The fun-loving folks blogging on behalf of the Large Hadron Collider offered a real treat to particle physics fans this week, especially those who know their history: an entire post devoted to playing with so-called "Feynman diagrams." These childlike scribbles are a huge aid in visualizing the complexity of photon/electron interactions, one more testament to the inspired genius of the man who invented them: physicist Richard Feynman.
The blackboard in Feynman’s Caltech office – where he taught from 1950 until his death -- had a phrase written at the very top: “What I cannot create, I do not understand.” He couldn’t understand quantum mechanics as it was presented in textbooks of the time. So he re-invented it. He streamlined the convoluted equations of quantum electrodynamics (QED) by creating a series of simple line drawings that precisely illustrated how electrons interact with photons. These are the Feynman diagrams.
Feynman envisioned those interactions as a theatrical
play. The “actors” are the photons and electrons, and there are three basic
actions from which all dramatic conflict – all phenomena related to light and
electrons -- arise: (1) a photon goes from place to place; (2) an electron goes
from place to place; and (3) an electron emits or absorbs a photon. Think of light passing through a window, or bouncing off a mirror. At the subatomic level, Feynman argued, light isn't really passing through or bouncing off a surface. Instead, incoming photons are absorbed by the
electrons in the atoms inside the glass, and new photons are emitted. Those new
photons are the ones we “see.”
Feynman’s diagrams serve as a mathematical shorthand. In the simplest interaction, a pair of electrons moves toward each other. The electron on the right emits a photon and is pushed outward. That photon is then absorbed by an electron, which is also pushed outward, just like the recoil effect what happens when a fired bullet hits a tin can, knocking it backward. It’s all one big, elaborate subatomic game of catch. “Photons do nothing but go from one electron to another,” Feynman said. “Reflection and transmission [of light] are really the result of an electron picking up a photon, ‘scratching its head,’ so to speak, and emitting a new photon.”
This is why you sometimes get partial reflection. If you’re sitting inside with a lamp on in the room, and you look out the window in the daytime, you will be able to see not just the things outside through the glass, but also a dim reflection of the lamp. Why does this happen? Feynman explained that an incoming photon actually interacts with electrons it encounters throughout the glass, not just on the surface, but the net result is the same as if the photon only hit the surface. If light hits the surface of a piece of glass at a 90-degree angle, for every 100 photons that hit the glass, 96 go through (transmission), while 4 hit the glass and bounce back (reflection). The result is the same even if the photons travel through many layers of glass.
How does the photon make up its mind which way to go? We don’t know. Physicists can only calculate probability: out of every 100 photons, four will be reflected back; they can’t predict which specific photons that will be. As Feynman himself said in his lectures on QED, this is how Nature works. "You don't like it? Go somewhere else!" Preferably to an alternate universe.
Not everyone was a fan of the Feynman diagrams. Schwinger, who shared the Nobel Prize with him, avoided using the diagrams altogether, sniffing dismissively that the stick figures had brought “computation to the masses.” But being able to visualize the many possible interactions between electrons and photons has rescued many a physics graduate student from the brink of despair at ever being able to make sense of it all. And you'd better believe Feynman relished the thought of his humble "scribbles" being published in the world's most prestigious journals.
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