Boxes

Some more Feynman diagrams from quantum electrodynamics (QED), with squiggly lines representing photons and straight lines representing electrons or positrons (their anti-particle), depending upon which way you read the arrows. The process represents an entirely quantum process, with no classical counterpart, of light scattering off light. You can read the diagrams any way you like, running time in your chosen direction, and you’ll get a sensible story. For example two photons approach each other, (looking at the first of the three box diagrams) and one turns itself into an electron-positron pair, the positron goes off and absorbs with the other photon… meanwhile the electron goes off and emits a new photon and then it meets the positron again and they annihilate producing a photon again. So, two photons in, two photons out, all orchestrated by the dance of matter and antimatter. I’ll leave it to you to find the alternative stories told by the other two diagrams. The plus sign says that they all happen, and the answer is the sum of all the possibilities…

This was the subject of one of yesterday’s midterm questions for the class. Their main goal was to show that a piece of each of these stories (the “divergent” bit) actually cancels out when you add them together… because of the properties of this thing that I won’t explain: [tex] g^{\rho\sigma}g^{\nu\mu}+g^{\rho\mu}g^{\sigma\nu}-2g^{\sigma\mu}g^{\rho\nu}[/tex]

-cvj