This October, I鈥檒l be speaking at 麻豆传媒 Live about the wonders of dark matter, the invisible matter that dominates galaxies. When I give talks like this, I emphasise the difference between dark matter and visible matter, the stuff we are made of. To highlight the point, I stress that humans are a collection of quarks and electrons 鈥� as are most things in the universe. This is maybe a funny way of describing humans, which we tend to think of as biological in nature. If you ask a physiologist what a human is made of, they鈥檇 probably say something like, 鈥淲e are 60 per cent water and about 20 per cent carbon, plus a smattering of other things.鈥� In other words, we are mostly hydrogen and oxygen.
That鈥檚 technically correct, but a physiologist sees it somewhat differently from a physicist, who drills down beyond the atomic level. Consider hydrogen, the simplest atom. In its most basic form, hydrogen is composed of a proton in its atomic nucleus, with an electron in orbit. Hydrogen can also have one or more neutrons. Meanwhile, oxygen has eight protons in its nucleus, up to eight electrons and as many as 20 neutrons. Every atomic element is some combination of these subatomic particles, so you might think a particle physicist would answer, 鈥淗umans are made of protons, neutrons and electrons.鈥�
But one of these things is not like the others: electrons are fundamental, indivisible particles. Protons and neutrons are, by contrast, composites. A proton is made of two up quarks and one down quark, while a neutron is the reverse, two down quarks and one up quark. A quark, like an electron, can鈥檛 be broken down into anything smaller. So, returning to the question of what humans are made of, I have frequently told people that we are collections of quarks and electrons.
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As I鈥檝e toured for my new book, The Edge of Space-Time, I鈥檝e started to think more and more about whether this is a fair rendering of the situation. There鈥檚 another particle that I should be including 鈥� well, maybe. The complication comes in part from how quarks are bound together. Quarks, uniquely, never exist on their own. They are always in combinations with other quarks. Their most stable combinations are the three-quark combinations we know as protons and neutrons. There are many other three-quark composite particles, as well as two-quark and pentaquark (five-quark) combinations, but all of them are unstable and short-lived. But what you鈥檒l never see is a lone quark and that鈥檚 because of the way the strong force works.
The strong nuclear force, which holds quarks in place, is so powerful that if we try to pull a quark out of, say, a proton, the energy involved would be enough to create another quark and an anti-quark, which is effectively an antimatter version of a quark. Same mass, opposite electric charge. In the end, the quark is still bound to something, never escaping at all. That鈥檚 because of another particle: gluons.
Yes, we have particles that glue quarks together called glue-ons! So you might think that I should really be telling people that humans are electrons, quarks and gluons. The strong force happens because quarks interact via gluons, so including them in the ingredient list would seem to make sense. But there鈥檚 a catch: gluons are a little different from quarks and electrons because they aren鈥檛 real.
That鈥檚 not a typo. Gluons play a meaningful physical role in holding matter together, but they are also complete weirdos: they have no meaningful physical existence. In fact, we call them 鈥渧irtual鈥� particles because they are so incredibly short-lived that it鈥檚 like they don鈥檛 exist at all. Virtual photons also pop up in atoms, as part of electromagnetic interactions, and I鈥檓 not at all ready to claim people are made of particles of light. So, when I explain that we are collections of electrons and quarks, I leave out gluons.
The strangeness of gluons also highlights one of the ways in which 鈥減article鈥� isn鈥檛 really a good way to talk about what happens inside of an atom. The idea of a particle conjures the image of a tiny billiard ball. But this visual metaphor doesn鈥檛 serve us particularly well when trying to imagine something that鈥檚 barely there. For this reason, I鈥檒l probably stick to saying we are phenomenal collections of quarks and electrons, but I guess we鈥檒l all see in real time in October whether I鈥檝e changed my mind about the importance of including the elusive yet powerful gluon.
