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AnthonyC comments on [FINAL CHAPTER] Harry Potter and the Methods of Rationality discussion thread, March 2015, chapter 122 - Less Wrong
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I wasn't really concerned with his points against the story, and his points against the story's didactic purposes felt weak at best. What I found most pertinent in his critiques were the points about HPMoR's science. I'm no physicist, but from what I could tell, most of it seemed pretty sound (though the thing about the levitation vs. cat transformation in Chapter 1 was, I felt, pretty lazy reading). Any domain experts around here that could chime in, specifically on the physics?
EDIT: Poor wording in the above paragraph. What I meant was that the guy's critiques seem pretty sound, not that the science in HPMoR seems pretty sound.
Sure, the problems with the physics are right in there with bothersome things that Harry says that you could still justify, starting with the non sequiturs about conservation of energy when McGonogall turns into a cat.
I disagree with su3su2u1 (the tumblr author) about levitation; that doesn't violate conservation of energy if it's mediated by a force, and why shouldn't it be? On the other hand, turning into a cat violates conservation of mass (or would appear to, and that should be easy to check with a bathroom scale), which (via E = mc²) translates into a huge energy violation. But bringing up the quantum Hamiltonian? FTL signalling? Su3su2u1's analysis is correct.
The justification for this is that Harry is 11 and has only a vague idea about how physics actually works. But then it's hard to tell what we should learn from Harry and what we should ignore. (For that matter, I don't even know if Eliezer knows better than Harry or not.)
"But bringing up the quantum Hamiltonian? FTL signalling? Su3su2u1's analysis is correct."
Is it? Noether's theorem implies the Hamiltonian is conserved. The Hamiltonian is the quantum operator that give you the energy of a system. If energy conservation is violated, either the basic equations of quantum mechanics don't hold, or (magical) physics is not time invariant. I'm not saying Harry is being technically precise but he's not completely wrong, either.
No, he's certainly not completely wrong, but he's bringing up irrelevant complications and missing the main point. Quantum vs classical has nothing to do with it, for example.
The fact that quantum mechanics conserves energy is stronger evidence for the hypothesis that reality conserves energy than the fact that classical mechanics conserves energy. He is saying "our best model of reality conserves energy" which is very relevant.
If quantum mechanics allowed for small violations of energy conservation (which sometimes people even say that it does, on short time periods, although this is not really correct), then McGonagall's tranformation would still violate physical law as we know it. In physics, you don't always push everything down to the most fundamental theory, which is a good thing, since we don't actually have a most fundamental theory of physics. There is no such thing as ‘our best [single] model of reality’; there are some ways in which our quantum models are (so far) worse than our classical ones.