Does the MWI require "creation" of multiple worlds?

[A. Neumaier]

Shut up and calculate is what is meant by Copenhagen.

Then you should say so, and not call it Copenhagen. Otherwise nobody would guess that when you say Copenhagen, shut-up-and calculate is meant.

The usage of the term ''Copenhagen interpretation'' is very ambiguous, and your particular usage as expressed in the above quote seems to be unique to you.

The Wikipedia article on this says (in my opinion correctly):

According to an opponent of the Copenhagen interpretation, John G. Cramer, "Despite an extensive literature which refers to, discusses, and criticizes the Copenhagen interpretation of quantum mechanics, nowhere does there seem to be any concise statement which defines the full Copenhagen interpretation."

There is no uniquely definitive statement of the Copenhagen interpretation. It consists of the views developed by a number of scientists and philosophers during the second quarter of the 20th Century. Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics. Bohr once distanced himself from what he considered to be Heisenberg's more subjective interpretation.

Different commentators and researchers have associated various ideas with it. Asher Peres remarked that very different, sometimes opposite, views are presented as "the Copenhagen interpretation" by different authors.

And then they list lots of stuff not belonging to shut-up-and calculate as ''basic principles generally accepted as part of the interpretation''.

 

[atyy]

https://physicstoday.scitation.org/doi/10.1063/1.1768652
Could Feynman Have Said This?
N. David Mermin

"...I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”

In the intervening years, I’ve come to hold a milder and more nuanced opinion of the Copenhagen view, but that should be the subject of another column. ..."

 

[bhobba]

Well, general relativity does not contain Newtonian gravity.

Martin I think you need to elaborate it a bit more - I think I know what you are trying to say but would rather you say it.

Thsnks
Bill

 

[A. Neumaier]

https://physicstoday.scitation.org/doi/10.1063/1.1768652
Could Feynman Have Said This?
N. David Mermin

"...I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”

In the intervening years, I’ve come to hold a milder and more nuanced opinion of the Copenhagen view, but that should be the subject of another column. ..."

Ok. So in 1989 there was a second person equating Copenhagen with “Shut up and calculate!” (qualifying it as an uncomfortable forcing into one sentence), but by 2004 Mermin changed his views, as your citation shows.

So since 2004 you are alone with your views - especially since you declared in this thread the collapse to be part of your version of Copenhagen, whereas it is explicitly not part of “Shut up and calculate!”

 

[bhobba]

The usage of the term ''Copenhagen interpretation'' is very ambiguous, and your particular usage as expressed in the above quote seems to be unique to you.

Indeed it is. As Atty knows even Ballentine got it wrong - or at least didn't express himself well. The 6 I gave is just my view of it - but what a Copenhagenist like Bohr or Heisenberg would say I don't really know.

I actually think it has morphed a bit over the years and Decoherent Histories, which claims to be Copenhagen done right would be a better view of it. But I remember a Copenhagenist that posted here at one time was very against that view.

Thanks
Bill

 

[atyy]

Ok. So in 1989 there was a second person equating Copenhagen with “Shut up and calculate!” (qualifying it as an uncomfortable forcing into one sentence), but by 2004 Mermin changed his views, as your citation shows.

So since 2004 you are alone with your views - especially since you declared in this thread the collapse to be part of your version of Copenhagen, whereas it is explicitly not part of “Shut up and calculate!”

Well, then there is no big bang either!

 

[martinbn]

Martin I think you need to elaborate it a bit more - I think I know what you are trying to say but would rather you say it.

Thsnks
Bill

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.

 

[Derek P]

However, none of the other interpretations are uncontroversially solutions for all of quantum mechanics.

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.

 

[martinbn]

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.

 

[A. Neumaier]

https://physicstoday.scitation.org/doi/10.1063/1.1768652
Could Feynman Have Said This?
N. David Mermin

"...I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”

In the intervening years, I’ve come to hold a milder and more nuanced opinion of the Copenhagen view, but that should be the subject of another column. ..."

by 2004 Mermin changed his views, as your citation shows.

Later in the paper you cited, he is even more explicit about this:

Until quite recently, I had no memory of ever having written such a childishly brusque dismissal of such an exquisitely subtle point of view, much less of having published it in so widely read a venue.

Thus, though Mermin had coined the term “Shut up and calculate!”, he now explictly distinguishes it from the Copenhagen views.

 

[bhobba]

My guess is that atyy is referring to solutions of the measurement problem. An interpretation may solve it, but it will introduce other difficulties.

That's true. And if that's what is trying to be said then of course I agree.

Thanks
Bill

 

[Derek P]

My guess is that atyy is referring to solutions of the measurement problem. An interpretation may solve it, but it will introduce other difficulties.

Well difficulties are there to be overcome. The implication of atyy's remark is that they are insuperable. I query whether such difficulties exist.

 

[martinbn]

Well difficulties are there to be overcome. The implication of atyy's remark is that they are insuperable. I query whether such difficulties exist.

My view about interpretations changes all the time. Sometimes I think that if a problem exists in some interpretations and not in others, then it is not a problem. And it doesn't matter if there could be or not an interpretation without any problems. In a way one can think of the interpretations of the theory as coordinate charts of a manifold, say a sphere. In some coordinates a point of the sphere will not be covered, in others it will, but in those charts other points will not be, so that may seem like a problem. But in the case of the manifold there is no problem.

 

[bhobba]

Well difficulties are there to be overcome. The implication of atyy's remark is that they are insuperable. I query whether such difficulties exist.

That's not what Atty is saying - but best if he says it himself.

Formally there are no difficulties with QM - its what it means that's at issue and what the various interpretations grapple with. They all have problems - every single one of them.

I think Dr Neumaier has a good point - QFT may indeed be a better place for interpretations. I do not know enough of his thermal interpretation to comment on its specifics.

As a mentor I must point out this thread is getting off topic - can we please get back to its original intent?

Thanks
Bill

 

[A. Neumaier]

can we please get back to its original intent?

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.

 

[Derek P]

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]

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]

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]

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]

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]

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]

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 modeled outside.

 

[stevendaryl]

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]

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...

 

[Derek P]

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]

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]

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.

 

[Derek P]

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 modeled 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 :)