 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. Next we'll talk about the strong force. The strong force acts between quarks, and as you might recall, only acts over very small distances, but 10 to the minus 15 meters, on the order of the size of the atomic nucleus. The exchange bosons for the strong force are called gluons. Like photons, they're massless, and they're only exchanged between quarks and anti-quarks. This time, though, the exchange process is a bit more complicated, because there's the third property, known as color, that determines whether certain quarks can be drawn together via the strong force. There are three possible colors that quarks can have, red, blue, or green. The baryon that each set of quarks builds must be colorless, which means you can have one of each type of quark, a red, a blue, and a green. This is an analogy to light. If you combine red, blue, and green light, you get white. This constrains how you can make baryons. Here you can have a quark-anti-quark pair of the same color. This constrains how you can make mesons. Now, quarks don't really have color in the visual sense. The color here is a visual tool we can use to think about which quarks can bind together. The theory that underlies this explanation is known as quantum chromodynamics, or QCD. And the color is basically another quantum number. It's a handy one, too. It allows us to put quarks, which are fermions, and therefore can't live in the same quantum state, in configurations that might be impossible otherwise. These impossible configurations were, in fact, how color was discovered in the first place. But that's a story for another time, because we have one more force to go. The weak force is next. If you recall, it's called the weak force because it only acts over an extremely short range, about 10 of the minus 18 meters. It happens to involve two exchange bosons, the W and Z bosons. These two, unlike the previous exchange bosons we learned about, have mass. Actually, they're pretty heavy, about 100 times the mass of the proton. This is one of the reasons the weak force acts over such short ranges. Imagine how hard it must be to check huge bosons back and forth. The W boson can be positively or negatively charged, and we denote both types W plus and W minus respectively. The Z boson is uncharged. One good example of the weak force in action is the nuclear beta decay. Let's take a look a bit more closely at how beta decay works on the quark level. We'll choose neutron decay to focus on, rather than look at the beta decay of the specific nucleus. Note that the neutron consists of one up and two down quarks, while the proton is two up and one down quark. In the beta decay process, a down quark emits a W minus boson, causing it to transform into an up quark. The W minus boson then decays into an electron and an anti-electron neutrino, leading to the familiar beta decay products. The W and Z exchange particles were actually predicted by Sheldon, Glashow, Stephen Weinberg, and Abdus Selam in their Electro-Week theory, which combined the electromagnetic and weak forces into one model. They won the Nobel Prize for this work in 1979. The W and Z bosons were discovered at CERN in 1983 by Carlo Rubia and Simon Vandermeer, who won the Nobel Prize for this work in 1984. I should note Carlo Rubia and Simon Vandermeer won the Nobel Prize for this work, but there were actually quite a few people involved in this experiment. Now, we associated the weak forces range with the size of its associated exchange particles, but the strong forces also short range, and its exchange particle, the gluon, has no mass. So what's going on here? Well, the range of the strong force has to do with the fact that we never see isolated quarks in nature. The force keeping them together is indeed quite strong. Now, back to the last force, gravity. It turns out to be the only force that is in part of the standard model. The reason for this is we don't have a quantum model for gravity, and we have found no evidence for messenger particle for this force. We do, however, already have a name if such a particle is ever discovered, the graviton. Now, evidence for gravitational waves was just found by the LIGO collaboration, including some members of the ANU. This means that Einstein's theory of general relativity was right about the existence of these classical waves. But we don't yet have an idea of how to build up an experimentally verifiable quantum theory that is consistent with these observations, as well as all the other observations confirming Einstein's theories of relativity. So there's still more work to be done on this front. Everything we've learned up until now was first put together into a standard model of particle physics in 1974. To this day, the standard model is the most complete quantum model explaining what matter consists of and how it interacts. When we test for physics beyond the standard model, we're searching for clues that the theory we've come up with thus far is perhaps derived from a bigger, possibly more elegant and, more importantly, experimentally verifiable story. In the next video, we're going to learn about how the standard model of physics can be used to understand what might have happened during the birth of our universe as we know it.