 The standard model was introduced in an attempt to put quantum mechanics and special relativity together. It turns out that a classical way of thinking of forces as instantaneous actions over a distance is incompatible with special relativity, which tells us that no physics should happen faster than the speed of light. For an example, let's look at a system of two negatively charged particles. Because they have light charges, they will repel each other. You can think of this as one charge creating an electromagnetic field that the other particle interacts with. If you move one particle, it will create ripples in the field that will travel to the other particle and cause it to interact. The reason this interaction has to work like this is special relativity. If the interaction did not happen through ripples in a field, the force would be felt instantaneously by the particles. But when you looked at special relativity with Ruby, you found that the idea of happening at the same time doesn't actually mean anything. And for different observers in different reference frames, things happen in different orders. For example, if the sun suddenly disappeared, gravity wouldn't immediately switch off. It would actually take about 8 minutes before the ripples of the gravitational field reach Earth and are felt. Now in the quantum mechanics section, you learned that the electromagnetic field comes in discrete chunks or quanta, which we call the photon. In fact, all fundamental particles that we know of are actually discrete excitations of underlying fields. When particle physicists talk about particles, they don't actually mean point masses. But really what they're talking about are discrete chunks or discrete ripples or quanta of fields that propagate through space and time. A nice way to think about this is to take the two-dimensional case and to picture the field as a matrix. The quanta of this field look like lumps in this matrix and they move in space and time. Every fundamental interaction works in the same way, through discrete ripples of an underlying fundamental field. These discrete ripples we call exchange bosons, and we have exchange bosons for every fundamental force. For example, when we look at the strong force, which is the force that binds quarks together in protons and neutrons, it turns out there are exchange bosons called gluons that mediate that interaction. An intuitive way to visualize how interactions work by exchanging particles is to consider Alice and Bob playing catch while floating in space. When Alice throws the ball towards Bob, Alice will experience a push in the opposite direction due to conservation of momentum. The ball will then go flying through empty space and then land in Bob's hands. Because the ball is travelling with some momentum, when Bob catches it, he will begin to move to the right. As Alice and Bob continue to throw the ball back and forth to each other, they'll move further and further apart through this process. In the context of the standard model, Alice and Bob are fundamental particles and the ball they are passing back and forth is the exchange boson, which remember is just an excitation of an underlying field that mediates the interaction. In the following videos, I will introduce you to Feynman diagrams, which are a simple way to visualize interaction processes under the standard model. We will begin with the simplest type of fundamental interaction, which is the electromagnetic interaction.