Let us be a little more concrete, now. Let's limit ourselves to only two
kinds of particles: electrons and photons. We'll also have to throw in the
electron's antiparticle, the positron. (Photons are their own antiparticles.)
Imagine first a dull world where a bare electron wishes to propagate from
point A to point B, as Zeno did in my Three-Part Invention. A physicist would
draw a picture like this:
A •• ------... _----........ B
There is a mathematical expression which corresponds to this line and its
endpoints, and it is easy to write down. With it, a physicist can understand
the behavior of the bare electron in this trajectory.
Now let us "turn on" the electromagnetic interaction, whereby elec-
trons and photons interact. Although there are no photons in the scene,
there will nevertheless be profound consequences even for this simple
trajectory. I n particular, our electron now becomes capable of emitting and
then reabsorbing virtual photons-photons which flicker in and out of exis-
tence before they can be seen. Let us show one such process:
A •• .. .B
Now as our electron propagates, it may emit and reabsorb one photon after
another, or it may even nest them, as shown below:A •• • .B
The mathematical expressions corresponding to these diagrams-called
"Feynman diagrams"-are easy to write down, but they are harder to
calculate than that for the bare electron. But what really complicates mat-
ters is that a photon (real or virtual) can decay for a brief moment into an
electron-positron pair. Then these two annihilate each other, and, as if by
magic, the original photon reappears. This sort of process is shown below:The electron has a right-pointing arrow, while the positron's arrow points
leftwards.(^144) Recursive Structures and Processes