Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of QED and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics. For his contributions to the development of QED, Feynman, jointly with Julian Schwinger and Sin-Itiro Tomonaga, received the Nobel Prize in Physics in 1965. He developed a widely used pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, which later became known as Feynman diagrams. During his lifetime, Feynman became one of the best-known scientists in the world.

When you listen to Feynman it is impossible not to be taken by simplicity of his thinking and marvelous insight this man had. He looks a bit like a dirty Harry of science, but listening to his interviews and lectures on youtube is something I could do for days probably. BBC also placed, now in high res, parts of "Fun to imagine" on their site. There other sites too (example).
Listening to Feynman is very inspirable experience and will hardly leave you without any emotion.
Feynman was also known for many quotes he has spoken during his life. He was also known as the 'Great Explainer' because of his passion for helping non-scientists to imagine something of the beauty and order of the universe as he saw it. It is exactly his will to describe things in simple way, but understandable to people that drove him to create set of diagrams explaining relationships in particle physics. Today we call those - Feynman diagrams. |

The interaction of sub-atomic particles can be complex and difficult to understand intuitively, and the Feynman diagrams allow for a simple visualization of what would otherwise be a rather arcane and abstract formula. Feynman first introduced his diagrams in the late 1940s as a bookkeeping device for simplifying lengthy calculations in QED. Soon the diagrams gained adherents throughout the fields of nuclear and particle physics. Not long thereafter, other theorists adopted - and subtly adapted - Feynman diagrams for solving many-body problems in solid-state theory. By the end of the 1960s, some physicists even used versions of Feynman's line drawings for calculations in gravitational physics. With the diagrams' aid, entire new calculational vistas opened for physicists. Theorists learned to calculate things that many had barely dreamed possible before WW II. It might be said that physics can progress no faster than physicists' ability to calculate. Thus, in the same way that computer-enabled computation might today be said to be enabling a genomic revolution, Feynman diagrams helped to transform the way physicists saw the world, and their place in it.

Feynman introduced his novel diagrams in a private, invitation-only meeting at the Pocono Manor Inn in rural Pennsylvania during the spring of 1948. Twenty-eight theorists had gathered at the inn for several days of intense discussions about problems they were trying to address and Feynman offered his view using his diagrams. If you into the details, David Kaiser did great overview of it which is online and can be found here. The simplicity of these diagrams has a certain aesthetic appeal, though as one might imagine there are many layers of meaning behind them. The good news is that’s it’s really easy to understand the first few layers and today you will learn how to draw your own Feynman diagrams and interpret their physical meaning. You do not need to know any fancy-schmancy math or physics to do this which for most of people reading this is a good news.

A Feynman diagram is a representation of quantum field theory processes in terms of particle paths. You can draw two kinds of lines, a straight line with an arrow or a wiggly line.

You can draw these pointing in any direction. The rules are:

- straight line, going from left to right, represents electron
- straight line, going from right to left, represents positron (electron's anti-particle)
- wiggly line is photon
- you may only connect these lines if you have two lines with arrows meeting a single wiggly line
- up and down (vertical) displacement in a diagram indicates particle motion, but no attempt is made to show direction or speed, except schematically
- any vertex (point where three lines meet) represents an electromagnetic interaction

Of course, Feynman rules are much broader, but then again this is not class of physics.

So, these diagrams tell us a story about how a set of particles interact. We read the diagrams from left to right, so if you have up-and-down lines you should shift them a little so they slant in either direction. This left-to-right reading is important since it determines our interpretation of the diagrams. Matter particles with arrows pointing from left to right are electrons or any other fermion if noted. Matter particles with arrows pointing in the other direction are positrons or any other anti-matter particle if noted. In fact, you can think about the arrow as pointing in the direction of the flow of electric charge.

But here comes a cool thing; the interaction with a photon information about the conservation of electric charge: for every arrow coming in, there must be an arrow coming out. Not just that, we can also rotate the interaction so that it tells a different story; we will take as an example electron and positron anihilation example from above and rotate it Here are a few examples of the different ways one can interpret the single interaction (reading from left to right):

In essence, we rotated picture and created 4 new interactions. Dare to say what they are? It's easy. These are to be interpreted as:

- an electron emits a photon and keeps going
- a positron absorbs a photon and keeps going
- an electron and positron annihilate into a photon
- a photon spontaneously produces an electron and positron

Because Feynman diagrams represent terms in a quantum calculation, the intermediate stages in any diagram cannot be observed. Physicists call the particles that appear in intermediate, unobservable, stages of a process "virtual particles". Only the initial and final particles in the diagram represent observable objects, and these are called "real particles".

On diagrams above, on the left side of a diagram we have "incoming particles" - these are the particles that are about to crash into each other to do something interesting. For example, in accelerators where protons and neutrons are collided, these "incoming particles" are the quarks and gluons. On the right side of a diagram we have "outgoing particles", these are the things which are detected after an interesting interaction. Bot "incoming particles" and "outgoing" are "real particles". What about the internal lines? These represent "virtual particles" that are never directly observed. They are created quantum mechanically and disappear quantum mechanically, serving only the purpose of allowing a given set of interactions to occur to allow the incoming particles to turn into the outgoing particles.

Again, using electron-positron example, we can this describe using diagram as:

In the first diagram the electron and positron annihilate into a photon which then produces another electron-positron pair. In the second diagram an electron tosses a photon to a nearby positron (without ever touching the positron).

In physics, Compton scattering is a type of scattering that X-rays and gamma rays (both photons with different energy ranges) undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, called the Compton effect. Part of the energy of the X/gamma ray is transferred to a scattering electron, which recoils and is ejected from its atom (which becomes ionized which means atom is no longer neutral, but rather charged), and the rest of the energy is taken by the scattered, "degraded" photon. Inverse Compton scattering also exists, in which a charged particle transfers part of its energy to a photon. In such case, electron can to become "virtual particle" as seen below.

Above we see a process where light (the photon) and an electron bounce off each other and it is electron as well that is "virtual particle" here.

By reading these diagrams from left to right, we interpret the *x* axis as time. You can think of each vertical slice as a moment in time. The *y* axis is roughly the space direction. The path that particles take through actual space is determined not only by the interactions (which are captured by Feynman diagrams), but the kinematics (which is not). For example, one would still have to impose things like momentum and energy conservation. The point of the Feynman diagram is to understand the interactions along a particle’s path, not the actual trajectory of the particle in space.

Feyman diagrams can be used to show some rather complicated relationships in rather easy way. Without getting into those, I will close this example section with 3 more diagrams; 2 for how Higgs may be produced at LHC and one special.

First above is Feynman diagram of one way the Higgs boson may be produced at the LHC; two gluons convert to two top/anti-top quark pairs, which then combine to make a neutral Higgs. Second one is another way the Higgs boson may be produced at the LHC; two quarks each emit a W or Z boson, which combine to make a neutral Higgs. Third one is easy and just a joke.

Feynman developed two rare forms of cancer dying shortly after a final attempt at surgery aged 69 in 1988. His last recorded words are noted as "I'd hate to die twice. It's so boring." What a genious!

Credits: Wikipedia, BBC, David Kaiser, CERN, Flip Tanedo