Problems with Many Worlds Interpretation

[bohm2]

This is way too long of a thread to go through all of it and a quick search of the first author of the paper below didn't bring up anything...here is a fairly recent paper arguing against Everett interpretation:

In other words: POD (Parallel Occurrence of Decoherence) points out the existence of not only one, but two mutually independent and irreducible Brownian particles that are subsystems of the same composite system. As long as this is a consistent quantum mechanical picture, we show that this makes for the apparent inconsistency in the very foundations of the Everett Interpretation. In Section 5 we show that the inconsistency can be removed if there is a privileged spatial structure of the model-universe (only one Brownian particle is physically realistic). The absence of a particular rule/prescription or a criteria for choosing the preferred structure forces us to conclude that the highlighted inconsistency is not removed.

We demonstrate that the Everett Interpretation is not consistent with the universally valid quantum mechanics, as long as the Everett-worlds are considered physically realistic. This inconsistency follows from the recent results of Entanglement Relativity and the Parallel occurrence of decoherence (provided for the Quantum Brownian Motion-like models) as corollaries of the universally valid quantum mechanics. In simplified terms: the Everett worlds splitting (branching) is not allowed for the realistic Everett worlds. Thus, we conclude: Unless there is a privileged, spatial structure (decomposition into subsystems) of the model-universe, Everett Interpretation appears either to be not correct or the Everett-worlds (the Everett ”branches”) are not physically real. The interpretational consequences as well as some ramifications of our findings are yet to be explored.

http://arxiv.org/PS_cache/arxiv/pdf/1109/1109.6424v1.pdf

 

[juanrga]

We can explicitly see how nonunitary dynamics emerges from unitary dynamics for certain model systems. So if one agress, that there is no good reason to assume that measurement devices obey different laws than electrons (which is based on the fact that the classical laws can be derived from QM), the assumption that collapse could be explained in principle by decoherence seems reasonable. Don't you agree?

This doens't lead directly to MWI, however.

Collapse is not derivable from decoherence. Neither decoherence has solved the measurement problem

http://arxiv.org/abs/quant-ph/0112095

The key point is that nonunitary dynamics are irreducible (i.e., are not derivable from) unitary dynamics.

 

[kith]

But that type of collapse never presented any problems, because it is still perfectly unitary on the whole system, and projections onto subspaces never had to be unitary, even in formal Schroedinger evolution.

For me, the important point is that decoherence shifts the measurement problem from pure states to mixed states. Decoherence explains how a system with a well-defined property (beeing the eigenstate of an observable) evolves into a system about which we can make only statistical statements. This can be interpreted in the way that the state of the system doesn't contain enough information to determine a unique outcome. And this is a something classical.

Maybe the crucial point is that the CI assumes reductionism?

The collapse that is at the heart of "the measurement problem" is something different-- it is the nonunitary collapse that occurs when you interpret a mixed state as not a projection from a decohered superposition, but as a particular outcome that has occurred even if you don't know it yet.

How can a mixed state be a particular outcome? Do you use outcome in a different meaning than outcome of a single experiment?

It is a purely philosophical difference, [...]

Yes, I agree. But a proponent of the Lorentz ether theory can use the same argument to claim that his view should be treated equal to SRT. Yet the overwhelming majority of physicists thinks SRT is the better interpretation. We agree that interpretations can't be proven, but still some are more plausible than others. And if there's an unambiguous reasonable way to motivate the appearance of collapse from the other axioms, most people are probably willing to adopt this view.

 

[juanrga]

This is way too long of a thread to go through all of it and a quick search of the first author of the paper below didn't bring up anything...here is a fairly recent paper arguing against Everett interpretation:http://arxiv.org/PS_cache/arxiv/pdf/1109/1109.6424v1.pdf

Thanks by the link.

What I do not understand is why it is needed to show that the Everett interpretation is internally inconsistent and does not agree with experiments when this has been done in the past in many occasions.

That is so weird for me as if it was needed to prove today that the ancient Weber electrodynamics suffers from the same kind of defects. If you look to Goldstein textbook, in some footnote page he merely says that Weber electrodynamics is inconsistent but he does not need to prove this (neither its failure to explain experiments) because it was already done in the past.

 

[kith]

juanrga, I think our views are quite incompatible when it comes to interpretations of QM, because you don't seem to acknowledge any other interpretation than the orthodox one. So I don't see the point in discussing with you here.

However, thanks for the paper. I have saved it, but there are other decoherence-related paper's I am going to read first.

 

[juanrga]

juanrga, I think our views are quite incompatible when it comes to interpretations of QM, because you don't seem to acknowledge any other interpretation than the orthodox one. So I don't see the point in discussing with you here.

However, thanks for the paper. I have saved it, but there are other decoherence-related paper's I am going to read first.

I know a physicist who does not accept Maxwell electrodynamics, neither SR or GR; affirms that are nonsensical theories, that would be abandoned by 'rational' minds; and promotes the outdated, inconsistent, and falsified weber electrodynamics (see also #664) as the 'rational' theory that every physicist would embrace.

He has been shown to be completely wrong in many occasions, but he cannot accept this and ignoring any criticism he has extended this kind of wrong theory to gravity as well. His theory of gravity is even poor and, as today, it cannot explain simple stuff as Mercury perihelion anomaly and light bending at once (although his theory contains a free parameter) unless you abandon energy conservation principle

http://arxiv.org/abs/gr-qc/9708047

I have discussed with him in the past and showed many mistakes in his papers and books, but he ignores any criticism, because he has philosophical prejudices about how nature reveals itself in experiments.

I find the same kind of attitude regarding QM.

Some few people has philosophical prejudices about how nature reveals itself in experiments and want to substitute QM by some other pseudo-theory satisfying their philosophical beliefs/credo because do not accept QM: the theory developed by Bohr, Heisenberg, vonNeumann, Schrödinger... using scientific methods.

This same people is also blind to any experiment disproving their 'theory', do not read the criticism done, neither care about the inconsistencies found in their 'theories'.

This people reveals their own inconsistency when sometimes they claim that MWI is just another interpretation of QM (although the contrary has been shown in literature), whereas sometimes they claim that is a theory that does different predictions than QM (although none of their new predictions have been verified).

Decoherence community is very related to MWI community and make bold claims about deriving nonunitary dynamics from unitary one, about solving the measurement problem and so on. However, all those bold claims are shown to be plain false (when one checks the details on the papers and preprints) again and again.

I have no problem with the possible existence of other 'interpretations' of QM different from that found in main textbooks. I have a problem when some few people claims that MWI (Everett, Deutsch, and friends), ensemble (Ballentine), Bohmian (Broglie, Bohm)... are merely another interpretations of the QM that one finds in textbooks, when none of them is even close.

I am sorry by saying what some people here want to ignore, but well, guys this is the post #666 here

 

[bohm2]

I thought this French physicist's (Gisin) reasons for being against MWI were kinda of cool:

I won't try to present the many-worlds view any further; from the little above it should already be clear that I am not sympathetic with this view. Two reasons.

First, all the assumptions presented in the previous section have an explanatory power. Moreover they could even be experimentally tested (and - even better for me - using technologies available in my lab!). On the contrary, I do not see any explanatory power in the many worlds: it seems to be made just to prevent one from asking (possibly provocative) questions. Moreover, it has built in it the impossibility of any test: all its predictions are identical to those of quantum theory. For me, it looks like a "cushion for laziness" (un coussin de paresse in French).

And there is a second, decisive, reason to reject the many-worlds view: it leaves no space for free will. I know that I enjoy free will much more than I know anything about physics. Hence, physics will never be able to convince me that free will is an illusion. Quite the contrary, any physical hypothesis incompatible with free will is falsified by the most profound experience I have about free will.

Are There Quantum Effects Coming from Outside Space-time? Nonlocality, free will and "no many-worlds"
http://lanl.arxiv.org/PS_cache/arxiv/pdf/1011/1011.3440v1.pdf

And the video version [the latter part (~16:35) discusses his criticism of MWI]. Even if one doesn't agree with him, you got to like this guy's spunk:

https://www.youtube.com/watch?v=9WnV7zUR9UA

 

[Ken G]

For me, the important point is that decoherence shifts the measurement problem from pure states to mixed states.

But this holds in every interpretation of QM, it does not single out many-worlds in any way. What you need an interpretation to do is to interpret the meaning of a mixed state. Many worlds says that any mixed state is a set of real states that we are only going to be able to perceive one, 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). Hence, the many-worlds version is still unitary, but has no accounting for why we perceive only one, whereas the Copenhagen version is nonunitary, but easily accounts for why we perceive only one. Choose your poison.

Maybe the crucial point is that the CI assumes reductionism?

I think one could either hold to, or drop, reductionism with either CI or MWI. Here's how it might look:
CI, drop reductionism: the "collapse" is controlled by holistic factors we have no scientific access to.
CI, keep reductionism: the "collapse" is fundamentally random in character, so there is nothing missing from the reduced perspective-- random is just random.
MWI, drop reductionism: there is no "collapse", but we perceive a collapse because of some holistic connection between all the minds in the many worlds, which somehow "mete out" experiences between them all.
MWI, keep reductionism: there is no "collapse", but we perceive one because each thread of the many-worlds splitting contains the bare ingredients to create a mind within that branch.

How can a mixed state be a particular outcome? Do you use outcome in a different meaning than outcome of a single experiment?

A mixed state is always conceived of as a particular outcome in CI, or indeed any interpretation other than MWI. But by "outcome", I just mean what actually happened, independently of our knowledge of it. If one uses the more restricted meaning that an "outcome" is only what I know happened, then my outcomes can be different ones from yours, so that isn't the kind of meaning I have in mind for the term. MWI, on the other hand, does not recognize our perceptions as being definitive of what actually happens, so in MWI a mixed state comprises a whole bunch of outcomes in a whole bunch of independent (mutually incoherent) worlds. So in MWI, the entries of a density matrix are interpreted as weighting the worlds, whereas in CI, the entries are interpreted as giving the probabilities of what actually occurs.

Yes, I agree. But a proponent of the Lorentz ether theory can use the same argument to claim that his view should be treated equal to SRT. Yet the overwhelming majority of physicists thinks SRT is the better interpretation.

But I have see no contradiction there. Yes, a proponent of LET can definitely make that argument, and be perfectly corrct, and yes, most physicists think SRT is a better interpretation. Those are both facts, no contradiction there at all. Indeed, a century from now something might happen and the majority of physicists will shift over to LET, so it's important to realize that LET is perfectly viable-- it's just not preferred at present. So it is with MWI or any other QM interpretation.

We agree that interpretations can't be proven, but still some are more plausible than others. And if there's an unambiguous reasonable way to motivate the appearance of collapse from the other axioms, most people are probably willing to adopt this view.

Yes, if the theory changes in some way, it might become clear which interpretation was superior, but in the mean time, it's always just going to be a matter of personal opinion-- and some might not care what the majority think, they might still hold to their own view and be perfectly successful in doing so.

 

[Ken G]

I thought this French physicist's (Gisin) reasons for being against MWI were kinda of cool:

And those are perfectly valid reasons for him. But I don't see anything in them that are compelling in general, and indeed it seems to me that he makes the common mistake of confusing an interpretation of a theory for some kind of authoritative world view-- a practice we should have dispensed with centuries ago.

My only objection to MWI is that I feel it is unscientific to take it as a world view, like some kind of true ontology, instead of what it actually is-- an interpretation of the unitarity postulate of QM. But the same can be said for any of the other interpretations as well-- they are each a way of thinking about a theory, that's what an interpretation of a theory is. So it is not a problem that it doesn't make predictions, and it's not a problem that it rules out free will. Interpretations don't make their own predictions, they give you a way to think about the predictions of some theory. And no physical theory rules out free will, any more than Newtonian gravity ruled out black holes. Theories don't rule things out, they make successful predictions. The predictions of QM have really nothing to do with free will, we have no idea how to connect those two things. Thus no interpretations of QM can say the least thing about free will, given our current understanding (or lack thereof) of the latter.

Thus, I claim it is certainly not true that MWI "rules out" free will while CI allows for it-- neither have anything to say on the matter, because the predictions of QM stand squarely between any of these interpretations and free will, so the real question is, what do the predictions of QM say about free will? I believe the answer to that is "nothing at all at present", but even if someone does think they have something to say, it will be the same statement that all the interpretations make-- at least as long as the interpretations are treated as what they should be (ways to think about the predictions of a theory, not demands on how what is currently unknown will have to work out). I could easily imagine some "next theory" that allowed for both many worlds and free will, the obvious example being the will to choose among the worlds.

 

[juanrga]

I thought this French physicist's (Gisin) reasons for being against MWI were kinda of cool:
Are There Quantum Effects Coming from Outside Space-time? Nonlocality, free will and "no many-worlds"
http://lanl.arxiv.org/PS_cache/arxiv/pdf/1011/1011.3440v1.pdf

And the video version [the latter part (~16:35) discusses his criticism of MWI]. Even if one doesn't agree with him, you got to like this guy's spunk:

https://www.youtube.com/watch?v=9WnV7zUR9UA

It is not needed to appeal to some kind of personal experience for disproving determinism. A simple analysis of the scope of science and a review of the available theories would be enough.

When cosmologists as Sean Caroll state

http://blogs.discovermagazine.com/cosmicvariance/2011/12/05/on-determinism/

My personal suspicion is that the ultimate laws of physics will embody something like the many-worlds philosophy

them fail to consider the simplest technical details of the theories (classical mechanics, chaos, QM...) that he is naming

http://blogs.discovermagazine.com/c...on-determinism/comment-page-1/#comment-202663

and fails to consider even the most direct philosophical consequences derived from these details

http://blogs.discovermagazine.com/c...on-determinism/comment-page-1/#comment-203643

 

[bohm2]

It is not needed to appeal to some kind of personal experience for disproving determinism.

If I'm understanding you, I agree. I think the issue of determininsm/indeterminism has very little to do with the issue of free will. Even if the universe is non-deterministic, I can't see how that would help the "free will" position anymore than a deterministic universe. Wouldn't that just lead to some sort of "random will" versus truly "free will"? In my opinion, the strongest argument put forth for the possibility of "free will" is positions that are able to challenge the following premise by Carl Hoefer:

For reasons that Kant first realized, indeterminism at the microphysical level does not seem to help. The randomness, if any, in microscopic phenomena does not seem to “make room” for free will, but rather only replaces a sufficient physical cause with (at least in part) blind chance. The presumption in favor of upward causation and explanation (from microphysical to macrophysical) that comes with causal completeness is what cuts free agency out of the picture, whether this causation is deterministic or partly random.

Some have argued Bell's theorem and violations does do this by allowing some type of bi-directional causality, etc. but I'm not sure?

The classical picture offered a compelling presumption in favour of the claim that causation is strictly bottom up-that the causal powers of whole systems reside entirely in the causal powers of parts. This thesis is central to most arguments for reductionism. It contends that all physically significant processes are due to causal powers of the smallest parts acting individually on one another. If this were right, then any emergent or systemic properties must either be powerless epiphenomena or else violate basic microphysical laws. But the way in which the classical picture breaks down undermines this connection and the reductionist argument that employs it. If microphysical systems can have properties not possessed by individual parts, then so might any system composed of such parts...

Were the physical world completely governed by local processes, the reductionist might well argue that each biological system is made up of the microphysical parts that interact, perhaps stochastically, but with things that exist in microscopic local regions; so the biological can only be epiphenomena of local microphysical processes occurring in tiny regions. Biology reduces to molecular biology, which reduces in turn to microphysics. But the Bell arguments completely overturn this conception.

For Whom the Bell Arguments Toll
http://faculty-staff.ou.edu/H/James.A.Hawthorne-1/Hawthorne--For_Whom_the_Bell_Arguments_Toll.pdf

 

[Dmytry]

Here's the problem though. Physics is not a purely rationalistic endeavor-- we don't just introspect the mathematical aesthetics that we like. Instead, we must check our theories against experiment, and MWI simply provides no accounting for the nonunitarity of what we actually perceive. MWI in effect subjugates the action of the observer to the theory about the observer, rather than using the action of the observer to substantiate the theory. The result is that MWI only requires itself to not contradict observations, but physics is built on something deeper than just not contradicting observations, it is built on receiving evidence from observations. Any number of pseudoscientific claims from ghost stories to UFO landings are based on the weak requirement of merely not contradicting observations, and while MWI certainly has a much more scientific footprint than these, it shares this fundamental problem. But we always know that, it is the default assumption. The sticky problem is how to use the approaches of science to get at a principle that underlies the whole structure of how science is done. Some reworking of what we even mean by science is going to be needed to do that, and it must be done so as not to throw out what is good about science, and what separates it from pseudoscience. Faith that MWI will lead us to that principle is fine for an individual to have, but it is hardly some kind of arguable benefit of MWI.
I think this stems from a common misconception about CI. When Bohr says "there is no quantum world", he means "there is no need to continue to advance physics to try to go deeper than quantum mechanics." I don't think Bohr would ever have suggested such a non-scientific idea. What he really meant, I believe, is that the problem of using physics to describe the observer/system interaction encounters a fundamental difficulty that is independent of quantum mechanics or any particular theory-- the act of observation is a kind of filter, and whatever does not come through that filter is not going to be able to be put into a physics theory. This problem was with us all along, but quantum mechanics is the place where we smacked right into it. MWI is essentially what you get when you try to ignore this problem and hope that it will go away, but the bizarre and almost mystical elements of MWI are the consequence of that attitude. It is really the place where science meets pseudoscience, and all Bohr was trying to do was retain the firewall between them.

Sounds like a very bad argument in favour of retaining non-unitary behaviour rather than trying to derive it from the unitary theory. Yes, the observer effects are fundamental but we didn't try to introduce human eye into the optics, we predict what happens in the human eye.

With regards to 'evidence based' research, there is presently no evidence of wavefunction collapse and other FTL stuff. None whatsoever.

The problem with MWI really is that people can't shut up their useless philosophical nonsense (which has never been shown to be useful in science in the first place, the free will being a great example), and consider a theory that is more compact and doesn't make use of concepts with immense hidden complexity, like that of 'observer'. The best parallel I can find is the 'god' concept, whereby you replace the complexity of the world with complexity hidden into the word 'god'. Likewise, in CI, you replace the complexity of the system that is observer, and the complexity that arises from the Schrodinger's equation, with the verbal concept of 'observer' that seems very simple at the surface.

 

[Ken G]

Sounds like a very bad argument in favour of retaining non-unitary behaviour rather than trying to derive it from the unitary theory.

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. That has never been true in the entire history of science. Instead, theories attempt to explain behaviors, we don't start trying to reinterpret behaviors to fit our theories. But that's just exactly what MWI does when it is taken to the extreme beyond what it actually is: what it actually is is a way to think about the theory, not a way to think about the behavior. The behavior is highly non-unitary, that's how we perceive it. Nothing in MWI changes this fact, it merely gives us a way to rationalize the behavior that we actually perceive. Rampant rationalization is not good science-- but there is certainly a role for rationalization, which is in the generation of new theories. If someone wants to use MWI to create a new theory that is not just quantum mechanics, more power to them, but no one has actually done that yet, so there is no actual demonstrable benefit of the rationalization exhibited by MWI.

Yes, the observer effects are fundamental but we didn't try to introduce human eye into the optics, we predict what happens in the human eye.

What we are talking about actually has nothing to do with the human eye, it has to do with what the human eye is even capable of interacting with, recording, or perceiving in any way at all. That's quite a different matter, and is fundamental to any empirical endeavor-- like physics. That's what Bohr was saying.

With regards to 'evidence based' research, there is presently no evidence of wavefunction collapse and other FTL stuff. None whatsoever.

No, that is incorrect. There is nothing "FTL" about wavefunction collapse, you are again mistaking the behavior for the explanations of the behavior within some theory. That is a very typical problem for rationalists. But collapse is not a theory, it is an observed behavior-- outcomes are eigenvalues of the observables (and that means the theory labels the outcomes as eigenvalues and goes from there). MWI then takes this observed fact and weaves it into a theory that builds onto it whatever scaffolding is necessary to retain the concept of unitarity. It certainly does not act in the opposite direction, in the way it is oversold to do.

The problem with MWI really is that people can't shut up their useless philosophical nonsense (which has never been shown to be useful in science in the first place, the free will being a great example), and consider a theory that is more compact and doesn't make use of concepts with immense hidden complexity, like that of 'observer'.

Again you are missing the crux of the issue. 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. What is certainly demonstrable is that MWI is "useless" (at the moment-- it certainly has no uses) and it is philosophical (that's obvious to anyone who knows what philosophy is). Whether or not it is "nonsense" depends on whether or not it is important for deriving the next theory. I would say the same for any interpretation of QM that is elevated to the level of a world view-- the world views are useless and philosophical, but might become real science depending on the next theory, or how they can be used to motivate observations that current QM cannot predict.

The best parallel I can find is the 'god' concept, whereby you replace the complexity of the world with complexity hidden into the word 'god'. Likewise, in CI, you replace the complexity of the system that is observer, and the complexity that arises from the Schrodinger's equation, with the verbal concept of 'observer' that seems very simple at the surface.

Yet to me, it is MWI that is the classic example of "god perspective" thinking. The real world rarely works out the way we want to imagine it does, whenever we have tried to take the "god perspective" in the past.

 

[Dmytry]

If I'm understanding you, I agree. I think the issue of determininsm/indeterminism has very little to do with the issue of free will. Even if the universe is non-deterministic, I can't see how that would help the "free will" position anymore than a deterministic universe. Wouldn't that just lead to some sort of "random will" versus truly "free will"? In my opinion, the strongest argument put forth for the possibility of "free will" is positions that are able to challenge the following premise by Carl Hoefer:

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

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? Philosophy is full of concepts that contain dangling references and are thus undefined, but are argued as if they were well defined concepts on par with those in sciences.

If what you decide could have been determined by something different than you before you make your decision, then indeed that would exclude free will.
However, outcomes of even very simple 'deterministic' processes capable of universal computation (cellular automata, Turing machines) can not be calculated other than by running those exact processes. If such process was to make a decision, this decision is made by that process, and by nothing else. Furthermore that process would make that particular decision, and no other decision. If someone had to make an exact copy of you to predict your choice, they aren't predicting your choice, they're just letting you choose it yourself.
Sounds free enough to me; indeed such deterministic automations are more free than living human brains that would make decision either way depending to the thermal noise at the synaptic junctions; all your reasoning is only able to bias the probability.

I think the 'but we have free will' resistance to determinism arises from some sort of reversal of above reasoning. You start with the notion of system that evolves by deterministic steps, then you slip up a little and say that everything is predetermined, and it happens to be true that if outcome of a process really is pre_{running of that process}determined_{by someone}, then intuitively, this process would be very simple and uninteresting; definitely not intelligent.

 

[Dmytry]

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.

Sorry, but you're just incorrect. The theory is designed so that all the observed behaviours result from the theory. 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).
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).

 

[bohm2]

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

 

[Hurkyl]

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[/size]

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.

 

[Dmytry]

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?).

 

[Ken G]

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.

 

[Ken G]

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!

 

[Dmytry]

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.

 

[juanrga]

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.

 

[Dmytry]

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.

 

[Dmytry]

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.

 

[Samshorn]

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.

 

[Ken G]

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.

 

[Ken G]

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.

 

[Samshorn]

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.

 

[Ken G]

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.

 

[Samshorn]

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.