Problems with Many Worlds Interpretation

[Ken G]

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?

 

[Samshorn]

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.

 

[Ken G]

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.

 

[kith]

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.

 

[Len M]

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

 

[Ken G]

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!

 

[Samshorn]

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.

 

[Ken G]

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.

 

[Samshorn]

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.

 

[Ken G]

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.

 

[Samshorn]

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.

I think you're mistaken about that. Here's a little excerpt from a discussion between Deutch and Paul Davies (reproduced in the book "The Ghost in the Atom"):

Deutch: "When Everett first put forward his interpretation, he believed that it was a pure interpretation in the technical sense of the word. In other words, that the physical predictions of quantum theory under his system were precisely identical with those under any other system. Now, I believe that this is not so, and I have recently done some work trying to elaborate the exact experimental difference between the Everett and the conventional 'interpretations'. I now have to say 'interpretations' in quotes because I believe that there are actually different formal structures for quantum theory."

Davies: "So we're talking, not about two different ways of looking at the same theory, but two completely different theories?"

Deutch: "Yes..."

So I think you have not really understood Deutch's position - and I would argue that the same applies to other prominent advocates of MWI. It's fairly clear that the explicitly unitary version of MWI (which is all anyone cares about) is a distinct theory from QM.

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... that evolves via the Shroedinger equation. Then we just do everything that CI does when we refer to decohered subspaces of that closed system...

Your first sentence describes the unitary (top down) version of MWI, but your second sentence describes bottom up version of MWI. It isn't legitimate to claim unitarity from the first version and consistency with QM from the second version. I know you think the two versions are equivalent, and I've tried to explain in various ways why they are not equivalent, and I've cited at least one prominent advocate of MWI who contends they are not equivalent... but none of this seems to make any impression on you. Maybe we can make some progress by examining this statement:

...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...

I would say both clauses of that sentence are wrong. First, I think it's wrong to say an idea can qualify as an interpretation of a theory involving Hamiltonians and initial conditions even if that idea is incapable of ever identifying the applicable Hamiltonian or initial conditions. This gets us back to our fundamental difference over whether or not an interpretation is required to actually make some kind of rational sense. Second, I think it's wrong to say that CI likewise fails to make such an identification... the whole point of Bohr's insistence on the need for the measuring instruments to be treated as classical objects is because he recongnized that without this we just have "one hand clapping", and can never hope to identify the Hamiltonian and initial conditions and potential functions for any specific physical situation. CI is a (relatively, though not entirely) well-defined interpretation of QM as a theory that describes how a quantum system interacts with a classical system. This is what gives CI whatever degree of well-definedness it possesses. But MWI lacks this.

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.

We strongly disagree about this. As I said before, every scientists and almost every philosopher of science I know would not accept such a lax definition of "interpretation" for a physical theory - and furthermore, even under this (to me) ridiculously lax definition, MWI STILL doesn't qualify as an interpretation, unless you go on to define "cognitive resonance" to mean "whatever anyone thinks is cognitive resonance". And even FURTHERmore, if we were to accept all these "whatever floats your boat" (WFYB) definitions, it would surely be permissible to criticize "interpretations" in this context.

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.

Of course interpretations are not unique, but I would differ with the "above all", because I think above all is the requirement for the concept of an interpretation to be meaningful and well-defined. Obviously if we were to apply the WFYB definition of "interpretation", then it would never even occur to anyone that they might be unique. The interesting point is that even with meaningful and well-defined interpretations it turns out there is non-uniqueness. But we wouldn't be able to see this interesting point if we held to the WFYB definition of "interpretation".

That's not what I mean by "mean". The example I gave above is how x(t) emerges from classical trajectory calculations...So interpretations are simply not what you ask them to be.

I don't see the disagreement. What you described there is exactly what I described, in terms of the contrast between operational definitions of x and t versus the abstract concepts of time and three-dimensional space. If there is any difference in our views about this, I guess it's that I think there is a fairly meaningful and well-defined correspondence between the operational definitions of x & t and the conceptual model of 3D space and time, and that this degree of correspondence between operational variables and concepts is lacking in MWI.

...that same criticism is leveled by every person who rejects a given interpretation.

I don't see it as a criticism of one interpretation versus another, I see it as an aspect of interpretations in general, i.e., they are a way of placing something within some conceptual context, and they tend not to be regarded as satisfactory unless the conceptual context is one with which people are already comfortable.

MWI enthusiasts say CI is backward-looking because it cannot accept that reality might transcend our ability to perceive it...

I don't think that's true. The rap against CI hasn't traditionally been that it is reactionary, but rather that it is wooly and adventurous and even quasi-mystical. It is an exceptional interpretation precisely because it denies the quo ante categories. Those are the features that repell people, and that motivate things like MWI, which sees itself as dispensing with Bohr's mystical dualism and tries to eliminate "those damned jumps" and restore the classical basis of a deterministic continuous differential equation. There's nothing unclassical about imagining infinitely many "sub-worlds". It's extravagant, but not unclassical. So I would still say that MWI is a (so far unsuccessful) reactionary idea for an interpretation of QM in classical terms, and I think most advocates of MWI would actually agree with this, which they regard as its motivation - eliminating Bohr's mystical dualism.

 

[Ken G]

I think you're mistaken about that. Here's a little excerpt from a discussion between Deutch and Paul Davies (reproduced in the book "The Ghost in the Atom"):

But that's just what I meant when I said "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"-- Deutsch doesn't think MWI is an interpretation of QM, he thinks it is a new theory that is more powerful, that is how he tries to sell it. I don't say that's wrong, I say I am unconvinced by the argument. To me, the claims that MWI (or Bohm) makes different predictions are very flimsy. They might be true in some formal sense, but physical theories are idealized anyway, so the distinctions are lost in the idealizations.

So I think you have not really understood Deutch's position - and I would argue that the same applies to other prominent advocates of MWI.

I think I do understand Deutch's position, but it doesn't matter-- since the MWI I refer to is an interpretion of quantum mechanics, I don't view Deutsch as the oracle of MWI, I view him as a proponent of some new theory that has no observational support.

It's fairly clear that the explicitly unitary version of MWI (which is all anyone cares about) is a distinct theory from QM.

Unless Deutsch is wrong, and Everett was right. I don't know the technical details Deutsch is talking about, but they may represent certain philosophical positions that go beyond simple unitarity. I'd have to see Deutch's arguments for why you cannot simply take CI and treat the renormalized amplitudes of a "collapsed" wavefunction as conditional on the experiences of the observers, and embed them into larger unnormalized wavefunctions that are not so conditioned, and retain unitarity via the complicated coherences of the full wave function that are not represented in the mixed-state projection onto the subsystem. That seems like it would work just fine, if you could ever actually write the wavefunction of the full system and evolve it according to the Schroedinger equation.

I know you think the two versions are equivalent, and I've tried to explain in various ways why they are not equivalent, and I've cited at least one prominent advocate of MWI who contends they are not equivalent... but none of this seems to make any impression on you.

You have claimed they are different, and you have cited an authority who sees them as different, and you have claimed that anyone who holds to MWI is basically an idiot. In total, I find that set of arguments unconvincing, as do many.

First, I think it's wrong to say an idea can qualify as an interpretation of a theory involving Hamiltonians and initial conditions even if that idea is incapable of ever identifying the applicable Hamiltonian or initial conditions.

I understand you think that's wrong, yet this is quite a typical assumption for rationalistic interpretations of any physics theory. The rationalistic interpretation holds that the universe must have both a Hamiltonian and some initial conditions, because the theory says a universe without a Hamiltonian and initial conditions would not know how to evolve in time, yet we all know these could never be stipulated in detail. This is quite consistent with many other interpretations-- Newton thought the universe needed forces and initial conditions, even though he knew quite well they could not be stipulated either. So it's just not true to object to an interpretation on the grounds that it holds the reality cannot be stipulated. For my own part, I object to imagining that any of those things exist in the reality, but it is just fine to say they exist in the intepretation of the theory, even though they cannot be stipulated.

This gets us back to our fundamental difference over whether or not an interpretation is required to actually make some kind of rational sense.

Now you are equating "makes rational sense" with "is able to be completely stipulated." Yet I would argue that any claim that the elements of any theory could be completely stipulated is what does not make rational sense. Give me any interpretation of any physics theory that you think allows a complete stipulation of the elements of that theory.

Second, I think it's wrong to say that CI likewise fails to make such an identification... the whole point of Bohr's insistence on the need for the measuring instruments to be treated as classical objects is because he recongnized that without this we just have "one hand clapping", and can never hope to identify the Hamiltonian and initial conditions and potential functions for any specific physical situation.

No, Bohr knew quite well that we would never hope to do that, regardless of the classical treatment of the measuring devices. How is treating the instruments as classical going to allow the Hamiltonian of any subsystem to be completely specified? That never happens, in any interpretation, physics theories always need idealization to be useful.

CI is a (relatively, though not entirely) well-defined interpretation of QM as a theory that describes how a quantum system interacts with a classical system. This is what gives CI whatever degree of well-definedness it possesses.

CI is completely nondescript about the interection between the quantum system and the classical system! That's what people don't like about it. Personally, that doesn't bother me-- it simply chooses not to try to describe that interaction, beyond the usual vagaries of "decoherence."

And even FURTHERmore, if we were to accept all these "whatever floats your boat" (WFYB) definitions, it would surely be permissible to criticize "interpretations" in this context.

One can criticize interpretations, but only on pedagogical grounds. Not that they are invalid, if they suffice for someone who is adept in the theory.

The interesting point is that even with meaningful and well-defined interpretations it turns out there is non-uniqueness. But we wouldn't be able to see this interesting point if we held to the WFYB definition of "interpretation".

I think your characterization of "well defined" is artificial, it seems to basically boil down to your favored interpretations. I don't think "well defined" is a useful word with interpretations, they're all pretty vague.

I don't see the disagreement. What you described there is exactly what I described, in terms of the contrast between operational definitions of x and t versus the abstract concepts of time and three-dimensional space. If there is any difference in our views about this, I guess it's that I think there is a fairly meaningful and well-defined correspondence between the operational definitions of x & t and the conceptual model of 3D space and time, and that this degree of correspondence between operational variables and concepts is lacking in MWI.

Yes, I would not agree on that well-defined correspondence. Indeed, someday we may find that the operational definition of x & t is continuous (we already have that), yet the best conceptual model of 3D space and time is discrete. What would that mean, that a well-defined interpretation is suddenly not well defined? It was always not well defined, I could interpret classical mechanics right now as being a theory for a discrete spacetime and still do classical mechanics just fine, you couldn't even tell.

I don't see it as a criticism of one interpretation versus another, I see it as an aspect of interpretations in general, i.e., they are a way of placing something within some conceptual context, and they tend not to be regarded as satisfactory unless the conceptual context is one with which people are already comfortable.

Yes, but there can be a tradeoff, as there is with CI and MWI. CI connects better with empirical experience, MWI connects better with rationalistic and analytic experience.

I don't think that's true. The rap against CI hasn't traditionally been that it is reactionary, but rather that it is wooly and adventurous and even quasi-mystical.

Lots of people criticize CI for being reactionary. It is the same as the mysticism-- it is mystical because it insists on connecting reality to classical experience (empiricism), but empiricism is reactionary in the 1900s (witness the string theory phenomenon).

There's nothing unclassical about imagining infinitely many "sub-worlds". It's extravagant, but not unclassical.

I agree, I would say it is un-empirical more so than un-classical. But I view the empiricism as more central to CI than the classicism. Classicism is more the crux of Bohmian interpretations.

 

[juanrga]

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).

I agree with almost all what you say. I would only emphasize two points:

First, the 'mathematics' behind decoherence are suspicious. The non-unitary evolutors do not follow from doing approximations to unitary evolutors, but by ad-hoc substitution of groups by semigroups. Van Kampen liked to call this kind of derivations «mathematical funambulism».

Second, the so close-enough-for-all-practical-purposes mantra (Penrose uses the acronym FAPP in a delicious way to criticize decoherent people as Zurek) works when one focuses in typical non-Markovian, second order coupling evolutors, because experience in this regime is so broad that allows one to apply the correct equations in the correct way using certain ad-hoc procedures and rules.

Outside of this simple regime, nobody knows how to derive/apply the equations or when certain approximations are valid or are not, due to the lack of underlying systematic methods.

 

[Ken G]

Van Kampen liked to call this kind of derivations «mathematical funambulism».

The problem with this criticism is that all of physics is mathematical funambulism, as it has always been. Physics borrows from mathematics, but physics is not mathematics.

Second, the so close-enough-for-all-practical-purposes mantra (Penrose uses the acronym FAPP in a delicious way to criticize decoherent people as Zurek) works when one focuses in typical non-Markovian, second order coupling evolutors, because experience in this regime is so broad that allows one to apply the correct equations in the correct way using certain ad-hoc procedures and rules.

Yet all of physics is ad-hoc procedures and rules. What's the problem? Which physics entity is not ad-hoc, the properties of what element of physics follows directly from that element, in some kind of obvious or logical way?

Outside of this simple regime, nobody knows how to derive/apply the equations or when certain approximations are valid or are not, due to the lack of underlying systematic methods.

But since when has physics ever been about underlying methods? The methods of physics are clear enough: observe, guess, observe some more, theorize, observe, unify, lather, and repeat. Nothing underlying at all, it's all right there on the surface. Yes it's amazing that this works as well as it does, but that doesn't prove there is something underlying it, that's essentially just personal philosophy, or even wishful thinking.

 

[juanrga]

The problem with this criticism is that all of physics is mathematical funambulism, as it has always been. Physics borrows from mathematics, but physics is not mathematics.

Here you confound his «mathematical funambulism» with the general lack of mathematical rigor that you can find in physics except, probably, in mathematical physics. Van Kampen did not confounded both concepts, neither I do.

Yet all of physics is ad-hoc procedures and rules. What's the problem? Which physics entity is not ad-hoc, the properties of what element of physics follows directly from that element, in some kind of obvious or logical way?

It is not true that all of physics are ad-hoc procedures and rules. Large parts of physics are axiomatized or are in the process of being axiomatized.

But that was not the point, the point is that the FAPP-mantra of the decoherent community fails almost always when one looks for the details, as Penrose knows. Their so-called FAPP is really a FAPP-where-I-known-the-correct-answer-before.

But since when has physics ever been about underlying methods? The methods of physics are clear enough: observe, guess, observe some more, theorize, observe, unify, lather, and repeat. Nothing underlying at all, it's all right there on the surface. Yes it's amazing that this works as well as it does, but that doesn't prove there is something underlying it, that's essentially just personal philosophy, or even wishful thinking.

I emphasized, in bold-face, that they lack systematic methods, a word that you have completely avoid when engaging in an useless discussion.

Useless, because when I was referring to the methods I was referring to the specific procedures associated to a given theory as decoherence (of course, the concept can be applied to a whole discipline e.g. "methods of thermodynamics" is a classic).

I was not referring to the general concept of scientific methods (what it seems that you call the «methods of physics»).

 

[Ken G]

Here you confound his «mathematical funambulism» with the general lack of mathematical rigor that you can find in physics except, probably, in mathematical physics. Van Kampen did not confounded both concepts, neither I do.

I see, you imagine good funambulism, and bad funambulism, based around your own philosophical preferences. All the same, countless numbers of physics theories use approaches that do not stem directly from any postulates. It is much more common for physics theory to be like that, just pick up any journal in which physics theory is being used to analyze some phenomenon that doesn't fit nicely into one of the boxes that sound like course titles. One random example might be the radiative reaction force, and consider for example the funambulism Feynman mentions in regard to that-- and that's just classical electrodynamics, one of the simplest theories we have. Never mind the divergences of QED! My question is: why do people seem to expect quantum mechanics to work in a more pure or mathematical way than all the rest of the theories of physics? Why should decoherence in quantum mechanics look totally different from something like line broadening mechanisms in a dense plasma, where note that the latter is perfectly rife with FAPP-inspired funambulism?

It is not true that all of physics are ad-hoc procedures and rules. Large parts of physics are axiomatized or are in the process of being axiomatized.

Some parts yes, but not "large" parts. Very little physics works like that, by which I mean, if you go down the hall in a physics department and look at the posters of their research. They are not going to look like mathematical theorems based on a set of postulates!

One of the most obvious examples of my point is the need for idealizations. There is never a formal treatment of what is being neglected, we just say "we'll neglect X, Y, and Z", and let our results tell us if we can get away with that. This even applies to something as simple as the Coulomb logarithm, another example of mathematical funambulism that is nevertheless used all the time and with no apology because that's just what physicists do, it would be hard to go to work without being allowed to do that.

But that was not the point, the point is that the FAPP-mantra of the decoherent community fails almost always when one looks for the details, as Penrose knows. Their so-called FAPP is really a FAPP-where-I-known-the-correct-answer-before.

I can agree that reasoning to produce a desired outcome is not the same thing as a mathematical derivation of said result. My point is merely that physics does FAPP all the time, it's nothing new, and it's not an inappropriate way to do physics unless it is sold as something else.

I emphasized, in bold-face, that they lack systematic methods, a word that you have completely avoid when engaging in an useless discussion.

And what do you think "systematic" means? There are vast arrays of journals which are loaded with systematic FAPP-type analyses, it is more or less what research is. One might try to argue that the problem you refer to is that FAPP should not appear at the fundamental level of pure theory, but it's not at all clear that anyone who is using FAPP with decoherence is claiming they are doing anything different from standard physics-- trying to understand some phenomenon using effective treatments, where the "system" is essentially time-honored trial and error.

Useless, because when I was referring to the methods I was referring to the specific procedures associated to a given theory as decoherence (of course, the concept can be applied to a whole discipline e.g. "methods of thermodynamics" is a classic).

Yes, it can be applied to thermodynamics, and is. So what is so terrible about applying it to the decoherence in quantum mechanics? Thermodynamics is some different kind of physics than quantum mechanics? I just think some people start imagining that physics is some new and different field when they get into fundamental new theories built around quantum mechanics, and I see no justification. The methods of physics are just like those used in thermodynamics-- and are rife with FAPP, it's just not always admitted.

I was not referring to the general concept of scientific methods (what it seems that you call the «methods of physics»).

Well, perhaps you have something more specific in mind where you object to the FAPP concept, but decoherence is not a nice canned field of physics, it is one of those messy, tricky and evolving fields, like quantum thermodynamics. FAPP would seem to have its place, that's all I'm saying.

 

[juanrga]

I see, you imagine good funambulism, and bad funambulism, based around your own philosophical preferences.

You 'see' stuff that nobody said. Also I was not appealing to «philosophical preferences». I am discussing in a physics forum and my criticism of both MWI and decoherence is purely technical.

All the same, countless numbers of physics theories use approaches that do not stem directly from any postulates.

All the established basic theories of physics follow from postulates: GR, QM, thermodynamics, mechanics, SM...

It is much more common for physics theory to be like that, just pick up any journal in which physics theory is being used to analyze some phenomenon that doesn't fit nicely into one of the boxes that sound like course titles. One random example might be the radiative reaction force, and consider for example the funambulism Feynman mentions in regard to that-- and that's just classical electrodynamics, one of the simplest theories we have.

You must be misreading him. Feynman never said that CED lacks postulational basis. Feynman said that the Maxwell-Lorentz formulation of CED fails when is applied to certain kind of problems.

Feynman and Wheeler corrected this defect of the old formulation in their AAAD formulation of CED. For instance, in their new formulation a single particle does not radiate and thus lacks pre-accelerations and other unphysical stuff traditionally associated to the Maxwell-Lorentz formulation. Of course, they present the set of basic postulates for their formulation of CED.

Never mind the divergences of QED! My question is: why do people seem to expect quantum mechanics to work in a more pure or mathematical way than all the rest of the theories of physics? Why should decoherence in quantum mechanics look totally different from something like line broadening mechanisms in a dense plasma, where note that the latter is perfectly rife with FAPP-inspired funambulism?

QED, as rest of QFT, has a postulational basis. The divergences only arise when you forget that QED is an effective field theory and you try to apply it outside its range of validity. It is now acknowledged that the divergences of QED are eliminated in more fundamental theories that introduce a kind of cut-off, beyond the which QED does not apply.

Quantum mechanics, of course, has a postulational basis. Just open a standard textbook to see the postulates.

I can agree that reasoning to produce a desired outcome is not the same thing as a mathematical derivation of said result. My point is merely that physics does FAPP all the time, it's nothing new, and it's not an inappropriate way to do physics unless it is sold as something else.

The point was not reasoning vs. mathematical derivation, but mathematical derivation vs. «mathematical funambulism».

The rest of your discussion also avoids my main points and pretend to debate about stuff that I have never said. Whereas you repeat mistakes corrected (again you confound the specific methods associated to a discipline as thermodynamics with the general methods of science: the so-called scientific methods).

 

[Ken G]

I am discussing in a physics forum and my criticism of both MWI and decoherence is purely technical.

And I'm questioning the validity of that objection. All kinds of advances are made in physics, literally every day, that involve FAPP thinking, and which all could be objected to on "technical" grounds. So what? That claim by itself is not a valid objection to doing FAPP thinking, one would need to be able to argue on technical grounds that what is being argued is FAPP is actually not FAPP. Show where it leads to a demonstrably wrong conclusion in general practice, for example.

All the established basic theories of physics follow from postulates: GR, QM, thermodynamics, mechanics, SM...

Sure, but most of physics research is not just one of those. The vast majority of all physics applications in actual research are going to go beyond these theories, or there would be no need to do the research in the first place. The place where the research goes beyond the postulates will generally be covered in the early stages of the paper, where the unique assumptions of that paper are outlined.

You must be misreading him. Feynman never said that CED lacks postulational basis. Feynman said that the Maxwell-Lorentz formulation of CED fails when is applied to certain kind of problems.

One defines QED (or any mathematical theory) by its postulates, so the issue is not whether it lacks postulational basis, the issue is whether or not one can just stick to those postulates to use QED in physics. The answer is that one cannot-- one encounters divergences that are handled by the kinds of funambulism that physics is impossible without.

Feynman and Wheeler corrected this defect of the old formulation in their AAAD formulation of CED. For instance, in their new formulation a single particle does not radiate and thus lacks pre-accelerations and other unphysical stuff traditionally associated to the Maxwell-Lorentz formulation. Of course, they present the set of basic postulates for their formulation of CED.

It is not uncommon for new theories to correct the funambulisms of prior theories. What is quite rare is for new theories to lack their own funambulisms when applied in practice.

QED, as rest of QFT, has a postulational basis. The divergences only arise when you forget that QED is an effective field theory and you try to apply it outside its range of validity.

Translation: the divergences only arise when you try to do physics. I realize this-- that's what I meant that physics is not mathematics. All theories of physics are effective theories.

It is now acknowledged that the divergences of QED are eliminated in more fundamental theories that introduce a kind of cut-off, beyond the which QED does not apply.

And those more fundamental theories have no issues of their own?

Quantum mechanics, of course, has a postulational basis. Just open a standard textbook to see the postulates.

Again, all physics theories are defined by their postulates, so they all have a postulational basis. That has nothing to do with what I'm talking about, I'm talking about doing physics. We can agree that decoherence is not yet something that can be called a physics theory, it is a way of understanding what is happening in the world that helps us see why quantum mechanics (the mathematical theory) applies, and how to apply it. In short, without the FAPP of decoherence thinking, one cannot do physics with quantum mechanics and have any idea how it should apply to the real world. The funambulism there is what is needed to make the theory make sense, and there is nothing unusual about that state of affairs in physics. That doesn't mean Penrose and others shouldn't try to find a better more fundamental theory that eliminates the need for that particular brand of funambulism, but of course any such new theory will have its own issues, and funambulisms, and hence the need for yet newer theories. Such is physics, thoughout all its history. One does not need to reject FAPP approaches to press forward on new theories and new formulations, the two work perfectly well hand-in-hand.

 

[juanrga]

And I'm questioning the validity of that objection.

Yes, by systematically ignoring the technical remarks made and trying to discuss the issue at some philosophical level.

Sure, but most of physics research is not just one of those. The vast majority of all physics applications in actual research are going to go beyond these theories, or there would be no need to do the research in the first place. The place where the research goes beyond the postulates will generally be covered in the early stages of the paper, where the unique assumptions of that paper are outlined.

Of course, but don't forget that the new theories need to be backward compatible with the previous one. Precisely, the main fiasco of the MWI is that is not compatible with QM.

One defines QED (or any mathematical theory) by its postulates, so the issue is not whether it lacks postulational basis, the issue is whether or not one can just stick to those postulates to use QED in physics. The answer is that one cannot-- one encounters divergences that are handled by the kinds of funambulism that physics is impossible without.

I am not sure if I would reply to this, correcting what you say, or if merely would notice that I was writing about CED (not about QED) in the paragraph that apparently you are replying

It is not uncommon for new theories to correct the funambulisms of prior theories. What is quite rare is for new theories to lack their own funambulisms when applied in practice.
Translation: the divergences only arise when you try to do physics. I realize this-- that's what I meant that physics is not mathematics. All theories of physics are effective theories.

In this part you reply a paragraph from mine where I have not used the word funambulism. Why? Because, as remarked before, the use that you give to the term is not the use given either by van Kampen or by me.

You can discuss with yourself about your own meaning/usage of the words, but I am not interested.

And those more fundamental theories have no issues of their own?

Of course that they have. I am convinced that the perfect theory or TOE exists only in the imagination of some naive physicists, but again that was not the point.

The point is that those more fundamental theories lack issues of the older and less-fundamental theories that have improved/corrected.

This is not the case with MWI. MWI does not improve/correct QM, but MWI is unable to explain what QM explains so well since 100 years ago. This is why MWI is considered nonsense and useless, as many posters have said to you before.

Again, all physics theories are defined by their postulates, so they all have a postulational basis.

No exactly. String theory (and specially M-theory) lacks axiomatic foundations because, as even its more fierce proponents agree, nobody knows still what the theory is. MOND theory also lacks axiomatic foundation, at least in its original Milgrom form, and would be best considered an empirical theory.

That has nothing to do with what I'm talking about, I'm talking about doing physics. We can agree that decoherence is not yet something that can be called a physics theory, it is a way of understanding what is happening in the world that helps us see why quantum mechanics (the mathematical theory) applies, and how to apply it. In short, without the FAPP of decoherence thinking, one cannot do physics with quantum mechanics and have any idea how it should apply to the real world. The funambulism there is what is needed to make the theory make sense, and there is nothing unusual about that state of affairs in physics. That doesn't mean Penrose and others shouldn't try to find a better more fundamental theory that eliminates the need for that particular brand of funambulism, but of course any such new theory will have its own issues, and funambulisms, and hence the need for yet newer theories. Such is physics, thoughout all its history. One does not need to reject FAPP approaches to press forward on new theories and new formulations, the two work perfectly well hand-in-hand.

Here you continue using terms in an ambiguous non-technical way and continue confounding stuff.

Decoherence, as a physical phenomenon, is real. I do not object that. But what people as Zurek usually call decoherence theory is an instance of what van Kampen calls mathematical funambulism and what Penrose criticizes as non-serious.

At the same time, QM and extensions of it are genuine physical theories, whereas MWI is a collection of internally-inconsistent 'theories' that cannot reproduce know phenomena. Enough references were given.

 

[Ken G]

Yes, by systematically ignoring the technical remarks made and trying to discuss the issue at some philosophical level.

What technical remarks? I saw none, only claims that they exist. In the absence of any actual technical issues around flaws in the descriptions of decoherence, the discussion is already at a more conceptual level.

Of course, but don't forget that the new theories need to be backward compatible with the previous one. Precisely, the main fiasco of the MWI is that is not compatible with QM.

Many theoretical physicists that I have been exposed to are proponents of MWI, so I can hardly intepret that as evidence that it is not compatible with QM. I guess your reasoning there must be too "technical" for them to understand. But you might find that neither Stephen Hawking nor Roger Penrose agree with your technical objections: in the Wiki on MWI, Hawking's opinion is quoted as saying that MWI is trivially a possible interpretation of QM, but he doesn't think interpretations can really tell us reality (which is my own view as well), and Penrose is purported to agree with Hawking that QM applied to the universe implies MW, but the Wiki says that "Penrose considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM." In other words, MW is QM, but the "technical" problem with both is the same problem. I'm afraid that all makes more sense to me than any of the arguments you've presented so far, but perhaps you haven't made your strongest case yet.

The point is that those more fundamental theories lack issues of the older and less-fundamental theories that have improved/corrected.

No, the point is that the more fundamental theories make different predictions that work. That is all that physicists are interested in, and funambulism comes with the territory. I agree that mathematical inconsistencies are often signposts to the need of new theories, but that only justifies the search for new theories, not the way you used it: to criticize solid physics on the grounds that it is not mathematically closed. Physics has never been that. All physics theories encounter difficulties in their application that the practicing physicist must navigate in many clever yet purely "effective" ways, that is the art of doing physics.

This is not the case with MWI. MWI does not improve/correct QM, but MWI is unable to explain what QM explains so well since 100 years ago. This is why MWI is considered nonsense and useless, as many posters have said to you before.

Yes, many posters have made that claim, and failed to justify it as completely as you have. I prefer to believe Hawking and Penrose than those posters. For my own part, the valid objections I see to MWI are primarily philosophical, and appear when people lose sight of the differences between theories, interpretations of theories, and claims about reality. That is the valid landscape for discussing the various interpretations, not vague allusions to problems that are apparently "too technical" to even discuss!

No exactly. String theory (and specially M-theory) lacks axiomatic foundations because, as even its more fierce proponents agree, nobody knows still what the theory is.

Then it is not actually a theory at all, in the literal sense. Your argument here is apparently that theories don't need to be defined by their postulates if someone hangs the label "theory" in the phrase "string theory"! No, "string theory" is just a term, it doesn't mean anything. Theories are still defined by their postulates.

MOND theory also lacks axiomatic foundation, at least in its original Milgrom form, and would be best considered an empirical theory.

And what defines an "empirical theory"? Empirical postulates! You can't have a theory without postulates, they just might not be mathematically closed, in the sense that everything you do with the theory stems directly from the postulates. I already mentioned that this is generally true of physics-- it is quite generally not mathematically closed, it is wide open, and physicists do all kinds of things starting from the basic theories, some that don't pan out at all, others that are effective at understanding some phenomenon in a way that does not follow strictly from any set of postulates of some theory, but rather require additional assumptions that are not laws.

Decoherence, as a physical phenomenon, is real. I do not object that. But what people as Zurek usually call decoherence theory is an instance of what van Kampen calls mathematical funambulism and what Penrose criticizes as non-serious.

Your argument has become internally inconsistent. Above you said that "theories" don't have to stem from postulates, they can be "empirical" in nature. Presumably you can recognize the value in pursuing ideas like string theory and MOND. So now your argument hangs on the contradiction that it can be valuable to work beyond what you can trace back to a set of clear postulates when thinking about strings or gravity, but if you do it around decoherence, you are guilty of doing non-serious funambulism. That is precisely the stance I objected to in the very start, you have added nothing to that position beyond simply repeating it. Can you actually argue it with evidence? Quoting Penrose won't work, because as I showed you above, Penrose quotes refute other claims you have made already.

Enough references were given.

Huh? You can prove an argument by referencing a select group of opinions and ignoring the references to the opposite opinion? News flash: the validity of MWI is a hotly debated issue, as this very thread shows. "References" to one side of the story prove nothing, all that is of value is constructing an actual argument with insights that move the discussion forward.

 

[juanrga]

What technical remarks? I saw none, only claims that they exist. In the absence of any actual technical issues around flaws in the descriptions of decoherence, the discussion is already at a more conceptual level.

This proves my point that you ignore the technical arguments...

I recall citing a preprint called «Why decoherence has not solved...». I recall citing a paper explaining why MWI is pure nonsense. I recall citing an online FAQ from a mathematician explaining why «MWI is a smokescreen without a consistent mathematics behind.»

I remember myself citing a paper from Brussels group showing how the evolutor \Lambda, explaining decoherence effect among others, is not reducible to U. The paper also shows how up to second order in the coupling and Markovian regime the real evolutor looks as U, although is not U, which is at the origin of the decoherence myth that starting from U one can obtain decoherence from tracing (one can only if one uses «mathematical funambulism», but using funambulism one can prove anything).

And so on and so on.

Many theoretical physicists that I have been exposed to are proponents of MWI, so I can hardly intepret that as evidence that it is not compatible with QM. I guess your reasoning there must be too "technical" for them to understand. But you might find that neither Stephen Hawking nor Roger Penrose agree with your technical objections: in the Wiki on MWI, Hawking's opinion is quoted as saying that MWI is trivially a possible interpretation of QM, but he doesn't think interpretations can really tell us reality (which is my own view as well), and Penrose is purported to agree with Hawking that QM applied to the universe implies MW, but the Wiki says that "Penrose considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM." In other words, MW is QM, but the "technical" problem with both is the same problem. I'm afraid that all makes more sense to me than any of the arguments you've presented so far, but perhaps you haven't made your strongest case yet.

MWI continues being a minority view and claims about its popularity are done by citing some unscientific polls or by saying Hawking-supports-it. Well, Hawking has been supporting all kind of bizarre ideas for decades and shown wrong very often. Recall his recent lost of the bet about QM and information, for instance. For decades, he believed that starting from a unitary evolution, more some horizon, and some hocus pocus information was lost, but evidently information cannot be lost via unitary evolution (by virtue of a well-known theorem). After several decades trying to teach him this, finally he accepts now that he was plain wrong. Hawking has never understood QM, sorry.

Regarding the Wiki, I find interesting that you avoid FAQs, papers, books and preprints, whereas use a Wiki as main support for your claims.

That Wiki affirms that «Penrose agrees with Hawking that QM applied to the universe implies MW» and gives reference 64, but in this 'reference' (an interview of 1991) Penrose says something different. He is not supporting MWI. He says that superposition (a part of QM) on a cat or person would give a many-world view. And in the same reply, it is emphasizing that this particular view about QM is wrong.

Penrose is well-known for his criticism of MWI. In his recent works and his latest books, he affirms that guys as Hawking are not really serious about QM. Penrose correctly states the existence of two irreducible evolutions in QM: U and R. And he correctly notices that people who do not understand this and pretends that only U matters (e.g. Hawking) are not really serious. Sorry, Hawking has never understood QM.

You pretension of that Penrose supports the nonsense of MWI, is in contrast with what Penrose says

[the 'many-worlds' view] is not a very economical description of the Universe but I think things are rather worse than that for the many-worlds description. It is not just its lack of economy that worries me. The main problem is that it does not really solve the problem.

https://www.amazon.com/dp/0521785723/?tag=pfamazon01-20

Effectively, MWI does not agree with observations. I added in this thread that MWI is also internally inconsistent.

No, the point is that the more fundamental theories make different predictions that work. That is all that physicists are interested in

It depends. If your interest is in developing a theory beyond its scope of application, then yes, with the new theory giving new predictions.

If your interest is in correcting inconsistencies of a theory, then new predictions are not in the menu. Example Wheeler and Feynman electrodynamics. Their goal was to correct deficiencies of Maxwell-Lorentz, not to make new predictions.

Yes, many posters have made that claim, and failed to justify it as completely as you have. I prefer to believe Hawking and Penrose than those posters. For my own part, the valid objections I see to MWI are primarily philosophical, and appear when people lose sight of the differences between theories, interpretations of theories, and claims about reality. That is the valid landscape for discussing the various interpretations, not vague allusions to problems that are apparently "too technical" to even discuss!

The claim was proven but you have systematically ignore the proofs. Penrose is also known for not supporting MWI. In his own book he renders himself along with the people who has discredited MWI. Penrose puts himself very far from the Hawking/Everett minority camp (https://www.amazon.com/dp/0521785723/?tag=pfamazon01-20).

https://p.twimg.com/AituXViCQAAfhom.png

Then it is not actually a theory at all, in the literal sense. Your argument here is apparently that theories don't need to be defined by their postulates if someone hangs the label "theory" in the phrase "string theory"! No, "string theory" is just a term, it doesn't mean anything. Theories are still defined by their postulates.

No, a theory of physics does not need to be axiomatized. This is specially true for theories under development, for which would be ridiculous to search axioms when tomorrow the theory can change radically.

And what defines an "empirical theory"? Empirical postulates! You can't have a theory without postulates, they just might not be mathematically closed, in the sense that everything you do with the theory stems directly from the postulates. I already mentioned that this is generally true of physics-- it is quite generally not mathematically closed, it is wide open, and physicists do all kinds of things starting from the basic theories, some that don't pan out at all, others that are effective at understanding some phenomenon in a way that does not follow strictly from any set of postulates of some theory, but rather require additional assumptions that are not laws.

An empirical theory is the collection of empirical laws together with their methods. No paper or book about MOND that I know use the term «postulates», still less «empirical postulates». If you have invented your own terminology, this is fine for you... but I have never used such.

Your argument has become internally inconsistent. Above you said that "theories" don't have to stem from postulates, they can be "empirical" in nature. Presumably you can recognize the value in pursuing ideas like string theory and MOND. So now your argument hangs on the contradiction that it can be valuable to work beyond what you can trace back to a set of clear postulates when thinking about strings or gravity, but if you do it around decoherence, you are guilty of doing non-serious funambulism. That is precisely the stance I objected to in the very start, you have added nothing to that position beyond simply repeating it. Can you actually argue it with evidence? Quoting Penrose won't work, because as I showed you above, Penrose quotes refute other claims you have made already.

Effectively, theories can be empirical. Apart from the examples given before, thermodynamics and electrodynamics both born as empirical theories, before their axiomatization was done.

The contradiction that you allude is only in your mind. I have not objected to empirical descriptions of the decoherence effect. What I have said, and proved with references, is that the so-called decoherence theory by Zurek et al. is based in what van Kampen calls mathematical funambulism. Mathematical funambulism has nothing to see with postulates, but with the fact that decoherence is irreducible to U. Penrose knows this very well. That is why he critizes decoherence guys as Zurek. See figure above again.

 

[Ken G]

Let me briefly summarize what I disagree with from your last post:

1) You seem to think I said Penrose was a proponent of MWI. Obviously, I did no such thing, Penrose is well known to not be an advocate of MWI, nor did I say anything else. What I did say is that Penrose views MWI to be consistent with the postulates of quantum theory "if taken seriously" (his words). The reason Penrose objects to MWI is that it does not connect the theory with what we observe, a problem that he does not think the current postulates of QM resolve at all-- he clearly thinks that new postulates are needed to do that, and he thinks that the new postulates will also include quantum gravity. We are not talking about whether MWI is good physics, or if it will survive a theory of quantum gravity, we are simply discussing whether MWI is a valid interpretation of the existing postulates of quantum theory. You have produced zero evidence that Penrose thinks MWI is not consistent with that, and indeed I have cited evidece that he does, he merely thinks that the current postulates are in a state of flux and require improvement. I also cited evidence that Hawking thinks the postulates are consistent with QM, and he doesn't think Penrose disagrees with that, but he is less bothered by the flaws in the current postulates because his own brand of realism is less demanding on the theories-- they merely need to work in practice. Apparently your arguments rests on the claim that you understand the existing postulates of quantum theory better than both Hawking and Penrose, and I don't see evidence of that either.

2) You argue that the existing postulates of QM require that U and R be mutually irreducible, refuting MWI, and you claim that Penrose makes this argument. However, Penrose does not argue that MWI is logically inconsistent with the postulates of current quantum theory-- that is only an argument that you make. Penrose's argument is much more philosophical-- he simply doesn't think MWI accomplishes the goal that a physics theory should have, and indeed he doesn't think quantum theory yet does that either! That's why he is looking for a new theory. If he actually agreed with you, that MWI is logically inconsistent with the current postulats of quantum theory, he would not need to mention quantum gravity in his objections to MWI. But he does. The natural conclusion is that you are misinterpreting Penrose when you think that he agrees with you that MWI is not consistent with the current postulates of quantum mechanics, instead Penrose thinks MWI won't be consistent with the improved theory he is working toward. Only time can tell on that score.

3) You have cited papers that are relevant to your conclusions, but you do not make the argument for your conclusion. Citing conclusions from papers only demonstrates one thing: you agree with some of the experts . However, citing different conclusions from different papers, or the opinions of Hawking and Penrose as I have done, demonstrates another thing: you don't agree with some of the other experts. This is hardly surprising-- we should certainly know by now that experts disagree on the validity of MWI, and even on why it should be considered valid or invalid. So citing a few experts tells us nothing of interest-- what would be interesting is actually making a case that brings some new insight to the question. All you do is claim that anyone who doesn't agree with you (like Hawking) "doesn't understand quantum mechanics." That is quite a convenient stance-- argument by dismissal.

4) You have said that decoherence theory isn't serious on the grounds that it cannot yet be rigorously traced to the fundamental postulates of any existing theory (the only possible meaning of "funambulism"). In the same breath, you have said that other endeavors, like MOND and string theory, which also cannot be traced to fundamental postulates of any existing theory, are much more serious. Thus, what you consider to be "serious" has no objective criterion, like traceability to fundamental postulates, it is simply that what you personally like is serious and what you don't like isn't serious. I have said that any mathematically closed theory is defined by its postulates, but that is only the rather small subset of physics called mathematical physics-- most of the rest of physics is both quite serious, and also funambulism. So the selective association you choose to make between funambulism and non-seriousness is completely arbitrary, and you have not given it any logical basis, it still boils down to nothing but argument by dismissal.

 

[juanrga]

Let me briefly summarize what I disagree with from your last post:

1) You seem to think I said Penrose was a proponent of MWI. Obviously, I did no such thing, Penrose is well known to not be an advocate of MWI, nor did I say anything else. What I did say is that Penrose views MWI to be consistent with the postulates of quantum theory "if taken seriously" (his words). The reason Penrose objects to MWI is that it does not connect the theory with what we observe, a problem that he does not think the current postulates of QM resolve at all-- he clearly thinks that new postulates are needed to do that, and he thinks that the new postulates will also include quantum gravity. We are not talking about whether MWI is good physics, or if it will survive a theory of quantum gravity, we are simply discussing whether MWI is a valid interpretation of the existing postulates of quantum theory. You have produced zero evidence that Penrose thinks MWI is not consistent with that, and indeed I have cited evidece that he does, he merely thinks that the current postulates are in a state of flux and require improvement. I also cited evidence that Hawking thinks the postulates are consistent with QM, and he doesn't think Penrose disagrees with that, but he is less bothered by the flaws in the current postulates because his own brand of realism is less demanding on the theories-- they merely need to work in practice. Apparently your arguments rests on the claim that you understand the existing postulates of quantum theory better than both Hawking and Penrose, and I don't see evidence of that either.

Penrose does not claim that MWI is consistent with the postulates of quantum theory, you did.

Penrose emphasizes the existence of two evolutions (U & R) in QM, how U and R are mutually irreducible, and how MWI ignores R.

What Penrose is trying to do with his quantum gravity approach is to obtain a dynamical description of R. Other people in the same camp is trying to do the same, but without appealing to exotic quantum gravity effects.

Penrose agrees that MWI is a failed theory, that it does not solves what was supposed to solve, and that is promoted by people who is not really serious about QM. He makes some funny remarks about Zurek. Those remarks are related to the discontinuous arrow that he draws in this figure

https://p.twimg.com/AituXViCQAAfhom.png

If you did not read the book, I will explain you. Penrose notices how Zurek starts from an initial position, claiming that decoherence solves measurement in QM, and that everything what one needs is U. But when Penrose asks to Zurek about the technical details (for example how you reproduce the R of QM using a theory only with U as MWI), for checking if Zurek's claims are to be trusted or not, then Zurek, lacking any serious mathematical/physical response, moves himself towards the MWI camp and avoids to answer the questions. This is another important remark by Penrose: Zurek avoids to answer the technical questions asked to him.

2) You argue that the existing postulates of QM require that U and R be mutually irreducible, refuting MWI, and you claim that Penrose makes this argument. However, Penrose does not argue that MWI is logically inconsistent with the postulates of current quantum theory-- that is only an argument that you make. Penrose's argument is much more philosophical

Here again you pretend to deviate the discussion from the technical level, where MWI has been proven wrong, to some kind of philosophical level, where MWI can be discussed up to exhaust.

The arguments done by Penrose against MWI are technical and of a relatively high mathematical level. Concretely, he uses GR arguments about localization of energy to try to break superposition at space-time level in some kind of speculative quantum gravity scenario.

Penrose gives an equation for estimation of the breaking of a spacetime superposition. His formula predicts that cats and larger objects never follow Sch evolution. The universe as a whole does not follow the Sch law, which goes against one of main MWI assumptions.

Philosophical arguments are only in the MWI camp.

3) You have cited papers that are relevant to your conclusions, but you do not make the argument for your conclusion. Citing conclusions from papers only demonstrates one thing: you agree with some of the experts . However, citing different conclusions from different papers, or the opinions of Hawking and Penrose as I have done, demonstrates another thing: you don't agree with some of the other experts. This is hardly surprising-- we should certainly know by now that experts disagree on the validity of MWI, and even on why it should be considered valid or invalid. So citing a few experts tells us nothing of interest-- what would be interesting is actually making a case that brings some new insight to the question. All you do is claim that anyone who doesn't agree with you (like Hawking) "doesn't understand quantum mechanics." That is quite a convenient stance-- argument by dismissal.

I would agree, except because Penrose agrees with me and because Hawking is not an expert. In the above figure Penrose puts Hawking very far from known experts in QM (what he calls people really serious about QM). The contributions of Hawking to the topic of QM are easily summarized: zero. Therefore, this is understandable.

Indeed, most what Hawking has said about QM and QG has been shown wrong. I illustrated the example of information paradox in QM, which Hawking lost, because is very relevant to the failure of MWI.

I explained to you that Hawking mistake regarding the information paradox was due to Hawking unability to understand unitarity and theorems derived from it (as conservation of information).

In essence, Hawking pretended that information was lost via an evolution law conserving information. This evidently is nonsense. He tried to support his opinion via mathematical funambulism. Many of us said that Hawking was wrong, but he did need several decades to understand this topic. It is fair to say that he nows acknowledge that he was wrong about unitarity and he now claims that information is not lost.

Hawking makes essentially the same mistake regarding quantum measurements. Well-known mathematical results prove that starting from U, one cannot obtain R. Myself cited a paper where some of those theorems are given.

The fathers of QM knew this and added the collapse postulate to the Sch law. Penrose knows this and accepts both R and U. The serious people in the above figure know all this as well. Only Hawking and some other few cosmologists ignore this.

4) You have said that decoherence theory isn't serious on the grounds that it cannot yet be rigorously traced to the fundamental postulates of any existing theory (the only possible meaning of "funambulism"). In the same breath, you have said that other endeavors, like MOND and string theory, which also cannot be traced to fundamental postulates of any existing theory, are much more serious. Thus, what you consider to be "serious" has no objective criterion, like traceability to fundamental postulates, it is simply that what you personally like is serious and what you don't like isn't serious. I have said that any mathematically closed theory is defined by its postulates, but that is only the rather small subset of physics called mathematical physics-- most of the rest of physics is both quite serious, and also funambulism. So the selective association you choose to make between funambulism and non-seriousness is completely arbitrary, and you have not given it any logical basis, it still boils down to nothing but argument by dismissal.

I have not said that «decoherence theory isn't serious on the grounds that it cannot yet be rigorously traced to the fundamental postulates of any existing theory». And I do not want to discuss about your systematic reinterpretation of my words.

 

[Ken G]

What Penrose is trying to do with his quantum gravity approach is to obtain a dynamical description of R. Other people in the same camp is trying to do the same, but without appealing to exotic quantum gravity effects.

Yes that's just what I said-- all those people think the current postulates of quantum mechanics require improvement, because they do not provide a dynamical description of R. This clearly demonstrates that the current postulates of QM don't provide that, or there would be no need for this effort. And you are arguing that MWI is inconsistent with those current postulates because it also doesn't provide that, so your argument that MWI is inconsistent with QM does not logically follow.

What I hear Penrose and Hawking as saying are the following. Penrose is saying the current postulates of QM are unsatisfactory because they lack an overarching description of how to connect the predictions of the theory with what we actually experience-- that connection is currently done in various ad hoc ways, which each interpretation gives its own accounting of. Hawking is agreeing with that, but doesn't see it as so much of a problem, so long as the predictions can be obtained by hook or by crook.

Note that none of this implies that MWI is inconsistent with the existing quantum theory, even though both lack a rigorous way to connect predictions and observations that stem directly from some set of postulates, absent any "funambulism." I suspect that both Penrose and Hawking would take a similar stance about any of the other interpretations of QM-- none of them would be satisfactory to Penrose because QM isn't satisfactory, and all are basically OK by Hawking because they all make the same predictions at the end of the day.

Penrose agrees that MWI is a failed theory, that it does not solves that was supposed to solve, and that is promoted by people who is not really serious about QM (that is an elegant way to say people-who-do-not-understand-QM).

That is all entirely consistent with everything I said about Penrose's position. In particular, at no point does it imply that MWI is not a valid interpretation of the current quantum theory, and at no point does it imply that decoherence theory is not a valid effort to understand why the current quantum postulates are as effective as they are. It is also completely consistent with the pervasive fact that Penrose's objections all trace back to his fundamental discontent with current quantum theory, as I said. Once it is clear that Penrose is "serious" about fixing quantum mechanics, it is obvious that he will view as "non-serious" any attempts to "patch" those flaws with informal (or "empirical") efforts to understand why QM is effective despite its blemishes. What that all reflects is Penrose's personal views about what direction mathematical physics should be taking in its usual efforts to underpin already existing theories with greater mathematical rigor, and his hopes that in the process, quantum gravity can be achieved.

Here again you pretend to deviate the discussion from the technical level, where MWI has been proven wrong, to some kind of philosophical level, where MWI can be discussed up to exhaust (but without changing a bit the technical conclusions).

No interpretation of QM has been "proven wrong"-- as I said, the issue with proving anything about any theory is that physics is not closed, and so there simply is no postulational basis that anyone can use to "prove" anything about QM without making additional assumptions, usually quite philosophical in nature (as in Penrose's idea that a theory must be able to connect its predictions to what we perceive, directly from the postulates of the theory-- absent funambulism, even though actual physics doesn't do that in practice). This is the fact you keep ignoring-- your entire argument is that anyone (like Hawking, or Zurek, or Gell-Mann, or many of the other names in that figure) who does not share the philosophical priorities that make "technical" proofs possible in the first place, must not understand quantum mechanics. The real problem is, you do not understand the role of philosophy, you are just another one of those who think that philosophy is about other people's opinions, but your own philosophy is buried in "technical" issues.

The arguments done by Penrose are evidently technical and of a relatively high mathematical level. Concretely, he uses GR arguments about localization of energy to try to break superposition at space-time level in some kind of speculative quantum gravity scenario.

And this proves my point completely. You just mentioned GR in a "technical" proof about the postulates of current quantum theory! Once again: Penrose is not making proofs about quantum mechanics, he is making proofs under assumptions about how quantum gravity might work. He is imposing his view of what the next theory needs to do, and is making proofs about how it would have to do it. That's a very valuable thing to do, especially if Penrose's mission turns out to be possible, but it certainly isn't what you are misquoting it to be.

You claim that his arguments are «philosophical», but this is plain wrong, of course. Penrose gives an equation for estimation of the breaking of a spacetime superposition. His formula predicts that cats and larger objects never follow Sch evolution. The universe as a whole does not follow the Sch law, which goes against one of main MWI assumptions.

So now you are claiming that Penrose can prove how the universe must work without making any philosophical assumptions about how the universe works? That's just what I mean about the logical holes in your stance. No, what Penrose is quite clearly doing is applying philosophical beliefs to find the direction of the next theory, just as Einstein did in originally deriving GR.

I explained to you that Hawking mistake regarding the information paradox was due to Hawking unability to understand unitarity and theorems derived from it (as conservation of information).

And I explained to you that no one knows the answer to that bet, because there is no theory of quantum gravity. All that happened was Hawking was convinced to change his mind about how the next theory might work. You pretend that the answer is already known, just as you pretend that Penrose can prove how reality will work based on "estimations of the breaking of spacetime superpositions." Sorry, this is all philosophy until there is demonstrable evidence of the success of a new theory, regardless of how "technical" the philosophical arguments can become. One would think you don't even realize that mathematics is used in philosophy too!

Hawking makes the same mistake regarding quantum measurements. Well-known mathematical results prove that starting from U, one cannot obtain R. He pretends that one can but he is plain wrong. I have given the arguments before, I haver given the papers before, I have given the mathematical FAQ before...

No need to repeat, true?

True, I don't expect you to rehash things you have already put a lot of work into establishing, and it is certainly clear you have spent a lot of time on the issue and have achieved a high degree of technical mastery. All the same, it is not at all uncommon for people with high levels of technical mastery to have lost sight of the basic philosophical assumptions they have made along the way, and those assumptions show up in the proofs (for example, that U cannot obtain R. On what closed system? Is the observer in there? Is the way the observer perceives and thinks in there? Is the way the observer tested the postulates of the theory in there?)

I know that some people believes that Earth is flat. No paper, book, website, theorem, argument, photograph, experiment, or anything makes they change their opinion. I am well aware that this also happen with some MWI people who never will accept that MWI is nonsense.

I am not an "MWI person", so I don't have to feel personally insulted by this remark, but all the same I have pointed out the flaws in your argument that make it impossible for you to call this a logical conclusion. It's actually just your opinion-- the simple truth is, no one has the slightest idea whether MWI will be consistent with the postulates of the next theory, whether it unifies QM and GR or QM and the way the mind does physics. But I can certainly agree that based on nothing other than simply being consistent with the current quantum theory, that is no justification to use MWI to make claims about reality.

 

[juanrga]

Yes that's just what I said-- all those people think the current postulates of quantum mechanics require improvement, because they do not provide a dynamical description of R. This clearly demonstrates that the current postulates of QM don't provide that, or there would be no need for this effort. And you are arguing that MWI is inconsistent with those current postulates because it also doesn't provide that, so your argument that MWI is inconsistent with QM does not logically follow.

Contrary to your misconception, there is not need to demonstrate limits of the current postulates of QM today, because the limitations of the postulates are well-known since that QM born. Everyone who has studied QM knows that the projection postulate is not a dynamical postulate. In fact, there is not even variable time in that postulate.

The extension or improvement of QM was not the point. The point I was making was about your incorrect claim that Penrose supports/likes/accepts MWI, when he does not clearly.

I am not «arguing that MWI is inconsistent with those current postulates because it also doesn't provide» a dynamical description of R. What I said is totally different. I said that MWI is both internally inconsistent and incompatible with QM by other reasons. I said, for instance, that MWI cannot give R, because R is irreducible to U.

I think this is rather easy to understand. If you start with a theory like MWI that uses only U, you cannot obtain R.

If you look to the figure above you can see that really serious people (i.e., people who understand QM) uses both R and U. People who do not understand QM (fortunately a tiny minority), believes that MWI is another interpretation of QM, but, as has been shown in this thread, MWI is a misinterpretation of QM.

Your following misconceptions and rewrites of Penrose were already corrected before.

The rest of your post is a new collection of misconceptions and systematic rewrites of what I and others said. I will be brief,

Nowhere I said that Gell-Mann does not understand QM. You said it.

Nowhere I said that GR is needed to prove the postulates of QM. You said it.

Nowhere I said that Penrose can prove how reality will work based on estimations of the breaking of spacetime superpositions. You said it.

And so on.

In fact, apart from not saying anything of that, I have said the contrary, which makes still more funny your tactic. For instance, regarding Penrose's work in spacetime superpositions I wrote:

What Penrose is trying to do with his quantum gravity approach is to obtain a dynamical description of R. Other people in the same camp is trying to do the same, but without appealing to exotic quantum gravity effects.

Which is a way to say that those exotic effects are not needed for understanding R.

There are many more misconceptions and very serious mistakes in the rest of your post...

 

[Ken G]

Contrary to your misconception, there is not need to demonstrate limits of the current postulates of QM today, because the limitations of the postulates are well-known since that QM born.

Tell me, just where do you imagine that I said there is a need to "demonstrate" the limits of the postulates today? Nowhere. What I actually said is that some people are working to fix these limitations, hoping to create a new set of postulates that are not necessarily the ones that MWI is consistent with (my whole point here), such as Penrose, while others, such as Hawking, are not bothered by the limits of those postulates because the experiments can be predicted either way. That is just what Hawking meant when he said that Penrose is a Platonist (someone who would be bothered by limits in the postulates because he believes the postulates are trying to be the reality) whereas he is a positivist (someone who believes that all we can ever say about reality is the outcome of our experiments, so if we predict those, that's the best we can ever hope to do.) You are not hearing what I'm saying, you seem to need to keep replacing it with something else so you can refute what I never said.

Everyone who has studied QM knows that the projection postulate is not a dynamical postulate. In fact, there is not even variable time in that postulate.

Yes of course, anyone who has studied the current postulates of QM, like you, like me, like Hawking, etc., does indeed know that, because it is perfectly obvious. It also has nothing to do with anything I've said in this whole thread. The question is, why do you think it does? Because you're not hearing anything I'm saying?

The extension of QM was not the point. The point was your incorrect claim that Penrose supports/likes/accepts MWI, when he does not.

Please quote me where you imagine I said that Penrose likes or accepts MWI! I never said (or thought) any such thing, and indeed I clarified several times that I never said that. One more time: what I said, and continue to maintain, is that the reason Penrose rejects MWI is the same as the reason he feels the postulates of QM are unsatisfactory, to wit, the theory is not directly connected to what we perceive without ad hoc nondynamical treatments of wave function collapse. What's more, the sole thing that distinguishes the various interpretations of QM is how they handle the ad hoc character of this collapse that is in the current postulates of QM. This is also why they are all perfectly consistent with QM, they simply do what is ad hoc (their funambulisms) in different ways. Someone like Hawking, who only cares that it works, isn't too bothered by that; someone like Penrose, who wants the postulates to be a description of the reality itself, is bothered by that. What this all means is that Penrose would object to all the interpretations of QM, because he objects to QM itself-- until its postulates can be "fixed", and hopefully include gravity.

I have not said that the problem with MWI is a lack of dynamical description of R. What I said is that MWI cannot give R, because R is irreducible to U.

And I have said that this theorem requires essentially philosophical assumptions that any MWI enthusiast would reject. That's what I meant when I said you are simply overlooking your own philosophical biases. I asked you where in the proof of the theorem you quote is there an observer, where is there an observer's mind, and where is there the epistemology that observer chooses to use to decide what it means for a theory to be valid. Until you put those things in your proof, you have not gone beyond your own philosophical assumptions. So we return to my original remark: physics is not mathematics. We can now append this remark: mathematical physics is impossible to connect with the rest of the body of physics without philosphical assumptions, and much of the debate that has people like Penrose saying Deutsch is "not serious" while Deutsch says Penrose is doing "aesthetics not physics", is all around their different philosophical assumptions and objectives for physics.

I think this is rather easy to understand. If you start with a theory that uses only U, you cannot obtain R.

If this were as categorically true as you imagine, then no one would have thought MWI is a reasonable interpretation of QM. So you now have an argument on the table that says "it is easy to understand that MWI could never be an interpretation of QM." Can you see the logical flaw in your position? I can. It holds that any proponent of MWI is an idiot. I think you actually believe that, but your logic is faulty because you have not tracked your own assumptions.

The actual truth is that if you start with only U, you cannot obtain R without altering your philosophical preconceptions about what physics should be trying to do, or how we should regard the connection between physics and reality, especially in regard to the importance of empiricism. It you want it 'more "technically", just look at the proof of any theorem that says you can't get R from U, and look where the philosophical assumptions come in. You might need to see outside your box a bit more.

If you look to the figure above you can see that really serious people (i.e., people who understand QM) uses both R and U. ...

Um, I think you should have a look at your own figure again. Penrose puts the "only U" camp under "serious about psi". Perhaps your own definition of "serious" is a little different from Penrose's? As near as I can tell, you define it as "agrees with me". (But you are right that I misplaced Gell-Mann.)

Nowhere I said that GR is needed to prove the postulates of QM. You said it.

I did? News to me. How does one "prove a postulate"? I might imagine proving the consistency of two different postulates, but of course no one thinks the postulates of GR are consistent with the current postulates of QM.

For instance, regarding Penrose's work in spacetime superpositions I wrote...
Which is a way to say that those exotic effects are not needed for understanding R.

Yet Penrose thinks those exotic effects are needed for understanding R. So like I said, the experts do not agree at the frontier-- so simply citing their opinions does not prove anything. But even more to the point, Penrose does not think that the current postulates of QM allow us to understand R! That is all I need to establish my argument-- if Penrose really thought that MWI is not consistent with the current postulates of QM (and there is no evidence he does, and evidence to the contrary like what Hawking said about Penrose's position), then whey would he bother to criticize MWI on the grounds that you can't use it to get R from U dynamically without going to new postulates? The answer is, all he cares about is the latter issue, it is of no concern to him if MWI is consistent with the current postulates, and indeed I believe he thinks it is.

There are many more misconceptions and very serious mistakes in the rest of your post...

Correction, there are many more mistakes in your furtive imagination applied to my posts. You first must demonstrate that you can actually hear the words I am using before you can critique them effectively. Communication is the hardest thing.

What Penrose thinks is that the current postulates of QM need modifying, in ways that, once they accomplish what Penrose's philosophical preferences convinces him they should, will leave no room for MWI. That is not a claim that MWI is inconsistent with the current postulates of QM, and it not a claim that Penrose's program will work. I can't say it any clearer than that.

 

[juanrga]

That is just what Hawking meant when he said that Penrose is a Platonist (someone who would be bothered by limits in the postulates because he believes the postulates are trying to be the reality) whereas he is a positivist (someone who believes that all we can ever say about reality is the outcome of our experiments, so if we predict those, that's the best we can ever hope to do.)

And Penrose correctly points that the central issue «has very little to see with Platonism/positivism». Moreover, Penrose does not consider himself a Platonist...

What's more, the sole thing that distinguishes the various interpretations of QM is how they handle the ad hoc character of this collapse that is in the current postulates of QM. This is also why they are all perfectly consistent with QM, they simply do what is ad hoc (their funambulisms) in different ways.

This is all untrue. You also continue using the term «funambulisms» in a different way to how I introduced the term here.

Someone like Hawking, who only cares that it works, isn't too bothered by that; someone like Penrose, who wants the postulates to be a description of the reality itself, is bothered by that.

Hawking believes that MWI works, but he, of course, has never proved such thing. However, the contrary thing, --i.e., that that MWI does not work-- has been proven.

Penrose is bothered because MWI does not work (bold face from mine):

[MWI] is not a very economical description of the Universe but I think things are rather worse than that for the many-worlds description. It is not just its lack of economy that worries me. The main problem is that it does not really solve the problem.

I asked you where in the proof of the theorem you quote is there an observer, where is there an observer's mind, and where is there the epistemology that observer chooses to use to decide what it means for a theory to be valid. Until you put those things in your proof, you have not gone beyond your own philosophical assumptions.

I wonder why you think that the mathematical proof that R and U are irreducible needs of such inputs as an «observer mind». I know that most of philosophical literature is rather confused about QM and still believe that collapse of wavefunctions is caused by the mind of an observer, but this all is the usual philosophical nonsense that lead to the well-known physicists reply: «Shut up and Calculate».

If this were as categorically true as you imagine, then no one would have thought MWI is a reasonable interpretation of QM.

There are many instances of correct statements that, however, are denied by some minority of persons with philosophical prejudices.

For instance, «Earth is not flat» is a correct statement. Still some people today think that their Flat Earth 'theory' is a reasonable interpretation of the properties of our planet, and they even join in a Society

http://en.wikipedia.org/wiki/Flat_Earth_Society

My point here is that it does not matter how many times MWI is proven wrong, useless or nonsensical; you probably will always find someone who supports MWI.

The actual truth is that if you start with only U, you cannot obtain R without altering your philosophical preconceptions about what physics should be trying to do, or how we should regard the connection between physics and reality, especially in regard to the importance of empiricism. It you want it 'more "technically", just look at the proof of any theorem that says you can't get R from U, and look where the philosophical assumptions come in. You might need to see outside your box a bit more.

Evidently the mathematical proof that U and R are irreducible has nothing to see with «philosophical preconceptions» neither with «philosophical assumptions».

Um, I think you should have a look at your own figure again. Penrose puts the "only U" camp under "serious about psi". Perhaps your own definition of "serious" is a little different from Penrose's? As near as I can tell, you define it as "agrees with me". (But you are right that I misplaced Gell-Mann.)

Not only I am right on that you misplaced Gell-Mann, but you continue misreading and confusing everything.

If you were to read what I wrote, then you would see that I wrote «really serious». Although you think that is my definition, it is not mine. Penrose explains, in his book, who in that diagram is really serious (Penrose own words) about QM and who is not.

For instance, Penrose correctly claims that Hawking is not «really serious» about QM.

It seems evident that you confound «serious about psi» with my «serious about QM». The «serious about psi» label is different. Penrose uses this label to differentiate those who believe that Psi is something about our mind of those who understand that Psi gives the physical state of the quantum system.

 

[Ken G]

Hawking believes that MWI works, but he, of course, has never proved such thing. However, the contrary thing, --i.e., that that MWI does not work-- has been proven.

Penrose is bothered because MWI does not work (bold face from mine):

Often you have claimed that MWI is inconsistent with the current postulates of QM, and you've claimed this has been proven (I already told you that any such proofs will express philosophical priorities that need not be accepted), and now you actually bold-face a sentence by Penrose as if it supported this stance, yet it clearly says nothing of the sort! That's your evidence? What Penrose said is that MWI doesn't solve the problem with QM. That is just precisely what I told you that Penrose felt about MWI. Yet you see no importance in what Penrose did not say: that MWI is inconsistent with the current postulates of QM! You are an intelligent person, probably extremely so, so why can you not see that if Penrose thought MWI was inconsistent with the postulates of QM he would have said so?

I think it is pretty clear what Penrose is actually saying-- he is saying that MWI doesn't solve the problems that the the current postulates of QM have. That means, he doesn't think MWI goes beyond those postulates, in the way his own intepretation attempts to do.

I wonder why you think that the mathematical proof that R and U are irreducible needs of such inputs as an «observer mind».

Simple. The proof will make assumptions about what it is talking about. Anyone who simply rejects those assumptions, say on the grounds that it does not make any account of the observer's mind (which is obviously part of any complete accounting of physical phenomenon), and thus reject the "proof." That is the nature of proofs: they are only as good as their assumptions.

I know that most of philosophical literature is rather confused about QM and still believe that collapse of wavefunctions is caused by the mind of an observer, but this all is the usual philosophical nonsense that lead to the well-known physicists reply: «Shut up and Calculate».

Apparently you are unaware that "shut up and calculate" is also a philosophical stance. Most people do realize that. More to the point, shutting up and calculating means use QM to make predictions, which we already know how to do and nothing that Penrose is doing has anything to do with that (he hopes his approach can be empirically demonstrated, but that is certainly not why he is doing it, and frankly there is fairly little likelihood that it will be empirically demonstrated any time soon).

My point here is that it does not matter how many times MWI is proven wrong, useless or nonsensical; you probably will always find someone who supports MWI.

Well, you might think that comparing Hawking and DeWitt to Flat Earth society members is a cool rhetorical device, but I think it is meaningless hooey.

Evidently the mathematical proof that U and R are irreducible has nothing to see with «philosophical preconceptions» neither with «philosophical assumptions».

That is where you are quite wrong, as I explained above. Maybe we would make better progress if you would cite such a proof, and then I will show you the philosophical assumptions involved.

It seems evident that you confound «serious about psi» with my «serious about QM». The «serious about psi» label is different. Penrose uses this label to differentiate those who believe that Psi is something about our mind of those who understand that Psi gives the physical state of the quantum system.

See? Your philosophical preferences are showing already, yet you seem blind to them. Your position is evidently that it is an objective fact that Psi gives "the physical state of a quantum system." What is a physical state? What is a quantum system? Please prove to me what these things are. Use pure mathematics please, no "funambulism". Oh, and no philosophy either, obviously.

 

[juanrga]

Often you have claimed that MWI is inconsistent with the current postulates of QM, and you've claimed this has been proven (I already told you that any such proofs will express philosophical priorities that need not be accepted), and now you actually bold-face a sentence by Penrose as if it supported this stance, yet it clearly says nothing of the sort! That's your evidence?

I have bold-faced a sentence by Penrose by another reason, evidently.

The proofs that MWI is internally inconsistent and disagree with QM predictions were given many messages ago and repeated often...

Simple. The proof will make assumptions about what it is talking about.

This is untrue.

Anyone who simply rejects those assumptions, say on the grounds that it does not make any account of the observer's mind (which is obviously part of any complete accounting of physical phenomenon), and thus reject the "proof."

Philosophers are very confused about this topic.

It is not needed to appeal to observer's mind to understand quantum measurement, just as it is not needed to appeal to observer's mind to explain how a thermometer works.

The idea that quantum measurement has something to see with a human mind is pure nonsense.

Apparently you are unaware that "shut up and calculate" is also a philosophical stance.

You seems unaware that the slogan was invented to avoid the rambling endlessly about the philosophical implications of QM, and is appropriate to use when there is an imbalance between the amount of philosophy and the amount of calculation.

Maybe we would make better progress if you would cite such a proof, and then I will show you the philosophical assumptions involved.

The mathematical proofs were given to you several times before, and you systematically ignored. Although you continue disputing them using philosophical pseudo-arguments.

See? Your philosophical preferences are showing already, yet you seem blind to them. Your position is evidently that it is an objective fact that Psi gives "the physical state of a quantum system." What is a physical state? What is a quantum system? Please prove to me what these things are. Use pure mathematics please, no "funambulism". Oh, and no philosophy either, obviously.

Once again the «philosophical preferences» are only in your mind.

If you open a textbook in QM (I cited some), you can learn why Psi gives the physical state of a quantum system. Examples of quantum systems and their properties are given as well in textbooks.

Of course, the concept of physical state is more general. For instance, any textbook on thermodynamics will explain you what is the physical state of a thermodynamic system at equilibrium.

Such books are to be found in the Physics section (not in the philosophy section) of your favorite library.

 

[Ken G]

I have bold-faced a sentence by Penrose by another reason, evidently.

And evidently, I'm missing the bold-faced sentence where Penrose actually says what you keep claiming he is saying: that MWI is inconsistent with any theory that can make the same tested predictions as the current postulates of QM that can be found in standard textbooks. Your argument lacks evidential support in a very blatant and obvious way, yet in all your posts, you cannot correct this gaping flaw.

The proofs that MWI is internally inconsistent and disagree with QM predictions were given many messages ago and repeated often...

And I repeated many times that to be what you claim, such proofs must stick entirely to the postulates of QM, with no philosophical assumptions at all. As such, they must use the postulates of QM to present a complete accounting of everything that is involved in a "QM prediction", including how such predictions are confronted with observations (to give the concept of "prediction" meaning in the first place). As such, they must account for the observer involved in verifying the prediction. They don't. Now, you might think that's a technicality, but obviously it isn't, as this is precisely the crux of many of the distinctions in the interpretations of QM. But I'm repeating myself.

This is untrue.

So you claim, but you are wrong. To see this, all we need to do is what I suggested we do: present me with such a "proof", and I will show you where the philosophical assumptions appear, and how an MWI proponent could reject those assumptions. It's really quite simple-- you can't get R from U, but the MWI proponent doesn't think you ever get R. You only get R when you adopt certain philosophical assumptions about what a prediction is, and how physics should work. Go ahead, show me your favorite proof, this won't take long.

Philosophers are very confused about this topic.

When it comes to issues like this, everyone is a philosopher. You are just another one of those who defines "philosophy" as "everything I don't agree with", and "scientific fact" as "everything I do." But that's not actually what philosophy is. I'm sure Penrose understands the role of philosophy, as Hawking clearly does as well. Penrose merely adopts the label "very serious about QM" for people who share his philosophical objectives. Many others, like Mermin, do not-- they don't think QM is anything but a system for making predictions, and there is no dispute among anyone about how to use QM to do that, at least in regard to everything that has already been observed that is used to support QM.

It is not needed to appeal to observer's mind to understand quantum measurement, just as it is not needed to appeal to observer's mind to explain how a thermometer works.

Interesting philosophical opinions. I would say they are quite naive, as is typical of people who don't know when they are using philosophy.

The idea that quantum measurement has something to see with a human mind is pure nonsense.

More philosophical opinionating. But I would say it is perfectly demonstrable that all of physics, not just quantum measurement, as "something to do with the human mind." Indeed, I would say that is quite obvious, but what is obvious to me and what is obvious to you can be very different things, and since we are both intelligent, this must trace back to our different philosophical priorities and assumptions. The main difference between us is that I realize this and you do not.

You seems unaware that the slogan was invented to avoid the rambling endlessly about the philosophical implications of QM, and is appropriate to use when there is an imbalance between the amount of philosophy and the amount of calculation.

I know perfectly well why the slogan was invented. I also know the words of Aristotle: "If you will philosophize, then you will philosophize. If you will not philosophize, then you will philosophize." Or those of Blaise Pascal: "To ridicule philosophy is really to philosophize." Either ponder on those, or dismiss the intelligences of Aristotle and Pascal. The truth is that "shut up and calculate" is not a means for avoiding philosophy, it is a philosophy, it is a very deep statement about the limitations of science that most scientists like yourself given your claim that those who understand QM know that a wave function is truly a "physical state", while dodging the request to even define that term) are loathe to believe, yet think they are "serious" in their disbelief.

The mathematical proofs were given to you several times before, and you systematically ignored. Although you continue disputing them using philosophical pseudo-arguments.

Pick your favorite one, anyone will suffice, and cite the assumptions that it uses. I may have to uncover the implicit ones, though.

Once again the «philosophical preferences» are only in your mind.

I realize you believe this, simply repeating your beliefs is rather pointless. Or is that what you call a logical argument?

If you open a textbook in QM (I cited some), you can learn why Psi gives the physical state of a quantum system. Examples of quantum systems and their properties are given as well in textbooks.

Now your argument is just plain silly. Of course I know all about QM textbooks, as do all the other people who don't think that psi is a "physical state" (they begin by attempting to define the term, which of course is the whole point-- by not even trying to do that, you expose the frailty of your position). The fact is, textbooks are not interested in establishing their philosophical assumptions, they are all implicit. This is simply because textbooks are not trying to probe the philosophical foundations of QM that would let you actually prove the things you claim that QM can prove. Instead, textbooks are interested in laying out the theory well enough that it can be used to make calculations, but it always dodges the issue of collapse. This is the entire reason that we have so many different interpretations of QM in the first place! You appear to think we can sector those interpretations into two camps: those that agree with your implicit philosophical assumptions, which you call the "serious" ones, and those that do not, which you dismiss. That is actually quite typical behavior of people who don't understand philosophy, I see it on all sides of the interpretation debates. That's why I pointed out that Deutsch claims Penrose is "doing aesthetics not physics", while Penrose says Deutsch is "not serious." Obviously, these people cannot agree, and the clear reason is they have not recognized how their philosophical opinions have colored their judgements. What gives Penrose the right to judge what is serious? What gives Deutsch the right to judge what is physics? These are issues that physicists do not agree on, it's just that simple-- deal with it!

Of course, the concept of physical state is more general. For instance, any textbook on thermodynamics will explain you what is the physical state of a thermodynamic system at equilibrium.

And in the process, will make idealizations and philosophical assumptions. The point is, I can very easily recast thermodynamics, or any physics theory, into something that looks completely different, yet makes all the same predictions that can actually be tested, yet proves very different things about what that theory can and cannot do, by simply adopting a very different philosophical stance. This is precisely what you do not realize.