Chapter 7 of Book 7: Quarks to Quasars begins with a quote from Lords and Ladies, a book by the most marvellous Terry Pratchett. This pleases me immensely – not just because I am a big Discworld fan, but for reasons that will hopefully become clear.
“It was here that the thaum, hitherto believed to be the smallest possible particle of magic, was successfully demonstrated to be made up of resons (Lit.: ‘Thing-ies’) or reality fragments. Currently research indicates that each reson is itself made up of a combination of at least five ‘flavours’, known as ‘up’, ‘down’, ‘sideways’, ‘sex appeal’ and ‘peppermint’.” Terry Pratchett
Firstly, this description of sub-magic particles is not so far from our description of subatomic particles. Including the flavours.
Secondly, “reality fragments” is not just a poetic way to describe the fundamental particles that make up the matter of the Universe, but is also pretty accurate. Reality fragments can be put together into larger and larger particles, as the stuff of the Universe is created in star factories.
In our world, until fairly recently (50 years ago or so), it was accepted that the Universe was built from protons, neutrons, electrons and electron neutrinos. Electrons and electron neutrinos, together with their antiparticles (everything has an equal and an opposite), are indeed fundamental particles. They cannot, as far as we know, be broken down further.
Electrons and electron neutrinos are in the lepton family, along with four other fundamental particles: the muon (about 200 times heavier than an electron) and its associated neutrino; and a tauon (about 3,500 times heavier than an electron) plus its neutrino.
So, there are six flavours of lepton. The electron, the muon and the tauon, which all have a negative charge, plus their neutrinos, which are neutral. And just to really confuse matters, there are also six antileptons, with a positive charge but the same mass.
The word “lepton” comes from the Greek leptos, meaning “thin” or “lightweight”, which is reasonable really when you consider just how tiny these things are…
So are these the only fundamental particles? No. We now know that if two nucleons (a proton or a neutron) are picked apart, smaller bits fall out.
Now, let’s give the nucleons another name – just as a test of memory. Protons and neutrons are examples of hadrons. They are not the only hadrons – there are also baryons and mesons.
What makes up hadrons? Quarks!
(As an aside: if you google “quark” in images, you get the Star Trek character. This pleases me.)
This is where it becomes really fun, and has led me to believe that particle physicists are a bunch of hippies at heart. It wouldn’t surprise me if they loaf around smoking pot and drinking absinthe while pondering the nature of the Universe (and there’s nothing wrong with that). You see, quarks, too, have flavours. Sadly not “peppermint” or “sex appeal”, but Terry wasn’t far off.
The quark flavours are: up, down, charm, strange, top and bottom (or, on a particularly fuzzy day, top and bottom are known as “truth” and “beauty”). The up, charm and top quarks have a charge of +2/3e and the down, strange and bottom quarks have a charge of -1/3e. And don’t forget that each quark has its corresponding antiquark…
A hadron can consist of three quarks (a baryon), three antiquarks (an antibaryon) or one quark and one antiquark (a meson); and it always has a whole number charge, so you can determine the recipe.
For example, a proton has a charge of +e and is composed only of up and down quarks. The only way to produce a net charge of +e with up and down quarks is with the recipe up, up, down (uud): 2/3e + 2/3e – 1/3e = +e.
It is now accepted that these are all fundamental particles; that they cannot be broken down further. However, particle physics is moving at lightning speed, and boundaries are being pushed all the time, so who knows what else will turn up?
It is incredible that we have drilled down into the very fabric of the Universe, and pulled out particles that are so small they are incomprehensible. Much like trying to imagine the immense distances between the stars, numbers and sizes become almost meaningless at this point, and it’s much more helpful to think in abstract terms.
Perhaps this is why physicists have come up with such whimsical names for the particles… at this stage, it may as well be pixie dust!