Mechanics of Gravitational Waves

Jillian's Guide to Gravitational Waves : Mechanics

The effects of a gravity wave are seen perpendicular to the direction in which the wave is travelling. Hunh? If a wave is moving towards you---dead on, straight atcha---you'll see the effects of the wave left and right and up and down, not forwards and backwards. As much as people compare gravity waves to sound waves (for some good reasons---later!), which are just compression waves, the comparison is not complete. You always see such waves in the movies; you know, an oil truck blows up and the heroine gets thrown away from the truck to land splat! on the asphalt further back. That's a compression wave---it affects stuff parallel to the direction of propagation (that is to say, it rattles stuff back and forth in the same direction in which it moves). Gravity waves are not oil truck explosions! If you stuck gravity waves in the place of the explosion from the oil truck, the heroine would have been rattled left and right and up and down---but she would not have moved one inch away from the truck!

I keep mentioning ripples in jello molds and sound waves and water ripples, but those are all little atoms jiggling around like mad. Those are just compression-wave analogies to describe what a gravitational wave does. The gravitational wave is, upon closer examination, much more like a light ray than a sound wave. D'you get it? Electromagnetic field/gravity well --- electromagnetic wave/gravity wave (AND transverse effects to propagating waves!) --- photon/graviton. Graviton? Want a more detailed run-through of that? Check out the parallels section, which is after this one.

"Rattled left and right and up and down"---what does that mean, exactly? That's just what a gravitational wave does. Take the unfortunate heroine-near-the-oil-truck. Say there's this extremely powerful gravitational wave whooshing towards her (at the speed that it moves, whooshing is a woefully understated description). By the by waves this powerful aren't anything we could experience. Before the wave hits she's 6 feet tall and 2 feet wide. As the wave hits, she is first stretched vertically to 8 feet and squished horizontally to 1.5 feet, returned to normal, squished vertically and stretched horizontally, and returned to normal. That was one complete oscillation of one particular type of gravitational wave. Here's the best part: not only was the heroine stretched in such painful ways, but everything around her was, too. I should stress that this particular gravitational wave was extremely strong. We don't get waves that strong on the earth. Another thing is that even though everything was stretched and compressed, it still would have hurt a lot for the heroine. The actual stretching-and-compressing of the gravitational waves is more complex that this, but I saved that for a part in the parallels section.

Say our heroine wished to avoid the gravity wave speeding towards her. Could she do it? How fast does a gravitational wave move, and does it matter what it moves through? Fair enough, I mean, compression waves travel at different speeds as they move through different consistencies. Say I pound the bottom of a table, right underneath a glass bowl filled with jello. The wave travels through the table at once speed, enters the glass and changes speeds, enters the jello and changes speeds again, and pops into the air, changing speeds yet one more time. All those different speeds! What speed is right for a gravitational wave? C, the speed of light: 3 x 10^ 8 meters per second. Ah, so, perhaps our heroine won't be able to avoid the wave. What about putting up a jello-mold barrier so that the wave slows down enough for her to out run the wave? Does a gravitational wave slow down as it changes media? No. Should it be through gas, star, galaxy, black hole, or jello, gravitational waves move at the speed of light---constantly. Funny thing is that not much affects these waves. Very few things would change a gravitational wave, making them rather handy echo-locations for astronomers, should they detect them, which they haven't; but it'll be really cool when they do, but they haven't. Why not? It has to do with the strength of the signals.

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