- #176
PeterDonis
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MWI proponents claim this, but I have never seen any of them actually write down such a state.
you can so describe the two coarse grained states in a single pure state
Not a valid objection. Ever seen the state for an elephant? Does that stop elephants existing?MWI proponents claim this, but I have never seen any of them actually write down such a state.
What degrees of freedom does this pure state encompass? Also strictly speaking you can't, in QFT no finite volume system has a pure state.Yes you can so describe the two coarse grained states in a single pure state.
Her objection doesn't make sense to me. And would apply to all interpretations. She seems to be objecting to standard nomenclature.Where is she "playing with words" specifically? Also, where does she complain about deriving the Born Rule? She seems to address that in the comments where she elaborates:
http://backreaction.blogspot.com/20...howComment=1569568819019#c6700348820680074317
Do you ever see the wood for the trees?What degrees of freedom does this pure state encompass? Also strictly speaking you can't, in QFT no finite volume system has a pure state.
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.Do you ever see the wood for the trees?
Ever seen the state for an elephant? Does that stop elephants existing?
Do you ever see the wood for the trees?
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.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.
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?)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.
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.You may question the existence of such a pure state, but what is your basis?
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...)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.
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.
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."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.
That finite volume objects don't have pure states? See here:And what is the basis of this further assertion?
Interpretational issues (such as dead and alive cats existing in the same superposition) are not resolved by experiment.
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.
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.
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.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.
consistency, logic and extrapolation are important criteria
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.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.
This may be your personal opinion. But it is far removed from real quantum physics.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.
If this is true, then interpretational issues are not physics. I'm not too eager to contradict you here ;-)).Interpretational issues (such as dead and alive cats existing in the same superposition) are not resolved by experiment.
Yes, a personal opinion, held for 40 years, shared by many other physicists.This may be your personal opinion. But it is far removed from real quantum physics.
And collapse is part of MWI, the difference being that in MWI the illusion of collapse is a derived epiphenomenon. Not an assumption.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.
No, you're quoting out of context. I said the reverse.If this is true, then interpretational issues are not physics.
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.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.
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.
How are they unresolved definitional issues? They're mathematically proved.And how do unresolved definitional issues in QFT
https://en.m.wikipedia.org/wiki/Haag's_theoremHow are they unresolved definitional issues? They're mathematically proved.
What has Haag's theorem got to do with any of this?
If you're saying nothing, I am happy with that.What has Haag's theorem got to do with any of this?
I mean what has it got to do with the statement that states are mixed in QFT?If you're saying nothing, I am happy with that.
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.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.
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 mean what has it got to do with the statement that states are mixed in QFT?
I linked a paper above explaining it on this thread in #190.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 ;-)).
It's true, Bell provided a nice framework for testing the EPR predictions of QM.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.
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.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.