- Mar 24, 2005
While I understand what you're saying, I'll also point out that gravity indicates a "down" direction right here in the 3-D world we experience.Up and down are just subjective ways of visualizing the pinching and stretching of spacetime.
If there were such a thing, up and down would be along a fourth dimensional axis.
But I think it a mistake to confuse the common visualization (model) of spacetime with the reality. The math is where it's at, but I have not messed around with tensors since grad school, so that's not happening.
So, what construct would indicate to us an "upward" direction?
The "stretching" of spacetime you mention is called positive curvature. It makes a "saddle shaped" universe at its basis, like a stack of infinitely large Pringles chips. Each chip being a 2-d representation of a moment.
There is no antigravity associated with that kind of curvature in any cosmological theory. Maybe because there is no construct that causes it. There's nothing to be forced "up" from.
The negative curvature caused by mass is relative. For example (according to General Relativity,) if the Moon could think, it would think it was traveling in a straight line. A pair of observers moving on parallel routes through a positively curved space would find they are getting farther apart, even though they are both traveling in straight lines. Maybe you could translate that as a force separating them, but they (like the Moon) would not experience that force themselves. Just the force required to keep their courses parallel.
I suppose you could think of this constant separation of the two courses (and the constant need for correction) as antigravity, but, given that the positive curvature is not caused by a thing, the idea is useless for any practical application.