Sunday, October 31, 2010

On emptiness

It's hard to read much popular science writing without learning that atoms - and consequently anything made of atoms, such as a chair - are pretty much made up of nothing. A hydrogen atom, say, has a nucleus consisting of a single proton and a single electron buzzing around it as a quantum probability cloud. All the space between the two has no matter in it at all. (Sure you want to sit in that chair?)

But just how empty is an atom? The guy teaching my DVD lecture course on astronomy did some math. It turns out that our hydrogen atom is empty indeed. It is 99.999999999999 percent empty. That's 14 nines. (Still going to sit in the chair?)

But there is emptiness, and there is emptiness. The teacher (Alex Filippenko, University of California, Berkeley) did some more math and found that the ratio of the radius of that electron cloud to the radius of that proton is about 50,000. Then, rather arbitrarily, he compared that result to the Milky Way Galaxy by finding the ratio of the distance to the nearest star (4.2 light years) to our sun's radius of 700,000 kilometers. That ratio turns out to be about 60 million. Considering just the stars, anyway - which of course are a rather small part of the whole considering the dark stuff - our galaxy is 1,200 times emptier than an atom!

And then there's my brain. But let's not go there.

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