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The Atomic Universe

Week of October 12, 1998
Nature can be remarkably frugal. For example, the spherical shape can be found everywhere in Nature, from things as small as a drop of water floating freely in space to giant galaxies hundreds of thousands of light years across. Maybe even more remarkable is that two completely different forces are responsible for those shapes. Electromagnetic force between the molecules of water in the surface of the drop force the drop to become spherical (as long as there are no other forces on it, which is why I put it in space). Gravity is the force that reaches across countless trillions of kilometers and sculpts the galaxy into a ball. This happens because both electromagnetism and gravity behave in similar manners. They are radial forces (that is, they work in a way that points to the source) and both drop in strength with distance in the same way.

Some people see this similarity and wonder if it goes even farther. The Earth has a lone moon orbiting it, and an atom of hydrogen has a lone electron going around it. Maybe the whole Universe is nothing more than a giant atom!

I see this analogy on the internet sometimes, and while poetic, it's flawed. First, electrons do not really orbit the nucleus of an atom. That is an old-fashioned way of saying it, and does not include what we call quantum mechanics. The Universe, on a very small scale, does not behave in ways that make much sense to us. Details are very difficult to explain here, but the way it works out, electrons can only be at very specific distances from the nucleus, called "preferred energy levels". A planet, on the other hand, can be at any distance from its star.

Worse, gravity and electromagnetism have a fundamental difference: while you can have positive and negative electricity (and north and south magnetic poles), gravity only has positive mass. Electromagnetism can either attract or repel, but gravity can only attract. Finally, electromagnetism is tremendously stronger than gravity; it is about 1040 times stronger! That's a 1 followed by 40 zeros, or a ten thousand trillion trillion trillion times stronger, if I didn't drop a zero somewhere. Think of it this way: a tiny magnet can attract an iron needle off a table, even though the entire mass of the Earth is pulling it the other way using gravity!

So it would be nice to be able to make an Atomic Universe analogy, but as usual, Nature is more subtle than we can easily know.



©2008 Phil Plait. All Rights Reserved.

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