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Saturday, January 24, 2015

The very large, the very small, and the emptiness

The phrase "a picture is worth a thousand words" and similar phrases in the English language were popularized only 100 years ago, but the truth is universal. This fantastic website proves the point. I've meant to blog about it since I first saw it in 2012.

Click Start. You see the slider centered at a length of one meter, a distance that human beings can easily comprehend. Drag the slider left and right, and you'll see the entire universe as we measure it — from the inconceivably small to the inconceivably large, a span of 62 orders of magnitude. The same information is available in list form, but words and numbers aren't as compelling a medium as animation.

To physicists the universe of 62 orders of magnitude is a happy hunting ground. The rest of us, including myself, find it difficult if not impossible to grasp fully the wideness of that range. One conclusion is clear: the universe is basically an empty place. This depiction of a lithium atom is worse than inaccurate, but we saw it so many times in the 1950s and 1960s that it's burned into our brains.

Electrons don't have fixed orbits, for one thing. But the most egregious mistake is scale. It makes the electron, neutrons, and protons look large relative to the diameter of the atom. False! The Huang brothers' website gets it right. The atomic diameter is roughly 100 picometers, but the nuclear diameter is roughly 10 femtometers. That's a ratio of 10,000:1. In other words, the interior of an atom is nearly all empty space. And if you break apart the neutrons and protons, their constituent quarks are orders of magnitude smaller. Again, the interior of the protons and neutrons is nearly all empty space.

At the macro level, we know that our solar system is quite spread out. Think it's a long way from Earth to Pluto? It is. But the nearest star system, Alpha Centauri, is 6500 times farther away from us than Pluto. In between, there is empty space aside from a molecule here or there. From the Huang website we see that Alpha Centauri is figuratively touching our eyeballs compared to the diameter of our galaxy, the Milky Way — most of which is empty. And the distance to the next galaxy is, again, several orders of magnitude of empty space.

Mind-boggling, don't you agree?