B1 中級 1670

 Suppose you have a ball covered entirely with hair and you're trying to comb the hair so that it lies flat everywhere along the surface. If the ball were a donut, or it existed in two dimensions, this would be easy! But in three dimensions? Well, you're going to run into trouble. A lot of trouble. A big hairy ball of trouble. That's because of a theorem in algebraic topology called the "Hairy Ball Theorem" (and yes, that's it's real name) which unequivocally proves that at some point, the hair must stick up. Now don't go wasting your time playing around with a hairy ball trying to prove the theorem wrong - this is math we're talking about. It's proven - done - QED! Technically speaking, what the Hairy Ball theorem says is that a continuous vector field tangent to a sphere must have at least one point where the vector is zero. So what does this have to do with reality apart from uncombable hairy balls? Well, the velocity of wind along the surface of the earth is a vector field, so the Hairy Ball Theorem guarantees that there's always at least one point on earth where the wind isn't blowing. And it doesn't really matter that the object in question is ball-shaped. As long as it can be smoothly deformed into a ball without cutting or sewing edges together, the theorem still holds. So the next time a mathematician gives you trouble, ask them if they can comb a hairy banana.

# 看動畫理解毛球定理！（The Hairy Ball Theorem）

1670
Calvin Chen 發佈於 2013 年 1 月 2 日

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