• Question: What is a quark?

    Asked by sarahtimp to Ben, Jony, Katharine, Mark, Peter on 19 Nov 2011.
    • Photo: Mark Basham

      Mark Basham answered on 18 Nov 2011:

      A quark is one of the fundamental particles and are the building blocks from which Protons and Neutrons are made. We know all about them from particle accelerators and breaking the nuclei of atoms apart. Lots of info about them is available on-line, for example http://en.wikipedia.org/wiki/Quark.

    • Photo: Peter Williams

      Peter Williams answered on 19 Nov 2011:

      They are tricksy little things, much harder to pin down than photons or electrons. They never exist on their own, only in groups, three in things like a proton or neutron (these are called hadrons), two in things like a pion or kaon (called mesons). Recently they have been found in groups of more than 3, but these things are very unstable.
      Even what i have said above is a bit of an oversimplification. If you look inside the proton, what you see depends on how you look at it. The only analogy i can think of is – if you use red light to look at an object, then look at it again in blue light you find that the constituents of the object have changed! It’s only when you average over all colours that you find there is an excess of quarks with an average number of 3. The other things that protons are made of – gluons, average out to zero.
      This behaviour is connected to the nature of the force that binds quarks and gluons together, called the strong nuclear force. It has the wierd property that it increases with the distance between the quarks – the total opposite to gravity or electromagnetism (these two are 1/r^2 force laws, the force decreases in proportion to the square of the distance between the objects). The strength of the strong force increases linearly with the distance between the objects – a bit like an elastic band.
      It’s this that causes “confinement” – the fact that you never see one on it’s own.

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