The Many-Worlds Interpretation of QM

[bhobba]

MWI drives me to it.

No problemo - the only reason I mention it if you have a philosophical bent the answers from a guy like me, and I suspect some others who regularly post around here, may not satisfy.

Thanks
Bill

 

[Maui]

This can't be right, since (1) doesn't change any of the theory's predictions.

Different interpretations propose different routes to the same predictions. If the route is found to be wrong, so is the interpretation. The (1) assumption seems invalid in light of the experiments i cited earlier.

 

[Maui]

It can't invalidate it since MWI is simply bog standard QM with the measurement postulate removed - instead of the wavefunction changing it simply 'splits'. Thanks
Bill

That's the whole point - the experiments i mentioned in post 46, namely

the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc.

all require a measurement postulate based on the which-way information being available or not.

 

[bhobba]

That's the whole point - the experiments i mentioned in post 46, namely
all require a measurement postulate based on the which-way information being available or not.

Since they conform to bog standard QM they can't falsify it. Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

I am afraid if you are to convince me, and I suspect others that post around here, it has been falsified you will need to detail exactly in what way when decoherence occurs that one outcome is not selected from the ensemble (ie mixture), but rather all occur simultaneously in different worlds, in anyway leads to an experimentally different outcome in any of those worlds that is different to if it was selected as the only one. Since there is no way of telling the difference as an observer in one of those worlds there is no way to tell the difference.

What you have posted from my perspective doesn't tell me how its falsified - a lot more detail is required for me to take it seriously.

Thanks
Bill

 

[Maui]

Since they conform to bog standard QM they can't falsify it. Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

The MWI conforms to the standard QM via a different "route". That the interpretations reach the same target(predictions) doesn't mean that the hypothetical route(s) of the MWI or other interpretaions is right. The mathematical formalism says nothing about this route.

I am afraid if you are to convince me, and I suspect others that post around here, it has been falsified you will need to detail exactly in what way when decoherence occurs that one outcome is not selected from the ensemble (ie mixture), but rather all occur simultaneously in different worlds, in anyway leads to an experimentally different outcome in any of those worlds that is different to if it was selected as the only one. Since there is no way of telling the difference as an observer in one of those worlds there is no way to tell the difference.

No, again they all lead to the same predictions, but the way to the predictions is different for the different interpretations. There is nothing(AFAIK) that prevents decoherence to take place when information about an otherwise contextual system becomes available(you isolate it from the environemnt - and no information can be extracted and the system returns to its quantum state - sometimes measured and verfied through weak measurements). When you don't isolate it(and information is constantly available) the system is mostly in its classical, particle-like state(coherence is destriyed).

I have no agenda or point to prove. As many of the others here i just want to know better what is going on, that's all.

Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

If you take seriously the weak measurements experiments that 'show' superpositions of states, i am afraid the BI has to go as well.

 

[bhobba]

No, again they all lead to the same predictions, but the way to the predictions is different for the different interpretations.

If they all lead to the same predictions how can it be falsified compared to another theory that has the same predictions?

Specifically take the MSI (Minimal Statistical Interpretation) of the experiments you cite - since this is bog standard QM it must predict the results of those experiments - if it didn't that would be big news because QM would have been falsified. Now the only difference between MSI and MWI is that the wave function did not collapse - instead it split into a number of worlds with each world experiencing a different possible outcome. To an observer in any of those worlds there is no way - none - zero - zilch - for them to tell the difference between MWI and the MSI. Because of that since the experiments you cite all conform to the MSI it must conform to MWI - there is no way it can't - its impossible.

To put it another way - yes the MWI is a different route than the MSI but the MWI has been deliberately concocted to be indistinguishable from the MSI.

Just to be 100% clear take for example the theory that forces do not cause objects to move, but rather it causes ghosts to move the objects. Its obvious this theory is a different route to objects moving but its just as obvious there is no way to tell the difference.

Thanks
Bill

 

[Fredrik]

Different interpretations propose different routes to the same predictions. If the route is found to be wrong, so is the interpretation. The (1) assumption seems invalid in light of the experiments i cited earlier.

That first sentence is only true about interpretations that change the mathematics of the theory. My (1) is a non-mathematical assumption that's added on top of QM. So every bit of QM remains intact. (1) is just a guess about what it all means.

 

[Quantumental]

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

 

[Maui]

If they all lead to the same predictions how can it be falsified compared to another theory that has the same predictions?

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. MWI requires something(interaction between wavefunctions to give the impression of 'particles') and that is not what the experiments i listed earlier show.

Specifically take the MSI (Minimal Statistical Interpretation) of the experiments you cite - since this is bog standard QM it must predict the results of those experiments - if it didn't that would be big news because QM would have been falsified.

You obviously think that the role of an interpretion is to make correct predictions, whereas its role is to explain the behavior of the respective system while arriving at the same predictions as the formalism. I have no idea why this point isn't obvious.

 

[Maui]

That first sentence is only true about interpretations that change the mathematics of the theory.

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

My (1) is a non-mathematical assumption that's added on top of QM. So every bit of QM remains intact. (1) is just a guess about what it all means.

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

 

[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.