Problems with Many Worlds Interpretation

[Ken G]

I don't see how. CI's compatibility with QM depends on you not being part of the system, so that it makes sense to place a cut between it and the classical you.

The place where CI puts the cut is not between the QM system and the classical me. Remember, Bohr said there's no such thing as the QM system, so there's no need to place any cut there. Instead, the cut is simply between what I experience, and what I think about what I experience. That is the standard cut of empiricism, it has nothing to do with any particular physical theory, and there is no theory that cannot have a cut put there, including string theory. So string theory is perfectly compatible with CI.

... but only between collapses. I do sometimes like to make the point that even a staunch CI still ought to learn some MWI to understand how the wave-function behaves between collapses.

And the correct response of the CI person is: wave functions don't "behave" at all! They evolve in the parameter time, in exactly the way QM says they do, this is no issue for any CI person, nor does it have the least bit of connection to adopting a many-worlds view of reality. The very connection between the letter "t" you put in your expression for a wave function, and your experience of time, is just another type of collapse, in the CI view, and the only reason you can even associate the two is because you experience it to work. The experience is the justification for everything-- that's CI, that's empiricism.

As far as I can tell, nothing is observationally different between a person who says "Oh, the wave-function is a mathematical object that contains all of the information about reality, and we update that information via unitary evolution or sometimes collapse" and "Oh, the wave-function is a mathematical object that corresponds to a real entity, and the time evolution of that real entity agrees with unitary evolution or sometimes collapse" except for the particular choice of words they used.

I agree completely-- the difference between CI and MWI is entirely the way the window is dressed. But that's the whole problem-- since when did window dressing justify a world view? This has been my point all along, not that CI is demonstrably a better way to think about reality than MWI, but rather that CI is simply more honest about our motivations. We want to believe we know, but we can still resist the lure of wanting to create a world view based on what we only want to believe we know to be true. Again, look at the history of physics, and look at how scientists generally characterize the desire to believe in the absence of evidence one way or the other.

And, for the record, information updated sometimes by unitary evolution and sometimes by collapse is somewhat more ad-hoc and unsatisfying than information that is updated consistently by unitary evolution. From this point of view, the reason CI needs collapse is because it's not asking the right questions.

All physics theories are ad hoc, they just invent whatever they need. Need an electron? Invent one! That's how physics is, it's ad hoc. But you are right that CI and MWI are conditioned by the questions we feel are important. CI is motivated by the question "what explains what I perceive." MWI is motivated by the question "how can I describe a system in terms different from what I perceive, yet in a mathematically unified way?" Take yer pick-- mathematical unity or description of experience, it's rationalism vs. empiricism.

No, that's syntax.

Correct, you get it-- to an empiricist, all physical theories are syntax, whereas reality is experience. To a rationalist, all experience is some kind of illusion, and theory is truth. So has it been for thousands of years. But note that the reason science is usually grounded in empirical truth is that it is the only type that is demonstrable in an unambiguous way, and when experience disagrees with what seems reasonable, experience always wins. Even so, we cannot understand our experiences without theories, so we need both, and the tug-of-war goes on.

No, they use different approaches to gaining knowledge.

For an empiricist, knowledge can be derived from observation and experiment.
For a rationalist, knowledge can be derived through logic.

A scientist must be both an empiricist and a rationalist.

You are talking about the epistemology. Scientists have no real trouble with epistemology-- we all know we need to combine those two ways of knowing to get anywhere. Nor does either MWI or CI have any disagreement around how to do the scientific method. The place where we find the real issue between MWI and CI is not their identical epistemologies, it is their different ontologies. When someone asks you, do you think many worlds really exist, that is a classic ontological question, and that is exactly where the debate should center. What is the evidence you will call forth to make the claim that you are not just believing something because you like to believe it? Is there really anything more that can be said about MWI as something other than just doing quantum mechanics?

 

[Hurkyl]

This isn't a property of the CI or any other interpretation of QM. It's a property of all theories of physics.

Eh? This needs clarification, because I've never seen that in any of my science texts, and I've never observed any difficulty including myself in a system under study.

 

[Hurkyl]

You are talking about the epistemology.

Yes. That's all "rationalism" and "empiricism" are -- epistemological positions.

And the correct response of the CI person is: wave functions don't "behave" at all! They evolve in the parameter time, in exactly the way QM says they do,

Er, that's a behavior.

I agree completely-- the difference between CI and MWI is entirely the way the window is dressed.

Except that CI asserts collapse.

We want to believe we know,

I don't want to believe I know. I just want to learn and understand a physical theory, and how it connects to observation.

we can still resist the lure of wanting to create a world view based on what we only want to believe we know to be true.

Right. A world view to be based on knowledge (be it rational, empirical, or other) is more likely to be accurate and useful than one based on desire or biases.

Again, look at the history of physics, and look at how scientists generally characterize the desire to believe in the absence of evidence one way or the other.

As far as I can tell, those that see past their biases tend to prefer to do things like preferring Special Relativity to the equivalent Lorentz Ether Theory -- e.g. not to insist on the correctness of a particular frame of reference.

That's how physics is, it's ad hoc.

Ad hoc is necessary, but bad. The more ad-hoc a theory is, the less its ability is to make precise predictions, which in turn diminishes the value of any evidence favoring the theory.

MWI is motivated by the question "how can I describe a system in terms different from what I perceive, yet in a mathematically unified way?"

No. MWI is motivated by the question "Can a mathematically unified method explain what I perceive?"

Individual people may have other motivations. I'm not talking about those people. If there was ever a branch of MWI that had a different motivation, I'm not talking about that branch.

Correct, you get it-- to an empiricist, all physical theories are syntax, whereas reality is experience. To a rationalist, all experience is some kind of illusion, and theory is truth.

I think you mean some sort of Platonism, rather than rationalism.

Formally, at least, theories are syntax and truth is semantics, there's no if's, and's, or but's about it. And I'm enough of a formalist to believe that anyone who claims otherwise really just hasn't learned to mentally separate the ideas of "theory" versus "interpretation".

(I'm fine with a conventional usage of 'theory' to refer to a theory together with a particular interpretation of the theory -- as long as a person can admit they are doing that!)

You would only ever catch me saying "theory is truth" if I really meant something about deduction or provability, but got lazy when faced with the fact natural language is poorly equipped to talk about the subtler details of the topic.

it is the only type that is demonstrable in an unambiguous way,

I strongly disagree. It is very, very difficult to consider an empirical "truth" without having first filtered it through one's intellect. I would be so bold to claim it impossible to do so completely.

The unambiguity only (seemingly) appears between people that filter their experiences in similar ways.

 

[Fredrik]

Except that CI asserts collapse.

You seem to have a very precise opinion about what the CI is. There are many different ideas that are all considered to be "the CI" by someone. The one that you call "the CI" is the one that I'm the least willing to call "the CI" myself, because it involves an essentially supernatural element that actually contradicts the theory it's supposed to "interpret". An interpretation is supposed to tell us, in language we can understand, what the theory is saying is actually happening to the system. It's not supposed to tell us how the theory is wrong.

The idea that QM doesn't describe reality and just assigns probabilities to verifiable statements, is one of the ideas that can be called "the CI". However, people tend to call it an "ensemble interpretation", or a "statistical interpretation" these days.

I think it's really hard to talk about "the CI" at all, because a lot of what we can read about it are silly misunderstandings of what Bohr actually said. For example, Bohr acknowledged the fact that we wouldn't consider something a "measurement" if we can't read a "result" off of an indicator part of the measuring device, and people have misunderstood this to be a claim that the laws of QM don't apply to measuring devices at all.

 

[Fredrik]

Eh? This needs clarification, because I've never seen that in any of my science texts, and I've never observed any difficulty including myself in a system under study.

OK, I see that my statements look pretty weird in that context. Not sure if I can do much better though. One of the reasons is that I can't figure out why you think that string theory would be more compatible with MWI than CI. Since I don't understand that, I don't know what I should be saying in response.

 

[Fredrik]

There are (at least) TWO main ways to understand what a theory is:

a) as a description (of the system, the universe, or whatever...)
b) or as an interaction tool (for learning about "the system", the universe, or whatever...)

in (a) you test the theory by clean poppian style falsification. A theory that in retrospect fails to "DESCRIBE the future" is wrong - there is no "theory" for how to produce a new hypothesis.

I have some issues with the word "describe". I would choose to talk about making accurate predictions about results of experiments instead. More importantly, your (b) contradicts science as we know it. (That's not entirely clear from what you're saying here, but we have been talking about this before, so I know what you mean). I'm not saying that it definitely is a bad idea. I haven't ruled out that something similar to what you like to talk about can be developed into a useful generalization of the term "theory". However, I don't think it's necessary to mention this in every thread about quantum mechanics, which clearly is a theory in the sense of what I like to talk about: It assigns probabilities to verifiable statements.

I also wouldn't describe a speculative new idea as one of the two "main" things that the term "theory" can refer to.

 

[Ken G]

Yes. That's all "rationalism" and "empiricism" are -- epistemological positions.

Not when the knowledge in question is about "what exists." This is the point of contact between epistemology and ontology, and it is exactly the place where CI and MWI differ. They just don't differ anywhere else. They certainly don't differ in the issue of how to know if a theory makes correct predictions or not, and they don't differ in the issue of whether or not observers and theorists need to confer on a joint agreement of what is a good theory. So they simply don't differ in any of the ways in which scientific knowledge is judged or obtained, except one: they differ on the issue of what constitutes true knowledge about what exists.

Er, that's a behavior.

Certainly not, not to the empiricist. Your claim here is, categorically, that how a mathematical entity depends on a variable t, makes a one-to-one claim on how some system behaves with the experiential and measurable concept of time. That just isn't true. You are missing that we are already affixing an interpretation to a physical theory that the parameter t in the theory corresponds to the empirically measurable concept of time. But it is not a necessary part of any theory that we must fail to distinguish between an experimental measure and a parameter in the theory (note for example that t is not an operator of quantum mechanics). It is part of the judgement of the value of the theory to connect that variable t to the empirical observation of experienced time, but only in the same empirical context that all theories are judged. No one ever says "I criticize your interpretation that t is time on the basis that it just doesn't seem like time to me", no, any criticism of that interpretation must be based on empirical comparisons of how the parameter t acts under experimental conditions. But we already know that the t that appears in the Schroedinger equation will not correspond to any experiential version of time if t is small enough, so the entire idea that "t" in QM is a continuous reflection of real time is simply false.

Ad hoc is necessary, but bad. The more ad-hoc a theory is, the less its ability is to make precise predictions, which in turn diminishes the value of any evidence favoring the theory.

MWI is just as ad hoc, but the ad hoc nature comes at a different level in the goals of the theory. CI adds an ad hoc physical postulate that some mysterious process, not covered by the theory, chooses which outcome we actually experience, MWI adds an ad hoc metaphysical postulate (as you did above) that our particular individual experience is a question that physics should not be interested in. CI sees that as cheating-- it's not surprising that greater unification can be achieved by allowing ourselves to cheat on what a theory should describe (rather than on how it should describe it, that was Einstein's objection about dice).

No. MWI is motivated by the question "Can a mathematically unified method explain what I perceive?"

It can't be that, because it fails to do that. Also, note that CI is just as mathematically unified as MWI. It has to be, it's all the same mathematics. MWI is not content with mathematical unification, it wants ontological unification. And it can only accomplish it by dodging the question of why I perceive what I perceive. I still haven't seen your answer to that. Your scenario doesn't answer it, because we don't have an observer getting <heads,tails>, we have a bundle of perceptions connected with the sentient being I'll call observer1, and a bundle of perceptions (according to MWI) connected with sentient being observer2, whose perceptions do not overlap and they do not perceive each other. So what you'd have to say is, you have <observer1,observer2> reporting <heads,tails>. If you do that, the mathematical description is entirely unitary, but you have no way to account for why it did not come out <observer1,observer2><tails,heads>. All you could say is that you don't care about the difference there-- but try telling that to observer1 if "heads" means he loses his.

I think you mean some sort of Platonism, rather than rationalism.

Platonism is quite a bit different from the way MWI is normally expressed. Indeed, I would have no problem with MWI as a form of Platonism, that is the sense in which MWI makes reasonable claims. The key difference is that Platonism draws a distinction between what is physically real and what is abstractly real. If people want to imagine that the many worlds are abstractly real, as some form of perfect concepts, I would have no issue with them other than being a bit idealistic. It is the claim on physical reality that I feel should require empirical demonstration. When someone can empirically demonstrate the existence of many worlds, only then would I find it appropriate for us to conclude that they are physically real.

Formally, at least, theories are syntax and truth is semantics, there's no if's, and's, or but's about it. And I'm enough of a formalist to believe that anyone who claims otherwise really just hasn't learned to mentally separate the ideas of "theory" versus "interpretation".

I completely agree, that was the flavor of Godel's proof-- syntax and semantics can never be the same thing in any formal system rich enough to be suitable for our purposes. Indeed that is my entire issue with how MWI is normally expressed-- it improperly crosses that dividing line, mistaking a syntactic interpretation for a semantic one. That's also the place where ontology creeps in.

I strongly disagree. It is very, very difficult to consider an empirical "truth" without having first filtered it through one's intellect. I would be so bold to claim it impossible to do so completely.

That's true, it's the bugbear of formal empiricism that brainless entities can't do it. All the same, it is pretty clear when a consistency of perception has been identified. That's why reading of pointers can be done by almost anyone, but predicting those readings can be done by rather few. The rationalistic perspective about what is true knowledge regarding what exists is quite elitist, and suffers the flaw that a much more intelligent species than we will likely form a completely different view, one that we simply cannot understand any better than most plumbers understand string theory. Yet the plumber knows what he experiences, so the empiricist version is a more accessible ideal about what constitutes truth. Empiricism also avoids the troubling "truth is only as good as your current theory" problem that dogs rationalism.

Still, I grant you that neither rationalism nor empiricism can make a self-contained case, and that's probably why we need a combination to do science. Perhaps we have more to learn from the tension between the CI and the MWI, than we have to learn by marrying one or the other. But I completely agree with Fredrik that you are mischaracterizing CI when you say that it adopts essentially supernatural claims about the ontology of collapse-- instead, CI adopts a solipsistic perspective on collapse, it merely accepts collapse as real on the basis that we experience it, and unitarity as unreal on the basis that we do not experience it. CI takes no other position on the matter, there is no sense that "a miracle happens" when collapse occurs-- instead, collapse is what happens, the two are exactly the same concept in every way. That's why I said that unitary evolution is not a behavior at all in CI, it is just the language of how to predict, statistically, the next behavior, the next collapse. There is nothing in the empirical meaning of our word "behavior" that is not also in the word "collapse," so there is no need to attribute any mystical or miraculous elements to the latter word.

 

[Ken G]

MWI is certainly capable of talking about whether a subsystem is in a pure state or approximately in a pure state or not.

I suppose that depends on what one means by a subsystem. Many people, including on this thread earlier, talk about subsystems in MWI as associated with a particular particle that has some identifiable consistency across branches. In that language, no subsystem is ever in a pure state-- if I pass an electron through a Stern-Gerlach and accept all the ones taking the spin up path, none of them are in the state "spin up", they are all in a mixture of <spin up, accepted> and <spin down, rejected>. That's not a pure state, because there is no definite phase relationship between those terms, the phase is decohered. All the same, the experimenter doing quantum mechanics will deal only in the pure state <spin up, accepted> when he/she talks about the electrons in the accepted path, and MWI can deal with the fact that the projection onto that experimenter's acceptance decision will look like a pure state. But in MWI, that experimenter is under an illusion that the reality of the particle state is something different than what it actually is, because the experimenter's version cannot result from unitary evolution prior to the experiment, even as he/she uses unitary evolution to predict the subsequent behavior.

The big idea is that nobody has a birds-eye view. Therefore, any philosophical assertions about how the birds-eye have no scientific basis. I assert the analogy:

CI is to MWI as Lorentz Ether Theory is to Special Relativity​

I take your point about nobody having a birds-eye view, so that the ontological description must in a sense be cobbled together from all the birds-eye views. However, I reach the opposite conclusion along that same logical path! I see the analogy as:

MWI is to CI as Lorentz Ether Theory is to Special Relativity​

We agree that a consistent ontology needs to be patched together from objective observer's perspectives, but what goes into the patchwork? LET attempts to find a single truth that in some sense regulates all the observations, the aether, whose existence relies entirely on a rationalistic desire to unify the reality of all observers into a single description of "what is actually happening even though we never see it." That's a lot like what MWI does, it takes an ontological stance on what is happening in a way that we never actually see. But SR, like CI, rejects any ontology that involves entities we never see. SR elevates to the level of a metaphysical principle that if nature conspires to keep some ontological entity hidden, then it must be part of our interpretation that such an entity does not exist.

On the other hand, the role of decoherence as seen in MWI sounds an awful lot like how Lorentz imagined that the aether must be monkeying with our experimental apparatus to fool different observers into seeing different things, all within a single unified mathematical description. In other words, Lorentz interpreted his contractions ontologically, whereas Einstein made it a core value that if there is no empirical imperative for the contraction, then the contraction was not ontological at all, it was merely relative to the observer. In short, the set of observers that can share notes with each other, and their observations, is what defines the reality, reality is not tricking them or leading them into illusions. Yet that's just what it is doing in the MWI, so to me, the MWI sounds a lot more like LET in its stress on a consistent ontology instead of just querying a demonstrable collection of observers and take their reports at face value.

What I really want to do is to do the very thing my analogy above suggests: once we can entertain the notion that unitary evolution of wave-functions can adequately describe our experiences, we can then take the next step and notice there are many wave-functions^* that are empirically indistinguishable, and so we can switch between them at our leisure.

I can agree that what reality does not distinguish as different is not different. I don't yet see why that has a different flavor in MWI than in CI.

In particular, collapsing a wave-function when you observe something just becomes an example of changing your frame of reference.

I will grant you that is a valid insight, I can see elements there that would justify your analogy. If, for some reason, all the observers in our universe had to always be in the same one inertial frame, they might quite likely have come up with something like LET rather than SR. Their frame would have seemed very special, and when elementary particles at high speed took a long time to decay, they might have assumed something was affecting them, like a Lorentz aether. They would never have encountered an observer in a different frame, so they would not have cast the result in terms of invariants for that other observer, but rather in terms of physical effects on the elementary particle. But to that I have two immediate reactions:
1) if all the observers in the universe had to all be in the same inertial frame, there would be good justification for LET over SR, because there would seem to need to be a darn good reason for being all in the same frame, and
2) if we will think of MWI as a kind of transformation to observers in other reference frames, then what is the equivalent to the invariants that SR builds reality from? There is no equivalent to the norm of a 4-vector, which is empirically constructed. Instead, the only invariant I can see is the unitarity, but that is not empirically constructed, it is purely a rationalistic invariant. There is no function that takes as inputs each observers findings and generates a common scalar out of them, such that we could say our theories must only refer to those scalars.

How would I tell the difference? Not only do I not possesses any empirical evidence that I don't hear the words "<heads,tails>", I have no idea what such evidence would look like.

But that's just the point-- that you have no idea what such evidence would look like is the proof that you do not experience it.

I would perceive "heads or tails" because <heads,tails> is what really happened.

That's not what you perceive when you flip a coin one time-- you do not experience "heads or tails", you experience heads, or you experience tails, which is something different .

 

[Fra]

I have some issues with the word "describe". I would choose to talk about making accurate predictions about results of experiments instead.

Yes you're right, but I deliberately used the word describe to sharpen the point.

But there is no difference in what I refer to as "describing the future" and "predicting the future"; except in the way you understand what the point of "prediction" IS, and that's exactly my point (a) vs (b).

However, I don't think it's necessary to mention this in every thread about quantum mechanics, which clearly is a theory in the sense of what I like to talk about: It assigns probabilities to verifiable statements.

I also wouldn't describe a speculative new idea as one of the two "main" things that the term "theory" can refer to.

I don't mention it every thread. But the discussion in this thread is IMHO not so much about physics as it is about philosophy of physics. After all these discussions about the same old interpretations often (to me) seems non-constructive as no one rarely makes a point that aims to make a difference.

Also, it seems to me that I'm one of the few that represent these ideas on here, so for the benefit of a health discussion at least I feel I'm providing a fresh (possibly constructive, each one can judge that on their own) perspective to the discussed topic.

/Fredrik

 

[Eqblaauw]

I think it would be philosophy if we talked about the consequenses on our daily lives if mw where true. Wich are pretty big, especially if you talk about the splitting variant, that lives on while being denied by prominent proponents of mw. For example when someone dies in a quantum (chance) event you will feel a lot different when you believe he/she lives on in most of the (many, many) worlds. Sure it's sad you don't see him/her anymore, but that certainly isn't all we grieve about at funerals,
also it makes every history book seem pretty trivial
But making speculative claims about the physical existense of things isn't philosophy, it isn't even bad philosophy. Just speculative science (good or bad, it's up to you. Bad I would say, but hey).

 

[Dmitry67]

It sounds like there are two "me"s here, an "F" me and a "B" me. Will the real me please stand up! When did the "B tree of me" begin, anyway?

Sure, but please provide a definition of "being real" first.

Your tree had started when you were born. In some alternatives you were born earlier/later and in different conditions, I don't know how operation = (equals) works for the consiousnesses, so I can't say if they also belong to "you"

The same issue you have in Infinite Universe even without MWI, just alternative/copies are separated spacially.

 

[Eqblaauw]

semantic philosopher when someone says you get an icecream and you get one it is real when someone says that and he gives you nothing it is not real, there is nothing hard about something being real
and don't say define icecream

 

[Ken G]

Sure, but please provide a definition of "being real" first.

That's indeed the question. To a CI proponent, "being real" means "demonstrable by experiment"-- what is real is what is experienced by objective observers that we can (in principle) communicate with. To a MWI proponent, being real means fitting in with a mathematical structure that is generated by the minimal and most unifying conceptual principles. So to be consistent, both of the interpretations must build their models of what "I" am from similar stuff. That's going to be a lot easier for CI, because "me" is whatever I perceive myself to be, and the experiences of "me" are nonunitary.

Your tree had started when you were born.

Not when I was conceived? What if I am born dead in some branch, and alive in another, do I need to be born alive to count in the "me" tree? When does the "me tree" end, if there is always some branch in which I am still alive, no matter how rare? What if I'm a vegetable in some tree, does that suffice to keep the "me tree" going?

In some alternatives you were born earlier/later and in different conditions, I don't know how operation = (equals) works for the consiousnesses, so I can't say if they also belong to "you"

Well, that's certainly going to be a problem you will need to work out before you can claim that MWI can account for "my" perceptions. Note how much easier it is for CI-- there, my perceptions are just my perceptions.

The same issue you have in Infinite Universe even without MWI, just alternative/copies are separated spacially.

Not in CI, because alternative copies are irrelevant in CI-- my perceptions are just what they appear to be, and I don't perceive any copies of me, so I have no reason to account for them. I don't need them to recover my unitariness, I can be just as nonunitary as I perceive myself to be.

 

[Eqblaauw]

That's indeed the question. To a CI proponent, "being real" means "demonstrable by experiment"-- what is real is what is experienced by objective observers that we can (in principle) communicate with. To a MWI proponent, being real means fitting in with a mathematical structure that is generated by the minimal and most unifying conceptual principlesborn alive to count in the "me" tree?

Yes but what dimitri is implying is that there are different definitions for 'being real'. That's gibberish, both miw-ers and ci-ers have exactly the same definition of 'being real' they just think it applies to different things.

 

[Fra]

With the high risk of beeing misunderstood...

To a CI proponent, "being real" means "demonstrable by experiment"-- what is real is what is experienced by objective observers that we can (in principle) communicate with.

At first sight, this means "real" is defined by whatever the set of any classical observers will agree with infer from "experiments". The idea is that even if there are some differences, due to inertial frame, in principle one can classsically recover an equivalence group of observers that defines "reality" as invariants of the set of observers whose observations are related by known transformations. It's in this specific sense that "reality" exists independnet of observation EVEN in CI!

Thus the "realism" here is applied to the set of agreements between classical observers. Ie. this is even take to be a "consistency requirement".

Yes but what dimitri is implying is that there are different definitions for 'being real'. That's gibberish, both miw-ers and ci-ers have exactly the same definition of 'being real' they just think it apply's to different things.

Someone else get to elaborate for MWI, but there is similary some structures where "realism" is applied

So in this sense I see what you say.

there is nothing hard about something being real

But I think the above is a simplification.

Since I consider myself coming from CI, while currently beeing more radical I can most easily explain what I mean by noting what's wrong in the CI picture above:

The core point is objectivity. Essentially the laws of physics MUST be the same to all observers. Thus realism applies to physical law. But this a subtle breach with the scientific ideal of empirism, which seems to more advocate effective laws, rather than "objective laws".

Instead of thinking of the indeed sounds (almost obvious) requirement of a scientific theory to be observer independent (or scale covariantly) in a predictable way, and thus consider objectivity to be

- a LOGICAL CONSTRAINT on the theory; prime examples are special as well as general relativity. Here realism as "observer independence" is a CONSTRUCTING principle. Einsteins insight was that, it does not make sense of different observers "sees" different laws - thus in the search for a covariant theory this is a key insight.

This is by the way the "normal" and by far dominating way to understand it, even today, even in QM.

- or as a form of equilibrium condition for emergent objective, in terms of observer democracy. This picture seems to be preferred by a empirist perspective since after all, laws are abductions from experiments, not deductions. I do not anyone can deny that.

It sounds like Eqblaauw's point is that both CI and MWI talk contains realism, just in different forms, here I agree.

But we don't have to settle with this, so here I disagree that what's real is a triviality.

/Fredrik

 

[Eqblaauw]

yes my point is both have the same definition of 'real', but they just think other things are real based upon the method/interpretation they choose.
I'm aware I speak for MIW, but this is just in the best faith.
If some theory chooses another definition of real then this is only bound to give confusion, and they should use another word.

 

[Samshorn]

...what you'd have to say is, you have <observer1,observer2> reporting <heads,tails>. If you do that, the mathematical description is entirely unitary, but you have no way to account for why it did not come out <observer1,observer2><tails,heads>. All you could say is that you don't care about the difference there-- but try telling that to observer1 if "heads" means he loses his.

What is the significance of your observer labels 1 and 2? I think a MWI advocate would say that, if we have a sequence of 50 coin tosses, there are 2^50 distinct observers, one for each possible sequence of outcomes, and we identify those observers with their respective sequence of outcomes. So it's self-contradictory to talk about permuting the observers to different sequences of outcomes. The sequences of outcomes ARE the observers.

By the way, Deutsch has argued that observers (and universes) not only proliferate, they must also re-aggregate, and hence it is possible in principle for an observer to remember a superposition of histories. So there are versions of MWI that are compatible with observers perceiving superpositions, and there are versions of MWI that are compatible with observers NOT perceiving superpositions. Then, applying my "second quantization" meta-interpretation MMWI (Many Many Worlds Interpretation) [Trademark] the universe is actually a superposition of Many Worlds's, in some of which the identities re-aggregate and in some of which they do not. (I've already trademarked all the other MM...MWIs too, so don't even think about it.)

 

[Fredrik]

I don't think it makes sense to ask for definitions of terms like "real", "exists" and "describes" when we're talking about definitions of QM. They should be considered primitives (i.e. terms left undefined).

If I had to define it, I would say that a "mathematical thing" in the theory is "real" if it's directly measurable, like momentum. But this is obviously not what anyone has in mind when they discuss interpretations of QM. They mean "real" in the sense of how you have understood the term since you were a kid. Polar bears are real, Santa Claus isn't.

This explanation isn't a definition, since it doesn't associate the term with a specific mathematical or physical object. It's an "elucidation". (That's what a book I own calls an explanation of a primitive).

 

[Eqblaauw]

Polar bears are real, Santa Claus isn't.

This seems a pretty good example to me. But I have to stand up for the Tegmark-like MWIers who think there probably is an immortal ('since we all are immortal') Santa Claus somewhere, riding his fuled sleigh to make all the little children happy.
And no this isn't a strawman

 

[Ken G]

The idea is that even if there are some differences, due to inertial frame, in principle one can classsically recover an equivalence group of observers that defines "reality" as invariants of the set of observers whose observations are related by known transformations. It's in this specific sense that "reality" exists independnet of observation EVEN in CI!

I think I see what you are saying, but I would elaborate on it a little. Relativity doesn't actually say that reality is something other than what the observers perceive, it says two things:
1) what an observer experiences is not separable from that observer, so reality must include only an account of dual entities like < observer_i | observation_n >
2) any theory used to describe or predict reality must be built from a special type of invariant, a type involving a mapping from the entities < observeri | observation_n> into some observer-independent scalar [special invariant]. The [special invariant] is free of any need to refer to an < observer|, but note that <observer_i|observation_n> is already an invariant, all observers will agree that that observer got that result. But the theory will not refer to invariants like that because they involve explicit mention of some observer, so relativity theory is built not just from any old invariant, like <observer_i|observation>, but only from norms of 4-vectors, whose components look like <observer_i|observation_n> in the basis <observer_i|, for a special class of n. Those components are not invariants when regarded as coordinates that stay the same in any basis, but they are invariants when regarded as the projection of an observation onto an observer.

In short, some invariants are frame-dependent, not in that they change from frame to frame, but that they require the identification of a frame in order to exist. Relativity doesn't use that type of invariant to build its laws about reality, but it does not deny that such invariants are part of reality. So I would say the lessons of relativity are:
1) a consistent concept of reality must be an account for all the invariants <observer_i | observation_n> so also any function f(<observer_i | observation_n>)
2) a description of the laws on such a reality must be built from a higher-order form of invariant, which is a function on the invariants that do reference an observer frame, and the function produces a scalar output that is the same for any (inertial) observer frame referenced. So we have the kind of "invariant" that relativity talks about, which is really any sum of <observer_i | observation_n>² with the appropriate signature as n ranges over the indices of a 4-vector. It is a special construction from the invariants that do refer to a reference frame, resulting in a special invariant that does not so refer.

Now, relativity says the laws have to be constructed from these 4-vector-norm type of invariant, but they still have to predict all the invariants, like the <observer_i | observation_n>. The latter requires something more than the laws, it requires boundary and/or initial conditions-- the "hidden" element of any physical theory that doesn't get counted as part of the laws of the theory, but the theory doesn't work without them.

So I would say that reality in relativity only exists independently of observations if one is strictly considering the laws of relativity, and ignoring the essential role of boundary conditions to condition the outcomes that we will actually test in order to judge if the theory works. When we generalize the theory to include the way it processes boundary conditions, then we need to include all the invariants, including the ones that refer to a particular reference frame, since they will be needed for the theory to do what we want it to do-- predict invariants that do refer to particular reference frames. The reality, then, is still the latter-- not the laws. That's the empiricist stance on relativity-- the reality always refers to reference frames, but the laws do not. The MWI/rationalist analog would be to say that the laws are the reality, and the boundary conditions and outcomes we use to test them are just some kind of arbitrary illusion, not important to the reality (but essential to testing the theory).

The core point is objectivity. Essentially the laws of physics MUST be the same to all observers. Thus realism applies to physical law. But this a subtle breach with the scientific ideal of empirism, which seems to more advocate effective laws, rather than "objective laws".

Interesting point, it sounds like you are an empiricist with rationalistic leanings! The pure empiricist does not have an issue translating between effective and objective laws-- they simply say that the two are the same, for a law to be effective it must be objective. The objectivity still stems from the empirical set of <observer_i | observation_n> entities that define reality (where the observers can be hypothetical, the issue here is to generate a language about reality), but it is natural to require that any laws that effectively govern or predict those objective entities must themselves be objective, even if they are just effective laws. The rationalist always asks, how can reality do anything but obey laws, and the empiricist responds, how would reality know how to obey anything, especially not laws that we keep changing-- reality just is.

 

[Ken G]

What is the significance of your observer labels 1 and 2? I think a MWI advocate would say that, if we have a sequence of 50 coin tosses, there are 2^50 distinct observers, one for each possible sequence of outcomes, and we identify those observers with their respective sequence of outcomes. So it's self-contradictory to talk about permuting the observers to different sequences of outcomes. The sequences of outcomes ARE the observers.

Yes, exactly-- I expect that many (most?) MWI advocates simply define the observer by what they observe. That means you are defined by your perceptions, you are not a kind of "vial" that gets filled with perceptions. That's what I mean by a "model of me" that is required in MWI. The problem is, whenever we make a model for one purpose, like interpreting quantum mechanics, we have to live with that same model in every other way, if our claim is going to be that we have a consistent and complete view. So then we need to ask, is this really the "model of me" that works, that I am defined by my experiences? Note how completely that undoes the standard empiricist perspective on reality-- we take ourselves as given, and use our experiences to say what reality is. That's what separates the scientist from the mystic. We don't take reality as given, and use our experiences to say what we are, that's essentially a mystic stance. You can do it either way and everything works, but you have to accept the full package of the rationalist perspective if you're going to go that road. I have no objection to MWI other than that many of its proponents seem to think they are just doing physics, rather than adopting a fairly radical rationalistic philosophy of who we are and what knowledge of reality is. Are you simply defined as the person who lives in a scientific reality?

 

[Eqblaauw]

I would like to give zeno's paradox as an example about something that makes mathematical sense for centuries, but we simply know by experience it is false. I,m not saying mwi directly controdicts are experiences, but there aren't any experiences that account for it. To paraphase brian greene (not a fan) math tells us what may be possible, expirements have to tell us what is. There may be some experiments in the future (mwiers sometimes say this as if this itself alone is an argument for the theorie) but now there isn't. So now it's a hypothesis, and nothing more, and if you want to say it's true you just have to wait till there is any empirical evidence whatsoever. If there ever will be.

 

[fleem]

Well, the thing about Zeno's paradox is that it does what I'd call "reverse-normalization" (abnormalization?). Normally if we have infinities in a problem we find a way to remove them. Zeno's paradox is just an indication that if we're crazy enough we can intentionally add infinities to any problem.

 

[Hurkyl]

Your scenario doesn't answer it, because we don't have an observer getting <heads,tails>, we have a bundle of perceptions connected with the sentient being I'll call observer1, and a bundle of perceptions (according to MWI) connected with sentient being observer2, whose perceptions do not overlap and they do not perceive each other.

(There was only one sentient being on my scenario. The mathematical description is identical to a different scenario with two observers that we talk to simultaneously -- but we're not doing that thought experiment, we're doing a thought experiment with a single observer in a universe that admits indefinite outcomes)

but try telling that to observer1 if "heads" means he loses his.

Me: *applies hypnosis* You will kill yourself if your first coin flip was heads.
Experimenter: <okay. *bang*, phew good thing I got tails>
Me: So, how are you feeling?
Experimenter: <no response, pretty good>
...​

Your scenario doesn't answer it, because we don't have an observer getting <heads,tails>

That's because it was the starting point. If one really wanted to (and had the details), one could wind back in time and find the state of 5 seconds in the past, and then see that <heads,tails> was the (deterministic!) time evolution of that state.

If that's not an answer to your question, then I think you're starting to ask questions akin to "Why was the universe created in such a way so that we're sitting here posting on an internet forum during the September of 2011?"

Also, note that CI is just as mathematically unified as MWI.

Really? CI has found a way to unify unitary evolution and collapse? Can you provide a reference?

And it can only accomplish it by dodging the question of why I perceive what I perceive. I still haven't seen your answer to that.

The answer is trivial -- you perceive what you perceive because it's encoded in the wave-function. And the wave-function is what it is because it was the unitary evolution of the wave-function of the past.

You're rejecting the answer because you want reality to adopt a definite outcome, and you don't think a frog's eye view of definite outcomes is good enough, you are insisting that they are definite in the bird's eye view as well.

It can't be that, because it fails to do that.

It sure looks like it does to me. Consistency of observation is explained. That different observers agree upon what is observed is explained. Empirical frequencies are explained.

So what is left unexplained? The thing you keep insisting is unexplained is a quality of reality that is imperceptible.

Certainly not, not to the empiricist.

In your own words:

And the correct response of the CI person is: ... They evolve in the parameter time, in exactly the way QM says they do,​

If, for whatever strange reason, you define the English word "behave" in a way that the above isn't a behavior, then whenever you see me using the word, you should mentally substitute whatever word you find more appropriate.

MWI adds an ad hoc metaphysical postulate (as you did above) that our particular individual experience is a question that physics should not be interested in.

You've changed the question again. Our individual experiences are interesting. What is not interesting is the supposition that reality adopts a definite outcome.

MWI is not content with mathematical unification, it wants ontological unification.

I'm not interested in whatever straw man you are intent on setting up to attacking. I'm interested in the the hypothesis that collapse is not required to get the correct predictions from QM -- in particular the qualitative behavior of our observations.

Platonism is quite a bit different from the way MWI is normally expressed.

Where the heck did this train of thought come from?

Indeed that is my entire issue with how MWI is normally expressed-- it improperly crosses that dividing line, mistaking a syntactic interpretation for a semantic one.

I can't even begin to make sense of this.

CI adopts a solipsistic perspective on collapse, it merely accepts collapse as real on the basis that we experience it,

This does not follow without the additional ontological postulate that reality adopts a definite outcome.

and unitarity as unreal on the basis that we do not experience it.

What evidence do we have that we do not experience it? What experiment could possibly give different results between the two cases of definite and indefinite outcomes?

instead, collapse is what happens

Yes, that is the interpretation of QM.

the two are exactly the same concept in every way.

And this is an ontological postulate about an imperceptible aspect of reality.

 

[Hurkyl]

You seem to have a very precise opinion about what the CI is.

Precise, but I don't believe very narrow. CI is

• The wave-function is a mathematical object that contains all the information needed to make predictions about the system
• Unitary evolution is sometimes applied to wave-functions
• Collapse is sometimes applied to wave-functions
• Our observations are of definite states (which are the results of collapse)

The idea that QM doesn't describe reality and just assigns probabilities to verifiable statements, is one of the ideas that can be called "the CI".

Agreed, but it's still an interpretation in which the information about reality is encoded in the wave-function. And quite honestly, the only practical difference I've seen out of this particular variation is that its adherents find it very easy to rationalize things away. (Collapse is ad-hoc? But it's just information, so there's no problem!)

However, people tend to call it an "ensemble interpretation", or a "statistical interpretation" these days.

But I haven't spent much time breaking down these variants. I suspect my final opinion would be that they are doing pretty much the exact same thing as MWI, except they'll using different words and shy away the idea that one can speak meaningfully about an individual system.

OK, I see that my statements look pretty weird in that context. Not sure if I can do much better though. One of the reasons is that I can't figure out why you think that string theory would be more compatible with MWI than CI. Since I don't understand that, I don't know what I should be saying in response.

The "part of the system" thing. According to CI, my experiences are most definitely definite from the bird's eye. Therefore, either you cannot apply a quantum mechanical theory to learn about, say, our galaxy, or you have to first cut a "me"-shaped hole out of the universe to avoid predicting that my experiences will be indefinite. The latter doesn't sound especially promising.

 

[Hurkyl]

In short, some invariants are frame-dependent, not in that they change from frame to frame, but that they require the identification of a frame in order to exist. Relativity doesn't use that type of invariant to build its laws about reality, but it does not deny that such invariants are part of reality.

Hrm. I think it, at least, strongly suggests they are not part of reality -- strong diffeomorphism invariance.

The fix is, assuming our invariant really is an invariant, noting that the "identification of a frame" is done in terms of physical matter or somesuch. So (in principle) we can translate any dependence upon identification of the frame into frame-free dependence on actual physical matter, and *poof* -- it's not that the invariants you thought were frame-dependent aren't part of reality, it's that you were actually wrong about labeling them as frame-dependent.

 

[Ken G]

(There was only one sentient being on my scenario.

Not so-- there were two. This is quite important-- there was the person answering, and there was you. And if you really play that game, you never hear answers like <heads, tails>, that's my point. Why don't you? Because you don't have the birds-eye perspective either. However, you pretended, in your device, that you did. That's the "cheat" of MWI right there, it pretends it is possible to have a birds-eye view, in the same breath that it admits there is no such thing. Special relativity never did that-- that's what LET does (where the "aether" is the "birds-eye frame").

If that's not an answer to your question, then I think you're starting to ask questions akin to "Why was the universe created in such a way so that we're sitting here posting on an internet forum during the September of 2011?"

I don't need to return to the origin of the universe to ask the question I'm asking, only back to the last coin flip. I have heard plenty of ways to understand why I get <heads, tails>, but I already know that this is what MWI gives me. I have a different challenge for MWI: I just paused typing and flipped a coin. It came up "heads". You say "no it didn't, it came up <heads, tails>, but the perceiving agent you imagine that you are is somehow only in perceptual contact with the result heads." I say, "yes, I realize that, that isn't what I'm asking. I'm asking why the perceptual agent tha I imagine I am is somehow only in perceptual contact with the result heads. We agree this is the case, now give me an accounting for that using MWI."

Really? CI has found a way to unify unitary evolution and collapse? Can you provide a reference?

What I said was that the mathematics of CI is exactly identical in the smallest detail to the mathematics of MWI. The only difference is in the interpretation of the purpose of that mathematics! The purpose of the mathematics in CI is to use the Born rule to predict a measurement. The purpose of the mathematics in MWI is to generate an ontological description of reality. Those are not mathematical differences, those are philosophical differences in what the mathematics means. So no, there is no need for a reference, this is just obviously a true statement. If you think the mathematics of CI is any different from the mathematics of MWI, you need to show me a mathematical entity used by MWI that is not used in CI (and please note that von Neumann entities like |pointer 1>|state 1> + |pointer 2>|state 2> do not meet my challenge-- they are certainly used in CI!).

The answer is trivial -- you perceive what you perceive because it's encoded in the wave-function. And the wave-function is what it is because it was the unitary evolution of the wave-function of the past.

What is encoded in the wave function is more than I perceive. I didn't ask why I perceive the part I do perceive, I asked how you account for my perception being different from the wave function. I have given two challenges:
1) find a mathematical entity used by MWI that is not used by CI
2) find an accounting within MWI for why I do not perceive the unitary evolution of the wavefunction, but instead a nonunitary projection. If you say "that's just how it is", you fall victim to the very aspect of CI that you criticize: ad hocness.

You're rejecting the answer because you want reality to adopt a definite outcome, and you don't think a frog's eye view of definite outcomes is good enough, you are insisting that they are definite in the bird's eye view as well.

No, my objection is that if all we have is a frog's eye view, plus what all the frogs can tell each other (as in relativity), then the birds-eye view is a rationalistic fantasy that we have no reason to believe or expect really exists at all, unless we buy off on that very same rationalistic dream.

So what is left unexplained? The thing you keep insisting is unexplained is a quality of reality that is imperceptible.

Yes, I thought you'd say that-- "that's just how it is." How is that any different from CI saying that collapse is "just how it is"? Your last remark admits to exactly the same type of incompleteness that CI is much more honest about, that's my whole objection to MWI-- it is disingenuous.

In your own words:

And the correct response of the CI person is: ... They evolve in the parameter time, in exactly the way QM says they do,​

If, for whatever strange reason, you define the English word "behave" in a way that the above isn't a behavior, then whenever you see me using the word, you should mentally substitute whatever word you find more appropriate.

You seem to have missed my point. In QM, t is a continuous parameter that can take on any value, no matter how small. Is this true of time in the "behavioral" sense?

I'm interested in the the hypothesis that collapse is not required to get the correct predictions from QM -- in particular the qualitative behavior of our observations.

It is obvious that collapse is not required, any number of interpretations accomplish that. That's all you are interested in?

 

[Eqblaauw]

I would like to give zeno's paradox as an example about something that makes mathematical sense for centuries, but we simply know by experience it is false. I'm not saying mwi directly contradicts our experiences, but there aren't any experiences that account for it. To paraphrase brian greene (not a fan) math tells us what may be possible, expirements have to tell us what is. There may be some experiments in the future (mwiers sometimes say this as if this itself alone is an argument for the theorie) but now there isn't. So now it's a hypothesis, and nothing more, and if you want to say it's true you just have to wait till there is any empirical evidence whatsoever. If there ever will be.

I don't know if zeno is a good example, but the rest of this post still stands (a part from my flawed english) . I think now I will stop debating about this. It's becoming, or it was in the first place, a bit obsessive. This is no argument against mwi, but I think that when you go to a shrink and tell him that you believe that during the conversation both of you where split more then a 1000 times and there by you where creating an equal amount of universes which all contain 6 milliard people, well I think you will be locked up for quite some time. Living with this 'knowledge' must be quite a burden. I don't want to put anymore energy in this because it makes me sick. I want to conclude with a call to everyone here (including the mwi-ers) to make the wikipedia mwi reception section less biased, because that was what got me in this mess. This page is pure propaganda, and if you look at the talk cite of wikipedia there is a other view of the reception that is equally justified, and completely ignored. If you look carefully at how it's presented you will find out that it's nothing short of propaganda. I suspect most mwi-ers want people/layman to make up their minds about this on the basis of objective information.
And i suspect that there are more people like me (not copies), who get a little confused when they read a theory, they never heard of, which supposedly has almost a majority in the relevant science community. Especially when this theory is altering your worldview and view of yourself in such a dramatic way as many-worlds does. This it self isn't a bad thing if it where true, but it's not. And you find this out when you properly investigate this.
The poll of David Raub for example, is conducted by a fan, it's unfindable, highly contradicting every other poll, very vague (for some reason the question is 'do you think mwi is 'true' and not 'do you think many-worlds is true'), it says Gell-Mann believes in the theory (that may be true but his view certainly doesn't involve any parallel universes, this information is conveniently left out) and sadly quoted by almost every site referencing mwi.
So let us all for the sake of truth MWI-ers, non MWI-ers, hand in hand, restore this ugly piece of propaganda in the name of truth.

 

[Fredrik]

Really? CI has found a way to unify unitary evolution and collapse? Can you provide a reference?

I'm guessing that Ken G is just using a different definition of "the CI" than you. This is a good example of why I don't like to talk about "the CI". (Edit: I wrote my reply before seeing Ken G's post above).

QM (decoherence) tells us that after a measurement, the combined system (specimen+device) will be approximately in a mixed state, which I'll write as $\rho=\sum_i p_i|\psi_i\rangle\langle\psi_i|$. My preferred version of the CI (an ensemble interpretation) would say that this is just a way of saying that the probability that we got the ith result is p_i. It seems to me that your version of the CI would say (something like) that exactly one of the terms in this post-measurement state operator describes the combined system after the measurement. This is where the interpretation contradicts the theory. There's nothing in QM that allows the state of the combined system to go from what it was before the measurement to being just one of the terms in the post-measurement state. This is why you need collapse to make sense of "the CI", and we (or at least I) don't.

I'm interested in the the hypothesis that collapse is not required to get the correct predictions from QM

Consider the assumptions

(a) QM describes reality
(b) there's only one world​

These assumptions look incompatible to me. One way to remove the contradiction is to invoke collapse. We simply change (a) to

(a') QM describes reality, except when things are being measured.​

I consider this an extremely ugly solution, since I don't want an interpretation of QM to say that QM is wrong. Another way to remove the contradiction is to just drop (b). A third way is to replace (a) with its negation. (Then you can do anything with (b): Keep it, drop it, or replace it with its negation).

I don't consider your version of "the CI" (defined by (a') and (b)) to be a very rational position. Does anyone actually hold this position? I mean, someone who has actually thought about it? ("QM describes what's happening, except when a collapse is going on").

The "part of the system" thing. According to CI, my experiences are most definitely definite from the bird's eye. Therefore, either you cannot apply a quantum mechanical theory to learn about, say, our galaxy, or you have to first cut a "me"-shaped hole out of the universe to avoid predicting that my experiences will be indefinite. The latter doesn't sound especially promising.

OK, I understand what you mean now. But I don't have a good answer right now. I will think about it.

 

[Hurkyl]

Not so-- there were two. This is quite important-- there was the person answering, and there was you. And if you really play that game, you never hear answers like <heads, tails>, that's my point. Why don't you? Because you don't have the birds-eye perspective either.

Oh, I misunderstood both aspects of your comment: -- I thought you were suggesting there were two copies of the experimenter, and I thought you were referring to a hypothetical in-the-universe me that was different from the external-to-the-universe me.

The point of the game is that we have a bird's eye view and can see the indefiniteness of the toy universe, and by interacting with the experimenter, you could understand that the experimenter still only perceives one outcome, and while his belief in definite outcomes -- that his experiences are definite -- and you can never trap him in a contradiction.

If you would prefer, you could play the game without us interacting with the observer, and instead just pick his actions or something, so that we watch how his universe plays out without ever actually interacting with it.

The game was not an equivalent to MWI -- it's supposed to help learn the principle that the an indefinite universe doesn't contradict our perceptions of definite outcomes are not. There's no point in someone entertaining MWI if he can't wrap his head around that fact -- and it's probably easier to understand it in the simpler and more familiar classical context.

It came up "heads". You say "no it didn't, it came up <heads, tails>,

What I would actually say is "We can't tell if it was 'heads' or '<heads,tails>' or '<heads,tails,edge>' or '<heads,tails,edge,a sparrow flew away with the coin>'".

But if, instead, we are considering a qubit which I had reason to believe was spin up in the x direction and you measured it around the Z axis and said "I measured spin up", I would fairly confidently believe that, conditioned on my knowledge the original state was in the x direction, we are now currently in the indefinite state of a mixture of one half "you saw spin up" and "you saw spin down". And I would agree with you: "I see spin up", and by the MWI, have high confidence that both of our usages of the word "up" is indefinite.

Of course, as I mentioned earlier, I really want to use MWI + indistinguishable states are equivalent. So at this point, unless I had some particular reason to do otherwise, I'd probably put the indefiniteness out of my head since it doesn't matter.
But if it doesn't matter, why bother thinking about the issue at all? The big point is that I can understand how everything in the above happened in terms of the unitary evolution of quantum states; the switch to the collapsed state has no significance beyond computational convenience.

I don't need to keep two separate notions in my head of "quantum states evolving by unitary evolution" and "things collapse onto a definite state".

2) find an accounting within MWI for why I do not perceive the unitary evolution of the wavefunction, but instead a nonunitary projection. If you say "that's just how it is", you fall victim to the very aspect of CI that you criticize: ad hocness.

You do perceive the unitary evolution of the wave-function.

By relative states and decoherence, unitary evolution of the wave-function looks like a transition to a mixed state.

By probability theory, mixed states look like definite outcomes obeying statistical laws.

You would describe your perceptions as being definite, and obeying statistical laws, right?

In QM, t is a continuous parameter that can take on any value, no matter how small. Is this true of time in the "behavioral" sense?

And in various circumstances, I would find it perfectly natural to paraphrase that with words like "t behaves like a continuous variable" or "t behaves like a real number".

Did you mean for "time" and "t" to be different? I don't really have any better way to understand the notion of "time" than physics, so I can't really appreciate the point. Unless, of course, you were simply allowing "time" to cover other forms of time that appear in physics.

It is obvious that collapse is not required, any number of interpretations accomplish that. That's all you are interested in?

It's not obvious at all. For decades they were pretty convinced it was required, and even today you still get lots of people who swear seven ways to Sunday that our perception of single outcomes is definitive proof that collapse occurs.

And even once one wraps one's head around indefinite outcomes, you have the new problem of "Okay, we've refuted all the counter-arguments, but we haven't yet developed sufficiently powerful machinery (theoretical or physical) to check if QM really does work on these scales".

 

[Eqblaauw]

Originally Posted by Eqblaauw
'I would like to give zeno's paradox as an example about something that makes mathematical sense for centuries, but we simply know by experience it is false. I'm not saying mwi directly contradicts our experiences, but there aren't any experiences that account for it. To paraphrase brian greene (not a fan) math tells us what may be possible, expirements have to tell us what is. There may be some experiments in the future (mwiers sometimes say this as if this itself alone is an argument for the theorie) but now there isn't. So now it's a hypothesis, and nothing more, and if you want to say it's true you just have to wait till there is any empirical evidence whatsoever. If there ever will be.'I don't know if zeno is a good example, but the rest of this post still stands (apart from my flawed english) . I think now I will stop debating about this. It's becoming, or it was in the first place, a bit obsessive. This is no argument against mwi, but I think that when you go to a shrink and tell him that you believe that during the conversation both of you where split more then a 1000 times and there by you where creating an equal amount of universes which all contain 6 milliard people, well I think you will be locked up for quite some time. Living with this 'knowledge' must be quite a burden. I don't want to put anymore energy in this because it makes me sick. I want to conclude with a call to everyone here (including the mwi-ers) to make the wikipedia mwi reception section less biased, because that was what got me in this mess. This page is pure propaganda, and if you look at the talk site of wikipedia there is another view of the reception that is equally justified, and completely ignored. If you look carefully at how the current site is presented you will find out that it's nothing short of propaganda. I suspect most mwi-ers want people/layman to make up their minds about this on the basis of objective information.
And i suspect that there are more people like me (not copies), who get a little confused when they read about a theory, they never heard of, which supposedly has almost a majority in the relevant science community. Especially when this theory is altering your worldview and view of yourself in such a dramatic way as many-worlds does. This it self isn't a bad thing if it where true that it's the dominant view of the relevant science community, but it's not. And you will see this when you properly investigate this.
The poll of David Raub for example, is conducted by a fan, it's unfindable, highly contradicting every other poll, very vague (for some reason the question is 'do you think mwi is 'true' and not 'do you think many-worlds is true'), it says Gell-Mann believes in the theory (that may be true but his view certainly doesn't involve any parallel universes, this information is conveniently left out) and sadly quoted by almost every site referencing mwi.
So let us all for the sake of truth MWI-ers, non MWI-ers, hand in hand, restore this ugly piece of propaganda in the name of truth.

 

[Ken G]

The point of the game is that we have a bird's eye view and can see the indefiniteness of the toy universe, and by interacting with the experimenter, you could understand that the experimenter still only perceives one outcome, and while his belief in definite outcomes -- that his experiences are definite -- and you can never trap him in a contradiction.

That game could be useful for someone who doesn't understand MWI. I already knew all that-- my objection to MWI was never that you can get trapped in a contradiction using it, I completely accept that MWI is self-consistent, it merely espouses a different set of priorities about the kinds of questions that physics should be trying to answer, and then pretends that it isn't moving those goal posts. The empiricist simply says, "why do you care if the universe is unitary or not, aliens on alpha Centauri are using a more advanced theory right now that dropped unitarity a millennium ago. It is enough to explain what we experience in terms of aesthetically pleasing theories, there is no need to explain what we do not experience in ways that only allow us to imagine our theories can dictate to reality. No rationalist was ever right, but at least the observations don't change."

If you would prefer, you could play the game without us interacting with the observer, and instead just pick his actions or something, so that we watch how his universe plays out without ever actually interacting with it.

Watching is interacting, that's the CI position. It's empiricism-- the "interaction" does not need to be a physical influence, it is the simple fact that we know by experiencing, so we know about the universe you are inventing by experiencing it in some way. That constrains us to be observers, and not of the birds-eye variety, that is just a make-believe entity in physics. Even Einstein's gedankenexperiments always involved a possible physical system, that is quite important.

The game was not an equivalent to MWI -- it's supposed to help learn the principle that the an indefinite universe doesn't contradict our perceptions of definite outcomes are not.

It is a useful device for those who don't already know that. I already know that. Note that I never said we can empirically rule out many worlds, I said that it is anti-empiricist to invoke them at all, when the motivation for doing so is to preserve unitarity yet our experiences are a litany of non-unitariness.

What I would actually say is "We can't tell if it was 'heads' or '<heads,tails>' or '<heads,tails,edge>' or '<heads,tails,edge,a sparrow flew away with the coin>'".

Exactly, we can't tell. That is indeed my whole point. When the empiricist is confronted with something that "we can't tell", he/she does not say "so choose whichever version gives you the greatest aesthetic pleasure." That's just precisely what Lorentz did, and is why MWI is more analogous to LET. The empiricist takes the stance of SR-- a difference that we cannot tell is not a difference at all. Reality is what is perceived by observers, not what is perceived by observers plus whatever number of angels fits on the pin, doing the "dance of unitarity." While they dance, the empirical meaning of time is the interval between measurements, that's all it ever meant. Nothing happens between measurements, what happens is the measurements. Call that a corollary to Bohr's "there's no such thing as the quantum world." That's what I mean when I talk about the CI, not some magical collapse property that is "actually happening to the real wave function."
You raise some other points I'll look at in a bit.

 

[Hurkyl]

Exactly, we can't tell. That is indeed my whole point. When the empiricist is confronted with something that "we can't tell", he/she does not say "so choose whichever version gives you the greatest aesthetic pleasure." That's just precisely what Lorentz did, and is why MWI is more analogous to LET.

It's my point too -- except that you have the analogy wrong.

Lorentz Ether Theory represents maintaining prior views -- e.g. shaping the interpretation to retain the notion of absolute time that was present in previous physical theories.

CI is the same -- shaping the interpretation so as to retain the prior notion of definiteness.Special Relativity, on the other hand, represents taking the physical theory seriously enough to warrant reshaping our views on reality and reject^* absolute time.

MWI (at least, the form of MWI I care about) is the same -- to take the physical theory seriously enough to reshape our views to reject definiteness.

*: I do not mean "deny", I mean "reject".

The empiricist simply says, "why do you care if the universe is unitary or not, aliens on alpha Centauri are using a more advanced theory right now that dropped unitarity a millennium ago.

Or he might say "why do you care if the universe is definite or not, aliens on alpha Centauri are using a more advanced theory right now that dropped definiteness a millennium ago. It is enough to explain what we experience in terms of aesthetically pleasing theories, there is no need to deny what we do not experience in ways that only allow us to imagine our theories can dictate to reality. No rationalist was ever right, but at least the observations don't change."

our experiences are a litany of non-unitariness.

How can you tell? In what perceptible way does a unitary experience differ from a non-unitary one?

 

[jensa]

Fredrik/KenG:
Sorry to jump into the middle of the discussion. I usually don't participate in interpretational issues but have found your posts on the matter interesting. I am just curious if the following summarizes your viewpoints on the issue (or whether I am missing some points).


Let me start with points I think everybody can agree on (I hope)

• After the system of interest interacts with the measurement apparatus the combined system evolves (unitarily) into state approximately described by a mixture $\rho=\sum_ip_i|\psi_i\rangle\langle\psi_i|$. We can already here talk about a mathematical branching.
• Of the different parts of the mixture I, for whatever reason, perceive only one result, let's say $|\psi_j\rangle$.
• Mathematically, we may keep all terms in the mixture but keep track of the one corresponding to the perceived result $\rho=\sum_{i\neq j}p_i|\psi_i \rangle \langle\psi_i|+p_j|\psi_j \rangle \langle \psi_j|$
• Now we can apply unitary evolution to the whole thing, which by linearity will only affect each term of the mixture individually.
• Upon a new measurement we again experience a mathematical branching and we again perceive only one result $|\psi_{j'}\rangle$. The whole thing can be iterated over and over and you may imagine a tree of mathematical branchings with only one sequence of branchings, $j,j',j'',j''',\ldots$ corresponding to the perceived results.
• As far as I know there is no way to "derive" the born rule, and applying this tells us that the probability of ending up in the branch I currently perceive is given by $p_jp_{j'}p_{j''}p_{j'''}\ldots$

So far there is no interpretation at all just application of pure QM. Neither MWI nor CI can explain why we only perceive one particular result. There has also been no mention of collapse, or non-unitary evolution.
The only difference (as far as I can tell) between the two interpretations is a (semantic/philosophical) question about which object to assign the notion of reality. In MWI all branches are considered "real" and for every possible measurement result/branching there is an observer which perceives just that. In CI only the branch which I recall to have perceived is considered "real" and in fact all other branches can just as well be discarded since they don't describe reality. In fact, in the spirit of CI we can just take the wave function corresponding to the result of one measurement and use it as a starting point of further evolution, i.e. projecting our wave function onto this state (I suppose this step is what people refer to as "collapse").

The argument for the CI can then be viewed as a removal of baggage: Why should we consider (and keep track of) all the other branches, that we don't perceive and which cannot influence future perceptions, as "real"?

 

[Fredrik]

Neither MWI nor CI can explain why we only perceive one particular result.

Actually, I think that MWI may be able to explain that. I have some ideas about how, but no actual proof. The ensemble interpretation certainly doesn't explain it.

The only difference (as far as I can tell) between the two interpretations is a (semantic/philosophical) question about which object to assign the notion of reality.

I don't think it's quite that simple. I think the "assignment of reality" to the whole state operator can be considered a definition of a MWI, but I don't think it really makes sense to "assign reality" to a single term. If we do, we have to imagine an unknown process called "collapse" that eliminates the other terms. (Other people may have less of a problem with this than I do). So I consider the main option to the MWI to be the idea that nothing in the theory is "assigned reality". This is to assume that QM doesn't describe reality, that it just tells us how to associate probabilities with verifiable statements.

In fact, in the spirit of CI we can just take the wave function corresponding to the result of one measurement and use it as a starting point of further evolution, i.e. projecting our wave function onto this state (I suppose this step is what people refer to as "collapse").

This is what "collapse" refers to in the ensemble interpretation, but it can also refer to a hypothetical physical process that violates the rules of QM during measurements.

Why should we consider (and keep track of) all the other branches, that we don't perceive and which cannot influence future perceptions, as "real"?

Because it would be nicer to have a theory that in addition to telling us how to associate probabilities with verifiable statements, also describes reality. Since QM assigns probabilities so well, it would be weird to not even consider the possibility that whatever QM describes is reality. Without a physical process that eliminates the other terms in the post-measurement state operator, or a radical change in the laws of logic, I don't see how QM can be said to describe a single world.