Thursday, August 27, 2009

Interlude: Anticipating the 2007 Many Worlds conference

For many years, I knew it was coming. You just had to do the math: Hugh Everett III had published his thesis, which introduced the Many Worlds Interpretation (MWI) of quantum mechanics, in 1957. So, somewhere, there would be a 'Many Worlds at 50' conference in 2007. And I would be there.

-------------------------------------------------------------------------------------

Back in 2000, I attended the conference ‘One Hundred Years of the Quantum: From Max Planck to Entanglement’ at the University of Puget Sound, which commemorated Planck's paper which first introducted the concept of energy quantization, used to explain why the equilibrium density of thermal radiation is not infinite.

I had already started exploring the concepts behind the Many Computations Interpretation (MCI). [I called it the 'Computationalist Wavefunction Interpretation' (CWI) but that just didn't have the same ring to it.] It grew out of David Chalmer's suggestion, in the last chapter of his book The Conscious Mind, that applying computationalism to quantum mechanics was the right way to make sense of the MWI. But I knew that computationalism had to be made more precise before that could be done, and I knew that the Born Rule would be the key issue.

I submitted a short paper about it for the conference book. The paper is still available online at
http://www.finney.org/~hal/mallah1.html

At the conference I met a few well known physicists, the most famous of whom was James Hartle. At the time, the 'Consistent Histories' approach to interpretation of QM was getting a lot of attention, and Hartle and Murray Gell-Mann had written a book about it. As far as I was concerned, that approach was not of much interest, because it pretended that single-world-style probabilities could be assigned to terms in the wavefunction 'once decoherence occurred' despite the fact that decoherence is never truly complete. (Probabilities can not generally be assigned in the sense that, prior to decoherence, interference effects can occur and only be understood as showing the simultaneous existance of multiple terms in the wavefunction.)

It was also maddeningly vague about what exactly was suppposed to really exist, and declared that some questions must not be asked. It was not clear whether it was really just the MWI in drag, deliberately using vague language so as not to scare away those who thought the MWI is too weird, or if it was some new variant of the single world Copenhagen Interpretation. Its advocates publically claimed inspiration from both sources!

I got the chance to ask Hartle a question. I asked him two things:

1) Is Consistent Histories the same as the MWI?

He said it is. That provoked a gasp from the audience! You see, Consistent Histories was looked on quite favorably by many physicists at the time, while the MWI was still largely dismissed as material for science fiction.

2) Is it the same as the Pilot Wave Interpretation?

He said it's not. The second question was necessary because some people, especially those who like the Copenhagen Interpretation, consider experimental predictions to be the only thing that matters - so that they would consider all interpretations which give the same predictions to be the same thing. Now I knew that was not the case with him, so the first answer really did mean something.

Anyway, after that conference I resolved to try to make my interpretation of QM precise in time to discuss it at the inevitable 2007 conference. Seven years should be enough time, right? Of course, it was never my day job, just a hobby of sorts.

--------------------------------------------------------------------------------------

In 2002 I attended ‘Towards a Science of Consciousness’ (TSC), a yearly philosophy conference which was held at the University of Arizona that year and every even year. That was interesting in its own right, as I met interesting people and learned about issues and thought experiments in philosophy of mind which I had not previously been exposed to. (I don't think it would be as interesting to attend another TSC, because many of the issues are the same every year, unless I have published something of my own that will be talked about. But it's not bad so perhaps I will.)

At that 2002 TSC, I participated in the poster session, with a poster called “What Does a Physical System Compute?” which laid out my ideas about an implementation criterion for computations. It got little attention, except that David Chalmers himself was kind enough to stop by and consider it. He made some comments and criticisms. I'd had many false starts at formulating a criterion, and had discussed it by email with him, so he knew what it was about. The criteria I listed weren't good enough, and we both knew it, but I believed it was a step in the right direction.

[Some of the other posters there were interesting, but I remember only one, because it stood out as being the most crackpot idea I'd yet encountered - and I'd encountered many on the usenet newgroups. This guy was combining the kooky notion that humans only became conscious when language was invented, with the crazy idea that only consciousness causes wavefunction collapse, to argue that _the biblical age of the Earth is correct_ (a few thousand years) because that's when the first wavefunction collapse brought the universe into real existence! Quite a combination!]

--------------------------------------------------------------------------------------

So, years passed by and before I knew it the 2007 Perimeter Institute conference Many Worlds @ 50 was approaching. This was it; the conference I'd been looking forward to for so long, in which I hoped to discuss my ideas about the MWI with other supporters of the interpretation. Would I be ready? I'd had some success in refining my implementation ideas, and scrambled to write up what I had.

The Born Rule still eluded me, though. I had hoped that once I found the precise criteria for existence of an implementation, I could apply it to quantum mechanics and the Born Rule might pop out. After all, it's actually fairly easy to get the Born Rule to pop out if you impose certain simple requirements such as conservation of measure. People have been doing it for years without even realizing they'd made unjustified assumptions. All I had to do was find a reason to justify an assumption like that for the counting of implementations.

I didn't find that justification, and time was getting short. I turned to an unusual approach for inspiration - Robin Hanson's 'Mangled Worlds' papers. He had a rather innovative approach to the MWI, in which large terms in the wavefunction 'mangle' small ones, leading to an effective minimum amplitude, and he argued that the Born Rule followed from counting worlds (lumps of wavefunction) in the distribution of survivors. The world-counting appealed to me, as it could easily be translated into implementation-counting, but I did not believe his scheme could work: large worlds would not 'mangle' worlds they had decohered from nearly as much as Hanson had assumed.

To get that kind of thing to work, I had to assume new physics, contrary to Everett. But the new physics was fairly simple: random background noise in the wavefunction (which could be part of the initial conditions rather than new dynamics) could 'mangle small worlds' and if it does the Born Rule pops out (in an interesting new way). There were still some real questions about whether this could work out right, so I explored a more direct approach as well in which I tried to rig the way implementations are to be counted in order for it to come out right. That turned out to be easier said than done, and it remains an open question about whether it can or should be done, though I regard it more favorably now. All of this will be discussed in later posts.

I also discussed other alternatives, such as an MWI with hidden variables, and other ways that a minimum amplitude could be introduced. The basic conclusion was that computationalism strongly favors some kind of MWI over single-world interpretations, even if both have hidden variables, but the details are unknown (and might always remain so).

I wrote all this up and added criticisms of the incorrect attempts to derive the Born Rule in the MWI, including the one based on decision theory, which was widely considered the strongest of the attempted derivations although it had its critics. This became my MCI paper, which I placed on the preprint arxiv: http://arxiv.org/abs/0709.0544

I knew that I was cutting it close, so I emailed some of the people who had written about the MWI and who would attend the conference to tell them about my paper on the arxiv.

It was time to go to Canada and see if the 2007 MWI Perimeter Institute conference would live up to the anticipation.

No comments:

Post a Comment

Featured Post

Why MWI?

Before getting into the details of the problems facing the Many-Worlds Interpretation (MWI), it's a good idea to explain why I believe t...

Followers