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Gravity 101 

I happened to walk into the middle of a conversation in my local coffee hangout the other day. Two of the regulars were engaged in an animated discussion about alien abductions, psychic surgery and the end of the Earth (we've got three more years). As I got up for a refill, one of the participants noted that I didn't seem to go in for such weird metaphysics. I said, "It's just that I'm more interested in the really weird stuff," I said. "Like what?" he asked. "Well ... gravity, for instance."

I'm not kidding. Tales of the paranormal are just too easy to explain away -- hey, we all want to be famous and get interviewed on Coast to Coast AM and sell first-person books, right? But gravity? Try explaining that to Art Bell. Even Isaac Newton, the man who figured out that an apple falling from a tree and the moon orbiting the Earth were acted on by the same force, even he gave up trying to actually explain it. Having unified apples and moons by postulating that the attractive force between two bodies diminishes by the square of the distance between them, he added the prophetic caveat, "That gravity should be innate, inherent, and essential to matter, so that one body may act upon another, at a distance through a vacuum ... is to me ... an absurdity."

Let a pebble fall from your fingers to see what he was talking about. It goes downward, of course, or more precisely, it heads to Earth's center of gravity. The question that bothered Newton, and would probably bother us had we lived our lives in the weightlessness of space before landing on Earth to do the experiment, is this: how does the pebble know which way is down? No little arrows in the air, no disembodied voices telling it which way to fall. It just knows, as if an invisible rubber band was connecting it to the Earth's center. It is, as Newton wrote, an absurdity.

It took Einstein to replace Newton's notion of bodies somehow acting remotely on each other with his picture of "curved" space, in which massive objects "deform the fabric of space": mass tells space how to curve, while space tells mass how to move. The beauty of Einstein's metaphor is that it does away with spooky forces acting at a distance, replacing them with local curvature. As an analogy, when you go down a playground slide, your body reacts to the immediate (local) slope of the slide, rather than being pulled to the ground below by some force acting at a distance.

When you let go of your pebble, it was obvious that it felt Earth's gravity. What wasn't so obvious is that the Earth felt the pebble's gravity -- as the pebble fell to Earth, the Earth rose slightly to the pebble -- very slightly, in proportion to the mass of the pebble divided by the mass of the Earth. Now that's weird!

Barry Evans is enjoying his time between the weightlessness of the oceans (from whence came his forebears) and the weightlessness of space (where humanity will spend most of its history). He lives in Old Town Eureka.

CAPTION: Einstein's picture of space near the Earth. Neither the apple nor the moon are "aware" of Earth's presence, all they "know" is the curvature of their local space.

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About The Author

Barry Evans

Barry Evans

Barry Evans lives in Old Town Eureka with his girlfriend (and wife) Louisa Rogers, several kayaks and bikes, and a stuffed gorilla named “Nameless.” A recovering civil engineer, he is the author of two McGraw-Hill popular science books and has taught science and history. His Field Notes anthologies are available... more

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