Problems with Many Worlds Interpretation

[Hurkyl]

A little off topic, but since you raised it, this is new to me. I've always considered proper time along a timelike world line as an invariant observable. Can you clarify what you mean?

I've heard it a couple times, but I didn't think to keep a reference, and I couldn't find one with a brief search -- IIRC it's either something akin either to the hole argument or something to the effect of the stress-energy tensor (including time evolution) isn't enough to determine the metric (and thus isn't enough to determine the proper time of a time-like path)


But even if I'm flat out wrong, it still makes a good mental exercise for the topic under discussion.

 

[Ken G]

I'm not entirely sure I would agree with the claim that "the weights on the components of a mixed state correspond to the probabilities observed in experiment" is ad-hoc, but I can sort of see it.

Well I'm glad we can kind of agree that this is an ad hoc element of both MWI and CI! I really don't see any difference in the two interpretations in this regard, frankly-- to me, CI imagines a statistical distribution of single outcomes witnessed by a single observer (which certainly feels like what is happening), and MWI imagines a statistical distribution of single observers witnessing a single event that embodies all the possible outcomes. Which end you put your statistical distribution on, and whether or not you imagine an ontological unity behind the distribution, is pretty irrelevant to the physics. It all comes down to one's own philosophical preferences, largely rationalist vs. empiricist.

Hypothetically^* speaking, would your opinion change (or, at least, weaken) if one could theoretically derive that the weights on the components of a mixed state

Yes, if the Born rule had a derivation in one interpretation, that wasn't "rigged" like using anthropic thinking, then I would indeed see that as a huge advantage for said interpretation.

But I don't see your point -- what does this have to do with your assertion that anyone who talks about something that isn't an eigenvalue cannot care what frogs see? Are you claiming that measurements involve spontaneous decoherence which cannot be the product of a system interacting with its environment through unitary evolution?

No, it is quite apparent that measurements involve spontaneous (in the thermodynamic sense, not some magical sense) decoherence that is describable through unitary evolution of the closed system. Projections onto open subsets produce the mixed states. That's just quantum mechanics, it has nothing to do with any interpretation of quantum mechanics. The interpretation doesn't even enter until you specify what you think is the ontological message of the mixed state. In CI, the message is, "one of the outcomes actually occurred, and you just don't know which, but you will when you look." In MWI, the message is, "all the outcomes occurred, so there is no need to know which, even when you look, because then you are just entering into some kind of illusion." Decoherence has exactly nothing to do with that difference, it merely clarifies why we get to the place where we need to address the difference, which we already knew we'd get to.

Yes it should -- a theory with greater degrees of it has less ability to make precise predictions, and correspondingly experimental evidence for the theory becomes less meaningful.

I agree-- I meant that the existence of ad hoc elements shouldn't bother us, what should bother us is if there are too many such elements. I claim that CI involves no more ad hoc elements than does MWI, the "collapse" is not ad hoc, it is what we see. It is no more "ad hoc" to try to create a theory that connects to mystifying aspects of our experience than it is ad hoc to pretend our experiences fit the axioms of the theory and we just don't know that they do, which is more or less what you are saying.

The CI is more specific than that -- depending on the version, it further asserts that a mixed state is a matter of ignorance or non-determinism, that one should specifically not interpret it as corresponding to the physical system.

A mixed state in CI is an absence of information about the state, I agree. It is completely dual to the way MWI treats a mixed state as an absence of information about the observer's perception. I really don't see any difference there, except in one's priorities about what one thinks physics is for.

(I refer to the individual case, since I haven't thought through ensemble variations beyond the initial impression that they represent "giving up" in some sense)

I agree, the ensemble approach is the place where CI and MWI are really indistinguishable even in principle, because an ensemble is already a kind of "many worlds." So in going to an ensemble, one is dodging the need for an interpretation.

On the statistics, MWI says that one should think of the mixed state as corresponding to the physical system.

Yes, but in effect the distribution is then over the observers interacting with the mixed state. It's no more different than the Schroedinger vs. Heisenberg difference in whether the wave functions evolve or the operators. Dynamics in the dual space of states vs. observations is equivalent, and so are statistical distributions in that same dual space. It's nothing but philosophical priorities, and the advantage of CI is that it isn't motivated by a need for the theory to be ontologically exact because it adopts the empiricist attitude that theories are tools. This makes it much more flexible, unless QM axioms happen to be exactly correct.

On the origin of the weights, they come from decoherence, which MWI posits is the effect unitary evolution, rather than being spontaneous.

There's still no difference there, it is not the least bit important to CI what causes those weights to appear. Spontaneous doesn't mean magical, it just means that some very high-dimensional phase space is maximizing its entropy. That's just exactly what decoherence says too.

Does your variety of CI posit that measurement involves spontaneous decoherence? Or causal decoherence followed by spontaneous collapse?

The former. Remember, in CI collapse isn't a "happening", the observation is the happening. The collapse is what explains the observation, and quantum mechanics explains the collapse. All we need the interpretation for is to say what the collapse means. Your objections to CI sound like ones I often hear from people that I don't think really understand CI-- if I thought of it the way you do, I wouldn't like it either.

But, I expect the above is a diversion, and doesn't actually address the point we disagree upon.

I'm not sure what point we disagree on, so let me clarify what I'm saying:
1) CI and MWI are scientifically equivalent, in that they use all the same mathematics, make all the same predictions, and suggest the same observations to test these predictions. They only differ in their philosophical priorities, and in how they might guide us toward new theories that are not QM. Thus, it behooves us to recognize these different philosophical priorities, to help us understand the guidance these interpretations are giving us.
2) The main philosophical priorities that differ is that CI takes the perception of the observer as the crux of the purpose of doing physics, and MWI takes the axiomatic structure of the theory as the crux of the purpose of doing physics. This leads CI to treat the axiomatic structure as a kind of tool or effective theory, designed only to make predictions about outcomes. It also leads MWI to regard the perceptions of observers as illusory, even though they will find that the theory does predict what they do see on a statistical basis, because the actual reality must fit the axioms not the perceptions. In a nutshell, this means CI is motivated by the empiricist fondness of the relative concreteness of experience and measurement, and MWI is motivated by the rationalistic fondness of the aesthetic beauty of unified postulates.

Now, what aspect of the above are we not agreeing on? I realize that some of what I said was critical of MWI, but mostly it was not a logical inconsistency kind of criticism, it was a failure to recognize the philosophical and extra-scientific elements of the interpretation. So about the only way you could really disagree with me is if you think that MWI is "just good science", and does not represent the rather one-sided philosophical priorities that I suggested it does.

CI is extremely antagonistic to the idea that a measuring device or an observer obeys the laws of quantum mechanics.

Yes, that is true. Indeed, like any empiricist interpretation of any physical theory, it is antagonistic to the very idea that there is any such thing as "obeying laws" in the first place. To the empiricist, the very words "obeying laws" have a much weaker meaning-- it just means "allows us to interpret the outcome of observations by mathematically manipulating the measurables according to some highly unifying yet idealized principles that are presumed to be only approximate." Since the role of "laws" is as approximation tools for the observer, it does not even make sense to imagine that the observer obeys laws-- the observer is the master of the laws, not their slave. All the same, observers can observe each other, so a law is not useful if it cannot also be used to explain observations made on other observers. That does not require that the observers "obey" laws in any way but the weak sense already described.

I honestly cannot see how a collapse-as-reality interpretation can survive if quantum mechanical theories start expanding their domain to scales which include measuring devices and observers.

I have no doubt that quantum mechanics works fine on that domain, yet collapse-as-reality has no difficulty with any of that. It would all fit just fine with the statements I just made that characterize the crux of empiricism, and often summarized as "the map is not the territory."

A collapse-as-being-just-as-good-of-a-description-of-reality-as-a-mixed-state interpretation would likely survive, but that's not CI.

I don't understand, if collapse is just as good as mixed state, then one can adopt either. That means one can still use either CI or MWI, which would certainly be my expectation. If your basis for accepting MWI is that you don't think CI is workable, and that motivates you to tolerate the more bizarre ontological constructs of MWI, then I suggest the problem is with your understanding of CI. I have no doubt that you understand the MWI profoundly, and it probably helps you do QM, which is reason enough for it to be a good interpretation for you. But even an interpretation has two meanings-- the harmless one, which is a picture that one uses while one is applying a theory, and a more insidious one, which is a picture that generates a devoutness to a world view. Devoutness to world views is probably not supposed to be a goal of science, though we certainly all have our own personal reasons for doing it.

Yes, for the same reason we still do Netwonian mechanics. MWI can only fall apart if quantum thermodynamics doesn't work out.

If any aspect of QM doesn't work out, then both CI and MWI are wrong, but they only "fall apart" if one uses them for more than they should be used for in the first place. I learned Newtonian mechanics, then I learned it wasn't right, but it never "fell apart" for me because I was never devout about it and I knew I could still use it in all the same situations.

Empiricism now is about the experiences of imaginary people?

The word "imaginary" is problematic, I prefer "hypothetical." But yes, empiricism would be a completely powerless perspective on science if it did not support the concept of a hypothetical observer. The point of empiricism is not that we have to interpret reality based on what actually observers really saw (the tree in the woods business), it is that we have to interpret reality based on what actual observers are capable of observing and communicating to others-- whether they were there or not. However, whether they are there or not is part of the reality, so we recognize that a reality that contains a hypothetical observer might be quite a bit different from one that contains no observer. This rarely comes up in relativity though, hence the concept of a coordinate chart based on the concept of a reality with hypothetical observers being the same as one without. But yes, the whole issue of hypothetical observers in empiricism is just as misunderstood as the CI, and there may be "hard core" empiricists who reject the concept, just as there are "hard core" CI proponents who think the wave function is real and it really collapses. I am not speaking for the hard-liners, rather the garden variety empiricists.

Er, yes. So do you, it seems:

• people just experience things
• putting your hand on a stove is a painful

This is the point you keep returning to, and it is the reason you do not understand empiricism. I have already agreed with you that there is no such thing as "pure" empiricism, because rocks don't make measurements, thinking beings do. Similarly, there are no "raw" experiences, and your experience of the pain of a stove might not be the same as mine. But they don't need to be, all that is required for empiricism is the ability to establish consistencies of experience. Measurable outcomes like "heads" when a coin is flipped, that's all that is required, because it allows us to do science on those experiences without bothering to characterize them any more than saying "the distance is X" or the "coin was heads." So as long as you cannot understand experiencing "heads" or "tails" when you look at a flipped coin, you will never understand empiricism.

 

[Ken G]

I've heard it a couple times, but I didn't think to keep a reference, and I couldn't find one with a brief search -- IIRC it's either something akin either to the hole argument or something to the effect of the stress-energy tensor (including time evolution) isn't enough to determine the metric (and thus isn't enough to determine the proper time of a time-like path)


But even if I'm flat out wrong, it still makes a good mental exercise for the topic under discussion.

And notice how rationalistic are all those answers! You are not really saying the proper time isn't the fundamental observable in GR (it is, along with local distances), you are saying the concept isn't bulletproof in GR. Empiricists are never surprised to find any theory is not bulletproof! But we still find it telling that the beating heart of GR is the observable proper time, that's the most important scalar to know about any world line.

You see, empiricists don't much care when a theory says that an observation cannot be made, we care about making the observation. If the theory was wrong, then the observation is still the reality, and if the theory was right, then the theory is describing a fundamental limit about reality. Neither is a problem for the empiricist, we would still just say that the mathematical structure of GR is constructed around the concept of proper time, which dovetails with an observable, and empiricists find that quite telling. If that concept doesn't always work, we say, welcome to the world of the limitations of either physics theories, or reality itself, depending on whether or not the observation is actually possible. (What's more, an empiricist of my own brand doesn't think observations are actually reality, I just think that they are what we have to do science with. So I would call the observations the scientific reality, and what actual reality is is essentially mysterious. I don't object to MWI on the grounds that its ontology is mysterious, just that it is not supported by empiricism.)

 

[Ken G]

I think "no" is a more accurate short response then "yes". I would say that the main assumption is that it makes sense to talk about the wavefunction of the universe, and that this wavefunction describes reality. Density operators are still needed, in particular to describe the correlations between subsystems after decoherence (and therefore to describe the frog's view), but the wavefunction of the universe is what "describes reality" (the bird's view anyway).

But doesn't the density operator preserve all the useful information in the wave function, both before and after decoherence? I realize that the mapping from the wave function to the density operator is not invertible, but is any relevant information lost? For example, surely even MWI can't believe that the global phase of the wave function is physically part of the state of the system, as that would really be angels on a pin, even for MWI!

 

[Hurkyl]

Note that it is no solution to consider bigger and bigger observers (so the big observer can describe the original observer) quantum mechanically.

You're referring to the argument that considering a quantum system presupposes an observer that observes the system?

I've heard the argument, but I've never heard a convincing argument for the presupposition. I don't need to posit Wigner's existence in order to consider the evolution of a quantum system that includes Wigner's friend. The point of also having Wigner in the thought experiment is so that you can have two observers disagreeing on their description of reality. For example, one reason would be to refute of the paired hypotheses

• All observers are equal
• Collapse is something that happens at the interface between our experience and the external world

I think instead the challange is to reconstruct measurement theory so that it does make sense from the point of view of a general observer.

The general approach I'm familiar with is:

• Construct a theory that is easy to understand and work with
• Extract what a general observer sees

For example, in Newtonian mechanics, it's rather effective to:

• suppose the universe is described by three-dimensional Euclidean geometry
• work out the theory in that context using ordinary methods of Euclidean geometry (rather than, e.g., trying to always work in observer-centered coordinate charts)
• work out what general observers can observe -- e.g. he can't observe position, but can observe the distance between two positions

You could try to build that last point in right into the beginning -- but interestingly, this is a case of less is more. The only good mathematical structure I know for doing such a thing boils down to adding structure -- to equip everything in the mathematical universe with an action by the Euclidean group (rotations, translations, and reflections), and then insist everything must respect symmetry.

Despite the appearance more machinery, the requirement of symmetry really does result in the effect we've removed structure -- for example, in this setup, Euclidean space doesn't have any points at all! (picking a point doesn't respect symmetry)

 

[PAllen]

I've heard it a couple times, but I didn't think to keep a reference, and I couldn't find one with a brief search -- IIRC it's either something akin either to the hole argument or something to the effect of the stress-energy tensor (including time evolution) isn't enough to determine the metric (and thus isn't enough to determine the proper time of a time-like path)


But even if I'm flat out wrong, it still makes a good mental exercise for the topic under discussion.

Well, the hole argument I recall was a famous mistake of Einstein's that delayed GR a couple of years.

The stress energy tensor is equal to the Einstein tensor, and determines the metric modulo diffeomorphism and boundary conditions.

If the metric is unknown GR has no observables or predictions at all. So either proper time on a world line is an observable, or GR has not content at all.

 

[Ken G]

I agree it's a problem that the formalism only makese sense for "classical obervers". But as I see it this is a problem of QM itself, not of interpretations.

The way I like to put what might be a similar idea is that if electrons could do physics, they wouldn't do quantum mechanics, because quantum mechanics is manifestly a theory for determining the outcomes of macroscopic couplings. So it's not that quantum mechanics doesn't work in the classical world, it's that quantum mechanics only works in the classical world. This idea might not be consistent with your ideas about generalized observers, or with Hurkyl's qubit observer, so it would be interesting to follow where that leads-- perhaps on its own thread!

I think instead the challange is to reconstruct measurement theory so that it does make sense from the point of view of a general observer.

What worries me about this program is that I see physics as fundamentally a language. It is a language that involves mathematical structures, and observational testing, but it's a language all the same. If so, then it suffers from the same limitations of all languages-- it can never do anything more than draw connections between experiences. That's all any dictionary can do, it cannot really explain what any words mean, it can only connect words to experiences by connecting words to other words that are already connected to experiences. So we might have a very complex language of particles and waves and mathematical postulates, but at the end of the day we are just connecting some experiences to other experiences, and finding patterns. How do we ever get the "classical" out of that situation, given that our experiences are the experiences of beings with a vast number of interacting subsystems? If Penrose is right that we do think "quantum thoughts", in effect, then perhaps I am wrong. But if we are fundamentally classical thinkers and perceivers, I think we're stuck with that, and no language involving a "general observer" will ever really mean anything that we can understand because we only understand what we can connect to our experiences.

The problem starts when we try to understa unification and cosmologial theories, then we can't keep referring to classical observers anymore. but these problems was not things taken into considerations when QM was constructed.

I believe that we are referencing classical observers when we construct models like unification and the early moments of the Big Bang. Certainly matter as we know it would not have existed then, so we cannot embed a Stern-Gerlach apparatus into the first femtosecond of the Big Bang, but I claim that this is essentially what we do all the same. It's because although the Stern-Gerlach and its brethren aren't there on the scene, they still provide us with all the elements of the language we will be using to talk about those early times. So the ghosts of those classical apparatuses are everywhere in the machine of the early universe, implicitly, just in ways that we can all too easily overlook if we take a highly rationalistic perspective on what is happening there. How could it be otherwise? If the language we learned from our own apparatus is no useful agent for analyzing the early universe, then it would be like saying physics changes with time, and if the language is useful, then it's still fundamentally classical.

The way I see it the collapse isa matter of perspective, and thus obsever independent. What comes as a collapsing surprise to one observer, might well be expect by anotther one. In this sense the collapse is not real as in "objective". It is however real in the sense of phenomena since the objective implication of this, is that one can imagine a mechanism (whe nreconstructing measurement theory) where the ACTION of a system, depends on that it experiencs a collapse, because at each collapse the action changes in a non-trivial way, and if you consider a sequence of collapses resulting in rational information updates - this looks just like a dynamical evoluion to a different observer.

That's an interesting insight. I guess I'd need to see an example.

 

[Fredrik]

But doesn't the density operator preserve all the useful information in the wave function, both before and after decoherence? I realize that the mapping from the wave function to the density operator is not invertible, but is any relevant information lost? For example, surely even MWI can't believe that the global phase of the wave function is physically part of the state of the system, as that would really be angels on a pin, even for MWI!

D'oh, for some reason I forgot to consider phase factors when I wrote that post. It's more accurate to say that a 1-dimensional subspace describes the universe than to say that a unit vector |\psi\rangle in that subspace does. The 1-dimensional subspace can be represented by its projection operator |\psi\rangle\langle\psi|, which happens to satisfy the definition of a state operator. So I guess I have to take back what I said. The state of the universe is represented by |\psi\rangle\langle\psi|, not |\psi\rangle.

 

[Eqblaauw]

a serious question to people who believe in mwi,
do you feel less sad when a beloved one dies in an unlikely quantum chance event, for then he will live on in many worlds, then in a death less chance related?

 

[Fra]

it's that quantum mechanics only works in the classical world.

Good way of putting it. Makes good sense to me. Indeed QM (as it's currently formulated) ONLY works given a classical context. This is simply how it is. This is however not a statement of nature, it's more a statement of the current state of the theory.

My ambition is to improve of course, but reinterpretations doesn't solve anything as far as I can see.

That's an interesting insight. I guess I'd need to see an example.

Actually I won't deny that this is non-trivial. Like I said this isn't an pure interpretation, it's rather an interpretation that "understand QM" valid only for limiting cases. So in the extension taking this seriously really is a deep conjecture that may or many not bear fruit.

Until I am prepare to be explicit, I suppose I was hoping that the conceptual point would get across? The conceptual point is that if you consider a physical system as a player in a game, whose actions follow more or less "rationally" from what he infers from his experience in the game, then it should be clear the "description" of how this player acts, from the point of view of an outside observer will contain evolution rules suchs as hamiltonians that are in principled originating from the ignorance of the system of it's own environment.

So while no eternal observer sees the actual collapse, we can see that the system behaves just as if it did see a collapse, given the above conjecture.

Suppose you see a ghost. I see you and infer that "you look just like you've seen a ghost". It means I didn't see a ghost, but I did see someone that acts in consistency with having seen a ghost (ie. there I am connecting the presumed experience of the collapse with a rational REaction). So maybe the ghost isn't real to me, but your behaviour is. And that all that matters. This is why in the external "descrpition" the ghost is gone! Yet it's essential for understanding the action.

But this is also why I think this suggestion is non-trivial. It's not just something that makes no difference, it's rather an "interpretation" suggest a research direction that I personally see as rooted in the Bohrs view. Just that maybe we try to do what Bohr "should have done" if he knows what we knows today at that time. That's how I see it.

/Fredrik

 

[Ken G]

The conceptual point is that if you consider a physical system as a player in a game, whose actions follow more or less "rationally" from what he infers from his experience in the game, then it should be clear the "description" of how this player acts, from the point of view of an outside observer will contain evolution rules suchs as hamiltonians that are in principled originating from the ignorance of the system of it's own environment.

I do think that is an interesting perspective to take, it sort of reverses the old "god made man in his own image" into "intelligence makes perception into its own image." So perhaps we would not say the systems are themselves intelligent negotiators with their environments, but we are, so we must cast our perceptions of nature into a similar form if we are to gain understanding. So understanding is not entering into "the way nature thinks", as we often picture the process, but instead it is finding a workable way to "get nature to think like we do." It's still science because it must pass tests, but we recognize from the outset that the tests are rigged-- they have to make sense to us.

So while no eternal observer sees the actual collapse, we can see that the system behaves just as if it did see a collapse, given the above conjecture.

That sounds similar to what I would call the role of the "hypothetical" observer in science. It's a form of counterfactuality.

Suppose you see a ghost. I see you and infer that "you look just like you've seen a ghost". It means I didn't see a ghost, but I did see someone that acts in consistency with having seen a ghost (ie. there I am connecting the presumed experience of the collapse with a rational REaction). So maybe the ghost isn't real to me, but your behaviour is. And that all that matters. This is why in the external "descrpition" the ghost is gone! Yet it's essential for understanding the action.

Yes, I think that is a fitting metaphor for more or less everything that physics does. May I use it in informal settings? (On another thread, we are discussing just how "real" are or are not "virtual" particles-- your metaphor would be quite apt there.)

But this is also why I think this suggestion is non-trivial. It's not just something that makes no difference, it's rather an "interpretation" suggest a research direction that I personally see as rooted in the Bohrs view.

I agree that the real value of a good interpretation is not how it helps you understand the current theory (that's subjective), but rather how it guides you to the next one.

 

[Eqblaauw]

i'm posting it again

a serious question to people who believe in mwi,
do you feel less sad when a beloved one dies in an unlikely quantum chance event, for then he will live on in many worlds, then in a death less chance related?

 

[Hurkyl]

I don't understand, if collapse is just as good as mixed state, then one can adopt either. That means one can still use either CI or MWI, which would certainly be my expectation. If your basis for accepting MWI is that you don't think CI is workable, and that motivates you to tolerate the more bizarre ontological constructs of MWI, then I suggest the problem is with your understanding of CI.

Be aware that you are the first person I've ever encountered who says they prefer CI but could also admit that retaining the mixed state is just as good as collapsing it. (even if you prefer not to think in the former way)

I mentioned it earlier, but I think it warrants bringing it up again -- my actual take on the matter is that when an observer collapses a wave-function after measurement, it's nothing more significant than a change of "reference frame".

So when we want to understand how systems evolve, we probably consider the process of unitary evolution, without the notion of collapse entering our minds. Later, when we want to condition on past observations, we might invoke collapse to simplify the both the physical questions we ask and (possibly) calculations needed to answer them.

 

[Eqblaauw]

why don't you answer the question hurkyl? Maybe you don't explain why, maybe you do explain why, I'm curious

 

[Hurkyl]

why don't you answer the question hurkyl? Maybe you don't explain why, maybe you do explain why, I'm curious

I haven't responded to what you've written because I've been busy conversing with Ken G, and when skimming your posts, I didn't see anything that made me feel like jumping into another thread of conversation.

And to be honest, fairly or not, I felt you had been somewhat pushy and was mildly irritated. I had actually planned to make a brief response to your question, but I was put off by your insistence and it was about time to leave for work so I didn't bother.

I hadn't planned to make a big deal out of it, but since you asked directly...

And the question is pretty much purely psychology anyways. (My response would have been basically this sentence, possibly with a little more elaboration)

 

[Eqblaauw]

ok sorry for being pushy, but I think it's a relevant question, though purely psychology since when you believe a theory, it's interesting how much it affects your life,
and since we're human, it's interesting to know the human consequences of a theory

 

[Fra]

I mentioned it earlier, but I think it warrants bringing it up again -- my actual take on the matter is that when an observer collapses a wave-function after measurement, it's nothing more significant than a change of "reference frame".

I think I see your point, but there is another thing that only makes this view a partial answer IMO.

I certainly agree that collapse or not is observer dependent. This is what I said in some previous post too.

The difference that makes this still different from the usual picture of references frames of say SR is that the set of all most general observers and it's "transformations" are not konwn. I'd even say that it's impossible for an observer to defined any observer invariants, because any attempt to do so still contains another level of observer dependence and thus in principle the equivalents of higher level collapses. This is because the bird is just a big frog.

So if the big frog is the primary observers (from his picture) then all the small frogs he is watching are secondary observers. What this big frog can do, is to define invariants with respect to the secondary observers, since from the perspecive of the big frog these are like gauge choices.

This is clear, but once you acknowledge that no real observer is infinite or omnipresent, there is still an evolution of the picture the big frog has. This has implications for when you consider two big frogs interacting.

So I think I agree with that the collapse is a matter of "observer choice", but this does not mean that it's possible to fundamentally get rid of it (like we get rid of the references frames in SR). This situation is much more complex, because the covariance of the secondary pictures needs to be inferred withing the primary picture (which again is a choice).

So I'd say my picture is this: there is an hierarchy of coupled gauges (like micro and macro gauges):

In conventaional models, there is usually a "classical domain" where things are wrapped up and you get rid of all gauges and establish the invariants. In my view this is an idealisation that ignores the cosmological interactons and it's selection on the laws. /Fredrik

 

[kith]

I would say that the main assumption is that it makes sense to talk about the wavefunction of the universe, and that this wavefunction describes reality.

[...] if one goes the MWI road, then purely classical physics is also subject to the MWI interpretation.

Thanks KenG and Frederik! I'm not sure, if my issues are already solved. ;-)

First of all, it now seems clear to me that the fundamental state is a pure state also in MWI, while I was assuming that it could be mixed when I wrote my last post.

But we can't choose whether we represent this fundamental state as a ray in Hilbert space or as a density operator in MWI, because the density operator is required for assigning reality to the subsystems.

So my current understanding is this: in CI, a density operator describes the state of an ensemble of single systems. In MWI, a density operator describes the state of the single system itself (even if it's mixed). However, we cannot obtain the fundamental state by combining the states of all subsystems, due to entanglement. This gives rise to probability distributions in classical mechanics. Does this sound correct?

 

[Fredrik]

But we can't choose whether we represent this fundamental state as a ray in Hilbert space or as a density operator in MWI, because the density operator is required for assigning reality to the subsystems.

A ray is a 1-dimensional vector subspace. (Some prefer to define it differently, but this definition is good enough for this discussion). 1-dimensional subspaces are closed sets. Closed vector subspaces corrrespond bijectively to projection operators. The projection operator for a 1-dimensional subspace that contains the unit vector |\psi\rangle, is |\psi\rangle\langle\psi|. This projection operator is a state (i.e. density) operator for a pure state, and every pure state operator can be expressed in the form |\psi\rangle\langle\psi| for some unit vector |\psi\rangle. So there's a bijective correspondence between rays and pure state operators.

So my current understanding is this: in CI, a density operator describes the state of an ensemble of single systems.

Right, and the members of the ensemble don't have to exist at the same time. It could be the systems that participate in the measurements you do when you run the same experiment over and over.

In MWI, a density operator describes the state of the single system itself (even if it's mixed). However, we cannot obtain the fundamental state by combining the states of all subsystems, due to entanglement. This gives rise to probability distributions in classical mechanics. Does this sound correct?

I don't understand what you're saying here. This is my (possibly flawed) understanding of MWI and decoherence: The state of the universe is always pure. If we decompose the universe into subsystems, say "specimen+everything else", and choose to express the state of the universe in a basis that's consists of eigenvectors of an operator that commutes with the part of the Hamiltonian of the universe that describes the interaction between the specimen and the rest of the universe, then the state operator will be approximately diagonal (after the interaction). So even though it's still pure, it will be practically indistinguishable from a mixed state.

 

[Ken G]

Be aware that you are the first person I've ever encountered who says they prefer CI but could also admit that retaining the mixed state is just as good as collapsing it. (even if you prefer not to think in the former way)

What I mean is, to an empiricist, the "state of a system" means identically this: "the way a physicist with certain information characterizes their expectations about the behavior of that system." The point being, the "state" depends on the physicist, and their information. Now, we might assert the presence of a physicist (hypothetical or real) who has access to "all the information possible in the actual reality", and give special weight to the "state" as they would describe it, but all the same, some other physicist at that same time will not mean that as the "state" of the system-- they will mean their own version, because that is what they can use. Good thing too-- generally, there is no physicist who ever has the complete information.

Given this, I can't possibly see how any CI-user, or indeed any empiricist, would see any difficulty in having a state that is "mixed" for one physicist be "just as good" for their needs, and "collapsed" for another, being "just as good" for theirs. We know what we know, and that's what we want physics for, not some imaginary situation where we know more than we know. The first physicist has seen the coin flip, but not the coin, the second has seen the coin. Neither is a "better way" to talk about that coin, one just involves having more knowledge about the coin. It is only the rationalist who demands that "the state of the coin" be something absolute, and that priority forces them to choose the mixed state over the collapsed state simply because the mixed state fits into a unitary representation of the closed system. That also forces the rationalist to discount the extra information that the physicist who has seen the coin has, as some kind of illusion. Ironic that-- the extra information they have, from having seen the coin, is treated by the rationalist as a loss of true knowledge of the coin-- before they looked, they could treat the coin in its "correct" mixed state, but now that they have looked, they must enter into their own illusion in order to function in their world! Are you starting to see the radical character of rationalism, and what empiricism is?

I mentioned it earlier, but I think it warrants bringing it up again -- my actual take on the matter is that when an observer collapses a wave-function after measurement, it's nothing more significant than a change of "reference frame".

Actually, that analogy really doesn't hold if you dig into it. An observer in a reference frame is not treated as having incomplete information about a system-- because they can transform their information into any other frame. In your analogy, the person who has looked at the coin is entering into an illusion that the coin is "heads", they cannot transform their information into any frame where the coin is "tails". In relativity, observer A can say "tell me my relationship to observer B, and I will tell you what they see". This means that A and B are seeing essentially the same thing, just in a different lexicon, they are not seeing incomplete elements of something larger than either. But if I see "heads" on a coin, there is no observer that I can say will see "tails", except exactly the observers that I imagine see tails. No other information makes that transformation for me, no communication, nothing-- it's completely tautological, those who see tails are defined by those I imagine as seeing tails. Zero empirical content, not at all like relativity. The analogy does not hold.

Indeed, I claim you have the analogy to LET exactly backward. In relativity, the "equivalence class" is all the sets of ways of talking about an event that are really saying the same thing (the invariant), so we don't need an aether because the aether embodies something different that never shows up-- the speed through the aether is not an invariant, it's not even an observable-- it is irrelevant. That's exactly the relationship we have to the "tails" result the instant we perceive "heads"-- the class of observers who see "heads" have no way of communicating or taking information of any kind from other observers to determine if they saw tails-- other than asking them what they saw (which is impossible because there's no communication across worlds). No matter what experiment the "heads" camp does to try to make contact with the existence of a "tails" camp, reality thwarts them-- no way they can detect the presence of the "tails" camp. That sounds an awful lot like an aether to me. Note that LET, just as with MWI, is not falsifiable by experiment-- but it is rejected on the grounds that if nature foils your every attempt to detect something, like the aether or anyone seeing "tails", then the empiricist takes it as a law of nature that what nature renders invisible, does not exist. That's what happened in relativity, the rationalist-motivated expectation that there should be an aether lost to the empiricist recognition that what cannot be detected should not be said to exist.

So when we want to understand how systems evolve, we probably consider the process of unitary evolution, without the notion of collapse entering our minds. Later, when we want to condition on past observations, we might invoke collapse to simplify the both the physical questions we ask and (possibly) calculations needed to answer them.

Absolutely. Said just like a CI user, except that in CI, "how systems evolve" means "how our expectations of the system change with time". Then the "notion of collapse" only enters our minds in the sense that collapse defines the very nature of what we mean by unitary evolution in the first place.

 

[psypher]

i don't claim to know much about this stuff, but i do understand the many worlds theory to a degree.

your flipping coin explanation doesn't really make sense because the coin only generates 2 possible outcomes. i think you would have to consider that in one world you would flip a coin and it would land on heads. in another you may flip the coin a little higher and it will land on heads. in another world it hits a table and lands on heads with all the worlds generating alternate realities and alternate worlds also. worlds being created exponentially. and sloppily you could say with your example 99% more worlds exponentially on the heads side and 1% worlds exponentially on the tails side.

 

[Hurkyl]

then the empiricist takes it as a law of nature that what nature renders invisible, does not exist. That's what happened in relativity, the rationalist-motivated expectation that there should be an aether lost to the empiricist recognition that what cannot be detected should not be said to exist.

Empiricism is the method of using experience to gain knowledge. If you can't gain experience about a thing, then you have no knowledge regarding it. In particular, you don't have knowledge of its non-existence.

You mis-describe relativity. The recognition that a choice of reference frame is invisible did not lead to reject the use of reference frames, nor did it cause people to start insisting that a good empiricist should only use a self-centered reference frame. Instead, it just meant that people stopped attaching any significance to the choice.

A heliocentric reference frame is just as good (arguably better) for studying the solar system as a geocentric reference frame.

(Incidentally, the thing in 1905 was the proposition that special relativity would match observation better than Galilean relativity)

By the way, reference frames have things like origins and axes. Things that cannot be detected. Does your radical empiricist reject making use of them?

Given this, I can't possibly see how any CI-user, or indeed any empiricist, would see any difficulty in having a state that is "mixed" for one physicist be "just as good" for their needs, and "collapsed" for another, being "just as good" for theirs.

But I'm further arguing that the mixed state is just as good my own needs. If I saw heads, then any mixed state with the corresponding heads component will make the same predictions when conditioned on me seeing heads.

Zero empirical content, not at all like relativity.

If there's zero empirical content, then by definition empiricism has nothing to say on the issue.

But it's not zero -- in principle, I could lock you in a laboratory, seal it away from the surrounding environment, and measure in a basis for which the partial traces of basis elements contain both "you saw heads" and "you saw tails" components. It's just the experiment is too hard to do with human-sized components. (it is, of course, not too hard to do with qubit-sized components)

Then the "notion of collapse" only enters our minds in the sense that collapse defines the very nature of what we mean by unitary evolution in the first place.

I didn't really expect your admittance to last; you had already previously been equating an unobservable bird's-eye (or god's eye) assertion about reality with the notion of experience.

Although, your particular phrasing here seems self-contradictory.

 

[dm4b]

Empiricism is the method of using experience to gain knowledge. If you can't gain experience about a thing, then you have no knowledge regarding it. In particular, you don't have knowledge of its non-existence.

I don't think it's quite that simple, as it applies to science today. It's a bit broader meaning derived from experience OR experiment, and science applies more to the experiment part, than it does to the experience part. If you cannot detect something via experiment, THEN it does not exist, EVEN IF an individual may have experienced it.

It's a reason many people don't consider psychology a "real" science, even though it deals with what people are directlty experiencing.

The most controversial example I can think of falls under the study of consciousness - Yoga.

Yoga (specifically raja Yoga, as outline by Patanjali in the Yoga Sutras) traditionally was basically a step-by-step procedure for obtaining a direct experience of higher states of consciousness in a predictable, repeatable and verifiable way, achieved by many folks over the past 1000's of years. This is why many practioners of yoga have considered it to be "scientific."

Yet modern day science hardly acknowledges it, even though it is knowledge of a thing derived through (conscious) experience, in other words "empirical".

Why? For one, science doesn't understand consiousness, pretty much doesn't acknowledge "higher states of consciousness" and sometimes disregards consiousness all together, even though we experience it on a daily basis. But, mainly because we don't know how to quantify it, or measure it in a lab, or rather, you cannot derive knowledge about it through an experimental setup under the currently accepted materialistic paradigm. And, the mere mention of things like Yoga can make people shudder because they're taboo, for the very reasons mentioned.

So, mainly empiricism, as it manifests under modern science, is knowledge gained through experiment performed under a materialistic paradigm . Direct experience is put under a vastly inferior role to that.

So, yeah, that's a controversial example, becuase, well I like to be controversial sometimes, but you can think of many other simple examples of how "direct experience" is given much less credence in modern science to "experimental verification".

 

[Hurkyl]

But it's not zero -- in principle, I could lock you in a laboratory

More than this. A subsystem approximate in a mixture state of "classical" states is not a subsystem exactly in a mixture of "classical" states. I don't actually know what the difference would look like, though -- but if we are serious about decoherence-based interpretation it warrants looking into.


Also, a subsystem is not a whole. It may also be possible to detect and make inferences about the whole that have real, observable effects, in direct contrast to the case of statistical classical mechanics where the components of a mixed state are eternally and perfectly independent of one another no matter how inclusive your analysis is.

Again, this is another aspect I don't know much about, but one IMO warrants the research -- and it's a question that, as far as I can tell, the CI of quantum mechanics rejects as not even making sense.

 

[Ken G]

Empiricism is the method of using experience to gain knowledge. If you can't gain experience about a thing, then you have no knowledge regarding it. In particular, you don't have knowledge of its non-existence.

You mis-describe relativity. The recognition that a choice of reference frame is invisible did not lead to reject the use of reference frames, nor did it cause people to start insisting that a good empiricist should only use a self-centered reference frame. Instead, it just meant that people stopped attaching any significance to the choice.

OK, that entire point was one long straw man. Let's take it straw by straw. First of all, empiricism is not the statement that you can gain knowledge by experience, it is the statement that knowledge comes fundamentally from experience. Hence, empiricism is certainly not agnostic on the issue of knowledge about things you cannot perceive, it regards them all as angels on a pin. So you are not talking about empiricism here, but I am.

Second, I do not mis-describe relativity, you simply reinvent what I said. I have no idea how you got the idea that I said relativity "rejects" reference frames! Empiricism requires reference frames, because it requires observers, and all observers have reference frames. What relativity actually does, and what I said it did, was to provide the lexicon that translates between the reference frames. Having this lexicon unifies empiricism into a single description, instead of a disunion of different experiences, so relativity is crucial to empiricism-- there is no empirical need to unify some imagined class of experiences that cannot be detected even in principle by the observer in question. That's why empiricism must always view MWI as an inquiry into the "angels on the pin". I would have thought that would be obvious to anyone who understands empiricism, so again you just keep establishing that you do not. So what, you don't need to understand empiricism, you're a rationalist-- but that's what I'm trying to get you to understand.

The point is, your analogy between relativity and MWI fails completely for any empiricist. The empiricist can explicitly demonstrate that the lexicon provided by relativity does indeed unify the reference frames of observers who are in communication, at least in principle, and that's why they are not angels on a pin. We can demonstrate that by querying observers in the other reference frames, as we did in the Michelson-Morely experiment as the Earth passed between different frames and communicated outcomes between them. When Michelson-Morely was envisaged as a thought experiment under Galilean relativity, it got the wrong answer-- but the "right" answer could only be obtained by actual communication between the reference frames. Try that in your analogy with many worlds!

A heliocentric reference frame is just as good (arguably better) for studying the solar system as a geocentric reference frame.

Obvious to any empiricist.

By the way, reference frames have things like origins and axes. Things that cannot be detected. Does your radical empiricist reject making use of them?

What would make you think that a radical empiricist would reject using mathematical tools in physics? You just don't seem to get empiricism at all. Empiricists love mathematical tools, they love how they simplify the making of predictions and forming of pictures and models that help us understand the reality of our observations. They especially love it when the mathematical tools come with postulates that say "only invariants are real", when combinations like |observer A><observation a| are what the invariants are. Given all this, they are perfectly fine when a mapping from these "raw" invariants into rationalistic 4-vector norm invariants is discovered-- they hoped such a thing would have to exist, because that is the union of objectivism and empiricism, and so they are glad when it is found.

Once again: empiricism is the statement that reality is what you observe, and theories exist in your mind to make sense of what you observe. That observations require a mind to say what constitutes an observation is the core inconsistency of empiricism, but it lives with it. The core inconsistency of rationalism is that theories keep changing, but observations don't. In the history of science, the latter has been a much larger issue than the former.

But I'm further arguing that the mixed state is just as good my own needs. If I saw heads, then any mixed state with the corresponding heads component will make the same predictions when conditioned on me seeing heads.

Yes I know it is just as good for your needs. That's because "your needs" are purely rationalistic. "Your needs" start by rejecting what you actually experience as the actual reality, which is what forces you to "condition on what you see." For most people, "conditioning on what they experience" is a perfectly redundant step of science, it doesn't require any "if.. then" in front of it. Your approach needs the "if ... then" or it doesn't work. That's exactly why it is so radically un-empiricist, as I keep saying. For some reason, you seem to keep arguing that you are not being un-empiricist by making more and more un-empiricist statements.

If there's zero empirical content, then by definition empiricism has nothing to say on the issue.

Again, no. That would be like saying that science has nothing to say about the existence of invisible unicorns. Instead, science is not agnostic about invisible unicorns, it says that if you have zero evidence for something, it is silly to claim it exists. We see this all the time-- a scientific claim that is made on reality requires some evidence be produced that there is an influence on reality. If you want to instead say that MWI makes non-scientific claims on reality, then we have no dispute, that is exactly how I see MWI.

But it's not zero -- in principle, I could lock you in a laboratory, seal it away from the surrounding environment, and measure in a basis for which the partial traces of basis elements contain both "you saw heads" and "you saw tails" components. It's just the experiment is too hard to do with human-sized components. (it is, of course, not too hard to do with qubit-sized components)

You have no idea that this is true. Indeed, in my opinion, the fact that I am sentient is the reason that your experiment would never succeed, not even in principle. But since you cannot produce on your claim, your speculation is irrelevant.

I didn't really expect your admittance to last; you had already previously been equating an unobservable bird's-eye (or god's eye) assertion about reality with the notion of experience.

Although, your particular phrasing here seems self-contradictory.

Then you have not understood. I introduced the "god's eye" view expressly to contrast it with the notion of experience. The "god's eye" view is the unitary evolution of the wave function, and we never experience that. Instead, we experience observable eigenvalues, and we invent the intervening unitary evolution to correctly predict (statistically) what we observe. That is quite demonstrably just exactly what physics did, pick up any history book. The rationalist can claim that it means something else than that, but that's what actually happened. I have no idea what you think I am now "admitting."

 

[kith]

So there's a bijective correspondence between rays and pure state operators.

Ok, they are mathematically equivalent. But whether they mean the same thing physically, is a matter of interpretation. In CI, I would say they don't. A ray describes the state of a single system, a pure state operator an ensemble of such systems. Else, the evolution from pure to mixed states makes no sense.

I don't understand what you're saying here.

Don't you agree that mixed states describe two fundamentally different things in CI and MWI? If we have an electron beam with density operator ϱ ~ |+><+| + |-><-|, this is an ensemble where 50% of the electrons are in state |+> and 50% in state |-> in CI. In MWI, ϱ is the "real" state of a single electron (disregarding issues with indistinguishability). There exist two distinct universes and in one of them, the electron has definite spin +h/2 wrt to the quantization axis and in the other -h/2 .

This is my (possibly flawed) understanding of MWI and decoherence: The state of the universe is always pure. If we decompose the universe into subsystems, say "specimen+everything else", and choose to express the state of the universe in a basis that's consists of eigenvectors of an operator that commutes with the part of the Hamiltonian of the universe that describes the interaction between the specimen and the rest of the universe, then the state operator will be approximately diagonal (after the interaction). So even though it's still pure, it will be practically indistinguishable from a mixed state.

I find it odd to speak of decoherence in the context of the state of the universe, because this state evolves coherently and all you do is to change the basis. Decoherence usually refers to the decay of the off-diagonal elements of density matrices describing open subsystems like our specimen, transferring an initially pure state into a mixed one due to interactions with the environment. I'm not sure what you mean by saying that the density matrix of the universe gets 'approximately' diagonal. What gets diagonal is the density matrix of the specimen. I don't see how this diagonality translates into an approximate diagonality of the whole density matrix. Can you please elaborate?

 

[Hurkyl]

Second, I do not mis-describe relativity, you simply reinvent what I said. I have no idea how you got the idea that I said relativity "rejects" reference frames!

I take the words you say and follow them to the logical conclusion.

Empiricists love mathematical tools, they love how they simplify the making of predictions and forming of pictures and models that help us understand the reality of our observations.

Except for mixed states (as corresponding to observation), apparently.

They especially love it when the mathematical tools come with postulates that say "only invariants are real"

Except when they are conditional probabilities, apparently.

Again, no. That would be like saying that science has nothing to say about the existence of invisible unicorns.

It doesn't. Asserting the non-existence of things you have no evidence against is pure dogma.

Eliminating IPU's is an application of Occam's razor, and nothing more -- when faced with two observationally equivalent alternatives, all that matters are non-empirical concerns (such as simplicity) so recognize science has nothing to say and make your choice based on the non-empirical ones.





How a state is mixed in classical statistical mechanics is unobservable. For most purposes, applying Occam's razor and only bothering with pure states is a reasonable thing to do.

But when the point is to understand how mixed states do just as good of a job as pure ones -- that even the very language used by "internal" observers is agnostic on the issue -- applying Occam's razor to eliminate the mixture is wholly inappropriate.



When we look at QM, the way the state of a subsystem is mixed does have an influence on the dynamics. Occam's razor no longer applies.

You have no idea that this is true. Indeed, in my opinion, the fact that I am sentient is the reason that your experiment would never succeed, not even in principle. But since you cannot produce on your claim, your speculation is irrelevant.

I have some idea.

However, I do recognize that the fantastic successes of QM might falter when pushed to these scale. Or maybe there's an odd theoretical quirk that would prevent such a thing even in principle.

But "it can't work because I'm a human being?" Seriously?

 

[BruceW]

I agree with Hurkyl that just because someone is human, it doesn't mean that it wouldn't be possible to do some kind of quantum experiment with them (for example interference or quantum teleportation). They've done quantum teleportation with atoms, and they've had a superconductor with currents going in a superposition of different directions, so it seems like they will keep making experiments with bigger and bigger systems.

A human is of course an incredibly complex system, so its unlikely they will ever quantum teleport a human. But this doesn't mean it couldn't be done in principle.

Hurkyl raises another good point that there may be some other theory that takes over from QM when we have gotten to such scales. This is why the debate of MWI against CI is very premature.

 

[Ken G]

I take the words you say and follow them to the logical conclusion.

Actually, I explained in detail why that is exactly what you did not do. I guess you are just set in your opinions though.

Except for mixed states (as corresponding to observation), apparently.
Except when they are conditional probabilities, apparently.

No one both counts, empiricists like those situations too. Hard though this may be for you to quite get, empiricists see mathematical tools of all kinds as essential to science. They just see them as tools-- the maps, not the territories. I've no doubt Sir Edmund Hillary was quite fond of his map of Mount Everest, but that doesn't mean he could have just stayed home and climbed the map.

Asserting the non-existence of things you have no evidence against is pure dogma.

Here I fear you have a rather nonstandard understanding of the word "dogma." Most people, when they use that word, mean believing in something purely on the basis of having been told it enough times, or in an authoritative enough setting, without any actual evidence for it being supplied. It is often repeated by rote, rather than paraphrased to express an individual understanding of the ideas. By contract, the rejection of believing in something, on the grounds that there is no evidence for it, is actually much closer to the opposite of dogma. Here I think you have even left the fold of the rationalists.

Now, while I might agree with you that no one has a right to tell someone else they are wrong unless they can produce evidence, it is certainly incorrect to label the skeptical stance as "dogmatic." That's just what the people who believe in ghosts say. Instead, a skeptical stance is a personal choice to accept a certain standard of belief, a standard that requires evidence rather than just desire to believe. In that spirit, I would never tell you that there aren't many worlds, I would merely say that you have not adopted the skeptical stance that scientists normally include in their repertoire of tools. Doubting what has not been verified has proven vastly useful in the history of science.

Eliminating IPU's is an application of Occam's razor, and nothing more -- when faced with two observationally equivalent alternatives, all that matters are non-empirical concerns (such as simplicity) so recognize science has nothing to say and make your choice based on the non-empirical ones.

The issues between MWI and CI were never issues involving Occam's Razor. O.R. is a widely valued principle of science, it just says that if the goal is to understand, then choose a description that is easiest to understand. But as I've said, MWI vs. CI invokes much deeper metaphysical priorities, to wit, rationalism vs. empiricism. The rationalist and the empiricist will encounter their disagreement long before they even get to O.R., indeed O.R. would only be used to adjudicate between various versions of MWI and CI, it is never used to adjudicate between fundamental epistemological priorities. (It would be like telling Sir Edmund Hillary that he should have just climbed his map, because it's "much simpler.")

How a state is mixed in classical statistical mechanics is unobservable. For most purposes, applying Occam's razor and only bothering with pure states is a reasonable thing to do.

But when the point is to understand how mixed states do just as good of a job as pure ones -- that even the very language used by "internal" observers is agnostic on the issue -- applying Occam's razor to eliminate the mixture is wholly inappropriate.

I don't think Occam's razor is ever invoked to eliminate the mixture. The mixture is eliminated directly by observation, that's empiricism. However, as I said above, it is only eliminated for the physicist with access to that observation-- any that do not will still invoke the mixture, so the mixture has not been "eliminated" in some absolute way. The whole concept of mixture vs. pure state never existed anywhere except in the heads of the people who are using the concepts effectively.

When we look at QM, the way the state of a subsystem is mixed does have an influence on the dynamics. Occam's razor no longer applies.

There's no such thing as "the dynamics" of a system, there's only the way our information about the system, and our expectations of that system, are changing. CI has no difficulty tracking how that information and expectations evolve, invoking Occam's Razor in the usual ways.

I have some idea.

No, you don't, you have none at all. Any plausible reason you can give for why that experiment, if possible to do, would come out that way, can be countered by an equally plausible reason why it would not.

However, I do recognize that the fantastic successes of QM might falter when pushed to these scale. Or maybe there's an odd theoretical quirk that would prevent such a thing even in principle.

Yes, maybe this time we got it exactly right, let's just ignore the history of our discipline and pretend it is something now that it has never been before but was always thought to be before by rationalists like you.

But "it can't work because I'm a human being?" Seriously?

Human? I'm not sure how humanity enters the picture. I was referring to sentience, by which I mean, "able to process information and do physics with it." And yes, I am quite serious when I suggest that physics might be fundamentally affected by having the ability to do it. Never a concept that rationalists are terribly comfortable with, I know that.

 

[Ken G]

I agree with Hurkyl that just because someone is human, it doesn't mean that it wouldn't be possible to do some kind of quantum experiment with them (for example interference or quantum teleportation).

Importantly, that was not quite all that Hurkyl claimed. The claim was "- in principle, I could lock you in a laboratory, seal it away from the surrounding environment, and measure in a basis for which the partial traces of basis elements contain both "you saw heads" and "you saw tails" components. It's just the experiment is too hard to do with human-sized components." The statement is internally inconsistent-- I have an enclosure that is sealed away, yet I'm doing measurements on it. That's just not logically possible.

Also, it was not a statement that we don't know how the experiment would come out if we could do it, no MWI proponent could really be happy with that, their world view is too radical for that. It is a statement that we do know, or the MWIers do anyway, we just can't do the experiment. I think that's an unscientific claim-- my claim is that we simply don't know how that experiment would come out unless we could do it, and we can't even talk about the outcome of an experiment that is internally inconsistent. Now the key question is-- which interpretation, CI or MWI, is more comfortable with not knowing the outcome of that experiment?

They've done quantum teleportation with atoms, and they've had a superconductor with currents going in a superposition of different directions, so it seems like they will keep making experiments with bigger and bigger systems.

Actually, there is something much bigger than that in those experiments-- the experimenters.

A human is of course an incredibly complex system, so its unlikely they will ever quantum teleport a human. But this doesn't mean it couldn't be done in principle.

It also doesn't mean it could be done in principle. We simply don't know if quantum mechanics still works when the processor applying the quantum mechanics is part of the system that the quantum mechanics is being applied to. We know it works to predict elements of such a system that are not treated as having processing ability, but that wouldn't foot the bill for teleporting humans. It's all about not pretending that we know our theories will work in situations that are fundamentally different from those the theory was built for.

Hurkyl raises another good point that there may be some other theory that takes over from QM when we have gotten to such scales. This is why the debate of MWI against CI is very premature.

Actually that has been a central theme I've raised all along. CI does not construct a world view, so it will have no problem if QM is shown wrong by the next theory-- empiricism says theories are to understand reality, not to dictate to reality. It is actually MWI that would end up looking pretty darn silly if unitary evolution is discovered to not be a universal principle, as treating it as a universal principle is the whole basis of the MWI. That's the problem with rationalism, that's why it hasn't worked yet when used to create world views (instead of what it should be used for-- to inspire new tools).