> >Orbital mechanics is just fine with > >that. > > No, it isn't. If the force of gravity was directly in line with where we > see the sun with light that is now 8 minutes old, it doesn't work. Period, > full stop. It only works if that vector force is pointed at where the sun > is now, not where we see it with that 8 minute old light. This is easiest to understand from the reference frame where the Sun is standing still (and the Earth moves). Sun generates the the gravitation force field that is static, ie. it does not change with time. So, whereever the Earth is at some particular moment, it feels the field at that point (approximating the Earth with a single point, which is ok in this case). The force is independent of time, so the whole thing has nothing to do with the time it takes for light to travel between the Sun and the Earth. Generally, the velocity of propagation of gravity has a net effect only when the field itself is varying with time, and in most Solar system mechanics it does not (it does, but very, very, very, veeeeeery, miserably slow). As a thought experiment, suppose that for some reason the Sun disappears at some instance of time, along with it's gravitational field and the light it emits (I know it's impossible, but for the moment imagine it was). Now, we on Earth would *see* that the Sun is gone 8 minutes later, when the light goes off. And the trajectory of Earth's orbit would change also 8 minutes later. The abrupt change in the gravitational field at the point where Sun resided would generate a time-varying gravitational wave that would take 8 minutes to reach Earth, if it moves with the speed of light. It would take shorter time if gravity would propagate faster, or more time if it propagated slower than light. But if there is no time-varying gravitational field, as is the case in orbital motion of Earth around the Sun, there is nothing to be propagated, so the velocity of propagation is irrelevant. Orbital mechanics is quite in agreement with any velocity of gravitational waves, be it slower, equal or grater then velocity of light. You can trust me on that one. :-) > The diff is > what, 700 miles but I forget the time frame, minutes or seconds. The Earth's orbital velocity is (roughly) 30 km/s, so in 8 minutes it moves for around 30*8*60 = 14400 kilometers. Now, IIRC, a mile is about two kilometers, so it amounts to around 7000 miles or so. I guess that was what you were reffering to. :-) > There is even a web page, complete with the math examples for those that > can double check the results, that while not offering a finite value for > the pv speed of gravity, comes to the conclusion that it cannot be less > than 10x C speed, and probably much faster than that, in order for the > orbital math to match reality. I was going to snip it from my ff > bookmarks, but it seems to have been a casualty of an upgrade. I found it > with a google search about 3 years ago. "Do not trust a man with a digital wrist watch." "Do not trust everything you read on the Web." Ancient Chinese sayings... :-) (As a side note, I do wear a wrist watch, but it is both analog and digital, so I don't know how much I should be trusted... :-) ) Best, :-) Marko Marko Vojinovic Institute of Physics University of Belgrade ====================== e-mail: vmarko@xxxxxxxxxxxx
