The quantum probability of electron-positron scattering. For Bhabha scattering, our two Feynman diagrams encode the following equation: There are a set of rules-called Feynman rules of course-on how to translate a given set of diagrams into an equation representing the likelihood of that interaction happening. To the trained eye, the diagrams are part of a systematic method to calculate the chance of a given interaction in ever-greater detail. The story on the left, with the original electron and positron scattering away, and the story on the right, with the original electron-positron pair replaced by a reincarnated copy, both happen.įeynman diagrams offer much more than tall tales of particles. Quantum mechanics says that every imaginable development of the story occurs. So which one really takes place? Well, here’s the incredible thing. Yet the narrative unfolds differently, like different choices in a choose-your-own-adventure. Finally, some time later, at the topmost junction, the photon vanishes and its energy spontaneously births a new electron-positron pair.īut why are there two diagrams, and not just one? After all, in both diagrams the story starts out the same, with the electron and the positron approaching each other. They’ve annihilated each other, leaving behind a wiggly line, which is our old friend, the virtual photon. Going further up the diagram, we see that the electron and positron are now gone. Feynman gave this bounce-back an astounding interpretation: he suggested that what appears to be two particles-the matter electron and the antimatter positron-can be thought of as just one single particle meeting its own future self going backwards in time. Notice how the diagram has the electron line going up and bouncing back into the positron line. As before, the electron comes in (bottom left) and meets the positron (bottom right). Next, let’s look at the diagram on the right. It thereby transmits momentum and energy between the electron and the positron, which both recoil as a result and scatter. The virtual photon connects the electron and positron lines. Since there’s no vertical component, it’s all taking place at the same moment in time: the virtual photon essentially travels from one place to another instantaneously-light traveling literally faster than light. Notice that the photon line runs purely horizontally. Take for instance the diagram on the left. Yet in their brief lives quantum mechanics allows these droplets of unseeable light to do extraordinary things. You cannot capture them, you cannot see them. These are virtual photons, wraith-like and ephemeral, flitting just beneath the threshold of existence. (Yes, really.) The wiggly lines denote photons, little droplets of light. That’s because, for Feynman, antimatter is secretly matter traveling backwards in time. Notice that the electron has an arrow pointing up but the positron has an arrow pointing down. In both diagrams, the electron (labeled e-) comes in from the bottom left, the positron (labeled e+) from the bottom right. We read these diagrams chronologically, from bottom to top as if we were reading a page from the bottom up, line-by-line, each line a snapshot of a moment in time. Of course, space is really three-dimensional and our picture has only two dimensions-one of which we’re already using to represent time-but we needn’t worry about that. On these diagrams, time runs vertically up, space horizontally. This particular pair of Feynman diagrams corresponds to a process called Bhabha scattering, the meeting of an electron and a positron-the electron’s antimatter identical twin. But it wasn’t until graduate school that I understood the significance of his diagrams-what they meant and what they were for.įeynman diagrams portray the interaction between particles in space and time. Feynman’s irreverent antics delighted high-school me. That book introduced me to the persona of Richard Feynman, the brilliant physicist who cracked jokes and cracked safes and played bongos to boot. I was in eleventh grade, a student in Bombay, puttering around my house during a teachers’ strike, when my mother, herself trained as a particle physicist, gave me a copy of Surely You’re Joking, Mr Feynman!. I learned of Feynman diagrams when I first encountered Feynman, not in person (unfortunately) but through his memoirs. In the process, they transform how physicists think about forces. Still, it nicely sums up the life stories of elementary particles. Perhaps you feel this stark biography has left out some important milestones of your life.
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