function. The wave function is a complete description of what is known about the quantum system it refers to (at least it's one way of describing what is known). However, the wave function is necessarily an incomplete description of the physical reality of the quantum system it refers to. Hence, no superluminality is implied. The assumption that Nature is local is based on strong theoretical arguments which have thus far not been falsified by experiment, so it remains.

Concerning the conservation laws you mention: it is very hard to obtain an anomaly free interacting QFT and these equations do usually not make much sense anymore...

I haven't learned QFT yet, so if your main points depend on this theory, then I must excuse myself from the discussion.

 

[Sherlock]

This should be only so for that part of B which interacted causally with A (which is de facto in the future lightcone of A), not the part that did not interact with A at all.
The real problem is that reduction of the state (in B) is an instantaneous non - local (causal) process and that is why the influence of A through B will travel to C.

I don't understand what you're saying here.

 

[vanesch]

I will come back to this tomorrow.

Maybe we should then start another thread, not hijacking this one ? This thread is about the non-locality or not of quantum theory, under the working hypothesis that QM predictions are correct, and it is NOT about whether or not this working hypothesis is acceptable.

 

[Careful]

Maybe we should then start another thread, not hijacking this one ? This thread is about the non-locality or not of quantum theory, under the working hypothesis that QM predictions are correct, and it is NOT about whether or not this working hypothesis is acceptable.

Ok, you start a new one although - as I mentioned before - the answer to the literal message of this thread is rather obvious (it might be pleasant to chat about it however and I am aware that recently some controversy about this has been on the Arxiv, entirely unnecessary and even misguided from time to time). People talk too much about the interpretation of QM and are afraid to really modify it, that's one good reason why progress (especially in quantum gravity) is slow...(not substantial) So, I will check this site later on.

 

[vanesch]

Ok, you start a new one although - as I mentioned before - the answer to the literal message of this thread is rather obvious

I don't think that the answer is obvious. QM presents us with a riddle: Bell locality is violated, but signal locality isn't.
If neither were violated, I think nobody would even think of saying that locality is violated by QM. If both were violated, again, it would be obvious that QM violates locality. But we're in between.

And then it depends on how you look at the internal workings of the theory to decide whether the mathematical operations you execute (and which you "believe" to be associated to an ontology or not) are respecting locality or not. So it depends on what exactly you understand by locality, and what exactly you assign a reality to in the mathematical framework of QM. This involves of course the interpretation you attach to it.

Concerning the predictions:
So if you say: a theory is local if it respects Bell locality, then, no, QM is not local (that's ttn's point of view).
If you say: a theory is local if it respects signal locality, then yes, QM is local (can't build a FTL phone that way).
Concerning the mathematical formalism and its relation to an ontology:
If you 1) assign a reality to the wavefunction and 2) consider the projection postulate as describing something that physically happens, then the inner workings of QM are bluntly non-local.
Denying 1) is the epistemological viewpoint of QM (just a technique for calculating outcomes of experiments) and you're back to the "predictions" side.
Denying 2) (like does MWI) allows you to consider the mathematical machinery of QM as respecting locality.
See, plenty of stuff to argue endlessly over, and spend time on PF :-)

 

[Careful]

I don't understand what you're saying here.

Look, it is very simple: locality in the operational sense means that a measurement at A cannot have measurable influence outside the lightcone of A. The example I gave you violates this. However, it is of crucial importance here that the measurement at B is non-local: such as the projection operator on a localized state or a Wilson loop, but not the integral of a local operator valued density. Such non-local operators are used all the time in QFT, so one cannot claim they are not physical. Therefore, if one assumes the validity of perfect von Neumann measurements and the existence of non-local observables, then one has to conclude that QFT is not local operationally. One could argue that such measurements are impossible, but then one has to develop an accurate measurement theory which respects locality. Such task has not been accomplished yet: therefore my statement is fair.

 

[Careful]

I don't think that the answer is obvious. QM presents us with a riddle: Bell locality is violated, but signal locality isn't.
If neither were violated, I think nobody would even think of saying that locality is violated by QM. If both were violated, again, it would be obvious that QM violates locality. But we're in between.
And then it depends on how you look at the internal workings of the theory to decide whether the mathematical operations you execute (and which you "believe" to be associated to an ontology or not) are respecting locality or not. So it depends on what exactly you understand by locality, and what exactly you assign a reality to in the mathematical framework of QM. This involves of course the interpretation you attach to it.
Concerning the predictions:
So if you say: a theory is local if it respects Bell locality, then, no, QM is not local (that's ttn's point of view).
If you say: a theory is local if it respects signal locality, then yes, QM is local (can't build a FTL phone that way).
Concerning the mathematical formalism and its relation to an ontology:
If you 1) assign a reality to the wavefunction and 2) consider the projection postulate as describing something that physically happens, then the inner workings of QM are bluntly non-local.
Denying 1) is the epistemological viewpoint of QM (just a technique for calculating outcomes of experiments) and you're back to the "predictions" side.
Denying 2) (like does MWI) allows you to consider the mathematical machinery of QM as respecting locality.
See, plenty of stuff to argue endlessly over, and spend time on PF :-)

I agree with you here except that you cannot send signals faster than light in QM. This is a much more subtle issue than just postulating commutation relations (see my previous post) at spacelike separated events. So, you should not talk but develop a theory of non-perfect Von Neumann measurements which respects locality in the operational sense.

 

[vanesch]

I agree with you here except that you cannot send signals faster than light in QM. This is a much more subtle issue than just postulating commutation relations (see my previous post) at spacelike separated events.

I have seen what you allude to, but I can't make much sense of it. I would think that what is sufficient is that the Green's functions (the propagators) vanish outside of the lightcone ? (and this is related to the commutation relations vanishing at spacelike separated events) How are you going to modify the field in a spacelike way if the Green's function is 0 ?

 

[Careful]

I have seen what you allude to, but I can't make much sense of it. I would think that what is sufficient is that the Green's functions (the propagators) vanish outside of the lightcone ? (and this is related to the commutation relations vanishing at spacelike separated events) How are you going to modify the field in a spacelike way if the Green's function is 0 ?

It is very simple: quantum field theory has NO measurement theory. There is NO principle of reduction of the state functional (that is why the vanishing of the Green function outside the lightcone is sufficient for your purposes) such as in standard QM (you should read Sorkin's paper). This is clearly unsatisfactory and the only thing QFT is good for is to compute S matrices. Summary: in QFT you are restricting yourself to a unitary evolution. Bringing in any discrete/non-unitary reduction of the state principle allows for the possibility of measurable correlations outside the lightcone at least when you do it in the naive Von Neumann sense. So it seems to me you have two possibilties: either (a) you admit that the idea behind QFT needs improvement in order to incoorporate for a suitable measurement theory or (b) you refuse fundamental investigations in the principles of QFT and accept either (i) superluminal signalling or (ii) the fact that QFT allows for a limited number of questions to be asked.

 

[vanesch]

Summary: in QFT you are restricting yourself to a unitary evolution.

But that is maybe a very very good idea

 

[Careful]

But that is maybe a very very good idea

No, it is not
Let me make the full reasoning: let X be the orginal density matrix
(a) you want f(x) and f(y) to commute when x and y are spatially separated since you want MEASUREMENT of f(x) and f(y) to be independent (otherwise there is no sense in doing this)
(b) Let A be in the past of B and C in the future of B but not in the future of A (A,B and C are domains in spacetime). Suppose a and c correspond to local operators on A and C that is : a = integral(f(x), x in A) and c = integral(f(y), y in C). Then clearly a and c commute and if I do a after c or c after a, it does not matter. However the causal relations impose a temporality on the order in which a,b and c have to be performed : that is c after b after a. Now if b is not a local operator (for example the integral of quasi local observables) then
sum_{i,j,k} P_i Q_j R_k X R_k Q_j P_i is not equal to sum_{i,j} P_i Q_j X Q_j P_i even when both density matrices are restricted to the complement of the lightcone of A (P_i Q_j R_k are the orthogonal projection operators associated to c,b and a respectively). Non local operators are for example integrated hamiltonian densities with respect to some observers.

This is clearly a problem. So I expect a better answer from you. On one hand you claim that f(x) and f(y) have to commute since measurements have to be independent and on the other you claim that you do not want to do state reduction when it becomes troublesome. Even funnier, if you would claim that no measurement can be made in QFT, then it is impossible to even tell something about this issue at all :-) It is clear that any quantum theory MUST have a consistent measurement theory for it to be taken seriously. So either you propose one, or otherwise I see no reason why superluminal signalling is banned.

 

[Tez]

No, I think you have it right.
For me personally, the confusion begins when you talk about an object outside the lightcone. Alice makes a measurement, which causes collapse of the shared wave function. Now you know something about something somewhere else, true, and that is outside the lightcone.
But what has happened that is really so weird? We project the knowledge we have back to the point at which the entangled particle pair was created. This is the same thing that happens when only one particle is involved, nothing strange about that. The particle acts as if it had that orientation from the last point something happened.
Say Alice sees a V orientation with a polarizer at 0 degrees. Naturally, all subsequent measurements will be consistent in EVERY WAY with this knowledge AS IF it was always that way from the creation of the particle. So in that sense there is absolutely nothing happening outside any light cone.
In other words, all quantum measurements find a particle in an eigenstate and its eigenvalue is consistent with the quantum measurement rules. Entangled particles are no different in this respect. So the real question to me is: why does a measurement at time T2 cause the particle to assume a specific value as if it had that value at time T1 (where T1 is before T2) ? Does that make oQM non-local? Or is that a case of backwards causality? I am not sure that anything physical occurs along with the collapse, and I think that is a relevant question too.
Naturally, some of these issues show up in our definition of locality. You can see that there is no information transfer which is FTL, and there is no clear causal effect which is FTL. Yet the Bell Locality condition is violated with a strict application of its definition. So what does that condition actually tell us? Of course, it fits with the Bell Inequality too so that is very important.
Inquiring minds want to know...

What you say here is one of the most common misunderstandings of what Bell's theorem tells us. It is categorically not the interesting nonlocality of QM.

Ask yourself if that type of nonlocality would enable you to "win" this game:

The game is you and a friend are imprisoned, and told you're going to be separated. Once separated you will each randomly be asked either "what is X?" or "what is Y?" to which you must answer either 1 or -1. If you are both asked the X question then you must give opposite answers, but in all other cases (one of you asked X the other Y, or both asked Y) you must give the same answer. You win the game, you get released.

A minutes thought will let you know that unless you can tell what question the other person is asked there's no way to guarantee you winning the game. Your best bet is simply to agree to always answer the same thing and rely on the 3/4 chance that this'll win you the game.

But wait: if you carried entangled particles you can delay the decision of what to answer to your captors - once asked the question you make a measurement on the particle and output 1 or -1 according to the outcome. This way your probability of being released goes up to 85%. How did the entangled particles do it unless they knew something about what the other particle had been "asked".

In a 3 prisoner version the probability of release can go up to 100%, despite every "logical" strategy allowing for a maximum of 75%.

 

[DrChinese]

But wait: if you carried entangled particles you can delay the decision of what to answer to your captors - once asked the question you make a measurement on the particle and output 1 or -1 according to the outcome. This way your probability of being released goes up to 85%. How did the entangled particles do it unless they knew something about what the other particle had been "asked".

But they don't really! (Obviously, otherwise we could use that for FTL signalling.)

All we know is that our captors communicated in our past light cone and we are using that locally transmitted knowledge to play a logic game. There is no superluminal anything over and above a normal interpretation of a Bell test. After all, our captors can't release us until they compare our answers.

 

[Careful]

But they don't really! (Obviously, otherwise we could use that for FTL signalling.)
All we know is that our captors communicated in our past light cone and we are using that locally transmitted knowledge to play a logic game. There is no superluminal anything over and above a normal interpretation of a Bell test. After all, our captors can't release us until they compare our answers.

Well, enlighten us with a measurement theory which bans superluminal signalling consistently. I did not meet anyone until now who can do this, perhaps a texan chinese can be the first one.

 

[Sherlock]

You can see that there is no information transfer which is FTL, and there is no clear causal effect which is FTL. Yet the Bell Locality condition is violated with a strict application of its definition. So what does that condition actually tell us?

The Bell Locality condition tells us that A and B aren't observationally or statistically independent. This has a local explanation via quantum theory which also tells us that A and B aren't independent. (Paired results are the macroscopic manifestation of quantum-level disturbances that came from the same emitter via the same emission process. They're thus related by the applicable conservation laws, and, when they're entangled, they're entangled due to the ambiguity of certain intermediate states described by the emission process model.)

My current understanding of, and answer to, your original question is that quantum theory is not inherently non-local.

EDIT: I think that maybe the Bell Locality condition is poorly named. Calling it the Bell Independence condition would be less confusing. The assumption that statistical independence of A and B is required in a local universe is, I think, incorrect.

 

[Sherlock]

How did the entangled particles do it unless they knew something about what the other particle had been "asked".

The entangled particles don't need to know anything about what the other particle had been asked. Their motions just need to be related in some way.

Entanglement implies a relationship between the motions of two particles. Exactly how they're related is unknown. But the assumption of some sort of relationship has a purely local basis, and this assumption is conceptually adequate to understand the predictable results.

The fact that a detailed description of the motions (the sub-microscopic evolutions) of the two particles is impossible according to the principles of quantum theory is why the theory can't be made to be explicitly local. But it certainly isn't explicitly non-local either.

 

[Sherlock]

Well, enlighten us with a measurement theory which bans superluminal signalling consistently.

Does Special Relativity qualify?

 

[vanesch]

(b) Let A be in the past of B and C in the future of B but not in the future of A (A,B and C are domains in spacetime).

I don't understand this: A and B are time-like connected (A is in the past lightcone of B). C and B are time-like connected (C is in the future lightcone of B). How the hell can A and C then not be time-like connected ??
Imagine a material particle traveling from A to B (is possible: timelike), and have it then travel from B to C (is possible: timelike). So overall, a material particle traveled from A to C, no ?

 

[Careful]

I don't understand this: A and B are time-like connected (A is in the past lightcone of B). C and B are time-like connected (C is in the future lightcone of B). How the hell can A and C then not be time-like connected ??
Imagine a material particle traveling from A to B (is possible: timelike), and have it then travel from B to C (is possible: timelike). So overall, a material particle traveled from A to C, no ?

Look Vanesh, A,B and C are spacetime REGIONS, you cannot speak about measurement theory for points since field operators are distributional, you need to smear it out by test functions (independently of this mathematical worry, locality in QFT must obviously also hold for observables living on such extended regions). I said that : A is in the past of B, this does not imply that B is in the future of A (this is however obviously true for points though). Moreover, sorry that I say this, it is a travesty to think that particles are points in QFT. To make everything crystal clear: I am not saying that it is impossible to construct a measurement theory which is consistent (although I am pretty much convinced it is impossible indeed), but it does not exist yet to my knowledge. Therefore, saying that superluminal signalling is excluded is unfounded.

 

[Careful]

Does Special Relativity qualify?

Sorry, but you should better study QFT first before you make such comments... I am trying to raise a serious issue here: that is the lack of a consistent measurement theory for QFT which is compatible with the demands of special relativity.

 

[DrChinese]

I am trying to raise a serious issue here: that is the lack of a consistent measurement theory for QFT which is compatible with the demands of special relativity.

Wow, you should consider starting a thread on this (as opposed to hijacking an existing one).

 

[Sherlock]

Sorry, but you should better study QFT first before you make such comments... I am trying to raise a serious issue here: that is the lack of a consistent measurement theory for QFT which is compatible with the demands of special relativity.

No offense intended. I saw your smily and thought I'd match it.

So there's no mathematically rigorous and consistent measurement theory for QFT in line with one of its principal components, SR -- and the possibility of superluminality in Nature can't be definitively ruled out. Ok.

Nothing said in this thread is ruling out the *possibility* of superluminality in Nature, afaik. However, the consensus seems to be that the considerations that go into considering whether to refer to QM as inherently non-local do not necessitate the assumption that there *is* superluminality in Nature either.

The problem of developing a consistent measurement theory for QFT which is compatible with the demands of special relativity is a problem for another thread. And I promise that I'll just sit back and watch that one.

As for the topic of this thread, I take it that you would consider quantum theory to be inherently non-local. Maybe one might say that it's kinematically, but not dynamically, non-local. But I think that such statements confuse the issue. The bases of quantum theory are local. It neither predicts ftl phenomena, nor does its formalism imply ftl phenomena.
Its principles do prohibit tracking the continuous evolution of quantum phenomena, thereby prohibiting hidden variable theories of the sort that would allow an explicitly local description of the phenomena responsible for the inequality-violating results of Bell tests.

 

[Careful]

Wow, you should consider starting a thread on this (as opposed to hijacking an existing one).

Sorry this issue is relevant ! You cannot claim that QFT forbids signalling faster than with the speed of light when you do not have an appropriate measurement theory. Instead of being so defensive, look at it as a challenge : you should solve the problem, not me, I am convinced it is a waste of time anyway. If, on the other hand, you might surprise me, then I shall praise you.

 

[Careful]

No offense intended. I saw your smily and thought I'd match it.
So there's no mathematically rigorous and consistent measurement theory for QFT in line with one of its principal components, SR -- and the possibility of superluminality in Nature can't be definitively ruled out. Ok.
Nothing said in this thread is ruling out the *possibility* of superluminality in Nature, afaik. However, the consensus seems to be that the considerations that go into considering whether to refer to QM as inherently non-local do not necessitate the assumption that there *is* superluminality in Nature either.
The problem of developing a consistent measurement theory for QFT which is compatible with the demands of special relativity is a problem for another thread. And I promise that I'll just sit back and watch that one.
As for the topic of this thread, I take it that you would consider quantum theory to be inherently non-local. Maybe one might say that it's kinematically, but not dynamically, non-local. But I think that such statements confuse the issue. The bases of quantum theory are local. It neither predicts ftl phenomena, nor does its formalism imply ftl phenomena.
Its principles do prohibit tracking the continuous evolution of quantum phenomena, thereby prohibiting hidden variable theories of the sort that would allow an explicitly local description of the phenomena responsible for the inequality-violating results of Bell tests.

HUH ? You are claiming for some time now that you *cannot* signal faster than with the speed of light and now you say that it does not matter or that it violates it kinematically while a measurement process is clearly dynamical. Do you know the mathematical foundations of logic?

 

[DrChinese]

1. Sorry this issue is relevant !

2. you should solve the problem, not me, I am convinced it is a waste of time anyway.

1. By that standard, we don't need threads at all. Everything in this subforum relates to QFT in SOME way.

2. Why would you ask me or any other person to expend effort for something you consider a waste of time?

 

[Sherlock]

HUH ? You are claiming for some time now that you *cannot* signal faster than with the speed of light and now you say that it does not matter or that it violates it kinematically while a measurement process is clearly dynamical. Do you know the mathematical foundations of logic?

The assumption that Nature obeys the principle of locality hasn't been falsified. Has it? Where did I say that it doesn't matter? The point is that, as far as is known, there are no superluminal phenomena in Nature. That doesn't mean that it's impossible for such phenomena to exist, does it? How would we know for sure?

That quantum theory is kinematically, but not dynamically, non-local is from something I read by H.D. Zeh.

 

[Hurkyl]

Careful:

I understand that the axioms of Algebraic Quantum Field Theory are derivable from doing things the ordinary way, and it's manifestly evident that in AQFT, that any space-like separated operators commute.

 

[Careful]

Careful:
I understand that the axioms of Algebraic Quantum Field Theory are derivable from doing things the ordinary way, and it's manifestly evident that in AQFT, that any space-like separated operators commute.

This is obvious and not the issue (I suspect you have to be careful when you take products of field operators and so on). What I say, is that this is not sufficient (while it is clearly sufficient in the case of two measurements). Check out the Sorkin 1994 paper. impossible measurements on quantum fields. THINK about it before you reply; I notice that Vanesh is thinking (or he is just absent for some reason, just noticed he was thinking).

 

[Careful]

1. By that standard, we don't need threads at all. Everything in this subforum relates to QFT in SOME way.
2. Why would you ask me or any other person to expend effort for something you consider a waste of time?

Because YOU think QFT is a worthwile enterprise while I have a dozen of other reasons to dispose of it. Look, I am not saying that on this forum, research problems should be solved, but at least we should make the effort in trying to ask the right questions. If you take the Wightman axioms as true, then you need to develop a consistent measurement theory. Vanesh, a few mails ago, said that QM poses us with a riddle and started to argue why we should talk about the different options. Hereby, he assumed that it is a FACT that the Wightman axioms imply that no signalling faster than with the speed of light is possible (which is the crucial assumption for what follows in the entire conversation) without caring for an accurate measurement theory. Now, I transported the Copenhagen scheme to QFT and showed that this cannot be right. So you need to do better and I do not believe such effort it is meaningful in the end. If you would tell an engeneer for example that correlations beyond the lightcone exist in your theory, but you cannot measure them ,then he would mock you and say that your measurement apparatus sucks.

 

[Careful]

The assumption that Nature obeys the principle of locality hasn't been falsified. Has it? Where did I say that it doesn't matter? The point is that, as far as is known, there are no superluminal phenomena in Nature. That doesn't mean that it's impossible for such phenomena to exist, does it? How would we know for sure?
That quantum theory is kinematically, but not dynamically, non-local is from something I read by H.D. Zeh.

No, this assumption has not been fasfied by EXPERIMENT (although this is a very delicate issue and another piece of conversation). The question is whether your THEORY allows for processes FTL and that is NOT known for the moment. We never know for sure if processes FTL exist or not, but if this would be the case then we can forget about relativity and go back to eather theories. I must confess I BELIEVE that this cannot be, since it would be impossible for the justice departement to convict anyone of murder (he could argue that the person in question were killed in a tachyon crime commited by a third person in the future of the event itself). You can find this example in John Bell's book: no, the causality axiom is certainly more holy than anything else (this is certainly also the consensus although I do not like to use such arguments). You should not repeat what doctors write in books and realize that in research, there are many conflicting ideas written by equally qualified doctors. THINK for yourself, that is what Sherlock did, he was not a doctor but much sharper than dr. Watson however.

 

[vanesch]

Look Vanesh, A,B and C are spacetime REGIONS,

Ah, so, if you allow me, we can in fact do things with A and C events (or small regions) and B an extended region somewhere in between, such that a part of B is in the future lightcone of A and a disjoint part of B is in the past lightcone of C.
Defining some observable a,b and c on each of these regions, we can then state:
[a,c] = 0
[a,b] is not 0
[b,c] is not 0
This case is in fact handled in "Modern Quantum Mechanics" by JJ Sakurai (p 33): the correlation between a and c is dependent on whether the b measurement is performed or not.
But, but: here our situation is subtly different:
the correlation of a and c IS NOT AVAILABLE to C because C is outside of the future lightcone of A. So what is only available to C is the REDUCED density matrix of the state, tracing out A and B (B also, because the result of B, being a region, is only available to an event which has the ENTIRE B in its past lightcone, let us call this event B', and B' must necessarily be outside the past lightcone of C). This is one of the reasons why it is in fact not necessary to consider extended regions, because their result of measurement can only become available at an event that has the ENTIRE region in its past lightcone (so only at that point one can say one has "performed the measurement" - if one insists on using the von Neumann picture ; me being an MWI-er, I insist on keeping everything unitary!).
Let us apply von Neumann's measurement scheme:
So you seem to claim that performing the measurement at a, or not, when the B measurement is performed, changes the outcomes of C ?
Let us take an initial state |psi> which is u|a+> + v|a->, |a+> and |a-> being the two eigenstates of A (and also of C, since they commute).
Now, if we perform the measurement at A, we have, with probability u^2, |a+> and with probability v^2, |a->
Now, if we perform the B measurement in the first case, we get, with probability
u^2 |(b+|a+)|^2 + v^2 |(b+|a-)|^2 the state |b+>
with probability u^2 |(b-|a+)|^2 + v^2 |(b-|a-)|^2 the state |b->
When C now performs its measurement (which is the same as A), we obtain:
with probability P_c(a+) =
(u^2 |(b+|a+)|^2 + v^2 |(b+|a-)|^2) |(a+|b+)|^2
+
(u^2 |(b-|a+)|^2 + v^2 |(b-|a-)|^2 ) |(a+|b-)|^2
the state |a+> (that will do, a- will be complementary).
On the other hand, if A does NOT perform his measurement, we have, for the B measurement:
|u(b+|a+) + v(b+|a-)|^2 probability to have b+ and
|u(b-|a+) + v(b-|a-)|^2 probability to have b-.
After C performs then his measurement, we have the probability at C to measure a+:
|u(b+|a+) + v(b+|a-)|^2 |(a+|b+)|^2 + |u(b-|a+) + v(b-|a-)|^2 |(a+|b-)|^2
The difference between both approaches (with A measurement and without A measurement) is then (we take u and v real):
Diff = u v ( (b+|a+) (a-|b+) + (a+|b+) (b+|a-) ) |(a+|b+)|^2
+ u v ( (b-|a+) (a-|b-) + (a+|b-)(b-|a-)) |(a+|b-)|^2
Writing this with U the unitary transformation matrix between the a set and the b set, we rewrite this as:
Diff = u v (U11 U11* U11 U21* + U12* U22 U12 U12* + CC)
If a is not to signal to c, this difference should vanish. Now, let us take that the basis transformation between the a set and the b set is unitary and unimodular (choice of overall phase):
U11 = x
U22 = x*
U12 = y
U21 = - y*
Now, after working this out I obtain:
Diff = u v (-x* x + y* y) (x y + x* y*)
which is, to my great surprise, not zero. I suspect I made an error somewhere...

 

[vanesch]

Now, after working this out I obtain:
Diff = u v (-x* x + y* y) (x y + x* y*)
which is, to my great surprise, not zero. I suspect I made an error somewhere...

I checked my calculation and I don't seem to find an error.
If this is true, this is amazing:
We have an initial state |psi> to which a can, or can not, apply a measurement (decision of a).
b applies always his/her measurement.
c applies the measurement (which is the same, or compatible, with the one done by a) and looks at the probability to get a certain result.
This probability (of c) seems to depend on whether a decided to measure or not (and NOT on the outcome of a), although a and c are spacelike connected points, which would mean that there is a FTL phone from a to c (a can decide, or not, to measure A, and c sees his probabilities change).
I admit being puzzled. There must be some quirk I didn't get.
I suppose that the trick is the spread of the B measurement, which can only be completed at an event which has the entire B section in its past lightcone (probably von Neumann's projection should only apply at that moment - at least, only at that moment I could entangle, in an MWI view, a local observer with the system according to the B measurement), and that this B measurement then doesn't occur BEFORE C.
But I admit, again, to be puzzled !
cheers,
Patrick.

 

[Tez]

But they don't really! (Obviously, otherwise we could use that for FTL signalling.)
All we know is that our captors communicated in our past light cone and we are using that locally transmitted knowledge to play a logic game. There is no superluminal anything over and above a normal interpretation of a Bell test. After all, our captors can't release us until they compare our answers.

I simply don't understand what you mean. Are you intimating that our captors cannot make a local, independent random choice as to which question to ask, and that therefore the particles can know before they're separated which question/measurement is going to come up? THis is the standard "no free will" loophole to Bell tests. And what is the "normal interpretation of a Bell test"?.

In case it wasn't clear, what I described is not an allegory - the game could be played by real prisoners and captors, and presuming the prisonors can carry concealed entangled particles and stern gerlach appartuses(!) their probability of being released goes up to 85%. And no, it doesn't allow for superluminal communication between the two prisoners, but it certainly would seem to require superluminal communication between the particles in order to achieve.

If you really don't see an issue with this, then perhaps you can outline how your understanding could help one tackle the question of why the probability of being released doesn't go up to 100%?

 

[Careful]

Ah, so, if you allow me, we can in fact do things with A and C events (or small regions) and B an extended region somewhere in between, such that a part of B is in the future lightcone of A and a disjoint part of B is in the past lightcone of C.
Defining some observable a,b and c on each of these regions, we can then state:
[a,c] = 0
[a,b] is not 0
[b,c] is not 0
This case is in fact handled in "Modern Quantum Mechanics" by JJ Sakurai (p 33): the correlation between a and c is dependent on whether the b measurement is performed or not.
But, but: here our situation is subtly different:
the correlation of a and c IS NOT AVAILABLE to C because C is outside of the future lightcone of A. So what is only available to C is the REDUCED density matrix of the state, tracing out A and B (B also, because the result of B, being a region, is only available to an event which has the ENTIRE B in its past lightcone, let us call this event B', and B' must necessarily be outside the past lightcone of C). This is one of the reasons why it is in fact not necessary to consider extended regions, because their result of measurement can only become available at an event that has the ENTIRE region in its past lightcone (so only at that point one can say one has "performed the measurement" - if one insists on using the von Neumann picture ; me being an MWI-er, I insist on keeping everything unitary!).
Let us apply von Neumann's measurement scheme:
So you seem to claim that performing the measurement at a, or not, when the B measurement is performed, changes the outcomes of C ?
Let us take an initial state |psi> which is u|a+> + v|a->, |a+> and |a-> being the two eigenstates of A (and also of C, since they commute).
Now, if we perform the measurement at A, we have, with probability u^2, |a+> and with probability v^2, |a->
Now, if we perform the B measurement in the first case, we get, with probability
u^2 |(b+|a+)|^2 + v^2 |(b+|a-)|^2 the state |b+>
with probability u^2 |(b-|a+)|^2 + v^2 |(b-|a-)|^2 the state |b->
..

You say : B also, because the result of B, being a region, is only available to an event which has the ENTIRE B in its past lightcone, let us call this event B', and B' must necessarily be outside the past lightcone of C.

If you mean by this that B can only effect C if the entire B is in the past of C, then this is utter nonsense. This is not even true classically (sorry but you are cryptic here).

Ah I see that you have posted again.. I was redoing your entire calculation :-) I was pretty confident you did it good since I have redone the sorkin calculations a few years ago and it came out right (moreover you are more ``quantal´´ in the computational sense than I am, I stopped doing this as soon as I realized a few things).

There is nothing mysterious about it however: as I said before the catch is that B is a non local operation in the sense that measurement of A is instantaneously coupled to something which is outside its lightcone (this is what B does). There is nothing wrong with the measurement setup I gave, you might indeed argue that you need to look for a better measurement theory (actually, that is your only way out). Now that you got this insight, you might start wondering WHY I say that it is probably impossible to make a realistic measurement theory which avoids this issue.

Cheers,

Careful

 

[vanesch]

You say : B also, because the result of B, being a region, is only available to an event which has the ENTIRE B in its past lightcone, let us call this event B', and B' must necessarily be outside the past lightcone of C.
If you mean by this that B can only effect C if the entire B is in the past of C, then this is utter nonsense. This is not even true classically (sorry but you are cryptic here).

Well, I'm an MWI-er, so I consider a "measurement" simply as a local entanglement of the observer body with the state, without projecting it. What I meant was that the measurement of B, over the entire region, can only be completed when this entire region is in the past lightcone of the observer who is going to observe this. So the observer doing this "B" measurement can only be completely entangled in this basis when he has the entire B region in the past. That doesn't mean that some unitary evolution cannot be initiated, but as everything here is unitary, I can clearly state that the lightcone will be respected in this way, and that whatever A gets entangled with at A will not influence what so ever at C.

So what I meant was that in the case that you want to apply a projection postulate a la von Neumann, you have in any case a difficult time, because you have to, somehow, take into account the partial unitary evolution during the B region itself, but you cannot have the entire result until all of this evolution was communicated to a (point-like) observer in some way or another, at which moment some magical "collapse" occurs (along a time slice in that pointlike observer's ref frame, I presume). So *IF* you want to do collapse stuff, you should only do it at that event ; but then the measurement at C already took place. It is this magic which makes me prefer the MWI view, BTW.

If I find some time I'll work out the problem from an MWI point of view...

cheers,
Patrick.

 

[Careful]

I simply don't understand what you mean. Are you intimating that our captors cannot make a local, independent random choice as to which question to ask, and that therefore the particles can know before they're separated which question/measurement is going to come up? THis is the standard "no free will" loophole to Bell tests. And what is the "normal interpretation of a Bell test"?.
In case it wasn't clear, what I described is not an allegory - the game could be played by real prisoners and captors, and presuming the prisonors can carry concealed entangled particles and stern gerlach appartuses(!) their probability of being released goes up to 85%. And no, it doesn't allow for superluminal communication between the two prisoners, but it certainly would seem to require superluminal communication between the particles in order to achieve.
If you really don't see an issue with this, then perhaps you can outline how your understanding could help one tackle the question of why the probability of being released doesn't go up to 100%?

Right, you are adressing the good question in my view. What physical mechanism can provide these correlations ?? They are all perverted. Quantum physicists try then to hide behind the no FTL signalling theorem, but as is clear from previous communications, this is by far not good enough! Moreover, QM does not offer any insight into the detailed dynamics of microworld, and this my greatest worry. My healthy peasant brain tells me that excluding faster than light communication is in fact not possible (but that is speculation) in any *natural* quantum theory; it is up to QFT theorists to prove me wrong.

 

[Careful]

Well, I'm an MWI-er, so I consider a "measurement" simply as a local entanglement of the observer body with the state, without projecting it. What I meant was that the measurement of B, over the entire region, can only be completed when this entire region is in the past lightcone of the observer who is going to observe this. So the observer doing this "B" measurement can only be completely entangled in this basis when he has the entire B region in the past. That doesn't mean that some unitary evolution cannot be initiated, but as everything here is unitary, I can clearly state that the lightcone will be respected in this way, and that whatever A gets entangled with at A will not influence what so ever at C.
So what I meant was that in the case that you want to apply a projection postulate a la von Neumann, you have in any case a difficult time, because you have to, somehow, take into account the partial unitary evolution during the B region itself, but you cannot have the entire result until all of this evolution was communicated to a (point-like) observer in some way or another, at which moment some magical "collapse" occurs (along a time slice in that pointlike observer's ref frame, I presume). So *IF* you want to do collapse stuff, you should only do it at that event ; but then the measurement at C already took place. It is this magic which makes me prefer the MWI view, BTW.
If I find some time I'll work out the problem from an MWI point of view...
cheers,
Patrick.

I do not know what MWI is (But I am a classical relativist and there we do not have interpretational clans since everything is crystal clear), but here are some objections to what you say:
(a) make your measurement procedure exact: you will have to apply a non local avaraging procedure as well in time as in space in order to interpret the result of this entanglement in a classical way.
(b) you say that it is only possible for a magical collapse to happen once an observer can have acces to the entire information of B. Now, this collapse is a non local procedure and happens on an entire spacelike hypersurface X containing this point like observer. It is no problem to put C to the future of this X unless X stays in the future lightcone of A which brings along other problems (so your claim is false there). Since this has to hold for any A your collapse has to happen on a null surface (and not even a differentiable one)!
(c) the only reasonable way to save your butt is by coupling realistic detector models to A,B and C and making a measurement theory for those. However, the dynamics to the quantum field under observation is not unitary anymore then (the total dynamics is of course) and the measurement theory itself is an entirely different issue. I presume you are just shifting the problem at this instant.

 

[vanesch]

I do not know what MWI is

MWI = Many Worlds Interpretation, a fancy word for assuming that the observer is just as well suffering quantum evolution as everything else, so that an observation does not give rise to a projection, but that everything (including observation) is simply one big unitary evolution.
initial state:
A0, B0 and C0 are the states of the observers before they got involved in the measurement (before they underwent an evolution that entangled them with our system).
|A0)|B0)|C0) (u |a+) + v |a-) )
A "measures":
|B0)|C0) (u |A+)|a+) + v |A-) |a-) )
A+ is the body state of observer A where he saw a + result, and A- is the body state of the observer A where he saw a - result.
B "measures":
B+ is the state of the body of observer B when he's informed about the entire result of the B-measurement (so here we see - see further - than in order to be so informed, B actually has to have the entire B region in his past lightcone, but we're now pretending that this must not be the case).
|C0) u |A+) (|B+) (b+|a+) |b+) + |B-) (b-|a+) |b-))
+ |C0) v |A-)(|B+) (b+|a-) |b+) + |B-) (b-|a-) |b-))
C "measures":
u |A+) (|B+) (b+|a+) (|C+) (a+|b+)|a+) + |C-) (a-|b+)|a-))
+ |B-) (b-|a+) (|C+)(a+|b-)|a+) + |C-)(a-|b-)|a-)) )
+ v |A-)(|B+) (b+|a-) (|C+) (a+|b+)|a+) + |C-) (a-|b+)|a-))
+ |B-) (b-|a-) (|C+)(a+|b-)|a+) + |C-)(a-|b-)|a-) ) )
= |C+) {u |A+) (|B+) (b+|a+) (a+|b+)
+ |B-) (b-|a+) (a+|b-) )
+ v |A-) (|B+) (b+|a-) (a+|b+)
+ |B-) (b-|a-) (a+|b-) )}|a+)
+ |C-) {u |A+) (|B+) (b+|a+) (a-|b+)
+ |B-) (b-|a+) (a-|b-) )
+ v |A-)(|B+) (b+|a-) (a-|b+)
+ |B-) (b-|a-) (a-|b-) )} |a-)
The probability to get C+ is then the total length of the state vector which has the C+ body state as a factor:
|u|^2 (|U11|^4 + |U12|^4) + |v|^2 (U11.U21.U11*.U21*+U22.U12*.U22*.U12)
which is the same result as our first calculation.
Note that a priori we're in the same deep s**t, because if we don't let
A measure, then |A0) stays factored out, and the |A+) and |A-) terms are not
orthogonal anymore, but just add as amplitudes:
B "measures":
|C0) u |A0) (|B+) (b+|a+) |b+) + |B-) (b-|a+) |b-))
+ |C0) v |A0)(|B+) (b+|a-) |b+) + |B-) (b-|a-) |b-))
C "measures":
|C+) |A0) {u (|B+) (b+|a+) (a+|b+)
+ |B-) (b-|a+) (a+|b-) )
+ v (|B+) (b+|a-) (a+|b+)
+ |B-) (b-|a-) (a+|b-) )}|a+)
+ |C-) |A0) {u (|B+) (b+|a+) (a-|b+)
+ (b-|a+) (a-|b-) )
+ v |A-)(|B+) (b+|a-) (a-|b+)
+ |B-) (b-|a-) (a-|b-) )} |a-)
which will us probably give the same result as using projection.
But now we understand why ! The so-called B measurement cannot have taken place completely when C measures, so the B interaction (unitary) has to be split in 2 parts:
the one in the future lightcone of A (BL), and the one in the past lightcone of C (BR). Both interactions (unitary evolutions) BL and BR commute, and BL commutes with C, while BR commutes with A. BL does not commute with A and BR does not commute with C however.
Clearly, my 2-state example is not sufficient in this case to implement these operators, so I give up here for the moment, but I think that this will solve the issue.
In a way, you can say that (typical of the MWI approach) this splitting in BL and BR is part of what you require "detailling the detection procedure".
As far as I can tell, because the only evolution that could possibly influence C (as unitary evolution, using Green's functions all the way within the detector, brain, whatever), is BR, and whatever happens to BL and A should normally factor out, hence not influencing the entanglement of C with the state.
But I should work it out, and I think it's going to take more work and time than I have.
Nevertheless, interesting problem !

 

[Careful]

MWI = Many Worlds Interpretation, a fancy word for assuming that the observer is just as well suffering quantum evolution as everything else, so that an observation does not give rise to a projection, but that everything (including observation) is simply one big unitary evolution.
initial state:

I thought you quantum physicists called this environmental decoherence (I made the MWI guess myself but I was confused over what you meant by it) I thought MWI was just a particular way of envisaging the Schroedinger equation in the path integral framework, but ok now we speak the same language...

 

[Careful]

But now we understand why ! The so-called B measurement cannot have taken place completely when C measures, so the B interaction (unitary) has to be split in 2 parts:
the one in the future lightcone of A (BL), and the one in the past lightcone of C (BR). Both interactions (unitary evolutions) BL and BR commute, and BL commutes with C, while BR commutes with A. BL does not commute with A and BR does not commute with C however.
Clearly, my 2-state example is not sufficient in this case to implement these operators, so I give up here for the moment, but I think that this will solve the issue.

The B measurement can have taken place before C happens (see my comments about state reduction). Your entanglement to an observer is really not going to solve anything, it is just going to make the notation more heavy. B is a measurement which *cannot* be split into two parts by definition since is measures a non local property.

 

[vanesch]

(a) make your measurement procedure exact: you will have to apply a non local avaraging procedure as well in time as in space in order to interpret the result of this entanglement in a classical way.

But that's exactly what you DON'T want to do in MWI: you only consider a (pointlike) observer, which gets LOCALLY entangled (that means, whose state can only suffer a unitary evolution involving whatever is local at the spot of the observer).
If you want to do this "averaging" you should in fact construct several local observers at the different locations of the B region, which you make then travel (at less than lightspeed) towards the final B observer, and make them interact with this final B observer when they get there. It is only when that final B observer has encountered locally all of his "messengers" that he is finally entangled with the "B measurement" which is a very coarse-grained operation.
So I made one extra step, and considered "messengers from the BL and the BR" region, BL being in the future lightcone of A, and BR being in the past lightcone of C, both regions being disjunct.

(b) you say that it is only possible for a magical collapse to happen once an observer can have acces to the entire information of B. Now, this collapse is a non local procedure and happens on an entire spacelike hypersurface X containing this point like observer. It is no problem to put C to the future of this X unless X stays in the future lightcone of A which brings along other problems (so your claim is false there). Since this has to hold for any A your collapse has to happen on a null surface (and not even a differentiable one)!

Yes, that's why I consider collapse bull**** Except that it is damn practical to do calculations and that it comes out all the same as the MWI approach

(c) the only reasonable way to save your butt is by coupling realistic detector models to A,B and C and making a measurement theory for those. However, the dynamics to the quantum field under observation is not unitary anymore

Oh but of course it is ! That's the entire issue of MWI: stay unitary until you die (and beyond ) Once you accept that ALL is unitary evolution, maybe the respect of the "lightcone" will occur to you.
It is exactly what I try to argue with EPR situations: if you treat it the MWI way, you can stay local and nevertheless obtain the EPR correlations ; only, you can only observe them when BOTH Alice and Bob are in the past lightcone of this famous "correlation observer" (because Bob will have to travel to Alice, and Bob is in two states !) MWI is (according to me) the only way to reconcile relativity with QM.
In MWI, it is not you who collapses the state of the world, it is the world who entangles your body (and you only consciously experience one of those states, according to the Born rule) with the state of the world.
Now, I could argue of course endlessly over this, but I challenge you (for a change): describe me a way, in principle to DO this extended B measurement, so that we can turn it into a real FTL phone.
You can use screens, detectors, whatever. A plane wave (photon) is coming in, and A is going to decide to do, or not do, a measurement, while I'm C, doing a measurement, and you have to make me (at C) find out the result of your decision at A to do, or not, your measurement.

 

[vanesch]

B is a measurement which *cannot* be split into two parts by definition since is measures a non local property.

Ok, but then the exact unitary dynamics of that "measurement" will involve non-local hamiltonians and it will not happen using electroweak or strong interactions.

If you have such a physical process which can do something non-locally, you ALREADY screwed up relativity, and you've a preferred foliation of spacetime. The very definition of your measurement interaction did this. But, as I said, you're not going to be able to construct such a measurement apparatus whose function is based upon electroweak or strong interaction.

 

[vanesch]

I thought you quantum physicists called this environmental decoherence

There are subtle differences between the two concepts, but it is true that environmental decoherence does not make much sense if you do not adhere to an MWI-like view, because both are based upon the same idea: that what's called "measurement" is nothing special, and involves just unitary interaction (using hamiltonians in the usual way). This is in fact even present in the von Neumann view, and he calls this unitary interaction the "pre-measurement interaction". Only, von Neumann states that *at a certain point* (between the system and conscious observation) we have to make a break, and apply the projection postulate. Using the results of decoherence, one can then show that this comes down (FAPP = for all practical purposes) to just applying the projection postulate already on the system level - as it is taught in elementary textbooks.

MWI takes this one single step further, and allows everything (even your body, your brain and all that) to take part in the pre-measurement interaction, WITHOUT collapse. Problem is then of course that we've lost the Born rule. People have been struggling with that, I just assume (as others did) that we can just state that we consciously observe only one branch and that the probability of observing this is given by the Born rule.
There's not much difference between the von Neumann view and this view, in fact (FAPP, the calculations give the same results). It is only on the conceptual level that MWI is the only way to AVOID entirely this collapse, which is indeed highly non-local, badly defined (when exactly does it happen, and in what spacelike foliation?) and at least weird in that my brain can change the state of the universe somehow.Environmental decoherence comes of course to its "full glory" within such a view when there's no collapse... and also looses a part of its meaning: because environmental decoherence tries to explain the Born rule, by using the Born rule on a higher level of complexity. So contrary to what is sometimes claimed, environmental decoherence does not EXPLAIN the appearance of the Born rule in MWI. It just transports it from a high level of complexity down to the system level (as such, justifying the elementary textbook procedures). But the Born rule still has to come from some place. In von Neumann, that's clear. In MWI, you have to do it with what you call consciousness bull****

 

[Careful]

I can appreciate your comment about *collapse bull ***** since it leads to even further problems than I have mentioned. However, your interaction picture is unphysical (measurement does not happen at a spacetime point, it does need a spacetime region, an apparatus registers only when there is peak which goes over a certain threshold). If you want to interpret your measurement results, you have to trace out the degrees of freedom of the field under observation, the resulting dynamics of the traced out density matrix is not unitary although the total dynamics (field + observer) is. Similarly the dynamics of the field under observation is not unitary when you trace out the degrees of freedom of the detector. Therefore, neither of both fields have to statisfy causality constraints if you want to hint that unitarity implies causality. Moreover, there is no theorem which says that unitarity implies causality and vice versa (otherwise the wightman axioms would be abundant). The most you can argue is that unitarity is less troublesome than reduction postulates, but then again you have other problems. So unless you come up with a theorem, your argument is empty.

 

[ttn]

The probability to get C+ is then the total length of the state vector which has the C+ body state as a factor:

The *probability*??! To **get**?!

Maybe you should elaborate (for those who don't know it already) *your* "measurement" axioms which give these concepts meaning in (your version of) MWI.

 

[Careful]

Environmental decoherence comes of course to its "full glory" within such a view when there's no collapse... and also looses a part of its meaning: because environmental decoherence tries to explain the Born rule, by using the Born rule on a higher level of complexity. So contrary to what is sometimes claimed, environmental decoherence does not EXPLAIN the appearance of the Born rule in MWI. It just transports it from a high level of complexity down to the system level (as such, justifying the elementary textbook procedures). But the Born rule still has to come from some place. In von Neumann, that's clear. In MWI, you have to do it with what you call consciousness bull****

Ok, so that kills it off... You know what I hate the most about this kind of arguments, is that you always leave something unexplained (something weird, magical has to be there). The next step you have to take is to explain conciousness by a physical theory which uses consciousness as a fixed, postulated, concept. So, actually, you are not solving anything, you are just pushing a perverse scheme a step further. I would like to know from you where your consciousness was in the beginning of the universe, since clearly something must have reduced the state there (the universe is entirely classical...). Moreover, your consciousness does not solve many problems : I do not see for example how you would get out the second law of thermodynamics (this is much nastier at the quantum level than the classical one). If you like Penrose in that respect, then you must realize that the scheme he has for quantum gravity is not covariant ...

I also think that gravity is playing an important part in quantum mechanics, but then CLASSICAL gravity not some undefined dream as QUANTUM gravity.

 

[vanesch]

The *probability*??! To **get**?!

"The probability for the conscious observer to be associated with the body state who saw C+."