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I'd always wondered about the second panel myself... - John [entries|archive|friends|userinfo]

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I'd always wondered about the second panel myself... [May. 7th, 2011|10:43 am]

[User Picture]From: wcg
2011-05-07 06:04 pm (UTC)
I can answer that!

Spacetime is distorted by the presence of mass. The rubber sheet analogy is only a 2d approximation. But it's one that most people can visualize. The "force" we call gravity is, in General Relativistic terms, the natural inclination of any object possessing mass to fall into the "gravity well" of any other massive object. You can also draw a map of equi-gravi-potential curves, kind of like a topographical map of a valley. Anywhere on a given curve the strength of the gravitational field would be equal.

Where we approach a quantum model of gravity is when we ask "What is it that exchanges information between two -- or more -- massive objects about their mass?" Is there some intermediate vector boson, like the photon for light, which mediates the interaction? Newton famously wrote, "I make no hypothesis," about the apparent action at a distance. GR explains it as a gravity field with curvature that is influenced by the presence of mass. In a purely GR interpretation, it's the field itself that mediates the interaction of the interacting masses. But that's unsatisfactory in that it does nothing to explain the other known forces in the universe. So we want to get to a quantum model of gravity too. But we're not there yet.
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[User Picture]From: fourgates
2011-05-08 05:01 am (UTC)
I ran into this today: Einstein for Everyone
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[User Picture]From: johnpalmer
2011-05-08 05:41 am (UTC)
That looks like some good reading. But, to make my point clear, what actually made me giggle at the xkcd was the second panel. At one point, I realized that the imagining of a stretched rubber sheet was dependent on gravity. And I never had anyone to explain "well, yes, it doesn't work, it's just a model to help people understand what it's *like*." Eventually, I figured it out on my own, and I was tickled to see that exact issue pop up in an xkcd cartoon. It makes me feel a tiny bit more interesting than I had before :-).
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