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A-Z of thought: Quarks
John Polkinghorne
Quarks are the fundamental entities out of which we believe that
nuclear matter is composed. They were given the name by Murray
Gell-Mann, a Nobel physicist who has made many fundamental
contributions to particle physics. A rather bewildering array of
different forms of nuclear matter had been discovered and Gell-Mann
recognised that taxonomic order could be introduced into this 'zoo'
if these forms were organised into simple patterns which could be
generated mathematically by considering different arrangements of
three fundamental units.
Gell-Mann called these units 'quarks', recalling a line from
Finnegan's Wake, 'Three quarks for Muster Mark'. He tells us that
he felt there was a quart pot atmosphere about the context and so
he pronounced quarks with a long 'a', to rhyme with corks. Some of
us feel, however, that the euphony of the line requires a short 'a'
to rhyme with sparks. This difference of opinion persists among
particle physicists.
Gell-Mann saw that quarks would possess some unusual properties,
including fractional electric charge. But no one had ever seen a
fractionally charged object. So Gell-Mann was cautious about the
physical status of the quarks themselves, laying emphasis on the
patterns that they generated and calling the quarks 'presumably
mathematical'. However, experiments were performed later where high
energy projectiles such as electrons struck a nucleus and some
recoiled at wide angles. This behaviour was interpreted as the
electrons striking some small hard entities within the nuclear
matter (think of bullets striking sixpences within a Christmas
pudding.) A similar phenomenon at much lower energy had been
observed by Rutherford in 1911 in his discovery of the point-like
nucleus within the atom. These experiments strongly supported
the idea of the existence of constituents within nuclear matter and
when the properties of the constituents were analysed they
corresponded precisely to those expected of quarks. Thus it became
possible to drop the qualifier 'presumably mathematical'. (Later
three more exotic kinds of quarks were also discovered.) But still
no one has ever seen a fractionally charged particle. This is
explained by assigning to the quarks a new kind of physical
property, 'confinement'. The idea is that the forces that bind the
quarks together to form nuclear matter are so strong that no impact
is ever strong enough to expel an isolated quark.
The moral of this tale is that scientists are quite content to
believe in the existence of unseen realities, provided that belief
makes intelligible a great deal of directly accessible
experience.
John Polkinghorne