Matter | Pages 74-75 | See Linked Version

Subatomic Particles

By smashing atoms at ever-higher energies to produce increasingly exotic subatomic particles, physicists have learned that most of the tiny specks are actually combinations of a small number of fundamental objects. According to current theory, the two most fundamental subclasses of particles are fermions, named for the Nobel laureate Enrico Fermi, and bosons, named for the Indian physicist Satyendra Bose. Fancifully illustrated here are the fermion members of the subatomic family along with their antimatter counterparts. Bosons, which transmit the four known forces between fermions, are shown on page 76.

The most fundamental fermions are classified as either leptons or quarks. Quarks are bound together in threes by the strong nuclear force to make neutrons and protons. Less common particles such as pions and kaons are made up of two quarks. Experiments suggest that quarks come in six varieties, which physicists have named up, down, charm, strange, top, and bottom.

Leptons, which are all low-mass objects, do not combine with each other or with other particles except under special circumstances. They come in six varieties: Three have a negative charge and three have no charge. Charged leptons include electrons, muons, and taus. The heaviest is the tau, with nearly twice the mass of a hydrogen atom. Leptons with no charge are called neutrinos ("little neutral one" in Italian) and usually accompany a charged counterpart. Thus, they include the electron-neutrino, the muon-neutrino, and the tau-neutrino. Once presumed to be massless, neutrinos are now suspected of possessing just a smidgeon of mass.

A generic fermion is shown here with its antiparticle, which has the same mass, although all other properties are reversed.

The electron, carrier of electric current and of negative charge in the atom, has as its antiparticle the positively charged positron.

The neutrino and antineutrino have no charge and are believed to have minuscule mass.

Generic quarks and antiquarks are bound by the strong nuclear force to form composite particles.

apply those rules to a host of chemical processes on Earth, such as predicting the noxious compounds that form when car exhaust drifts into the atmosphere. The rules work equally well in space. For instance, astrophysicists can determine which elements combine in cooling interstellar gas clouds. The new molecules created in this way eventually lead to new stars and planetary systems.

The periodic table is so basic to understanding the chemistry of the cosmos that a panel of physicists, archeologists, artists, and sociologists recently devised a surprising way to use it. The panel was charged with creating warning systems for the Waste Isolation Pilot Plant (WIPP), an underground storage facility in New Mexico for low-level radioactive waste. The experts had to envision systems that would warn people against digging at the WIPP site for at least 10,000 years because its contents will stay hazardous for that long. Languages and cultures are likely to come and go in that time, but WIPP will remain. Panel members designed frightening sculptures, earthworks, and other symbols of danger. They also proposed a chamber containing an engraved reproduction of the periodic table, with highlights marking the squares for uranium, plutonium, and other radioactive elements. Any future scientists would recognize the hazard, the panelists reasoned, because the periodic table is likely to endure.

The Scarcity of MATTER

The periodic table is a tool for us to understand how matter behaves, and Rutherford's model of the atom helps us realize that all matter is mostly empty space. We also have seen that matter is rare in the universe--just a few atoms per cubic yard of space, on average. But by a different reckoning, it seems there is plenty of matter to go around. There are perhaps 100 billion galaxies in the universe, each containing perhaps 100 billion stars. Every person on Earth would have to count five stars per second for about 10,000 years to tally all of those stars, not to mention the atoms that compose them. How is it possible for so much matter to add up to so little?

The key is to grasp the vast distances between objects in the universe. Just as we constructed a model of an atom with a tiny ball bearing in the center of the Louisiana Superdome, we can imagine scale models of planets and moons in our solar system, stars in our galaxy, and groups of galaxies in the cosmos as a whole.