Re: How do force carrier particles exert attractive forces?

Sarah,

I am asked this question often, and my quick answer is "throw a basketball with negative mass"! In the case of electromagnetism where a massless photon is exchanged, this answer can be easily modified to read "throw a photon with negative momentum". With a little thought, you can see that these answers are essentially the same. When you "throw" a particle with negative momentum, you would recoil in the direction that you threw it. "Catching" the particle would again cause a momentum change in the direction that the exchange particle came from.

This is, of course, only a crude analogy of the physics of virtual particles. A more complete plain-English explanation can be found in part 4 of the sci.physics FAQ (item 32). There, the explanation of attractive forces uses the fact that a well-known momentum transfer between two particles implies a virtually unknowable spatial position of the exchange particle, due to the Heisenberg Uncertainty Principle. This, the explanation goes, makes it possible for the exchange particle's "trajectory" to point from the receiving particle to the emitting one. While this explanation is closer to the "truth" than my own quick answer, mine is simply a more simplistic way of describing the same mathematics.

The "moral" to be learned from all of this is that the lines in Feynman diagrams are not to be thought of as literal trajectories of real particles. I encourage you to ponder these and other explanations, and come to your own understanding to share with your students. If you would like further explanation, or need someone to bounce ideas off of, please feel free to send me an email at silver@physto.se.

Hope this helps!

Sam