icarus wrote:Okay, so the rest (not the spacecraft) of the mass of the universe moves (ever so slightly) with respect to the mass of the entire universe and the center of mass of the whole system is stationary?
And you don't see any problem with the logic here?
Nope. I'm starting to wonder if you've ever actually worked a simple vector mechanics problem...
universe includes the spacecraft. That's the key.
You have (I trust) no problem with the concept of a spacecraft accelerating, provided that a force is applied to it?
Another facetious, rhetorical question ... give it up, you were busted.
I was trying to lead into the rest of the explanation. Yes, I'm getting impatient and it's affecting my style. You seem to be too busy being contemptuous to actually understand what I'm talking about.
How about you come out and explain what it is you think is wrong with the conservation-of-momentum argument, instead of hinting around that I'm some kind of idiot for not figuring it out myself?
darn right it is a question of scale. Now you have involved the whole of the rest of the mass of the universe, (but not the entire universe I should remember which universe I'm talking about here it seems). So now tell me how quickly is this information that rest of the mass of the universe has moved is transmitted to the center of mass telling it to stay where it is? (and at the same time perhaps you'll explain how fast the information travelled from the spacecraft to the rest of the universe telling it that it should move)
Hint: the answer can not be instantaneously as per GR.
That's a completely different issue and you know it. Talk to Paul March about gravinertial fields and reversed-time waves; I'm not going to get into that.
how quickly is this information that rest of the mass of the universe has moved is transmitted to the center of mass telling it to stay where it is
is nonsense; either it was posted too quickly or you are somehow under the impression that the "center of mass" is somehow a separate, discrete thing rather than just the abstract concept of a mass-average of the "rest of the mass" and the spacecraft...
Bear in mind that according to current theory the universe doesn't have
a centre of mass in the location-in-3D-space sense, only in the velocity/momentum sense...