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  #1  
Old 07-26-2012, 11:23 PM
Frylock Frylock is online now
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Backward Causation in Quantum Physics

I just read a nearly twenty year old book by a philosopher which argues that we can keep local hidden variables in quantum physics so long as we allow for backward causation. The measurement problem also goes away if we allow for backward causation. And, of course, he also argues that we should allow for backward causation--that objections against it don't work.

I check the author's website and see he's still active and still pushing much the same line.

In the book he discusses some conversations he's had with physicists by mail, and he implies that at least some of them were at least friendly to his ideas. But that is all I can glean about the status of ideas like this in the actual practice of Physics.

My GQ is, are there physicists who buy into the idea that an interpretation of quantum mechanics which allows for backward causation:

1. ...gives us the possibility of local hidden variables and avoids the measurement problem, and
2. ...is useful or plausible or should be taken on, whether because of 1 or for some other reason

?

Last edited by Frylock; 07-26-2012 at 11:24 PM..
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  #2  
Old 07-26-2012, 11:33 PM
Frylock Frylock is online now
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I should note:

You also get, he suggests, a QM that's compatible with special relativity (he thinks that any model involving simultaneous collapse, such as those models most QM theorists build around entanglement phenomena, are not compatible with special relativity since they require a privileged point of view).

And I think he might have just said backward causation saves locality, not "local hidden variables," but I'm not sure now. I'd have to reread the relevant section.
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Old 07-27-2012, 01:04 AM
Trinopus Trinopus is online now
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I'm not very good at quantum physics, but I have had some college physics that covered it, and I have a decent grasp of the basics...

Accepting "backward causality" is a darn high price to pay, simply to defend "hidden variables," for which there isn't any need anyway.

Hidden Variables is (as I understand it) the idea that there are internal mechanisms that "explain" the random nature of quantum events. For instance, the standard model says that a Uranium atom will decay randomly. It might happen now, or a year from now. All we can really say is that if you have a million Uranium atoms, half of them will decay in a certain definite period of time (the "half life" of Uranium.)

The Hidden Variables idea says that there are little thingamajigs inside the atom that tell it when to decay. A kind of "fuse" that burns down slowly; a little internal "alarm clock" that goes off and make the decay happen.

No one has ever found any evidence for this. The usual QM theories don't require it in any way. They just say, "It's random."

To accept some form of "time travel" -- a signal moving backward in time -- simply to defend Hidden Variables is a really gross complicating assumption. Hidden Variables alone, as a theory, is unnecessarily complicating, but then to throw away the fundamental principle of causality -- that's certainly gilding the crocodile!
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Old 07-27-2012, 02:48 AM
Frylock Frylock is online now
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He thinks that the price is considerably lower than people usually suppose--that, in fact, the possibility of backward causation falls out almost automatically from the fact that physics is (with the exception of one particle) time-symmetric. The apparent massive time-assymetry we seem to see is an artifact of the fact that our local universe has low entropy in one direction (the past) and high entropy in the other (the future). This fact gives us the impression that causation can only go one way. (And in our practical macro-realm, this is basically true.) But it's only an impression, and an inaccurate one.

If almost all physical interactions are time-symmetric (and they are) then when you look at simple micro-physical interactions you might as well say the future state caused the past state rather than the other way around. This doesn't force you to say it, of course. But if, in certain special cases, you do say it, he argues, you get the results of Quantum Mechanics out of a model that avoids all action at a distance, gets around sticky issues like the measurement problem, and generally takes "god's dice" out of the picture (though it doesn't make us privy to the underlying information that gives rise to the "dicelike" appearance of quantum reality. There is backward causation, but not in a way that makes us able to see the future).

I should note that he generally avoids the phrase "backward causation" since he thinks our concept of causation is really only properly applicable at the macroscale (I think!) and the "backward causation" I've been discussing is mostly a micro-physical phenomenon (though of course you can magnify it to the macroscale with the right detector setup). I may be failing to do him justice in a way. He uses the phrase "advanced action" instead of "backward causation". But--my current thinking is that this is a potato/potato issue, that really, what he's talking about is backward causation even if he'd prefer to avoid the phrase.

His argument hinges on a claim that there has been a presupposition underlying physics for which there is no evidence, namely:

At the microphysical level, temporally forward-looking influences are coherent, and temporally backward-looking influences are not.

To explain what that means, I'll discuss a similar idea about the macrophysical level. At the macro level, for any particular event, the most usual course of things is for an event's causes not to be particularly coordinated with each other, while the event's effects are highly coordinated. Throw a stone into a pool, and the effect--the radiating waves--show a high degree of correlation with each other. Not so much the cause of the process--the stone dropping into the pool doesn't show much coordination with anything else.

At the macro level, if you watch a film in which causes are coordinated and effects are not, you soon realize you're watching a film played backwards. What you're seeing is physically possible, but appears bizarrely improbable, becuase we know that at the macro level, coordination of influence goes forward in time, not backward in time.

Price argues that we have generally assumed the same holds at the micro level as well. But, he argues, there's literally no evidence that this is so. Another GQ I have then (which I'm not sure will be answered since it's buried here!) is whether he's right that there's no direct evidence that this principle holds at the micro level.
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Old 07-27-2012, 03:52 AM
eburacum45 eburacum45 is offline
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This sounds like Cramer's 'Transactional Interpretation';
http://en.wikipedia.org/wiki/Transac...interpretation
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The transactional interpretation of quantum mechanics (TIQM) describes quantum interactions in terms of a standing wave formed by retarded (forward-in-time) and advanced (backward-in-time) waves.
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Old 07-27-2012, 06:58 AM
Half Man Half Wit Half Man Half Wit is offline
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The transactional interpretation doesn't have hidden variables, and features a probabilistic wave-function collapse, though, IIRC. Similiarly, the other time-symmetric framework I'm (somewhat) familiar with, the Aharonov-Vaidman two-state-vector formalism, isn't a HV theory either.

As to the specific claim, that you can get quantum (Bell) correlations with backwards causation without violating local realism, I don't have any trouble believing it -- basically, you should be able to prepare any kind of correlation that way, since you can essentially 'cheat' and look at the future outcome, and modify the present outcomes accordingly. (I think that causality is actually a -- perhaps implicit -- assumption in Bell's theorem, but I'm not perfectly sure about that.)

However, as for the time-symmetry of fundamental laws, there's at least one exception, which occurs in weak interaction decays, most famously in that of the neutral Kaon. So, empirically, physics isn't perfectly time-symmetric. And this does have macroscopic consequences: it's widely expected to be the reason for the domination of matter over antimatter in the universe, thus, if the effect didn't exist, there ought to have been exactly as much matter as antimatter produced in the big bang, which subsequently would have annihilated, leaving a universe chiefly populated with radiation.

Also, there's always the danger of causality violations if you admit retrocausality; there's ways around that, but I to me, this makes things sufficiently undesirable to stick with an ordinary notion of causation.
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Old 07-27-2012, 12:50 PM
Half Man Half Wit Half Man Half Wit is offline
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Incidentally, I just noticed that a recent issue of Philosophy Bites features Huw Price talking about his views, if you're interested, you can find it here; I haven't had the chance to listen to it yet, though.
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Old 07-29-2012, 06:48 AM
Half Man Half Wit Half Man Half Wit is offline
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I'm probably just talking to myself here, but another issue I just thought of is how, actually, retrocausality is 'better' than non-locality -- an even A can receive influence from an event B if B retrocausally influences an event C in the mutual past lightcone of both A and B, which then in turn influences A via ordinary causation; or A can influence B by influencing an event D in both A and B's future lightcone, which then retrocausally influences B. So why not allow direct nonlocal influence between A and B?

In fact, I'd expect that in order to get Bell correlation between, say, two photons at A and B, something like the above would have to happen, where measurement at B influences the preparation of the state at C, where C is the source of both photons, in order to insure the right outcome at A...
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Old 07-29-2012, 09:54 AM
Frylock Frylock is online now
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Originally Posted by Half Man Half Wit View Post
I'm probably just talking to myself here, but another issue I just thought of is how, actually, retrocausality is 'better' than non-locality -- an even A can receive influence from an event B if B retrocausally influences an event C in the mutual past lightcone of both A and B, which then in turn influences A via ordinary causation; or A can influence B by influencing an event D in both A and B's future lightcone, which then retrocausally influences B. So why not allow direct nonlocal influence between A and B?

In fact, I'd expect that in order to get Bell correlation between, say, two photons at A and B, something like the above would have to happen, where measurement at B influences the preparation of the state at C, where C is the source of both photons, in order to insure the right outcome at A...
I think he likes it better for reasons of parsimony--backward influence at least gives us a model for influence that we can assimilate to models of influence we already have (just remove the temporally assymetric restriction). He talks a lot about how Einstein hated nonlocality and Bell wasn't a fan and only went for it because he thought it was a forced conclusion, and Price seems to present his idea as a resolution of that problem (he seems to concieve it as a problem anyway).

The advantages are all conceptual, in that the backward-influence model doesn't give you any predictions different than any other model. So what makes for a "conceptual" advantage? (Either an interesting question or an empty question...)

Another conceptual advantage is that the backward influence model obviates any need to solve a "measurement problem." Once again it's a case of preferring parsimony in the sense of preferring a model which posits phenomena for which there are clear analogues in other realms. In this case, instead of there being a mystery as to just what constitutes a "measurement" and how such a thing can have the influences it does in the Copenhagen (and some other) interpretations of QM, we now can just treat measurement intutively as we always have--as simply an epistemological change, a change in the amount and kind of knowledge we have of an independently existing object of measurement.

It has the practical advantage over many-worlds of preventing any more philosophically inclined person from climbing into a Schroedinger machine on the belief that he's destined to experience one of the sets of branching universes in which he survives an arbitrary number of rounds in the machine...

I think I recall that he summarizes a response to the question (why prefer any model, much less this one, if they all give the same predictions) toward the end of the book. I'll take a look.

(One disadvantage I've discovered concerning the Kindle--much harder to simply browse through than a physical book would be...)

Last edited by Frylock; 07-29-2012 at 09:55 AM..
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Old 07-29-2012, 11:00 AM
Half Man Half Wit Half Man Half Wit is offline
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Well, it's probably largely a matter of taste -- to me, it doesn't seem that much of a difference whether A and B influence one another directly (no matter what exactly that's supposed to mean...) or only via some intermediary in either their mutual causal past or future. In both cases, spatially separated events influence one another. And the measurement problem is solved in any hidden variables theory, as the measurement simply reveals the value of the hidden variable, as in any classical theory. (I'm not sure why retrocausal influences would be needed here, incidentally -- other than in setting the values of the HVs.)

I think the problem I have with retrocausality is that either it permits sending information into the past -- but then, one would have to contend with all the classical time-travel paradoxes, which I think is unattractive. Or, it doesn't -- but then, how is it 'retrocausality' in any objective way? You can describe the same physical facts without appealing to the concept...

Nevermind that I think there's lots about time in general, and causality specifically, that we still don't understand properly (or perhaps don't think about in the right way).
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Old 07-29-2012, 07:52 PM
Trinopus Trinopus is online now
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Originally Posted by Half Man Half Wit View Post
I'm probably just talking to myself here . . .
I'm reading attentively! I'm only getting a tiny fraction of what's going on here, alas. The thread QUICKLY surpassed anything I ever learned in undergrad physics!

I was taught that "hidden variables" was dead as phlogiston. Is it still around and kicking?
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Old 07-30-2012, 09:54 AM
Chronos Chronos is offline
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It seems to me that any sensible definition of "local" in itself already implies a lack of backwards causality. Or to put it another way, backwards causality is itself inherently nonlocal.

EDIT: And hidden variables aren't dead per se. We know what the math of quantum mechanics says, but how that math is interpreted is a philosophical question, not a scientific one. Bell proved that any interpretation consistent with the math must do away with some feature or another that we'd consider intuitive, but that doesn't say which of those intuitive features we have to toss out. One possible thing you can toss out is determinism, in which case you don't have hidden variables. Alternately, though, you can throw out locality, and retain hidden variables as long as they're nonlocal hidden variables. There's no way to tell the difference, and thus most physicists nowadays don't bother worrying about the interpretations at all.

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Old 07-31-2012, 02:06 AM
allotrope allotrope is offline
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How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
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Old 07-31-2012, 03:48 AM
iamnotbatman iamnotbatman is offline
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How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
Particles A and B are entangled, in a superposition {UP, DOWN}.

If by non-locality: there is some rule that particles A and B are following, such that if A is measured to be UP, then B will be measured to be DOWN. Even if we don't make a measurement, we know the rule, and can say definitively what would have happened. It turns out that ultimately in order for this to work, for us to be making true statements about things which have not been measured, particle A must communicate instantaneously with particle B to tell it what to do, which violates relativity.

If by non-reality: an example would be the many-worlds interpretation. Here there is no definite attribution you can make to the state of particles A and B before measurement, since they no longer represent the state of a particle in a single definite reality. There is some universe where A will be be UP and B will be DOWN, and another universe in which the opposite happens. There is no need to communicate faster than light, but there is also no single, definite state to which we can assign reality before a measurement takes place.
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Old 07-31-2012, 04:12 AM
allotrope allotrope is offline
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There is no need to communicate faster than light, but there is also no single, definite state to which we can assign reality before a measurement takes place.
Can you please explain this statement?
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Old 07-31-2012, 06:27 AM
iamnotbatman iamnotbatman is offline
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Can you please explain this statement?
In the MWI it makes no sense to claim definite knowledge of the state of a particle that has not been measured, because until measurement you don't even know what universe you will find yourself in. In one universe the particle might be UP, in another it might be DOWN, but until you make a measurement you don't know which universe you are in, and therefore you can make no claim on what the state of a particle is. All you know is what the schrodinger equation tells you, which in this case would be the density of universes in which the particle is UP or DOWN, so that you can make a statistical prediction about what universe you will find yourself after you make a measurement.
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Old 07-31-2012, 04:07 PM
Trinopus Trinopus is online now
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Obviously, this is more a question of opinion than of fact, but, in general, is the Many Worlds Interpretation gaining support among physicists, or is it still pretty much a minority view? I read about it more often as time goes by, which makes me think it may be "catching on."

(In the thread on science as opinion, I mentioned my old cosmology prof, who told about how astronomy conferences used to hold votes on whether people thought Cygnus X-1 was a black hole or not. As time went by, more and more people thought it was. This obviously ain't science, but it is interesting to see the way scientific opinion sways.)
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Old 08-01-2012, 01:59 AM
allotrope allotrope is offline
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In the MWI it makes no sense to claim definite knowledge of the state of a particle that has not been measured, because until measurement you don't even know what universe you will find yourself in. In one universe the particle might be UP, in another it might be DOWN, but until you make a measurement you don't know which universe you are in, and therefore you can make no claim on what the state of a particle is. All you know is what the schrodinger equation tells you, which in this case would be the density of universes in which the particle is UP or DOWN, so that you can make a statistical prediction about what universe you will find yourself after you make a measurement.
I don't really see how that answers the question and I don't see what the probabilities have to do with it either.

You know that one will be up and one will be down. It doesn't matter which or when. The critical point is that as soon as one is measured, the other assumes the opposite value - instantaneously.

If you mean that each particle's value is predetermined in any given universe, and the probabilities apply between universes for any given event, which I assume you do, that certainly makes more sense, at least to me. Except now the number of universes becomes at least partly a function of how many times you do the experiment.

I don't mean to be rude, but that seems more than a bit bogus.

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Old 08-01-2012, 03:11 AM
iamnotbatman iamnotbatman is offline
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You know that one will be up and one will be down. It doesn't matter which or when. The critical point is that as soon as one is measured, the other assumes the opposite value - instantaneously.
Not in the MWI.

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If you mean that each particle's value is predetermined in any given universe, and the probabilities apply between universes for any given event, which I assume you do, that certainly makes more sense, at least to me. Except now the number of universes becomes at least partly a function of how many times you do the experiment.
Doing an experiment leads to decoherence, which indeed, depending on how you define "number of universes", causes their number to increase. You may find this "absurd" or "ridiculous", but I assure you it is not ridiculous if you really understand what is going on.

The point here is that there is a version of "you" in all these different "universes," and until you make a measurement you don't know whether your version of "you" is in the universe with the particle being UP or DOWN. Even after you make a measurement and find that the particle is UP, you cannot say that is was UP in your universe until you made the measurement, because before the measurement the version of "you" corresponding to the universe in which the measurement had been made was not yet anthropically selected. Remember, there is a "you" in all these universes, and you don't know which "you" you are.
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Old 08-01-2012, 03:32 AM
iamnotbatman iamnotbatman is offline
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Obviously, this is more a question of opinion than of fact, but, in general, is the Many Worlds Interpretation gaining support among physicists, or is it still pretty much a minority view? I read about it more often as time goes by, which makes me think it may be "catching on."

(In the thread on science as opinion, I mentioned my old cosmology prof, who told about how astronomy conferences used to hold votes on whether people thought Cygnus X-1 was a black hole or not. As time went by, more and more people thought it was. This obviously ain't science, but it is interesting to see the way scientific opinion sways.)
Decoherence, consistent histories, and MWI (which are all roughly equivalent) are together the preferred interpretation among theoretical physicists. Since I don't want to get into a very long debate about the equivalence of the above interpretations, I will also say that MWI specifically seems to be the slightly preferred interpretation. In my experience a significantly smaller fraction of experimental physicists are MWI-believers. I think that comes from the fact that their education on the subject is little more than osmosis from terrible popular attempts to popularize physics by inaccurately dramatizing the "many worlds" in MWI. "Interpretations of QM" is generally not taught in grad school, so in many cases a NOVA special or a Brain Greene book is a physicist's only exposure to the subject.

All the MWI ultimately is (as well as the decoherence and consistent histories interpretations), is the idea that the Schrodinger equation is correct (bold and wild and absurd, I know). The Copenhagen interpretation, most should agree, is a sad joke, postulating some non-linear and ill-understood collapse-mechanism with no dynamical explanation or relation to the Schrodinger equation, which magically comes into play during a "measurement" (a concept still completely ill-defined in the Copenhagen interpretation) but not at other times. Furthermore, the Copenhagen interpretation leaves out the effect of including the measurement apparatus in the Schrodinger equation, and in this sense is explicitly an incomplete description of reality. The Copenhagen interpretation even lacks both counterfactual-definitiveness and local-realism. It's just a real disaster, and the founders of QM understood this. The idea is to "shut up and calculate", and not worry about the "interpretation", even though they knew the description was ad-hoc. Had the MWI been discovered in the 1920's, I have no doubt it would have become the canonical interpretation. But it was discovered in the late 1950's, after everyone had moved on.
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Old 08-01-2012, 04:01 AM
allotrope allotrope is offline
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Decoherence, consistent histories, and MWI (which are all roughly equivalent) are together the preferred interpretation among theoretical physicists. Since I don't want to get into a very long debate about the equivalence of the above interpretations, I will also say that MWI specifically seems to be the slightly preferred interpretation. In my experience a significantly smaller fraction of experimental physicists are MWI-believers. I think that comes from the fact that their education on the subject is little more than osmosis from terrible popular attempts to popularize physics by inaccurately dramatizing the "many worlds" in MWI. "Interpretations of QM" is generally not taught in grad school, so in many cases a NOVA special or a Brain Greene book is a physicist's only exposure to the subject.
Thank you for the background and insight, but I'm actually more interested in mechanics at the moment.

I had a different image of how the manifold universes were created, being of the believe that they would already have to exist, even if infinite in number.

I see now that you mean instead that a new universe is spontaneously created for each and every quantum event and/or possibility and it comes into existence for the sole purpose of accommodating that event/possibility.

IOW, “our” universe is constantly branching off into an infinite number of other universes at an incomprehensible rate.

So a) is that close to being accurate and b) if so, can you please clarify my amalgam “event and/or possibility” by indicating if one, the other or both items are included?

Last edited by allotrope; 08-01-2012 at 04:03 AM..
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Old 08-01-2012, 06:46 AM
iamnotbatman iamnotbatman is offline
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Thank you for the background and insight, but I'm actually more interested in mechanics at the moment.

I had a different image of how the manifold universes were created, being of the believe that they would already have to exist, even if infinite in number.

I see now that you mean instead that a new universe is spontaneously created for each and every quantum event and/or possibility and it comes into existence for the sole purpose of accommodating that event/possibility.

IOW, “our” universe is constantly branching off into an infinite number of other universes at an incomprehensible rate.

So a) is that close to being accurate and b) if so, can you please clarify my amalgam “event and/or possibility” by indicating if one, the other or both items are included?
Consider the case where your quantum theory has no interactions. You have, say, a single particle's wave function. Suppose it is some continuous function, like a Gaussian, in the position basis. In the MWI, you could say that there is a universe corresponding to each of the uncountably infinite points along the x-axis where the wave function is non-zero. As you time-evolve the wave-function (Schrodinger's equation), it's shape changes, spreads out. Your distribution of universes is changing, but are they "multiplying"? No. Your wave function is normalized to unity. The "number" (if it is remotely sensible to use that word) of universes is constant. Please note, BTW, that when I say "universe" in the above, it is true in just a strictly mathematical sense. There is no need to start worrying that our description is becoming bloated or silly; the point of the MWI is to just observe that a wave function is in some sense mathematically equivalent to a statement about a distribution of particles described by non-interacting delta-functions (the Schrodinger equation is linear), and therefore the use of term "separate universe" to describe each of these delta functions is perhaps an appropriate semantical leap. One way of describing the MWI, is continuous wave function = infinite linear superposition of delta functions.

Now, let's consider the case where our quantum theory has interactions. Each of these infinite separate parts (delta-functions) of each particles' wave function now can interact with each of the infinite separate parts of every other particles' wave functions. After each part interacts with another part, certain possibilities start contradicting other possibilities, due to conservation laws. For example, suppose one (of the infinitely many) parts of a particle's wave function interacts with one (of the infinitely many) parts of another particle's wave function, causing those parts of the wave function to scatter. One part goes one way with momentum p1, then other goes another way with momentum p2. Momentum is conserved. Now any further interactions with those parts of the wave function have to be consistent with that previous interaction. Correlations start developing between different parts of the wave function, and after some interactions, certain parts of the wave function become so uncorrelated with other parts, that they can effectively be called "separate universes". This is the sense in which universes "multiply," and the principle is called quantum decoherence, a principle that by now is completely canonical and agreed-upon among physicists.

So you see it may not be as simple or clear-cut as you described, but hopefully I've clarified questions a) and b). A new universe is not "spontaneously created for each and every quantum event and/or possibility", at least not by magic, it happens as the universe's wave function naturally evolves and interacts with various parts of itself, creating correlations and anti-correlations among different parts of its wave function, effectively isolating them from each other. Each of these parts continue to evolve, and the process continues ad infinitum. If you isolate parts of the universe and don't let them interact at all with other parts, then they "stop multiplying". For example, if you keep Schrodinger's cat in the box, completely isolated from the rest of the universe, then there will never be a split into two universes (one in which the cat is dead, one in which the cat is alive) because you will never open the box so that each part of the cat's wave function can become correlated with parts of your wave function. The various parts of the cat's wave function (dead parts and alive parts) will just continue to evolve in the box. But if you open the box, the dead part can become correlated with a part of you, and the alive part can become correlated with a part of you, and those two parts of you will be anti-correlated with each other so that they cannot interfere with each other and will effectively represent two different universes.
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Old 08-01-2012, 01:52 PM
Half Man Half Wit Half Man Half Wit is offline
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I was taught that "hidden variables" was dead as phlogiston. Is it still around and kicking?
Well, depends on what you mean by 'around and kicking', I suppose. There's for instance the de Broglie-Bohm theory, which is a HV theory that is known to reproduce all predictions of quantum mechanics (at least in the non-relativistic case; the relativistic regime gets a bit more muddled). In principle, you can always adjoin some 'hidden' dynamics and make it fit, if you feel like it; the question is just whether or not it makes much sense to do so. Generally, Bell's and similar theorems put such stringent conditions on the allowed hidden variable theories that you don't really gain much insight or understanding from them -- they necessarily contain so many counterintuitive features that you might just as well accept quantum mechanics as-is (or at least, so a lot of people feel).

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Originally Posted by allotrope View Post
How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
You don't need to appeal to things like the MWI, or specific interpretations in general, to understand this. Basically, entanglement can be considered as 'correlation + superposition', in a sense. Here, 'correlation' just means the following: you have two balls, one red, and one green, and two boxes to put them in. I give you one box, and keep the other; if you open yours, you will instantly know what color the ball in mine will be, no matter where in all of space and time I happen to be. So far, there's no non-locality involved -- the information just stems from the correlation put into the system due to its preparation.

Now, quantum systems can not only be in states like 'red' and 'green', which we can consider 'real states', or 'elements of reality', or 'value definite', but also in combinations of the two, typically written as |red> + |green>, where the weird brackets just mean 'this is a quantum object; tread carefully', for our purposes. It is important to realize that there is no 'underlying reality' to these states in the sense that the state is really |red> (or |green>), and we just describe it in this weird way due to our ignorance of the real fundamental level, but in this description, there actually is no real value to the balls' color property.

So, let's consider the possibilities. In the classical case, the state of the two balls is either |red, green> or |green, red>, where the first entry denotes the color of your ball, and the second the color of mine. Thus, if you open your box, find the ball to be green, you instantly know that the system's state is |green, red> and that thus, my ball must be red, without any nonlocal influence.

Now, in the quantum case, the system can also be in the state |red, green> + |green, red>. This means that neither my ball nor your ball has any definite color, independently of what you find once you check -- there is no element of reality attached to the balls' color. However, once you check, you will find a definite color -- say red -- and again, instantly know the color of my ball, since then, only the state |red, green> is still compatible with the outcome of your checking. But there has again be no nonlocal influence at work, for the same reason there hasn't been any in the classical case; what we have added was the element of superposition, the unique property of quantum systems to not have a definite, real value to all their properties at all times. (Note that I'm not talking about the 'collapse of the wave function/state vector', which could indeed be considered to be something of a distinctly nonlocal flavor. Instead, think of it more in the sense of conditional probabilities: once your knowledge becomes conditioned on your finding a particular color of ball in your box, the other possibility is no longer consistent with your (local) knowledge. Whether you want to consider this in terms of state-vector collapse, wave function branching, or whatever else tickles your fancy does not enter into things at this point.)

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Originally Posted by iamnotbatman View Post
All the MWI ultimately is (as well as the decoherence and consistent histories interpretations), is the idea that the Schrodinger equation is correct (bold and wild and absurd, I know).
This may be putting a too fine point on things, but I think this is overly simplified. Everett's original relative-state interpretation -- which also does not add any excess ontological baggage to the Schrödinger equation -- can well be considered agnostic on the front of whether or not there actually are 'many worlds', and so can a number of other modern interpretations; just considering the Schrödinger equation 'as is' does not automatically lead to the idea of branching universes (in fact, it was only Bryce DeWitt's later popularization of Everett's ideas that introduced the many-worlds concept in the sense of worlds that actually are there in some sense). Personally, I tend to agree with Everett (and with you re Copenhagen, which I've always considered the interpretation for people who haven't really thought about the interpretation of quantum mechanics), but I'm less certain about DeWitt/Deutsch et al.'s many worlds.
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Old 08-01-2012, 02:09 PM
allotrope allotrope is offline
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The Stanford web site has a good overview of MWI but I'm not really inclined to wade through it today. Honestly, the idea of having a universe pop up just for the purpose of avoiding the issue of non-locality seems unduly contorted. Of course perhaps the math sings of its existence like a choir of angels. It wouldn't be the first time they've testified to the bizarre. It's just not the sort of thing that would ever be my first choice.

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Old 08-01-2012, 03:17 PM
SCSimmons SCSimmons is offline
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Originally Posted by Chronos View Post
It seems to me that any sensible definition of "local" in itself already implies a lack of backwards causality. Or to put it another way, backwards causality is itself inherently nonlocal.
Really? I don't see that necessarily follows. Locality, as I understand relativity, demands that events separated by a spacelike interval cannot causally effect each other. If locality is violated, then causality definitely will be, because the temporal order of events with spacelike intervals is reference frame-dependent.

I think that, for the general description of this theory, we don't need to have these sorts of events affect each other. Rather, it's that with two events separated by a timelike interval, the subsequent event can conceivably affect the prior event. (Temporal order of events separated by timelike intervals is not frame-dependent, even though the time difference is.) That violates causality, but not locality.

Or am I misunderstanding something?
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Old 08-01-2012, 04:08 PM
iamnotbatman iamnotbatman is offline
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Originally Posted by Half Man Half Wit View Post
You don't need to appeal to things like the MWI, or specific interpretations in general,
(note: true, but examples can help)

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Originally Posted by Half Man Half Wit View Post
to understand this. Basically, entanglement can be considered as 'correlation + superposition', in a sense. Here, 'correlation' just means the following: you have two balls, one red, and one green, and two boxes to put them in. I give you one box, and keep the other; if you open yours, you will instantly know what color the ball in mine will be, no matter where in all of space and time I happen to be. So far, there's no non-locality involved -- the information just stems from the correlation put into the system due to its preparation.

Now, quantum systems can not only be in states like 'red' and 'green', which we can consider 'real states', or 'elements of reality', or 'value definite', but also in combinations of the two, typically written as |red> + |green>, where the weird brackets just mean 'this is a quantum object; tread carefully', for our purposes. It is important to realize that there is no 'underlying reality' to these states in the sense that the state is really |red> (or |green>), and we just describe it in this weird way due to our ignorance of the real fundamental level, but in this description, there actually is no real value to the balls' color property.

So, let's consider the possibilities. In the classical case, the state of the two balls is either |red, green> or |green, red>, where the first entry denotes the color of your ball, and the second the color of mine. Thus, if you open your box, find the ball to be green, you instantly know that the system's state is |green, red> and that thus, my ball must be red, without any nonlocal influence.

Now, in the quantum case, the system can also be in the state |red, green> + |green, red>. This means that neither my ball nor your ball has any definite color, independently of what you find once you check -- there is no element of reality attached to the balls' color. However, once you check, you will find a definite color -- say red -- and again, instantly know the color of my ball, since then, only the state |red, green> is still compatible with the outcome of your checking. But there has again be no nonlocal influence at work, for the same reason there hasn't been any in the classical case; what we have added was the element of superposition, the unique property of quantum systems to not have a definite, real value to all their properties at all times. (Note that I'm not talking about the 'collapse of the wave function/state vector', which could indeed be considered to be something of a distinctly nonlocal flavor. Instead, think of it more in the sense of conditional probabilities: once your knowledge becomes conditioned on your finding a particular color of ball in your box, the other possibility is no longer consistent with your (local) knowledge. Whether you want to consider this in terms of state-vector collapse, wave function branching, or whatever else tickles your fancy does not enter into things at this point.)
I'm a bit confused what point you are making. Unless you misspoke you seem to be arguing that there is no such thing as a "non-locality" problem. Perhaps you can elaborate.

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Originally Posted by Half Man Half Wit View Post
This may be putting a too fine point on things, but I think this is overly simplified. Everett's original relative-state interpretation -- which also does not add any excess ontological baggage to the Schrödinger equation -- can well be considered agnostic on the front of whether or not there actually are 'many worlds', and so can a number of other modern interpretations; just considering the Schrödinger equation 'as is' does not automatically lead to the idea of branching universes (in fact, it was only Bryce DeWitt's later popularization of Everett's ideas that introduced the many-worlds concept in the sense of worlds that actually are there in some sense). Personally, I tend to agree with Everett (and with you re Copenhagen, which I've always considered the interpretation for people who haven't really thought about the interpretation of quantum mechanics), but I'm less certain about DeWitt/Deutsch et al.'s many worlds.
I disagree; but I think this is largely an issue of semantics. What I am calling "separate universes" are what are largely agreed upon by the physics community to come about in the process known as decoherence. Whether you call these decorrelated parts of the wave function "universes" is semantics, but I think it is silly to avoid the term, since they satisfy the usual definitions one might have for "universe."
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Old 08-01-2012, 04:11 PM
iamnotbatman iamnotbatman is offline
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Originally Posted by allotrope View Post
The Stanford web site has a good overview of MWI but I'm not really inclined to wade through it today. Honestly, the idea of having a universe pop up just for the purpose of avoiding the issue of non-locality seems unduly contorted. Of course perhaps the math sings of its existence like a choir of angels. It wouldn't be the first time they've testified to the bizarre. It's just not the sort of thing that would ever be my first choice.
As I've tried to drive home, that's not how it works. There is no postulate that "a universe pops up." Everything is just a consequence of assuming that the universe is a wave function evolving according the the Schrodinger equation. A consequence, not an assumption.
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Old 08-01-2012, 04:17 PM
allotrope allotrope is offline
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Originally Posted by iamnotbatman View Post
As I've tried to drive home, that's not how it works. There is no postulate that "a universe pops up." Everything is just a consequence of assuming that the universe is a wave function evolving according the the Schrodinger equation. A consequence, not an assumption.
I'll let you step into the cage with Stanford.

from my link

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The fundamental idea of the MWI, going back to Everett 1957, is that there are myriads of worlds in the Universe in addition to the world we are aware of. In particular, every time a quantum experiment with different outcomes with non-zero probability is performed, all outcomes are obtained, each in a different world, even if we are aware only of the world with the outcome we have seen. In fact, quantum experiments take place everywhere and very often, not just in physics laboratories: even the irregular blinking of an old fluorescent bulb is a quantum experiment.
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Old 08-01-2012, 04:24 PM
iamnotbatman iamnotbatman is offline
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Originally Posted by allotrope View Post
I'll let you step into the cage with Stanford.

from my link
Everything in that paragraph is correct except the first sentence. The first sentence is just plain wrong. Completely wrong. Read the paper yourself if you are so inclined (it is fantastic):
http://www.univer.omsk.su/omsk/Sci/E...paper1957.html
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  #30  
Old 08-01-2012, 04:37 PM
leahcim leahcim is offline
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Originally Posted by iamnotbatman View Post
As I've tried to drive home, that's not how it works. There is no postulate that "a universe pops up." Everything is just a consequence of assuming that the universe is a wave function evolving according the the Schrodinger equation. A consequence, not an assumption.
I think part of the problem is that science fiction conjures up a lot of ideas about "parallel universes" that aren't true of the "worlds" of MWI.

In a "universe" something is either in the universe with you or it isn't. If Evil Kirk is in the same universe as Evil Spock, and Evil Spock is in the same universe as Evil McCoy, then Evil McCoy must be in the same universe as Evil Kirk.

But in the Schrodinger's cat experiment, both Live Cat and Dead Cat are both in the same world as us, even though Live Cat and Dead Cat would consider each other to be in different worlds. If MWI's "worlds" were just like SciFi's "universes" this could not happen.

Similarly, if you're in a SciFi parallel universe, you're in that universe forever, eternally completely separated from any other universe that currently exists (barring transporter accidents). MWI's worlds don't entirely work like that -- if all histories in two worlds lead to the same state, the worlds could be said to have recombined entirely.

MWI's worlds are a lot more porous than SciFi's parallel universes.
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Old 08-01-2012, 04:54 PM
Frylock Frylock is online now
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One thing I failed to mention is this.

We all know that the universe is time(/charge/parity)-symmetric except for the neutral kaon. But Price claims that while at the macro-scale, there is an assymetry of entropy, giving us a basis upon which to feel a flow of time, nevertheless at the micro-scale, there is not generally such an assymetry. What he argues that this implies is that, while on the macro-scale correlated events generally trace to an event that lies in the past, on the micro-scale correlated events might trace to an event that lies either in the past or the future.

And in the book, Price points out that if people had arrived at this understanding of time-symmetry on the micro-scale before quantum physics, they would have predicted exactly the kinds of strange phenomena quantum mechanics predicts, precisely because of the backward influence that exists in a time-symmetric world. QM would have been seen as confirming things already known, rather than as seeming to introduce new and spooky metaphysics. (If you think the backward influence itself is spooky metaphysics, remember all this is supposed to be in the context of seeing things, fundamentally, "block universe" style, with no objective flow of time. Not popularly intuitive, but far from spooky as it's an idea with a fairly old provenance.)

In this way, QM interpreted as involving backward influence can be seen to cohere with the rest of physics in a particularly parsimonious way.

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Old 08-01-2012, 05:02 PM
Half Man Half Wit Half Man Half Wit is offline
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Originally Posted by allotrope View Post
The Stanford web site has a good overview of MWI but I'm not really inclined to wade through it today. Honestly, the idea of having a universe pop up just for the purpose of avoiding the issue of non-locality seems unduly contorted. Of course perhaps the math sings of its existence like a choir of angels. It wouldn't be the first time they've testified to the bizarre. It's just not the sort of thing that would ever be my first choice.
Well, I've tried to give an explanation that doesn't appeal to many worlds above...

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Really? I don't see that necessarily follows. Locality, as I understand relativity, demands that events separated by a spacelike interval cannot causally effect each other.
And if you have backwards causation, they can: A and B are spacelike separated, and A influences B via exerting retrocausal influence on C, an event in A's and B's mutual causal past, which in turn influences B; picture two photons of an EPR pair, the measurement of one influences the preparation of the state at the source, which in turn influences the total state (and consequently, the other photon).

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I'm a bit confused what point you are making. Unless you misspoke you seem to be arguing that there is no such thing as a "non-locality" problem. Perhaps you can elaborate.
Not sure what's confusing, although I don't know what exactly you mean by nonlocality 'problem'... It's unambiguous that there is no local realistic model that reproduces the predictions of quantum theory, so you have to reject one or the other: either get a model that has definite values to all properties, which has to include nonlocal influences in order to reproduce Bell correlations, or abandon those definite values (or 'counterfactual definiteness').

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I disagree; but I think this is largely an issue of semantics. What I am calling "separate universes" are what are largely agreed upon by the physics community to come about in the process known as decoherence. Whether you call these decorrelated parts of the wave function "universes" is semantics, but I think it is silly to avoid the term, since they satisfy the usual definitions one might have for "universe."
All I mean is just that there are Everett-like interpretation in which there are multiple copies of 'me' experiencing different outcomes of branching events (which I would call genuinely 'many worlds'), and some in which that isn't the case.
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Old 08-01-2012, 05:04 PM
Chronos Chronos is offline
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Quoth SCSimmons:

Really? I don't see that necessarily follows. Locality, as I understand relativity, demands that events separated by a spacelike interval cannot causally effect each other.
Except that, in a typical topology, even if A and B are spatially separated, you can just find some event C that's in the common past (or equivalently, common future) of both A and B, and therefore timelike separated from both. Then just go backwards from A to C, and forwards from C to B, and you've got A and B influencing each other.

Half Man Half Wit, you're correct that, in the experiments you described, there's no need to invoke non-locality (or anything else equivalently weird). That only comes in when you do slightly more complicated experiments. Let's transition to a real quantum mechanical example: You've got two photons entangled so as to be in opposite polarization states. If you have vertical polarizers in front of both of your detectors, you'll pass the photon through and detect it in one of them, and so if you detect your photon, the other guy won't detect his, and vice-versa. Likewise, if one detector has a vertical polarizer and the other has a horizontal polarizer, then either both photons will be detected, or neither will, so again, as soon as you make your measurement, you know the results of the other one. So far, we still don't need anything weird. Where the weirdness comes in is if you have the two polarizers at some intermediate angle, say, one vertical and the other diagonal. Now, based on the results of your experiment, you can't predict for sure whether the other guy will detect the other photon, but you can calculate the probability that he'll detect it. But what Bell proved is that the probabilities you'd calculate from quantum mechanics couldn't possibly be the result of any local hidden variables in the particles. And the experiment has actually been done, and agrees with the predictions of quantum mechanics.

Finally, I wouldn't say that the MWI is the dominant interpretation among physicists. The plurality interpretation among physicists, and possibly the majority, is the "shut up and calculate" interpretation, which is to say not bothering with any particular interpretation at all.
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Old 08-01-2012, 05:10 PM
Frylock Frylock is online now
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But what Bell proved is that the probabilities you'd calculate from quantum mechanics couldn't possibly be the result of any local hidden variables in the particles.


Ahem assuming the influence involved in these hidden variables goes only forward in time

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Old 08-01-2012, 05:26 PM
iamnotbatman iamnotbatman is offline
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Not sure what's confusing, although I don't know what exactly you mean by nonlocality 'problem'... It's unambiguous that there is no local realistic model that reproduces the predictions of quantum theory, so you have to reject one or the other: either get a model that has definite values to all properties, which has to include nonlocal influences in order to reproduce Bell correlations, or abandon those definite values (or 'counterfactual definiteness').
I may have misunderstood you, but you seemed to be repeatedly emphasizing, in your own words "there has again be no nonlocal influence at work". It sounded like you were arguing that the model you were describing was not nonlocal. I found this odd, and didn't really get what point you were making. Chronos also seems to have noticed this in his most recent post responding to yours.

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All I mean is just that there are Everett-like interpretation in which there are multiple copies of 'me' experiencing different outcomes of branching events (which I would call genuinely 'many worlds'), and some in which that isn't the case.
OK, I guess I'm not familiar with those. Or I have a different interpretation of those interpretations than you.
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  #36  
Old 08-01-2012, 05:26 PM
allotrope allotrope is offline
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Let's transition to a real quantum mechanical example:
I would have gone with the quantum eraser experiment myself.
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Old 08-01-2012, 05:35 PM
iamnotbatman iamnotbatman is offline
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Finally, I wouldn't say that the MWI is the dominant interpretation among physicists. The plurality interpretation among physicists, and possibly the majority, is the "shut up and calculate" interpretation, which is to say not bothering with any particular interpretation at all.
In my experience the preference of theorists and astrophysicists for MWI is near a majority ('dominant' would be too strong a word), while experimentalists are universally in the "shut up and calculate" camp. Overall, I would agree it is not dominant.

Here is an interesting quote from Tegmark:

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Originally Posted by Max Tegmark
At the quantum mechanics workshop to which theseproceedings are dedicated, held in August 1997 atUMBC, the participants were polled as to their preferred interpretation of quantum mechanics. The results are shown in Table 1.

Interpretation Votes
Copenhagen 13
Many Worlds 8
Bohm 4
Consistent Histories 4
Modified dynamics (GRW/DRM) 1
None of the above/undecided 18

Although the poll was highly informal and unscientific (several people voted more than once, many abstained, etc), it nonetheless indicated a rather striking shift in opinion compared to the old days when the Copenhagen interpretation reigned supreme. Perhaps most striking of all is that the Many Worlds interpretation (MWI), proposed by Everett in 1957 but virtually unnoticed forabout a decade, has survived 25 years of criticism and occasional ridicule to become the number one challenger to the leading orthodoxy...
Note: I would include "Consistent Histories" in with "Many Worlds", as I think they are equivalent, but that is debatable.
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Old 08-01-2012, 05:59 PM
Half Man Half Wit Half Man Half Wit is offline
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Half Man Half Wit, you're correct that, in the experiments you described, there's no need to invoke non-locality (or anything else equivalently weird). That only comes in when you do slightly more complicated experiments. Let's transition to a real quantum mechanical example: You've got two photons entangled so as to be in opposite polarization states. If you have vertical polarizers in front of both of your detectors, you'll pass the photon through and detect it in one of them, and so if you detect your photon, the other guy won't detect his, and vice-versa. Likewise, if one detector has a vertical polarizer and the other has a horizontal polarizer, then either both photons will be detected, or neither will, so again, as soon as you make your measurement, you know the results of the other one. So far, we still don't need anything weird. Where the weirdness comes in is if you have the two polarizers at some intermediate angle, say, one vertical and the other diagonal. Now, based on the results of your experiment, you can't predict for sure whether the other guy will detect the other photon, but you can calculate the probability that he'll detect it. But what Bell proved is that the probabilities you'd calculate from quantum mechanics couldn't possibly be the result of any local hidden variables in the particles. And the experiment has actually been done, and agrees with the predictions of quantum mechanics.
Yes, I'm not questioning that. But there is no non-local influence at work in Bell correlations if you do not assume counterfactual definiteness, or whatever else you want to call it. There's no 'action at a distance'. (There's not even any interaction Hamiltonian!) What you have is non-factorizability: you can't describe those experiments using product distributions. This may be considered some kind of 'holism' or 'non-separability', but that's not the same as nonlocality in, for instance, Bohmian mechanics, where the quantum potential actually and causally changes its value in a non-local fashion. Typically, the meaning of non-locality in these discussions is that you measure at point A, and as a result of this measurement, the distribution at point B instantaneously changes.

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I may have misunderstood you, but you seemed to be repeatedly emphasizing, in your own words "there has again be no nonlocal influence at work". It sounded like you were arguing that the model you were describing was not nonlocal.
Well, there is no nonlocal influence: i.e. what happens at A does not 'change' anything at B. In a sense, the description is nonlocal, in so far as there is no description that takes into account only the subsystems on their own that reproduces quantum mechanics, but I think this is a very different kind of nonlocality from the kind we mean when we talk about 'nonlocal influences'.
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Old 08-02-2012, 02:15 AM
iamnotbatman iamnotbatman is offline
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Well, there is no nonlocal influence: i.e. what happens at A does not 'change' anything at B. In a sense, the description is nonlocal, in so far as there is no description that takes into account only the subsystems on their own that reproduces quantum mechanics, but I think this is a very different kind of nonlocality from the kind we mean when we talk about 'nonlocal influences'.
Your response emphasizing that there is no nonlocal influence (without ever mentioning the lack of counterfactual definitieness) was in response to the question:

Quote:
How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
So I think it would be worth emphasizing in the example you gave the reason you have to abandon counterfactual definiteness if your are explaining the behavior of particles via a local description.
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Old 08-02-2012, 03:13 AM
Half Man Half Wit Half Man Half Wit is offline
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So I think it would be worth emphasizing in the example you gave the reason you have to abandon counterfactual definiteness if your are explaining the behavior of particles via a local description.
I think we must be talking past each other... I have repeatedly emphasized that there is no measurement-independent reality, i.e. no 'real value', to be assigned to a state in superposition, so I'm not sure where the problem lies.
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Old 08-02-2012, 03:25 AM
iamnotbatman iamnotbatman is offline
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I think we must be talking past each other... I have repeatedly emphasized that there is no measurement-independent reality, i.e. no 'real value', to be assigned to a state in superposition, so I'm not sure where the problem lies.
It seems you are positing that fact rather than showing it to be necessary in the case of a local description. Obviously in various HV models there is a measurement-independent reality even for what in QM can be described by the word "superposition." So your simply defining that "superposition" = "lack of counterfactual definiteness" does not really address the question you were responding to.
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Old 08-02-2012, 03:38 AM
Half Man Half Wit Half Man Half Wit is offline
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It seems you are positing that fact rather than showing it to be necessary in the case of a local description.
Of course I'm positing that fact -- that's what the question was! ('How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness)...?')

The work in showing it to be necessary in case of a local (realist) description was already carried out by Bell...
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Old 08-02-2012, 03:47 AM
iamnotbatman iamnotbatman is offline
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Of course I'm positing that fact -- that's what the question was! ('How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness)...?')

The work in showing it to be necessary in case of a local (realist) description was already carried out by Bell...
Gah. It seemed to me the question was asking for an explanation rather than a "Bell already showed it" sort of comment, but whatever, this is not a productive dialog.
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Old 08-02-2012, 05:38 AM
Half Man Half Wit Half Man Half Wit is offline
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Gah. It seemed to me the question was asking for an explanation rather than a "Bell already showed it" sort of comment, but whatever, this is not a productive dialog.
Agreed (though in all fairness, that's also not the question you seemed to be answering in your first response). Perhaps allotrope can clarify what he intended to ask.
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Old 08-02-2012, 09:10 AM
allotrope allotrope is offline
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Agreed (though in all fairness, that's also not the question you seemed to be answering in your first response). Perhaps allotrope can clarify what he intended to ask.
Huh?

I asked what he meant by the sentence I quoted.
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Old 08-02-2012, 09:19 AM
Half Man Half Wit Half Man Half Wit is offline
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I meant re your first post in this thread...
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Old 08-02-2012, 10:10 AM
allotrope allotrope is offline
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I meant re your first post in this thread...
Ah, OK.

For the viewers at home:
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Originally Posted by allotrope View Post
How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
My impression and I'm pretty sure it's accurate, is that the 'spooky action at a distance' of entangled particles is generally regarded as a de facto if not de jure violation of locality. Sure, you can't actually use that violation to do anything interesting so meh, no one gets a case of bunched up panties, but even so, it is what it is.

If on some level we happen to be worried about Bell breathing down our necks in addition to Albert, then we can tell him to bugger off as well since we've duly gored ourselves on one of his dilemma's horns.

IOW, all is right with the world, or at least as right as things get round these parts.

But heaven forbid that there should be no controversy, so I was just wondering, 'well, what if we thought it would be more fun to impale ourselves on the other horn?' Let's say we wanted to hang on to the notion of locality and were willing to give up the idea of counterfactual definiteness. How would that play out? How would we still get something like the quantum eraser experiment to work in precisely the same way as it does now by throwing CFD to the wolves instead of locality?

Last edited by allotrope; 08-02-2012 at 10:11 AM..
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  #48  
Old 08-02-2012, 11:57 AM
Chronos Chronos is offline
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Quoth Half Man Half Wit:

Yes, I'm not questioning that. But there is no non-local influence at work in Bell correlations if you do not assume counterfactual definiteness, or whatever else you want to call it. There's no 'action at a distance'. (There's not even any interaction Hamiltonian!) What you have is non-factorizability: you can't describe those experiments using product distributions. This may be considered some kind of 'holism' or 'non-separability', but that's not the same as nonlocality in, for instance, Bohmian mechanics, where the quantum potential actually and causally changes its value in a non-local fashion.
Yeah, that's why I specified "non-locality (or anything else equivalently weird)". You have to do something weird, whether it be non-locality, or lack of definiteness, or "holism", or whatever. There are many different choices one can make for the "something weird" that are all consistent with quantum mechanics.
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  #49  
Old 08-02-2012, 01:40 PM
Indistinguishable Indistinguishable is offline
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Originally Posted by iamnotbatman View Post
Everything in that paragraph is correct except the first sentence. The first sentence is just plain wrong. Completely wrong. Read the paper yourself if you are so inclined (it is fantastic):
http://www.univer.omsk.su/omsk/Sci/E...paper1957.html
That link seems riddled with (minor, but distracting) typos. A better link is here (though, warning, PDF, etc.)
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  #50  
Old 08-02-2012, 04:47 PM
iamnotbatman iamnotbatman is offline
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Originally Posted by Indistinguishable View Post
That link seems riddled with (minor, but distracting) typos. A better link is here (though, warning, PDF, etc.)
Thanks for finding it!
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