Monday, September 7, 2009

Interlude: The 2007 Perimeter Institute conference Many Worlds @ 50

As explained in the previous post, I had long been anticipating a conference on the MWI in 2007, and attended the Perimeter Institute conference Many Worlds at 50, armed with a copy of my then-new eprint on the Many Computations Interpretation.

When I arrived at my hotel the night before the conference, an older couple was checking in at the same time as I was. Someone asked the clerk for directions to the Perimeter Institute. It turned out that this couple was also attending the conference, and they were a couple of the friendliest and most interesting people I met there.

George Pugh had worked with Hugh Everett (founder of the MWI) at a defense contractor, Lambda Corp. (The work Everett did there is not so famous as his MWI but was actually important during the Cold War.) George and his impressive wife Mary had talked about the MWI with Everett himself, and they support it. They asked me which side I was on, as both pro- and con- people were attending the conference. I told them I was in favor of the MWI. They liked to hear that. We ended up having meals together on several occasions over the course of the conference.

The conference itself consisted mostly of lectures in a classroom-like atmosphere, followed by questions from the audience. Appropriately, most of the talks focused on the question of probability in the MWI.

However, and unfortunately, they mainly focused on the attempt to derive the Born Rule from decision-theoretic considerations. That approach was proposed by David Deutsch in 2000, and further developed by Simon Saunders and especially by David Wallace. Saunders and Wallace gave talks that mainly reiterated what is in their papers. There were also talks that (correctly, though of course this was not accepted by Wallace's supporters) pointed out the failures of that approach, such as those by Adrian Kent and David Albert.

The only other approach to the Born Rule that was presented at a talk was that of W. Zurek, who talked about his (equally fallacious) 'envariance' approach. Most people seemed to agree that Zurek's approach was similar to Wallace's. There was little discussion of it beyond that. When Zurek was asked about Wallace's approach during an informal discussion, he basically said that he didn't know if Wallace's approach was correct also, but he didn't seem to think it matters much, because his own approach showed that the Born Rule followed from the MWI. When I tried to point out to him why his approach fails - a task made all the more difficult by his somewhat intimidating large physical presense and lion-like bearded appearance - he didn't understand my point and soon ended the conversation.

Max Tegmark was a speaker, and he briefly discussed his heirarchy of many-worlds types, up to the Everything Hypothesis for which he is known.

Besides that, the only other controversy addressed in the talks was that of the legitimacy and meaning of talking about probability in the deterministic MWI, which is a seperate question than the quantitative problem of deriving the Born Rule. This focused on Hilary Greaves' 'caring measure' approach. She is sometimes lumped in with the decision theoretic approach to the Born Rule, because she uses decision theory in another way, but in fact her ideas are independent of that and are basically correct though not the full story.

The official speakers were basically divided into two camps: Those MWI-supporters who supported Wallace's attempted derivation of the Born Rule or who were considered allies of it (like Zurek and Greaves), versus those who not only rejected it but also were against the MWI in general (like Kent and Albert). Tegmark was neither but his one talk was largely ignored, and he did not address the Born Rule controversy.

Among the attendees, however, the situation was more complicated. I was not the only one who supported some kind of MWI, and considered understanding the Born Rule to be the key issue of interest, but utterly rejected the approaches to the Born Rule that had been presented. The alternatives that we wanted to discuss involved some form of observer-counting as the basis for probabilities in an MWI, even if it required some new physics. This led to a minor rebellion, in which a few of us tried to talk about our ideas during a lunch period in the room set aside for the conference lunch. The only official speaker that we got any help from was Hilary Greaves. We were able to speak in the lunchroom for a little while, but it didn't get much attention.

There was another young woman by the name of Hillary, I think a physicist studying at the Institute, who also helped us set up the lunchtime discussion.

The 'counter' camp included Michael Weissman, who proposed a modification of physics in order for world-counting to yield the Born Rule. His scheme involved sudden splitting of existing worlds into proposed new degrees of freedom, with a higher rate of such splitting events for higher amplitude worlds. This was interesting, but I was skeptical, and after thinking about it for a while I found the fatal flaw in it. If new worlds were constantly being produced, then the number of observers would be growing exponentially. The probability of future observations, as far into the future as possible, would be much greater than that of our current observations. Thus, the scheme must be false unless we are highly atypical observers, which is highly unlikely. While false, Mike's model serves as a good way to discuss the need for approximate conservation of measure for a successful model. In any case, Mike proved to be a good guy to talk to.

Also among the 'counters' was David Strayhorn, who proposed that an indeterminacy in General Relativity could lead to a Many Worlds model in which spacetime topologies were distributed according to, and formed the basis for, the Born Rule. His ideas did not seem fully developed, and I was skeptical of them as well, but we had interesting discussions.

Another guy with us was Allan Randall. He supports Tegmark's Everything Hypothesis, and is also interested in transhumanism and immortality. As I explained to Allen and to Max Tegmark, I wasn't sure about the Everything hypothesis, because of the problem of what would determine a unique measure distribution, but I used to support it and still like it. I think it's important and maybe useful. After all, and like many supporters of the hypothesis, I discovered a version of it on my own long before I ever heard of Tegmark.

Which brings me to a subject that received little official mention at the conference, the 'Quantum Immortality / Quantum Suicide' fallacy which Tegmark had publicized. This is the belief, which many MWI supporters have come to endorse, that the MWI implies that people always survive because some copies of them survive in branches of the wavefunction. I had always regarded this as the worst form of crackpot thinking, and had hoped to discuss it at the conference as something that MWI supporters must crush before it gets out of hand. My brief discussions about it at the conference convinced me that it was not getting the condemnation that it deserves. This ultimately led me to write my own eprint against it, Many-Worlds Interpretations Can Not Imply 'Quantum Immortality', despite my misgivings that even discussing the subject could give the dangerous idea extra publicity.

I also had interesting discussions with Mark Rubin, who had shown an explicit local formulation of the MWI using the Heisenberg picture, which is something I still need to study more. Mark and I had dinner with the Pughs. I liked the Swiss Chalet restaurant and Canadian beer.

I also happened to run into a friend of mine from NYU, where I got my Ph.D. in physics. Andre is a Russian who came to the US to study, and he had a postdoc at the Perimeter Institute. He's not an MWI supporter or really into interpretation of QM, but he knew that I am, so he was not too surprised that I showed up at the conference. I was lucky to run into him, because the next day he was heading to England for a postdoc there, studying quark-gluon plasmas using the methods he learned from models of string theory. He said he might never return to the US.

All in all, it was certainly an interesting experience. Ultimately, though, it was disappointing because I didn't get to discuss my paper much, and I never was able to have a substantive discussion with the well-known figures in the field who were there to present their own work. It was largely a lecture series rather than an egalitarian discussion group. Some discussion took place on the sidelines, such as at meals, but that was limited in who you happened to be next to. Well-known people mainly talked to each other.

One thing that grew out of the discussions on observer-counting was that a group of us decided to continue the discussion on-line. This led to the creation of the OCQM yahoo group, which included David Strayhorn, Michael Weissman, Allan Randall, Robin Hanson, and myself. Robin had not been at the conference, but he was the originator of the Mangled Worlds approach to the Born Rule, and accepted our invitation to join the group. In practice, however, posts to the group largely came from just David and myself. We all supported some form of observer-counting, but our approaches were quite different. We had some very interesting discussions, and it was a good place to 'think out loud', but ultimately even David's posting to the group petered out and it seems dead at this point.

I gave the Pughs my printed copy of the MCI paper. They were compiling a book in which they would quote various people about why Everett's interpretation of QM was important, so I wrote a few lines for them. Ultimately they decided not to use it though. I think they didn't like my criticism of the current status of the Born Rule in the MWI.

2 comments:

  1. Jack- I just rediscovered this site. I'm now inclined to support your style of interpretation. It is not entirely clear, however, that even it has a satisfactory answer to your question "why is it now?" Have you solved for the time evolution of the the background noise which seems to be the essential ingredient in justifying the emergence of Born probabilities from linear dynamics in your approach?

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  2. Good to hear from you, Michael.

    If the background noise exists I tend to assume it would be at or near thermal equilibrium, so it would have a constant amplitude and spectrum on average. There would still be time evolution and correllations, though. I have not studied that.

    One problem with 'noise' is that it could lead to spontaneously formed, short-lived 'Boltzmann brains'. I'm not sure how much of a problem that is.

    Other than the 'noise' idea, other possibilities are that the amplitude cutoff is due to some kind of discreteness rather than noise, or that the right way to count implementations leads to a more direct Born rule. Or, of course, that Everett was wrong about there being no hidden variables, but even in that case I believe he was still right about there being many worlds (e.g. continuum Bohmian mechanics).

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