Stupid Questions Open Thread Round 3

OpenThreadGuy

From Costanza's original thread (entire text):

"This is for anyone in the LessWrong community who has made at least some effort to read the sequences and follow along, but is still confused on some point, and is perhaps feeling a bit embarrassed. Here, newbies and not-so-newbies are free to ask very basic but still relevant questions with the understanding that the answers are probably somewhere in the sequences. Similarly, LessWrong tends to presume a rather high threshold for understanding science and technology. Relevant questions in those areas are welcome as well. Anyone who chooses to respond should respectfully guide the questioner to a helpful resource, and questioners should be appropriately grateful. Good faith should be presumed on both sides, unless and until it is shown to be absent. If a questioner is not sure whether a question is relevant, ask it, and also ask if it's relevant."

Meta:

How often should these be made? I think one every three months is the correct frequency.

Costanza made the original thread, but I am OpenThreadGuy. I am therefore not only entitled but required to post this in his stead. But I got his permission anyway.

Meta:

I still haven't figured out a satisfactory answer to the previous meta question, how often these should be made. It was requested that I make a new one, so I did.
I promise I won't quote the entire previous threads from now on. Blockquoting in articles only goes one level deep, anyway.

From the last thread:

From Costanza's original thread (entire text):

"This is for anyone in the LessWrong community who has made at least some effort to read the sequences and follow along, but is still confused on some point, and is perhaps feeling a bit embarrassed. Here, newbies and not-so-newbies are free to ask very basic but still relevant questions with the understanding that the answers are probably somewhere in the sequences. Similarly, LessWrong tends to presume a rather high threshold for understanding science and technology. Relevant questions in those areas are welcome as well. Anyone who chooses to respond should respectfully guide the questioner to a helpful resource, and questioners should be appropriately grateful. Good faith should be presumed on both sides, unless and until it is shown to be absent. If a questioner is not sure whether a question is relevant, ask it, and also ask if it's relevant."

Meta:

How often should these be made? I think one every three months is the correct frequency.

Costanza made the original thread, but I am OpenThreadGuy. I am therefore not only entitled but required to post this in his stead. But I got his permission anyway.

Meta:

I still haven't figured out a satisfactory answer to the previous meta question, how often these should be made. It was requested that I make a new one, so I did.
I promise I won't quote the entire previous threads from now on. Blockquoting in articles only goes one level deep, anyway.

This article (PDF) gives a nice (and fairly accessible) summary of some of the issues involved in extending MWI to QFT. See sections 4 and 8 in particular. Their focus in the paper is wavefunction realism, but given that MWI (at least the version advocated in the Sequences) is committed to wavefunction realism, their arguments apply. They offer a suggestion of the kind of theory that they think can replace MWI in the relativistic context, but the view is insufficiently developed (at least in that paper) for me to fully evaluate it.

A quick summary of the issues raised in the paper:

In NRQM, the wave function lives in configuration space, but there is no well-defined particle configuration space in QFT since particle number is not conserved and particles are emergent entities without precisely defined physical properties.
A move to field configuration space is unsatisfactory because quantum field theories admit of equivalent description using many different choices of field observable. Unlike NRQM, where there are solid dynamical reasons for choosing the position basis as fundamental, there seems to be no natural or dynamically preferred choice in QFT, so a choice of a particular field configuration space description would amount to ad hoc privileging.
MWI in NRQM treats physical space as non-fundamental. This is hard to justify in QFT, because physical space-time is bound up with the fundamentals of the theory to a much greater degree. The dynamical variables in QFT are operators that are explicitly associated with space-time regions.
This objection is particularly clever and interesting, I think. In MWI, the history of the universe is fully specified by giving the universal wavefunction at each time in some reference frame. In a relativistic context, one would expect that all one needs to do in order to describe how the universe looks in some other inertial reference frame is to perform a Lorentz transformation on this history. If the history really tells us everything about the physical state of the universe, then it gives us all the information required to determine how the universe looks under a Lorentz transformation. But in relativistic quantum mechanics, this is not true. Fully specifying the wavefunction (defined on an arbitrarily chosen field configuration space, say) at all times is not sufficient to determine what the universe will look like under a Lorentz transformation. See the example on p. 21 in the paper, or read David Albert's paper on narratability. This suggests that giving the wavefunction at all times is not a full specification of the physical properties of the universe.

On the other hand, my understanding is that QFT itself doesn't exist in a rigorous form yet, either.

I assume you're referring to the infinities that arise in QFT when we integrate over arbitrarily short length scales. I don't think this shows a lack of rigor in QFT. Thanks to the development of renormalization group theory in the 70s, we know how to do functional integrals in QFT with an imposed cutoff at some finite short length scale. QFT with a cutoff doesn't suffer from problems involving infinities. Of course, the necessity of the cutoff is an indication that QFT is not a completely accurate description of the universe. But we already know that we're going to need a theory of quantum gravity at the Planck scale. In the domain where it works, QFT is reasonably rigorously defined, I'd say.

This article (PDF) gives a nice (and fairly accessible) summary of some of the issues involved in extending MWI to QFT.

Thanks for that; it's quite an interesting article, and I'm still trying to absorb it. However, one thing that seems pretty clear to me is that for EY's intended philosophical purposes, there really is no important distinction between "wavefunction realism" (in the context of NRQM) and "spacetime state realism" (in the context of QFT). Especially since I consider this post to be mostly wrong: locality in configuratio... (read more)

0Quantumental14y

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?

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Stupid Questions Open Thread Round 3

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Stupid Questions Open Thread Round 3

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