 In the previous videos we have described how the standard model describes the fundamental objects of the universe as fields rather than particles. When we talk about the elementary particles what we are really talking about are quanta or discrete ripples of these fields. We have also discussed how interactions between the different fundamental fields work by exchange of some of these particles and how we can represent them with Feynman diagrams. This type of theoretical framework is called quantum field theory. When particle physicists use quantum field theory to calculate the probability of fields evolving from some initial state to some final state they get a bunch of mathematical terms that can be interpreted as particles appearing and disappearing in different states. When Richard Feynman was working on quantum field theory he realized that each individual mathematical term tells a different story about what happens between the initial state and the final state and he came up with a way to describe them with diagrams. Which we now call Feynman diagrams. For example, if I were to calculate the probability of two electrons scattering from each other I can draw a Feynman diagram for each mathematical term that shows up. Here is what some of these terms might look like. In fact for every valid Feynman diagram we can draw there will be a corresponding mathematical term. This poses a challenge as there might be an infinite number of increasingly complex diagrams we can draw with the same initial state and the same final state. For the electromagnetic and weak interactions the simplest diagrams contribute the most to these calculations. So we only need to concern ourselves with the diagrams that have the fewest vertices which in general is a finite amount. This is not true for the strong interaction however where the more complex diagrams can have significant contributions to the calculation so using Feynman diagrams is not as helpful. It is in fact an active area of research in physics today trying to figure out how best to do these calculations for the strong interaction. As a side note, Feynman diagrams everything between the initial state and the final state are called virtual particles. Virtual particles are often misunderstood as invisible particles or not real particles but they are simply discrete ripples of the field that do not live for very long. This essentially makes them undetectable for the most part which is why we call them virtual.