 Earlier in this module, we reviewed the fundamental forces that can act on particles. As it turns out, special particles called exchange bosons play a fundamental role in how at least three of the fundamental forces act. Particles can feel each other's presence because they exchange these specific bosons with each other. It's like they're sending messages to each other as a way of figuring out whether they should get closer to each other or move further away. Let's look at some of these exchange bosons and think about how they explain some of the fundamental properties of each force. We'll start with the electromagnetic force. For the electromagnetic force, the exchange boson is a photon, sometimes known as a virtual photon, since these exchanges happen so quickly that we don't actually detect them. The photon is a boson, as the name suggests, and is massless, charge-less, and has spin one. They're only exchanged between charged particles. We can draw this kind of exchange in the form of a Feynman diagram, like this. The diagram can be translated into a specific mathematical equation that describes the collision process. Don't worry, we're not going to get into that here, but the theory behind this is known as quantum electrodynamics, or QED.