# Does the MWI require "creation" of multiple worlds?

• A
As a mentor I must point out this thread is getting off topic - can we please get back to its original intent?
I couldn't agree more. But as you have addressed your remark to me, I must point out that in post 43 I tried to do exactly that. Atyy's statement was "none of the other interpretations are uncontroversially solutions" which implies that at best MWI fails to deliver solutions and at worst it is an inconsistent mess. Discussing whether MWI requires the creation of multiple worlds can't proceed if there is no agreement that MWI is coherent and complete. Or rather any discussion will be vacuous.

stevendaryl
Staff Emeritus
But which property should it be if the measurement device is described by quantum mechanics (and hence, according to MWI, only by a wave function)? Clearly, this property must be a function of the wave functtion (the only thing that exists in MWI). But what to call measurement result is left unanswered by MWI and requires another interpretation.
That's true, but none of the orthodox interpretations really answer that.

You give no mechanism that makes the macroscopic state behave such that measurement is possible - i.e., that it correctly reflects in the measurement apparatus a property of the microscopic state of the measured system.
I'm not claiming any such thing. I don't think it's necessary to specify which macroscopic state corresponds to a measurement.

You need to postulate (and this is the extra interpretive step) that the macroscopic state is a classical probabilistic state, and you need to justify why the observed (objective) frequencies, measured on individual systems with their individual states produce the correct probabilities.
What I suggested has the elements of the MWI, except that I'm not actually proposing anything as ambitious as deriving probabilities from unitary evolution. I'm just taking as a postulate that for macroscopic states, my $P_j(t)$ is the probability of being in state $j$. Not the probability of measuring anything.

stevendaryl
Staff Emeritus
Well, it is a bit off topic, and I, being a curmudgeon, had to complain.

Of course the two theories in some limiting cases give very close predictions. But they (the theories) are so different that I wouldn't say that one contains the other. An analogy, if you look near a point on a parabola it looks very close to a straight line, but I disagree with the statement that a parabola contains a straight line.
It's hard to say when one theory is strictly a generalization of another. Newtonian gravity can be formulated (Newton-Cartan theory) as a theory of spacetime curvature in which mass is the source of curvature. Then Einsteinian gravity can be seen as a tweak to that.

stevendaryl
Staff Emeritus
Well, to answer the original question one first needs clear definitions of the concepts involved:
1. What precisely constitutes a world in MWI?
2. Are these worlds just ''points of view'' (independent of reality), or are they dynamical objects in time?
3. What precisely constitutes a split of one of these worlds? What triggers a split?
4. When precisely do these splits happen? Do they happen at all? Is it observer-dependent?
5. For an observer as a quantum object in the MWI for the whole universe, how is its perceived world characterized among all possible worlds?
6. Do different observers perceive different worlds? If yes, why?
7. What object inside a quantum universe described by MWI qualifies as an observer? What as a measurement? What constitutes a measurement result?
Precise statements about such basic terms, all stated in terms of the wave function of the universe - which is all that evolves, are needed since a reference to an external classical world is not meaningful in MWI - its virtue is supposedly that it applies to everything!

Lacking precise statements makes a useful discussion impossible.
Here's what I've noticed about QM and interpretations of QM. There are certain unresolved and seemingly unresolvable problems in QM and its interpretation. They are always present. However, people tend to only bring them up as arguments against interpretations of QM that they dislike.

A. Neumaier
I don't think it's necessary to specify which macroscopic state corresponds to a measurement.
Then your account of MWI says nothing about measurement, since it does not say how to recognize the relation between measurements and the interpretation.
I'm just taking as a postulate that for macroscopic states, my $P_j(t)$ is the probability of being in state $j$. Not the probability of measuring anything.
This is an extra interpretational ingredient compared to MWI.

stevendaryl
Staff Emeritus
I think Dr Neumaier has a good point - QFT may indeed be a better place for interpretations.
I actually disagree with that. I don't think that any of the conceptual difficulties with QM are resolved by QFT. From a certain perspective, QFT is a specific quantum theory, quantum theory being the more general category that includes QFT and nonrelativistic QM as special cases.

stevendaryl
Staff Emeritus
Then your account of MWI says nothing about measurement, since it does not say how to recognize the relation between measurements and the interpretation.
It's not intended to say anything about measurement. What we observe is certain macroscopic facts about the world: I set up such and such a device. Later, I found it in such and such a state. If you have a theory that predicts probabilities for macroscopic states, that's sufficient.

stevendaryl
Staff Emeritus
This is an extra interpretational ingredient compared to MWI.
If you go back to Everett's original paper, he did the analysis completely within the framework of standard quantum theory. What was different about his approach was that he considered the observer and measurement devices to be quantum-mechanical, as well. So from that point of view, MWI is not a new theory of QM, it's really a consequence of good old fashioned QM, if you try to apply it to macroscopic objects.

The question of how do probabilities arise in MWI is interesting, but it isn't critical.

A. Neumaier
There are certain unresolved and seemingly unresolvable problems in QM and its interpretation. They are always present. However, people tend to only bring them up as arguments against interpretations of QM that they dislike.
But questions about multiple worlds (such as my questions 1-6) do not apply to any other interpretation than the MWI. Only 7 is more general, but it does not apply to interpretations that negate being applicable to the whole universe.

However, MWI must claim to be about the whole universe, as without that its basic assumption of unitarity is experimentally invalid, because dissipation is everywhere. Thus it must answer question 7 to be a good interpretation. It cannot take recourse to a classically modelled outside.

stevendaryl
Staff Emeritus
But questions about multiple worlds (such as my questions 1-6) do not apply to any other interpretation than the MWI.
The question of how QM applies to a macroscopic object is inherent in any interpretation of QM.

A. Neumaier
If you go back to Everett's original paper, he did the analysis completely within the framework of standard quantum theory.
I read his original paper in detail, and found it wanting in essential aspects. See my critique of Everett here.
The question of how QM applies to a macroscopic object is inherent in any interpretation of QM.
But it is answered easily in the various versions of the Copenhagen interpretation, where it applies as long as you can measure it from the outside. MWI has no such recourse.

stevendaryl
Staff Emeritus
But it is answered easily in the various versions of the Copenhagen interpretation. it applies as long as you can measure it from the outside. MWI has no such recourse.
Well, I disagree. I think that the motivation for considering MWI is precisely because the questions are not answered by any other interpretations of QM.

A. Neumaier
Well, I disagree. I think that the motivation for considering MWI is precisely because the questions are not answered by any other interpretations of QM.
Only because one would like to have an interpretation that applies to the whole universe. No version of Copenhagen claims that, and hence is immune from this problem as far as its self-proclaimed realm of applicability goes. Once one demands the unrestricted validity of quantum mechanics in the universe as a whole, one needs an interpretation that can settle precisely this issue. MWI claims to cater for the whole universe, hence pretends to have answers....

Well, to answer the original question one first needs clear definitions of the concepts involved:
1. What precisely constitutes a world in MWI?
2. Are these worlds just ''points of view'' (independent of reality), or are they dynamical objects in time?
3. What precisely constitutes a split of one of these worlds? What triggers a split?
4. When precisely do these splits happen? Do they happen at all? Is it observer-dependent?
5. For an observer as a quantum object in the MWI for the whole universe, how is its perceived world characterized among all possible worlds?
6. Do different observers perceive different worlds? If yes, why?
7. What object inside a quantum universe described by MWI qualifies as an observer? What as a measurement? What constitutes a measurement result?
Precise statements about such basic terms, all stated in terms of the wave function of the universe - which is all that evolves, are needed since a reference to an external classical world is not meaningful in MWI - its virtue is supposedly that it applies to everything!

Lacking precise statements makes a useful discussion impossible.
There are plenty of precise statements in MWI. It's not exactly a new theory - and if it was it would be off-topic.

A. Neumaier
There are plenty of precise statements in MWI. It's not exactly a new theory
Please point to a reference where the above 7 points are made precise. They are directly relevant for the question posed in #1.

stevendaryl
Staff Emeritus
Only because one would like to have an interpretation that applies to the whole universe.
I disagree. The issue is how to apply QM to macroscopic objects. That's been the issue since nearly the beginning (with Schrodinger's cat, Wigner's friend, etc.) None of the interpretations gave good answers.

The reason that "the wave function of the universe" comes in is because decoherence implies that a macroscopic system can't really have a quantum state unless it is isolated, and only the universe as a whole can be isolated.

A. Neumaier
I disagree. The issue is how to apply QM to macroscopic objects. That's been the issue since nearly the beginning (with Schrodinger's cat, Wigner's friend, etc.) None of the interpretations gave good answers.
Wigner's friend is not a problem for Copenhagen, only the infinite iteration of Wigner's friend, which is impossible in a bounded domain.
The reason that "the wave function of the universe" comes in is because decoherence implies that a macroscopic system can't really have a quantum state unless it is isolated, and only the universe as a whole can be isolated.
But this also applies for a microscopic system. It can only be approximately isolated. So whatever quantum mechanics predicts based on isolation is an approximation only.

stevendaryl
Staff Emeritus
Exactly. The interpretation problems with MWI are exactly the interpretation problems of any theory of QM. It doesn't introduce any new ones.

I feel like your 7 questions are actually based on a straw man version of MWI. A careful description of MWI does not mention worlds splitting---there is no such notion in MWI. It's part of the layman's intuitive story of what's going on in MWI, but not part of the mathematics. So your questions about what constitutes a world are moot.

But questions about multiple worlds (such as my questions 1-6) do not apply to any other interpretation than the MWI. Only 7 is more general, but it does not apply to interpretations that negate being applicable to the whole universe.

However, MWI must claim to be about the whole universe, as without that its basic assumption of unitarity is experimentally invalid, because dissipation is everywhere. Thus it must answer question 7 to be a good interpretation. It cannot take recourse to a classically modelled outside.
The interaction that creates an improper mixture is distinct from the interaction of an outcome with an observer. You simply factorize the state into as many subsystems you like. Two of them interact and create an entanglement, then the rest pile in and interact with each of the components to create a superposition of consistent products, aka worlds.

atyy
1+1=2 will be controversial to some people. So short of quasi-philosophical objections, what solutions does the interpretation referenced in this thread fail to deliver? The content of what MWI says, I mean, not the fanciful name given to it.
My guess is that atyy is referring to solutions of the measurement problem. An interpretation may solve it, but it will introduce other difficulties.
@martinbn, I was not saying that an interpretation that solves the measurement problem necessarily introduces other difficulties (except possibly aesthetic ones, but as we say in classical relativity: nature does not care about what you like!). I am saying, that at present, that all attempts to solve the measurement problem either have technical difficulties or lack a sufficiently clear exposition of technical solutions to convince even proponents of the approach.

1. Bohmian mechanics and other hidden variable theories lack explicit constructions that will reproduce the standard model.
2. MWI may be technically sound (eg. Aharonov and Rohrlich), but some MWI proponents (Wallace and Carroll) remain unsure if there is a correct justification of how probabilities enter the theory.
3. Decoherent histories is likely to be technically correct, but it is not a realist solution to the measurement problem (traditionally, the measurement problem only admits realist solutions; if necessary - just add the qualifier "realist" to my remarks at the right places) because of the lack of a single fine-grained history (eg. Gell-Mann and Hartle). Here may I borrow bhobba's characterization: defining your difficulties away :)

2. MWI may be technically sound (eg. Aharonov and Rohrlich), but some MWI proponents (Wallace and Carroll) remain unsure if there is a correct justification of how probabilities enter the theory.
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Last time I wrestled with this issue it seemed to me to come down to explaining
1. why a naive observer might mistake frequencies in a history for actual probabilities
2. why the statistics follow the Born rule
3. what is meant by a "typical" history
They all seemed rather trivial to me but someone, I think it was Ruth Kastner (apologies @rkastner if I'm wrong), was adamant that #3 cannot be resolved without invoking actual probability. I see the point. Even if the frequency in a real experiment converges to the ensemble expectation value, that only occurs in "most" cases. We could still be in a rogue world. But as that would apply to classical probability, indeed to all of science (because induction and probability are inseparable), it would seem to be a philosophical matter. We can eliminate physical probability but we need to be able to justify the assumption that the world I am in is probably fairly typical. The trouble is "the world I am in" does not have a probability measure. Or does it? A symmetrical finite-way split should result in equal probabilities, so the problem becomes easier to think about. And then the map from amplitude to frequentist probability necessarily follows Born's Rule because of the way orthogonal vectors add. But is the assumpion of equal weighting justifiable? My brain hurts.

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stevendaryl
Staff Emeritus
1. Bohmian mechanics and other hidden variable theories lack explicit constructions that will reproduce the standard model.
2. MWI may be technically sound (eg. Aharonov and Rohrlich), but some MWI proponents (Wallace and Carroll) remain unsure if there is a correct justification of how probabilities enter the theory.
3. Decoherent histories is likely to be technically correct, but it is not a realist solution to the measurement problem (traditionally, the measurement problem only admits realist solutions; if necessary - just add the qualifier "realist" to my remarks at the right places) because of the lack of a single fine-grained history (eg. Gell-Mann and Hartle). Here may I borrow bhobba's characterization: defining your difficulties away :)
My feeling is that the measurement problem is a red herring. For something to count as a measurement, a microscopic quantity must be amplified (typically using a metastable system) so that it has a macroscopic effect. So for empirical adequacy, a theory only needs to predict probabilities for macroscopic quantities. It doesn't need to have any special rules for measurement. However, for intellectual coherence, it's unsatisfying that macroscopic quantities should be treated as special. Decoherence explains why superpositions of states with different macroscopic properties will be problematic, but I don't think that actually solves the question of what's special about macroscopic quantities. It says: If you have to choose just one basis for describing things, it doesn't make sense to choose one other than one where macroscopic quantities have definite values. But it doesn't say why a basis needs to be chosen at all.

Please point to a reference where the above 7 points are made precise. They are directly relevant for the question posed in #1.
That's just prevarication. The thread has moved on from simply finding a reference for the OP. (I'd have given it if I had one.)

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Mentor
Shut up and calculate is what is meant by Copenhagen.
I don't think there is an agreed meaning for "Copenhagen interpretation" to begin with, but I certainly don't think most of the people using that term mean it this way.

Also, if it is meant this way, then "Copenhagen observer" would just mean "shut up and calculate observer", which obviously any interpretation of QM contains. So I don't understand your claim that MWI doesn't contain "Copenhagen observers".

Mentor
That's verging on being insulting. Please stick to analysing the physical theory before implying that its
proponents are delusional !
Please do not overreact. "In the head of the believer" is not implying any kind of delusion. It's just referring to a person's beliefs as opposed to what those beliefs are about. Even if "what those beliefs are about" does not exist, unless you are going to claim that normal humans never have false beliefs, it's hard to see such a statement as an insult; it's just a description of the human condition.