Quarks possess a property to which has been given the name color. Just as electric charge originates electromagnetic interactions, color originates strong interactions. Thus color can also be called ``strong charge´´. Quarks come in three different states of color, conventionally called red, green and blue. A proton or a neutron is made of three quarks of different colors. Since equal amounts of red, green and blue result in white, neutrons and protons can be said to be white. In fact, according to the theory of strong interactions, only white objects can exist in isolation. Colored particles - such as quarks - can only exist as components of white objects. This property is called confinement. The region of space to which quarks are confined to form a proton or a neutron - represented by the crystal ball in the picture - is usually called a bag.


As far as is presently known, there exist six different types of quarks. These types are known as flavors and have been given the following names: up, down, strange, charm, top and bottom. One property which differs from one flavor to the other is mass. The up and down quarks have the smallest masses. These are the only flavors which constitute ordinary stable matter. The other flavors are present only in unstable particles produced in collisions. In the present book, the up quark is represented as an upper hemisphere and the down quark is represented as a lower hemisphere. This is a mere visualization convention; in fact, quarks are ``point-like´´ in the standard theory of elementary particles. The proton is made up of two up quarks and one down quark, while the neutron is made up of two down quarks and one up quark. The up quark has positive electric charge equal in absolute value to 2/3 that of the electron. The down quark has negative electric charge equal in absolute value to 1/3 that of the eletron. Check that these charges add up correctly to give, for the neutron, zero total charge and, for the proton, positive charge equal in absolute value to that of the electron.


Protons and neutrons are made of three quarks. Quarks, like protons and neutrons, have spin s=1/2. These three spins must add up to form the spin of the proton or neutron. According to quantum mechanics, in order to obtain a proton or neutron with spin up, we must combine two quarks with spin up with one quark with spin down. Symmetrically, in order to obtain a proton or neutron with spin down, we must combine two quarks with spin down with one quark with spin up. These rules are illustrated in the pictures.