well, don't tell me, tell tell him:
"Suppose that we are trying to calculate the probability (or,
actually, the probability amplitude) that some amount of momentum, p,
gets transferred between a couple of particles that are fairly well-
localized. The uncertainty principle says that definite momentum is
associated with a huge uncertainty in position. A virtual particle
with momentum p corresponds to a plane wave filling all of space, with
no definite position at all. It doesn't matter which way the momentum
points; that just determines how the wavefronts are oriented. Since
the wave is everywhere, the photon can be created by one particle and
absorbed by the other, no matter where they are. If the momentum
transferred by the wave points in the direction from the receiving
particle to the emitting one, the effect is that of an attractive
force."
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
