Problems with Many Worlds Interpretation

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  • #676
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Free will is a pair of dangling references: free from what? Will of who? Predetermined is a pair of dangling references too: pre, before what? determined by whom?

I like this definition by Klemm:

Analysis of the controversy requires clear definitions of a few terms, which unfortunately are often used colloquially with poor precision. To a degree, such problems are inevitable. Nonetheless, operational definitions are helpful. Free will could be defined in various ways. Will is herein operationally defined here by such synonyms as intent, choice, or decision, and it can be accomplished consciously or subconsciously. Free implies a conscious causation in which an intent, choice, or decision is made among alternatives that are more or less possible of accomplishment and are not constrained by either external or internal imperatives for the embodied brain.

Free will debates: Simple experiments are not so simple
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2942748/pdf/acp-06-047.pdf

I think the core of issue is that the determinism does not imply that everything is 'predetermined' or 'predeterminable'.

Gisin has argued that there is, in fact, no conflict between predetermined properties (realism) and an open future:

Realistic true randomness is some sort of nondeterministic force, or propensity of physical systems to manifest such and such properties under such and such conditions. Realistic random events reflect preexisting properties, as required by realism, simply the reflection is not deterministic; still, the preexisting properties determine the propensities of the different possible events.

Is realism compatible with true randomness?
http://lanl.arxiv.org/PS_cache/arxiv/pdf/1012/1012.2536v1.pdf
 
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  • #677
Hurkyl
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The problem in your claim here is your use of the implication that we "derive behaviors from theory." Physics does not work that way, instead, it works the opposite way: it derives theories from behavior. Your expressed bias is very common for MWI enthusiasts, it is a starkly rationlistic framing of what physics is: the theory is right, the behavior happens because of the theory.
I don't think the word "derive" means what you think it means.

From Merriam-Webster:
to ... obtain especially from a specified source​
In this context, a derivation means we obtain some assertion based on a logical argument from a specified set of hypotheses. e.g. we obtain a claim about behavior from the hypotheses of a theory.

That one can derive X from Y does not entail that X is true, that Y is true, that we already knew X is true before the derivation, or that we were not aware Y is true before the derivation (and all of the dual statements for "false").


If you want to use theories to do anything other than rationalize experience, you have to make derivations. A civil engineer, for example, uses his knowledge of statics* to derive how much weight a bridge can handle without collapsing.

The scientific method itself demands that one make predictions -- that one use the theory to derive specific assertions about how things will behave -- so that the theory may be tested and validated. If one cannot make such derivations from a theory, it's not a scientific theory at all!

*: Yes, I know I'm probably simplifying what's involved


Instead, theories attempt to explain behaviors, we don't start trying to reinterpret behaviors to fit our theories.
Those clauses look to be quite synonymous -- the only difference is what connotations I think you are trying to attach to the words involved.
 
  • #678
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I like this definition by Klemm:
Well that's quite vague, with regard to what constitutes internal imperatives. I have no time to look into this but I would bet that for deterministic beings (or which ever beings he want to declare lacking of free will) he takes the liberty of portraying any decision of being itself as another internal imperative.

Consider this machine:
It doesn't have free will, right? You can tell that it'll just switch itself off.

Now consider a variation of this machine which runs rule 110 cellular automation to determine whenever it wants to switch itself off (it keeps running rule 110 automation and changes one cell depending to the switch, and other cell's value determined the closing or not closing).
Now you can't tell what it will do without running rule 110 or equivalent yourself. You can only lament that it is still deterministic and thus in principle it's behaviour is pre-determined, yet at the core of this machine you have something which escapes any 'pre' determination; you won't determine what it will do prior to running rule 110 for first time. The only thing that it's behaviour is now un-free from, is the rule 110 itself. It'd be silly to describe what this machine does as 'will', but it is most definitely as free as it ever gets (can't be free from yourself, can you?).
 
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  • #679
Ken G
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Sorry, but you're just incorrect. The theory is designed so that all the observed behaviours result from the theory.
The theory is quantum mechanics. This has nothing to do with MWI, because nothing you just said is unique to MWI.
Indeed, the theory is derived from behaviour, but it is derived using highly informal process of human reasoning, involving hunches, guesses, and mental visualization, while the observed behaviours must be produced by theory via a very direct and straightforward calculation (otherwise the theory is demonstrably incorrect or incomplete).
Still all about quantum mechanics, nothing about MWI.
Furthermore a more compact theory is chosen over a less compact theory (for example, Einstein's general relativity would be chosen over someone dull who would be simply fitting polynomial curve to observations, edit: even though former is based on a hunch and a belief that there's something to the equivalence of inertial and gravitational mass, whereas latter was straightforwardly derived from behaviour).
There is no consensus whatsoever that MWI is a "more compact theory" (I believe you mean "more compact interpretation" of a theory). So this point is not relevant.
 
  • #680
Ken G
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If you want to use theories to do anything other than rationalize experience, you have to make derivations. A civil engineer, for example, uses his knowledge of statics* to derive how much weight a bridge can handle without collapsing.
I don't see the importance of this highly semantic point, but I would not choose the word "derive" in that sentence, I would simply use "infer." The meaning would be clear enough either way, but if one is trying to be clear, then "derivations" can be reserved for the act of creating a theory based on tracing the ramifications of certain postulates, and "inferences" can be reserved for applying the theory to reach conclusions about some specific situation where the theory is expected to perform well. What is relevant to the issue of the MWI interpretation is simply that we do not take observed behaviors and try to retrofit them into some interpretation that can be made consistent with them, instead we take the observations as the givens, and ask, "does this interpretation help us understand why we obtain this outcome?" That is precisely where MWI is badly lacking, even though it doesn't make MWI wrong because MWI is still consistent with the observations in the sense that it is not refuted by the observations. That's rather a low bar, however. Still, as I"ve said, it all depends on how rationalistic one wants to be-- if one simply does not care that there is no accounting for what is actually perceived, but there is a lovely mathematical framework for describing a reality that is not refuted by the observations, then one is perfectly happy with MWI. The only error comes when such a person assumes that MWI is likely to be correct in some absolute way that makes it more than just one possible interpretation of quantum mechanics-- that is quite an unlikely assumption, simply on the authority of the history of physics. But there is no harm in using that assumption as a kind of working hypothesis when trying to derive the next theory, I have no problem with that at all because it is a purely personal and subjective choice to make.
The scientific method itself demands that one make predictions -- that one use the theory to derive specific assertions about how things will behave -- so that the theory may be tested and validated. If one cannot make such derivations from a theory, it's not a scientific theory at all!
I don't disagree, but I would not use "derive" in that sentence. When I do a "derivation" for a class, I am never just solving some physics problem. I call that applying a theory. But again, the distinction is only important in some contexts-- my point about not "deriving behavior" is simply about how MWI is an interpretation of a theory in which the theory itself does not force us to change what we think a perception is, or what a prediction of a perception is either; but the interpretation does. So I would say we do not use our theories to derive behaviors, nor to derive predictions, we derive our theories from the postulates that successfully predict behaviors-- but the behaviors require no derivation at all, they merely require observation to determine what we are trying to get the theory to predict in the first place.
Those clauses look to be quite synonymous -- the only difference is what connotations I think you are trying to attach to the words involved.
Yes, it is the connotations that are the key-- to the rationalist, like yourself and Dmytry, those would seem perfectly synonymous. But to an empiricist, there is a world of difference there. It all comes down to what one regards as the proper authority for attributing what a "behavior" is in the first place-- the empiricist feels that behavior is what is observed, period, and the rationalist feels that behavior is what we imagine to be the reasons behind what actually happened. That's why a rationalist will always think that understanding the reasons is understanding the behavior, not realizing that the reasons always change with the next theory. Not that I mean to harsh on rationalists-- when the next theory is derived, it will be derived by someone wearing a rationalist hat, and they will probably take it too seriously as usual!
 
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  • #681
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Well, the correct theory in physics is not formally defined like this:
take observations, apply well defined process, obtain correct theory.
but like that:
take theory, apply well defined process, obtain predictions of observations which match actual observations.

It's unfortunate that MWI got named 'interpretation'. It's not really an interpretation, more of an application of quantum mechanics to the observer.

"the empiricist feels that behavior is what is observed, period"
What is "observed" is the basic qualia in the head. The raw signal your eye sends to your brain perhaps. Anything beyond this is theoretical, "what we imagine to be the reasons behind the qualia". When you see a cube, you experience some retinal stimulation, and the cube is what you imagine to be the reason behind that stimulation. It's not even innate, you had to develop a theory as an infant to imagine the correct reasons for your qualia.
Empiricists usually just draw arbitrary line in sand somewhere, proclaim that what's on the line is really observed, whereas anything to the right of this line is imaginary reasons for what's on the line, and take for granted all the theory on the left, between the basic qualia and this line.
 
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  • #682
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It's unfortunate that MWI got named 'interpretation'. It's not really an interpretation, more of an application of quantum mechanics to the observer.

MWI is a collection of different misapplications of quantum mechanics. Yes, it is unfortunate that got named 'interpretation', but this has the simple explanation of that their proponents (Everett et al) never understood quantum mechanics.
 
  • #683
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MWI is a collection of different misapplications of quantum mechanics. Yes, it is unfortunate that got named 'interpretation', but this has the simple explanation of that their proponents (Everett et al) never understood quantum mechanics.
"Yeah, well, that's just, like, your opinion, man.". QM seem to pretty straightforwardly lead to the person who's observing the phenomena ending up in a superposition of states.
 
  • #684
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Ahh, and by the way:
whereas Copenhagen says that any mixed state is a condition of lack of knowledge (only one state actually occured, we just don't have knowledge of it yet).
No. CI says no such thing. That'd be some form of a hidden variable theory.
 
  • #685
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For most physicists, it is MWI itself that represents the useless philosophical nonsense. I don't go that far-- I say it is simply a valid interpretation of a theory, it only becomes useless philosophical nonsense when it is mistaken for a world view of how some behavior actually happens.

I think there are two very distinct forms of MWI that often get confused. One is the "top down" MWI, and the other is the "bottom up" MWI. TopDownMWI is manifestly unitary (because that is one of its postulates), but it is very far from being manifestly consistent with the empirical content of quantum mechanics. BottomUpMWI is manifestly consistent with the empirical content of quantum mechanics (because that is one of its postulates), but it is very far from being manifestly unitary. Enthusiasts for MWI tend to think that MWI possesses both manifest unitarity and manifest consistency with the empirical content of quantum mechanics, but that isn't true. They can legitimately claim one or the other, but not both - but of course without being able to claim both, MWI is trivial and pointless.


BottonUpMWI simply stipulates all the empirical content of ordinary quantum mechanics as given by the projection postulate, etc., and then imagines that if we stitched together a sufficient multitude of possible sequences of outcomes (probably for the entire universe, since it is unclear whether any sub-systems could be truly isolated, at least for sub-systems of a certain complexity) consistent with this basis, we would arrive at an aggregate that, taken as a whole, is a unitary solution of Schrodinger's equation for the entire universe for some suitable Hamiltonian and boundary/initial conditions. Now, this is a gigantic leap, and we have nothing like a rigorous - or even very plausible - justification for it, nor can we ever get one, because we don't (and can't) know the Hamiltonian and initial conditions of the universe.

TopDown MWI simply stipulates that the universe evolves unitarily in accord with Schrodinger's equation, and then imagines that decoherence will somehow naturally lead to a partitioning of the wave function into essentially distinct components (worlds) within which the experience of a "person" sub-system will so closely approximate the empirical predictions of ordinary quantum mechanics that we cannot (presently, and perhaps not ever) distiniguish them. Note that the mathematics of TopDown MWI are very different from the mathematics of ordinary quantum mechanics. In particularly, there is no projection postulate, there is only the Schrodinger equation, from which it is argued that something approximating the projection postulate with sufficient accuracy for all practical purposes (Bell's FAPP) would appear to some suitable model of an observer. Much effort has been put into trying to substantiate this notion, but I would argue that ultimately it can never really be fully substantiated, basically for the same reason that BottomUpMWI can never be substantiated - without being able to actually write down the Hamiltonian and suitable initial conditions for the universe, or even something as simple as a human being (!) along with a suitable model of consciousness, we can never really demonsrate the connection between unitary evolution and the empirical content of quantum mechanics.

That's why I say MWI is not an interpretation for quantum mechanics, it's an idea for an interpretation of quantum mechanics - and I doubt it can ever be more than that.
 
  • #686
Ken G
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CI says no such thing. That'd be some form of a hidden variable theory.
I have often found that the loudest critics of CI don't understand it at all.
 
  • #687
Ken G
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BottonUpMWI simply stipulates all the empirical content of ordinary quantum mechanics as given by the projection postulate, etc., and then imagines that if we stitched together a sufficient multitude of possible sequences of outcomes (probably for the entire universe, since it is unclear whether any sub-systems could be truly isolated, at least for sub-systems of a certain complexity) consistent with this basis, we would arrive at an aggregate that, taken as a whole, is a unitary solution of Schrodinger's equation for the entire universe for some suitable Hamiltonian and boundary/initial conditions. Now, this is a gigantic leap, and we have nothing like a rigorous - or even very plausible - justification for it, nor can we ever get one, because we don't (and can't) know the Hamiltonian and initial conditions of the universe.
Yes, this is what I think of as MWI. It is not unusual for physics to describe an idealized system, rather than the reality, though many seem to imagine otherwise. So I don't hold it against MWI that it does the same thing-- it imagines a truly isolated system, regardless of how large, and the key postulate is that such a system must evolve unitarily, even if the system includes sentient beings that construct perceptions of sub-worlds in which nonunitary outcomes occur. Since the projection of a pure state can be a mixed state, it is a perfectly natural element of quantum mechanics that subsystems should be describable as mixed states. Where MWI, or any interpretation, comes in is in the interpretation of the meaning of the mixed state-- in particular, the elements of the mixed state that are not perceived by the physicist. MWI simply asserts that the unperceived elements of the mixed state are just as real as the perceived elements-- the "blame" for nonperception lies with the sentient processing agent. Empiricists never think they are to "blame" for their observations, they think their observations are definitively what is real. I see flaws in either perspective, but the empiricist can point to the fact that the observations don't change, and the theories do.
TopDown MWI simply stipulates that the universe evolves unitarily in accord with Schrodinger's equation, and then imagines that decoherence will somehow naturally lead to a partitioning of the wave function into essentially distinct components (worlds) within which the experience of a "person" sub-system will so closely approximate the empirical predictions of ordinary quantum mechanics that we cannot (presently, and perhaps not ever) distiniguish them. Note that the mathematics of TopDown MWI are very different from the mathematics of ordinary quantum mechanics.
I don't see that, to me the MWI enthusiast would always claim that top-down and bottom-up arrive at the same destination because they are both their interpretation of what is really happening.
In particularly, there is no projection postulate, there is only the Schrodinger equation, from which it is argued that something approximating the projection postulate with sufficient accuracy for all practical purposes (Bell's FAPP) would appear to some suitable model of an observer.
There is still the projection postulate, otherwise MWI would be useless. The projection postulate is very simply required any time anyone is using quantum mechanics to do anything practical, and of course any MWI enthusiast will recognize this. They merely think that the projection postulate is a kind of "rule of thumb" that must be traced to some deeper principle that has not yet actually been discovered or elucidated. I imagine it would require better understanding of how intelligence works, but then, I believe that when we have such a better understanding, something quite a bit different from quantum mechanics will emerge from it. If and when such a "next theory" comes along, it is not currently clear which interpretation of QM will retain the greatest relevance. That right there is the reason to retain them all.

Much effort has been put into trying to substantiate this notion, but I would argue that ultimately it can never really be fully substantiated, basically for the same reason that BottomUpMWI can never be substantiated - without being able to actually write down the Hamiltonian and suitable initial conditions for the universe, or even something as simple as a human being (!) along with a suitable model of consciousness, we can never really demonsrate the connection between unitary evolution and the empirical content of quantum mechanics.
I agree that this is the stumbling block, and I share your skepticism that it will ever be overcome. But that doesn't make MWI invalid as an interpretation-- it merely undercuts its claims to being a superior interpretation. All the others have their own issues, and each person who holds to a given interpretation always sees its "issues" as features, while the rest see them as bugs. Vive la difference.
That's why I say MWI is not an interpretation for quantum mechanics, it's an idea for an interpretation of quantum mechanics - and I doubt it can ever be more than that.
I'm not sure any of the interpretations are more than ideas for interpretations. We need some observation to discriminate them, but it must be an observation that QM does not predict, because all the interpretations are consistent with the predictions of QM.
 
  • #688
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I don't see that, to me the MWI enthusiast would always claim that top-down and bottom-up arrive at the same destination because they are both their interpretation of what is really happening.

The two approaches are completely distinct, and there's no warrant for the belief that they arrive at the same destination. You apparently accept uncritically the claim that the bottom-up and the top-down approaches are equivalent, and hence that MWI is a viable interpretation of quantum mechanics, and that the empirical content of quantum mechanics is compatible with a unitary interpretation. I'm saying you accept far too much. There is no proof that top-down and bottom-up lead to the same result, and even most serious proponents of MWI realize this, which is why some of them have devoted many years to trying (without success) to establish that correspondence.

There is still the projection postulate, otherwise MWI would be useless. The projection postulate is very simply required any time anyone is using quantum mechanics to do anything practical, and of course any MWI enthusiast will recognize this.

The projection postulate is only available to someone who either postulates it or else can deduce it from his postulates. The Bottom-Up approach does indeed invoke the projection postulate, but it's not manifestly unitary. The TopDown approach postulates unitary evolution, but isn't manifestly consistent with the projection postulate. Surely you don't really believe that the MWI enthusiast has the right to invoke the projection postulate even if that postulate is inconsistent with his other postulates? You stated that unitarity is the key postulate of MWI. I'm saying that the use of the projection postulate for doing practical quantum mechanics is not manifestly consistent with unitarity. It might conceivably be consistent, but it certainly has not been established.

I'm not sure any of the interpretations are more than ideas for interpretations. We need some observation to discriminate them, but it must be an observation that QM does not predict, because all the interpretations are consistent with the predictions of QM.

It would be more accurate to say that all the putative interpretations aspire to be consistent with the predictions of quantum mechanics, but the question is whether a given putative interpretation actually IS consistent with the predictions of quantum mechanics. There are some minimalist interpretations that can hardly be inconsistent in any very significant way, but MWI (by which you mean TopDownMWI) could conceivably be very inconsistent with the predictions of quantum mechanics... we have no way of even assessing this, because no one can make any actual predictions from the postulates of TopDown MWI. Whenever they make predictions they invoke BottomUpMWI, but then when they claim unitarity it is on the basis of TopDownMWI... with no proof that these are consistent. It's a shell game.

I suspect this all comes back to an earlier discussion, where we concluded that our irreducible difference is that (in my admittedly tendentious paraphrase) you believe ANY idea qualifies as a viable interpretation of quantum mechanics, no matter how half-baked it is, even if it makes no rational sense, and even if there is no clear and definite correspondence between the terms of the putative interpretation and the empirical content of the theory. I honestly think that hardly anyone would agree with your loose criteria for what qualifies as a viable interpretation of a physical theory - not even proponents of MWI. Every scientist and almost every philosopher of science I've ever known would say that a set of ideas qualifies as an interpretation of a physical theory only if the ideas correspond in some clear and definite way to the empirical content of the theory. MWI does not do this.
 
  • #689
Ken G
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You apparently accept uncritically the claim that the bottom-up and the top-down approaches are equivalent,
I don't see that as apparent at all from anything I said. Instead, what I said is that the MWI enthusiast sees them as equivalent, and you have not proven that they aren't. I don't see them as equivalent at all, but it is a matter of opinion-- the distinction is not proven.
and hence that MWI is a viable interpretation of quantum mechanics, and that the empirical content of quantum mechanics is compatible with a unitary interpretation.
Yes, I think there are plenty of MWI enthusiasts who certainly understand QM well enough to know what it predicts.
There is no proof that top-down and bottom-up lead to the same result, and even most serious proponents of MWI realize this, which is why some of them have devoted many years to trying (without success) to establish that correspondence.
But I am perfectly aware that there is no proof the two are equivalent. Your argument appears to hinge on excluding the middle-- you hold that if there is no proof they are equivalent, and there is no proof they are not equivalent, then anyone who says they might be equivalent is adopting an uncritical stance. I don't see that logic. I don't think they are equivalent, but I cannot prove they are not equivalent, so I must allow that they could be equivalent.
The projection postulate is only available to someone who either postulates it or else can deduce it from his postulates. The Bottom-Up approach does indeed invoke the projection postulate, but it's not manifestly unitary. The TopDown approach postulates unitary evolution, but isn't manifestly consistent with the projection postulate.
That is all true. But what you must recognize is the MWI person is usually both of those at once, they merely hold that what is not manifestly true is true all the same. There is nothing illogical in that stance, as long as it is recognized as a kind of hopeful position. A critic might even call it wishful thinking, as I have done above, but that still doesn't make it wrong, it makes it wishful.
Surely you don't really believe that the MWI enthusiast has the right to invoke the projection postulate even if that postulate is inconsistent with his other postulates?
You have not shown that it is inconsistent with the MWI postulates. If it could be shown to be inconsistent, then MWI enthusiasts could not do QM. But they do.
You stated that unitarity is the key postulate of MWI. I'm saying that the use of the projection postulate for doing practical quantum mechanics is not manifestly consistent with unitarity. It might conceivably be consistent, but it certainly has not been established.
I agree it has not been established, but that is not what you said a moment ago-- you said it was inconsistent. What is not established is not the same thing as what is inconsistent.
There are some minimalist interpretations that can hardly be inconsistent in any very significant way, but MWI (by which you mean TopDownMWI) could conceivably be very inconsistent with the predictions of quantum mechanics... we have no way of even assessing this, because no one can make any actual predictions from the postulates of TopDown MWI.
Yes, I agree, we have no way of assessing this. That still doesn't make MWI wrong, because it only aspires to the weak standard of not being manifestly inconsistent with observations. I think that's a low bar, but there is no proof that MWI doesn't get over that bar. It is an interpretation that is held for other, highly rationalistic, reasons, and is held by those who like it as long as there is no direct inconsistency.
Whenever they make predictions they invoke BottomUpMWI, but then when they claim unitarity it is on the basis of TopDownMWI... with no proof that these are consistent. It's a shell game.
That is a valid criticism of the arguments that MWI is a superior interpretation, or even a different theory, than CI or others. I feel that the only valid argument for holding to MWI is that it is a philosophically preferred way to think about the predictions of ordinary QM, generally by rationalists.
I suspect this all comes back to an earlier discussion, where we concluded that our irreducible difference is that (in my admittedly tendentious paraphrase) you believe ANY idea qualifies as a viable interpretation of quantum mechanics, no matter how half-baked it is, even if it makes no rational sense, and even if there is no clear and definite correspondence between the terms of the putative interpretation and the empirical content of the theory.
Well, you have certainly not proven that MWI makes "no rational sense." Indeed, I've argued that the only people it does make sense to are extreme rationalists, so it would be easy to argue that MWI makes the most "rational sense" of all the interpretations, but only in contrast to "empirical sense", not in contrast to "irrational sense." To qualify as a valid interpretation, all you need is someone who is successful at quantum mechanics by applying that way of thinking about it.
I honestly think that hardly anyone would agree with your loose criteria for what qualifies as a viable interpretation of a physical theory - not even proponents of MWI.
Yes, that's probably true-- but then, they all think their own interpretation is superior, and that's the problem-- most people really don't seem to understand what an interpretation actually is, but it becomes a whole lot clearer what an interpretation is when some other theory comes along. Just look at how clear the interpretations of Newtonian mechanics became when quantum mechanics came along. In my view, it is quite clear that an interpretation does not need to be a way of thinking about what reality is actually doing-- it is clearly just a way of thinking about a theory, which does convey some understanding of reality through the successes of the theory, but only through that pathway.

Every scientist and almost every philosopher of science I've ever known would say that a set of ideas qualifies as an interpretation of a physical theory only if the ideas correspond in some clear and definite way to the empirical content of the theory.
Yet it is actually observations that have empirical content, theories just predict and understand the observations, and an interpretation gives us a way to think about what the theory is doing. But there is no requirement for direct connections between the theory and the observations, indeed I would argue that any such direct connections are pure fancy, a fact that the history of physics is quite clear on. Like the fancy of imagining that there really are such things as forces while one is using Newton's laws. There is not a direct connection between a force in Newton's laws, and what happens in an experiment, there is only a demonstrable value in imagining such a connection, and a demonstrable limitation to that imagining. The force of gravity being a particularly appropriate example.
 
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  • #690
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I don't see that as apparent at all from anything I said. Instead, what I said is that the MWI enthusiast sees them as equivalent, and you have not proven that they aren't. I don't see them as equivalent at all, but it is a matter of opinion-- the distinction is not proven.

I would say the burden of proof is on any putative interpretation of a physical theory to prove that its concepts can indeed be consistently applied to yield the predictions of the theory. Since this has never been (and probably can never be) done for MWI, it remains just a vague wishful idea for an interpretation. It's true that many putative interpretations of quantum mechanics (not just MWI) are really just ideas for interpretations, e.g., the transactional interpretation. The foundational issues of quantum mechanics are notoriously swampy. But in other physical theories throughout history the interpretations have been more cogent... which is precisely why we tend to feel dis-satisfied (by comparison) with the interpretations of quantum mechanics.

Yes, I think there are plenty of MWI enthusiasts who certainly understand QM well enough to know what it predicts.

Sure, but the point is they don't understand MWI well enough to know what it predicts. They like to think it predicts the same thing, but believing and espousing something because we "like to think it" isn't very scientific.

I don't think they are equivalent, but I cannot prove they are not equivalent, so I must allow that they could be equivalent.

Yes, I allow that MWI - or rather something LIKE MWI but augmented with important new features to enable us to actually use it instead of just vaguely thinking about using it - could conceivably be consistent with the empirical content of quantum mechanics. But given the present state of affairs, I wouldn't say MWI qualifies as a viable interpretation of quantum mechanics. I think you would say it is, so that's where we differ.

A critic might even call it wishful thinking, as I have done above, but that still doesn't make it wrong, it makes it wishful.

I agree it's not wrong to be wishful - although being wishful about very implausible things isn't terribly sensible - but I contend that it is wrong to be wishful and claim that you are being more than wishful, i.e., for a person to claim that MWI is known to be a viable interpretation of quantum mechanics when in fact it is just a vague idea that he hopes or fantasizes is a viable interpretation.

You have not shown that it is inconsistent with the MWI postulates. If it could be shown to be inconsistent, then MWI enthusiasts could not do QM. But they do.

Here we disagree. When an MWI enthusiast "does QM" he is not making any use at all of MWI. True, he fantasizes that his calculations bear some relation to the idea of MWI, but my point is that he is deluded, because he doesn't have the SLIGHTEST capability of actually performing a calculation or making a prediction legitimately from the postulates of MWI (unitary evolution, etc).

That still doesn't make MWI wrong, because it only aspires to the weak standard of not being manifestly inconsistent with observations. I think that's a low bar, but there is no proof that MWI doesn't get over that bar. It is an interpretation that is held for other, highly rationalistic, reasons, and is held by those who like it as long as there is no direct inconsistency.

I think this discussion gets a bit confused, because TopDownMWI actually is a different theory from von Neuman quantum mechanics, i.e., the postulates and the mathematics actually are different, so at best the claim is that TopDownMWI matches the predictions of von Neuman QM (which are incorporated into BottomUpMWI by postulate) to sufficient accuracy that we can't rule out TopDownMWI based on the empirical success of von Neuman QM, just as general relativity is a distinct theory from Newton's theory, but it's predictions are close enough to explain why Newton's theory seems to work as well as it does. The difference here is that we can actually extract predictions from general relativity, and show that it does indeed reduce to Newtonian predictions in most cases, whereas we are utterly incapable of extracting any predictions from TopDownMWI.

That's where I think we differ - you believe MWI is well-defined and it may be right or wrong (i.e., may or may not be consistent with the empirical content of quantum mechanics), and you're willing to give it the benefit of the doubt until proven inconsistent, whereas I contend that it isn't even well-defined, so it can't even be wrong (let alone right).

Well, you have certainly not proven that MWI makes "no rational sense."

When I say it makes no rational sense I just mean it isn't well defined, and it makes no actual predictions at all.

As always, I have to qualify that remark by saying it refers to TopDownMWI, based on the unitary postulate. I would say your main critique of MWI is actually aimed at the other variant, which I call BottomUpMWI. This takes all of empirical quantum mechanics as a postulate, including the projection postulate, and then for purely rationalistic reasons it proposes to embed this empirical world of our experience conceptually within an uncountable infinity of other such worlds, in each of which ordinary QM is also postulated to be valid, and then asserts that this multiplicity of QM worlds is the real state of affairs. As I read your comments, you don't view this kind of unbridled rationalistic fantasizing very favorably - and neither do I. But it's worth remembering that this applies only to BottomUpMWI, which is not manifestly unitary. Without unitarity - i.e., without being able to say everything just evolves according to the Schrodinger equation - even the most ardant MWI enthusiast would agree that it is trivial and pointless, so they NEED to assert unitarity, but you don't get that from BottomUpMWI. To get unitarity, the MWI enthusiast adopts a completely different theory, based on the unitary postulate, with no projection postulate. This is a mathematically distinct theory, even when restricted to just what an individual observer would experience. But no one is competent to extract any actual calculations from this theory, because we have no way of knowing the applicable Hamiltonians and other constraints. So it's really a fatuous claim.
 
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  • #691
Ken G
Gold Member
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333
I would say the burden of proof is on any putative interpretation of a physical theory to prove that its concepts can indeed be consistently applied to yield the predictions of the theory.
And I don't think any interpretations do that much. It is never the interpretation that yields the predictions, it is always the theory itself that does that. That is why the "shut up and calculate" camp hold that theories require no interpretation whatsoever. Of all the interpretations, the "no interpretation" is on the most solid logical foundation, but it is essentially never actually used because it is simply unsatisfying (not because it is necessary to apply an interpretation to yield the predictions of a theory, but because interpretations convey a sense of meaning to predictions that can easily be made without them.)
But in other physical theories throughout history the interpretations have been more cogent... which is precisely why we tend to feel dis-satisfied (by comparison) with the interpretations of quantum mechanics.
That's true to a point, but I think that's largely because we simply haven't dug as deeply into other interpretations. Why must entropy increase? Because we define entropy in such a way that more likely types of configurations are counted as having higher entropy. So does this mean entropy really increases, or it is just an analysis technique? The interpretation of what entropy is, versus our role in creating the concept, gets swampy pretty quickly. Or take action-- we know that action is minimized in classical trajectories, but what interpretation should we give that? Shall we say that the trajectories happen because they minimize action, or do they happen for some other reason that also explains why they minimize action? And when certain consistencies in the minimization of action give rise to an equivalent concept of physical forces, does this imply that physical forces are what cause action to be minimized, or does the minimization of action simply create an illusion that forces are present? I'd say that even seemingly basic interpretations always get swampy when you poke and prod them enough.
I agree it's not wrong to be wishful - although being wishful about very implausible things isn't terribly sensible - but I contend that it is wrong to be wishful and claim that you are being more than wishful, i.e., for a person to claim that MWI is known to be a viable interpretation of quantum mechanics when in fact it is just a vague idea that he hopes or fantasizes is a viable interpretation.
This hinges on the criteria used to establish the "viability" of an interpretation. You seem to saying that viability requires that as soon as we lay out the details of an interpretation, the predictions of a theory should follow. But that doesn't actually happen. If I interpret acceleration as being due to physical forces, it still does not follow that F=ma. I can tell students that there are these things called forces out there which cause acceleration, but I haven't told them anything they can use-- I still never get F=ma until I assert F=ma, and when I do that, I don't really need the interpretation at all (except to give myself a sense of meaning to what I'm saying, which is quite different from the ability to predict observations).

Here we disagree. When an MWI enthusiast "does QM" he is not making any use at all of MWI.
Neither is anyone who invokes CI. If that were not true, it would be impossible to have a "shut up and calculate" camp. When you look over the shoulder of someone doing a QM problem, and watch the equations they manipulate on their paper, you never get any idea which interpretation is happening in their heads. This is a very important thing about interpretations. When someone writes down the Born rule, for example, we have no idea if they are thinking "and this must be true because it is a central postulate that has no underlying explanation because there is no quantum world", or if they are thinking "this is known to be true in practice, but must have some underlying explanation that emerges from some deeper principle in the many worlds."

That's where I think we differ - you believe MWI is well-defined and it may be right or wrong (i.e., may or may not be consistent with the empirical content of quantum mechanics), and you're willing to give it the benefit of the doubt until proven inconsistent, whereas I contend that it isn't even well-defined, so it can't even be wrong (let alone right).
I don't think it's well defined, I think it's vague enough to be consistent with QM. That's the main difference between QM interpretations and more classical ones-- in QM, the behavior we are explaining is quite vague, so the interpretations inherit that same vagueness. CI is also quite vague, for example-- it's hard to get more vague than "there is no quantum world", even though I think that's a valid insight of Bohr's.
To get unitarity, the MWI enthusiast adopts a completely different theory, based on the unitary postulate, with no projection postulate.
There is still a projection postulate in TopDownMWI, because even the concept of a pure state evolving unitarily must come with a concept of what it means to project onto a subspace (and that result is a mixed state). There is no interpretation necessary at this point, it's all pure quantum mechanics, even shut up and calculate QM. The interpretations only come in when you ask, what does a mixed state mean?
 
  • #692
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There is still a projection postulate in TopDownMWI, because even the concept of a pure state evolving unitarily must come with a concept of what it means to project onto a subspace (and that result is a mixed state). There is no interpretation necessary at this point, it's all pure quantum mechanics, even shut up and calculate QM. The interpretations only come in when you ask, what does a mixed state mean?

With that I completely disagree. The whole point of MWI is to dispense with the projection postulate, and to argue that the -approximate- appearance of QM with a projection postulate emerges purely from unitary evolution, taking decoherence and a bunch of other things into account - but NOT a projection postulate. The mathematics of decoherence is totally different from the mathematics of projection, and the correspondence is acknowledged even by proponents of MWI to be only approximate (i.e., close enough for all practical purposes). The off-diagonal terms of the density matrix are never exactly zero with decoherence. MWI based on the postulate of unitary evolution most definitely does not include a projection postulate - which is why it's consistency with the empirical content of quantum mechanics is not established (and, I argue, can never be established).

It is never the interpretation that yields the predictions, it is always the theory itself that does that.

A genuine interpretation (as opposed to vague hand-waving fantasizing) expresses and entails the theory that it represents, and it does so in a clear, self-consistent, and definite way. There needs to be a clear and definite correspondence between the calculations of the theory and the features of the interpretation. I wouldn't have thought this was controversial. Surely we would not accept just ANY arbitrary idea as a legitimate interpretation of a given physical theory.

Of all the interpretations, the "no interpretation" is on the most solid logical foundation, but it is essentially never actually used because it is simply unsatisfying (not because it is necessary to apply an interpretation to yield the predictions of a theory, but because interpretations convey a sense of meaning to predictions that can easily be made without them.)

I'd say there are different levels of interpretation, and there's no such thing as a "no interpretation", because even the bare theory must assert a correspondence between some terms of the calculations and some aspect of our experience. If it doesn't do this, it isn't a theory at all. This is already a necessary (and sufficient) interpretation. MWI doesn't satisfy this bare minimal requirement, so there isn't much point in going on to consider the higher level aspects of interpretation, which really involve model-building within some conceptual framework that we find appealing for some rationalistic reason, like the die-hard Cartesians who labored to interpret Newton's gravity in a Cartesian context as the shadow effect of a flux of ultra-mundane particles moving at high speed in all directions.

I'd say that even seemingly basic interpretations always get swampy when you poke and prod them enough.

The higher level interpretations, i.e., models, always get swampy, basically because the context of the model is ultimately no more justifiable or explicable than the thing being modeled. The "shadow gravity" example I just mentioned relied on inertia, but ultimately the primitive property of inertia is no more explainable than a primitive force of gravity, so invoking either one the "explain" the other (both have been tried) is sort of pointless.

This hinges on the criteria used to establish the "viability" of an interpretation. You seem to saying that viability requires that as soon as we lay out the details of an interpretation, the predictions of a theory should follow.

Yes, that's right.

But that doesn't actually happen. If I interpret acceleration as being due to physical forces, it still does not follow that F=ma. I can tell students that there are these things called forces out there which cause acceleration, but I haven't told them anything they can use-- I still never get F=ma until I assert F=ma, and when I do that, I don't really need the interpretation at all (except to give myself a sense of meaning to what I'm saying, which is quite different from the ability to predict observations).

An interpretation doesn't exclude the details, it encompases them. An interpretation is a superset of a theory. In other words, it is simply a description of the theory in terms of some context that seems to make sense or be appealing (like the mechanical billiard balls to the Cartesians). The algebraic equation "F=ma" is meaningless until it's terms are usage are defined, at least well enough that someone can check to see whether, in fact, F=ma. This correspondence between the terms of an equation and elements of our experience is what needs to be conveyed, and it is conveyed by an "interpretation". So one way of establishing that correspondence is to "tell students that there are these things called forces out there (which we can quantify in a specified way and call the number F) which cause acceleration (which can quantify in a specified way and call the number a) of a mass (which we can quantify in a specified way and call the number m). Once we've done all this, we have what can be called a fairly minimal interpretation of Newton's second law. The interpretation entails the theory.

Neither is anyone who invokes CI. If that were not true, it would be impossible to have a "shut up and calculate" camp. When you look over the shoulder of someone doing a QM problem, and watch the equations they manipulate on their paper, you never get any idea which interpretation is happening in their heads.

We're talking about two different levels of interpretation. You're talking about model building. I'm talking about the basic bare interpretational statements sufficient to establish the required correspondence between the terms of the calculation and the identifiable elements of our experience. We can dispense with model building (which tends to be pointless anyway), but we can't dispense with the clear and definite correspondence between our calculations and our experience.
 
  • #693
Ken G
Gold Member
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With that I completely disagree. The whole point of MWI is to dispense with the projection postulate, and to argue that the -approximate- appearance of QM with a projection postulate emerges purely from unitary evolution, taking decoherence and a bunch of other things into account - but NOT a projection postulate.
There's always projection, it is just part of quantum mechanics. If you have an entangled pair of particles, say in a Bell state, you still need to be able to talk about the outcomes of measurements on one of the particles (without necessarily identifying which particle if they are indistinguishable). That requires a projection. The projection postulate simply generalizes that crucial requirement to the situation where the entangled system itself includes a measuring apparatus. All interpretations must hold that a subsystem is a projection, and they all must hold that the projection yields a mixed state when part of the system involves sufficient decoherence to be considered a measurement.

That's all true in MWI as well, the only difference is that MWI sees the projection postulate as nothing fundamental, nothing requiring a separate "postulate" to treat, because it is pure quantum mechanics. CI, on the other hand, does not think quantum mechanics is meant to apply to the whole system, it is only meant to apply to the projection, so even though the projection is the same thing (a mixed state), since it is treated as fundamental (and the mixed state is interpreted very differently as a result, it is intepreted as the object of scientific realism), it reaches the level of a core postulate of the interpretation. In MWI, it's just as much a postulate, but now a kind of practical postulate, not a core one. The main point is, you still cannot tell if someone has CI or MWI in their heads when they carry out any QM calculation.

The mathematics of decoherence is totally different from the mathematics of projection, and the correspondence is acknowledged even by proponents of MWI to be only approximate (i.e., close enough for all practical purposes).
Decoherence and projection are two steps in the same mathematical process. One cannot understand what decoherence is without projection, for what is being decohered is the phase relationships between the different projections. That's why any decoherence has its own projective basis, each eigenstate of the observable. At issue is whether a projection shall be regarded as looking at only a piece of the whole, or if it will be taken to throw away everything orthogonal and scale up the amplitude of the projection to renormalize it to unit amplitude once it is registered as an outcome by an observer. The latter is required if the projection is going to be regarded as the new state of the subsystem, as in CI.

CI therefore interprets the amplitude renormalization as a physical process required to correctly treat the reality, whereas MWI interprets the amplitude renormalization as a non-real analysis technique, invoked by physicists but not present in the actual reality. That's it, that's the difference between CI and MWI right there, to my knowledge there is no other difference. This also explains why CI is nonunitary and MWI is unitary, because the amplitude renormalization and the discarding of the orthogonal terms in the projection are both nonunitary, but neither "really" happens in MWI, they are interpreted as illusions generated by the physicist's knowledge or lack thereof. CI thinks the physicist does not generate illusions, he/she generates physics.

The off-diagonal terms of the density matrix are never exactly zero with decoherence.
That is a very separate issue, dealing with the inevitable role of idealization in all physics. No theory of physics is immune to idealization, there's nothing special about decoherence or quantum mechanics that the off-diagonal elements are treated as exactly zero. Nothing anywhere in physics is "exactly" anything, only the idealizations are ever exact.

MWI based on the postulate of unitary evolution most definitely does not include a projection postulate - which is why it's consistency with the empirical content of quantum mechanics is not established (and, I argue, can never be established).
MWI does have a projection postulate, if it didn't no one could call it quantum mechanics. The only difference is how they intepret the meaning of the projection (which connects to some semantic issues around whether or not it is regarded as a core "postulate" of the theory, but it is certainly used either way). The interpretation-independent version of the projection postulate is simply this: certain measurements are regarded as measurements because they have the demonstrated property that outcomes of the measurement are always eigenvalues of the measurement. This arises because the measurement achieves substantial decoherence between the various projected eigenstates, where the projection is from the full system onto the subspace that is regarded as being measured. This is just quantum mechanics, it has nothing to do with any interpretation and without it quantum mechanics isn't quantum mechanics. The interpretations only give us a sense of what that projection means, and what it does not mean, and that is the issue of all the debate.
There needs to be a clear and definite correspondence between the calculations of the theory and the features of the interpretation.
This is our main point of difference-- I hold that no interpretations do that, not of any physics theory at all. Indeed, what tends to happen is the interpretation asserts more than the theory does, and people fail to recognize that they have left the theory and entered the interpretation. This has caused an enormous number of false conclusions throughout the history of science.

Surely we would not accept just ANY arbitrary idea as a legitimate interpretation of a given physical theory.
That's a straw man, there is no question that a lot of physics theorists use the MWI interpretation of QM. All that is required for an interpretation to be valid is that a rational and reasonable expert of some theory uses that interpretation to help them picture what the theory is doing, or how the theory helps them understand the reality it predicts. That's it, that is the sole requirement of a valid interpretation. Were that not so, we'd have to face endless debates about whether students should ever be taught that F=ma, or if they should only be taught the principle of least action. And just what interpretation do we have for the principle of least action, that could be called a "definite correspondence between the calculations of the theory and the features of the interpretation"? All we say is that for some essentially magical reason, action is minimized, and so that's not really much of an interpretation by your standards, yet it is generally viewed as more powerful than interpretations that invoke forces. I just don't think it's that much of a problem for an interpretation to actually be more of an idea for an interpretation.
I'd say there are different levels of interpretation, and there's no such thing as a "no interpretation", because even the bare theory must assert a correspondence between some terms of the calculations and some aspect of our experience.
It really comes down to what "interpreting" is, I agree. Even someone who is shutting up and calculating must assert what it is that they are calculating. But most would reserve the term "interpret" for going farther than just that-- they reserve it for associating some meaning with the predictions. If I predict a function x(t) using classical physics, the shut up and calculate type could say that x(t) is nothing but a prediction for a distance measurement at some clock reading, and there is no meaning to either "space" or "time" that is required to do that calculation and check that prediction. The interpretation takes the next step of giving the meaning that x is a location in space, not just a distance measurement, and t is a time, not just a clock reading. Those are interpretations expressly because they cannot be tested, but they do convey a sense of meaning, some kind of network of associations that convey a sense of understanding. That is the only reason we need interpretations-- we are not happy purely with prediction, we crave understanding. But understanding is subjective, and its only objective test is whether or not someone can get the answer right. I haven't known MWI enthusiasts to get QM answers wrong.

The higher level interpretations, i.e., models, always get swampy, basically because the context of the model is ultimately no more justifiable or explicable than the thing being modeled. The "shadow gravity" example I just mentioned relied on inertia, but ultimately the primitive property of inertia is no more explainable than a primitive force of gravity, so invoking either one the "explain" the other (both have been tried) is sort of pointless.
I agree, and that's why the goals of an interpretation should be rather minimal.
An interpretation doesn't exclude the details, it encompases them. An interpretation is a superset of a theory.
That is the evil of interpretations, it is when interpretations are taken too far and become an obstruction. That's the only problem I have with MWI-- that it gets taken as something more than quantum mechanics. I have the same problem when any other interpretation does that-- it's just not the right way to think about what an interpretation is. It is fine to treat it as a kind of hypothesis for the next theory, but then it should be called a hypothesis, not an interpretation of the previous theory, and indeed we have found that hypotheses should generally be expected to be wrong, but hopefully they are wrong in useful or insightful ways that motivate new discoveries. Thus I feel the right way to debate interpretations of QM is to ask which ones are most useful for generating hypotheses that can motivate new observations and new theories, and that is generally hard to anticipate until it actually happens.
The algebraic equation "F=ma" is meaningless until it's terms are usage are defined, at least well enough that someone can check to see whether, in fact, F=ma. This correspondence between the terms of an equation and elements of our experience is what needs to be conveyed, and it is conveyed by an "interpretation".
This is the fundamental source of our disagreement, we do not have in mind the same purpose for an interpretation. I would say it is not the role of an interpretation to connect the terms of an equation with things that can be measured, that is the role of the theory itself and must work exactly the same in every valid interpretation by definition. So the role of an interpretation is something else-- it is to provide meaning to the terms that have already been connected to observations but whose meaning is unclear. If I use F=ma to solve for x(t) and associate x(t) to distance measurements and clock readings, I am just using some theory. The role of the interpretation is to answer questions like "what is a force" or "what does x and t mean, independently of how they are measured". That's why there is a school that says not to do interpretations at all, they are a kind of delusion (this is probably more or less what Mermin, of "shut up and calculate" fame, would hold). But this is also why interpretations are invariably done-- we want to do more than predict, we want to extract meaning.
 
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  • #694
kith
Science Advisor
1,395
481
A genuine interpretation (as opposed to vague hand-waving fantasizing) expresses and entails the theory that it represents, and it does so in a clear, self-consistent, and definite way. There needs to be a clear and definite correspondence between the calculations of the theory and the features of the interpretation. I wouldn't have thought this was controversial. Surely we would not accept just ANY arbitrary idea as a legitimate interpretation of a given physical theory.
If the CI would meet your standard, there would be no need for alternate interpretations like the MWI. In the CI, I am not allowed to describe the measurement apparatus quantum mechanically. Yet it doesn't say where the applicability of QM ends.

The problem with interpretations of QM is that for many people, there is no satisfying interpretation at all.
 
  • #695
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This is our main point of difference-- I hold that no interpretations do that, not of any physics theory at all. Indeed, what tends to happen is the interpretation asserts more than the theory does, and people fail to recognize that they have left the theory and entered the interpretation. This has caused an enormous number of false conclusions throughout the history of science.



Just concerning interpretations, I would like to clarify things for myself.

Is it not the case that a physical interpretation gives rise to the mathematical model via the thought process of the physicist? The physicist thinks about experiences and how they might be related to each other and as part of that process all sorts of interpretations may arise as to what these experiences really are. So in that sense it is tempting to think that the mathematical model must have a proper mathematical relationship with the original full interpretation, even if that interpretation is not taken seriously.

But if we (say) forgot about f = ma as ever being formulated from any kind of interpretation, rather, suppose as a species we had no desire to think about “meanings” but we took great delight in establishing correlations between quantified experiences. We would experience force and we could build an instrument to quantify that observation, likewise for mass and acceleration. If we then simply looked for correlations between all of the measurements I’m sure eventually we would come up with a result of f = ma.

But it would still be important to track the correspondence those correlations have with the “experiences” (via the purposely designed measurement devices) in order to make use of the correlations. That is the very minimum correspondence required to make the correlations useful. The question then is, would this minimum requirement be classed as an interpretation?

I think this minimum requirement is not an interpretation if we go no further than accepting that the measurement of “force” is no more than an instrument reading that connects with something we experience. If however we start to analyse what “force” actually is then we do get into interpretations, but we could at any time strip away all of that baggage of interpretation and be left with the bare minimum correspondence between “experience” (in terms of the instrument reading) and the mathematical model.

So I think I see what you are saying about interpretations – you are not denying the required correspondence between calculations and experience, rather you are drawing a line in the sand between quantifying an experience and trying to “understand” that experience. The former is “shut up and calculate” physics, the latter is physics with interpretation. The latter plays no part in the usefulness of the calculations and is not required for that role. The former most certainly is required to make use of the physics, but only in terms of the minimum correspondence between the calculations and the measurements (the “experience”) that gave rise to the mathematical model. This is not interpretation, it is raw experience quantified as a measurement.

I picked this quote from another thread by Feynman which I quite liked.

"Many different physical ideas can describe the same physical reality. Thus, classical electrodynamics can be described by a field view, or an action at a distance view, etc. Originally, Maxwell filled space with idler wheels, and Faraday with fields lines, but somehow the Maxwell equations themselves are pristine and independent of the elaboration of words attempting a physical description. The only true physical description is that describing the experimental meaning of the quantities in the equation - or better, the way the equations are to be used in describing experimental observations. This being the case perhaps the best way to proceed is to try to guess equations, and disregard physical models or descriptions."


The Development of the Space-Time View of Quantum Electrodynamics
Richard Feynman, Nobel Lecture, December 11, 1965
 
  • #696
Ken G
Gold Member
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Is it not the case that a physical interpretation gives rise to the mathematical model via the thought process of the physicist?
It seems we have three terms here, the "theory", the "model", and the "interpretation." I would divide these meanings by their roles-- the role of a theory is to make testable predictions given certain idealizations that make the theory tractable, the role of a model is to organize those idealizations, and the role of an interpretation is to provide a sense of meaning to it all. So I would not say that the interpretation gives rise to the model, the model and the interpretation have rather different objectives and you can have either without the other.
The physicist thinks about experiences and how they might be related to each other and as part of that process all sorts of interpretations may arise as to what these experiences really are. So in that sense it is tempting to think that the mathematical model must have a proper mathematical relationship with the original full interpretation, even if that interpretation is not taken seriously.
You seem to envision a form of interpretation that precedes its organization into mathematical expressions. That's a kind of protoypical thinking process, but I mean something more specific for "interpretation" that can only appear after the theory is in a mature form. Essentially the question, "now that we have a good theory that we like, what does it mean, what is it trying to tell us about nature,
and what lessons shall we exract from it?" Those questions don't lead to theories or models, they only come last, and we should not be surprised when they are not unique or not widely agreed on, even when the good theories and the good models are widely agreed on.
But if we (say) forgot about f = ma as ever being formulated from any kind of interpretation, rather, suppose as a species we had no desire to think about “meanings” but we took great delight in establishing correlations between quantified experiences. We would experience force and we could build an instrument to quantify that observation, likewise for mass and acceleration. If we then simply looked for correlations between all of the measurements I’m sure eventually we would come up with a result of f = ma.
Yes, that would be the purest form of "shut up and calculate" physics. It would have all the practical benefits, but most would find it lacking in insight. Others argue the "insight" is illusory.
I think this minimum requirement is not an interpretation if we go no further than accepting that the measurement of “force” is no more than an instrument reading that connects with something we experience. If however we start to analyse what “force” actually is then we do get into interpretations, but we could at any time strip away all of that baggage of interpretation and be left with the bare minimum correspondence between “experience” (in terms of the instrument reading) and the mathematical model.
Yes, that is what I would say as well.
So I think I see what you are saying about interpretations – you are not denying the required correspondence between calculations and experience, rather you are drawing a line in the sand between quantifying an experience and trying to “understand” that experience. The former is “shut up and calculate” physics, the latter is physics with interpretation.
Exactly.

I picked this quote from another thread by Feynman which I quite liked.
Feynman is always brilliantly concise!
 
  • #697
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There's always projection, it is just part of quantum mechanics.

Yes, it's part of quantum mechanics, but it isn't part of MWI. That's the point.

That's all true in MWI as well, the only difference is that MWI sees the projection postulate as nothing fundamental, nothing requiring a separate "postulate" to treat, because it is pure quantum mechanics.

It is a postulate of quantum mechanics, but it isn't a postulate of MWI, so MWI advocates claim that something approximating the appearance of projection arises for an "observer" in MWI, but this claim is unfounded.

The main point is, you still cannot tell if someone has CI or MWI in their heads when they carry out any QM calculation.

As I've said before, to me the concept of "having MWI in your head while you carry about quantum mechanics calculations" is meaningless at best. The calculations of quantum mechanics have nothing whatsoever to do with the idea of MWI.

At issue is whether a projection shall be regarded as looking at only a piece of the whole, or if it will be taken to throw away everything orthogonal and scale up the amplitude of the projection to renormalize it to unit amplitude once it is registered as an outcome by an observer.

That's just the introductory preface to what's at issue. If that alone was the issue, then the whole discussion would be trivial and pointless. The real issue is whether a genuinely coherent theory can be constructed from that first option you mentioned, i.e., from regarding a projection "as looking at only a piece of the whole". In quantum mechanics, when we project down to a definite eigenvector (by the projection postulate) and renormalize to unit amplitude, this then establishes the initial conditions and to some extent the boundary conditions within a suitably defined Hilbert space for the future evolution... but if we do NOT project, and instead simply note that the original state vector of the universe can be decomposed in various ways, and we consider abstract projections of that vector onto various bases, one of which we might place into correspondence with some quantum mechanical world, the issue is how we are to identify such a correspondence and what reason we have for imagining that any such correspondence would persist. And when you talk about throwing away everything orthogonal, remember decoherence doesn't really make the different worlds orthogonal, except approximately. Now, you can argue that the mutual projections are small, but smallness of projections has no meaning once you decide to never renormalize your world-vectors. If you think it though carefully, MWI just collapses (so to speak) into an ill-defined mess with no definite content at all.

There's nothing special about decoherence or quantum mechanics that the off-diagonal elements are treated as exactly zero. Nothing anywhere in physics is "exactly" anything, only the idealizations are ever exact.

The issue isn't exactness or idealizations, the issue is whether unitary evolution taking decoherence into account leads to mathematically identical predictions for an observer as does quantum mechanics with the projection postulate. And the answer is no, it doesn't. The projection postulate results in the system being left in an eigenvector, but unitary evolution with decoherence does not. No one that I know of (outside of this forum) disputes this. What people dispute is whether unitary+decoherence yields predictions that are close enough to be empirically viable. But the mathematics and predictions are definitely distinct.

MWI does have a projection postulate, if it didn't no one could call it quantum mechanics.

MWI definitely does not have a projection postulate. I agree that no one can (legitimately) call it quantum mechanics. (Bear in mind that this refers to TopDownMWI, which is unitary. It is certainly true that BottomUpMWI has a projection postulate, and is observationally equivalent to quantum mechanics - but there's not good reason to think it is unitary.)

That's a straw man, there is no question that a lot of physics theorists use the MWI interpretation of QM.

I would say just the opposite: There is no question that NO physics theorists use the MWI interpretation. Some espouse it, but none USE it, because it is utterly ill-defined and perfectly unusable.

All that is required for an interpretation to be valid is that a rational and reasonable expert of some theory uses that interpretation to help them picture what the theory is doing, or how the theory helps them understand the reality it predicts.

I think that's an *extremely* lax standard for what qualifies as an interpretation, but even with that standard I would say MWI does not qualify, because it doesn't help anyone do or understand anything.

It really comes down to what "interpreting" is, I agree.

Yes, there are operational interpretations, and then there are conceptual models, and I think what you are talking about is conceptual models. There isn't really a sharp line, but we tend to distinguish between what we regard as raw sense perceptions and conceptual models. (Actually, even raw sense perceptions represent conceptual models, but we usually agree on a distinction.) So, for example, we may have an elaborate sequence of "uninterpreted" operational steps to quantify something called "distance" between two entities, and we may choose to encode this within a conceptual model of a 3-dimensional space with a Euclidean metric, and we find that this model (interpretation) works very well. This is an example of a genuine interpretation. It isn't necessarily the only interpretation that could be used to encode and coordinate the quantification and organization of the sense perceptions that we associate with "distance", but it is one that "works". MWI is nothing like this, because it doesn't "work", i.e., it doesn't accurately place our sense perceptions into any correspondence with our calculations.
 
  • #698
Ken G
Gold Member
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Doesn't it bother you that your position that MWI isn't quantum mechanics seems very far away from the views of all the QM experts who hold to the MWI? And by "use" it, all I mean is they use it to motivate the well-known process of doing quantum mechanical calculations-- that's all anyone uses any of the interpretations for, to help them decide what it means while they are doing all the same things. MWI is not a different theory, it is an interpretation of quantum mechanics. As such, all it needs to do is make some claim about what is the meaning of a mixed state, because that's all that any QM interpretations do. It does that also. Now, it is true that some people take MWI, or deBB, or other interpretations, and claim that they are actually different theories that make different predictions, it's just that no one can test the different predictions. I'm not impressed by such an argument, because I don't think any of them are different theories (then your criticisms would come to bear, they are too half-baked to be considered different theories), but I do think that when a truly different theory comes along, it may be inspired by one of those interpretations of quantum theory.
 
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Doesn't it bother you that your position that MWI isn't quantum mechanics seems very far away from the views of all the QM experts who hold to the MWI?

The most prominent advocates of MWI actually have views that are fairly consistent with my position, at least to the extent that they agree unitary evolution by itself (even augmented with a Born rule) is not sufficient to yield an intelligible interpretation, and is not self-evidently even consistent with quantum mechanics. For example, David Deutch explicitly says that some further ingredient is necessary, and that the necessary further ingredient leads to a theory in which a sufficiently sensitive observer actually CAN perceive superpositions - just as many critics of MWI had said from the start. Now, you would probably not call this quantum mechanics any more, you would call it a different theory... but that's my point. Here is one of the most prominent advocates of MWI, and I think you would agree that what he espouses really isn't quantum mechanics. Likewise each advocate of "MWI" seems to mean something different by MWI - and each of them regards all the other flavors of MWI as obvious nonsense (like the three Christs of Ypsilanti).

And by "use" it, all I mean is they use it to motivate the well-known process of doing quantum mechanical calculations...

Yes, I just don't think anyone "uses" MWI to do that. It doesn't motivate any QM calculations. TopDownMWI is devoid of any definite content at all, and BottomUpMWI is nothing but quantum mechanics performed by someone with a "MWI" button on their lapel.

--that's all anyone uses any of the interpretations for, to help them decide what it means while they are doing all the same things.

Hmmm... I'd say that "deciding what the calculations mean" in an operational sense is the role of the low-level interpretation of the terms of the equations, i.e., the bare minimum of establishing the correspondence between the terms and some features of our experience. In contrast, I think the kind of interpretations you have in mind are what I would call models, whereby our messy low-level operational raw processes and perceptions are placed in a rationalistic context of some kind, that makes them easier for our brains to grasp - almost like mnemonic aids - based on how our brains are wired. We seek visceral and spatio-temporal "pictures" in terms with which we are familiar - just the the 18th century Cartesians trying to model Newton's mysterious force of gravity in mechanistic terms of bouncing billiard balls, or the 19th century physicists trying to model electromagnetism in terms of some palpable mechanistic ether. We always try to represent unknown things in terms of familiar concepts - even though those familiar concepts are usually no more self-evident than the new unfamiliar ones. We can tell we're getting into trouble when our efforts to do this lead us to postulate fantastically elaborate contraptions - and usually we eventually decide to abandon our old familiar conceptual framework once it no longer serves a useful heuristic purpose.

I do think that when a truly different theory comes along, it may be inspired by one of those interpretations of quantum theory.

I agree that's possible - even though I'm inclined to think that models are usually backward-looking, i.e., they are attempts to represent new unfamiliar phenomena in terms of old familiar concepts. Often we succeed in fitting the new phenomena into old concepts, with some adjustments perhaps, and so we feel satisfied that we understand it. Ocassionally we can't find a satisfactory representation for new phenomena in terms of old concepts, and we go through a long period of feeling dis-satisfied, like we don't understand it. This happened with the concept of inertia, and with Newton's force of gravity, and with the phenomena of electromagnetism, and so on. In each case there was a long period of reactionary attempts to interpret the phenomena in terms of prior concepts. In this same tradition, I'd say MWI is a very backward-looking attempt (so far unsuccessful) to rationalize quantum phenomena in classical terms.
 
  • #700
Ken G
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The most prominent advocates of MWI actually have views that are fairly consistent with my position, at least to the extent that they agree unitary evolution by itself (even augmented with a Born rule) is not sufficient to yield an intelligible interpretation, and is not self-evidently even consistent with quantum mechanics. For example, David Deutch explicitly says that some further ingredient is necessary, and that the necessary further ingredient leads to a theory in which a sufficiently sensitive observer actually CAN perceive superpositions - just as many critics of MWI had said from the start.
Deutsch is trying to meet certain criticisms of the MWI to make it more acceptable. That is not a requirement of the MWI being a valid interpretation of quantum mechanics, it relates to whether or not it can be regarded as a preferred interpretation. In short, Deutsch is joining those who try to argue that MWI is objectively better than CI because it inspires a set of postulates that are more powerful than CI, and might even make different predictions. Any such claim is premature, and that's where Deutsch encounters his difficulties, not in the simple act of supplying QM with a valid interpretation like F=ma has a valid interpretation as a statement about physically mediated actions at a distance called forces. No interpretation can replace the theory or derive the theory, the theory must always come before its interpretation.

For example, I can very easily give an MWI interpretation that is as valid and consistent with quantum mechanics as CI-- we simply interpret all closed systems as having a Hamiltonian and a wave function (even if we can't stipulate either, that's why it's an interpretation and not a theory, but note CI doesn't stipulate them either so we have changed nothing but our way of thinking) that evolve via the Shroedinger equation. Then we just do everything that CI does when we refer to decohered subspaces of that closed system, except the resulting mixed substates are not viewed as expressions of our ignorance when we apply the concept of a state of the subsystem, they are viewed as the real projections of the full system, so the real states of the subsystem. This holds even when the full system includes physicists doing quantum mechanics, and even after they have perceived a particular outcome. The physicists simply reside in decohered subspaces-- all their thoughts and perceptions are islands of mutually decohered analysis and sensory processing. The "true amplitude" of each of these islands is spectacularly miniscule, but the physicists within them view their processed portion as true, so the miniscule amplitudes are renormalized to unity when treated as conditional amplitudes that are conditioned on the perceptions of that processing agent.

Also, the vastly complex coherences between those conditional amplitudes and the "real" amplitudes are simply ignored by the processing agents, because they have no effect on any predictions or the value of the theory. Nothing the least bit different from a CI-inspired calculation is then done by those processing agents, we simply reinterpret the full reality as that which is unitary, rather than that which is perceived by the individual agents. This is exactly like CI, minus the additional baggage of what is outside the individual processing agents, so I see no difference at all between CI and MWI except for the inverted prioritization about what is perceived and what is unitary, in regard to what is real. Rationalists will always regard the mathematical formalism (unitarity) as what is fundamentally real, empiricists will always regard observations (the perceptions of the processing agents) as what is fundamentally real. Personally, I don't think the phrase "fundamentally real" has any scientific meaning in the first place, so I don't think the differences between MWI and CI have anything to do with science except in regard to how they might help inspire the next theory that one or the other of them might not be a valid interpretation of.

Here is one of the most prominent advocates of MWI, and I think you would agree that what he espouses really isn't quantum mechanics.
Yes, but I don't see Deutsch as using MWI the way an interpretation is supposed to be used, I see him, like many, as mistaking an interpretation of a theory for some kind of world view. We should have long ago dropped the habit of associating successful physics theories with world views, yet we seem to need to relearn that lesson constantly. I feel that Deutsch needs to recast his approach from being an effort to understand reality given what we know now, to what it really is-- formulating new hypotheses that might guide the next theory, in ways that go beyond what we know now so might be wrong, but that's science. None of that should be confused with an interpretation of quantum mechanics, which does not assert anything beyond that theory, any more than the concept of forces asserts anything beyond F=ma.
Likewise each advocate of "MWI" seems to mean something different by MWI - and each of them regards all the other flavors of MWI as obvious nonsense
But that situation is not unique to MWI, almost everyone who has a preferred interpretation of QM believes that! They are all wrong, of course-- the interpretations must be scientifically equivalent or they cannot be correctly called interpretations. Where they differ is purely in their philosophical priorities, which is subjective in the absence of some new theory that actually adjudicates the differences. No such new theory exists, they are all just general directions to theories. So each interpretation can be used to formulate new hypotheses for the attributes of new theories, but then they are no longer interpretations of quantum mechanics.
Yes, I just don't think anyone "uses" MWI to do that. It doesn't motivate any QM calculations. TopDownMWI is devoid of any definite content at all, and BottomUpMWI is nothing but quantum mechanics performed by someone with a "MWI" button on their lapel.
I agree, but I don't think that's a problem-- I view the same as true of CI and Bohmian approaches too. An interpretation is not a theory, it is merely a way to achieve some personally satisfying degree of cognitive resonance while a theory is being used. There simply is no other demonstrable role of an interpretation, I would regard that statement as more or less the definition of an interpretation. Above all, we must recognize that interpretations are not unique, and we should never expect there to be a "correct" interpretation of any physical theory.
Hmmm... I'd say that "deciding what the calculations mean" in an operational sense is the role of the low-level interpretation of the terms of the equations, i.e., the bare minimum of establishing the correspondence between the terms and some features of our experience.
That's not what I mean by "mean". The example I gave above is how x(t) emerges from classical trajectory calculations. To use this function to make predictions, all we need to do is say that x is a distance measurement, and t is a clock reading. That's it, we just need to say how to measure these things, and satisfy ourselves that different people get usefully consistent results for these measurements, and we are done-- we have a mathematical description that makes testable predictions, and we have no interpretation at all. We have "shut up and calculate", or "shut up and measure." But we aren't happy with that, because we have no sense of what x(t) means. So we add additional concepts like space and time, which have no demonstrable connection with classical mechanics and are not at all required to check our theory or to build better mousetraps. They are mental pictures, which we adopt for entirely subjective reasons, and the skeptic is free to dispense with them without incurring any loss of generality in how they do classical mechanics. So interpretations are simply not what you ask them to be.

In contrast, I think the kind of interpretations you have in mind are what I would call models, whereby our messy low-level operational raw processes and perceptions are placed in a rationalistic context of some kind, that makes them easier for our brains to grasp - almost like mnemonic aids - based on how our brains are wired.
That is a good description of what I mean by an interpretation, but it's not what a model is. I see these roles as fairly clear: theories predict measurements, models organize the idealizations needed to make any theory practical, interpretations give us a sense of what the theory means-- what lessons it is trying to tell us, what conceptual messages help understand the theory. But understanding and lessons are subjective and nonunique, and that is not a problem, it is how it is supposed to be.
I agree that's possible - even though I'm inclined to think that models are usually backward-looking, i.e., they are attempts to represent new unfamiliar phenomena in terms of old familiar concepts.
I agree, but again we have to replace your word "model" with my word "interpretation". The goal of understanding is to make contact with what we already know, what we have already found to be useful or mastered in some way. The goal of an interpretation is to achieve that kind of understanding. A model is something different-- a model is like treating the Earth as a sphere or its orbit as a circle, so that we can simplify the calculations our theory requires that we make. The model doesn't just make understanding easier, it has to actually make the calculation easier-- it is a different calculation. Interpretations never change the calculations, they tell us what the calculations mean. That's why two people using different QM interpretations can do the exact same calculations but think in their heads that they are calculating something with a completely different meaning, much like one physicist applying F=ma and another applying the Euler-Lagrange equations.
In this same tradition, I'd say MWI is a very backward-looking attempt (so far unsuccessful) to rationalize quantum phenomena in classical terms.
But that same criticism is leveled by every person who rejects a given interpretation. MWI enthusiasts say CI is backward-looking because it cannot accept that reality might transcend our ability to perceive it, CI enthusiasts say Bohm is backward-looking because it demands we retain the concept of a classical trajectory. This is perfectly normal-- people use different interpretations for subjectively different reasons, which all boil down to what aspects of what they already know do they wish to preserve going forward.
 
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