Quarks: The Search for the Elementary Particle
The Quark Theory
Physicists are forever looking for ways to examine particles too small to observe directly. One such method is a cloud chamber. A fine mist is evenly spread in a box, and particles entering the mist react with it, leaving a trail behind them. This trail can be examined, and the properties of the particle determined. Using this method, 'cosmic' rays were discovered, which had previously been so diffused by the planet's atmosphere that they were undetected. These rays contained hundreds, even thousands of particles which were not made up of electrons, protons and neutrinos, but which were themselves "elementary particles". These new particles were divided into two groups; the leptons (which include the electron and neutrino) and bozons (which include the proton and neutron). The process by which the particles were divided up is too lengthy to go into here, but most of the particles were found to be bozons.
The bozons were found to possess many similarities in their properties, in a manner suggestive of the periodic table. A sort of 'periodic table' was formed for these new particles, called the eightfold way. It related groups of particles with similar properties. Like Mendeleev's periodic table, the eightfold way had many gaps, which were soon filled as physicists hunted for the specific particle which would fill a gap. The view of the universe had become complicated once more, and physicists objected. The periodic table of the elements existed because of the arrangement of subatomic particles in the elements. Could the periodic nature of the bozons also be explained by the actions of a few, tiny particles contained by all bozons?
These theoretical new particles were named quarks by Caltech physicists Murray Gell-Mann. The origin of the name is a quotation from James Joyce: "Three quarks for Muster Mark!" Originally, two types of quarks were postulated. These were called 'up' and 'down' quarks. The names mean nothing, as quarks have no particular orientation in space. They are simply a way of differentiating between the quarks. The up quark has a positive charge equal to two thirds of the charge of an electron, while the down quark has a negative charge equal to one third the charge of an electron. A proton is made up of two up quarks and one down quark, giving it a positive charge. The neutron is made up of two down quarks and an up quark, giving it a neutral charge.
To explain all of the bozons, there must be six quarks. These types, called 'flavours', are known as 'up', 'down', 'strange', 'charmed', 'top' and 'bottom'. (These names are nothing more than an indication of the sense of humour of physicists.) Each flavour of quark has some fraction of electric charge, as well as other properties which determine the behavior of the particle they make up. Each quark also has an 'anti-quark', with exactly the opposite properties.
But although the new quark theory explained all the observations in a simple way, quarks had never been seen. Many physicists spent years and a great deal of money hunting for a quark, to no avail. Eventually, it was suggested that quarks NEVER occur alone, but always in groups. And since a group of quarks simply forms one of the known particles, the particle would be observed rather than the quarks.
This theory became known as the 'colour' theory. There were three 'colours' of quarks: red, blue, and green. There was also the opposites, anit-red, anit-blue, and anti-green. The theory states that 'colour' can never been seen, but only groups of quarks composed of either one of each colour, or a colour and its anti-colour. These groups would form 'white'. Of course, since quarks are so much smaller than the wavelength of visible light, these designations were purely analogy. Of course, the search for a way to observe a single quark continues. Until one is observed, quarks will remain only a theory.
Quarks: The Search for the Elementary Particle
The Quark
Theory