Would studying MWI be a waste of time?

[zonde]

The aim of the natural sciences is to provide an as accurate as possible description of nature concerning reproducible phenomena.

I agree with stevendaryl about the role of predictions in testing our description of nature.

To me, the role of successful predictions is that it provides feedback as to whether your understanding is correct.

So the aim of natural sciences is to get descriptions that can be extrapolated with some limited confidence to other not yet observed phenomena i.e. to make better speculations about unobserved phenomena.

to introduce funny concepts like parallel universes which are not observable in principle is, from the point of view of physics, unnecessary and doesn't lead to any additional insight.

I agree with this.
But as a disclaimer I would like to say that I have heard the same argument about Bohmian trajectories and there I would not agree with that conclusion as sticking to Bohmian trajectories would make me very skeptical about any explanation of experiment with discontinuous particle trajectories.

 

[vanhees71]

Bohmian trajectories are as unobservable as parallel universes of the (popular-science flavor) of MWI. That's why I have no need for BM either ;-).

 

[stevendaryl]

There's no way to tell, why this is so, and to introduce funny concepts like parallel universes which are not observable in principle is, from the point of view of physics, unnecessary and doesn't lead to any additional insight.

In the case of MWI, it's not a matter of introducing parallel universes, it's a matter of trying to understand the implications of theories we already working with. Whether it leads to any additional insights is a matter of opinion.

 

[Denis]

Bohmian trajectories are as unobservable as parallel universes of the (popular-science flavor) of MWI.

Wrong. The real world you see around is, in dBB theory, described by its Bohmian trajectory. It is the wave function which is unobservable.

Have you ever seen a superposition of a dead and a living cat? But, given that in dBB theory you have no collapse of the wave function, this is what the wave function describes. The Bohmian trajectory is, instead, the cat as it is. Living or dead, but not above.

 

[Demystifier]

Bohmian trajectories are as unobservable as parallel universes of the (popular-science flavor) of MWI.

The unobservability of Bohmian trajectories is more similar to unobservability of dark matter. See
https://arxiv.org/abs/1703.08341 Sec. 4.1.

 

[durant35]

...why we should associate a probability, and which specific probability measure.

First of all, thanks for your extensive input. Now I feel like I have better insight in what's bothering you regarding collapse.

I agree that there are many challenges that you mentioned, but I feel ultimately physics needs to be challenged and realize that there's something more than a single equation which is in my opinion a pretty lazy solution that doesn't give meaningful answers by itself. Whether a collapse, Bohmian particle or something else is needed, neither of us knows, but the point is that the wavefunction is not enough.

In the quoted part I mentioned the part of the problem which I still view as a non-issue. I still feel that the probability has a straight-forward meaning in a probabilistic/collapse interpretation.

No, people don't do that. They (many) try very hard to understand.

Nobody's disputing that and their effort. The problem is the supstance they are dealing with - a deterministic theory where probability has a clear meaning. This is connected to the initial question that OP asked, is it a waste of time - for an expert as well as for the layman.

BTW, when I mentioned and referenced the MWI thread in one of the previous posts (https://www.physicsforums.com/threa...-the-many-worlds-interpretation.706927/page-5) I had a strong impression in that thread that you were insisting on right kind of questions regarding the lack of clear probability meaning, your posts were basically all the issues that don't satisfy me in the MWI.

May I ask, what has changed from then, that you now support MWI despite the counter-arguments you provided there?

 

[bhobba]

The problem is why the Born rule appears as a probability measure

Well a bloke called Andrew Gleason came up with this theorem. Maybe that has something to do with it.

There is a theorem in MW called the non-contextuality theorem so that isn't an out.

Its not a waste of time studying any interpretation. Currently Wallace has the most up to date presentation that gives the full technical details such as the proof of the above:
https://www.amazon.com/dp/0198707541/?tag=pfamazon01-20

I think MW is a load of rubbish personally - but that means nothing, nor did it stop me studying the above book.

Thanks
Bill

 

[vanhees71]

So why do you think it's worth wasting your time with rubbish? If it were at least some fun, I'd understand it, but what the heck...?

 

[bhobba]

So why do you think it's worth wasting your time with rubbish? If it were at least some fun, I'd understand it, but what the heck...?

Well you discover its basically the same as Decoherent Histories with each history in its own world as mostly the only difference. To me that was very interesting.

Maybe I am weird - the more I dislike something it sort of needles at me. Scientifically its as good as any other - so I shouldn't do that - I guess I feel I owe it to myself to at least know what it says.

Just watching a major event here in Aus - Stargazing Australia with Brian Cox - already learned something - why Britain can't see the center of the Milky way. Very interesting.

Thanks
Bill

 

[AlexCaledin]

A review of the book "The Emergent Multiverse..."

http://philsci-archive.pitt.edu/10940/1/Wallace_review_final_single-spaced.pdf

"... the quantum state is correctly regarded as describing a collection of quasi-classical worlds. Worlds do not appear as fundamental elements in the formalism, nor are they explicitly definable in terms of such elements. They have the status rather of emergent entities." (page 4)

- Prof. Hartle was saying practically the same, developing his cosmologic QM, e.g. here:
http://web.physics.ucsb.edu/~quniverse/papers/nlh92.pdf

 

[bhobba]

A review of the book "The Emergent Multiverse..."

http://philsci-archive.pitt.edu/10940/1/Wallace_review_final_single-spaced.pdf

A bit too 'philosophy' for my taste.

What I like about the book is its very theorem proof, theorem proof in style. It's interesting to see how it relates to the formalism and other interpretations - this especially applies to decoherent histories, the books that I have read leave some technical details out found in Walllace.

It didn't convince me of MW - in fact the opposite - since its mostly Decoherent histories without the many worlds why bother?

Thanks
Bill

 

[stevendaryl]

I consider MWI to be a research program, rather than a theory. That was basically the approach of Everett's original paper; it didn't introduce multiple universes or anything beyond ordinary quantum mechanics. It was an investigation into QM without a collapse hypothesis, and without the ad hoc splitting of the universe into observer + system. If the observer himself is a system described by quantum mechanics, then how can we make sense of our observations.

Everett's answer was to show how the appearance of collapse (or technically, how pure states can evolve into mixed states) can be explained from within unitary evolution.

To the extent that it can't be completely explained---that is an important fact about QM, regardless of whether you "buy" the MWI or not. Contrarily, to the extent that the appearance of collapse can be explained within unitary evolution tells us something about whether QM needs an additional assumption beyond unitary evolution.

If it's really true that MWI is nonsense--well, it's the usual QM with certain assumptions removed. Logically, if a theory is nonsensical, then it can't become more sensible by adding additional assumptions. If MWI is nonsense, then so is QM.

 

[durant35]

A bit too 'philosophy' for my taste.

What I like about the book is its very theorem proof, theorem proof in style. It's interesting to see how it relates to the formalism and other interpretations - this especially applies to decoherent histories, the books that I have read leave some technical details out found in Walllace.

It didn't convince me of MW - in fact the opposite - since its mostly Decoherent histories without the many worlds why bother?

Thanks
Bill

Mr Bill, could you explain the decoherent histories approach in short terms (without too much mathematical formalism, if that's possible) and differentiate it from the many worlds approach?

Thanks in advance.

 

[durant35]

If it's really true that MWI is nonsense--well, it's the usual QM with certain assumptions removed. Logically, if a theory is nonsensical, then it can't become more sensible by adding additional assumptions. If MWI is nonsense, then so is QM.

I would say that it in fact is nonsensical because additional assumptions are neccessary, in a certain way you can say that QM is incomplete.

It is similar to seeing a car drive from the outside, you can see all of the components move and you may describe the car as being drived even if you don't see the engine on the inside. But the engine is necessary to make the car drive and for the act of driving itself to make any sense. In a analogous way, the wavefunction shouldn't be everything, there must be something more to make sense of the measurements.

P.S. Sorry if the analogy is poor, it makes some sense to me

 

[bhobba]

Mr Bill, could you explain the decoherent histories approach in short terms (without too much mathematical formalism, if that's possible) and differentiate it from the many worlds approach?

In it QM is the stochastic theory of histories.

I can't explain a history without that. But it's simply a sequence of projection operators.

Thanks
Bill

 

[atyy]

Since you are an amateur, studying MWI would not be a waste of time. There is a lot of serious work in it, well worth studying to figure out why MWI remains problematic.

https://arxiv.org/abs/0712.0149
https://arxiv.org/abs/quant-ph/0209123

 

[stevendaryl]

I would say that it in fact is nonsensical because additional assumptions are neccessary, in a certain way you can say that QM is incomplete.

It's certainly possible that QM is incomplete.

 

[tom.stoer]

MWI takes "no collapse" idea to extreme and adds nothing else

Which is not bad ...

while Bohmian mechanics ... has other nice features ...

... like no clue for quantum field theory?

 

[tom.stoer]

The aim of the natural sciences is to provide an as accurate as possible description of nature concerning reproducible phenomena. It answers "how questions" rather than "why questions".

You are right regarding the answers, but not regarding the questions.

Reading the debates of Einstein, Bohr, Heisenberg and Weizsäcker you find - and I am sure you know - that they have been concerned with "why-questions".

There's no way to tell, why this is so, and to introduce funny concepts like parallel universes which are not observable in principle is, from the point of view of physics, unnecessary and doesn't lead to any additional insight. Some people feel the necessity for such additional "metaphysical" concepts ...

One does not introduce "parallel universes" (very polemic, by the way), one simply accepts them as predictions of quantum mechanics! These "branches" are there microscopically, their effects are well-known, visible and testable (e.g. double-slit).

In the case of MWI, it's not a matter of introducing parallel universes, it's a matter of trying to understand the implications of theories we already working with. Whether it leads to any additional insights is a matter of opinion.

Exactly!

What one introduces by hand is a magical collaps to get rid of macroscopic parallel branches, simply b/c one does not like them.

That's OK when looking at quantitative and testable predictions, but it does not tell us anything else but "quantum mechanics is working in practice". Since we know for decades that this is true, it might be the right time to ask new questions to get a deeper understanding on the meaning of quantum mechanics. This is what the Everett interpretation does.

 

[tom.stoer]

Well a bloke called Andrew Gleason came up with this theorem.

Gleason's theorem tells us that if we want to introduce a probability measure in QM then it has to comply with Born's rule; it's the unique probability measure in Hilbert space.

Gleason's theorem does not force us to introduce a probability measure at all, nor does it explain why we should do so.

 

[tom.stoer]

BTW, when I mentioned and referenced the MWI thread in one of the previous posts (https://www.physicsforums.com/threa...-the-many-worlds-interpretation.706927/page-5) I had a strong impression in that thread that you were insisting on right kind of questions regarding the lack of clear probability meaning, your posts were basically all the issues that don't satisfy me in the MWI.

May I ask, what has changed from then, that you now support MWI despite the counter-arguments you provided there?

Thanks for your post and for asking.

I am still concerned with these questions, meaning that I am still not satisfied with the "answers" I have received.

But my point is that asking questions is the best way to learn, so I am very disappointed reading "interpretations" not asking questions at all.

Another issue I have with the collaps is that logically (not historically) it "postulates away" mathematical structures that can be derived from the axioms of quantum mechanics (the reduced set of axioms w/o the collaps).

A very serious issue I see is the criticism that the Everett Interpretation introduces or postulates something new; it does not! So these arguments against the Everett Interpretation are fundamentally flawed.

All in all for me the "modern" variant of the Everett Interpretation - backed up by decoherence - with it's questions, open issues etc. is still more acceptable than other directions or interpretations trying to avoid or deny these key issues.

I am a Platonist, so I am convinced that the mathematical structures in our theories do reflect some aspects of external, independent but somehow "veiled" reality. This is impossible in any collaps interpretation, since the collaps contradicts the unitary time evolution; so not both can reflect reality. It is unsatisfactory in the ensemble interpretation since this interpretation does not make any statements regarding individual quantum systems. And it is unsatisfactory in purely instrumentalist interpretations providing no idea why (sic!) mathematical theories do describe nature.

Of course I admit that some of these reasons are irrelevant if one does not take up a platonistic position - but this is another story.

 

[Denis]

If it's really true that MWI is nonsense--well, it's the usual QM with certain assumptions removed. Logically, if a theory is nonsensical, then it can't become more sensible by adding additional assumptions. If MWI is nonsense, then so is QM.

Wrong. A theory may be nonsensical as a physical theory if it does not contain the world we observe around us. This state can be reached by removing the world around us, or essential parts of it, from the theory.

I cannot really judge if MWI is QM with certain assumptions removed, because it makes no sense to me - that means, I have never seen a more or less consistent, meaningful description of it which could be used as a base to justify such a claim or its rejection.

Whatever, or it contains the world I live in, in this case it has to contain also a lot of other words, thus, contains more than dBB, thus, more than quantum theory. Or it does not contain it, in which case it makes no sense. While, instead, dBB, which contains it, makes sense.

 

[A. Neumaier]

One does not introduce "parallel universes" (very polemic, by the way), one simply accepts them as predictions of quantum mechanics! These "branches" are there microscopically,

No. What is there are wave functions, not parallel worlds. Predicted by quantum mechanics are not parallel universes but probabilities for measuring things in the unique world that we know of. The whole talk about worlds is completely irrelevant for the predictions. The collapse is not a consequence of world splittings - nothing splits at all anywhere in the quantum formalism. instead it is - as decoherence shows - a consequence of the approximations made when one replaces an interacting multiparticle system by an idealized isolated system together with an instantaneously responding measurement device.

The problem is that if you do not study the Everett interpretation at least in some detail, you don't get the concept right.

I studied Everett's thesis in some detail and found that he assumed silently what he set out to prove. See my page http://arnold-neumaier.at/physfaq/topics/everett.html

 

[durant35]

Thanks for your post and for asking.I am a Platonist, so I am convinced that the mathematical structures in our theories do reflect some aspects of external, independent but somehow "veiled" reality. This is impossible in any collaps interpretation, since the collaps contradicts the unitary time evolution; so not both can reflect reality. It is unsatisfactory in the ensemble interpretation since this interpretation does not make any statements regarding individual quantum systems. And it is unsatisfactory in purely instrumentalist interpretations providing no idea why (sic!) mathematical theories do describe nature.

Of course I admit that some of these reasons are irrelevant if one does not take up a platonistic position - but this is another story.

Well, I am also unsatisfied with the shut up and calculate approach and as you I am seeking a decent description of external, independent reality out there. My only difference is that the MWI doesn't satisfy me as the theory of what's out there due to issues that are mentioned so many times already that you know them even before you notice my post. As I said, something more is neccessary.

I don't see how this is impossible in the collapse interpretation, after all the one selected branch after the measurement continues to evolve unitarily according to the Sch equation. Unitarity is still preserved in a kind of restrictive, exclusive way. This indeed satisfies me much more than imagining that each human evolves into nearly identical twins which are differentiated by possibly just one atom in a different position etc.. So saying that a collapse "contradicts" it is too hard. Sure, it doesn't explain what happens with other branches, but at least some of the elements of the Schrodinger evolution are preserved. Do you at least agree with the latter sentence?

And also, I appreciate that you as a supporter of MWI are indeed aware of some problems of it and criticisms. You openly aknowledge that more work is needed in whatever direction regarding QM, unlike some other members here which strike you with this incomplete idea of many-worlds which they present as a sure thing like gravity or some other natural law that we know. This only adds confusion to QM newbies who are striked by the fuzziness of QM formalism itself and presenting many-worlds in a "oh, it's a sure thing" kind of way is just a step too far.

 

[PeterDonis]

I studied Everett's thesis in some detail and found that he assumed silently what he set out to prove. See my page http://arnold-neumaier.at/physfaq/topics/everett.html

As I read this argument, it appears to prove that it is impossible to have a unitary evolution that entangles a measured system with a measuring device unless there is only a single possible outcome of the measurement. In other words, that "measurement" as it is normally understood (i.e., applying to cases where there is more than one possible outcome) cannot possibly be a unitary process. Is that the intended result?

 

[atyy]

... like no clue for quantum field theory?

QFT is only an effective field theory. So to the extent that QFT can be captured by lattice gauge theory, Bohmian mechanics should be able to get QFT. The main problem is that we still don't have lattice gauge theory for chiral fermions in non-abelian gauge theory.

 

[atyy]

Gleason's theorem tells us that if we want to introduce a probability measure in QM then it has to comply with Born's rule; it's the unique probability measure in Hilbert space.

Gleason's theorem does not force us to introduce a probability measure at all, nor does it explain why we should do so.

The problem is not the probability measure - even a deterministic process can be described by a probability measure (eg. Liouville evolution in classical mechanics).

The problem with Gleason's is it assumes non-contextuality.

 

[tom.stoer]

No. What is there are wave functions, not parallel worlds.

I was not talking about worlds but about "branches":

One does not introduce "parallel universes" (very polemic, by the way), one simply accepts them as predictions of quantum mechanics! These "branches" are there microscopically, their effects are well-known, visible and testable (e.g. double-slit).

And they are there microscopically in a rather trivial manner, e.g. |spin up> + |spin down>. All what happens is that this somehow induces a kind of "branch structure" macroscopically, but of course in one single quantum state.

Please note the quotes when talking about "branches".

 

[bhobba]

The problem is not the probability measure - even a deterministic process can be described by a probability measure (eg. Liouville evolution in classical mechanics). The problem with Gleason's is it assumes non-contextuality.

Sigh. Someone gets it.

Really - is it an assumption that the outcome will occur with some probability? I mean - really? Is it an assumption it will rain or not tomorrow with some probability - is that an assumption? I suppose you could argue it but it would be really hair splitting. I think people, including my professor, in my old probability modelling class would look at you rather strange going that route. Still to each his/her own I suppose.

The key point is non-contextuality. But as I pointed out there is a theorem in MW that it must be non contextual. Is it faulty? People argue about the decision theory approach, but I haven't seen anyone argue about that theorem. I have been through it - it looks OK to me - but then again so does the decision theory approach so I may have goofed.

Thanks
Bill

 

[Demystifier]

... like no clue for quantum field theory?

See http://lanl.arxiv.org/abs/1703.08341 Sec. 4.3 for an ambitious clue.

 

[vanhees71]

You are right regarding the answers, but not regarding the questions.

Reading the debates of Einstein, Bohr, Heisenberg and Weizsäcker you find - and I am sure you know - that they have been concerned with "why-questions".One does not introduce "parallel universes" (very polemic, by the way), one simply accepts them as predictions of quantum mechanics! These "branches" are there microscopically, their effects are well-known, visible and testable (e.g. double-slit).Exactly!

What one introduces by hand is a magical collaps to get rid of macroscopic parallel branches, simply b/c one does not like them.

That's OK when looking at quantitative and testable predictions, but it does not tell us anything else but "quantum mechanics is working in practice". Since we know for decades that this is true, it might be the right time to ask new questions to get a deeper understanding on the meaning of quantum mechanics. This is what the Everett interpretation does.

I don't need a collapse, I need Born's rule. That's it. There's no necessity for any additional "interpretation" to what's known as the minimal interpretation. The double-slit experiment with, say, electrons doesn't show anything additional than the probability distribution predicted by minimally interpreted QT. There's no additional input necessary to predict its outcome than Born's rule and solving the Schrödinger equation with the appropriate boundary conditions to get the wave function (or transition matrix elements if you treat it as a scattering problem in QFT) to apply Born's rule to.

 

[zonde]

I don't need a collapse, I need Born's rule. That's it.

How do you deal with three polarizers experiment? The one where we place three polarizers (first at 0°, second at 45° and third at 90°) in the path of light beam.

 

[Demystifier]

How do you deal with three polarizers experiment? The one where we place three polarizers (first at 0°, second at 45° and third at 90°) in the path of light beam.

Bayesian update of information encoded in the wave function interpreted as a thinking tool, and not as an objective physical entity. This is what @vanhees71 means when he says that there is no collapse.

 

[vanhees71]

An idealized polarizer by definition absorbs photons polarized in one direction with 100% probability and let's through photons in the perpendicular direction with 100% probability. It's an ideal filter. Where do I need a collapse here? It's just a device constructed to filter photons according to their polarization. Indeed, Demystifier is right in saying that I choose my description according to the preparation procedure. That's also done in classical physics, and nobody talks about a collapse there either. To call this "Bayesian" is just to make the argument sound more hip ;-)).

 

[Demystifier]

That's also done in classical physics, and nobody talks about a collapse there either.

Indeed, classical physics can be formulated in a quantum-like language and using a quantum-like philosophy, as I presented in
https://arxiv.org/abs/quant-ph/0505143