New user? Register here:
Email Address:
Password:
Retype Password:
First Name:
Last Name:
Existing user? Login here:
 
 
Icons

A-Z of thought: Quarks

John Polkinghorne

AZ.jpg

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