I Danger for the Many-Worlds Interpretation?

[DarMM]

Do you ever see the wood for the trees?

You're claiming there is a pure state. Three of us here with a good deal of knowledge of QM don't see how there could be such a pure state. Rather than provide a proper answer you suggest each of us is limited in our thinking.

Take non-relativistic QM, to ignore the issue of no pure states in QFT, what degrees of freedom is this a pure state of?

 

[PeterDonis]

Ever seen the state for an elephant? Does that stop elephants existing?

Of course not, but every elephant that anyone has ever observed existing has been either alive or dead. I'm not the one claiming that elephants can be in a superposition of being alive and being dead; you are. That's the state whose existence is being questioned.

 

[PeterDonis]

Do you ever see the wood for the trees?

The rules of this forum are clear that, while you can state your opinion, your opinion is not the same as fact. It is also not the same as actual math. Continuing to reiterate the same opinions while not offering any backup even when repeatedly asked for it is not permitted.

 

[Michael Price]

The rules of this forum are clear that, while you can state your opinion, your opinion is not the same as fact. It is also not the same as actual math. Continuing to reiterate the same opinions while not offering any backup even when repeatedly asked for it is not permitted.

And the opinion I gave once which you quoted (not given repeatedly, as you seem to imply) was that the approach taken by some is not helpful nor constructive. Your criticism, I might add, I *have* responded to elsewhere.

 

[Michael Price]

Of course not, but every elephant that anyone has ever observed existing has been either alive or dead. I'm not the one claiming that elephants can be in a superposition of being alive and being dead; you are. That's the state whose existence is being questioned.

You may question the existence of such a pure state, but what is your basis? The existence of such states is not contradicted by experiment, and is a logical extrapolation of microscopic physics and standard nomenclature, which is the whole point of MWI. (Unless you are simply saying coarse grained pure states do not exist by the collapse postulate, i.e. by intepretation?)

 

[DarMM]

You may question the existence of such a pure state, but what is your basis?

That not everything exists in a pure state (e.g. one member of an entangled pair) and that the actual algebra of observables does not support the pure state required.

 

[Michael Price]

That not everything exists in a pure state (e.g. one member of an entangled pair) and that the actual algebra of observables does not support the pure state required.

And what is the basis of this further assertion? Note my assertion was backed by reference to MWI, which has extensive supporting literature. If you're asserting dead and alive cats can't exist in a superposition, well there's plenty of literature that contradicts that assertion out there. (I would not normally belabour the literature issue on such foundational issues, but since it seems to cause distress if I don't...)

 

[PeterDonis]

If you're asserting dead and alive cats can't exist in a superposition, well there's plenty of literature that contradicts that assertion out there.

Really? There is experimental evidence for cats existing in a superposition of being dead and being alive?

Of course there is plenty of literature asserting support for the MWI, but in the absence of experimental evidence such assertions remain speculative.

Also, the issues @DarMM is raising are theoretical, and simply pointing out that there is literature asserting that the MWI is correct does not respond to such theoretical issues. We are looking for references to literature that responds specifically to the points @DarMM is raising.

 

[Michael Price]

Really? There is experimental evidence for cats existing in a superposition of being dead and being alive?

Of course there is plenty of literature asserting support for the MWI, but in the absence of experimental evidence such assertions remain speculative.

Interpretational issues (such as dead and alive cats existing in the same superposition) are not resolved by experiment. The Everettian perspective (which I share) is that all evidence supports the MWI stance, since MWI is the rational, minimalist explanation - indeed the only such explanation, IMO. Of course I realize that not everyone agrees with us. But I think David Deutsch put it most clearly when he said (paraphrasing): "Calling many worlds an interpretation is like calling dinosaurs an interpretation of the fossil record."

The take away message is that which intepretation you subscribe to very much colours or filters your view other theories and evidence. Everybody should remember that and not get upset when other people don't share your views on what constitutes evidence, or what is blindingly obvious or ridiculous.

 

[DarMM]

And what is the basis of this further assertion?

That finite volume objects don't have pure states? See here:
https://arxiv.org/pdf/1401.2652.pdfSection 4.1.2

 

[PeterDonis]

Interpretational issues (such as dead and alive cats existing in the same superposition) are not resolved by experiment.

That's true, and you should recognize what it means. It means that you cannot state your preferred interpretation as fact (and you will note that the rules for this forum say as much) and you cannot offer literature that favors your preferred interpretation as evidence for claims of fact. Literature that favors the MWI does not show that the MWI is true, and it certainly does not constitute evidence that cats can in fact exist in a superposition of being dead and being alive. All it shows is that there are people who believe the MWI is true. Which is no news to anyone.

The take away message is that which intepretation you subscribe to very much colours or filters your view other theories and evidence.

You just said interpretation questions can't be resolved by experiment. That means there can't be any evidence for or against any interpretation.

 

[Michael Price]

You just said interpretation questions can't be resolved by experiment. That means there can't be any evidence for or against any interpretation.

No experimental evidence. But still consistency, logic and extrapolation are important criteria.

 

[A. Neumaier]

You just said interpretation questions can't be resolved by experiment. That means there can't be any evidence for or against any interpretation.

No experimental evidence. But still consistency, logic and extrapolation are important criteria.

The Everettian perspective (which I share) is that all evidence supports the MWI stance, since MWI is the rational, minimalist explanation - indeed the only such explanation, IMO.

In my opinion, no evidence at all supports the MWI stance, since MWI is the irrational, minimalist non-explanation. It is neither consistent nor logical, and extrapolates from one existing world to innumerable in principle unobservable (and indeed very ill-defined) worlds. It is a ''many words'' interpretation with many words (sic!) added to the unitary dynamics to make it seem (to those with sufficiently diluted standards) to produce an explanation for the observation of unique measurement results in the only world we have access to.

 

[PeterDonis]

consistency, logic and extrapolation are important criteria

Yes, these are valid theoretical considerations. So are the other theoretical considerations raised by others in this thread.

 

[Michael Price]

In my opinion, no evidence at all supports the MWI stance, since MWI is the irrational, minimalist non-explanation. It is neither consistent nor logical, and extrapolates from one existing world to innumerable in principle unobservable (and indeed very ill-defined) worlds. It is a ''many words'' interpretation with many words (sic!) added to the unitary dynamics to make it seem (to those with sufficiently diluted standards) to produce an explanation for the observation of unique measurement results in the only world we have access to.

No doubt I'll be criticized for 'forum rules violation' if I don't respond, so... The 'many words' accusation, levied against MWI, is particularly laughable, since MWI does away with collapse - and collapse has generated realms of 'words' over the last 90+ years. A magical, ill-defined, vitalist, non-unitarity process, collapse has no place in quantum mechanics, or even science. The attempt, by many, to say collapse is just 'updating' the wavefunction with information shows a failure to understand or acknowledge the distinction between classical and quantum physics.

Notions of measurement, observers and observation have no place in fundamental physics. As Einstein said, in response to an early talk Heisenberg gave, these concepts should emerge from fundamental physics, not be hard-wired in. MWI takes Einstein's critique seriously, and remains the only rational, fully scientific resolution of the measurement problem in quantum mechanics.

 

[A. Neumaier]

MWI does away with collapse - and collapse has generated realms of 'words' over the last 90+ years. A magical, ill-defined, vitalist, non-unitarity process, collapse has no place in quantum mechanics, or even science.

This may be your personal opinion. But it is far removed from real quantum physics.

The collapse (and its more realistic generalization in the form of quantum instruments, quantum operations, or quantum channels) was from the start of modern quantum mechanics (and still is today) an indispensable, objective property of all finitely extended quantum systems (which are open and hence do not satisfy unitary dynamics) that may change when passing an instrument. There is nothing ill-defined - the parameters involved in their accurate description can be measured by quantum tomography.

 

[vanhees71]

Interpretational issues (such as dead and alive cats existing in the same superposition) are not resolved by experiment.

If this is true, then interpretational issues are not physics. I'm not too eager to contradict you here ;-)).

However, it's not completely true either. E.g., Einstein's hypothesis of deterministic Hidden-Variable models as an interpretation for the probabilities of QT is an idea to resolve interpretational issues he and many other physicists (including Schrödinger, one of the founding fathers of QT) have had with the probabilistic interpretation a la Born.

Now, obviously about alternative physical theories to some more or less established theory can only be decided by experiment. The problem was to find an experimental test to decide which theory is a better description of nature for quite some time, and it was Bell who provided a theoretically possible test in checking his inequality valid for local deterministic hidden-variable theories but contradicting QT. He considered this test as non-feasible at the time he published the idea, but feasible or not, it brought a hitherto completely philosophical question into the realm of hard science, i.e. an issue which in principle can be decided by experiment. It made the then not very favorable subject of interpretational issues (a job killer for young scientists, and Bell himself always told people who wanted to get involved with it to do so only after getting tenured, as he did too) a respectable scientific topic, and that's why experimentalists dared to take up the challenge (I guess the first one was Aspect using a atomic cascade to prepare entangled biphotons in a controlled way for the first time).

Nowadays the issue is settled with exceptional significance in favor of QT, and the experimental techniques developed are not in the realm of engineering with applications already there on a commercial basis (quantum cryptography) and some on the edge of being realized (quantum computers).

If think this indeed has proven the "interpretational issues" of yesterday to be the science of today and new technology in the (very near) future.

 

[Michael Price]

This may be your personal opinion. But it is far removed from real quantum physics.

Yes, a personal opinion, held for 40 years, shared by many other physicists.

The collapse (and its more realistic generalization in the form of quantum instruments, quantum operations, or quantum channels) was from the start of modern quantum mechanics (and still is today) an indispensable, objective property of all finitely extended quantum systems (which are open and hence do not satisfy unitary dynamics) that may change when passing an instrument. There is nothing ill-defined - the parameters involved in their accurate description can be measured by quantum tomography.

And collapse is part of MWI, the difference being that in MWI the illusion of collapse is a derived epiphenomenon. Not an assumption.

 

[Michael Price]

If this is true, then interpretational issues are not physics.

No, you're quoting out of context. I said the reverse.

I'm not too eager to contradict you here ;-)).

However, it's not completely true either. E.g., Einstein's hypothesis of deterministic Hidden-Variable models as an interpretation for the probabilities of QT is an idea to resolve interpretational issues he and many other physicists (including Schrödinger, one of the founding fathers of QT) have had with the probabilistic interpretation a la Born.

Now, obviously about alternative physical theories to some more or less established theory can only be decided by experiment. The problem was to find an experimental test to decide which theory is a better description of nature for quite some time, and it was Bell who provided a theoretically possible test in checking his inequality valid for local deterministic hidden-variable theories but contradicting QT. He considered this test as non-feasible at the time he published the idea, but feasible or not, it brought a hitherto completely philosophical question into the realm of hard science, i.e. an issue which in principle can be decided by experiment. It made the then not very favorable subject of interpretational issues (a job killer for young scientists, and Bell himself always told people who wanted to get involved with it to do so only after getting tenured, as he did too) a respectable scientific topic, and that's why experimentalists dared to take up the challenge (I guess the first one was Aspect using a atomic cascade to prepare entangled biphotons in a controlled way for the first time).

Nowadays the issue is settled with exceptional significance in favor of QT, and the experimental techniques developed are not in the realm of engineering with applications already there on a commercial basis (quantum cryptography) and some on the edge of being realized (quantum computers).

If think this indeed has proven the "interpretational issues" of yesterday to be the science of today and new technology in the (very near) future.

I'm not sure what your point is here, probably because you started of in the wrong direction (see above), but 'all' Bell provided was a test that showed whether or not the equations of quantum mechanics are obeyed. And then Aspect showed that QM passed this test. All fine of course, but not Earth-shattering.

 

[Michael Price]

Also, the issues @DarMM is raising are theoretical, and simply pointing out that there is literature asserting that the MWI is correct does not respond to such theoretical issues. We are looking for references to literature that responds specifically to the points @DarMM is raising.

And how do unresolved definitional issues in QFT relate to the collapse of the wavefunction in quantum mechanics? They don't seem to bear on the measurement problem.

 

[DarMM]

And how do unresolved definitional issues in QFT

How are they unresolved definitional issues? They're mathematically proved.

 

[Michael Price]

How are they unresolved definitional issues? They're mathematically proved.

https://en.m.wikipedia.org/wiki/Haag's_theorem

 

[DarMM]

https://en.m.wikipedia.org/wiki/Haag's_theorem

What has Haag's theorem got to do with any of this?

 

[Michael Price]

What has Haag's theorem got to do with any of this?

If you're saying nothing, I am happy with that.

 

[DarMM]

If you're saying nothing, I am happy with that.

I mean what has it got to do with the statement that states are mixed in QFT?

 

[vanhees71]

No, you're quoting out of context. I said the reverse.

I'm not sure what your point is here, probably because you started of in the wrong direction (see above), but 'all' Bell provided was a test that showed whether or not the equations of quantum mechanics are obeyed. And then Aspect showed that QM passed this test. All fine of course, but not Earth-shattering.

No what Bell provided was a prediction concerning all theories which contradict quantum mechanics in assuming local determinism. The empirical decision is clearly that any such theory is not describing nature accurately but QT does. The merit of this ingenious idea is to have brought a vague philosophical quibble of some (pretty eminent) physicists and also some philosophers into the realm of scientifically decidable alternatives, i.e., it made vague philosophical arguments into an empirically decidable scientific question. Why the philosophers and obviously also some physicists cannot accept this finding, I can't say.

 

[vanhees71]

I mean what has it got to do with the statement that states are mixed in QFT?

I hope you explain finally, or at least quote some papers, about this enigmatic statement. No matter, whether you have QT formulated in the 1st-quantization formalism or as a QFT, for a physicist the distinction between a pure and a mixed state is very simple: If the stat. op. is a projector, it describes a pure otherwise a mixed state. Why are you claiming that "states are mixed in QFT". At least the vacuum is a pure state (at least if it's non-degenerate ;-)).

 

[DarMM]

I hope you explain finally, or at least quote some papers, about this enigmatic statement. No matter, whether you have QT formulated in the 1st-quantization formalism or as a QFT, for a physicist the distinction between a pure and a mixed state is very simple: If the stat. op. is a projector, it describes a pure otherwise a mixed state. Why are you claiming that "states are mixed in QFT". At least the vacuum is a pure state (at least if it's non-degenerate ;-)).

I linked a paper above explaining it on this thread in #190.

Sorry I was being brief with @Michael Price , if you see #190 you'll see that it's finite volume states that are mixed. Global states like the vacuum can be pure. Although the purity of even global states is an open question for theories with massless particles.

 

[Michael Price]

No what Bell provided was a prediction concerning all theories which contradict quantum mechanics in assuming local determinism. The empirical decision is clearly that any such theory is not describing nature accurately but QT does. The merit of this ingenious idea is to have brought a vague philosophical quibble of some (pretty eminent) physicists and also some philosophers into the realm of scientifically decidable alternatives, i.e., it made vague philosophical arguments into an empirically decidable scientific question. Why the philosophers and obviously also some physicists cannot accept this finding, I can't say.

It's true, Bell provided a nice framework for testing the EPR predictions of QM.

 

[vanhees71]

I linked a paper above explaining it on this thread in #190.

Sorry I was being brief with @Michael Price , if you see #190 you'll see that it's finite volume states that are mixed. Global states like the vacuum can be pure. Although the purity of even global states is an open question for theories with massless particles.

Are you saying that in the finite-volume model, i.e., imposing periodic spatial boundary conditions with a cube as "quantization condition", there are no pure states? That doesn't make sense to me. To the contrary here the "plane waves" are even normalizable bona-fide Hilbert-space vectors, which is why this model is used to regularize the trouble with the infinite volume (which is an idealization of course too), where a lot of the formal trouble like the one provided by Haag's theorem is "regularized away". Of course the trouble is not gone, because to finally get a Poincare covariant theory you need to take the "appropriate infinite-volume limit", and that's far from being trivial.