I was reading the lectures compiled in "Six not-so-easy pieces" by the great Feynman. He was trying to explain the curvature of space(and subsequently space-time) through varying temperatures on a hot plate. Much like the curve of a sphere. Anyways I'm going to assume most of your are familiar with this, since Feynman is pretty famous. So he was saying, that if you reverse the temperatures on the hotplate(ie, cold in center, hot on out side, or vice versa) you would come up with negative curvature. So, the way I understood it, mass curves space-time "positively". So if you were to gather negative mass(tachyons?) in a location, you could produce perhaps, anti-gravity, and maybe travel backwards in time due to the "negative" curvature of space. I guess I need to say, that everytime I mention "anti-" or "negative" or "backwards" I mean relative to our ordinary FOR. I really don't know much of what I'm talking about, but it's nice to discuss with people who DO know. So how little do I understand this?