Rationality Reading Group: Part S: Quantum Physics and Many Worlds

This is part of a semi-monthly reading group on Eliezer Yudkowsky’s ebook, Rationality: From AI to Zombies. For more information about the group, see the announcement post.

Welcome to the Rationality reading group. This fortnight we discuss Part S: Quantum Physics and Many Worlds (pp. 1081-1183). This post summarizes each article of the sequence, linking to the original LessWrong post where available.

S. Quantum Physics and Many Worlds

229. Quantum Explanations—Quantum mechanics doesn’t deserve its fearsome reputation. If you tell people something is supposed to be mysterious, they won’t understand it. It’s human intuitions that are “strange” or “weird”; physics itself is perfectly normal. Talking about historical erroneous concepts like “particles” or “waves” is just asking to confuse people; present the real, unified quantum physics straight out. The series will take a strictly realist perspective—quantum equations describe something that is real and out there. Warning: Although a large faction of physicists agrees with this, it is not universally accepted. Stronger warning: I am not even going to present non-realist viewpoints until later, because I think this is a major source of confusion.

230. Configurations and Amplitude—A preliminary glimpse at the stuff reality is made of. The classic split-photon experiment with half-silvered mirrors. Alternative pathways the photon can take, can cancel each other out. The mysterious measuring tool that tells us the relative squared moduli.

231. Joint Configurations—The laws of physics are inherently over mathematical entities, configurations, that involve multiple particles. A basic, ontologically existent entity, according to our current understanding of quantum mechanics, does not look like a photon—it looks like a configuration of the universe with “A photon here, a photon there.” Amplitude flows between these configurations can cancel or add; this gives us a way to detect which configurations are distinct. It is an experimentally testable fact that “Photon 1 here, photon 2 there” is the same configuration as “Photon 2 here, photon 1 there”.

232. Distinct Configurations—Since configurations are over the combined state of all the elements in a system, adding a sensor that detects whether a particle went one way or the other, becomes a new element of the system that can make configurations “distinct” instead of “identical”. This confused the living daylights out of early quantum experimenters, because it meant that things behaved differently when they tried to “measure” them. But it’s not only measuring instruments that do the trick—any sensitive physical element will do—and the distinctness of configurations is a physical fact, not a fact about our knowledge. There is no need to suppose that the universe cares what we think.

233. Collapse Postulates—Early physicists simply didn’t think of the possibility of more than one world—it just didn’t occur to them, even though it’s the straightforward result of applying the quantum laws at all levels. So they accidentally invented a completely and strictly unnecessary part of quantum theory to ensure there was only one world—a law of physics that says that parts of the wavefunction mysteriously and spontaneously disappear when decoherence prevents us from seeing them any more. If such a law really existed, it would be the only non-linear, non-unitary, non-differentiable, non-local, non-CPT-symmetric, acausal, faster-than-light phenomenon in all of physics.

234. Decoherence is Simple—The idea that decoherence fails the test of Occam’s Razor is wrong as probability theory.

235. Decoherence is Falsifiable and Testable - (Note: Designed to be standalone readable.) An epistle to the physicists. To probability theorists, words like “simple”, “falsifiable”, and “testable” have exact mathematical meanings, which are there for very strong reasons. The (minority?) faction of physicists who say that many-worlds is “not falsifiable” or that it “violates Occam’s Razor” or that it is “untestable”, are committing the same kind of mathematical crime as non-physicists who invent their own theories of gravity that go as inverse-cube. This is one of the reasons why I, a non-physicist, dared to talk about physics—because I saw (some!) physicists using probability theory in a way that was simply wrong. Not just criticizable, but outright mathematically wrong: 2 + 2 = 3.

236. Privileging the Hypothesis—If you have a billion boxes only one of which contains a diamond (the truth), and your detectors only provide 1 bit of evidence apiece, then it takes much more evidence to promote the truth to your particular attention—to narrow it down to ten good possibilities, each deserving of our individual attention—than it does to figure out which of those ten possibilities is true. 27 bits to narrow it down to 10, and just another 4 bits will give us better than even odds of having the right answer. It is insane to expect to arrive at correct beliefs by promoting hypotheses to the level of your attention without sufficient evidence, like a particular suspect in a murder case, or any one of the design hypotheses, or that one of a billion opaque boxes that just looks like a winner.

237. Living in Many Worlds—The many worlds of quantum mechanics are not some strange, alien universe into which you have been thrust. They are where you have always lived. Egan’s Law: “It all adds up to normality.” Then why care about quantum physics at all? Because there’s still the question of what adds up to normality, and the answer to this question turns out to be, “Quantum physics.” If you’re thinking of building any strange philosophies around many-worlds, you probably shouldn’t—that’s not what it’s for.

238. Quantum Non-Realism—“Shut up and calculate” is the best approach you can take when none of your theories are very good. But that is not the same as claiming that “Shut up!” actually is a theory of physics. Saying “I don’t know what these equations mean, but they seem to work” is a very different matter from saying: “These equations definitely don’t mean anything, they just work!”

239. If Many-Worlds Had Come First—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.

240. Where Philosophy Meets Science—In retrospect, supposing that quantum physics had anything to do with consciousness was a big mistake. Could philosophers have told the physicists so? But we don’t usually see philosophers sponsoring major advances in physics; why not?

241. Thou Art Physics—If the laws of physics control everything we do, then how can our choices be meaningful? Because you are physics. You aren’t competing with physics for control of the universe, you arewithin physics. Anything you control is necessarily controlled by physics.

242. Many Worlds, One Best Guess—Summarizes the arguments that nail down macroscopic decoherence, aka the “many-worlds interpretation”. Concludes that many-worlds wins outright given the current state of evidence. The argument should have been over fifty years ago. New physical evidence could reopen it, but we have no particular reason to expect this.

This has been a collection of notes on the assigned sequence for this fortnight. The most important part of the reading group though is discussion, which is in the comments section. Please remember that this group contains a variety of levels of expertise: if a line of discussion seems too basic or too incomprehensible, look around for one that suits you better!

The next reading will cover Part T: Science and Rationality (pp. 1187-1265) and Interlude: A Technical Explanation of Technical Explanation (pp. 1267-1314). The discussion will go live on Wednesday, 10 February 2016, right here on the discussion forum of LessWrong.