Why Does E=mc2_ - Brian Cox [72]
Apart from the matter particles that we have just been talking about, and the mysterious φ, there are some other particles we need to mention. They are the W and Z particles, the photon and the gluon. We should say an introductory word or two about their role in affairs. These are the particles that are responsible for the interactions between all the other particles. If they did not exist, then nothing in the universe would ever interact with anything else. Such a universe would therefore be an astonishingly dull place. We say that their job is to carry the force of interaction between the matter particles. The photon is the particle responsible for carrying the force between electrically charged particles like the electrons and quarks. In a very real sense it underpins all of the physics uncovered by Faraday and Maxwell and, as a bonus, it makes up visible light, radio waves, infrared and microwaves, X-rays, and gamma rays. It is perfectly correct to imagine a stream of photons being emitted by a lightbulb, bouncing off the page of this book and streaming into your eyes, which are nothing more than sophisticated photon detectors. A physicist would say that the photon mediates the electromagnetic force. The gluon is not as pervasive in everyday life as the ubiquitous photon, but its role is no less important. At the core of every atom lies the atomic nucleus. The nucleus is a ball of positive electric charge (recall that the protons are all electrically charged, while the neutrons are not) and, in a manner analogous to what happens when you try to push two like poles of a magnet together, the protons all repel each other as a result of the electromagnetic force. They simply do not want to stick together and would much rather fly apart. Fortunately, this does not happen, and atoms exist. The gluon mediates the force that “glues” together the protons inside the nucleus, hence the silly name. The gluon is also responsible for holding the quarks together inside the protons and neutrons. This force has to be strong enough to overcome the electromagnetic force of repulsion between the protons, and for that reason it is called the strong force. We are really not covering ourselves in glory in the naming-stakes.
The W and Z particles can be bundled together for our purposes. Without them the stars would not shine. The W particle in particular is responsible for the interaction that turns a proton into a neutron during the formation of the deuteron in the core of our sun. Turning protons into neutrons (and vice versa) is not the only thing the weak force does. It is responsible for hundreds of different interactions among the elementary particles of nature, many of which have been studied in such experiments as those carried out at CERN. Apart from the fact that the sun shines, the W and Z are rather like the gluon in that they are not so apparent in everyday life. The neutrinos only ever interact via the W and Z particles and because of that they are very elusive indeed. As we saw in the last chapter, many billions of them are streaming through your head every second, and you don’t feel a thing because the force carried by the W and Z particles is extremely weak. You’ve probably already guessed that we’ve named it the weak force.
So far we have done little more than trot off a list of which particles “live” in the master equation. The twelve matter particles must be added into the theory a priori, and we don’t really know why there are twelve of them. We do have evidence from observations of the way that Z particles decay into neutrinos made at CERN in the 1990s that there are no more than twelve, but since it seems necessary to have only four (the up and down quarks, the electron, and the electron