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All atomic nuclei are composed of two types of particles: protons and their electrically neutral partners, neutrons. If a nucleus has too many of one type or the other, then the rules of quantum mechanics dictate that the balance has to be redressed and those excess particles will change into the other form: protons will become neutrons, or neutrons protons, via a process called beta-decay. This is precisely what happens when two protons come together: a composite of two protons cannot exist and one of them will beta-decay into a neutron. The remaining proton and the newly transformed neutron can then bind together to form an object called a deuteron (the nucleus of an atom of the heavy hydrogen isotopefn3 called deuterium), after which further nuclear reactions enable the building of the more complex nuclei of other elements heavier than hydrogen, from helium (with two protons and either one or two neutrons) through to carbon, nitrogen, oxygen, and so on. The key point is that the deuteron owes its existence to its ability to exist in two states simultaneously, by virtue of quantum superposition. This is because the proton and neutron can stick together in two different ways that are distinguished by how they spin. We will see later how this concept of ‘quantum spin’ is actually very different from the familiar spin of a big object, such as a tennis ball; but for now we will go with our classical intuition of a spinning particle and imagine both the proton and the neutron spinning together within the deuteron in a carefully choreographed combination of a slow, intimate waltz and a faster jive. It was discovered back in the late 1930s that within the deuteron these two particles are not dancing together in either one or the other of these two states, but in both states at the same time – they are in a blur of waltz and jive simultaneously – and it is this that enables them to bind together.fn4 An obvious response to this statement is: ‘How do we know?’ Surely, atomic nuclei are far too small to be seen, so might it not be more reasonable to assume that there is something missing in our understanding of nuclear forces? The answer is no, for it has been confirmed in many laboratories over and over again that if the proton and neutron were performing the equivalent of either a quantum waltz or a quantum jive, then the nuclear ‘glue’ between them would not be quite strong enough to bind them together; it is only when these two states are superimposed on top of each other – the two realities existing at the same time – that the binding force is strong enough. Think of the two superposed realities as a little like mixing two coloured paints, blue and yellow, to make a combined resultant colour, green. Although you know the green is made up of the two primary constituent colours, it is neither one nor the other. And different ratios of blue and yellow will make different shades of green. Likewise, the deuteron binds when the proton and neutron are mostly locked in a waltz, with just a tiny amount of jive thrown in. So
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