On 12/02/2010 10:58 AM, Terry Blanton wrote: > If gravity propagated at the speed of light, the earth would not orbit > the true position of the sun but where it was 930/1.86 = 500 seconds > ago. And if the sun winked out of existence, the earth would wait > 8-1/3 minutes before flinging off on a tangent. > Does science support these suppositions? >
No. Or, rather, yes. Or, well... In general, gravity can be viewed as not "propagating" at all, any more than a static electric field "propagates". Rather, *changes* to the gravitational field propagate at C, just as changes to the E field propagate at C. So, in particular, if you look at a uniformly moving body like the Sun, the G vector in space around it points directly toward the sun -- it doesn't lag. Similarly, if the sun has a negative charge, and you measure the E vector in space near it, you'll find it points directly toward the sun -- it also doesn't lag. Consequently the Earth orbits the current location of the Sun, not a "stale" location from 8 minutes ago. However, an *accelerating* body which is L units away has an E field which points to the position it *would* *have* *occupied* had it continued at uniform velocity from the position (and speed) it had L/C seconds ago. That goes by the rubric "abberation". In particular, if the Earth had an electric charge, then while the Earth would see the Sun as being where it currently is based on the Sun's electric field, the Sun would see the Earth as being slightly shifted from its orbit, based on the Earth's electric field. (Or at least I think it would.) Gravity, however, doesn't behave like an electric field, and for reasons I can't pretend to have a clue about, GR supposedly says that the Sun "sees" the Earth as being where it currently is, rather than where it would have been had it continued on a straight line for the last 8 minutes. Intuitively you can think of the situation as being that the Sun "sees" the Earth as being where it would have been if it had traveled along a *geodesic* rather than going in a "straight line" -- and, in fact, that's exactly what it does. And as to the Sun magically disappearing and leaving no mass behind -- the GR equations fall into pieces if you allow something like that to happen, so it's anybody's guess what the result would be. If you want a less garbled and far more intense answer, here's a paper by Steve Carlip which may explain it all (if I understood it I could be more definite about whether it really does): http://xxx.lanl.gov/abs/gr-qc/9909087 > T > >
