Relativity & Quantum Theory: Is Locality Violated?

[ttn]

The interesting thing is that MWI and Bohmian Mechanics BOTH hypothesize the existence of forces/wave/worlds which cannot, in principle, be observed directly. So why would one be "more plausible" than the other? Or more palatable? I think it simply comes back to personal preference, not logic.

There is a huge difference. In Bohm's theory, there is a very physical/tangible difference between branches of the wave function -- namely, most of them are "empty" while one is "full". That is to say, there's one branch that *in fact has the particles in it*. There's an *actual* configuration for all the particles. So there is no weird mystery about (say) Schroedinger's cat -- assuming that what you refer to with the word "cat" is all of the particles the cat is built from, then the cat really is definitely alive or dead, period. No ambiguity, no parallel universes, etc.

 

[ttn]

There is no dependency on the outcome at Alice (+ or -) based on a setting at Bob (measured in degrees). I.e. if you vary Bob's setting, you do not see any change in the +/- pattern at Alice. We have been over this a hundred times.

Yet you still remain confused about what's at issue. When you say "there is no dependency", what do you mean *exactly*? According to some particular theory, Alice's outcome "doesn't depend" on Bob's setting? If so, what theory? Or do you mean that the observed relative frequency of +/- is (empirically) independent of Bob's setting? These are very different claims, and it's not even clear how *either* is relevant to what we're talking about here.

Here's what is actually true: no theory respecting Bell's "local causality" condition (for which I can give the equations if anyone cares) can be in agreement with the observed correlations. So the results on one side *do* depend on the results on the other -- even if the dependence is "washed out" in the noise in a way that prevents *signalling*.

The only time a pattern emerges is when you correlate Alice's setting, Bob's setting, Alice's outcome, and Bob's outcome (4 observables). And the resulting pattern follows the predictions of standard QM, which respects special relativity.

So the collapse postulate is consistent with special relativity? What *exactly* do you mean when you say that "standard QM" "respects special relativity." Do you mean that standard QM is "signal local"? That it is "Bell Local"? Something else? It's vagueness over questions like this that have led to such decades-long confusion about what Bell proved. So do your part to achieve clarity by being precise!

And my description IS a generally accepted description of the physics.

So much the worse for "the physicists" then, since your description (so far) is very vague... But I hope you will fix this!

That said, none of this comprises an absolute disproof of non-local effects. Bell's Theorem allows a degree of latitude.

What does that mean? Maybe both sides are right? I don't think there's any "latitude". There's controversy, yes, but that just means somebody's confused. The claim I'm making (no bell local theory can agree with experiment, hence nature violates bell locality) is perfectly definite and is either true or false. I'm either right or wrong. No latitude.

Unless MWI is true, then maybe in one branch of the wf I'm right, and in another I'm wrong...

 

[LnGrrrR]

DrC,

Perhaps it is a preference thing. I personally find the idea of universes we can never know about distasteful. :)

 

[ttn]

The interesting thing is that MWI and Bohmian Mechanics BOTH hypothesize the existence of forces/wave/worlds which cannot, in principle, be observed directly. So why would one be "more plausible" than the other? Or more palatable? I think it simply comes back to personal preference, not logic.

After responding to this above, I realized I might have misunderstood you. I thought you were referring to the fact that, in Bohm's theory, the same "total wave function" exists that MWI says exists. So I pointed out that there's a huge difference, that the *actual particle configuration* in Bohm's theory picks out one branch of the wave function as dynamically special (in that *only* it affects the subsequent time evolution of the configuration).

But maybe you meant something more general -- namely, that Bohm's theory has "hidden variables" which you seem to think "cannot, in principle, be observed directly." If you remember that the "hidden variables" in Bohm's theory are *particle positions*, you will realize this is a silly claim. The weird, spooky, mysterious, dubiously-tangible, not-directly-observable entity in Bohmian Mechanics isn't the particle positions -- it's the wave function. But surely nobody can criticize Bohmian Mechanics on *that* point, unless they're able to put forward some version of quantum theory which doesn't have wave functions in it! My point: you're treading on very thin ice if, as an advocate of "conventional QM", you criticize Bohm's theory for containing "unobservables." The only unobservables in Bohm's theory are shared with conventional QM. And the *observables* in Bohm's theory (the particle positions) are precisely what allows that theory to solve the various ambiguities that plague the conventional theory (specifically, the measurement problem).

 

[UglyDuckling]

DrC,

Perhaps it is a preference thing. I personally find the idea of universes we can never know about distasteful. :)

I agree! The universe ought to be simple and elegant not distasteful.

Personally I believe the ideas of non-locality, MWI, Bohmian mechanics, entangles particles are all spawned from a near universal misunderstanding of the special theory of relativity.

The problems of quantum weirdness, particle entanglement, HUV and the apparent instantaneous collapse of the wave function should be exhaustively addressed within the framework of relativity before we allow ourselves the indulgency of these exotic fantasies.

 

[UglyDuckling]

Let me make sure I understand. Bell's Theorem proves that the outcomes in one wing of the apparatus must depend on the setting of the polarizer in the other/distant wing of the apparatus. Your point is that this dependence could be "mediated" by the particle source at the center -- i.e., the joint state of the particle pair could be affected in some way by the settings of (say) both polarizers, so that the particle on one side "knows about" the distant setting. Is that the idea? Presumably the information about the settings would travel ("backwards", from detectors to source) at the speed of light or slower. So then this would be a local (i.e., relativistically causal) mechanism by which the correlations could be explained.

Have I got that basically right?

You start off OK but then lose the plot about half way through when you start talking about information traveling backwards in time. Special relativity does not require a signal to go backwards in time. This is merely an illusion caused by the way we have to measure and represent distance and time.

The problem is this: In this mechanism, the distant setting that a given particle "knows about" isn't (necessarily) the *current* setting of that distant polarizer; it's the setting of that distant polarizer a time 2L/c ago (where L is the source-polarizer distance on each side). So, if the orientations of the polarizers were to be randomly set while the particles are in flight (i.e., after the particle pairs have been emitted) then the particles on each side will have "bad info" some of the time, and the QM correlations won't be able to be reproduced. This "loophole" (called sometimes the "locality loophole" in the literature) is well-known. It is because of it that the "delayed choice" experiments (in which the orientations are randomly flipped around while the particles are in flight) were crucial.

Again you misinterpreting what is happening in space-time. Events that are fixed in space and time have to be reinterpreted for space-time.

For Aspect's Experiment; in space-time the cascade event and detection events are contiguous and any communication between the events must be instantaneous. Therefore, again your argument about "bad info" because of the delay in the settings is incorrect. The apparent 2L/c delay is due to the way we measure and represent events in space and time.

Since when has quantum mechanics said anything about particles in flight?

Talk of altering the settings while the "particles are in flight" has no validity.

It's already been eliminated.

As your argument was so full of errors I think the validity of this statement is very doubtful.

The possibility of relativity acting as the agent of entanglement cannot be so easily dismissed.

One could of course still "modify" the definition of locality to still be able to say that nature is local. But that's just playing with words. What's important is what's actually established, not what you call it. And what's established is that there exist superluminal causal influences. And this makes relativity unhappy.

You cannot say that “superluminal causal influences exist”; especially on the strength the argument above!

Relativity is quite capable of encompassing the violation of Bell’s inequality.

In fact the violation of Bell’s inequality doesn’t make the universe non-local but makes locality more interesting and will eventually elevate the status special relativity when its full implications in explaining what’s going on in the quantum world are fully understood

 

[ttn]

You start off OK but then lose the plot about half way through when you start talking about information traveling backwards in time. Special relativity does not require a signal to go backwards in time. This is merely an illusion caused by the way we have to measure and represent distance and time.

I said "backwards" -- not "backwards in time." What I meant is "backwards" relative to the direction the particles go; the information in question flies from the polarizers to the particle source, not (like the particles) from the source to the polarizers, right? That was your idea.

Again you misinterpreting what is happening in space-time. Events that are fixed in space and time have to be reinterpreted for space-time.

For Aspect's Experiment; in space-time the cascade event and detection events are contiguous and any communication between the events must be instantaneous. Therefore, again your argument about "bad info" because of the delay in the settings is incorrect. The apparent 2L/c delay is due to the way we measure and represent events in space and time.

Since when has quantum mechanics said anything about particles in flight?

Talk of altering the settings while the "particles are in flight" has no validity.

Huh? Are you denying that it takes some time for the particles to get from the particle source to the polarizers (which are, at Innsbruck say, several kilometers away)?

In fact the violation of Bell’s inequality doesn’t make the universe non-local but makes locality more interesting and will eventually elevate the status special relativity when its full implications in explaining what’s going on in the quantum world are fully understood

Well, I don't agree. But I anxiously await your producing an example of a Bell Local theory (i.e., a relativistically-locally causal theory) that agrees with experiment.

 

[UglyDuckling]

I said "backwards" -- not "backwards in time." What I meant is "backwards" relative to the direction the particles go; the information in question flies from the polarizers to the particle source, not (like the particles) from the source to the polarizers, right? That was your idea..

The information dosen't have to fly anywhere. Three events are contiguous and therefore can/will share information about their states. Of course nature does not permit us to see what goes on at quantum level. We must rely on our ability to measure distance and time with rules and clocks and then crudely represent the locations of the events on inertially referenced space-time diagrams. Which gives us a very different view of the world to what quantum objects experience. Fortuneately special relativity enables us to piece together the relationship between the events and helps us understand why sets of spatially separated results can be correlated.

Huh? Are you denying that it takes some time for the particles to get from the particle source to the polarizers (which are, at Innsbruck say, several kilometers away)?

Yes I am denying, it takes some time for the particles to get from the source to the polarisers. Special relativity shows the source and the polarisers at three events to be superpositioned and therefore can freely interact. The perceived timing of the events is a function of the way we measure and represent their locations.

Well, I don't agree. But I anxiously await your producing an example of a Bell Local theory (i.e., a relativistically-locally causal theory) that agrees with experiment.

I don't have to. It already exists. Its called special relativity.

You just have to make the effort to understand it properly

 

[vanesch]

I personally find the idea of universes we can never know about distasteful. :)

I always had difficulties with derivatives... I really don't like them

I know. It is the eternal, and in fact the ONLY objection to the "straightforward" interpretation of QM: "Naaah, too crazy !"

History is full of "naah, too crazy" statements: men evolved from apes ? Naah, too crazy ! The Earth is round ? Naah, too crazy ! The Earth turns around the sun ? Naah, too crazy ! Spacetime is curved ? Naah, too crazy ! The universe once was a hot place that is expanding ? Naaah, too crazy ! Long ago there were big beasts running over the Earth's surface, which looked like dragons ? Naah, too crazy ! ...

My PoV is that we should learn some modesty concerning our intuitive concepts, which induced us all too often in saying "naah, too crazy", and instead learn to take at face value what the formalism of science tells us. Mind you, this is no excuse to expressly postulate crazy things, because crazy things must be true !
But, if you find a formalism which makes correct predictions (quantum formalism) ; principles which seem to hold (like Lorentz invariance) and you can build a system in which all these requirements are met, but the only objection you have is "naah, too crazy", well,... you should think twice.

 

[ttn]

The information dosen't have to fly anywhere. Three events are contiguous and therefore can/will share information about their states.

I don't think you know what "contiguous" means!

Yes I am denying, it takes some time for the particles to get from the source to the polarisers. Special relativity shows the source and the polarisers at three events to be superpositioned and therefore can freely interact. The perceived timing of the events is a function of the way we measure and represent their locations.

All right, now it's just painfully clear you don't know what you're talking about.

Thanks for the discussion.

 

[LnGrrrR]

I always had difficulties with derivatives... I really don't like them

I know. It is the eternal, and in fact the ONLY objection to the "straightforward" interpretation of QM: "Naaah, too crazy !"

History is full of "naah, too crazy" statements: men evolved from apes ? Naah, too crazy ! The Earth is round ? Naah, too crazy ! The Earth turns around the sun ? Naah, too crazy ! Spacetime is curved ? Naah, too crazy ! The universe once was a hot place that is expanding ? Naaah, too crazy ! Long ago there were big beasts running over the Earth's surface, which looked like dragons ? Naah, too crazy ! ...

My PoV is that we should learn some modesty concerning our intuitive concepts, which induced us all too often in saying "naah, too crazy", and instead learn to take at face value what the formalism of science tells us. Mind you, this is no excuse to expressly postulate crazy things, because crazy things must be true !
But, if you find a formalism which makes correct predictions (quantum formalism) ; principles which seem to hold (like Lorentz invariance) and you can build a system in which all these requirements are met, but the only objection you have is "naah, too crazy", well,... you should think twice.

Oh yeah, I'm not saying MWI can't be correct. I just find it distasteful.

The thing is, to me, that MWI is not very useful unless you can provide evidence for it. Now, I know that MWI does fix some 'problems' in QM, but I think we should give the other methods some more time to try to work on it before we have to relegate ourselves to believing in another realities that can not seen to be (as of now) empirically proven.

I can see the appeal of MWI (after reading up on it), but for me, it doesn't seem to be the definition that seems 'right'.

 

[UglyDuckling]

I don't think you know what "contiguous" means!

It means touching or next to. As far relativity is concerned it means the proper interval between events has zero magnitude.





All right, now it's just painfully clear you don't know what you're talking about.

Thanks for the discussion.

OK if I don't Know what I'm talking about! Explain to me why spatially separated quantum objects should not become super-positioned at locations on their world lines where their "proper" separation has zero magnitude.

 

[DrChinese]

Yet you still remain confused about what's at issue. When you say "there is no dependency", what do you mean *exactly*? According to some particular theory, Alice's outcome "doesn't depend" on Bob's setting? If so, what theory? Or do you mean that the observed relative frequency of +/- is (empirically) independent of Bob's setting? These are very different claims, and it's not even clear how *either* is relevant to what we're talking about here.

I'm confused? What IS the dependency between Alice's outcome and Bob's setting if I'm wrong? I have never heard any postulated. That is 2 specific observables, not 3, not 4, just 2.

My point is simple. This thread should be about the relativity and QM, not about your personal pet hypothesis - which we have already discussed in other threads.

 

[vanesch]

The thing is, to me, that MWI is not very useful unless you can provide evidence for it. Now, I know that MWI does fix some 'problems' in QM, but I think we should give the other methods some more time to try to work on it before we have to relegate ourselves to believing in another realities that can not seen to be (as of now) empirically proven.

I can see the appeal of MWI (after reading up on it), but for me, it doesn't seem to be the definition that seems 'right'.

Let me tell you a secret: to me neither ! Where it helps me, is to put aside, for the moment, considerations of which I *really* think they are totally misguided, and create a lot of problems and questions, where they shouldn't.

The first of these considerations is some positivist talk about "undescribable reality" and "what only counts is outcomes of measurement, not to say what "really" happens". This, to me, is a far greater offense to science - to give up on talking about what really happens - than any counter-intuitive ontology. Even though Popper is there to remind us that we should concentrate on the empirically verifiable, that doesn't mean that all there is to science, is just relationships between observations. If that were true, I really don't see the point in doing anything else but purely applied technological scientific research, where we can put these relationships to good use to improve our lives. What's the point in "knowing the relationship between observations with a telescope and a radiotelescope" ? We want, somehow, to get indications of what "really" goes on, no ?

The second of these considerations is the putting down of certain principles, which have nevertheless, served us extremely well. I'm thinking of Lorentz invariance. Never we've been able to find explicit violations of Lorentz invariance, and it is a MAJOR guiding tool in, say, considering different possible interactions in elementary particle theory (you start by writing a Lorentz-invariant Lagrangian). But for the sake of the co-existence of an intuitive ontology, and quantum mechanics, we'd have to kiss goodbye to this otherwise so powerful principle ?
I'm also thinking of the superposition principle (which is the founding principle of quantum theory). Never, when empirically challenged, the superposition principle has failed (in that one expected an interference pattern based upon the superposition principle, and one didn't observe one). Rather on the contrary: in EPR experiments, interferences have been observed which are a direct consequence of the superposition principle.

The rigorous application of these two principles leads, with our current tools, to something like MWI. All the rest falls or in one of the above considerations, or into speculation of what a future theory might look like. I can have some sympathy for the last viewpoint, which is: quantum theory makes correct predictions, but probably one day we will find what's REALLY going on, and this will then probably be totally different from what you think is going on. We will then also understand all our current confusions. My answer is: sure! One day, we will maybe know all that. When ? 100 years from now ? 500 years from now ? What does that help us NOW ? Would it have been helpful to tell people, 400 years ago, that all that talk about "the Earth is going around the sun" or vice versa is useless, counter-intuitive talk, and that what only counts is the GOOD AGREEMENT between observations theory ; that things like "the force of gravity" are hopelessly misguided concepts, that they would find out 300 years later that things are different than they seem to be and that, well, 400 years later, we'd have a "much clearer view on all this physics" ?

So once the homework is: accept the superposition principle, and accept Lorentz invariance as two fundamental guiding principles, and tell me now what fundamental ontology we can think of, I do not see many alternatives.

 

[ttn]

I'm confused? What IS the dependency between Alice's outcome and Bob's setting if I'm wrong? I have never heard any postulated. That is 2 specific observables, not 3, not 4, just 2.

Bell wrote down a precise mathematical condition which, he argued, should be satisfied by any "relativistically local" theory. It amounts to this: the probability that a theory assigns to a given event should depend *only* on facts in the past light cone of that event (in the sense that the probabilities shouldn't change if one, in addition, specifies something outside the past light cone). This is called Bell Locality. Any theory that violates this criterion obviously contains some kind of physical mechanism by which space-like separated events can causally affect one another (though what the mechanism *is* will of course vary from theory to theory).

*That* is the "dependency" that exists between Alice's and Bob's experiments.

You say you've never heard this postulated before, but that's definitely not true. Conventional QM contains just this kind of dependency, in the collapse postulate: which angle Alice sets her device to instantaneously influences the state of Bob's particle (it collapses to one or the other eigenstates of the relevant spin/polarization component). Bohm's theory also includes such a nonlocal mechanism (by which what Alice does at her end affects the state of Bob's particle), though what the mechanism is is different because of the different ontology of the theory.

Can you provide an example of a theory which does *not* contain such a nonlocal mechanism (i.e., which is Bell Local) yet which agrees with experiment? No. Because it's a *theorem* that all Bell Local theories which are capable of explaining the observed correlations for one class of possible experiments, are inconsistent with another class. That is: no Bell Local theories are empirically viable.

So what exactly are you disagreeing with?

My point is simple. This thread should be about the relativity and QM, not about your personal pet hypothesis - which we have already discussed in other threads.

"Relativity and QM" *is* what I'm talking about.

And why do you constantly focus on personalities rather than facts/proofs? First off, the argument I'm sketching above isn't "my personal pet hypothesis." This is precisely the view taken by Bell (just read any of his papers, e.g., Bertlmann's Sox, or La Nouvelle Cuisine), David Albert (read his beautiful book "Quantum Mechanics and Experience"), Tim Maudlin (read his beautiful book "Quantum Nonlocality and Relativity"), Shelly Goldstein (read his insightful encyclopedia article on Bohmian Mechanics), and a number of other well-known physicists and philosophers of science. No, the view is not universally accepted, but then neither are all sorts of eminently true things. And second, even if I were the only person in the world taking this view, that wouldn't make one bit of difference as to its truth. Scrutinize my argument and tell me what's wrong with it if you're going to so vehemently disagree. Unless the point of this discussion board isn't to clarify issues and help us all achieve truth, but simply to list, as dogma, widely accepted viewpoints whether they are logical and true or not. In which case, I'm outta here.

 

[vanesch]

Bell wrote down a precise mathematical condition which, he argued, should be satisfied by any "relativistically local" theory. It amounts to this: the probability that a theory assigns to a given event should depend *only* on facts in the past light cone of that event (in the sense that the probabilities shouldn't change if one, in addition, specifies something outside the past light cone). This is called Bell Locality. Any theory that violates this criterion obviously contains some kind of physical mechanism by which space-like separated events can causally affect one another (though what the mechanism *is* will of course vary from theory to theory).

Well, you can take that in different ways. If you only look upon the OUTCOMES, then clearly, the probability of what happened at Alice only depends of what is in this event's past lightcone: the probability to have a click or not, only depends on the source, and on her local setting. In most experiments, this probability is simply 50%.
IT DOESN'T MAKE SENSE TO TALK ABOUT A CORRELATION at this point !
Now, the same can be said for Bob. So Bob has only events to his disposal, of which the probability to click or not, is only dependent on the source and on HIS setting. Alice's settings do not influence Bob's probability of seeing a click or not.

The only way to find a "strange" result is by comparing Alice's and Bob's observations. But this must happen at a LATER event, where Bob and Alice's measurement actions are in the past lightcone of this "comparing" event. It is only NOW, at THIS EVENT that it makes sense to talk about the probabilities of hits which were seen simultaneously, or in opposition (correlation or anti correlation). So, at the event where it makes sense to talk about the "probability of a correlated hit", both Alice's and Bob's measurement are in the past lightcone. And at Alice's place, it doesn't make sense to talk about Bob's probabilities and vice versa, because these results are not available to her.

In fact, to arrive at Bell's statement, you have to make AN EXTRA hypothesis: that is that Bob's and Alice's POTENTIAL outcomes are part of an overall common probability measure, no matter their actual settings.

This extra hypothesis would then allow you to create a (hidden or not) variable, distributed according to this overall common probability measure, and reproduce the correlations ; and that's what Bell proved, cannot happen and agree with QM predictions.

Can you provide an example of a theory which does *not* contain such a nonlocal mechanism (i.e., which is Bell Local) yet which agrees with experiment? No. Because it's a *theorem* that all Bell Local theories which are capable of explaining the observed correlations for one class of possible experiments, are inconsistent with another class. That is: no Bell Local theories are empirically viable.

Yes, for the nth time: MWI ! What MWI "violates" is the extra hypothesis of the existence of an overall common probability measure of POTENTIAL outcomes, because the only terms appearing in the overall wavefunction are those that got correlated with the ACTUAL measurement apparatus (and whose Hilbert norms hence give you the probabilities of observation at Alice, at Bob, and later, when they come together, for their correlations). But it respects the probabilities of all observable things at a certain event, to depend only on what happens in the past light cone. As such, the CORRELATIONS don't "exist" until Alice and Bob come together, and the "potential measurement outcomes" are not considered. In other words, there is no overall probability measure. And hence no Bell theorem.

 

[LnGrrrR]

So once the homework is: accept the superposition principle, and accept Lorentz invariance as two fundamental guiding principles, and tell me now what fundamental ontology we can think of, I do not see many alternatives.

Yes, I can see your point from here. I also detest the idea of 'just worrying about the probabilities' for the same reason you do...it just seems like 'quitting'.

Now, you all certainly know much more than I (though I am slowly...ever so painfully learning), but I think I can see where you're coming from. Right now, MWI is certainly one of the, if not the most, 'valid' ideas, given what we know.

However, it almost seems to be a 'god of the gaps' idea (if you're not sure what this means, I'll let you know). Without having read up a great deal on Lorentz transformations or anything, it seems to me (again, more on a gut feeling than anything else) that QM right now seems to be where classic Newtonian physics was.

Newton did an amazing job describing how certain things function in our world, and for most things, it works great...however, Einstein of course improved upon Newton's mechanics and came up with a theory that not only covered what his could cover, but even more so.

I think that I'm 'holding out' for something like the Theory of Relativity to happen to QM...mayhaps you could say I'm looking for a 'hidden variable' of some sort. But I'm still holding out that we will discover a means that will rule out MWI.

It bothers me enough to think that there are things where we just CAN NOT know (HUP)...but to think that there are other REALITIES we can't get to? Far too 'unfair'...it's like reading one book by an author and finding it immensely satisfying. Then having joy upon hearing he's written THOUSANDS of books...and despair to realize you can't read any other one but the one in your hands. :)

I don't think I've done enough research to form an honest opinion about which 'interpretation' is best though.

 

[selfAdjoint]

Yes, I can see your point from here. I also detest the idea of 'just worrying about the probabilities' for the same reason you do...it just seems like 'quitting'

With the deepest respect I have to say this: when I was in grad school back in the sixties it was the era of the radical student movement. And the the leaders used to say "I know this{whatever they were urging } is not fair or prudent, but WE HAVE TO START SOMEWHERE. Otherwise we're just giving up!" In that case such reasoning led to bombing a campus building at UW Madison, and killing a mathematical researcher.

"Don't be a quitter, no matter what" is a little demon that whispers in peoples' ears to lead them into perdition.

 

[DrChinese]

*That* is the "dependency" that exists between Alice's and Bob's experiments.

You say you've never heard this postulated before, but that's definitely not true. Conventional QM contains just this kind of dependency, in the collapse postulate: which angle Alice sets her device to instantaneously influences the state of Bob's particle (it collapses to one or the other eigenstates of the relevant spin/polarization component).

As you vary Alice's setting, there is no change in Bob's outcome and vice versa. So where is the dependency? There is no prediction by oQM that there is any demonstrable or (non-local) physical connection between these. And there is no way to test Bob's particle alone and determine if it is a collapsed eigenstate or not. Only when you add more observables do you have anything to talk about.

 

[LnGrrrR]

With the deepest respect I have to say this: when I was in grad school back in the sixties it was the era of the radical student movement. And the the leaders used to say "I know this{whatever they were urging } is not fair or prudent, but WE HAVE TO START SOMEWHERE. Otherwise we're just giving up!" In that case such reasoning led to bombing a campus building at UW Madison, and killing a mathematical researcher.

"Don't be a quitter, no matter what" is a little demon that whispers in peoples' ears to lead them into perdition.

I fail to see how your first comment leads to anything but a non-sequitur...

Tell me, how do you know when the right to just 'start' somewhere is? What is wrong with trying to determine the methods behind QM? I personally see nothing wrong with trying to determine it.

Sure, there are some fundamental things we may never know (for instance, /why/ gravity occurs) but certainly we should try to investigate and determine answers about our world.

I don't see why scientists should 'quit' trying to determine answers to any subject. As far as demons...I don't believe in that stuff ;)

 

[vanesch]

Yes, I can see your point from here. I also detest the idea of 'just worrying about the probabilities' for the same reason you do...it just seems like 'quitting'.

Now, you all certainly know much more than I (though I am slowly...ever so painfully learning), but I think I can see where you're coming from. Right now, MWI is certainly one of the, if not the most, 'valid' ideas, given what we know.

However, it almost seems to be a 'god of the gaps' idea (if you're not sure what this means, I'll let you know). Without having read up a great deal on Lorentz transformations or anything, it seems to me (again, more on a gut feeling than anything else) that QM right now seems to be where classic Newtonian physics was.

I agree with all that. In fact, the major challenge is not the interpretation of quantum theory, but the unification, the clash, or no matter how you call it, between quantum theory and general relativity. As long as the jury is out on that one, I'd say: hold your bets.
But does that mean that in the mean time, we shouldn't have a picture to work with ?

Newton did an amazing job describing how certain things function in our world, and for most things, it works great...however, Einstein of course improved upon Newton's mechanics and came up with a theory that not only covered what his could cover, but even more so.

I think that I'm 'holding out' for something like the Theory of Relativity to happen to QM...mayhaps you could say I'm looking for a 'hidden variable' of some sort. But I'm still holding out that we will discover a means that will rule out MWI.

Sure. And usually, the surprise comes from an unexpected side. It's not said that things become simpler. Maybe. Maybe not.

It bothers me enough to think that there are things where we just CAN NOT know (HUP)...but to think that there are other REALITIES we can't get to? Far too 'unfair'...it's like reading one book by an author and finding it immensely satisfying. Then having joy upon hearing he's written THOUSANDS of books...and despair to realize you can't read any other one but the one in your hands. :)

I don't think I've done enough research to form an honest opinion about which 'interpretation' is best though.

It's like getting married, you know...

 

[ttn]

Well, you can take that in different ways. If you only look upon the OUTCOMES, then clearly, the probability of what happened at Alice only depends of what is in this event's past lightcone: the probability to have a click or not, only depends on the source, and on her local setting. In most experiments, this probability is simply 50%.

This involves an equivocation. There are two different things that "probability" could mean here. The way you're using it above, you mean the relative frequency of a certain occurrence. The way I was using it before was to refer to the probabilities that some *particular theory* assigns to a given happening. You're right that there's no way to just look at Alice's data and determine whether Bell Locality is violated. This is because Bell Locality is primarily a locality requirement *for theories*. Any claims made about nature (based on experiment) have to be filtered through (so to speak) an argument about theories. This is why I'm always careful to say things like: "no Bell Local theory can be in agreement with the observations -- thus nature violates Bell Locality."

IT DOESN'T MAKE SENSE TO TALK ABOUT A CORRELATION at this point ! Now, the same can be said for Bob. So Bob has only events to his disposal, of which the probability to click or not, is only dependent on the source and on HIS setting. Alice's settings do not influence Bob's probability of seeing a click or not.

Meaning: Alice's settings do not influence the relative frequency of Bob seeing a click. That's true. It's basically equivalent to what's usually called "signal locality." The deeper question (that Bell asked) is: can a locally causal theory explain the outcomes? The answer turns out to be no (modulo your usual objection about MWI... )

The only way to find a "strange" result is by comparing Alice's and Bob's observations. But this must happen at a LATER event, where Bob and Alice's measurement actions are in the past lightcone of this "comparing" event. It is only NOW, at THIS EVENT that it makes sense to talk about the probabilities of hits which were seen simultaneously, or in opposition (correlation or anti correlation). So, at the event where it makes sense to talk about the "probability of a correlated hit", both Alice's and Bob's measurement are in the past lightcone. And at Alice's place, it doesn't make sense to talk about Bob's probabilities and vice versa, because these results are not available to her.

But (like a good MWI-er ) you're then denying that the measurements really have outcomes prior to Alice and Bob meeting up later! This is like my saying: there's no fact of the matter about whether some distant star has supernova'ed or not (in say the rest frame of the earth) because facts about that distant star only become "real" when the "information" about them gets to my eyes.

What happened to the *realism* I thought we agreed about??

In fact, to arrive at Bell's statement, you have to make AN EXTRA hypothesis: that is that Bob's and Alice's POTENTIAL outcomes are part of an overall common probability measure, no matter their actual settings.

This extra hypothesis would then allow you to create a (hidden or not) variable, distributed according to this overall common probability measure, and reproduce the correlations ; and that's what Bell proved, cannot happen and agree with QM predictions.

You can look at it this way, but I think it's clearer to say that the "extra hypothesis" you need is simply that Alice's and Bob's individual experiments really do have definite outcomes, independent of whether the two of them get together later for coffee and to compare notes. With this assumption alone, the EPR argument gets you (from the postulate of Bell Locality) the existence of local hidden variables which determine (in advance, so to speak) the outcomes on each side. And then Bell's Theorem demonstrates that this kind of model is at odds with experiment. So from two assumptions (that the experiments on each side actually have definite outcomes, and Bell Locality) you get a contradiction with experiment. So either (what I always say, because I'm unwilling to deny the actual outcomes) nature violates Bell Locality... or (what you always say, because you're in love with locality) we have to give up the idea that Alice's and Bob's individual experiments had definite outcomes.

Yes, for the nth time: MWI ! What MWI "violates" is the extra hypothesis of the existence of an overall common probability measure of POTENTIAL outcomes, because the only terms appearing in the overall wavefunction are those that got correlated with the ACTUAL measurement apparatus (and whose Hilbert norms hence give you the probabilities of observation at Alice, at Bob, and later, when they come together, for their correlations). But it respects the probabilities of all observable things at a certain event, to depend only on what happens in the past light cone. As such, the CORRELATIONS don't "exist" until Alice and Bob come together, and the "potential measurement outcomes" are not considered. In other words, there is no overall probability measure. And hence no Bell theorem.

Yes, I think we agree about all this. We just disagree about which of the two premises (locality, or actual outcomes) is more reasonable to give up.

 

[ttn]

I don't think I've done enough research to form an honest opinion about which 'interpretation' is best though.

I thought I'd second my own earlier recommendation of David Albert's book, "Quantum Mechanics and Experience." It is far and away the best book out there if you want to really understand what the issues are and what the possible interpretations are all about. There are a few weird things in the book (speculations about how this or that interpretation might deal with a situation in which some crazy device is hooked up directly to some poor guy's brain, etc.) but for the most part the book is a 100% honest, and 100% clear presentation of the actual problems with the conventional view, and the various proposed ways of dealing with the problems (which means, basically: GRW type theories, MWI type theories, and Bohm type theories).

 

[ttn]

As you vary Alice's setting, there is no change in Bob's outcome and vice versa. So where is the dependency? There is no prediction by oQM that there is any demonstrable or (non-local) physical connection between these. And there is no way to test Bob's particle alone and determine if it is a collapsed eigenstate or not. Only when you add more observables do you have anything to talk about.

Sigh. I've tried so many times to explain this, but we just never seem to get anywhere. Should I try again? Maybe one last time...

You have to allow that there's a difference between two things:

1. Alice can affect the relative frequency of some occurence near Bob by turning a knob (or something else she has direct control over). For example, the frequency of a certain outcome of a certain experiment near Bob has one value if Alice's knob is turned to the left, and has a different value if Alice's knob is turned to the right. In the simplest case, the frequencies are 1 or 0, so, say, when the knob is turned left, a light bulb near Bob lights up; when it's turned right, the bulb goes dim.

2. Some particular theory says that the fundamental dynamical probability of a certain event depends on happenings outside the backwards light cone of the event.

The first involves (evidently) a *controllable* causal mechanism. It allows Alice to send a *signal* to Bob. And note that we don't have to know anything about what theory is or isn't true in order to demonstrate the existence of something like 1. We just discover (say) that turning a certain knob over here, causes some regular correlated happening over there. It's a purely *empirical* statement.

The second, on the other hand, is a statement not about data from an experiment, but about a *theory*. If some theory works this way, we would say that this theory is nonlocal, right? We'd say that this theory violated relativity's prohibition on superluminal causation, right?

Are you with me so far? If so, I'll continue later... If not, no point going further.

 

[UglyDuckling]

Help! Help! Help! Help! Help! Help! Help! Help! Help! Help! Help! Help! Help!

Will someone please put me out of my misery regarding “the locality loophole”? ttn says “I don’t know what I’m talking about”, I have tried to find a rational valid reason why he should make this allegation but so far it escapes me. So would someone please! please! tell me what I’m missing!

I will restate my reasoning so you can identify more easily where my logic is going wrong.

The assertion is; given experimental results that violate Bell’s inequality then “locality” can be recovered if we assume that certain events in the experimental procedure are, at quantum level, super-positioned.

For Aspect’s experiment these events are: -
1. The moment of the calcium atom decays.
2. The moments at the polarisers when by the conventional theory the “photons” would be passing through them.
3. Finally the moments at the detectors when they pick up a count.

The argument is the quantum systems at these events become super-positioned and act as a single entity. Communication between the components of the system becomes instantaneous and any changes in the quantum states of the component system can take place in accordance with the laws of conservation without special relativity being violated.

I’ve tried thinking of why ttn should so vehemently oppose this viewpoint and I’ve come up with the following possibilities.

1. The events in question are separated in space and time and therefore could not possibly be super-positioned in the manner proposed? The answer here is the events can all be connected by zero proper interval paths so mathematically there is no reason they should not be super-positioned.
2. Since the component quantum systems are super-positioned there is no requirement for a photon to mediate the electromagnetic force? As quantum mechanics does not provide an ontology this cannot be his grounds for objection. Super-positioning, for instance, as the mediator of electromagnetic force would have a neutral on the dynamics of QED.
3. The only other reason I’ve thought of is that the proposal is incompatible with his belief that super-luminal influences are an established fact and he may think it is inconsistent with Bohm’s interpretation of quantum mechanics. But I’m sure ttn would not let a fondness for a pet theory come in the way of rational debate. So that can’t be the answer!

So just what is it I’m missing about “the locality loophole” problem.

Again.help!

 

[LnGrrrR]

It's like getting married, you know...

Which is another issue I'm very undecided on...

I certainly see your point of "go with MWI now until we understand better otherwise". However, I think a part of me refuses to believe/acknowledge that idea as correct. Whether it's due to lack of information or just something about the way I'm configured, I have no clue. :)

 

[LnGrrrR]

I thought I'd second my own earlier recommendation of David Albert's book, "Quantum Mechanics and Experience." It is far and away the best book out there if you want to really understand what the issues are and what the possible interpretations are all about. There are a few weird things in the book (speculations about how this or that interpretation might deal with a situation in which some crazy device is hooked up directly to some poor guy's brain, etc.) but for the most part the book is a 100% honest, and 100% clear presentation of the actual problems with the conventional view, and the various proposed ways of dealing with the problems (which means, basically: GRW type theories, MWI type theories, and Bohm type theories).

Thanks for the recommendation! The bookstores near me...well...there aren't any anymore. (Thanks Katrina!) Maybe next time I go to Mobile, AL, I'll take a look...or just rely on Amazon.com :)

 

[DrChinese]

The first involves (evidently) a *controllable* causal mechanism. It allows Alice to send a *signal* to Bob. And note that we don't have to know anything about what theory is or isn't true in order to demonstrate the existence of something like 1. We just discover (say) that turning a certain knob over here, causes some regular correlated happening over there. It's a purely *empirical* statement.

The second, on the other hand, is a statement not about data from an experiment, but about a *theory*. If some theory works this way, we would say that this theory is nonlocal, right? We'd say that this theory violated relativity's prohibition on superluminal causation, right?

Are you with me so far? If so, I'll continue later... If not, no point going further.

I am with you so far...

 

[ttn]

I am with you so far...

Spectacular. OK. So, a violation of "1" from the previous post is a violation of what's usually called "signal locality." A violation of "2" from the previous post is a violation of "Bell Locality." This is all just defining of terms, so nothing really to worry about.

By way of continuing, here are some important facts, in no particular order:

* All experiments to date suggest that Signal Locality is true. There is every reason to believe that "the one true theory" (whatever that is exactly) should be Signal Local.

* It is possible for a theory to violate Bell Locality (and, I think we agreed, therefore be at odds with relativity's prohibition on superluminal causation) but nevertheless *respect* Signal Locality. Here's a silly example: suppose there are these two boxes and whenever you and a friend look into the two boxes, you see balls that are the same color. We just take that as an empirical fact. Now suppose somebody proposes the following theory: when nobody's looking, the balls are grey; but then as soon as the first person (you or your friend) look into one of the boxes, the ball in the looked-in box flips a coin and turns itself either red or blue (at random) -- *and*, simultaneously, the distant ball turns itself the same color. Let's leave aside the question of whether there could possiblly be any good reason to believe such a theory (probably not). My point here is simply that this theory violates Bell Locality (since the color of the one ball is determined, according to this theory, by the space-like separated outcome of that random coin flip) -- but that, despite violating Bell Locality, the theory is consistent with Signal Locality (basically, because what one person sees when he opens his box is either a red/blue ball, seemingly at random, and there's no way to infer anything about what happened or didn't happen at the distant box from this). If this theory is right, there exist relativity-violating interactions, but it is impossible to *use* these interactions to communicate with your distant friend (i.e., you can't send a signal).


So... still with me? Do we agree that, in principle, a theory can violate Bell Locality (and hence not be consistent with relativity) yet still not support the sending of signals faster than light?

 

[vanesch]

So... still with me? Do we agree that, in principle, a theory can violate Bell Locality (and hence not be consistent with relativity) yet still not support the sending of signals faster than light?

I know that you want to talk to Dr C. and I don't want to interrupt your discussion. But I wanted to make something clear. We know that signal locality is a less severe requirement than Bell locality. However, we should not forget where this "locality" requirement comes from: it comes from the idea that spacetime has a geometrical Minkowski structure ; from this, it follows that the laws of nature should not depend on how we LABEL the events in Minkowski space - which results that ALL physically relevant expressions should be writable in Lorentz-invariant form. In fact, we need a second assumption: that in all these different ways of labeling the events in Minkowski space, the "t" coordinate represents somehow time for A thinkable observer, which should not be placed before the fact that something that can be set up to be influenced at t1 can determine the probabilities of something at t2, with t2 < t1, for this particular observer ; this, simply because of the obvious paradox that would result: the observer could set up a device that determines the relevant probabilities at t2, and then wait until t1 to change things such that what he observed at t2 would not be true.
This last idea comes from the fact - the observed fact - that we can obtain results at a certain ta and THEN take decisions based upon that result, at a tb > ta, but not vice versa. In other words, we seem to be able to influence the future, but not the past.
This, plus the geometrical structure of Minkowski space, makes us "require locality". But BEFORE it even makes sense to talk about such a locality, one must adhere to the *Minkowski* GEOMETRY of spacetime ; and this, in turn, means, that all physically relevant quantities that could ever appear in a theory, must be representations of the Lorentz group.

As such, it doesn't make, IMO, much sense to *even talk* about signal locality if the Minkowski geometry of spacetime is not respected. There is not the remotest motivation to even think about signal locality if the Minkowski geometry is not there. Of course, there can be a kind of "conspiration" for a theory, which doesn't respect the Minkowski geometry, to nevertheless manifest requirements that ONLY MADE SENSE for that geometry, but there's absolutely no fundamental reason for this to happen. It is a bit as if one discovered that, say, all international banc transfer amounts in the last 20 years, expressed in dollars, are prime numbers.
Of course this is possible. But there's no reason to it. In the same way, a theory that doesn't respect the Minkowski geometry of spacetime (is not expressible in a Lorentz-invariant way) has absolutely no "reason" to respect information locality. It's of course possible. But it would be a very weird thing, as with the banc tranfers being prime numbers.

 

[LnGrrrR]

So... still with me? Do we agree that, in principle, a theory can violate Bell Locality (and hence not be consistent with relativity) yet still not support the sending of signals faster than light?

This is a question I'm certainly interested to hear the answer to from you folks, as it is one I have read about that seems counter-intuitive at first glance.

 

[ttn]

I know that you want to talk to Dr C. and I don't want to interrupt your discussion. But I wanted to make something clear. We know that signal locality is a less severe requirement than Bell locality. However, we should not forget where this "locality" requirement comes from: it comes from the idea that spacetime has a geometrical Minkowski structure ; from this, it follows that the laws of nature should not depend on how we LABEL the events in Minkowski space - which results that ALL physically relevant expressions should be writable in Lorentz-invariant form.

Patrick, very good and interesting post. This probably deserves its own thread... but I'll answer briefly here, and then participate if anyone wants to tear this off and give it its own thread.

Basically, I disagree about the hierarchical relation between "locality" and spacetime's Minkowski structure. You suggest that the only reason we *care* about requiring theories to be local is to enforce this underlying spacetime structure. If we knew directly from God that spacetime *did* have this structure, then I think you'd be right -- including being right about "signal locality" being a useless/irrelevant idea once you reject Bell Locality (which is much closer to the requirement you're after, that the dynamical laws respect the minkowski structure).

However, I do *not* think it's the case that God whispers this in our ear. That spacetime has a certain structure is an *inference* from more concrete experimental data, including such things as the measured invariance of the speed of light (e.g., the M-M experiment) and the failure of all attempts to send signals faster-than-light.

The point is, if we should someday encounter some empirical evidence that Bell Locality is violated (i.e., that it is impossible to formulate dynamical laws in a Lorentz invariant way) then we will simply have to accept that, despite the tons of evidence for this conclusion that was found up to this point, the conclusion turns out to have been premature and wrong -- Lorentz invariance is *not* the fundamental/final word in spacetime structure. ...which is, though surely *surprising*, not the end of the world (unless one has this crazy view that we know it is true a priori, from god).

This is basically nothing but our same old disagreement about whether or not locality is proved by experiment, whether or not MWI is a counterexample to my claims that nonlocality is proved by experiment. I take the outcomes of experiments ("naively" interpreted) as the rock-bottom givens. Theory, in my opinion, must always stand or fall with experimental data. So if (as I think) some experimental data is in conflict with the idea of Bell Locality (i.e., the fundamental principles of relativity theory) then it is so much the worse for locality/relativity. We must reject these ideas as wrong -- or at any rate non-universal. On the other hand, you seem to give this "untouchable rock-bottom" status to something very high up and abstract, namely the Lorentz invariance of dynamical laws. As I understand it your view is that this is sacred and untouchable, so if the experimental data "naively" appears to conflict with it, we need to find some way to reinterpret the experimental data so as to render it consistent with the sacred principle -- and hence is born this idea that, really, when Bob thinks his needle points left, he's deluded, and what's actually happening is that it's pointing both left and right at the same time for two different Bobs (or in two different universes or branches of the wf or whatever).

As a philosophical summary of all this, I'd say that I'm much more of an empiricist and you're much more of a rationalist. What's sacred to me is basic perceptual facts like Bob sees his needle go left; all the abstract stuff about Lorentz invariance and such is, if need be (i.e., if empirical data requires it), negotiable. What's sacred for you is the abstract principle, while all the nitty-gritty perceptual facts (Bob seeing the needle go left) are, if need be (i.e., if the principle requires it), negotiable.

And just to hint at what I think is wrong with your approach: as an empiricist (who doesn't believe we have *any* a priori knowledge, no revelations from God, etc.) I think that we only get to abstract principles by organization/interpretation of the closer-to-perception type data. So your whole approach strikes me as circular: you're willing to radically reinterpret something like Bob's perception of a hunk of aluminum in front of him, in order to "save" some abstract principle which (I would argue) we only believe in in the first place because we accepted as given such things as Bob's ability to correctly perceive bits of aluminum in front of him.

As such, it doesn't make, IMO, much sense to *even talk* about signal locality if the Minkowski geometry of spacetime is not respected.

I don't agree. Signal Locality is a summary of empirical facts. We know from experience that it isn't possible (by any mechanism studied so far) to send signals faster than light. It is not unreasonable to formulate this summary as a principle and hypothesize that it is general, i.e., to expect future theories to also respect it. But you're right that, without some kind of prior certainty about the underlying cause of signal locality (minkowski spacetime structure) we can't be *certain* of this extrapolation. And that is perhaps frustrating... but if one is an empiricist at least, that's just the way things are, the way science works.

There is not the remotest motivation to even think about signal locality if the Minkowski geometry is not there. Of course, there can be a kind of "conspiration" for a theory, which doesn't respect the Minkowski geometry, to nevertheless manifest requirements that ONLY MADE SENSE for that geometry, but there's absolutely no fundamental reason for this to happen.

I don't see how you could possibly know the latter. Minkowski geometry is one possible underlying cause for our in-practice inability to send signals FTL. But something like what you call a conspiracy is also a possible cause of this. The fact is, we just don't know for sure a priori. What the cause is of our inability to send signals FTL will have to be ultimately settled by experiment and (future) theories based on experiment. Your attitude seems to be that all the experiments prior to (say) 1950 constitute logically sufficient proof that spacetime has this minkowski geometry, and that this principle is therefore now untouchable, and that we therefore have to go to these ridiculous crazy MWI ideas in order to respect that principle. I completely disagree with that initial claim to certainty, though. It was known (or should have been known) all along that an "ether" type view was logically possible and consistent with all the empirical data (if inelegant). I'm not saying people should have believed in the ether, just that they shouldn't have foreclosed on it and claimed certainty about this so early. And, I think, that chicken is now coming home to roost, because the Bell/Aspect stuff (if you just accept all the experimental data at face value) proves that minkowski geometry is not the final word in spacetime structure.

It is a bit as if one discovered that, say, all international banc transfer amounts in the last 20 years, expressed in dollars, are prime numbers. Of course this is possible. But there's no reason to it.

Spoke like a true rationalist. =) If we *did* discover this, of *course* there'd be some reason, some cause, for it. It'd be too much of a coincidence to happen by coincidence. But just because no reason, no explanation, is apparent to us right now, doesn't mean we should just stipulate that it's impossible and then proceed to invent wild fantasies about how, really, when Joe Banker counts dollar bills, he is deluded into thinking there's 7 of them when really there's 10...

In the same way, a theory that doesn't respect the Minkowski geometry of spacetime (is not expressible in a Lorentz-invariant way) has absolutely no "reason" to respect information locality. It's of course possible. But it would be a very weird thing, as with the banc tranfers being prime numbers.

Bohmian Mechanics provides a nice counterexample of this claim. It violates Bell Locality (and the easiest way to formulate it is to have a preferred reference frame, contra Minkowski) and yet respects signal locality. Why? Because of the uncertainty in the initial locations of the particles -- uncertainty which turns out, as a *theorem* of Bohmian Mechanics, to be *absolute*. See

http://www.arxiv.org/abs/quant-ph/0308039

 

[ttn]

Oh, one other quick comment: all this debate about "signal locality" is interesting, but really beside the point. Signal Locality in fact plays no role whatever in Bell's argument for nonlocality. I only raised it (w/ Dr C) because I have too often been burned by people listening to the argument, only to play the switcheroo game at the end and saying: OK, but really there's no nonlocality here because you can't send a signal with it. So I wanted to make absolutely sure we agreed, up front, that there are two in-principle distinguishable questions -- whether you can send a signal FTL, and whether a given theory respects relativity's prohibition on superluminal causation.

The claim I am at pains to demonstrate (and it is not original to me, it is Bell's claim, which has been widely misunderstood) is that no theory respecting that latter condition (no causally local theory, no Bell Local theory) can be in agreement with the empirical data. Not just no "hidden variable theory" that is local, not just any "deterministic" theory that is local, but no theory *at all* that is local. No causally local theory *at all* can be in agreement with experiment. This is what I am convinced (by Bell) is true. And this claim has nothing whatever to do with signal locality -- except that it is perhaps a bit of a surprise that a Bell Non-Local theory can nevertheless prohibit superluminal signalling.

 

[vanesch]

However, I do *not* think it's the case that God whispers this in our ear. That spacetime has a certain structure is an *inference* from more concrete experimental data, including such things as the measured invariance of the speed of light (e.g., the M-M experiment) and the failure of all attempts to send signals faster-than-light.

Of course. Let's say that taking on this structure for spacetime gives us then IMMEDIATELY the reason for:
1) the constancy of the speed of light,
2) the failure to send signals faster than light
3) the transformation properties in special relativity
4) the EXPLANATION of why ALL LAWS OF NATURE seem to be able to be expressed as Lorentz invariant quantities.
...

In other words, this is ONE SINGLE principle, from which a WHOLE LOT of consequences can be derived, and which ALL have been verified experimentally.
So, if some reasoning, which also makes OTHER hypotheses, leads us to conclude, that after all, this structure of spacetime cannot be true, it takes a really convincing argument that it is THIS point, and not all the other hypotheses, that need to fail.

The point is, if we should someday encounter some empirical evidence that Bell Locality is violated (i.e., that it is impossible to formulate dynamical laws in a Lorentz invariant way) then we will simply have to accept that, despite the tons of evidence for this conclusion that was found up to this point, the conclusion turns out to have been premature and wrong -- Lorentz invariance is *not* the fundamental/final word in spacetime structure.

This would be true if Bell locality were a direct consequence of the spacetime structure of Minkowski space. But there are extra hypotheses needed to do so, and the most evident OTHER hypothesis is the denial of that other great principle: the superposition principle.

This is basically nothing but our same old disagreement about whether or not locality is proved by experiment, whether or not MWI is a counterexample to my claims that nonlocality is proved by experiment. I take the outcomes of experiments ("naively" interpreted) as the rock-bottom givens. Theory, in my opinion, must always stand or fall with experimental data. So if (as I think) some experimental data is in conflict with the idea of Bell Locality (i.e., the fundamental principles of relativity theory) then it is so much the worse for locality/relativity. We must reject these ideas as wrong -- or at any rate non-universal. On the other hand, you seem to give this "untouchable rock-bottom" status to something very high up and abstract, namely the Lorentz invariance of dynamical laws. As I understand it your view is that this is sacred and untouchable, so if the experimental data "naively" appears to conflict with it, we need to find some way to reinterpret the experimental data so as to render it consistent with the sacred principle -- and hence is born this idea that, really, when Bob thinks his needle points left, he's deluded, and what's actually happening is that it's pointing both left and right at the same time for two different Bobs (or in two different universes or branches of the wf or whatever).

Yes. For TWO reasons, not only one. We already found one great principle, which is the Minkowski structure of spacetime, as being able to explain naturally points 1,2,3 and 4 in my little list above. These points include A LOT of empirical evidence, and none EXPLICITLY against it (that is, a DIRECT derivation of a result based upon the spacetime structure being a Minkowski space, in contradiction with experiment ; say, the wrong life times of muons or so, or clocks not behaving as computed).
But we discovered also ANOTHER great principle, which is the superposition principle, which ALSO explained a lot of empirical results. It gave rise to all of quantum mechanics, in its "bare bones" applications, from atomic spectral lines, solid state stuff, ...
Applying "naively" the superposition principle to "remote Bob and his needle" would imply that indeed, Bob exists in two states. But somehow we don't want to see that, so we state that this shouldn't be so.
So now we MAKE THE ASSUMPTION that 1) the superposition principle DOES NOT APPLY TO remote Bob and his needle, and 2) the Minkowski geometry of spacetime and we arrive at a contradiction with a prediction of a theory based upon the superposition principle - and some experimental support for it: namely the violation of Bell locality.
So it seems that if, at a certain point, you DO NOT ALLOW FOR THE SUPERPOSITION PRINCIPLE anymore, and you assume the superposition principle for microscopic systems, that you run into problems with Minkowski spacetime, both theoretically and experimentally.
Conclusion: Minkowski space is dead. Is it ? Or is it in the *assumption of the non-application of the superposition principle* that the error resides ?

It sounds a bit weird that we conclude about the non-validity of two principles, namely the superposition principle, and the Minkowski structure of spacetime, which were otherwise empirically very successful, simply because at a certain point we REFUSE to apply the superposition principle, no ?

As a philosophical summary of all this, I'd say that I'm much more of an empiricist and you're much more of a rationalist. What's sacred to me is basic perceptual facts like Bob sees his needle go left; all the abstract stuff about Lorentz invariance and such is, if need be (i.e., if empirical data requires it), negotiable. What's sacred for you is the abstract principle, while all the nitty-gritty perceptual facts (Bob seeing the needle go left) are, if need be (i.e., if the principle requires it), negotiable.

That's a bit carricatural. Every great principle is negociable, ON THE CONDITION that we have a better, more encompassing principle to replace it. One that has MORE explanatory power.

And just to hint at what I think is wrong with your approach: as an empiricist (who doesn't believe we have *any* a priori knowledge, no revelations from God, etc.) I think that we only get to abstract principles by organization/interpretation of the closer-to-perception type data. So your whole approach strikes me as circular: you're willing to radically reinterpret something like Bob's perception of a hunk of aluminum in front of him, in order to "save" some abstract principle which (I would argue) we only believe in in the first place because we accepted as given such things as Bob's ability to correctly perceive bits of aluminum in front of him.

This is often the case, that, when things are better understood, we realize that our perception of things were not what we thought it was, but nevertheless at a certain point we had to go by there to arrive where we are now.

You could say the same of, say, the "atomic hypothesis", which was regarded as a very hypothetical idea in the 19th century, but of which one had to recon, one could deduce quite some observed facts. One could argue a bit like you do here: in order to make the atomic hypothesis (which includes that objects are made of tiny little things, with lots of empty space in between them), we'd need to consider that we are "deluded" in thinking that we have massive objects of continuous matter around us, while these are essentially "empty" pieces of space, with some tiny tiny matter points in them. And the funny thing is, that to even MAKE the atomic hypothesis, we have to use instruments that are EXACTLY MADE OF MASSIVE, CONTINUOUS MATTER.
Now, it might be that using the atomic hypothesis, you might have an explanation of why atomic matter *looks and feels* like massive, continuous matter, but isn't it strange that in order to "save this abstract principle of atomic matter" one has to deny the existence of the continuous matter which we used in the first place to arrive at this "atomic matter". We used "continuous" electrical wires, "continuous" pieces of metal, ...

I don't agree. Signal Locality is a summary of empirical facts. We know from experience that it isn't possible (by any mechanism studied so far) to send signals faster than light. It is not unreasonable to formulate this summary as a principle and hypothesize that it is general, i.e., to expect future theories to also respect it. But you're right that, without some kind of prior certainty about the underlying cause of signal locality (minkowski spacetime structure) we can't be *certain* of this extrapolation. And that is perhaps frustrating... but if one is an empiricist at least, that's just the way things are, the way science works.

But the great advancements of science are exactly when one realizes that a general principle encompasses a lot of empirical facts.

I don't see how you could possibly know the latter. Minkowski geometry is one possible underlying cause for our in-practice inability to send signals FTL. But something like what you call a conspiracy is also a possible cause of this. The fact is, we just don't know for sure a priori.

Yes, but when you get a lot of "conspiracies" that can find their explanation in a principle otherwise, one should really consider that principle, no ?

What the cause is of our inability to send signals FTL will have to be ultimately settled by experiment and (future) theories based on experiment. Your attitude seems to be that all the experiments prior to (say) 1950 constitute logically sufficient proof that spacetime has this minkowski geometry, and that this principle is therefore now untouchable, and that we therefore have to go to these ridiculous crazy MWI ideas in order to respect that principle.

First of all, if one takes the attitude that one can never formulate a principle because one day it might be falsified, one isn't going to make much progress!
I'm not stating that the Minkowski structure is there once and for all (hey, we already know it is not the case thanks to general relativity!) I'm saying that NOTHING has ever been found that contradicted it. The only "contradiction" that is found, is when you are using another principle and violate it at the same time: the superposition principle.
Entanglement is a consequence of the superposition principle ; and saying that Bob CANNOT be in two states is denying this. Now from this double standard, you derive that Minkowski spacetime must come into troubles. My answer is: apply RIGOROUSLY the superposition principle, and apply RIGOROUSLY the Minkowski spacetime structure, and you don't have the problems you're talking about.

I completely disagree with that initial claim to certainty, though. It was known (or should have been known) all along that an "ether" type view was logically possible and consistent with all the empirical data (if inelegant). I'm not saying people should have believed in the ether, just that they shouldn't have foreclosed on it and claimed certainty about this so early. And, I think, that chicken is now coming home to roost, because the Bell/Aspect stuff (if you just accept all the experimental data at face value) proves that minkowski geometry is not the final word in spacetime structure.

So, spacetime is not Minkowski, but behaves in almost all respects AS IF it were. The superposition principle is not valid, but things behave AS IF it were valid on microscales. We know that if spacetime IS Minkowski, and the superposition principle IS valid, that there is no problem and that all empirical data can be explained too, but this goes against our intuition. Hmmm...

Bohmian Mechanics provides a nice counterexample of this claim. It violates Bell Locality (and the easiest way to formulate it is to have a preferred reference frame, contra Minkowski) and yet respects signal locality.

Bohmian mechanics also violates a priori signal locality. It is only when we equip it (with much difficulty!) with Lorentz-invariant dynamics apart from the quantum potential that it doesn't. In other words, Bohmian mechanics has NO EXPLANATION for signal locality ; it has to be put in there by hand.

So does unitary quantum theory BTW. But the difference is that unitary QM can be made COMPLETELY Lorentz invariant. As such, Minkowski spacetime can be considered, and serves then as an explanation for it. In other words, quantum theory can be considered over Minkowski spacetime. In Bohmian mechanics, as its formulation does not allow for Minkowski spacetime to exist, and there are explicit violations of this formulation to be Lorentz invariant. But SOME expressions must be Lorentz invariant while others aren't. And if you do the mix in the right way - with no good reason as of WHY - you can get out signal locality.
So this framework has much less explanatory power than the combination of Minkowski spacetime and the superposition principle.

 

[DrChinese]

So... still with me? Do we agree that, in principle, a theory can violate Bell Locality (and hence not be consistent with relativity) yet still not support the sending of signals faster than light?

Yes, I guess I can see it conceptually. There could exist tachyon-like particles that do not otherwise interact with currently known particles except for their ability to synchronize chance events at space-like separated spaces. Were this the case, we would presumably need to extend/modify relativity to compensate.