 So, in this module, we're going to talk about some of the most important properties of particles. The first one that we're going to talk about is something called spin. Spin's kind of a funny quantity. It's kind of like angular momentum, except it's intrinsic to a particle. So, that means you can't change it. Spin is given by a spin quantum number times h-bar, where h-bar is Planck's constant over 2 pi. Now, what's important about spin is that the spin quantum number is quantized. So, what that means is it can either be half integer or integer. It can only come in discrete quantities. There's discrete steps. This is as opposed to continuous quantities where you could have any value for us. So, this spin quantum number is a quantum property of a particle. I've noted that spin can either be half integer or integer. We have special names for the particles that fall into each of these categories. So, the half integer particles we call fermions, and the integer particles we call bosons. One of the common fermions that you can think of are electrons. One of the common bosons that you would be familiar with is a photon. We'll learn about lots of different types of fermions and bosons later on, but for now we'll leave it there. Fermions are half integer spin, bosons are integer spin. The reason spin is important is that there is a particular principle that applies only to fermions and not to bosons, and that principle is the poly exclusion principle. Now what this principle says is that only one particle can sit in a particular quantum state at a time. Fermions have to obey this rule. Bosons don't have to, and what that means is that basically if you have lots of bosons that can all sit in the same quantum state, they'll probably choose the lowest energy quantum state. Fermions, they have to kind of keep stacking up to all the quantum states available. They can't all live in that same state. So that has some consequences about how those particles act within certain systems. So next we're going to talk about the forces that can act on particles. The first is something we're very familiar with on a day-to-day basis, and that's the gravitational force. So the gravitational force is what basically keeps us walking on the Earth's surface. All particles experience gravitational force. Now this might seem confusing to you because up until now you've learned that the gravitational force between two bodies is proportional to the product of their masses. Well, as it turns out, a massless particle can experience the gravitational force. It just doesn't actually create any gravitational force in and of itself. This is actually the reason why black holes are black. Photons, massless particles, can get sucked in by the gravitational force exerted by the black hole. The next force is the electromagnetic force. This is the force that keeps atoms and molecules together. Now the electromagnetic force only acts on charged particles. So for example, protons and electrons are both charged particles because protons are positively charged and electrons are negatively charged. They're going to be attracted to each other and the attraction is due to the electromagnetic force. Now the next force that particles can experience is called the strong force. This is the force that keeps atomic nuclei together. Now this force is called the strong force because over distances of about the size of a typical nucleus that's 10 to the minus 15 meters, the force is even more powerful than the electromagnetic force. Basically this is the reason why protons which are positively charged can all live together within the atomic nucleus. They can be bound together despite the fact that they are experiencing a repulsive force due to the electromagnetic force. Only certain particles can experience the strong force. We call these particles hadrons. Now there's one other force that we didn't mention. This force is known as the weak force. The weak force is kind of a misnomer. The reason is that the weak force is actually one of the strongest forces but it acts over the shortest range. So it acts only over a distance of 10 to the minus 18 meters. So it's even shorter range than the strong force. Now the weak force is responsible for turning protons into neutrons and vice versa in processes like nuclear beta decay. The weak force acts on all particles. We have a special name for particles that don't experience the strong force but do experience the weak force. We call these particles leptons. Later on we'll learn that these forces are actually mediated by certain particles and that's one of the reasons why the ranges over which these forces interact are so different. So there's one other essential property that we haven't yet talked about that we've sort of alluded to when we explored how the positron was discovered back in the first video. And that is whether we are dealing with a particle or an anti-particle. In other words whether we are dealing with matter or anti-matter. So the example that I just gave you, the positron, is the anti-particle of the electron. Now anti-particles have the same mass as their corresponding particles but have opposite charge. There's one other key distinguishing characteristic of anti-matter. If an anti-particle of mass M collides with its corresponding particle, they can annihilate. That is they can make each other disappear. In their place they emit two photons each with an energy equal to M times the speed of light squared. E equals MC squared is Einstein's famous mass energy equivalence equation. We can write this process out for an electron positron annihilation as E plus plus E minus goes to two photons. Each photon in this case has an energy of 511 kiloelectron volts. The mass of an electron or positron is equal to 511 kiloelectron volts divided by C squared. Basically the electron and positron can't just annihilate leaving nothing in their place. Instead the energy equivalent to two of their masses is carried by those two photons. One important thing to remember is that all particles have anti-particles. So for all particles there should be an anti-particle out there with the same mass and opposite charge that will cause some kind of annihilation process like this. Now one of the cool things is we can actually detect the gamma rays that come from one of these annihilation processes and so we can use that to look for matter and anti-matter collisions. Now just to summarize the three key properties we covered in this video of particles. The first is spin and the two key terms we need to keep in mind are that bosons have an integer spin quantum number, fermions have a half integer spin quantum number. We also learn that forces and the forces that act on particular particles are very important. The two key terms you need to remember are that hadrons are acted on by strong and weak forces whereas leptons are only acted on by the weak force. We also learn that there can be matter or anti-matter particles.