I just finished reading A Briefer History of Time, and somehow, for the most part, it made sense to me. Some questions did come up in my mind, though, so I figured I'd ask for some help. The biggest question I have is on force-carrying particles. In the book it says It then goes on to describe how these force-carrying particles are kind of like a cannonball being shot out of one particle into another, thereby changing the trajectories of both particles. I found this confusing because, while it makes sense to describe a repulsive force this way, it makes absolutely none to describe an attractive force this way. The only thought that came to mind was that it could be a repulsive force moving backwards though time. This seems rediculous to me, but I'm not really sure if that is completely off base, because there are stranger ideas mentioned in the book. Any help in clearing up this confusion would really be appreciated.