Problems with Many Worlds Interpretation

[Dmitry67]

Born rule was always the weak point of MWI, but in 2010 Born Rule was derived from QM by Max Tegmark in the assumption of an Infinite Universe.
http://arxiv.org/abs/1008.1066

 

[Eqblaauw]

Tegmark is known for saying 'Many worlds, or Many words', I think you're equally right when you say 'Many Gods or Many Words'. When you don't have a problem with many-worlds it is psychologial I would say.
And about the whole splitting, this is part of the reaction to an email that I send to David Deutsch:
Dear Edo Blaauw -- > Dear David Deutsch,> > I have a few questions about the many-worlds interpretation: > Are there according to most of the many-world believers many universes exactly the same until they differentiate, Yes. > or do most of the 'mwi-ers' believe we are continuously being split? That is an older view, of which I currently know of no supporters among people working on the theory.

 

[Fredrik]

The unequal probabilities are given by the Born rule. As I just said: So now a good question is "what is the probability of experiencing having either red or blue hair after the experiment?" and the answer is given by the Born rule.

The Born rule is used in both MWI and CI, I don't understand your objection to it.

Just to make clear, I'm using the definition of MWI as simply 'no non-unitary collapse happens', which is the most widely-held definition of MWI, as far as I'm aware.

He's objecting because it's very common to define the MWI as QM without the Born rule, applied to the whole universe. Of course, if you remove the Born rule, what you have left is no longer a theory, since it can't be used to make predictions. So someone who defines the MWI that way will have to find a way to derive the Born rule if they want to be taken seriously. There are lots of attempts to do that, but I haven't seen one yet that I find convincing. (Gleason's theorem is certainly convincing, but what it tells us isn't quite what the people who define the MWI this way are required to show. See my comments in post #159). I think more and more people are realizing that this approach just doesn't work.

On the other hand, I don't think there are any strong arguments against the idea that QM with the Born rule can be interpreted as describing many worlds. I think that if it describes anything at all, it would have to be either many worlds, or a single world where some of the normal rules of logic don't apply. However, there's also the possibility that it doesn't really tell us what's actually happening in every situation, i.e. that it's first and foremost a set of instructions that tells us how to assign probabilities to verifiable statements.

 

[BruceW]

He's objecting because it's very common to define the MWI as QM without the Born rule, applied to the whole universe. Of course, if you remove the Born rule, what you have left is no longer a theory, since it can't be used to make predictions. So someone who defines the MWI that way will have to find a way to derive the Born rule if they want to be taken seriously. There are lots of attempts to do that, but I haven't seen one yet that I find convincing. (Gleason's theorem is certainly convincing, but what it tells us isn't quite what the people who define the MWI this way are required to show. See my comments in post #159). I think more and more people are realizing that this approach just doesn't work.

Interesting, I didn't know that. Do you know why it is common to define MWI without any kind of Born rule? Maybe its because the MWI advocates wanted to get as far away from conventional QM as possible... I would personally have kept in the Born rule as a fundamental postulate of QM. (And just taken out the bit about non-unitary collapse).

 

[BruceW]

On the other hand, I don't think there are any strong arguments against the idea that QM with the Born rule can be interpreted as describing many worlds. I think that if it describes anything at all, it would have to be either many worlds, or a single world where some of the normal rules of logic don't apply. However, there's also the possibility that it doesn't really tell us what's actually happening in every situation, i.e. that it's first and foremost a set of instructions that tells us how to assign probabilities to verifiable statements.

I don't fully get what you're saying here. You were saying before that people who define MWI as QM without the Born rule would have to derive the Born rule. So what difference would there be between 'MWI with a derived Born rule' and 'MWI with a postulated Born rule'? (Apart from the fact that the derived Born rule is preferred, since it would mean there were less postulates in the theory).

 

[Ken G]

So now you ask "which was 'me'?" Well, clearly 'you' were not just one or the other, and 'you' weren't both. The only way to describe 'you' was as a superposition.

I don't believe that MWI asserts that. First let us not make a model of "me", let us simply make a model of "him." "He" is part of a system, but he is not a whole system. Thus "he" is not in a superposition state, he is projected from a pure state, and projections from pure states onto subsystems are mixed states. That much is true in either MWI or CI. The difference only appears when "I" enter the picture. because that is the only time that I have to account for a single outcome-- the place where the single outcome appears is purely in my perception. If there were no perceiving agents, MWI would be a complete description, but a description of what? What is a "description" when there are no perceiving agents interested in having something be described? The universe doesn't need physics, physics is for perceiving agents to try and understand their perceptions.

So now a good question is "what is the probability of experiencing having either red or blue hair after the experiment?" and the answer is given by the Born rule.

That is a question that is answered the same by CI or MWI, if MWI is using the Born rule (if it isn't, it's not even physics, as Fredrik pointed out). The important question is "what determined the outcome that I perceived?" Where is the complete accounting for that in MWI? I already said how CI accounts for that-- what actually happens determines what I perceive, so the perception is explained (and nothing explains what actually happened-- CI makes no pretense of being a complete description of a process it views as fundamentally mysterious.) How can MWI pretend it has removed the mystery there, without creating a model of me (which it does not do, and that's why we get incompletenesses like quantum suicide)?

So in my first example, I said 'you' were a superposition of hair colours. And in the second example, 'you' had just one hair colour. Both are viable definitions of 'you'. So in MWI, there is more than one possible definition of 'you'.

Now you are getting closer to creating a "model of me" that could work in MWI. But then we must ask-- does this model of me really work? It's simply not true that the model could have me be a superposition, as I said above, but I could perhaps be modeled as some kind of mixed state. That doesn't really work though, because it just pushes the question back to "if I'm a mixed state, why do I perceive myself as being in a definite state?" The need for a "model of me" has not been satisfied, that question remains as the fundamental incompleteness of MWI. CI, on the other hand, has no issue with its "model of me" (it asserts that I am a perceiving agent whose perceptions define reality, and the consistencies found in those perceptions are the domain of physics), but its incompleteness appears at the other end-- it cannot account for what happens, beyond a statistical description. So take your pick-- have trouble saying what determines what happens (CI), or have trouble saying why it happened to you. I don't see any great advance in creating the fantastical realm of the many worlds, other than allowing me to marry the postulates of a theory that will most likely be found to be incorrect at some future time.

 

[Ken G]

MWI redefines the very notion of "me". You are not a world line with a single history. You are a tree with all histories.

I don't think that will succeed very well as a model of me. It has two big problems:
1) it does not explain why I do not perceive such a tree, and
2) it does not account for how "I" diverge along different branches of the tree. What makes each branch the same "me", that there is someone in it with my genes? How much does my appearance or personality have to change before the branches are to be associated with different "me"s? And what about all the "yous", am I alone on my tree because the tree is what is actually me? This is the point I'm making, we must not imagine that MWI has a reasonable model of me without subjecting that model to scrutiny. Taking your model of me to the limit, all of reality is me. It's nice monism, but not really a very useful model of me.

 

[BruceW]

Now you are getting closer to creating a "model of me" that could work in MWI. But then we must ask-- does this model of me really work? It's simply not true that the model could have me be a superposition, as I said above, but I could perhaps be modeled as some kind of mixed state. That doesn't really work though, because it just pushes the question back to "if I'm a mixed state, why do I perceive myself as being in a definite state?"

'I' could have been a pure superposition of quantum states at an earlier time. 'I' as I perceive myself right now, am in my bedroom. But at an earlier time, my state could have been a superposition of 'in bedroom' and 'in kitchen'. As long as there is no information on whether I was in the kitchen or bedroom, then I could have been a superposition of both.

So It's true that I never see myself as a superposition, but that doesn't mean I don't have the ability to be in a superposition.

 

[BruceW]

I don't think that will succeed very well as a model of me. It has two big problems:
1) it does not explain why I do not perceive such a tree, and
2) it does not account for how "I" diverge along different branches of the tree. What makes each branch the same "me", that there is someone in it with my genes? How much does my appearance or personality have to change before the branches are to be associated with different "me"s? And what about all the "yous", am I alone on my tree because the tree is what is actually me? This is the point I'm making, we must not imagine that MWI has a reasonable model of me without subjecting that model to scrutiny. Taking your model of me to the limit, all of reality is me. It's nice monism, but not really a very useful model of me.

This is the thing with MWI, it doesn't give a precise definition of 'me, here, now'. Since if a few of my cells changed, then would I notice the difference? No. So If there were a pure quantum superposition of 'me plus a few cells' and 'me minus a few cells', then any linear combination of them could also be argued to be 'me, here, now'. But then if I went and counted all my cells, then the new definition of 'I' would have to be one or the other, not a superposition.

 

[Fredrik]

Interesting, I didn't know that. Do you know why it is common to define MWI without any kind of Born rule?

I can only speculate. My guess is that the two main reasons are a) since the whole MWI debate started with Everett, people like to use definitions that are similar to his, and b) people are used to viewing probabilities as "number of favorable outcomes"/"number of outcomes", so they were thinking that maybe that's exactly what the probabilities in QM are.

I don't fully get what you're saying here. You were saying before that people who define MWI as QM without the Born rule would have to derive the Born rule. So what difference would there be between 'MWI with a derived Born rule' and 'MWI with a postulated Born rule'? (Apart from the fact that the derived Born rule is preferred, since it would mean there were less postulates in the theory).

There wouldn't be any significant difference. I agree that a derived Born rule would be preferred. I just haven't seen any derivations that I consider valid. (Gleason's is valid, but his theorem doesn't explain why there's a need to assign probabilities at all, or why they should be associated with Hilbert subspaces). So I don't believe that there is a MWI without the Born rule.

Adrian Kent examined Everett's and similar approaches to the MWI in his Against many-worlds interpretations. His conclusion was very critical.

Everett’s original attempt at a many-worlds interpretation was an interesting and well motivated idea. Everett wanted simultaneously to simplify the axioms of quantum mechanics and to produce a realist physical theory (one which gives an explicit mathematical description of an external reality). However, it is clear that he did not succeed. Successive advocates of many-worlds interpretations, while usually claiming merely to clarify Everett’s position, have substantially abandoned his goals. No proponent of MWI (as far as we are aware) has actually produced a complete set of axioms to define their physical theory.​

 

[Ken G]

But at an earlier time, my state could have been a superposition of 'in bedroom' and 'in kitchen'. As long as there is no information on whether I was in the kitchen or bedroom, then I could have been a superposition of both.

And what about the "you" that is perceiving themself to be in the kitchen or bedroom-- they are both you? So what defines "you", is everyone and everything "you"? Remember, the pure state that you are projected from also includes entanglements with me, so you and I are not even separable in your model of you.

 

[Dmitry67]

I don't think that will succeed very well as a model of me. It has two big problems:
1) it does not explain why I do not perceive such a tree, and
2) it does not account for how "I" diverge along different branches of the tree. What makes each branch the same "me", that there is someone in it with my genes? How much does my appearance or personality have to change before the branches are to be associated with different "me"s? And what about all the "yous", am I alone on my tree because the tree is what is actually me? This is the point I'm making, we must not imagine that MWI has a reasonable model of me without subjecting that model to scrutiny. Taking your model of me to the limit, all of reality is me. It's nice monism, but not really a very useful model of me.

#1 is very easy.
You remember the past, not the future.
You don't communicate with other 'branches'
But as different branches share the same past, and your memory is always from the past, you have an illusion that world is a single-history.

#2 - I can answer what someone would give an answer, what a consiousness is.

 

[Eqblaauw]

No according to the current theory there are many-worlds each with only one timeline (partly the same until they differentiate)
once again:
David Deutsch:
Dear Edo Blaauw -- > Dear David Deutsch,> > I have a few questions about the many-worlds interpretation: > Are there according to most of the many-world believers many universes exactly the same until they differentiate, Yes. > or do most of the 'mwi-ers' believe we are continuously being split? That is an older view, of which I currently know of no supporters among people working on the theory.
Don't try to make people crazier then they are

 

[Ken G]

#1 is very easy.
You remember the past, not the future.

You said that I was a tree, not a linear branch of a tree. So my memories cannot be me either, in your model of me. My memories tell a story of nonunitary evolution, that's the whole problem. Memory is no kind of fix.

#2 - I can answer what someone would give an answer, what a consiousness is.

I agree-- maybe when someone can say what a consciousness is, MWI will be able to claim to be a complete description and we'll know what to do with quantum suicide. But that's just a pipe dream at the moment-- the incompleteness of the MWI world view persists at present.

 

[RandallBSmith]

I believe most physicists would be surprised to find an experiment that could distinguish Many Worlds vs. Copenhagen. (Trying to come up with such might be a useful way to understand these models. E.g.: Would transitioning into a universe in which someone spontaneously exploded disprove anything?)

If two theories make identical experimental predictions, then choose one based on philosophical taste or cognitive economy.

I see these polls as showing how smart and thoughtful people like to think about quantum mechanics, not about which model is more or less correct. An interpretation can be useful for efficient education or as a way to think about some quantum phenomenon ... stuff like that. Personally, I find one of the big disappointments about life to be that apparently we don't get to know which of two equivalent theories is "true," only that they both work to describe phenomena. Kind of a bummer, but oh well.

 

[Ken G]

Dear David Deutsch,> > I have a few questions about the many-worlds interpretation: > Are there according to most of the many-world believers many universes exactly the same until they differentiate, Yes. > or do most of the 'mwi-ers' believe we are continuously being split? That is an older view, of which I currently know of no supporters among people working on the theory.
Don't try to make people crazier then they are

Considerable elaboration is required to understand what Deutsch is saying. First of all, what is the point of having many universes that are exactly the same? I see no difference between that situation and one universe, it's an incoherent distinction. Second of all, if the many worlds are not being continuously split, how is unitarity being preserved? Maybe "split" is the wrong word, and "subsectioned" would work better for him, but I think most people intend the same meaning in those two words. Finally, just what "theory" is being worked on here? I was not aware that MWI was a theory, is there a suggestion of some kind of new prediction being generated?

 

[Ken G]

Personally, I find one of the big disappointments about life to be that apparently we don't get to know which of two equivalent theories is "true," only that they both work to describe phenomena. Kind of a bummer, but oh well.

Then let me suggest a less disappointing perspective: think of each theory, and its range of interpretations also, as a set of filters for looking at the world, each one revealing some different "color" if you will. Framed like that, the myriad of possibilities is not disappointing, it is enlightening. How boring is a world described completely by one approach, that's just a form of tunnel vision.

 

[RandallBSmith]

Then let me suggest a less disappointing perspective: think of each theory, and its range of interpretations also, as a set of filters for looking at the world, each one revealing some different "color" if you will. Framed like that, the myriad of possibilities is not disappointing, it is enlightening. How boring is a world described completely by one approach, that's just a form of tunnel vision.

Cute ... a nice "glass half full" way to look at our life with mystery.

 

[Dmitry67]

No according to the current theory there are many-worlds each with only one timeline (partly the same until they differentiate)
once again:
David Deutsch:
Dear Edo Blaauw -- > Dear David Deutsch,> > I have a few questions about the many-worlds interpretation: > Are there according to most of the many-world believers many universes exactly the same until they differentiate, Yes. > or do most of the 'mwi-ers' believe we are continuously being split? That is an older view, of which I currently know of no supporters among people working on the theory.
Don't try to make people crazier then they are

Before discussing it, you need to define what 'splitting' is exactly.
You will see that the definition of 'splitting' is fuzzy. The same as when 1 road has a fork, you can't locate a place where it splits up to 1 micrometer.

 

[Dmitry67]

You said that I was a tree, not a linear branch of a tree. So my memories cannot be me either, in your model of me. My memories tell a story of nonunitary evolution, that's the whole problem. Memory is no kind of fix.

Again you mix 'you' as a line, located in a branch, with a full collection of you (tree).
I suggest to prefix all objects with F (frog perspective, or a line) and with B (birds perspective, or a tree).

My-F memory-F tells a story of nonunitary evolution (F)
My-B (full collection of all) memories-B cover all possibilities and evolution-B is unitary
You-F don't have an access to memories-B
You-F feel always as You-F, never as you-B

 

[Hurkyl]

I just don't see how you can claim that MWI has any way to account for the particular outcome of an observation.

The important question is "what determined the outcome that I perceived?" ... what actually happens determines what I perceive,

I don't account for it. The question isn't important. The relevant point is that I consistently perceive a single outcome. One doesn't need to take such drastic action and insist on definiteness.Let's play a game. It's a classical universe, but with four-valued logic: <T,T>, <T,F>, <F,T>, and <F,F>. We are talking to an observer in the universe -- but we have the advantage of being external to the universe so that we can get the bird's eye view and see the four truth values.

To go along with the four-valued logic, everything gets treated the same way -- numbers, particles, observers, et cetera.

Your goal is to make the observer aware (with empirical observation!) that his perceptions are not determined.

In his responses, I will write anything definite normally, and anything indefinite with the angle brackets as I did with the truth values.I'll get started.

Me: Hi there!
Experimenter: Oh hi!
Me: What are you doing?
Experimenter: I'm flipping a coin.
Me: Oh, what was the result?
Experimenter: It came up <heads, tails>.
Me: Oh, so you didn't see a definite result, huh?
Experimenter: What do you mean? My result is clearly definite.
Me: Well, did you see heads?
Experimenter: <yes, no>.
Me: There you go!
Experimenter: What are you talking about? I <definitely, definitely did not> see heads.
Me: Let's try this again. Did you see heads?
Experimenter: <yes, no>.
Me: Did you see tails?
Experimenter: <no, yes>.
Me: So, did you see heads or did you see tails?
Experimenter: Yes.
Me: Har har, you're a comedian.
Experimenter: But in particular, I saw <heads, tails>.
Me: Well, flip it a few more times.
Experimenter: ...
Me: What are the results?
Experimenter: I got <53,51> heads, out of <101,100> flips in all.
Me: Er... 100 flips?
Experimenter: <No 101 flips, Yes>
Me: Er... can you make it 101 flips?
Experimenter: <I already did, ... Okay, 52 heads>​

 

[Eqblaauw]

Considerable elaboration is required to understand what Deutsch is saying. First of all, what is the point of having many universes that are exactly the same? I see no difference between that situation and one universe, it's an incoherent distinction. Second of all, if the many worlds are not being continuously split, how is unitarity being preserved? Maybe "split" is the wrong word, and "subsectioned" would work better for him, but I think most people intend the same meaning in those two words. Finally, just what "theory" is being worked on here? I was not aware that MWI was a theory, is there a suggestion of some kind of new prediction being generated?

what he says is very clear. If and how it works i don't know. David deutsch always said that mwi was actually a theory. I think when deutch says no-one who is an expert in this theory believes the splitting, but in many universes exactly the same until they differentiate that counts for something. There is also a dutch documentary about deutsch where he makes this claim. If you are really interested I can post the url.
To the post that says as a response to my post 'There is no actual hard-splitting' i would say no: there is no splitting

 

[Samshorn]

Considerable elaboration is required to understand what Deutsch is saying. First of all, what is the point of having many universes that are exactly the same? I see no difference between that situation and one universe, it's an incoherent distinction. Second of all, if the many worlds are not being continuously split, how is unitarity being preserved? Maybe "split" is the wrong word, and "subsectioned" would work better for him, but I think most people intend the same meaning in those two words.

I'm not an expert on what Deutsch thinks, but I believe he prefers the word "partitioned". He imagines a fixed (albeit possibly infinite) number of worlds, none being created or destroyed, and at any given time they are partitioned into equivalence classess of identical worlds, but these equivalence classes get re-shuffled, so two worlds that were equivalent before are not any more, and two worlds that used to be distinct can merge and become equivalent. So he imagines a constant re-partitioning of these worlds into these equivalence classes.

Finally, just what "theory" is being worked on here? I was not aware that MWI was a theory, is there a suggestion of some kind of new prediction being generated?

Yes, Deutsch argues (or at least he has in the past, not sure what he says today) that in fact a sufficiently sensitive observer actually WOULD perceive a superposition of states, because the worlds merge back together sometimes. He has suggested that this could be tested by some kind of artificial intelligence machine that is more sensitive to subtle memories than humans are. I guess there is some irony in the fact that a promiment proponent of MWI essentially agrees with all the past critics of MWI who always argued that an observer should see a superposition of states. What then becomes of all the iron-clad arguments of the other MWI proponents who claim that only an idiot would think that MWI implies the possible perception of superpositions?

 

[Eqblaauw]

Before discussing it, you need to define what 'splitting' is exactly.
You will see that the definition of 'splitting' is fuzzy. The same as when 1 road has a fork, you can't locate a place where it splits up to 1 micrometer.

well david deutsch agrees with many universes already there exactly the same until they differentiate so it's needless to think of what splitting may mean, he just doesn't agree with it, my dear semantic philosopher

 

[Hurkyl]

No. String theory is completely compatible with CI, for all the same reasons that quantum mechanics was.

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.

CI has all the same unitary evolution in the wave function as MWI!

... 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.

The sole difference is that CI does not interpret the thing that is evolving unitarily as the reality itself,

If it looks like interpreting the thing as reality itself and it quacks like interpreting the thing as reality itself, then as far as I'm concerned it's interpreting the thing as reality itself.

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.
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.

empiricist, the meaning of a theory is a kind of mental game, like chess

No, that's syntax.

outcome is found to be successful at predicting what is the actual reality-- the outcome of experiment.

But this is mapping elements of the theory to elements of reality -- an interpretation, in exactly the same sense the rationalist is using it.

Empiricism and rationalism use two completely different approaches to that "meaning"

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.

Can all observers agree on the different outcomes of the observations that appear in the many worlds?

Er, yes, this is fairly clear. This has never been a problem. I'll continue my game with a demonstration.

 

[Hurkyl]

...

Experimenter: I got <53,51> heads, out of <101,100> flips in all.
Me: Er... 100 flips?
Experimenter: <No 101 flips, Yes>
Me: Er... can you make it 101 flips?
Experimenter: <I already did, ... Okay, 52 heads>​

Continuing from where I left off...

Me: Hey, who's that?
Experimenter: Oh, that's my intern. Say hi to the mysterious voice!
Intern: Hi!
Me: Hey, can you flip your coin again?
Experimenter: Sure. It's heads.
Me: What do you see, intern?
Intern: Heads.
Me: Okay, flip again.
Experimenter: ... It's <heads, tails>.
Me: And intern, what do you see?
Intern: I see <heads, tails>.
Me: So, you both saw the same thing?
Experimenter: Yes.
Intern: Yes.
Experimenter: You're not going to try to tell me my outcomes are indefinite again, are you?
Me: But it wasn't! One of you saw heads, and the other you saw tails!
Experimenter: I'm sorry to disappoint, but there's just one of me, and I saw <heads, tails>.
Me: Fine, flip again.
Experimenter: Tails.
Me: Again.
Experimenter: <Heads, Tails>.
Me: And intern, what do you see?
Intern: <Heads, Tails>, exactly as the professor said.
Me: So you saw both heads and tails on that coin?
Intern: No, I only saw <heads, tails>. I didn't see <tails, heads>.
​

 

[Eqblaauw]

i don't know who you're trying to convince with your fictional sketches but it isn't really working

 

[Hurkyl]

Dmitry67 and BruceW, you should explain at some point where the unequal probabilities come from. On another website I recently proposed that MWI is like a car dealer who, asked to deliver 9 BMWs and 16 Rolls-Royces, instead shows up with one BMW with a "3" painted on it, and one Rolls-Royce with a "4" painted on it, saying you just have to square the numbers and you'll get the cars you ordered. If you are going to explain quantum mechanics by having a multiverse in which all possible outcomes actually exist, then you need to have an outcome which empirically occurs, say, twice as often, actually occurring twice as often in the multiverse.

And what about the strawman CI car dealer who shows up either with a car with a "9" painted on it, or a car with a "16" painted on it?

 

[Eqblaauw]

And what about the strawman CI car dealer who shows up either with a car with a "9" painted on it, or a car with a "16" painted on it?

does it make your story more valid? In holland we call this the 'but you' technique.

 

[Ken G]

Again you mix 'you' as a line, located in a branch, with a full collection of you (tree).

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?

 

[Fredrik]

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.

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

A bunch of statements is a theory only if it makes predictions about results of measurements. A measurement is an interaction with a measuring device, specifically an interaction of the type that will leave the measuring device in a state that indicates a number called "the result" of the measurement. The indicator component is always perceived as classical by a human. (A component that isn't would be of no use as an indicator).

So measuring devices, or at least their indicator components, will always be described in "intuitive" terms, no matter what theory we're dealing with. This means that there's a "cut" in every theory that doesn't describe the rest of reality in equally intuitive terms. I would say that there's a "cut" even in those theories, because measuring devices should always be thought of as essentially independent of the theory. Consider e.g. using a cesium clock to test the accuracy of predictions of classical special relativity. The theory doesn't describe the inner workings of the measuring device, but no one would say that this means that you're not allowed to use it.

 

[Ken G]

I don't account for it. The question isn't important.

Exactly, the only way MWI achieves completeness is by defining anything it doesn't do as "not important." But CI is just as free to play that same game-- CI simply asserts that having nonunitariness in the actual reality is not important! Which is isn't-- it is not important to take the postulates seriously, any more than it is important to take Newton's postulates seriously. Physicists still routinely use Newton's postulates, long after relativity and quantum mechanics, in exactly the same way they did 200 years ago when then took them seriously as an ontological description of reality. So the lesson is clear: don't take your physics postulates seriously as an ontological description of reality, it is both unnecessary and dubious! That's why I have no issue with MWI as a permitted interpretation of quantum mechanics, it is using it to fabricate a world view that I take issue with-- the pretense of knowing what is not known. CI is more clear that it is not a world view, and it makes no claims on completeness other than that it is a kind of folly to seek it in the first place.

In his responses, I will write anything definite normally, and anything indefinite with the angle brackets as I did with the truth values.

I appreciate your device of trying to create a specific example, but I don't understand how you differentiate what you are writing normally vs. in brackets. What do you see as the distinction between what is definite and what is indefinite? I would think that MWI would have no language to even make that distinction in any way. But I do understand what your device is attempting to accomplish-- as I said above, MWI has no difficulty at all with accounting for a model of "you". What it fails is a model of "me." For example, you cannot even have an experimenter, you have <experimenterH,experimenterT>. Try it again that way and it shows even more clearly that your problem is accounting for the experience of the individual! So your device works fine for anyone who has a "birds-eye" view, and accounts for the perceptions that appear along the branches of the many worlds. That's what I meant by "a model of you." What it does not account for is the perceptions that I have when I do experiments (where by "I", I mean, of course, you!).

So in your scenario, if you really were there, how come you never actually hear someone speak the words "<heads,tails>", if that is what you claim is really happening? You always have to go find someone else to be the bird's eye view to describe what is happening to you, you never actually get to be the person with that birds-eye view. That's what MWI cannot account for, why there can never be an actual observer with a birds-eye view, if that is what is really happening.

That's why MWI is incomplete, it has no accounting for your own experience, it can only give you an accounting of everyone else's experience by adopting the rationalist dream that there is any such thing as a birds-eye view that no perceiving agent ever gets. Is that really the mission of physics, to explain everyone else's experience but never your own? If <heads,tails> is what really happens, why when you flip a coin do you perceive heads or tails? I'm talking about your actual experience here, not your way of rationalizing that experience.

Thus any claim that MWI is complete first must reject empiricism. Ergo, it is an interpretation that is complete for rationalists, and incomplete for empiricists. That's all I'm saying here-- MWI is not a philosophy that is a complete way of looking at physics, it is only a complete philosophy under the condition that one has already adopted rationalism and rejected empiricism. Only then does MWI seem to be complete, so it is very restricted form of completeness, and one with a poor track record.

 

[PAllen]

What about the proposition that there are many Hugh Everetts but only one Niels Bohr?

 

[Hurkyl]

I would think that MWI would have no language to even make that distinction in any way.

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

So your device works fine for anyone who has a "birds-eye" view,

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 usually don't get to talk about the thing I really want to do, since in these discussions I'm usually involved in an uphill struggle to get people to admit MWI is valid, or (occasionally to get people to admit that CI is valid). But maybe you'll appreciate it.

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.

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

*: There's another bit of indefiniteness going on here if you want to pay careful attention to the bird's eye view

if you really were there, how come you never actually hear someone speak the words "<heads,tails>", if that is what you claim is really happening?

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.

If <heads,tails> is what really happens, why when you flip a coin do you perceive heads or tails?

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

 

[Fra]

I've made this point before but I'll make it again...

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

A bunch of statements is a theory only if it makes predictions about results of measurements. A measurement is an interaction with a measuring device, specifically an interaction of the type that will leave the measuring device in a state that indicates a number called "the result" of the measurement. The indicator component is always perceived as classical by a human. (A component that isn't would be of no use as an indicator).

So measuring devices, or at least their indicator components, will always be described in "intuitive" terms, no matter what theory we're dealing with. This means that there's a "cut" in every theory that doesn't describe the rest of reality in equally intuitive terms. I would say that there's a "cut" even in those theories, because measuring devices should always be thought of as essentially independent of the theory. Consider e.g. using a cesium clock to test the accuracy of predictions of classical special relativity. The theory doesn't describe the inner workings of the measuring device, but no one would say that this means that you're not allowed to use it.

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.

in (b) you test the theory by seeing "how much new information" you get from the system by applying it (ie by asking optimally rational questions as per the theories EXPECTATION of the future). A theory seen as an interaction tools is only "wrong" if it FAILS to serve the questioner in navigating in "hypothesis space".

Think about which of the two views that makes most sense in scenarious where for example the notion of ensembles and infinite repeats of the questions just can't be realized.

It's a property of all theories of physics.

I agree with this, if we're talking about all mainstream physical theories, but this observation does not make it right.

This is pretty much a dogma that is apparently VERY HARD for most physicists to release oneself from, but I think at some point we wil lbe force to. Note that (b) by no means weakens "science", it IMHO rather provides a deeper understanding of it. I personally think is one point that people like for example Popper either didn't understand or for some outlandish reason disagreed with.

One may wonder what this has to do with the discussion here, but I think it very much has, at a subtle deeper level, since this is much in line with the "solipsism at it's best", or at least my view of it.

/Fredrik