The essence of these diagrams is that they portrayed quantum events as trajectories. Richard Feynman developed a technique referred to as Feynman diagrams. Schmitz, in Physical Chemistry, 2018 1-10.5 Different Views: Niels Bohr and Richard Feynman If two paths are heading outwards, it does not imply that particles are repelling each other.Kenneth S. Side-to-side displacement in the diagrams has no meaning other than to show separate particles. Time goes vertically up the diagram (many sources have time horizontal). These are not consistent from source to source! In this episode, the following conventions are followed. If your specification requires Feynman diagrams, you will need to emphasise the rules for drawing them. If you deal with quark interactions later, the exchange particle is the gluon.Įpisode 536-1: Feynman diagrams (Word, 53 KB)Įpisode 536-2: Coat-hanger Feynman diagrams (Word, 37 KB) Discussion: Rules for Feynman diagrams For the weak interaction, there are three particles, depending on the changes in charge taking place. With one twist each time, it is possible to go from electron-electron interaction to electron positron interaction to positron-positron interaction to electron-positron annihilation.įor these electromagnetic interactions, the particle exchanged is a photon. The supporting shoulders of the coat hangers are the interacting particles, while the interlocked hooks constitute the vector boson. It is quick and easy to use cheap coat hangers linked by their hooks, with triangles of card attached midway across the shoulder of each. The key aspects – direction of time, transfer of the force-carrying boson, difference between particles and anti-particles – can be quickly illustrated for an electromagnetic interaction.Īs an example of these points (including the last), you may wish to use a simple physical model. Demonstration: Model Feynman diagramsįeynman diagrams can be introduced via a physical model that can be twisted to show different interactions. The weak interactions, which are harder to classify, and are similar in strength to the electromagnetic interactions, are associated with changes in the nature of particles. Here the exchange particle is the photon. The electromagnetic interaction, which consists of just the well-known attractions and repulsions of static electricity (pre-16 level), is a different interaction, much weaker than the strong interaction. In the last episode, it will be clear that a similar fundamental exchange works at a level that is more fundamental than mesons and baryons. This crude model in the demonstration above will illustrate the idea of an exchange particle originated by Yukawa, who suggested that a nuclear exchange particle (it turned out to be the pion) could explain the strong interaction between protons and neutrons. Discussion: Interactions of different types If the rules are changed so that, instead of throwing, each student pulls the object from the other’s hands in turn, then each will report feeling an inwards force both on gaining and on losing the particle. If two students throw (gently) a heavy object such as a schoolbag or football to each other, each will report feeling an outwards force both on throwing and on catching (Why? Conservation of momentum). Yukawa’s theory of an exchange particle to explain repulsive and attractive forces in nuclei is worth demonstrating with two students and a football or other large object.
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