# Ultraperipheral Collisions: the Feynman diagram

** Published:**

Hello again. Thanks for being so interested!

Last time (such a long time ago!) I actually only gave the definition of a UPC process. Essentially, the two interaction partners are so far away from each other that a photon is being emitted instead. This is shown here:

This figure here is actually something called a Feynman diagram… Physicists use them to compute the probability of a process happening. Of course, they also *look quite good*…

You see, a photon is there, and from the other side a grey block is emitted. That is something called a Pomeron. Wikipedia is being complicated about it, but essentially what it is is simply two or more gluons bundled together. Again, what is a gluon? Too complicated, but essentially they are the *force carrier* of the strong interaction.