dipper@utastro.UUCP (Debbie Byrd) (01/09/86)
We can tell up from down because of the force of gravity. More about gravity -- after this. January 9 Gravity Up and down. That's an easy concept. And yet we have an "up" and a "down" only because there is gravity in the universe -- one of the four great forces of nature -- a force that pulls. The Greek philosopher Aristotle explained up and down by saying that all bodies have a "natural place" in the universe. Earthly bodies go down toward the Earth, naturally. Gaseous substances, such as smoke, rise toward the less substantial spiritual realm. But Aristotle was wrong. It wasn't until the 17th century that Isaac Newton took the first step toward a scientific explanation of up and down when he published his theory of gravity. Albert Einstein further explained gravity earlier in this century in his general theory of relativity. Now we know gravity as a universal force. It acts not just on the Earth, but on all bodies in space -- including yours. Every particle in the universe pulls because of gravity. Huge collections of particles pull harder because the force of gravity accumulates where there are many particles, that is to say, large amounts of mass. Gravity binds planets in orbit around stars. It binds stars into galaxies. It keeps the stars themselves from "coming unglued" -- or simply floating away into space. But as one of the four fundamental forces of nature, gravity is really a feeble force. It's much weaker for example that binds electrons to the central nucleus of an atom. If the electrons in a hydrogen atom were bound by gravity, rather than electricity, the smallest electron orbit would be larger than the observable universe. Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin