LW - The goal of physics by Jim Pivarski
Description
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: The goal of physics, published by Jim Pivarski on September 3, 2023 on LessWrong.In grad school, I was a teaching assistant for a course called, Why the Sky is Blue. It was a qualitative introduction to physics for non-majors, covering a lot of the same topics as Physics I, such as forces, conservation of energy and momentum, electric charges and magnetic fields, in less detail, with not much math. The actual question about why the sky is blue was saved for the end. As the course dragged on and the students (who expected no math, rather than not much math) started to complain, "Are we ever going to find out why the sky is blue?" I watched the schedule slip and wondered the same thing.We skipped some sections and managed to wedge it into the last lecture: finally, we were talking about why the sky is blue! "The sky is blue because of Rayleigh scattering." Okay, that's not an answer we hadn't defined Rayleigh scattering, there wasn't time for it, so we said that air molecules absorb and re-radiate - effectively changing the direction of - blue light more than red light. Red light goes straight through the atmosphere, and blue light bounces around, making the whole sky glow blue. Conversely, sunrises and sunsets are red because you're looking at the light that has gone straight through a larger wedge of atmosphere. It lost most of its blue on the way to your eye.Pretty good explanation, for not being able to say(the 1/λ4 part affects small-λ blue light more than large-λ red light). We also showed pictures like this sunset:to demonstrate the effect of straight-through red light and bouncing-around blue light.So in the end, "Why is the sky blue?"Answer: "Because sunsets are red!""And why are sunsets red...?"It was understandably unsatisfying. One thing was only explained in terms of another thing. But even if we had the time to get into detail about Rayleigh scattering, they could reasonably ask, "Why does light scatter according to that formula?" We could go deeper and explain Lord Rayleigh's proof in terms of Maxwell's equations. And whyfore Maxwell's equations? Well, quantum electrodynamics, which is a quantum field theory with a local U(1) gauge symmetry, which is to say that every point in space has an extra degree of freedom, similar to a fourth spatial dimension except that this dimension can't be rotated with normal space like the other three, this dimension is connected to itself as a circle instead of being infinite (that's what the U(1) means), and neighboring points in 3D space try to minimize differences in this extra parameter, which leads to waves.The explanatory power is breathtaking: you can actually derive that photons must exist, if you assume that there's this U(1) symmetry laying around. But why is there a U(1) symmetry?Modern physics seems to be obsessed with symmetries. Even this U(1) symmetry is explained in terms of a more fundamental SU(2)×U(1) (different U(1)) and the Higgs mechanism. Physicists seem to be holding their tongues, avoiding saying, "This is the most basic thing," by saying, "This one thing is actually a manifestation that other thing." Answering the question, "Why do photons exist?" with "Because space has an internal U(1) symmetry" is a bit like saying, "The sky is blue because sunsets are red."Symmetry explanations collapse our description of the world onto a smaller description. They say that one thing is mathematically derivable from the other, maybe in both directions, but they don't say why either is there at all. Perhaps that's an unanswerable question, and the symmetry language is a way of acknowledging the limitation.To show what I mean, consider a universe that consists of nothing but a point in the exact center of a perfect circle. (I've been feeling free to say, "Consider a universe..." ever since a lecture...
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