Problems with Many Worlds Interpretation

[Ken G]

Then I just don't share your religion :)
Many people study the interior solutions of the Black Holes - and you just proclaim it waste of time...
Sorry.

You are not looking very closely at what I said. Empiricism does not demand there is no point in looking across EHs, it asserts that we note there is something important about the fact that we have no empirical knowledge of what is in there. As always, we cannot look under every rock, so all physicists have to make their best guess about what constitutes an observation that has basically already been done, and one that has not already been done. We don't have much reason to think observations on one side of an EH will be different from the other side, though we have every reason to believe observations near the singularity might be something we haven't seen. No religion involved, the key is simply keeping track of what we are actually doing, and the difference between what we know and what we don't know-- with perhaps a tiny bit of attention to the history of science.

 

[Ken G]

We can see only definite outcomes.
It is a result of Quantum Decoherence - purely mathematical, liek 2+2, not even physical.

You are talking about decoherence, that has nothing to do with the difference between MWI and CI. I find myself having to repeat this often, it seems. Both MWI and CI need decoherence, it explains why we need an interpretation of collapse in the first place. But it does not give us any interpretation of collapse. The difference between MWI and CI is how you treat the difference between flipping a coin and not looking (decoherence, pure QM, no interpretation needed), and flipping a coin and looking (now you need an interpretation to explain your experience of seeing the result). To put it succinctly-- MWI and CI differ not about the presence of mixed states, but about the meaning of mixed states. MWI says all outcomes in the mixed state actually occur, even in an individual outcome, and CI says all outcomes only occur for an ensemble, each individual trial yields only one. They have the same problem: they must account for why our perceptions don't match the mathematics of QM. The difference between MWI and CI is fairly simple-- MWI takes the mathematics as the reality and tries to modify how we think about perception, and CI takes the perception as the reality and tries to modify how we think about the mathematics. Either is valid, but perceptions don't change and mathematical treatments do.

 

[Hurkyl]

Let me ask you this: do you actually believe that when you, personally, flip a coin, and look at it, the outcome you perceive is a mixed state of heads and tails? And do you call that a definite or an indefinite outcome?

No -- my perception itself is in a mixed state of perceiving heads and perceiving tails. This is indefinite outcomes, because reality did not pick one of heads or tails, nor did it pick one of "I saw heads" or "I saw tails".

Do I "believe" it? In so much as I "believe" anything science. It's suggested both by statistical mechanics and by unitary evolution.

If it were just statistical mechanics, I might apply Occam's razor to justify not thinking about it since the mixtures don't play a part in time evolution -- not even to unobservable parts. (but then again, I might not -- the typical interpretation of random variables is IMO quite lacking when compared to the mathematical theory)

Nowhere near as radical as "when I flip a coin I perceive both outcomes I just don't know that I do", which is more or less what you seem to be saying.

But then, rejecting absolute time was also once a rather radical notion. There are people today who still refuse to make that paradigm shift.

Science doesn't tell you, language does. We experience outcomes. We define that word "definite" to go with that.

Then what I've been calling "indefinite outcomes" are actually definite by this definition. The question is (at least in the classical case) if we consider whether a physical proposition holds in "reality", does the "truth" of the proposition take values in the two-valued Boolean logic {true, false}, or does it take values in some other Boolean algebra?

Way off. The whole point of "proper time" is that it is just precisely time that is locally experienced. Absolute time was always the rationalistic notion, proper time was always the empirical version. That's what I mean that rationalistic theories like Galilean relativity disappear overnight, but observations, like proper time, do not.

Imagine how you would feal saying that to Chrono, and having him deny you at every turn, repeatedly stating that our experience is with an absolute notion of time, no matter how SR wants to deal with the topic.

That's how I've felt throughout this discussion.

 

[Hurkyl]

Here I would agree the bird's view assembles frog's views, including the views we consider hypothetically (like "if a frog were there, what would it see"). But note this is a very specific type of information, so even if we assemble it all, it's not all the information (unless we define this to be information, that's a subtle issue I won't take a position on). The key point is that the god's eye view is something quite different, because it does not ask "what would a frog see", it just asks "what is."

So, what would you call a view that has all of the information, but does care about what frogs can see? Because that is how I've been using the phrase "bird's eye view".

And I'm pretty sure labeling MWI as an interpretation that doesn't care about what frogs can see to be completely misguided -- except for the most basic stuff, nearly everything I've seen on the topic is ultimately concerned with the issue of trying to make sense of the question "what does a frog see?" in a unitary description of the universe, and also to check if the answer has the potential to match our experiences with reality.

 

[Ken G]

No -- my perception itself is in a mixed state of perceiving heads and perceiving tails.

So "your perception" is something different from "what you perceive," where by the latter statement, I just mean what you perceive. You are in effect saying that your perceptions are informed by your knowledge of quantum mechanics, but that is not the normal meaning to the term "your perception." So this is the basis of my claim that your stance is highly un-empiricist, perhaps even radically un-empiricist. You are reinterpreting your own perceptions using a rationalist filter-- that is more or less the definition of rationalism, of the most radical form I can imagine. Not that there's anything logically inconsistent with such a stance-- it was always my goal only to make you see you are not "just being a good scientist" here, you are quite purposefully adopting a highly rationalistic perspective on reality, just so that your highly rationalistic perspective could be interpreted as a valid description of reality. Fully self consistent, but you should recognize that this is simply not the standard meaning of "perception", and certainly not what any empiricist means by the word.

This is indefinite outcomes, because reality did not pick one of heads or tails, nor did it pick one of "I saw heads" or "I saw tails".

I didn't ask you what reality picked, I asked you what you perceive when you look at a coin. See the difference? It has to do with the definition of the word perceive, which you seem to feel you can alter at your own whim.

 

[Hurkyl]

So "your perception" is something different from "what you perceive," where by the latter statement, I just mean what you perceive.

How are they different? You gave a complete^* description of the coin -- a mixture of heads and tails. I replied with a complete description of my perception -- a mixture of seeing heads and seeing tails. We could go further and completely describe them jointly -- a mixture of 'heads and me seeing heads' and 'tails and me seeing tails'.

*: with respect to anything relevant to the discussion. (e.g. you didn't say whether it was a nickel or a penny)


From the setup you gave, there are no absolutes about particular outcomes -- neither "I saw heads" nor "I saw tails" hold. However, the absolute "I saw heads or I saw tails" does hold. More importantly, the relative statements "If the coin is heads, I saw heads" and "If the coin is tails, I saw tails" both hold^*, as do the more trivial claims "If I saw heads, I saw heads", "If I saw tails, I saw tails", and "If I saw heads, I did not see tails".

*: if my perceptions are valid, of course.

You are in effect saying that your perceptions are informed by your knowledge of quantum mechanics, but that is not the normal meaning to the term "your perception."

Is a perception a "raw experience" or is it one that has been processed by instinct, intuition, and reason?

I don't have a "raw experience" of seeing heads. Nobody does. The perception of heads only appears after things like passing through the image processing part of our brain, being pattern matched against familiar notions, being categorized into abstract notions like "coin" and "heads on a coin", and so forth.

My understanding of these abstract notions, like "here", "now", and "outcome" are informed by relativity and MWI.

And the details of all of the above are influenced by my a huge variety of factors -- education being one of them.

I didn't ask you what reality picked, I asked you what you perceive when you look at a coin. See the difference?

I see heads or I see tails.

If nobody talked about "what 'reality' picked", the question is completely vacuous. Alas, the split between CI and MWI is precisely about something equivalent^* to the question of "what 'reality' picked".

*: for purposes of this discussion

 

[Ken G]

You gave a complete^* description of the coin -- a mixture of heads and tails. I replied with a complete description of my perception -- a mixture of seeing heads and seeing tails.

It's clear that you do not distinguish the way you think about your perceptions from your perceptions themselves. You claim your perception of seeing a coin toss has changed since learning quantum mechanics, but I'm skeptical you really perceive anything different at all-- rather than just think differently about the experience. It's a pretty radical rationalist that can see no distinction between having an experience, and having a process of analysis about how to make sense of the experience.

Is a perception a "raw experience" or is it one that has been processed by instinct, intuition, and reason?

This returns us to the fact that mindless rocks do not perceive anything. There is no doubt that perception requires some kind of intelligent processing, but only the die-hard rationalist thinks that perceptions of coin flips change after you learn quantum mechanics. If I ask you what ice cream you prefer, you can certainly imagine that you are saying "<chocolate, vanilla>", but that just isn't going to be the sound that your ears hear come out of your mouth. Frankly it's fascinating to hear from such a completely radical rationalist, the complete invalidation of the most basic sensory experiences, based on the postulates of a theory that is only known to be highly accurate in certain situations, is quite a new discovery for me. I am quite serious that I see nothing logically wrong in your approach, but I also see it as nothing less than devoutness.

If nobody talked about "what 'reality' picked", the question is completely vacuous. Alas, the split between CI and MWI is precisely about something equivalent^* to the question of "what 'reality' picked".

Well, it normally starts with the basic recognition of what a perception feels like, and then moves on to what reality is. I'm afraid we got stuck at what a perception feels like.

 

[Ken G]

So, what would you call a view that has all of the information, but does care about what frogs can see? Because that is how I've been using the phrase "bird's eye view".

If it cares about what frogs can see, then it is not a wave function, it is comprised only of eigenvalues. It could be a mixture of eigenvalues, but not a superposition state. That's the god's eye view. The problem with what you're saying here is that to have what you call a bird's eye view, you must completely buy off on the god's eye view. So there really is no bird's eye view the way you describe it. If there is no god's eye view, then there is simply no justification to imagine that reality itself involves mixed states other than in the ensemble sense. The bird's eye view I talked about involves the union of frogs that could communicate in principle. Any other non-causally connected frogs cannot be seen by birds, but only by gods.

And I'm pretty sure labeling MWI as an interpretation that doesn't care about what frogs can see to be completely misguided -- except for the most basic stuff, nearly everything I've seen on the topic is ultimately concerned with the issue of trying to make sense of the question "what does a frog see?" in a unitary description of the universe, and also to check if the answer has the potential to match our experiences with reality.

Frogs see eigenvalues. That's what we experience. You have to enter into pretense to imagine something different, there's a ghost in that machine.

 

[Dmitry67]

You are not looking very closely at what I said. Empiricism does not demand there is no point in looking across EHs, it asserts that we note there is something important about the fact that we have no empirical knowledge of what is in there. As always, we cannot look under every rock, so all physicists have to make their best guess about what constitutes an observation that has basically already been done, and one that has not already been done. We don't have much reason to think observations on one side of an EH will be different from the other side, though we have every reason to believe observations near the singularity might be something we haven't seen. No religion involved, the key is simply keeping track of what we are actually doing, and the difference between what we know and what we don't know-- with perhaps a tiny bit of attention to the history of science.

Then General Relativity creates a problem for your Empiricism, because you have to define the 'shape' of your knowledge in the spacetime.

For example, ANYONE can jump into the BH to get an experience and to learn what is inside directly from the experiment. However, that knowledge won't be able to escape from the BH, so the poor experimenter won't be able to share it with the rest of the humankind (and as I remember in your definition it was important). So we can't learn what is inside the BH, right? Well, that conclusion is too atrophic. We prefer some area of spacetime just because it is more habitable.

So let me ask you, what is a “knowledge” – is it eternal mathematical truth, which can’t be located in spacetime (like the fact that the number 17 is prime) or is it an information (and in such case it can be located in spacetime)

 

[Dmitry67]

You are talking about decoherence, that has nothing to do with the difference between MWI and CI. I find myself having to repeat this often, it seems. Both MWI and CI need decoherence, it explains why we need an interpretation of collapse in the first place. But it does not give us any interpretation of collapse. The difference between MWI and CI is how you treat the difference between flipping a coin and not looking (decoherence, pure QM, no interpretation needed), and flipping a coin and looking (now you need an interpretation to explain your experience of seeing the result). To put it succinctly-- MWI and CI differ not about the presence of mixed states, but about the meaning of mixed states. MWI says all outcomes in the mixed state actually occur, even in an individual outcome, and CI says all outcomes only occur for an ensemble, each individual trial yields only one. They have the same problem: they must account for why our perceptions don't match the mathematics of QM. The difference between MWI and CI is fairly simple-- MWI takes the mathematics as the reality and tries to modify how we think about perception, and CI takes the perception as the reality and tries to modify how we think about the mathematics. Either is valid, but perceptions don't change and mathematical treatments do.

At first, what version of CI are you talking about? Bohr or Neumann flavor, or something else? CI is very old, was developed when Decoherence was not discovered yet, so the whole point of CI was to invent an artificial 'collapse' to explain why do we see definite outcomes. As an artificial creation, CI is not self-consistent and complete, it fails to give any predictions until you call (or not) any group of atoms a 'measurement device' or not.

 

[Fra]

invent an artificial 'collapse' to explain why do we see definite outcomes.

The collapse does not "explain" anything, it is just an honest acknowledgment of what happens when you ask a question: Given the experimenters of observers prior conception, she formulates and fire a question into your environment (make a measurement), and typically you will receive a backreation from your environment (or subsystem of it) and this is the answer. Typically this revises your prior conceptions. That's the whole point of making a measurement, you want to get new information, or "updated information", so your expectations are updated.

In what way can you possibly call this artificial? It's rather a very honest description of the situation.

To try to deny the concept of information updates seems to miss the original question, which is that the observer needs to make a decision and act, in order to maintain it's integrity and survive. This can be undertsood both in terms of a human scientists but also in terms of material observers, that can be destabilized by the environment unless it's in agreement with it.

I'd say that the concept of god's view and similar stuff, now that's artificial

/Fredrik

 

[kith]

I'm not very familiar with MWI, but is it correct, that it postulates the density operator ϱ to be the fundamental state which describes reality?

This suggests that MWI is nothing quantum mechanical, but also present in classical mechanics (points in phase space vs. probability distributions). As far as I can see, this removes all information-theoretic content from statistical mechanics, which seems very odd to me.

Any thoughts on this?

 

[Hurkyl]

If it cares about what frogs can see, then it is not a wave function, it is comprised only of eigenvalues.

This is just silly. It's like telling me that anyone who reads a real analysis textbook doesn't care about computing derivatives, that anyone reading The Lord of the Rings can't be interested in the geography of Middle Earth, or that anyone who makes use of a coordinate chart in classical mechanics cannot possibly care about what people see.

Caring about the frogs see means caring what the frogs see, not bending over backwards to try and encode what frogs see into eigenvalues, and not crippling ourselves by trying to avoid using anything else in our theories. Using a coordinate chart when working with classical mechanics isn't an act of radical rationalism, is it?

It could be a mixture of eigenvalues, but not a superposition state.

This is where the importance of relative state comes into play. Even if a whole system is in a pure^* state, its subsystems can be in mixed states -- in fact, they can transition from pure to mixed and back again. Overlooking that fact is the fatal flaw in the old argument that unitary evolution by itself is incapable of matching our experiences.

This observation was the origin of the MWI.

*: The term "superposition state" has no inherent meaning -- the notion of superposition only makes sense when viewing states as kets, and even then only after having chosen a basis.

Frankly it's fascinating to hear from such a completely radical rationalist, the complete invalidation of the most basic sensory experiences, based on the postulates of a theory that is only known to be highly accurate in certain situations, is quite a new discovery for me.

I find your view of my arguments completely baffling. As of yet, I have failed to discern any rhyme or reason to them, except by considering the hypothesis that you are either attacking a straw-man or have fallen victim to what you say I am doing -- that you have equated the very idea of experience with a particular philosophy and can't entertain the thought that they aren't literally as they are described classically.

 

[Ken G]

For example, ANYONE can jump into the BH to get an experience and to learn what is inside directly from the experiment. However, that knowledge won't be able to escape from the BH, so the poor experimenter won't be able to share it with the rest of the humankind (and as I remember in your definition it was important). So we can't learn what is inside the BH, right?

Observers can be outside each others' light cones without event horizons. They are just bearing witness to real events that cannot be shared, so each will form an incomplete picture of the whole. That is how empiricism works, we all get an incomplete picture, but we get the perceptions we get, and we build a reality from it. Some of us will never build a reality that has black hole singularities in it, that's empricism-- there's no problem there, it's just a pill we must swallow. Empiricism starts with the expectation that the reason we invoked physics in the first place was to predict the things that affect us, and use that to inform decisions about things that we can affect. The rest is angels on the pin.

So let me ask you, what is a “knowledge” – is it eternal mathematical truth, which can’t be located in spacetime (like the fact that the number 17 is prime) or is it an information (and in such case it can be located in spacetime)

The question here is what is the knowledge that physics gives us, not mathematics (mathematics knowledge is always purely tautological to the axioms, so it is an exercise in "knowing thy axioms." Physics for the empiricist is knowing , and gaining power over, thy experiences.). What is the purpose of quantum mechanics? I gave the answer that I think would be pretty standard for empiricism, what would you say is the purpose of quantum mechanics?

 

[Ken G]

In what way can you possibly call this artificial? It's rather a very honest description of the situation. To try to deny the concept of information updates seems to miss the original question, which is that the observer needs to make a decision and act, in order to maintain it's integrity and survive. This can be undertsood both in terms of a human scientists but also in terms of material observers, that can be destabilized by the environment unless it's in agreement with it.

I'd say that the concept of god's view and similar stuff, now that's artificial

I agree completely, well put.

 

[Ken G]

This is just silly. It's like telling me that anyone who reads a real analysis textbook doesn't care about computing derivatives, that anyone reading The Lord of the Rings can't be interested in the geography of Middle Earth, or that anyone who makes use of a coordinate chart in classical mechanics cannot possibly care about what people see.

Then you miss my point, which is this: frogs see eigenvalues. The formalism of quantum mechanics accounts for this via the Born rule, and no other way. The Born rule is completely ad hoc, it's just a "rule for using wave functions." That is completely true in either CI or MWI, we haven't even gotten to an interpretation yet. We don't get to the intepretations until we ask what does the Born rule mean, once we have recognized that our theory has this completely ad hoc character to it. (It shouldn't bother us, all theories are ad hoc, they just have different degrees of it.) CI says "the Born rule means this is what we will perceive, statistically for individual outcomes or more deterministically for an entire ensemble", MWI says "the Born rule means how the splitting outcomes get weighted, so is deterministic in either the individual case or the entire ensemble, it just connects better to our experience in the ensemble case." That is not any less ad hoc, the sole difference there is how strongly we value our perception in individual cases.

I actually see very little scientific difference between CI and MWI, though a huge philosophical difference, and what's more, if and when quantum mechanics is found to be just another effective theory like all the rest were, we should ask: will CI need to change its stance, that the Born rule predicts statistical outcomes? No it won't, CI won't care at all if a new theory comes along, for the same reason that we continue to do Newtonian mechanics. Can MWI say the same, when it is founded on an ontology that completely falls apart if nonunitary evolution of a wave function can occur?

Using a coordinate chart when working with classical mechanics isn't an act of radical rationalism, is it?

A coordinate chart is nothing but a set of observers and their perceptions about rulers and clocks, with some arbitrarily chosen rule to define that set. Thus it is already an inherently empiricist concept, with no rationalism involved at all. However, the rule that connects the observers is causal-- they can actually compare notes, they are interested in things they can affect and that can affect them. Other branches of decohered mixed states, interpreted ontologically, simply don't fall under that category.

This is where the importance of relative state comes into play. Even if a whole system is in a pure^* state, its subsystems can be in mixed states -- in fact, they can transition from pure to mixed and back again. Overlooking that fact is the fatal flaw in the old argument that unitary evolution by itself is incapable of matching our experiences.

That's just pure quantum mechanics, it has positively zero to do with any of the interpretations. Give me an example of what you are calling a mixed state of a subsystem that evolved back into a pure state, and I will explain how CI handles that situation using an empiricist-constructed ontology.

*: The term "superposition state" has no inherent meaning -- the notion of superposition only makes sense when viewing states as kets, and even then only after having chosen a basis.

I'd say that can be said much clearer in empiricist language. A "superposition state" has a perfectly fine meaning, it is simply a pure state that is not an eigenstate of some observation that we have in mind to make that designation. It is a pure state in a context that is relevant to this discussion, so is an appropriate term here.

I find your view of my arguments completely baffling. As of yet, I have failed to discern any rhyme or reason to them, except by considering the hypothesis that you are either attacking a straw-man or have fallen victim to what you say I am doing -- that you have equated the very idea of experience with a particular philosophy and can't entertain the thought that they aren't literally as they are described classically.

Once again you seem to feel the need to "describe" your experiences. Don't you realize that most people just experience things? I realize that we always pass our experiences through some kind of mental processing, but all the same, most people really are pretty comfortable with the idea that putting your hand on a stove is a painful experience, and don't have to conceptualize that experience as some kind of mixed state of hand on / hand off the stove. If quantum mechanics gets replaced by nonunitary evolution that appears unitary in certain conditions, you claim that what you feel when you put your hand on a stove is going to be radically different? Your rationalism runs so deep that you don't seem to even be able to conceptualize empiricism. I don't criticize that, actually I think it's fascinating, I'm just trying to get you to see how different that is from the general definition of the term "experience."

 

[Dmitry67]

Observers can be outside each others' light cones without event horizons. They are just bearing witness to real events that cannot be shared, so each will form an incomplete picture of the whole. That is how empiricism works, we all get an incomplete picture, but we get the perceptions we get, and we build a reality from it. Some of us will never build a reality that has black hole singularities in it, that's empricism-- there's no problem there, it's just a pill we must swallow. Empiricism starts with the expectation that the reason we invoked physics in the first place was to predict the things that affect us, and use that to inform decisions about things that we can affect. The rest is angels on the pin.

The question here is what is the knowledge that physics gives us, not mathematics (mathematics knowledge is always purely tautological to the axioms, so it is an exercise in "knowing thy axioms." Physics for the empiricist is knowing , and gaining power over, thy experiences.). What is the purpose of quantum mechanics? I gave the answer that I think would be pretty standard for empiricism, what would you say is the purpose of quantum mechanics?

I asked 2 yes/no questions and I did not expect to get a ... poetry instead of Y/N reply.
It does not answer my questions at all.

 

[Dmitry67]

The Born rule is completely ad hoc, it's just a "rule for using wave functions."

No, it is derived by Max Tegmark from QM in 2010 (even not all people agree)

 

[Ken G]

It does not answer my questions at all.

One question you asked was "what version of CI was I using." I'm not using any particular version of CI, rather I've been quite clear about what the salient features are about what I'm calling CI-- the interpretation that the reality we need to explain is the reality of our measurements and nothing else. Thus if we do a measurement, and get an eigenvalue, we do not contort our view of reality to accommodate other eigenvalues that we did not get, but instead, we regard QM as a system for understanding why we got that eigenvalue. When we do that, we find that QM is incomplete-- it cannot say why we got that eigenvalue, but it can say how often we'll get that eigenvalue if we repeat the same experiment. That's CI, pure and simple-- it is a choice about what QM is for. People have all kinds of crazy ideas about CI attributing magical properties to wave function collapse, but what they don't realize is that CI never saw the wave function collapse as anything but a step in a mathematical process, designed to reach statistical predictions around the observed fact that what we see are eigenvalues. It's not a bizarre ontology, it's simply the rejection of unnecessary ontologies in the first place. This seems to me to be the central spirit of virtually everything that Bohr said about interpreting QM.

 

[Dmitry67]

I don't understand how you can't care about the version of CI, for example, Neumann vs Bohr. Even they appear to be similar, they are absolutely not compatible, they are opposite, for example, in one wavefunction is "real" while in the others it is NOT REAL! How you can talk about the reality ignoring these key facts?

 

[Eqblaauw]

I just received a mail from Gerard 't Hooft, he says he thinks MWI is absolutely false: There is one World. QT gives us many-worlds, only because it's not capable of choosing one right world. Tough there is one world.
He also says he would have expected another outcome of the David Raub poll, because most of the physicists he knows are much more skeptical (tough he doesn't know what they would do when they get a poll).

 

[Eqblaauw]

I do know that of the few prominent dutch scientists I have contacted (Ewine van Dishoeck, Vincent Icke, Leo Kouwenhoven and Gerard t Hooft), none of them subscribe to the MWI. (And I didn't knew that in advence).
I also know that David Deutsch explicitly mailed to me he doesn't believe in the splitting of many-worlds, but in the differentiation of many worlds.

 

[Ken G]

I don't understand how you can't care about the version of CI, for example, Neumann vs Bohr. Even they appear to be similar, they are absolutely not compatible, they are opposite, for example, in one wavefunction is "real" while in the others it is NOT REAL! How you can talk about the reality ignoring these key facts?

Well, any interpretation that says the wave function is real is simply something I would not call CI. To me, the defining character of CI is empiricism.

 

[Ken G]

I'm not very familiar with MWI, but is it correct, that it postulates the density operator ϱ to be the fundamental state which describes reality?

Yes, I would say that the density operator is the fundamental way that quantum mechanics describes a state, and MWI takes that and translates it into the fundamental truth about the reality. In short, MWI takes the quantum mechanical state as being literally real, so in essence paints a reality that is beholden to the formal axiomatic structure of quantum mechanics, rather than the other way around. That's why MWI is so rationalistic.

This suggests that MWI is nothing quantum mechanical, but also present in classical mechanics (points in phase space vs. probability distributions). As far as I can see, this removes all information-theoretic content from statistical mechanics, which seems very odd to me.

Yes, since quantum mechanical principles continue to apply in classical physics, if one goes the MWI road, then purely classical physics is also subject to the MWI interpretation. In fact, any classical physicist could have made that same claim, without any inconsistency in their position. They just wouldn't have been taken seriously without the axiomatic structure of QM to support their viewpoint. There probably is some classical philosopher somewhere who maintained that it is impossible for a coin to come up either heads or tails, and so it must come up both-- philosophers are pretty clever that way.

I don't think this removes all information theoretic content, I think it simply forces one to reinterpret them, often by invoking the anthropic principle. In effect, instead of imagining the statistical distribution is over potential actualizations of some system that is doing only one of them, it is envisaged as being over the potential perceptions by a statistical distribution of observers who are experiencing a system that is doing all those things at once. What MWI proponents seem to ignore is that the Born rule is invoked, ad hoc, to characterize that distribution, no matter whether you interpret it as a distribution over potential outcomes, or a distribution over potential perceptions of a single unified outcome. Tomato, tomahto.

To me, the main difference is flexibility if the axiomatic structure is found wanting. In CI, the axiomatic structure was always taken as a handy device, so CI has no trouble with any cracks in it. MWI is essentially a radical world view that hangs entirely on the infallibility of the axiomatic structure, and since when has physics ever devised an infallible axiomatic structure?

 

[Hurkyl]

Then you miss my point, which is this: frogs see eigenvalues. The formalism of quantum mechanics accounts for this via the Born rule, and no other way. The Born rule is completely ad hoc, it's just a "rule for using wave functions."

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.

Hypothetically^* speaking, would your opinion change (or, at least, weaken) if one could theoretically derive that the weights on the components of a mixed stateBut 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?*: I don't think it's hypothetical at all.

(It shouldn't bother us, all theories are ad hoc, they just have different degrees of it.)

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.

CI says "the Born rule means this is what we will perceive, statistically for individual outcomes or more deterministically for an entire ensemble"

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.

(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)

, MWI says "the Born rule means how the splitting outcomes get weighted, so is deterministic in either the individual case or the entire ensemble, it just connects better to our experience in the ensemble case."

On the statistics, MWI says that one should think of the mixed state as corresponding to the physical system.On the origin of the weights, they come from decoherence, which MWI posits is the effect unitary evolution, rather than being spontaneous.

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

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

I actually see very little scientific difference between CI and MWI

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

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.

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.

Can MWI say the same, when it is founded on an ontology that completely falls apart if nonunitary evolution of a wave function can occur?

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

A coordinate chart is nothing but a set of observers and their perceptions about rulers and clocks, with some arbitrarily chosen rule to define that set.

Empiricism now is about the experiences of imaginary people?

Once again you seem to feel the need to "describe" your experiences.

Er, yes. So do you, it seems:

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

 

[Hurkyl]

As an aside, I've been reminded of a fact about General Relativity (assuming I recall correctly).

Proper time is not an observable in GR (for the same reasons that relative coordinate time is not observable in SR and coordinate velocity is not observable in Galilean Relativity). This is part of a larger measurement problem GR once experienced.

People eventually discovered things that could be observed in GR, which includes things like the classic 'light clock' of Einstein's original batch of thought experiments.

I'm curious to see how Ken G would react to such information.

 

[PAllen]

As an aside, I've been reminded of a fact about General Relativity (assuming I recall correctly).

Proper time is not an observable in GR (for the same reasons that relative coordinate time is not observable in SR and coordinate velocity is not observable in Galilean Relativity). This is part of a larger measurement problem GR once experienced.

People eventually discovered things that could be observed in GR, which includes things like the classic 'light clock' of Einstein's original batch of thought experiments.

I'm curious to see how Ken G would react to such information.

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?

 

[Ken G]

Proper time is not an observable in GR (for the same reasons that relative coordinate time is not observable in SR and coordinate velocity is not observable in Galilean Relativity). This is part of a larger measurement problem GR once experienced.

I would find that quite a problem yes, but like PAllen, I would like clarification. It sounds to me like the basic problem that "rigidity" is impossible in GR, but that always seems like a standard problem in physics, that the concept of measurement is always idealized. In effect, physics is structured to be an approximate science. So I would have thought that GR could treat proper time as an observer, simply by taking an idealized limit that instruments like clocks and rulers were small enough and rigid enough to avoid having to worry about tidal effects.

 

[Fredrik]

I'm not very familiar with MWI, but is it correct, that it postulates the density operator ϱ to be the fundamental state which describes reality?

Yes, I would say that the density operator is the fundamental way that quantum mechanics describes a state, and MWI takes that and translates it into the fundamental truth about the reality. In short, MWI takes the quantum mechanical state as being literally real, so in essence paints a reality that is beholden to the formal axiomatic structure of quantum mechanics, rather than the other way around. That's why MWI is so rationalistic.

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).

Proper time is not an observable in GR (for the same reasons that relative coordinate time is not observable in SR and coordinate velocity is not observable in Galilean Relativity). This is part of a larger measurement problem GR once experienced.

Can you elaborate? If this is accurate, I would like to know more about it. I like to think of the statement "a clock measures the proper time of the world line that represents its motion" as a part of the definition of GR.

 

[Fra]

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

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.

Note that it is no solution to consider bigger and bigger observers (so the big observer can describe the original observer) quantum mechanically. 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 non-classicaliy is just one problem here, the other problem is that a general observer typically must have limited information capacity to encode the theory.

Some as I understand MWI thinking, you consider the inside observer as described as a quantum system, but described from the bird's view. Which makes one of the problems worse: namely that you need even more information capacity - you espace this issue but somehow considering this information in some mathematial realm. (this is something I can't accept)

I see this related to the problem of constructing local observables as well.

Te point is however that FAPP; the classical context of a laboratory looking into say an atom, DOES provide the classical context the original theory needs. So the "classical observer" is justified as a limit ca, so the history of QM is understandable. 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. The MAIN "problem" way back was the failure of classical probability to explain experiment... hte other subtle issue we know of today was not known then.

So to me this is a deeper issue, it has little to do with interpretations but I agree with your issue.

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 wonder what definition of real we use here?

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.

But it would I think be a deep misake to think that this answers the original question which is to find the inside picture, which genereally always contains collapses.

/Fredrik