Feynman rule
In a Feynman diagram, the horizontal axis represents space and the vertical axis represents time, although the horizontal axis does not literally measure distance. A typical diagram shows electrons that approach each other, interact, and then move apart. Because the probability drops by roughly a factor of 100 with each additional vertex, the higher-order contributions fall off so quickly that they effectively tame the infinities.
Solving quantum mechanics: the process of removing infinities
By the uncertainty principle, apart from the observable incoming and outgoing particles, the infinitely many possible particles exchanged during an interaction can move in ways that ignore relativity, including particles that travel backward in time or faster than light. Mathematically, a particle moving backward in time is exactly equivalent to an antiparticle, although the antiparticle of a photon is the photon itself. Unlike Dirac, Feynman did not interpret negative energy as a sea of negative-energy particles; instead, he regarded it as a particle with positive energy but negative time, that is, an antiparticle.

Seong-lae Cho
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