What are force-carrying particles and how do they work in quantum mechanics?

[Unlockitall]

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

In quantum mechanics, the forces or interactions between matter partticles are supposed to be carried by particles.

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.

 

[NanakiXIII]

I think the analogy is only meant to work one way, don't break your head over it.

 

[tom.stoer]

The problem is that the cannon ball picture is very very misleading.

There is not one messenger particle, but sum over all possible messenger particles. The exchanged particles (summed over) differ by energy and momentum, they even violate certain properties classical particles have.

My picture is not really good, but perhaps it makes clear that it's not a problem with your understanding but with the missleading picture of messenger particles. First it looks good, but if you think about it (as you did) it's getting weird.

 

[K^2]

Yeah... The problem is that these aren't real particles. They are really fields that you break into components each of which works kind of like a particle. Them not being real particles allows them to do some strange things. One of these is being able to move in one direction, while carrying momentum that's pointing in a completely different direction. So in the cannon-ball example, you'd be shooting it, and the kickback from that would be in the same direction you fired.

Can't do that with real particles. Can do that as a mathematical trick with virtual ones.

 

[Unlockitall]

O.k., thanks. I knew there had to be something weird going on.

 

[Frimus]

As for an attractive force carried by particles, there exists a rough analogy. Imagine two skaters facing in opposite directions and throwing/catching a boomerang. A guy throws a boomerang and as a consequence he is pushed to his partner. The boomerang makes a loop and the other guy catches it, which causes him to be pushed to the first guy.