In 400 BC, the Greek Philosopher Democritis proposed the concept of the atom. He thought if you took a piece of matter and divided it repeatedly, eventually you would reach a point where it couldn't be divided any further.
Democritus also thought that the atoms of different substances had different sizes, weights, and shapes. Today, we know that he was exactly right.
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Around 2,000 years later, in 1814, English chemist John Dalton proposed that all matter consisted of atoms, and that individual elements can be characterized by the weight of their atoms.
In 1911, New Zealand-born physicist Ernest Rutherford proposed a model of an atom consisting of a positively-charged atomic nucleus comprised of protons and neutrons and orbited by negatively-charged electrons. By 1913, Danish physicist Niels Bohr had refined this model to include specific orbitals for the electrons. In 1926, Austrian physicist Erwin Schrodinger elucidated the "cloud model" for electrons.
"Three quarks for Muster Mark!"
Today, we know that the protons and neutrons in the atomic nucleus are comprised of quarks. That name was given to them by American physicist Murray Gell-Mann, who took the name from the novel Finnegan's Wake by James Joyce: "Three quarks for Muster Mark! Sure he hasn't got much of a bark. And sure any he has it's all beside the mark."
Gell-Mann and physicist George Zweig independently came up with the quark hypothesis, although Zweig called the particles "aces". While Gell-Mann, who died this past May, received the 1969 Nobel Prize in Physics for his work, Zweig has yet to be so honored.
According to the theory, quarks combine to form composite particles called hadrons. Protons and neutrons are stable forms of hadrons. Due to a phenomenon called color confinement, quarks cannot be directly observed, they are only found within hadrons.
Six "Flavors" of Quark
There are six types, or flavors, of quark, called up, down, charm, strange, top and bottom. For every quark flavor, there is a corresponding antiparticle, called an antiquark.
Antiquarks differ from quarks in that some of their properties, such as electric charge, have equal magnitude but opposite sign.
Quarks have certain intrinsic properties:
* Electric charge
* Color charge
It is this last attribute — color charge — that causes quarks to engage in the strong interaction, and the theory behind this interaction is called quantum chromodynamics (QCD).
The QCD theory states that there are three types of color charge — blue, green and red, and three types of anticolor — antiblue, antigreen and antired. Physicists represent these anti colors - antired, antigreen and antiblue - as cyan, magenta and yellow, respectively. Every quark carries a color, while every antiquark carries an anticolor.
It is this system of attraction and repulsion between quarks that are charged with different combinations of the three colors that is called the strong interaction, and the force is transmitted by force-carrying particles called gluons.
"The world of the quark has everything to do with a jaguar circling in the night" -- Arthur Sze
A quark having a single color value, let's say, blue, can form a system with an antiquark that has the corresponding anticolor, in this case, antiblue. The result is color neutrality, or "white" color, and the resulting two-quark meson has a net color charge of zero. A meson is any member of a family of subatomic particles that is comprised of a quark and an antiquark.
Baryons, of which the proton and neutron are the most common examples, are formed from three quarks. One quark must have a color charge of blue, one must have a color charge of green, and the third must have a color charge of red. The result is color neutrality, or "white".
The same goes for antibaryons, which are comprised of one antiquark having color charge antiblue, one antiquark having color charge of antigreen, and one antiquark having color charge antired.
While the color charge attribute of quarks and gluons is completely unrelated to the everyday meaning of color, it became a popular model due to its analogy to the primary colors. Richard Feynman, an American physicist who was not known to suffer fools gladly, referred to his colleagues as "idiot physicists" for choosing the confusing name, "color".
The particle that transmits, or mediates, the strong force between quarks is called the gluon. To allow for all the possible interactions between the three colors of quarks, there must be eight gluons, each of which carries a mixture of a color and an anticolor. For example, red and antigreen.
While the force-carrying particle for the electromagnetic force, the photon, can operate over vast distances, the gluon operates at close range — very close range. It is limited to a distance of about 10−15 meters, which is shorter than the diameter of an atomic nucleus.
This explains why quarks are confined in baryons, such as protons and neutrons, and in mesons, and cannot be observed independently. Even if you exert tremendous energy and knock a single quark out of a proton, instead of getting the quark, you would get a quark-antiquark pair, or meson.