Scott Aaronson's cautious optimism for the MWI

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Aug 19, 2012

http://www.scottaaronson.com/blog/?p=1103

Eliezer's gung-ho attitude about the realism of the Many Worlds Interpretation always rubbed me the wrong way, especially in the podcast between both him and Scott (around 8:43 in http://bloggingheads.tv/videos/2220). I've seen a similar sentiment expressed before about the MWI sequences. And I say that still believing it to be the most seemingly correct of the available interpretations.

I feel Scott's post does an excellent job grounding it as a possibly correct, and in-principle falsifiable interpretation.

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As far as I can tell (being a non-physicist), the Transactional Interpretation shares the mathematical simplicity of MWI. And here Kastner and Cramer argue that TI can derive the Born probabilities naturally, whereas MWI is said to need a detour through "the application of social philosophy and decision theory to subjectively defined ‘rational’ observers". So maybe TI is simpler.

The "possibilities" they posit seem quite parallel (pardon the pun) to the multiple worlds or bifurcated observers of MWI, so I don't see the philosophical advantage there, that they tout. But integrating the Born probabilities more tightly into the physics is a plus, if true.

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Mitchell_Porter

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With respect to Born probabilities, TI is on the level of MWI, it has no derivation for them. Similarly, its ontology is rhetorical rather than rigorous.

A central issue for any zigzag-in-time or retrocausal theory of QM would be vacuum polarization, which was the stumbling block for the most serious effort, by Feynman and Wheeler. But Feynman-Wheeler theory is also where the path integral was born, so TI advocates could say, we just need to go back and finish it properly.

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He says that the math is simpler under MWI.

Can someone explain why that's true (or false)?

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I think the short version is that you don't need math that covers the wavefunction collapse, because you don't need the wave function to collapse.

For a longer version, you'd need someone who knows more QM than I do.

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In non-relativistic MWI, the evolution of the quantum state is fully described by the Schrodinger equation. In most other interpretations, you need the Schrodinger equation plus some extra element. In Bohmian mechanics the extra element is the guidance equation, in GRW the extra element is a stochastic Gaussian "hit".

In Copenhagen, the extra element is ostensibly the discontinuous wavefunction collapse process upon measurement, but to describe this as complicating the math (rather than the conceptual structure of the theory) is a bit misleading. Whether you're working with Copenhagen or with MWI, you're going to end up using pretty much the same math for making predictions. Although, technically MWI only relies on the Schrodinger equation, if you want to make useful predictions about your branch of the wave function, you're going to have to treat the wave function as if it has collapsed (from a mathematical point of view). So the math isn't simpler than Copenhagen in any practical sense, but it is true that from a purely theoretical point of view, MWI posits a simpler mathematical structure than Copenhagen.

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In other words, MWI says: apply Copenhagen for anything useful.

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MWI says that you apply no more than one collapse in every experiment, and you know why it is a collapse from your point of view. Copenhagen requires you to decide without guidance whether to apply collapse inside the experiment.

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Yeah, just like statistical mechanics requires us to model systems as having infinite size in order to perform many useful calculations (e.g. phase transitions, understood as singularities in thermodynamic potentials, can only take place in infinite particle systems). It doesn't follow that we should actually believe that these systems have infinite size.

Also, the claim is not that MWI is mathematically identical to Copenhagen, just that it works out that way in most practical cases. The Copenhagen interpretation is sufficiently ill-defined that it's unclear what its mathematical structure actually is. But as Aaronson points out in the post, there are predictions that distinguish between MWI and Copenhagen.

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MWI says: apply Born's rule to get anything useful.

If that's what you call Copenhagen, then sure they're the same thing - but then why was Everett so scandalous and ridiculed? Something had to be different.

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why was Everett so scandalous and ridiculed

No idea, I don't find MWI ridiculous, just not instrumentally useful, given that you still have to combine unitary evolution with the Born rule to get anything done. This is a philosophical difference with EY, who believes that territory is in the territory, not in the map.

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... territory is in the territory.

Umm. That sounds... non-controversial. Did I read that wrong somehow?

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No, you read it right. However, instrumentally, the map-territory relation is just a model, like any other, though somewhat more general. It postulates existence of some immutable objective reality with fixed laws, something to be studied ("mapped"). While this may appear self-evident to a realist, one ought to agree that it is still an assumption, however useful it might be. And it is indeed very useful: it explains why carefully set up experiments are repeatable, and assures you that they will continue to be. Thus it is easy to forget that it is impossible to verify that "territory exists independently of our models of it", and go on arguing which of many experimentally indistinguishable territories is the real one. And once you do, behold the great "MWI vs Copehagen" LW debate. If you remember that territory is in the map, not in the territory, the debate is exposed as useless, until different models of the territory can be distinguished experimentally. Which will hopefully happen in the cantilever experiment.

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The territory is not in the map, because that is nonsense.

That does not beg the question against instrumentalism and jn favour.of realism, because the territory does not have to exist at all.

Realists and anti realists are arguing about whether the territory exists, not where.

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The territory is not in the map, because that is nonsense.

That's the standard reaction here, yes. However "that is nonsense" is not a rational argument. You can present evidence to the contrary or point out a contradiction in reasoning. If you have either, feel free.

That does not beg the question against instrumentalism and jn favour.of realism, because the territory does not have to exist at all.

I don't understand what you are saying here.

Realists and anti realists are arguing about whether the territory exists, not where.

Maybe so, then I am neither.

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I'll point out a contradiction: territory is defined as not-map.

"I am neither"

... in the sense that you are using the word territory in a way that no one else does.

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One can postulate that there is an and to a long stack of maps of maps which ends somewhere with a perfect absolute "correct" something. We call that the territory. I don't postulate that.

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