The Many-Worlds Interpretation of QM

[Fredrik]

Why? The MWI and the copenhagen interpretation both propose very different routes to classicality. How do they change the formalism?

They don't of course. Apparently we disagree about what a "route to the predictions" would be. I would say that since the calculation of a prediction is independent of whether the person doing the calculation prefers the CI or the MWI, the "routes to the prediction" are exactly the same.

I would not say that the route to the prediction is different in a given interpretation, unless the definition of the interpretation starts with a reformulation of QM using a different set of assumptions. For example, I don't really know the consistent histories approach, but it seems to me that its supporters are using the ABL rule as an assumption instead of the Born rule. This could be considered a different route to the predictions.

QM is certainly intact but the assumption is hard to support for the experiments cited earlier.

A non-mathematical assumption added on top of QM that doesn't change the theory's predictions can't be proved wrong by experiments.

 

[Fredrik]

This thread is missing discussion on the two most recent and relevant papers on the Many Worlds interpretation.

Here is a interesting paper showing that Many Worlds is incoherent: http://philsci-archive.pitt.edu/9542/1/Decoherence_Archive.pdf

Here is another one showing that the preferred basis problem has not been solved:
http://arxiv.org/abs/1210.8447

Has the first one been published? The average quality of papers on the MWI is far too low for me to consider reading unpublished preprints, especially when they're written by philosophers

 

[mfb]

@Maui: The basic idea of MWI is the validity of quantum mechanics everywhere. A falsification of MWI would need a process which does not follow quantum mechanics. Something like the observation of a non-unitary process (like a "collapse") in progress, or whatever. The standard experiments you listed do not give that.

Has the first one been published? The average quality of papers on the MWI is far too low for me to consider reading unpublished preprints, especially when they're written by philosophers

I agree.

And the second one is a nice trick: It hides the events in the definition of a very special basis. Following the same argument, "nothing happens in classical mechanics". While this is a valid point of view, I do not follow it. I think something can happen in classical mechanics, and it is the same in MWI.

 

[bohm2]

And the second one is a nice trick: It hides the events in the definition of a very special basis. Following the same argument, "nothing happens in classical mechanics". While this is a valid point of view, I do not follow it. I think something can happen in classical mechanics, and it is the same in MWI.

Demystifier summarized that paper here:

To define separate worlds of MWI, one needs a preferred basis, which is an old well-known problem of MWI. In modern literature, one often finds the claim that the basis problem is solved by decoherence. What J-M Schwindt points out is that decoherence is not enough. Namely, decoherence solves the basis problem only if it is already known how to split the system into subsystems (typically, the measured system and the environment). But if the state in the Hilbert space is all what exists, then such a split is not unique. Therefore, MWI claiming that state in the Hilbert space is all what exists cannot resolve the basis problem, and thus cannot define separate worlds. Period! One needs some additional structure not present in the states of the Hilbert space themselves. As reasonable possibilities for the additional structure, he mentions observers of the Copenhagen interpretation, particles of the Bohmian interpretation, and the possibility that quantum mechanics is not fundamental at all.

Many Worlds proved inconsistent?
https://www.physicsforums.com/blog.php?b=4289

Ilja summarizing his papers made the same point here:

MWI in it's current form simply becomes invalid, with or without Born rule, because it does not have an additional structure which is necessary to fix the preferred basis: The papers prove that different choices are possible, and lead to different physics. The Copenhagen intepretation solves this problem with its association of the operators p, q with classical experimental arrangements, but this solution is not available in the Everett interpretation. Thus, to make MWI a (viable) intepretation, you not only have to derive the Born rule, but also have to add some new structure to fix the canonical preferred basis.

Why MWI?
http://onqm.blogspot.ca/2009/07/why-mwi.html

 

[E=mc4]

They would be forced to admit that the observer causes the wave function to collapse.

The other alternative under the "many worlds" hypothesis is that the observer creates infinite universes and realities.

Both have at the center the "observer".

The "many worlds" by trying to deny the role of the observer, only made it bigger.

 

[bhobba]

Here is a interesting paper showing that Many Worlds is incoherent: http://philsci-archive.pitt.edu/9542/1/Decoherence_Archive.pdf

Here is another one showing that the preferred basis problem has not been solved:
http://arxiv.org/abs/1210.8447

Purport to show - please - purport to show.

Without reading the papers standard textbooks on decohrence such as Schlosshauer's are pretty clear that it does solve the basis problem - not the complete measurement problem - but the basis problem for sure.

There has been all sorts of criticisms of MW over the years that it has not been worked properly etc etc but there is no proof its invalid. Nor can there be - its specifically concocted to be observationally indistinguishable from bog standard QM as found in, for example, the MSI.

Thanks
Bill

 

[bhobba]

Specifically, there are experiments that say specifically that it's the which-way information that causes wavefunctions to collapse, not interactions as the MWI requires.

Since the MWI has no wavefunction collapse how does the MWI require interactions for a wavefunctin collapse?

Thanks
Bill

 

[bhobba]

Has the first one been published? The average quality of papers on the MWI is far too low for me to consider reading unpublished preprints, especially when they're written by philosophers

Too true, too true.

Getting philosophers to agree on anything is impossible.

Thanks
Bill

 

[bhobba]

And the second one is a nice trick: It hides the events in the definition of a very special basis. Following the same argument, "nothing happens in classical mechanics". While this is a valid point of view, I do not follow it. I think something can happen in classical mechanics, and it is the same in MWI.

Thanks for reading it. Papers supposedly refuting what is found in standard textbooks is not my idea of interesting reading - they almost surely have an error.

Just so people are not taking my word for it from page 73 of Schlosshauer's textbook on Decoherence:
'The preferred states of a system emerge dynamically as those states that are the least sensitive, or the most robust, of the interactions with the environment, in the sense they become least entangled with the environment in the course of evolution and are thus immune to dechorence.'

Thanks
Bill

 

[Fredrik]

standard textbooks on decohrence such as Schlosshauer's are pretty clear that it does solve the basis problem

I didn't read the whole article, but I think the point is that while the interactions (i.e. decoherence) do select a preferred basis given a decomposition of the universe into subsystems, there's no preferred decomposition.

Edit: The paper makes one more point. This is the one mfb is talking about. The paper claims that there's always a decomposition such that there's no interaction between the subsystems. So if we use this decomposition, nothing is happening in the "worlds". I still don't know what to think about this.

Specifically, there are experiments that say specifically that it's the which-way information that causes wavefunctions to collapse, not interactions as the MWI requires.

What provides the environment with which-way information if not interactions?

 

[bhobba]

They would be forced to admit that the observer causes the wave function to collapse.

The other alternative under the "many worlds" hypothesis is that the observer creates infinite universes and realities.

Both have at the center the "observer".

The "many worlds" by trying to deny the role of the observer, only made it bigger.

Exactly where do you get the idea the MWI forces the observer to come into it? One version is that the world does not split into multiple realities but rather observers simply only experience one at a time. That is just one version - and not the generally accepted one either - although it does sound less weird to me than this splitting into a different world.

In bog standard MW it splits into a number of worlds when decoherence occurs independent of an observer.

Thanks
Bill

 

[bhobba]

I didn't read the whole article, but I think the point is that while the interactions (i.e. decoherence) do select a preferred basis given a decomposition of the universe into subsystems, there's no preferred decomposition.

Sure - this is related to the issue of why we get any outcomes at all - that is a genuine issue - but to say because of it the preferred basis problem has not been solved is stretching it a bit. To be sure its more correct to say the preferred basis problem has been solved with some very minimal assumptions most people would be inclined to accept.

Added Later:
Gave the paper a quick scan. From my reading it is the why we get any outcomes at all issue in another guise - that is a genuine issue for sure but like I said its pushing it a bit IMHO to say it invalidates decoherence selecting a preferred basis.

To be even clearer as my Schlosshauer quote said the preferred basis (and hence a natural decomposition) comes from systems that are not affected by decoherence - the issue of why outcomes occur at at all is why such systems exist in the first place. This is one of the areas more work needs to be done - sure - and my understanding is its an active area of research - but an intractable problem - I am not so sure about that.

Thanks
Bill

 

[E=mc4]

Exactly where do you get the idea the MWI forces the observer to come into it? One version is that the world does not split into multiple realities but rather observers simply only experience one at a time. That is just one version - and not the generally accepted one either - although it does sound less weird to me than this splitting into a different world.

In bog standard MW it splits into a number of worlds when decoherence occurs independent of an observer.

Thanks
Bill

The MWI main hypothesis is that the wave function does not collapse, but that all the probabilities within the function come into being because they were measured. Until the moment of measurement they were only, mathematical probabilities.

So while in the Copenhagen Interpretation of QM the observer collapses the many mathematical probabilities into just one tangible reality, in the MWI, all the mathematical probabilities turn into parallel realities at the moment of measurement. In both interpretations the measurement is made by an observer.

The moment of measurement is when you open the box to see if the cat is dead or alive. In the Copenhagen Interpretation the wave function collapses into either dead or alive, in the MWI at the moment of opening the box, the wave function does not collapse into just one reality, instead a parallel universe is produced. Both required a measurement made by an observer. Opening the box.

I personally think the MWI is BS.

 

[bhobba]

The MWI main hypothesis is that the wave function does not collapse, but that all the probabilities within the function come into being because they were measured. Until the moment of measurement they were only, mathematical probabilities.

I personally think MWI is BS as well.

However its not at the moment of measurement it splits - its once decoherence has occurred.

In Schrodinger's Cat its long before someone opens the box - its prior to the detector that breaks the vial when decoherence localizes any particle that may be emitted.

Thanks
Bill

 

[Maui]

They don't of course. Apparently we disagree about what a "route to the predictions" would be. I would say that since the calculation of a prediction is independent of whether the person doing the calculation prefers the CI or the MWI, the "routes to the prediction" are exactly the same.

I would not say that the route to the prediction is different in a given interpretation, unless the definition of the interpretation starts with a reformulation of QM using a different set of assumptions. For example, I don't really know the consistent histories approach, but it seems to me that its supporters are using the ABL rule as an assumption instead of the Born rule. This could be considered a different route to the predictions.

No, the experiments i cited favor collapse interpretations whereas the MWI is not a collapse interpretation.

A non-mathematical assumption added on top of QM that doesn't change the theory's predictions can't be proved wrong by experiments.

The assumption can be proven wrong any time of the day by experiment(even if the predictions of the theory remain intact and correct).

 

[mfb]

So while in the Copenhagen Interpretation of QM the observer collapses the many mathematical probabilities into just one tangible reality, in the MWI, all the mathematical probabilities turn into parallel realities at the moment of measurement. In both interpretations the measurement is made by an observer.

The moment of measurement is when you open the box to see if the cat is dead or alive. In the Copenhagen Interpretation the wave function collapses into either dead or alive, in the MWI at the moment of opening the box, the wave function does not collapse into just one reality, instead a parallel universe is produced. Both required a measurement made by an observer. Opening the box.

That is not the way MWI works. You do not have to add any assumption about splitting of worlds.To clarify my argument posted yesterday:

And the second one is a nice trick: It hides the events in the definition of a very special basis. Following the same argument, "nothing happens in classical mechanics".

Here is an example: Consider a classical harmonic oscillator, expressed in the usual variables position and momentum: it has a potential $V(x)=\frac{1}{2}m\omega^2x^2$, the kinetic energy $E_{kin}=\frac{p^2}{2m}$ and the equations of motion $\dot{p}=\omega^2 x$ and $\dot{x}=\frac{p}{m}$. Do we all agree that "something happens"?

We can re-write the same system in terms of E and ϕ with $E=\frac{1}{2}m\omega^2x^2 + \frac{p^2}{2m}$ and $\phi = \mathrm{atan2}(xm\omega,p)-\omega t$ (with atan2).
The equations of motion become $\dot{E}=0$, $\dot{\phi}=0$. Nothing happens in a classical harmonic oscillator?
I think the motion is hidden in that specific basis.

 

[Fredrik]

No, the experiments i cited favor collapse interpretations whereas the MWI is not a collapse interpretation.


The assumption can be proven wrong any time of the day by experiment(even if the predictions of the theory remain intact and correct).

The only thing you can do with experiments is to find out how accurate the theory's predictions are. They can't possibly tell us anything else. What you're saying is clearly impossible.

 

[bhobba]

That is not the way MWI works. You do not have to add any assumption about splitting of worlds.

That's correct - its a natural part of the formalism by literally accepting the reality of the quantum state. Once decoherence occurs the mixed state evolves with no change - no collapse. The basis of the mixed state is simply interpreted as individual worlds where that particular basis outcome occurred.

Like I said its really an elegant solution.

Thanks
Bill

 

[bhobba]

I think the motion is hidden in that specific basis.

Nice - like it.

Thanks
Bill

 

[Fredrik]

Sure - this is related to the issue of why we get any outcomes at all - that is a genuine issue - but to say because of it the preferred basis problem has not been solved is stretching it a bit. To be sure its more correct to say the preferred basis problem has been solved with some very minimal assumptions most people would be inclined to accept.

Added Later:
Gave the paper a quick scan. From my reading it is the why we get any outcomes at all issue in another guise - that is a genuine issue for sure but like I said its pushing it a bit IMHO to say it invalidates decoherence selecting a preferred basis.

If I understand it correctly (and it's certainly possible that I don't), it says that decoherence does select a preferred basis, given a decomposition into subsystems. But there's no preferred decomposition. I don't know if the article explains clearly why it considers that a problem. Personally, I think the only "problem" with it is that it prevents us from saying that a preferred basis identifies "the worlds that make up the universe".

I don't think a choice of decomposition is a "very minimal assumption that most people would be inclined to accept". I would think that every choice is equally valid, like when we select a coordinate system in SR. Since each choice determines a way to view the universe as consisting of "worlds", this is a real problem for those who think that there's a unique set of worlds that make up the universe. The impression I've been getting from the MWI stuff I've read (admittedly not that much, because I got frustrated over how badly written everything was) is that its supporters do think that there's a unique set of worlds that make up the universe.

Personally, I think that this idea is untenable. If we just let it go, it seems very natural to me to (if we insist on trying to interpret QM as a description of the universe) postulate something like "every 1-dimensional subspace of the Hilbert space of the universe represents a world". This eliminates the preferred basis problem. Now we can label the worlds selected by decoherence (given a decomposition) as "especially interesting worlds", instead of as "the worlds".

To be even clearer as my Schlosshauer quote said the preferred basis (and hence a natural decomposition) comes from systems that are not affected by decoherence - the issue of why outcomes occur at at all is why such systems exist in the first place.

The Schlosshauer quote talks about "the preferred states of a system" and how they are determined by the system's interactions with its environment. So it only says that given a decomposition, there's a preferred basis. It doesn't suggest that there's a preferred decomposition.

 

[bhobba]

The Schlosshauer quote talks about "the preferred states of a system" and how they are determined by the system's interactions with its environment. So it only says that given a decomposition, there's a preferred basis. It doesn't suggest that there's a preferred decomposition.

It becomes clearer as you read more of the reference I quoted from. But basically it singles out the basis of observables that commute with the interaction Hamiltonian between the system and the environment that is causing the decoherence - obviously they are the ones that don't vary with time. Mostly, from what I have read, that is something like a coulomb interaction and evidently the observable that tends to commute with is position.

I guess the assumption is given any interaction Hamiltonian a unique observable and hence basis exists that it commutes with. That at least one exists is the problem of any outcomes at all - but there is another one - there may be more than one - although I am unaware of any such cases.

Regarding the other stuff such as a unique set of worlds I think we run into semantic issues with MWI. Basically there is really only one universe but when decoherence occurs the basis it produces as the mixture that continues to evolve is thought of as separate worlds. This however is just a way to describe the situation - its not really a separate world - there is really only one world or universe. This doesn't mean I hold to the MWI - its far too 'extravagant' as one guy I seem to recall said about it with all these new 'worlds' being created exponentially and with a different version of me in every one of them.

Thanks
Bill

 

[HomogenousCow]

QM is bizzare

-Is QM a true theory?, i.e. is the universe probabilistic at the core?
I find this question and its implications highly disturbing. Now if it were true that QM is simply a "front" for a true deterministic theory, where did we go wrong? How did we stumble upon a completely statistical theory when we were not even looking for one? One would expect that we would simply have a less accurate theory, not a theory with a completely different structure and physical output.

-Are interpretations simply human rationalizations?
I find this possibility even more disturbing, the fact that perhaps we will never understand the true nature of nature, because it is simply inaccessible to our intuition, if not our mathematics.

-Just what exactly is a measurement?
If I walk into a room with electrons, in a sense I am preforming a measurement on all of them, in the sense that my body is interacting with the electrons. The point is that we can always deduce something about a system no matter what, hence we are always measuring everything. The measurement axiom of QM is probably the least mathematically well defined axiom in all of physics.

 

[Fredrik]

It becomes clearer as you read more of the reference I quoted from.

I disagree. I mean, you're talking about things that we all agree about, but these are things that can't shed any light on the issue that I've brought up. I don't know what I should try to explain better, since we're talking about two different things.

I think you, me, Schlosshauer and that MWI article are all in complete agreement that the interaction between subsystems of the universe selects a basis. But you can't even begin to talk about the interaction between the subsystems until you have chosen a decomposition of the universe into subsystems. So the preferred basis is only preferred relative to that decomposition, and the decomposition is arbitrary.

Note that the subsystems are not the worlds. They are things like "the cat" and "the cat's environment". "Bill + Bill's environment" is one decomposition. "Fredrik + Fredrik's environment" is another. Both are equally valid. Each one gives us a preferred basis for the Hilbert space of "the system", but these are different systems, and their Hilbert spaces are two different subspaces of the Hilbert space of the universe. So a basis for one of them isn't a basis for the other one.

But basically it singles out the basis of observables that commute with the interaction Hamiltonian between the system and the environment that is causing the decoherence - obviously they are the ones that don't vary with time. Mostly, from what I have read, that is something like a coulomb interaction and evidently the observable that tends to commute with is position.

I understand these things, but the "interaction Hamiltonian between the system and the environment" will depend on which part of the universe that you choose to call "the system". Given a decomposition of the universe into "the system" + "its enviroment" , the interaction selects a basis. But the decomposition is an arbitrary choice made by the person doing the calculation.

Regarding the other stuff such as a unique set of worlds I think we run into semantic issues with MWI. Basically there is really only one universe but when decoherence occurs the basis it produces as the mixture that continues to evolve is thought of as separate worlds. This however is just a way to describe the situation - its not really a separate world - there is really only one world or universe. This doesn't mean I hold to the MWI - its far too 'extravagant' as one guy I seem to recall said about it with all these new 'worlds' being created exponentially and with a different version of me in every one of them.

I chose to call the single physical system that QM supposedly describes "the universe", lacking a better word. Penrose has suggested "the omnium", but I haven't seen anyone else use it. In my usage, a "world" is not synonymous to "the universe", because the latter refers to a single physical system, and the (many) "worlds" are aspects of its properties.

 

[Fredrik]

-Is QM a true theory?, i.e. is the universe probabilistic at the core?
I find this question and its implications highly disturbing. Now if it were true that QM is simply a "front" for a true deterministic theory, where did we go wrong? How did we stumble upon a completely statistical theory when we were not even looking for one? One would expect that we would simply have a less accurate theory, not a theory with a completely different structure and physical output.

If there's a deterministic underlying theory, then I think "what went wrong" is just that it was much easier to find a theory that assigns probabilities to measurement results than to find one that describes what's actually happening.

-Are interpretations simply human rationalizations?
I find this possibility even more disturbing, the fact that perhaps we will never understand the true nature of nature, because it is simply inaccessible to our intuition, if not our mathematics.

I find it pretty disturbing too. Even if it's accessible to intuition and mathematics, it may still be inaccessible to science, in the sense that we may find several theories that look like they might be descriptions of what's actually happening, but they that don't give us any new predictions.

-Just what exactly is a measurement?
If I walk into a room with electrons, in a sense I am preforming a measurement on all of them, in the sense that my body is interacting with the electrons. The point is that we can always deduce something about a system no matter what, hence we are always measuring everything. The measurement axiom of QM is probably the least mathematically well defined axiom in all of physics.

The problem is that each theory consists of a purely mathematical part and a set of correspondence rules that tell us how to interpret the mathematics as predictions about results of experiments. The correspondence rules are always non-mathematical, and never really well-defined. This is disturbing too, but it's just the way it is, and we will never be able to do anything about it.

The funny thing about correspondence rules is that so far, science has done pretty well without ever writing down a complete set of correspondence rules for any theory. It would actually be really hard to write them down. Consider e.g. a correspondence rule about how to measure time. It would have to describe what sort of things we are allowed to think of as "clocks", and tell us how to build clocks. So how do we do write down this rule for classical SR, when the best clocks rely on QM? The only answer I've been able to come up with goes like this: We define a hierarchy of theories. In the level 0 versions of SR and QM, we describe measuring devices that are extremely easy to build, but probably not very accurate. For every positive integer n, when we write down the level n versions of SR and QM, the correspondence rules must be such that they can be followed by a person who understands the level n-1 theories and has access to level n-1 measuring devices.

 

[Maui]

The only thing you can do with experiments is to find out how accurate the theory's predictions are.

No, there exist interpretations that are not part of the perdictive theory and these interpretations can be tested for viability in experiements. Look at the quantum eraser by Kim et al and you'll notice how there are detectors(D0--D0) at all possible photon paths all the time but you only get quantum behavior(interference pattern)when the which-way information cannot be obtained(D1 and D2). Whenever the which-way information can be obtained(D3 and D4) for the idler photon, there is no interference pattern when compared at the coincidence counter with the signal photon. So it cannot be the detector nor its photons that cause the wavefunction to collapse to an eigenstate of the observable. It makes no sense to apply a world splitting to something so obvious as the quantum eraser as it would explain nothing about why the interference pattern happens to disappear whenever information about what the system was doing between observations was available.

They can't possibly tell us anything else. What you're saying is clearly impossible.

No, the role of the interpretations is to tell us more than the theory(qm). They don't have any other role and they actually tell us a lot more about the inner workings of the world that is not visible in the theory. If one believes it though.

 

[kith]

No, there exist interpretations that are not part of the perdictive theory and these interpretations can be tested for viability in experiements.

Please provide a peer-reviewed reference for this, especially for your claim that the MWI is falsified by Kim's experiment. I think you either have a serious misconception about the scientific use of the word "interpretation" or don't understand how the MWI works.

 

[bohm2]

No, the role of the interpretations is to tell us more than the theory(qm). They don't have any other role and they actually tell us a lot more about the inner workings of the world that is not visible in the theory. If one believes it though.

I'm not sure if I understand you but I think I agree with you, for the most part. I think interpretation is important because it seems that there is something more to physical reality (or even our models of physical reality) over and above the mathematics. Surely, "...it is because of the unbending nature of the world that we find the need to move, for example, from classical to quantum physics; that we find the need to revise our theories in the face of recalcitrant experience." But the problem is that we can't get to the physical world without using mathematics because non-mathematical versions of scientific theories just seem to be practically very difficult to do. But, even though the mathematics may be indespinsible and the mathematical equations we use ultimately decide what we believe about the physical world there still seems to be this difference between the mathematics and what the mathematics represents and this just adds fuel to many of the interpretative debates in quantum mechanics because there seems to be many interpretations that are arguably equally compatible (or relatively so) with the math.

 

[Maui]

Please provide a peer-reviewed reference for this, especially for your claim that the MWI is falsified by Kim's experiment. I think you either have a serious misconception about the scientific use of the word "interpretation" or don't understand how the MWI works.

Which part of what i said about the which-path information causing wavefunction collapse in the Kim experiement didn't you understand, so i can focus on it? Here is the original peer-reviewed paper with detectors at all photon paths in case you need a reference:

http://arxiv.org/pdf/quant-ph/9903047v1.pdf

To be sure the interference
pattern disappears when which-path information is obtained.
But it reappears when we erase (quantum erasure)
the which-path information [3,4]. Since 1982, quantum
eraser behavior has been reported in several experiments
[5];

 

[mfb]

The impression I've been getting from the MWI stuff I've read (admittedly not that much, because I got frustrated over how badly written everything was) is that its supporters do think that there's a unique set of worlds that make up the universe.

I think that some (many?) opponents of MWI get the impression that supporters claim that, and argue against that self-made claim.

 

[kith]

Which part of what i said about the which-path information causing collapse in the Kim experiement didn't you understand, so i can focus on it?

This is not about your description of Kim's experiment. You claim that Kim's experiment refutes the MWI, correct? I just ask you to back this claim up by a reference.

 

[Maui]

This is not about your description of Kim's experiment. You claim that Kim's experiment refutes the MWI, correct? I just ask you to back this claim up by a reference.

I claim that in the Kim experiement it's the which-path information that causes wavefunction collapse. This is also what the authors claim and is visible from the setup and the results. From it follows that NEITHER detectors NOR interactions between photons in this particular setup creates or destroys the interference pattern.

It seems that since you cannot explain the results of this experiment in terms of the MWI(you would have if you could), you hope you can just skip the technical details of the experiement and appeal to authorities. Physics does not deal with interpretations and their refutals, so you are free to continue to stick to whatever interpretation you think is peer-reviewed or favored by a your chosen group. I am done here.

 

[mfb]

I claim that in the Kim experiement it's the which-path information that causes wavefunction collapse.

To rephrase this, you use a non-MWI interpretation (collapses), and get results different from MWI.
How does this invalidate MWI?

It seems that since you cannot explain the results of this experiment in terms of the MWI(you would have if you could)

Just let the wave function evolve unitarily (and calculate amplitude-squares after the experiment if you like), it will give the same result.

 

[Maui]

To rephrase this, you use a non-MWI interpretation (collapses), and get results different from MWI.
How does this invalidate MWI?

You can control the collapse and when it happens, so yes it's experimentally observed. See the paper i cited earlier.

Just let the wave function evolve unitarily (and calculate amplitude-squares after the experiment if you like), it will give the same result.

This is not an interpretation of the repetive no interference results when which-path information is available. It's a restatement of the shut up and calculate approach, which is most definitely not an interpretation of quantum theory.

 

[Fredrik]

I think that some (many?) opponents of MWI get the impression that supporters claim that, and argue against that self-made claim.

As I wrote that line, I was thinking about the many false claims about what MWI supporters believe that had already been made in this thread, and I was wondering if I was doing the same. That's why I used weak phrases like "it seems to me".

The main reason why it seems that way to me is that even the MWI supporters are talking about "the preferred basis problem" as if it really is a problem. I don't see any reason why someone who isn't assuming that there's only one set of worlds that make up the universe would consider "the preferred basis problem" anything more than an interesting feature.

There's also the fact that no one ever talks about worlds that don't correspond to possible measurement results (not even to dismiss them as irrelevant), or worlds that correspond to a different choice of decomposition of the universe into subsystems. OK, the guy who wrote that article did, but he was using it to argue against the MWI.

 

[Fredrik]

No, there exist interpretations that are not part of the perdictive theory and these interpretations can be tested for viability in experiements.

If you mean that there are interpretations that make different predictions than the theory, then I would consider them different theories. And this is irrelevant to what we've been talking about anyway.

If you mean that experiments can test an assumption that has no effect on the theory's predictions, then this is obviously incorrect.

Look at the quantum eraser by Kim et al and you'll notice how there are detectors(D0--D0) at all possible photon paths all the time but you only get quantum behavior(interference pattern)when the which-way information cannot be obtained(D1 and D2).

So? This is just what the theory predicts. So it's obviously consistent with all interpretations of the theory.

No, the role of the interpretations is to tell us more than the theory(qm). They don't have any other role and they actually tell us a lot more about the inner workings of the world that is not visible in the theory. If one believes it though.

Right. But they don't change the theory's predictions, and experiments only check the accuracy of the predictions. Experiments test theories, not interpretations.