Friday, June 5, 2015

Why Is the Sky Blue?

Physicists are often asked to explain such topics as Schrodinger's Cat or the theory of relativity. Once, I was asked what would happen to an astronaut suspended in space midway between two identical black holes. Let's just say it's not a situation you want to find yourself in.

One of the most common questions about the natural world is the question of why the sky is blue. In case you've ever wondered about this, a detailed physics answer for laypersons can be found here.

The Tyndall effect, Rayleigh scattering, and the electromagnetic spectrum—all of this is great physics, but doing justice to it can be difficult at a cocktail party. So what I usually say is this: "The sky is blue because air is blue." This reply captures a lot of the reality in just a few words, so it's efficient that way. It also gives me time to drink my drink while the questioner considers it.

I wonder what Socrates would make of that answer. I thought of him just now because of the following conversation in the Meno. Here Socrates and Meno are discussing color:
MENO: And how do you define color?
SOCRATES: [Flirts with Meno a little, and then says:] Would you like an answer à la Gorgias, such as you would most readily follow?
MENO: Of course I should.
SOCRATES: You and he believe in Empedocles' theory of effluences, do you not?
MENO: Wholeheartedly.
SOCRATES: And passages to which and through which the effluences make their way?
MENO: Yes.
SOCRATES: Some of the effluences fit into some of the passages, whereas others are too coarse or too fine.
MENO: That is right.
SOCRATES: Now you recognize the term 'sight'?
MENO: Yes.
SOCRATES: From these notions, then, 'grasp what I would tell,' as Pindar says. Color is an effluence from shapes commensurate with sight and perceptible by it.
MENO: That seems to me an excellent answer.
SOCRATES: No doubt it is the sort you are used to. ... Yes, it's a high-sounding answer, so you like it....
The Empedoclean explanation of color resembles the physics answer to the question of why the sky is blue, in the sense that both are mechanistic causal explanations given in terms of theoretical entities. Socrates isn't mocking the Empedoclean explanation so much as mocking people like Gorgias and Meno who are so impressed by it. Meno is eager for impressive explanations of things he can't even define. Disappointingly, in the Meno we never do get to hear Socrates' definition of color. And if anybody ever asked Socrates why the sky is blue, I'm not aware of it.

This morning I had an interesting dialogue with my kids about the color of the sky. It went like this:
DAD: ...about why the sky is blue.
ABIGAIL: I know why the ocean is blue!
CLAIRE: Because the ocean reflects the sky.
ABIGAIL: No fair! I was supposed to say it!
DAD: Well, the sky is blue because it's made of air, and air is blue.
ABIGAIL: Sometimes the sky is red.
DAD: Good point.
CLAIRE: No, air is clear. I know it because at night you can see through it to space.
DAD: Well that's true too.
My kids made some interesting points here. Maybe the next time somebody asks me why the sky is blue, I'll just say what Abigail said today: "Sometimes it's red." It's only three words, and it'll give me time to drink my drink.

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