and a neutron is udd. A meson is combination of a quark and an antiquark (not necessarily as the same type as the quark.) For example, a positive pion is ud, where the bar represents an antiparti-cle. The negative pion is ud, and the neutral pion is uu. All known hadrons (and there are over 100) can be constructed by these simple rules. Note that, though there are six quarks, the only ones we need for everyday life (that is to make the proton and neutron) are u and d. Similarly, the only leptons that we need for everyday life are the electron and the electron neutrino.
The quark theory, originally proposed by Murray Gell-Mann (who received the 1969 Nobel Physics Prize for this work) was immediately quite successful, but there were a few important problems left. One was why only the particular combinations mentioned are allowed. The other is that, despite considerable effort, no one has been able to detect a free quark.
Along with the quest for fundamental particles, physicists are also trying to understand the forces with which the particles interact. The concepts of the forces and particles are intimately tied together. Without forces, particles would have no meaning, since we would have no way of detecting the particles. Our current thinking is that there are four fundamental forces, summarized in Table 21.3. They are arranged in order of strength, and their strength is given relative to the strong nuclear force. Short range forces are felt primarily on the scales of nuclei. Long range forces have a 1/r2 falloff in the force, which allows them to be felt over a large distance.
It also appears that particles are necessary as carriers of the forces, as summarized in Table 21.3.
Table 21.3. I Forces and particles.
Strong Nuclear Electromagnetic Weak Nuclear Gravity
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