On Wednesday, July 21, 2010, wrote:
>Hang on, if your twin sees you as moving at .9c and you are seeing him
>move at .9c, why does anyone age differently?
That's why it's called the Twin Paradox. Special relativity
(which really deals *only* with motions at constant velocity in
empty space) says that there's *no* way to tell who's actually
moving (and thus no way to tell whose clock is actually going
faster), so in theory *each* twin sees *the other* as aging more slowly.
What breaks the degeneracy? Acceleration. The twin who
accelerated and then decelerated back into the same reference
frame is the one who ends up being younger.
General Relativity takes the same theory and then looks at
gravity. Simplifying drastically, it asks the question "Why is
inertial mass (the mass that resists acceleration, aka the m in
F=ma) always the same as gravitational mass (the mass that
affects gravitational attraction, aka both "m"s in F =
Gm1m2/r^2) whenever we measure them?" If they're actually
measuring different things (inertia vs. gravity), there's no
reason they should be. As such, GR postulates that they're the
*same* thing, and that the force of gravity, as experienced, is
essentially an acceleration (I'm not putting this very well, but
hopefully the essence will come through). Then you start having
to calculate stress-energy tensors, and Christoffel symbols, and
it's been a decade since I last took a general relativity course
so I hope you'll forgive me if I don't go into the math right now.
-Brian
--
Brian York <>
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