#15 Three Horizons

If you're standing on the edge of the beach, the ocean looks enormous, as if it stretches off forever. However, because of the Earth's curvature, the horizon is only three miles away (a little farther, if you're wearing platform shoes, or stilts and a clown costume). You can really only see a very small amount of the ocean. The Pacific Ocean, for example, is roughly nine thousand miles across.

When you look up at the night sky, it's fun to try to imagine how huge our galaxy is. It contains at least two hundred billion stars. However, because most of the stars are very dim and far away, you can usually only see several hundred of them without a telescope. The farthest star you can see is about a thousand light years away, and our galaxy is a hundred times larger than that. In short, you can't see squat. And if you live in a big city with lots of light pollution, you might as well have a paper bag over your head with a rotting fish in it. That's how much of our galaxy you can see.

The term "observable universe" refers to the portion of the universe that we could theoretically see, based on limitations of the speed of light, the age of the universe, its rate of expansion, and whether or not there's a tall person with a hat sitting in front of you. Current evidence suggests that this is a sphere about 95 billion light years across. That's getting pretty high up there in the overwhelmingly large department. However, the actual universe might be much, much larger than this. So far, there's no way to really know how large it is, and scientists can only guess. One theory suggests that the ratio between the size of the whole entire universe we can't see, and the smaller, merely observable universe might be the same as the ratio between the size of the Earth and a proton. The scientists who proposed this theory had to invent specialized equipment to achieve the exponentially large amounts of smugness that were required to discuss the theory in public with other scientists.

What I am really saying here is that as far as I'm concerned, you're fully justified in staying at home on the sofa, playing video games, eating ice cream, and getting super fat. If you were to bother to go outside, then even under the most ideal conditions, you wouldn't get to see very much of anything anyway.

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