Pam Mueller, age 12, of Sioux City, Iowa, for her question:

What is antimatter?

This fascinating aspect of physics takes us down to the minuscule realm of the Mev and the meson    and boggles the mind with a theory of colliding galaxies. Both inner and outer space are strewn with frozen forms of energy called matter. And the mortal enemy of matter is antimatter. When an electron particle meets its equal and opposite particle of antimatter, the pair annihilate each other in a Mev of energy.

Ordinary matter is composed of particles with positive, negative or neutral electric charges. We now know that there is an opposite partner of antimatter for each of these assorted particles. The two are mirror images of each other, spinning in opposite directions. They also bear opposite electrical charges. The ordinary electron bears a negative charge, its antiparticle is the positron which has a positive charge. The positive proton has an antiparticle with a negative charge. Even the neutral neutron has a mirror image antineutron. There are positive, negative and neutral particles of antimatter to match the mesons.

Actually these antiparticles are very very rare and in the world of ordinary matter they exist but a grief moment. They exist at the hazardous boundary between energy and mass, or matter     which is a concentrated, frozen state of energy. Physicists compute the mass of small particles in units of energy called electron volts    which actually are not related to electrons. One Mev is a million electron volts and the mass of one electron is 0.51 Mev.

When particle and antiparticle meet, they cross the boundary between matter and energy. Suppose a positron finds itself in a tank of water, surrounded by armies of enemy electrons. Being a tiny energetic particle amid swarms of widely separated tiny particles, it moves ahead. In its miniature world it may travel some distance before it meets its doom. With luck, in less than a millionth part of a second it might travel almost a centimeter.

Eventually a collision occurs. Instantly an electron and the positron annihilate each other, converting their combined masses into about one Mev's worth of energy. When confronted, a neutron and an antineutron convert their larger masses to about 1836 Mevs. Antiparticles that match the 30 or so other atomic particles meet similar fates. If whole atoms of molecules of antimatter exist in our realm, they would be annihilated promptly by the opposing majority.

Scientists speculate that suns, planets and perhaps whole galaxies of antimatter might exist in remote cosmic realms. There the fleeting minority would be particles of ordinary matter. We have telescope pictures of colliding galaxies. Who knows, perhaps they are pairs of matter and antimatter star systems, annihilating each other in stupendous cosmic conversions of matter into energy.