The last time, I made a brief remark about anti-particles. It is about time to illustrate this rather obscure notion.
What is meant, when we talk about anti-particles? Well, just from the experimental point of view, it is found that for every particle there exists another particle, which has (within experimental certainty) exactly the same mass. It has also the same properties when it comes to the way it spins, the so-called spin. This spin is also something I will explain sometimes else, what this mysterious property is.
However, considering everything else, it is exactly the opposite: If the particle has a negative electric charge, the anti-particle has a positive electric charge. If the particle has color red, the anti-particle has an opposite charge, which is called for the lack of a better name anti-red. And so on. The only exception to this rule are those particles which have, except for mass and spin, no other properties. An example is the photon, which is completely uncharged. In this case, the particle is its own anti-particle.
Now, these are rather surprising objects, but we have very good experimental proof that they exist. In fact, we know anti-matter so well that some experiments, like the old LEP at CERN, use matter and anti-matter routinely as a starting point: At LEP electrons and their anti-particles, the positrons, have been collided.
Matter and anti-matter show a very spectacular effect: Because one plus minus one is zero, it is possible for matter and anti-matter to annihilate each other when they are colliding into something else. For example photons. Or other particles. That happens very easily. Hence you may ask, why we do not annihilate whenever we touch something. The answer is surprisingly simple: Because everything around us is made from matter. If we want to use anti-matter or study it, we have to create it artificially. That is not simple, and we can only create very tiny amounts efficiently. Large amounts become rapidly very expensive, mostly because it is not simple to keep it away from matter, so that is not annihilating with it.
That seems a simple enough answer, but the real question baffling physicist is: Why is this so? If they are so equal, why do we not have the same amount of both (and thus vanish in a big photon cloud)? That is another of the questions we do not yet have a real answer to. Irritatingly, the problem is actually not that we do not know how this can be realized. In fact, in the standard model of particle physics, there is a very, very slight preference for matter over antimatter when it comes to the weak force. This implies that though matter and anti-matter are essentially equal, the forces make a difference between them. However, this effect is by far too small to explain why there is so a fantastically little amount of antimatter around us.
Ok, so you might say: Let us forget for the moment about the experimental evidence, and ask, do we really need anti-mater. Could this simple explanation just be a misinterpretation of the experiments, and what we think is anti-matter is really something else? Well, if this should be the case, we would have to rethink our complete view of how the standard model is described theoretically as well. Indeed, the mathematical structure of the standard model requires the existence of an anti-particle for each particle to work properly. If we would remove the anti-particles from the theory, the consequence would be dramatic. It would even be possible to obtain effects without cause or causes without having effects. This is not what we observe, but what we observe is described by the standard model with particles and anti-particles. Thus we take the experimental results as evidence for anti-particles, and everything fits together when we calculate something.
Of course, this means that we essentially double the number of particles. Up to exceptions like the photon, all particles are now accompanied by their anti-particles. And to every charge comes an anti-charge. However, this also provides new options for new phenomena. The last time last time, this gave us the option of a condensate of quarks and anti-quarks. Also, their are bound states of quarks and anti-quarks, the so-called mesons. The most famous and lightest of them are the so-called pions, of which there are three: One is uncharged, and there is one positively charged and one negatively charged. Th neutral one is again its own anti-particle. The reason is that it is made up out of a quark and the corresponding anti-quark. Thus replacing constituent particle by constituent anti-particle gives again the same bound state. The charged ones are each others anti-particle, because they contain an up and an anti-down quark and an anti-up and a down quark, respectively. Exchange particles by anti-particles yields an exchange of both bound states. So, one can have a lot of fun with building things out of particles and anti-particles.
You can also take a hydrogen atom, and exchanges its nucleus, a proton, by the anti-particle of the electron, a positron. Because the positron has the same electric charge as the proton, you get even something looking very much like an atom. This is called positronium, known for a very long time. Recently, it has also been possible to create true anti-atoms, made from an anti-nucleus and positrons. These are very important to test, whether we really have understood everything about anti-mater. If we have, they should behave in the same way as ordinary atoms. And whether this is the case the experimentalists right now try to find out.