This is a very good post, but I wonder: One of the authors in the paper you cite is David Wallace, perhaps the most prominent proponent of modern Everettian interpretation.
He just published a new book called “The Emergent Multiverse” and he claims there is no problem unifying MWI with QFT because interactions within worlds are local and only states are nonlocal.
I have yet to hear him mention any need for serious reformulation of anything in terms of MWI.
You said you suspect this is necessary, but that you hope we can recover a similar MWI, but isn’t it more reasonable to expect that at the planck scale something else will explain the quantum weirdness?
After all if MWI fails both probability and relativity, then there is no good reason to suspect that this interpretation is correct.
Have you given Gerard ’t Hoofts idea of cellular automata which he claims salvage determinism, locality and realism any thought?
You said you suspect this is necessary, but that you hope we can recover a similar MWI, but isn’t it more reasonable to expect that at the planck scale something else will explain the quantum weirdness?
When I talk about recovering MWI, I really just mean absorbing the lesson that our theory does not need to deliver determinate measurement results, and ad hoc tools for satisfying this constraint (such as collapse or hidden variables) are otiose. Of course, the foundations of our eventual theory of quantum gravity might be different enough from those of quantum theory that the interpretational options don’t translate. How different the foundations will be depends on which program ends up working out, I suspect. If something like canonical quantum gravity or loop quantum gravity turns out to be the way to go, then I think a lot of the conceptual work done in interpreting NRQM and QFT will carry over. If string theory turns out to be on the right track, then maybe a more radical interpretational revision will be required. The foundations of string theory are now thought to lie in M-theory, and the nature of this theory is still pretty conceptually opaque. It’s worth noting though that Bousso and Susskind have actually suggested that string theory provides a solid foundation for MWI, and that the worlds in the string theory landscape are the same thing as the worlds in MWI. See here for more on this. The paper has been on my “to read” list for a while, but I haven’t gotten around to it yet. I’m skeptical but interested.
Have you given Gerard ’t Hoofts idea of cellular automata which he claims salvage determinism, locality and realism any thought?
I know of ‘t Hooft’s cellular automata stuff, but I don’t know much about it. Speaking from a position of admitted ignorance, I’m skeptical. I suspect the only way to construct a genuinely deterministic local realist theory that reproduces quantum statistics is to embrace superdeterminism in some form, i.e. to place constraints on the boundary conditions of the universe that make the statistics work out by hand. This move doesn’t seem like good physics practice to me. Do you know if ’t Hooft’s strategy relies on some similar move?
’t Hooft’s latest paper is the first in which he maps a full QFT to a CA, and the QFT in question is a free field theory. So I think that in this case he evades Bell’s theorem, quantum complexity theorems, etc, by working in a theory where physical detectors, quantum computers, etc don’t exist, because interactions don’t exist. It’s like how you can evade the incompleteness theorems if your arithmetic only has addition but not multiplication. Elsewhere he does appeal to superselection / cosmological initial conditions as a way to avoid cat states (macroscopic superpositions), but I don’t see that playing a role here.
The mapping itself has something to do with focusing on the fractional part of particle momentum as finite, and avoiding divergences by focusing on a particular subspace. It’s not a trivial result. But extending it to interacting field theory will require new ideas, e.g. making the state space of each individual cell in the CA into a Fock space, or permitting CTCs in the CA grid. Surely you need radical ingredients like that in order to recover the full quantum state space…
Aha, I see.
So you do not share EY’s view that MWI is “correct” then and the only problem it faces is recovering the Born Rule?
I agree that obviously what will end up working will depend on what the foundations are :)
I remember that paper by Buosso and Susskind, I even remember sending a mail to Susskind about it, while at the same time asking him about his opinion of ‘t Hoofts work.
If I remember correctly the paper was discussed at some length over at physicsforums.com (can’t remember the post) and it seemed that the consensus was that the authors have misinterpreted decoherence in some way.
I don’t remember the details, but the fact that the paper itself has not been mentioned or cited in any article I have read since then indicates to me that there has had to have been some serious error in it.
Also Susskinds answer regarding ’t Hoofts work was illuminating. To paraphrase he said he felt that ’t Hooft might be correct, but due to there not being any predictions it was hard to hold a strong opinion either way on the matter. So it seems Susskind was not very sold on his own idea.
This is a very good post, but I wonder: One of the authors in the paper you cite is David Wallace, perhaps the most prominent proponent of modern Everettian interpretation. He just published a new book called “The Emergent Multiverse” and he claims there is no problem unifying MWI with QFT because interactions within worlds are local and only states are nonlocal. I have yet to hear him mention any need for serious reformulation of anything in terms of MWI.
You said you suspect this is necessary, but that you hope we can recover a similar MWI, but isn’t it more reasonable to expect that at the planck scale something else will explain the quantum weirdness? After all if MWI fails both probability and relativity, then there is no good reason to suspect that this interpretation is correct.
Have you given Gerard ’t Hoofts idea of cellular automata which he claims salvage determinism, locality and realism any thought?
When I talk about recovering MWI, I really just mean absorbing the lesson that our theory does not need to deliver determinate measurement results, and ad hoc tools for satisfying this constraint (such as collapse or hidden variables) are otiose. Of course, the foundations of our eventual theory of quantum gravity might be different enough from those of quantum theory that the interpretational options don’t translate. How different the foundations will be depends on which program ends up working out, I suspect. If something like canonical quantum gravity or loop quantum gravity turns out to be the way to go, then I think a lot of the conceptual work done in interpreting NRQM and QFT will carry over. If string theory turns out to be on the right track, then maybe a more radical interpretational revision will be required. The foundations of string theory are now thought to lie in M-theory, and the nature of this theory is still pretty conceptually opaque. It’s worth noting though that Bousso and Susskind have actually suggested that string theory provides a solid foundation for MWI, and that the worlds in the string theory landscape are the same thing as the worlds in MWI. See here for more on this. The paper has been on my “to read” list for a while, but I haven’t gotten around to it yet. I’m skeptical but interested.
I know of ‘t Hooft’s cellular automata stuff, but I don’t know much about it. Speaking from a position of admitted ignorance, I’m skeptical. I suspect the only way to construct a genuinely deterministic local realist theory that reproduces quantum statistics is to embrace superdeterminism in some form, i.e. to place constraints on the boundary conditions of the universe that make the statistics work out by hand. This move doesn’t seem like good physics practice to me. Do you know if ’t Hooft’s strategy relies on some similar move?
’t Hooft’s latest paper is the first in which he maps a full QFT to a CA, and the QFT in question is a free field theory. So I think that in this case he evades Bell’s theorem, quantum complexity theorems, etc, by working in a theory where physical detectors, quantum computers, etc don’t exist, because interactions don’t exist. It’s like how you can evade the incompleteness theorems if your arithmetic only has addition but not multiplication. Elsewhere he does appeal to superselection / cosmological initial conditions as a way to avoid cat states (macroscopic superpositions), but I don’t see that playing a role here.
The mapping itself has something to do with focusing on the fractional part of particle momentum as finite, and avoiding divergences by focusing on a particular subspace. It’s not a trivial result. But extending it to interacting field theory will require new ideas, e.g. making the state space of each individual cell in the CA into a Fock space, or permitting CTCs in the CA grid. Surely you need radical ingredients like that in order to recover the full quantum state space…
Aha, I see. So you do not share EY’s view that MWI is “correct” then and the only problem it faces is recovering the Born Rule? I agree that obviously what will end up working will depend on what the foundations are :) I remember that paper by Buosso and Susskind, I even remember sending a mail to Susskind about it, while at the same time asking him about his opinion of ‘t Hoofts work. If I remember correctly the paper was discussed at some length over at physicsforums.com (can’t remember the post) and it seemed that the consensus was that the authors have misinterpreted decoherence in some way. I don’t remember the details, but the fact that the paper itself has not been mentioned or cited in any article I have read since then indicates to me that there has had to have been some serious error in it. Also Susskinds answer regarding ’t Hoofts work was illuminating. To paraphrase he said he felt that ’t Hooft might be correct, but due to there not being any predictions it was hard to hold a strong opinion either way on the matter. So it seems Susskind was not very sold on his own idea.
Gerard ‘t Hooft actually does rely on what people call “superdeterminism”, which I just call “full determinism”, which I think is also a term ’t Hooft likes more. At least that is what his papers indicate. He discuss this some in a article from 2008 in response to Simon Kochen and John Conway’s Free Will Theorem. You might want to read the article: http://www.sciencenews.org/view/generic/id/35391/title/Math_Trek__Do_subatomic_particles_have_free_will%3F After that you might want to head on over to arxiv, ’t Hooft has published a 3 papers the last 6 months on this issue and he seem more and more certain of it. He also adress the objections in some notes in those papers. Link: http://arxiv.org/find/quant-ph/1/au:+Hooft_G/0/1/0/all/0/1