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Astazha comments on [FINAL CHAPTER] Harry Potter and the Methods of Rationality discussion thread, March 2015, chapter 122 - Less Wrong Discussion
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Comments (162)
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.)
Lifting someone does work. Where is that energy coming from?
I'm no physicist, but I took that bit to mean that the difference in energy involved in lifting someone is so much smaller than the difference in energy required to convert a 135-pound woman into a 10-pound cat without leaving 125 pounds of squishy McGonagall-bits by the wayside that it's far more plausible for it to be coming from some non-obvious source.
Which seemed reasonable enough to me at the time. But looking back at it, I'm no longer sure it really holds water. Harry's family at that point is dealing with a process with almost totally unknown properties; if you can get energy out of the Source of Magic to levitate people with, it's not too much of a stretch to imagine that you could also move mass into it and then back out. We don't e.g. see the floorboards creaking under McGonagall when she lifts stuff with her mind, so there's no particular reason in context to assume that the interaction has to involve local-ish forces.