A Brief Description of Spacetime

Jillian's Guide to Gravitational Waves : Spacetime Ripples

Best if I start at the beginning (not the big bang, that's later). I'm sure you've heard the much bandied about description of this universe wrapped up with the neat little label 'spacetime.' I'm going to ask you to imagine that we're living in a two-dimensional world just like living in a piece of paper. Why? See, spacetime for a three dimensional world plus the time dimension is a 4D shape, which, understandably, humans have a tough time picturing. Spacetime for a 2D universe like that paper would be a 3D thing, which I can describe to you rather well (if I do say so myself!). Thus, please, dear surfer, don't tax my poor brain by asking me to describe anything higher than three dimensions.

So, we've got this infinite 3D jello-mold of the universe, here, to serve as my analogy (wobble, wobble). Let's take a slice---schlok! What does this slice represent? This is the shape of spacetime at a certain time. See, over there is a star---see how the jello bends down? Over yonder is a black hole---look at the point on that sucker! See, there, that double-bumped dip is a binary system. These dips in spacetime are called gravity wells. The presence of matter or energy (lots and lots of energy) warps spacetime. Anyway, when spacetime bends, it manifests as something we call 'gravity.' The more stuff, the more bend. The more bend, the more gravity. If you get enough stuff within a certain area, you pop! right through the jello slice and get a black hole, but that's material for another section.

For a long time people believe this was a calm, peaceful, orderly universe, where stars danced eternally in stately paths through inky empty darkness. Now, of course, we've seen the contrary. New stars ignite and begin to burn themselves up in their nuclear bonfire, dying stars explode and viciously shove off skads of gas and leave sullen crisped orbs in their place, high-energy particles scream away at fractional-light speeds from the cores of galaxies and black holes, and in between everything is a fog of dust and gas, spiced by the flickers of virtual particles. Certainly an interesting place!

What's this got to do with our slice o' jello-mold-turned-spacetime? It means that objects do some interesting things over time. Take that supernova. Watch it as it happens, and you'll see a ripple move out from the star. What was that shockwave?! Look more closely at that black hole and you'll see it vibrate ever-so-slightly. Take the binary---it's radiating ripples the way a bobbing soccer ball creates ripples in water. Waves...in spacetime?

Recall how warped spacetime is detected as gravity? Well, the principle holds with this wave, too. Just like the stationary dips in spacetime are gravity wells, those travelling waves of spacetime are also manifestations of gravity. We know what a gravity well feels like---we life with the tug of gravity all of our lives; but what does a gravitational wave feel like when it hits? How fast does it move? If it's a wave, what's the equivalent of the jello particles that bounce around? These are all good questions!! Firstly, the mechanics (or, what a gravitational wave feels like when it hits).

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