electromagnetic force. Quarks carry the new charge in what we can
define to be the positive form, and so antiquarks will carry the same
amount but with negative charge. The attraction of opposites then
brings a quark and an antiquark together: hence the qq¯ bound
states that we call mesons. But how are baryons, which are made of
three quarks, formed?
It turns out that there are three distinct varieties of the strong
charge and to distinguish among them we call them red (R), blue
(B), and green (G). As such they have become known as colour
charges, though this has nothing to do with colour in its familiar
sense – it is just a name. As unlike colours attract, and like repel, so
would two quarks each carrying a red colour charge, say, mutually
repel. However, a red and a green would attract, as would three
different colours, RBG. Bring a fourth quark near such a trio and it
will be attracted to two and repelled by the third which carries the
same colour charge. The repulsion turns out to balance the net
attraction such that the fourth quark is in some sort of limbo;
however, should it find two other quarks, carrying each of the two
other colour charges, then this trio can also tightly bind together.
Thus we begin to see the attractions of trios, as when forming
protons and neutrons, is due to the threefold nature of colour
charges. As the presence of electric charges within atoms leads to
them clustering together to make molecules, so do the colour
charges within protons and neutrons lead to the clusters that we
know as nuclei.
The underlying similarity in the rules of attraction and repulsion
give similar behaviour to the electromagnetic and strong forces at
distances much less than the size of an individual proton or
neutron; however, the threefold richness that positive or negative
colour charges have in comparison with their singleton electric
counterparts leads to a different behaviour in these forces at larger
distances. The colour-generated forces saturate at distances of
around 10
−15
metres, the typical size of a proton or neutron, and are
very powerful, but only so long as the two particles encroach to
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The forces of Nature