What is the current status of Many Worlds?

[Demystifier]

... (even if some founders like Griffiths put forth an ontology). CH, as a minimum project ...

So by "minimum project" you mean CH which does not put forth an ontology, am I right? Or are there also other ways how CH can be non-minimal?

 

[Quantumental]

It sounds to me like CH is then by default just a more elaborate "Shut up and calculate using these tools" rather than an actual interpretation of QM

 

[gentzen]

It sounds to me like CH is then by default just a more elaborate "Shut up and calculate using these tools" rather than an actual interpretation of QM

Would you also say that Copenhagen is not an actual interpretation of QM? And what is wrong with "... calculate ..."? All those calculations felt quite positive to me on my first encounter with CH, because they gave me a nice illusion of understanding:

In 1998, I had to endure a QM 1 course at university, and I didn’t manage to connect at all to QM. ... I couldn’t create a picture or film in my head of how to use this to describe nature. ... Occasional attempts to read material discussing interpretation of QM failed quite early, I couldn’t penetrate into the material and words at all. Around 2005, I read (or rather browsed) “Understanding Quantum Mechanics” by Roland Omnès, and it was the first time that I felt that the material was presented in a way that I would understand it, if I invested the time to work through it. It felt like “let me calculate and explain” as opposed to “don’t ask questions, nobody understands QM anyway”.

 

[Morbert]

So by "minimum project" you mean CH which does not put forth an ontology, am I right? Or are there also other ways how CH can be non-minimal?

Yes, in this convo, by minimum I just mean the development of the formalism without commitment to an ontology. So e.g. if we take a typical case of a microscopic system $s$ and a measuring device $M$ that measures observable $A = \sum a_i \Pi_{a_i}$ with pointer states $\{M_i\}$, the conventional approach would be to write down a Hilbert space for the microscopic system, and to compute the probability of a measurement result $a_i$ as $$p(a_i) = \mathrm{Tr}\left[\Pi_{a_i}(t)\rho_s\right]$$A consistent historian would be perfectly comfortable expanding the Hilbert space to include the measurement device and computing the probability for the result $a_i$ and immediately after, the pointer outcome $M_i$ $$p(M_i\land a_i) = \mathrm{Tr}\left[\Pi_{M_i}(t+\delta t)\Pi_{a_i}(t)\rho_s\otimes\rho_M\Pi_{a_i}(t)\right]$$They could then go further and show that $p(a_i) = p(M_i) = p(M_i\land a_i)$ which establishes the logical relation $a_i\iff M_i$ which identifies a measurement scenario in the interaction between $M$ and $s$.

What is normally an implicit measurement context is made explicit by the consistent historian. They can do all this without committing to $a_i$ as a real property that exists before (or after) measurement. They can limit their ontic commitment to $M$ if they like.

 

[Demystifier]

What is normally an implicit measurement context is made explicit by the consistent historian. They can do all this without committing to $a_i$ as a real property that exists before (or after) measurement. They can limit their ontic commitment to $M$ if they like.

That's very much similar to my solipsistic hidden variables, where one can choose objects for which to postulate the ontic Bohm-like trajectories: everything including the measured system, or the apparatus but not the measured system, or perhaps only the state of brain responsible for consciousness. And of course, all this has roots in von Neumann theory of measurement where one has similar freedom where to put the collapse postulate.

 

[gentzen]

A consistent historian would be perfectly comfortable expanding the Hilbert space to include the measurement device and computing the probability for the result $a_i$ and immediately after, the pointer outcome $M_i$
$p(M_i\land a_i)=\ldots$

The formula for $p(M_i\land a_i)=\ldots$ looks wrong: $\Pi_{a_i}(t)$ appears two times in the formula, and the rightmost occurence feels like a typo to me.

 

[Morbert]

The formula for $p(M_i\land a_i)=\ldots$ looks wrong: $\Pi_{a_i}(t)$ appears two times in the formula, and the rightmost occurence feels like a typo to me.

If I work in an explicit history space $\mathcal{H}_t\otimes\mathcal{H}_{t+\delta t}$ the rightmost term is not necessary. But otherwise, histories made up of chains of projectors like $\Pi_{M_i}(t+\delta t)\Pi_{a_i}(t)$ are not necessarily projectors themselves and so it is good practice to avoid any time-ordering commutation issues. This is why you will see authors like Omnes (in "Understanding QM") write the probability of a history like so

as opposed to just $p(a) = \mathrm{Tr}(C_a E_0)$

 

[Minnesota Joe]

Such a view is rather obsolete. The quantum potential is as obsolete as e.g. relativistic mass in modern formulation of special relativity. Wave function is fundamental but not ontic, in the same sense in which Hamiltonian in classical mechanics is fundamental but not ontic.

I find it really difficult to understand how the configuration space wavefunction could be real (not that my difficulties are an argument for anything), so I was intrigued to find this paper by Norsen, Marian, and Oriols (2015, Synthese, or https://arxiv.org/abs/1410.3676) that sounds a little like your description. They argue that the single particle conditional wave functions are real but the configuration space wave function is not; instead, it plays a role analogous to the role the Hamiltonian plays in classical mechanics (the part similar to what you wrote). Is this the kind of approach you are hinting at or something else?

Thanks,

Joe

 

[PAllen]

I think most people would consider logic and inductive reasoning to be general tools applicable to a variety of disciplines, not philosophy. If they are part of any particular discipline, I think most people would say that discipline is mathematics.

Well, most universities, at least when I went, offered formal logic as part of philosophy. That was how some geeks got to take a “math” course for a humanities distribution requirement. And many of the most eminent figures in the history of logic (e.g. Quine), certainly considered themselves philosophers.

 

[Demystifier]

Is this the kind of approach you are hinting ... ?

Yes.

 

[Demystifier]

CH is not about what really goes on in the world. Instead, it is about what we can cay about the world, if we require that the things we say obey the following rules:
- It is logically consistent and precise. (Unlike some vague variants of Copenhagen and statistical ensemble interpretations.)
- We don't need any additional mathematical objects except those which are already there in standard QM. (Unlike Bohmian trajectories or GRW modifications of the Schrodinger equation.)
- There is no a priori preferred basis in the Hilbert space.
- Measurement and observation do not play any fundamental roles.
- There are no many worlds.

An even more concise explanation of the main idea behind CH is: complementarity without measurement.

Personally, I don't like this idea. I think complementarity arises precisely because of the measurement and has no meaning without measurement. That's why I don't like CH. But at the same time, I can understand why some people find the idea of complementarity without measurement attractive, which helps me to understand why one may find CH attractive.

 

[vanhees71]

I don't like all this "Bohrian weirdness" (wave-particle duality or complementarity), which leads to all these debates since the beginning of QT. It's simply the observed fact that quanta behave (in the probabilistic sense of QT of course) more like a classical field or more like a particle depending on the preparation and measurement setup of the experiment done.

 

[Morbert]

Presumably cosmology is a major motivator behind a quantum language for closed systems, so objectively evaluating these interpretations might be possible by measuring the impact they have on fields like cosmology. I.e. It's less about satisfying the philosopher, and more about offering useful intuitions that further a field.

 

[Demystifier]

Presumably cosmology is a major motivator behind a quantum language for closed systems, so objectively evaluating these interpretations might be possible by measuring the impact they have on fields like cosmology. I.e. It's less about satisfying the philosopher, and more about offering useful intuitions that further a field.

In quantum cosmology, I think MWI has been more influential than any other specific interpretation (beyond pure shut up and calculate). But of course, it doesn't mean that other interpretations cannot be used in quantum cosmology.

 

[Morbert]

I'm claiming this one for CH, even thought it was as much a byproduct as it was a motivator.

 

[Minnesota Joe]

Yes.

Thank you, that helps me understand your first post to this thread and what you mean by 'nomological'. I'm glad you mentioned it because I was under the impression that both MWI and Bohmian mechanics posit an real configuration space wave function. I looked for the Norsen paper because of my own difficulties understanding that and because he hinted at the approach in his quantum foundations book. Good to learn of this progress.

As someone who's been following its developments for the past 17 years closely I am left feeling that "Many Worlds" is similar to Platonism. You have a lot of mathematicians who adhere to some form of Platonism, but when you inquire about specifics it turns out that they all disagree on what platonism even means. In this spirit of confusion I'd love to hear what the thoughts on Many Worlds are in 2021 by everyone here at PF.

I don't know how much can inferred from their disagreements, but the problems with MWI seem bad enough.

Part of the draw is that they claim to need only the wave function. But that limits what they can add to explain the difference between an experiment that results in two outcomes, A and B, with A observed 50% of the time and an experiment that results in the same two outcomes, with A observed, say, 75% of the time.

Then there is the question of how the three dimensional world we observe, the world of "medium-sized dry goods" to steal a phrase, emerges from the universal wave function alone. Other interpretations can appeal to real things of which our observed world is made, bottom up, but MWI has only the wavefunction and somehow all our experiences are supposed to emerge as it evolves.

It seems that more than the wave function is necessary, but they could be right even if they can't defend it. I don't understand how we could ever know. I get the impression that they are hoping to make great strides in other areas (quantum gravity, cosmology) so that the successes speak for themselves.

 

[Demystifier]

I'm claiming this one for CH, even thought it was as much a byproduct as it was a motivator.

One author is adherent of CH, the other of MWI. The paper, I think, is interpretation independent.

 

[*now*]

Differently perhaps, RQM is said to be more easily followed with gravity having already learned LQC and LQG. But similarly, although published earlier than the DH links provided, I think the paper linked in post 88 in some ways explains a possible extension of interpretations like DH concerning possible questions raised, e.g. page 18? The paper also begins like this- “This point of view is not antagonistic to... Copenhagen [Heisenberg 1927, Bohr 1935], consistent histories [Griffiths 1984, Griffiths 1996, Omnes 1988, Gell-Mann and Hartle 1990], many-worlds [Everett 1957, Wheeler 1957, DeWitt 1970],,... This paper is based on a critique of a notion generally assumed uncritically. As such, it bears a vague resemblance with Einstein’s discussion of special relativity, which is based on the critique of the notion of absolute simultaneity”.
https://arxiv.org/abs/quant-ph/9609002

 

[Morbert]

Differently perhaps, RQM is said to be more easily followed with gravity having already learned LQC and LQG. But similarly, although published earlier than the DH links provided, I think the paper linked in post 88 in some ways explains a possible extension of interpretations like DH concerning possible questions raised, e.g. page 18? The paper also begins like this- “This point of view is not antagonistic to... Copenhagen [Heisenberg 1927, Bohr 1935], consistent histories [Griffiths 1984, Griffiths 1996, Omnes 1988, Gell-Mann and Hartle 1990], many-worlds [Everett 1957, Wheeler 1957, DeWitt 1970],,... This paper is based on a critique of a notion generally assumed uncritically. As such, it bears a vague resemblance with Einstein’s discussion of special relativity, which is based on the critique of the notion of absolute simultaneity”.
https://arxiv.org/abs/quant-ph/9609002

As an aside, I think Rovelli makes a similar mistake Kent makes when talking about CH, frameworks, and facts.

To put it pictorially (and a bit imprecisely): I do not care about a science that tells me that my airplane will not crash “in one framework”; I want a science that will tell me that my airplane will just not crash!

If a framework let's you conclude that your airplane will not crash, then the science tells you your airplane will just not crash. This confidence cannot be undone by e.g. a selection of some alternative framework. There is no framework where your airplane will crash.

 

[Demystifier]

If a framework let's you conclude that your airplane will not crash, then the science tells you your airplane will just not crash. This confidence cannot be undone by e.g. a selection of some alternative framework. There is no framework where your airplane will crash.

Yes, but there is a framework in which the question "Will my airplane crash?" is meaningless. CH claims that this is a perfectly legitimate framework, yet it's hard to understand what's the meaning of such a framework. It's somewhat like a unicorn, in the sense that it's legitimate to consider it just because it's logically consistent.

 

[Morbert]

CH claims that this is a perfectly legitimate framework, yet it's hard to understand what's the meaning of such a framework.

There is a classical analogue to this question. You roll a die and want to know the probability of landing a "2 or 3". In either quantum or classical theories, a framework is constructed from a sample space and an event algebra so we can construct the framework from the sample space

[die lands 1] (p = 1/6)
[die lands 2] (p = 1/6)
[die lands 3] (p = 1/6)
[die lands 4] (p = 1/6)
[die lands 5] (p = 1/6)
[die lands 6] (p = 1/6)

or we can construct a framework from

[die lands 1 or 2] (p = 1/3)
[die lands 3 or 4] (p = 1/3)
[die lands 5 or 6] (p = 1/3)

The first framework let's us compute the probability for the event [die lands 2]+[die lands 3] = 1/3. The question is not meaningful in the 2nd framework, and it does not permit us to compute the probability for the event. Does this mean the framework is inherently meaningless? No, it's just meaningless wrt the question we were interested in answering.

 

[Demystifier]

The first framework let's us compute the probability for the event [die lands 2]+[die lands 3] = 1/3. The question is not meaningful in the 2nd framework, and it does not permit us to compute the probability for the event. Does this mean the framework is inherently meaningless? No, it's just meaningless wrt the question we were interested in answering.

Interesting analogy! But the two frameworks are not on an equal footing, the first framework is more fundamental because it has finer graining. In classical physics, the finest possible graining is unique. CH, on the other hand, says that the finest possible graining is not unique. A Bohmian would say that the finest possible graining corresponds to primitive ontology, so in this language one would say that in CH primitive ontology is not unique. A classical analogy would be using the fact that a classical field $\phi(x)$ can equivalently be represented by its Fourier transform $\tilde{\phi}(k)$ and saying that $\tilde{\phi}(k)$ is not any less real than $\phi(x)$. The claim that, in classical physics, $\tilde{\phi}(k)$ is as real as $\phi(x)$ is very seducing mathematically, but at the same time it's very hard to swallow it from an intuitive physical point of view. CH interpretation of QM is hard to swallow for exactly the same reason.

 

[vanhees71]

Sure, you cannot deduce probabilities for a situation that describes more details from probabilities of a more coarse grained description. By coarse graining you through away information.

 

[martinbn]

My view on the many worlds is that, when it talks about many worlds existing and many copies of everything, it is just a pile of words. On the other hand the idea of a relative state interpretation without the philosophical nonsense is very atractive.

Here is a version of Everett's interpretation without the philosophical mambo jumbo.

https://www.jstor.org/stable/2214880

 

[gill1109]

This is one of the most frequent misconceptions about many worlds. The additional worlds are not postulated. They are derived, from the assumptions that the wave function is real (ontic) and that the unitary evolution is always right.

I disagree. Unless we disagree about what is "real". If the wave function is real and there is no collapse of the wave function then nothing is real at all. There is just the wave-function of the universe, evolving deterministically.

That's my personal opinion. I'm a mathematician, not a physicist, so you can call it an amateur opinion. Opinions in the "foundations of physics" community are very divided. A few years ago most practising practical-minded physicists thought that there were no more foundational issues in quantum mechanics any more and they all claimed to subscribe to the many worlds interpretation. But actually what they subscribed to was the bare standard empirical predictions of QM including the Born law, as it has stood for a long time. Really, most practicising physicists practice "shut up and calculate", and rely on the authority of the leaders of the field. I have the impression that in most Foundations of QM conferences these days, MWI is a minority opinion, unless an MWI protagonist is the organiser, of course. Still today, most people in Q Foundations argue that MWI has not succeeded in *deriving* the Born law.

 

[Demystifier]

If the wave function is real and there is no collapse of the wave function then nothing is real at all.

It doesn't make sense, even for a mathematician. If wave function is real, then something is obviously real. If wave function is real and if wave function has many branches, then the branches are real.

 

[WernerQH]

If wave function is real, then something is obviously real.

With respect to MWI nothing is real, because it is based on fiction. q.e.d.

 

[*now*]

As an aside, I think Rovelli makes a similar mistake Kent makes when talking about CH, frameworks, and facts.If a framework let's you conclude that your airplane will not crash, then the science tells you your airplane will just not crash. This confidence cannot be undone by e.g. a selection of some alternative framework. There is no framework where your airplane will crash.

I’m sorry, I don’t know Kent’s argument, would you tell me what it is please?

 

[Morbert]

Interesting analogy! But the two frameworks are not on an equal footing, the first framework is more fundamental because it has finer graining. In classical physics, the finest possible graining is unique. CH, on the other hand, says that the finest possible graining is not unique. A Bohmian would say that the finest possible graining corresponds to primitive ontology, so in this language one would say that in CH primitive ontology is not unique. A classical analogy would be using the fact that a classical field $\phi(x)$ can equivalently be represented by its Fourier transform $\tilde{\phi}(k)$ and saying that $\tilde{\phi}(k)$ is not any less real than $\phi(x)$. The claim that, in classical physics, $\tilde{\phi}(k)$ is as real as $\phi(x)$ is very seducing mathematically, but at the same time it's very hard to swallow it from an intuitive physical point of view. CH interpretation of QM is hard to swallow for exactly the same reason.

I should probably continue this convo in the ontology thread so I'll do that. Here I'll just say that yeah in a classical theory there is always a common refinement of the state space that can be identified as privileged, unlike in QM. I only bring it up to stress the point that the meaningfulness of a framework is not determined by reality, but by the events we want to compute probabilities for.

 

[Morbert]

I’m sorry, I don’t know Kent’s argument, would you tell me what it is please?

I'll answer this in the ontology thread to avoid going too far off topic here.

 

[physika]

Wave function is fundamental but not ontic

what getaway !
Who knows if is not ontic ?

 

[Demystifier]

what getaway !
Who knows if is not ontic ?

It was in the context of Bohmian mechanics, in another interpretation it can be different.

 

[*now*]

I'll answer this in the ontology thread to avoid going too far off topic here.

Does Kent claim DH probability is ontic, and is that mistaken? Anyway, I don’t think that claim was made in the paper I linked, e.g. …“The beauty of the histories interpretations is the fact that the prob[1]ability of a sequence of events in a consistent family of sequences does not depends on the observer, precisely as it doesn’t in classical mechanics. One can be content with this powerful result of the theory and stop here”…. (https://arxiv.org/abs/quant-ph/9609002)

 

[Morbert]

Does Kent claim DH probability is ontic, and is that mistaken? Anyway, I don’t think that claim was made in the paper I linked, e.g. …“The beauty of the histories interpretations is the fact that the prob[1]ability of a sequence of events in a consistent family of sequences does not depends on the observer, precisely as it doesn’t in classical mechanics. One can be content with this powerful result of the theory and stop here”…. (https://arxiv.org/abs/quant-ph/9609002)

Oops, forgot to answer this.

Consistent histories says different sets won't yield contrary inferences. Dowker and Kent argue that they will.
https://arxiv.org/abs/gr-qc/9412067

To predict the moon’s trajectory within the formalism, we need the independent assumption that the moon will be described by the right sort of projection operator. If the projection operators are those onto densities of chemical species within small volumes of the moon’s orbit, then they will describe the moon’s expected orbit, whose predictability follows from the quasiclassicality of those operators. However, there are almost certainly other descriptions, consistent with the quasiclassicality of the Earth but inconsistent with that of the moon.

I.e. The question of whether or not the moon proceeds in a quasiclassical manner is contingent on the set we choose to answer the question.

The resolution: These different sets are describing different properties. They are not all describing the same property labelled "the moon", and the hydrodynamic variables that make up what we call the moon will proceed quasiclassically.

 

[*now*]

That is helpful, thanks very much.