The Many-Worlds Interpretation of QM

[bhobba]

In an earlier era before QM would/could/should have MWI possibly been advanced to explain why dice rolls come up as random? Why or why not? Why is MWI better than just throwing up your hands?

Even in an earlier era it was known the reason dice came up random was lack of knowledge about the factors that went into it - such is not the case with QM - or rather if it is no one has been able to find them and certain theorems exist that show such if they existed would operate in a bit of a weird way eg contextually and non locally.

Its better than throwing up your hands because it takes the formalism of QM literally and to its logical conclusion.

Thanks
Bill

 

[1977ub]

Even in an earlier era it was known the reason dice came up random was lack of knowledge about the factors that went into it - such is not the case with QM - or rather if it is no one has been able to find them and certain theorems exist that show such if they existed would operate in a bit of a weird way eg contextually and non locally.

Its better than throwing up your hands because it takes the formalism of QM literally and to its logical conclusion.

Under superdeterminism one does not need to pick between contextual or nonlocal, correct?

 

[bhobba]

Under superdeterminism one does not need to pick between contextual or nonlocal, correct?

Super-determinism bypasses bells theorem and you can preserve locality, but it doesn't help with non-contextuality.

To the best of my knowledge no one has yet exploited the loophole and actually constructed such a theory - in fact from what I can gather many think its highly unlikely such a theory even exists - but I am no expert in the area and may stand corrected.

Thanks
Bill

 

[wuliheron]

In an earlier era before QM would/could/should have MWI possibly been advanced to explain why dice rolls come up as random? Why or why not? Why is MWI better than just throwing up your hands?

No explanation whatsoever means there is no particular place to begin investigating. That might be nice for artists, but doesn't cut it when you talk about building the largest machines in the world. Investors want to know the subject will be systematically analyzed.

 

[Fredrik]

In an earlier era before QM would/could/should have MWI possibly been advanced to explain why dice rolls come up as random? Why or why not? Why is MWI better than just throwing up your hands?

You seem to think that the MWI is just the idea "hey what if all the possibilities actually happen". That idea could certainly have been put forward before QM. Just say that each point of the phase space of the universe is a world that actually exists. This would have been a not entirely unreasonable speculation about what's really going on, but it's not suggested by the theory itself. You can certainly view classical mechanics as a description of what's actually happening to the universe without being led to the conclusion that all possibilities are real.

However, if we assume that QM is a description of what's actually happening to the universe, then we immediately find ourselves in many worlds territory. See e.g. how I argued against what I at the time thought people meant by "the Copenhagen interpretation" in the quote below. (Now I know that there are so many different ideas that are claimed to be "the CI" that the term is hardly useful anymore).

The main assumption of the CI is that state vectors can be identified with physical systems, i.e. that each state vector describes all the properties of the system it represents. Let's label that assumption (1). I said that if we add this on top of QM, we get a contradiction, but that's not quite right. What we get is many worlds. So QM+(1) contradicts the assumption that there's only one world. Let's label that assumption (2). Obviously, (2) should also be considered part of the definition of the CI.

So I'm not going to argue that QM+(1) is logically inconsistent, I'm going to argue that CI=QM+(1)+(2) is. The argument can't be made rigorous, since the assumptions (1) and (2) aren't mathematical statements. An informal argument is the best anyone can do.

The Schrödinger's cat thought experiment has taught us that the linearity of the SE implies that if microscopic systems can be in superpositions, then so can macroscopic systems. The details of this part of the argument are included both in Ballentine's 1970 article and in his more recent textbook. (Section 9.2).

(A calculation that includes decoherence effects would change the argument somewhat, but not enough to solve the problem).

Suppose that we prepare a large and complicated system, e.g. a system that includes you, in a state like |this>+|that>, where |this> and |that> describe two different experiences you can have in there. Now the problem is that (1) says that |this>+|that> is a complete description of the physical system. Clearly this means that neither |this> nor |that> can be a complete description of the physical system, and this means that what you actually experience as a part of that system is no more than half the story. If the complete description includes both of your possible experiences, then so does reality. Otherwise it wouldn't be a complete description.

Therefore QM+(1) implies that there are many worlds. This means that QM+(1)+(2) is inconsistent.

 

[Maui]

Quote by Fredrik

The main assumption of the CI is that state vectors can be identified with physical systems, i.e. that each state vector describes all the properties of the system it represents. Let's label that assumption (1). I said that if we add this on top of QM, we get a contradiction, but that's not quite right. What we get is many worlds.

Why would there be a contradiction?

lol Occam's razor is a guideline not a law of nature! Anyway I don't really see how MWI goes against it at all...

I never stated or implied it was a law.

Would you like to elaborate on why you think so?

As William of Ockham once said - "Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity."

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 and it also leads to quantum immortality which has never been observed(in this universe).

 

[Fredrik]

Why would there be a contradiction?

I think you stopped reading too soon or something. The next thing I said is "but that's not quite right. What we get is many worlds". The statement about the contradiction (in QM+(1)) was a reference to an incorrect statement I had made earlier in that thread. The contradiction is in QM+(1)+(2), as I explained.

As William of Ockham once said - "Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity."

But what really is "plurality"? An example that's often brought up in this context is the information required to specify a single integer vs. the information required to specify all the integers. There's no upper bound on how much information can be required to specify an integer, but you can specify all the integers with a simple recursive statement. The point of this is that it's not always the alternative that involves the larger number of "somethings" that's more complicated.

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 and it also leads to quantum immortality which has never been observed(in this universe).

There's nothing wasteful about it. QM consists of a few simple statements about a simple mathematical structure, and the MWI is essentially just the assumption that QM describes reality.

It would be at least as valid to argue that the MWI is favored by Occam's razor, because the alternative is that either there's a completely different piece of mathematics that describes what actually happens, or reality isn't described by mathematics at all. Both of these options can be considered unnecessary complications.

I'm not saying that we should believe in the MWI because of Occam or any other reason. I don't think Occam is of any help here.

I consider the quantum immortality argument to be nonsense. I don't believe it makes sense to assign a probability to the possibility that "you will survive" in that scenario with that very unorthodox definition of "you". And even if I'm completely wrong about that, the MWI doesn't predict that quantum immortality will be observed.

 

[1977ub]

Is MWI falsifiable? How is it better than MMRNG (many mysterious random number generators) ? What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

 

[Fredrik]

Is MWI falsifiable?

It's not. That's why it's called an interpretation, and not a theory.

How is it better than MMRNG (many mysterious random number generators) ?

How is that an interpretation of QM?

The MWI is simply the assumption that QM describes the universe.

What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

There's no new matter. (I don't care what that old "Everett FAQ" says). The worlds are "aspects" of the properties of a single physical system.

 

[1977ub]

It's not. That's why it's called an interpretation, and not a theory.

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

There's no new matter. (I don't care what that old "Everett FAQ" says). The worlds are "aspects" of the properties of a single physical system.

Do these "properties" have different subjective experiences & lives? (in this "interpretation").

 

[Fredrik]

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

All of the interpretations of QM.

If you don't want to make unfalsifiable assumptions, you need to focus on the fact that the only thing we know for sure about QM is that it's an assignment of probabilities to measurement results that agrees very well with experiments.

There is some room for interpretation of some classical theories as well. I think both SR and GR have, as an alternative to the standard geometrical stuff, an interpretation that says that the results of experiments and observations aren't caused by the geometry of spacetime, but by changes to the properties of the measuring devices and the objects on which we do measurements. I don't know this stuff well enough to elaborate. Such an interpretation of GR was mentioned in "Black holes and time warps: Einstein's outrageous legacy" by Kip Thorne.

Do these "properties" have different subjective experiences & lives? (in this "interpretation").

There's nothing subjective about the properties of that single physical system (here called "the universe", lacking a better word). But if we choose a decomposition of the universe into subsystems (e.g this piece of matter + everything else), then we are able to interpret what's happening to the universe as a description of what's happening in many different worlds.

I don't think I'd be able to elaborate much more than this. I don't think anyone has ever written anything really good about these things.

 

[bohm2]

All of the interpretations of QM.

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

 

[1977ub]

There's nothing subjective about the properties of that single physical system (here called "the universe", lacking a better word). But if we choose a decomposition of the universe into subsystems (e.g this piece of matter + everything else), then we are able to interpret what's happening to the universe as a description of what's happening in many different worlds.

I meant in the sense of observers possessing subjectivity. We have a cat who experiences being alive in one infinity of universes and dying ones in a different infinity. Branching off from the one problematic measurement, all the infinities being experienced each in its own classical way by different sentient observers - this is what I was thinking of.

 

[Nugatory]

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

Yes, it is true that some interpretations have been successfully falsified. Another example is that Bell-type experiments have pretty much excluded all interpretations that consider QM to be a statistical description of local hidden variables.

Of course if an an interpretation is falsified, we no longer consider it a valid interpretation... So every once in a while a proposed interpretation drops by the wayside. But this doesn't change Fredrik's basic point, which is that there a number of interpretations that are not currently falsifiable, MWI is one of them, and the only thing we can verify is that the statistical predictions of QM agree with experiment.

(It is, of course, not a coincidence that Bell's argument is statistical in nature).

 

[bohm2]

Of course if an an interpretation is falsified, we no longer consider it a valid interpretation... So every once in a while a proposed interpretation drops by the wayside. But this doesn't change Fredrik's basic point, which is that there a number of interpretations that are not currently falsifiable, MWI is one of them, and the only thing we can verify is that the statistical predictions of QM agree with experiment.

(It is, of course, not a coincidence that Bell's argument is statistical in nature).

I agree about MWI, but he argued that all of the interpretations of QM are unfalsifiable. GRW can be considered an alternative theory not just an interpretation, I think, because it is theoretically testable.

 

[Maui]

I think you stopped reading too soon or something. The next thing I said is "but that's not quite right. What we get is many worlds". The statement about the contradiction (in QM+(1)) was a reference to an incorrect statement I had made earlier in that thread. The contradiction is in QM+(1)+(2), as I explained.

Your (1) assumption is almost certainly wrong as has been found by the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc. As it turns out, it's the availability of the which-way information that destroys the interference pattern and not interactions with detector's photons or other obstacles. 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.

 

[Nugatory]

I agree about MWI, but he argued that all of the interpretations of QM are unfalsifiable. GRW can be considered an alternative theory not just an interpretation, I think, because it is theoretically testable.

I think we're all largely in agreement here... Once an interpretation becomes falsifiable we reclassify it as a theory not an interpretation (as suggested by your comfort with word "because" above) so it's a tautology to say that interpretations are not falsifiable.

At the risk of putting my words in Fredrik's mouth, I expect that he would consider a falsified proposition to be a dead theory not a live interpretation.

 

[Fredrik]

Your (1) assumption is almost certainly wrong as has been found by the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc.

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

 

[Fredrik]

I think we're all largely in agreement here... Once an interpretation becomes falsifiable we reclassify it as a theory not an interpretation (as suggested by your comfort with word "because" above) so it's a tautology to say that interpretations are not falsifiable.

At the risk of putting my words in Fredrik's mouth, I expect that he would consider a falsified proposition to be a dead theory not a live interpretation.

Yes, this is what I'm thinking.

 

[bhobba]

Is MWI falsifiable? How is it better than MMRNG (many mysterious random number generators) ? What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

MWI is as falsifiable to the extent QM is falsifiable. There however is no way to experimentally tell it from any other of the myriad of interpretations.

MWI does not postulate the creation of new matter and energy in the sense we generally think of it - it is preserved in any world it splits into - it simply takes literally the existence of a quantum state and has it as very real with an objective existence and doesn't have quantum state collapse as the result of an observation.

It is in fact a very elegant and mathematically beautiful interpretation - its just that many people like me can't stomach what it postulates and think its way too weird and not at all required. But that is a belief, an opinion, I have about how nature is - science is not about beliefs - its about correspondence with experiment. Just because I think it weird does not mean its not correct - it may well be.

Thanks
Bill

 

[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.