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Understanding science: what we cannot know
Understanding science: what we cannot know

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3.4 The strong force

The current theory of electromagnetic, weak nuclear and strong nuclear forces (but not gravity) in the world of subatomic particles is called the ‘Standard Model’. It is based on three symmetry groups, which are generalisations of rotation groups. One of these is Abelian (electromagnetic interactions), while the other two are non-Abelian groups.

Perhaps the most intriguing of these is the strong nuclear force, which acts on quarks. The interaction is much more complicated than gravitation or electromagnetism. For the strong force, there are six different ‘charges’ which are commonly labelled as primary colours: the quarks are r (red), r (green) and b (blue), and the antiquarks are denoted by the corresponding ‘anticolours’ r macron, g macron and b macron. Note that the use of ‘colours’ here is nothing more than an analogy to help us describe the properties of this force. It was also chosen because light of these three basic colours combines to produce white light. In the proton, three quarks of different colours combine to form a ‘colourless’ particle. Indeed, the theory predicts that all particles that can be observed are ‘colourless’ – which means that we can never encounter a single quark on its own.

As well as mixing all three colours, there’s another way to produce colourless particles: by matching a colour with its corresponding anticolour.

This combination into a ‘colourless’ system that cannot be broken up may seem strange, but here’s another analogy for this behaviour. If you have a magnet, it will have a north pole and a south pole. Imagine cutting it down the middle – you might expect that you’d end up with two pieces, one being a north pole and the other a south pole. However, this is not the case – there is no magnetic ‘monopole’ (single pole). Instead, both pieces are magnetic, each with a north and a south pole. The colour charge of quarks is similar, in that the proton as a whole is colourless. Even when smashed in a particle accelerator, the products observed will again be colourless. Nevertheless, the substructure can be detected indirectly, by its effects on other particles – much like observing the effect the magnet’s two poles have on other magnets.