[SEQ RERUN] If Many-Worlds Had Come First

MinibearRex

Today's post, If Many-Worlds Had Come First was originally published on 10 May 2008. A summary (taken from the LW wiki):

If early physicists had never made the mistake, and thought immediately to apply the quantum laws at all levels to produce macroscopic decoherence, then "collapse postulates" would today seem like a completely crackpot theory. In addition to their other problems, like FTL, the collapse postulate would be the only physical law that was informally specified - often in dualistic (mentalistic) terms - because it was the only fundamental law adopted without precise evidence to nail it down. Here, we get a glimpse at that alternate Earth.

Discuss the post here (rather than in the comments to the original post).

This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Collapse Postulates, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.

Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.

Today's post, If Many-Worlds Had Come First was originally published on 10 May 2008. A summary (taken from the LW wiki):

If early physicists had never made the mistake, and thought immediately to apply the quantum laws at all levels to produce macroscopic decoherence, then "collapse postulates" would today seem like a completely crackpot theory. In addition to their other problems, like FTL, the collapse postulate would be the only physical law that was informally specified - often in dualistic (mentalistic) terms - because it was the only fundamental law adopted without precise evidence to nail it down. Here, we get a glimpse at that alternate Earth.

We should consider another counterfactual, "If The Transactional Interpretation Had Come First On Less Wrong".

Some "interpretations", like Bohmian mechanics and objective collapse theories, are recognizably physical theories in the sense of being perfectly explicit hypotheses about what exists and how it changes, which reduce to quantum mechanics in some limit.

Then there are interpretations which are still just at the stage of being a qualitative hypothesis about the nature of reality. These are the ones which can't derive the Born rule, for example. Many-worlds is one of these. There is no standard derivation of the Born rule within many worlds, just competing claims and ideas about how to do it. As things stand, to employ many-worlds as a predictive theory, your theory really needs to be "Many worlds + Born rule". Many-worlds is your metaphysics, your conception of reality, and the Born rule is a further assumption (about the frequencies of the worlds) that you have to make in order to calculate anything.

The transactional interpretation is another instance of this. I would prefer to regard it as just one representative of a long-running genre of quantum interpretation, in which quantum mechanics is explained as due to causality operating future-to-past as well as past-to-future. In such a world, there are no literal superpositions. The peculiar features of the quantum probability calculus, such as the destructive interference (existence of dark zones where the particle does not arrive) in the double-slit experiment, are attributed to the ordinary calculus of conditional probability being at work in both "directions": the presence of closed chains of probabilistic dependence that loop in time imposes consistency conditions on what can happen, conditions which appear anomalous or nonlocal.

So far as I can see, transactional interpretations are at least as plausible as many-world interpretations. The whole Quantum Physics Sequence could have been written around them, with no change in any Less Wrong epistemological doctrine. The only difference would be, that LW discussion of QM would proceed from the premise that basic physics is all about an interference between procausal and retrocausal chains, not about an ensemble of parallel worlds.

At least, I can't see any difference that substituting John Cramer for Hugh Everett would make. There is nothing here to cause one to judge LW principles any differently, just evidence that they weren't applied perfectly in this case. But perhaps someone more devoted to LW epistemology and methodology could check this for me. Does anything fundamental change (from the perspective of "refining human rationality") if it turns out that the central factual claim of this sequence is wrong?

How do other theories get the born rule? Do any of them make less assumptions than MWI?