What is Feynman diagram?
[[File:Feynmann Diagram Gluon Radiation.svg|thumb|287px|In this Feynman diagram, an [[electron]] and [[positron]] destroy each other, producing a virtual [[photon]] which becomes a [[quark]]-[[antiquark]] pair. Then one radiates a [[gluon]]]] A '''Feynman diagram''' is a [[diagram]] that shows what happens when [[elementary particle]]s [[Collision|collide]].<ref>Martinus Veltman. 1994. ''Diagrammatica: the path to Feynman diagrams''. Cambridge Lecture Notes in Physics. {{ISBN|0-521-45692-4}}.</ref> Feynman diagrams are used in [[quantum mechanics]]. A Feynman diagram has ''lines'' in different shapes—straight, dotted, and squiggly—which meet up at ''points''. The points in Feynman diagrams represent a meeting between three particles at the same point in space at the same time. The lines in a Feynman diagram represent the probability amplitude for a particle to go from one place to another. In Feynman diagrams, the particles are allowed to go both forward and backward in time. When a particle is going backward in time, it is called an [[antiparticle]]. The meeting points for the lines can also be interpreted forward or backwards in time, so that if a particle disappears into a meeting point, that means that the particle was either created or destroyed, depending on the direction in time that the particle came in from. All the lines and points have an amplitude. When you multiply the probability amplitude for the lines, the amplitude for the particles to go from wherever they start to wherever they meet, and to the next meeting point, and so on, and also multiply by the amplitude for each meeting point, you get a number that tells you the total amplitude for the particles to do what the diagram says they do. If you add up all these probability amplitudes over all the possible meeting points, and over all the starting and ending points with an appropriate weight, you get the total probability amplitude for a collision in a particle accelerator, which tells you the total probability of these particles to bounce o