Quantum Diaries

I am itching to tell you about the ALICE experiment at the LHC so I am finally going to start. As my previous post alluded to, we are interested in improving our understanding of one of the forces of the universe. When most people hear “force” they thing of “weight” – gravity. This is a force that you feel if you have mass, but you have to be pretty massive to have a big effect (Earth-sized, for example) because it is ridiculously weak.

What others do we know about? Electromagnetism. Now, that one is more familiar than you might think. To feel this one, you need a magnetic field, like magnets, or an electric charge, like the electrons (negative charge, think electricity) or protons (charge equal in magnitude but positive) in our atoms. In both cases, opposites attract (a north and south pole of a magnet, or positive and negative electric charges, for example) and likes repel. It’s a good thing too, because atoms are mostly empty space – the protons sit tightly packed in a very small nucleus. If the force wasn’t so much stronger than gravity, we would fall through the floor. As it is, electromagnetism “likes” (I am not implying it has feelings here) to keep things neutral (hence the equal amounts of electrons and protons).

Hang on a minute. The protons are positive, but they stay tightly bound in the nucleus of the atom. What happened to the electromagnetic force? Well, there’s another force at work, and that’s the one we care about. It’s stronger than all the others, so we call it the “strong” force. Not that original but a very good description. So what do you need to feel that force? Well, that’s the problem. You need another kind of charge, one that we don’t see any more. We call it “colour” charge (hence the name Quantum Chromo-Dynamics). It has nothing to do with actual colour, but, like light’s 3 primary colours, there are three of these charges which, when all mixed together, give you something “white”, or “neutral”. Now, the strong force, like the electromagnetic force, “likes” to keep things neutral. The problem is, as we have implied, it is MUCH stronger, so it does a much better job.

To demonstrate, rub a balloon on your head and you’ll see how easy it is to defy the electromagnetic force. You rub off electrons and leave your hairs with a bit of electric charge, and they stand on end annoyingly, repelling each other. However, the strong force has been winning for billions of years. The particles that have this colour charge, quarks (and gluons), are not so easily liberated. One quark of each colour is bound in particles like the proton, so we never see quarks on their own. Not since the big bang, for that few millionths of a second where everything was trillions of degrees, were quarks free to fly about the place in the wonderful stuff we call quark-gluon plasma. That’s why we know very little about the strong force! We don’t know how the quarks became imprisoned in “colour neutral” particles like the proton, or why they only make up a few % of its mass. It’s a complicated mystery.

But have no fear! ALICE is going to recreate this environment, reproduce that ridiculous temperature by slamming lead ions into each other. For an even smaller fraction of time, those quarks will be free once more, and we might stand a chance of working out this big puzzle!