|MadSci Network: Physics|
Since you're asking not to have math, we'll back up a bit on this question. You can think of gravitation as the exchange of gravitons, just like you can think of the attraction or repulsion of charged bodies as the exchange of photons...but there's not even a complete theory of quantum gravitation in existence yet so we won't discuss the possibility of gravitons (which have never been observed). Discussing gravitons simply isn't necessary in this case. We'll start with gravitation as simple waves and static fields. The concept you mention has been purveyed by science fiction authors before, Alan Dean Foster comes to mind with something in one of his books called a KK drive. Don't recall what KK stood for, but as usual there was a grain of science in the science fiction author's notion of a spaceflight drive. You could indeed focus gravitational waves (not quite the same concept) from some source along the path of a spaceship and drive it. The problem comes in generating these waves and getting them to focus. Gravitational waves spread out over distance. But if some mechanism for producing them (we're getting beyond the realm of what science knows how to do) and focusing them was created then it could plausibly stretch spacetime in the direction that you wanted the spaceship to travel, causing an acceleration. Such a mechanism, because of the coupling strength of gravitation, would most likely involve fast-moving masses at high speeds, it's an open question as to whether or not you could even get it to work at short range and whether or not it would be powerful enough to be useful. But even weak thrusts over long times/distance (google "solar sail" for recent news of the development of such a propulsion system) can get you places in space.
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