One reason to write a trivia question is to get straight for yourself something you feel you really should know. That’s why today we’re looking at something rather fundamental: the modern notion of what reality is made of. The question below might find its way into a physics quiz on TriviaPark one day, or even a quiz on particle physics in particular. (Of course, it will show up in our general knowledge quiz rotation eventually too.) Feel free to suggest in the comments ideas for other questions you think a (particle) physics quiz should contain.
Here’s the question:
Nearly all the ordinary matter in the observable universe — the stars, the planets, ourselves — is made of just two kinds of elementary particle. What are they?
- Electrons and quarks
- Mesons and baryons
- Neutrinos and hadrons
- Protons and neutrons
The elementary particles that make up ordinary matter are the electron and the quark. The neutrino is also an elementary particle, but is not a constituent of matter. The other particles mentioned are not elementary, all being combinations of quarks. Any particle composed of multiple quarks is, by definition, a hadron. The hadrons are divided into two groups: the mesons, each formed from a quark and an antiquark, and the baryons, each a combination of three quarks or three antiquarks. Because there are many possible ways of combining three quarks, the baryons are a diverse family, but two of its members are the familiar proton and the neutron, which make up the nuclei of atoms. These, together with the much lighter electron, account for virtually all directly detectable matter. However, scientists now widely accept that most of the matter in the universe — about three-quarters — is of a type not directly detectable. The actual composition of this ‘dark matter’ is far from settled, but it appears unlikely that it can be explained in terms of the familiar ingredients of ordinary atoms.
