The Many-Worlds Interpretation of QM

[1977ub]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

 

[bhobba]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

Well let's see - what you are asking is if someone believes in something some think is too weird to be true could this sap ones motivation for finding other interpretations others think is less weird? Maybe they are scientist enough to not let such things get in the way but who knows. If such things interest you maybe psychology would be a better subject to study.

Thanks
Bill

 

[bhobba]

Interesting. Can you list some other ideas in physics on this same level - unfalsifiable interpretations?

Yea - Lorentz Ether Theory (LET) vs Special Relativity (SR).

Your choice depends on how you think the world works but most reject LET because of its unobservable ether. Also when its extended to Quantum Field Theory you require extra ad-hoc assumptions. In fact the ether is an ad-hoc assumption like if you were to postulate forces cause unobservable ghosts to move objects rather than forces themselves - you can't prove it wrong - but most reject it as - well - silly - which it is.

There are probably others about as well.

Such things have more of a psychological origin than physical.

I have answered a number of your queries and they all are more along the lines of philosophy than physics - maybe a philosophy forum would be a bit more appropriate for what interests you. I have zero problems with answering them but I am a bit of an anti philosophy type sort of like Feynman was and the kind of answers I give may not satisfy those of a more philosophical bent.

Thanks
Bill

 

[bhobba]

GRW is falsifiable as it makes slightly different predictions than QM. So, in principle, such theories can be tested against the standard one.

Yea that's true - some of what are called interpretations are in fact different theories (eg primary state diffusion as well as the GRW you mention) - but sometimes physicists are a bit lax in terminology.

Thanks
Bill

 

[bhobba]

As far as i can see, it thus invalidates the MWI as well, which requires that interactions between wavefunctions create particle-like detections and classicality in an endless world splitting.

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'. To be more specific MWI includes decoherence (in fact it was the first decoherence based interpretation - by which is meant it includes the phenomena of decoherence in its foundations) so a superposition is transformed into an improper mixture. Instead of an observation selecting one of the states of the mixture they all exist simultaneously but in different worlds. Extremely neat way of resolving the problem - very neat - but can you stomach its implication - that's the issue.

Actually in discussing it in this thread I can see why its so appealing to its supporters - it really is elegant.

Thanks
Bill

 

[Nugatory]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

Because interpretations are not falsifiable, you can subscribe to whichever one you find most satisfying, and as long as it is satisfying to you there's no particular motivation to go looking for another one. But is this a problem? We're talking about an individual's aesthetic preference, which is why two different people speaking of MWI can say

it really is elegant.

and

If you need the generation of 100 billion different universes just to justify the eating of a seed by a mouse, the theory is as wasteful as a human being can ever imagine. Nothing could ever be more wasteful and uneconomical...

I see no reason to argue with either position. It's discussions like this one that send me back into my tiny cave where can I huddle in the darkness and clutch my favorite interpretation ("shut up and calculate") to my breast

 

[bohm2]

Another model that can be considered an alternative theory to QM is nonequilibruim Bohmian mechanics. But its hopes lie in finding such nonequilibrium violations of QM in cosmic microwave background, etc. Should such violations be discovered that would be evidence for Bohmian mechanics as a new post-quantum theory; however, without any such discovery Bohmiam would just be an interpretation like many of the others and arguably less so according to Valentini. Of course, there's quite a few "if"s:

Should inflation be very firmly established, and should it be found that the predictions of quantum theory continue to hold well at all accessible lengthscales during the inflationary era, then this would constitute considerable evidence against the hypothesis of quantum nonequilibrium at the big bang (though of course, nonequilibrium from an earlier era might simply have not survived into the inflationary phase). Furthermore, it would rather undermine the view that quantum theory is merely an effective description of an equilibrium state. In principle, one could still believe that hidden variables exist, and that the hidden variables distribution is restricted to quantum equilibrium even at the shortest distances and earliest times. But in the complete absence of nonequilibrium, the detailed behaviour of the hidden variables (such as the precise form of the trajectories in de Broglie-Bohm theory) would be forever untestable. While exact equilibrium always and everywhere may constitute a logically possible world, from a general scientific point of view it seems unacceptable, and the complete ruling out of quantum nonequilibrium by experiment would suggest that hidden-variables theories should be abandoned. On the other hand, a positive detection of quantum nonequilibrium phenomena in the early universe (or indeed elsewhere) would be of fundamental interest, opening up a new and deeper level of nature to experimental investigation.

Inflationary Cosmology as a Probe of Primordial Quantum Mechanics
http://lanl.arxiv.org/pdf/0805.0163.pdf

 

[bhobba]

It's discussions like this one that send me back into my tiny cave where can I huddle in the darkness and clutch my favorite interpretation ("shut up and calculate") to my breast

Otherwise known as the Minimum Statistical Interpretation (MSI)- but even that has slight variations - but its basically what I hold to as well.

I find it a bit of an unnerving experience discussing interpretations I don't hold to - but still interesting.

Thanks
Bill

 

[bhobba]

Another model that can be considered an alternative theory to QM is nonequilibruim Bohmian mechanics.

Yes work is being done all the time on what is usually thought to be an interpretation but may be a different theory.

A while ago there was a lot of discussion on if Bohmian Mechanics was a different theory and some thought it was with papers written about how it could be tested. In fact someone even did an experiment that supposedly falsified it.

I originally thought that was the case but it wasn't until I had a chance to discuss it with some on this forum I found the error - it had to do with a mistaken use of the Dirac Delta function. But it does show how difficult this stuff actually is.

Thanks
Bill

 

[1977ub]

Yea - Lorentz Ether Theory (LET) vs Special Relativity (SR).

Your choice depends on how you think the world works but most reject LET because of its unobservable ether. Also when its extended to Quantum Field Theory you require extra ad-hoc assumptions. In fact the ether is an ad-hoc assumption like if you were to postulate forces cause unobservable ghosts to move objects rather than forces themselves - you can't prove it wrong - but most reject it as - well - silly - which it is.

There are probably others about as well.

Such things have more of a psychological origin than physical.

I have answered a number of your queries and they all are more along the lines of philosophy than physics - maybe a philosophy forum would be a bit more appropriate for what interests you. I have zero problems with answering them but I am a bit of an anti philosophy type sort of like Feynman was and the kind of answers I give may not satisfy those of a more philosophical bent.

Thanks
Bill

MWI drives me to it.

 

[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.

 

[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.