The Moon

Why does mass exert gravity?

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November 11, 2007 7:31PM

The teacher's manual answer is that mass warps space. But the real answer is "Just Because" it's what mass does. == It's interesting in this day and age when we're on the threshold of discovering the very basic nature of the universe by virtue of our understanding of matter and spacetime that we don't know more about gravity. For as long as I can remember, the best answer for the question "what is gravity?" is this one: We don't really know. It's still true. Accept it. Say it with me: we don't really know. See? It didn't stop working just because we made that admission. We know what gravity does. There is a wonderful history associated with its quantification. We can sure measure its effects, can't we? Those elegant sets of calculations for, say, the music of the spheres, are nothing short of breathtaking. But what is it that is accessible to the rest of us that lets us get a handle on gravity? There really isn't too much. Let's review. Matter has mass and occupies space. By virtue of just existing, matter warps spacetime. It puts a dent in it. But that doesn't provide too much extra help. Spacetime is (hold on to your seat) a mathematical construct. Darn it! That ain't any help! Perhaps an experiment would help. Try something that's been passed along to teachers here and there. The necessary equipment list includes a bed sheet and two modestly heavy balls like croquet balls. Lighter balls like tennis balls won't work as well, but a bit of creative thinking and experimenting will allow one to make a good substitute for the wooden spheres. Four students hold a bed sheet by the corners and stretch it out. Hold it steady! Put a ball in near the middle. See the dent? That's what mass (the ball) does to spacetiime (the sheet). This is a two-dimensional model, but it is really quite good. Now add the second ball near the first one. (A bit of practice might be required because there are limits on what the bed sheet will allow. Try short sheeting the setup from one or from two adjacent sides.) The second ball makes a dent like the first one, but the space between them is deformed, and the balls will roll together. If they are placed too far apart, the demonstration breaks down, but that's where the practice comes in. Lastly, it's absolutely wonderful and exciting that we don't know more about gravity! We know all this other stuff. All this other science and technological stuff. And yet here is gravity! It is so big, so broad in its effect, that it is the large scale organizer of the whole universe itself! We have spent our entire lives in it, and we react to it without thinking when we move. And yet it stands in defiance of our understanding! Hey! Anybody wanna Nobel prize? Wanna become world famous? Wanna have hotties asking you for your phone number? We have this little problem that needs solving....