Steve Proctor, age 13, of New Holland, Pennsylvania, for his question:

Does antimatter really exist?

Nowadays, antimatter is one of the hottest topics in physics. Researchers are finding more evidence that indeed it does exist. But in our corner of the cosmos it does not exist very long    maybe about one millionth part of a second. However, in some places out in the remote regions of our own and other galaxies, antimatter may be more common than our ordinary matter. In which case, a stray particle of our ordinary type matter would meet with speedy annihilation.

The theory of antimatter begins down at the level of infinitesimal atomic particles. The basic building block of matter is the atom. This dynamic midget is invisibly small, but indirect evidence proves it to be an orderly unit of electrically charged particles. Its tight fisted nucleus is a bundle of positive protons, neutral neutrons, mesons and perhaps a couple of dozen other smaller thansmall particles.

Ordinary matter is made from these ordinary atoms and their packaged molecules. We know the weight, the magnetic and electrical and other properties of about 30 atomic particles. We also know now that particles of antimatter exist to match all those of ordinary matter. An ordinary electron has a negative charge. Its anti particle is the positron, with an equal and opposite charge. The positive proton's opposite is the negative antiproton. Both the neutron and the antineutron are electrically neutral. But their magnetic and other properties are opposites.

You would think that assorted anti particles could be assembled to create whole atoms and molecules of antimatter. However, matter and antimatter are mortal enemies. When the opposites meet, they annihilate each other. An electron and a positron are converted into about half a million electron volts' worth of gamma and other radiation. When neutron meets antineutron, they are converted into about 1836 Mev of energy. A stray positron in a tank of water might travel one centimeter in a millionth of a second before it collided with an electron    and met its doom.

Nevertheless, scientists have succeeded in assembling a few nuclei of antimatter. They have created the nucleus of antideuterium from antiprotons and antineutrons, also an antimatter nucleus to match an isotope of helium. These nuclei need shells of positrons to become whole atoms of antimatter    atoms that would be annihilated when they contacted matching atoms of ordinary matter.

Obviously antimatter has only a slim chance of existing on this planet of ordinary matter. But elsewhere in the universe the reverse may be true. It is suggested that quasars may be powered by conflicts between large quantities of matter and antimatter. Some stars or even galaxies may be entirely antimatter, where stray atoms of ordinary matter would be annihilated. A recent theory suggests that the universe may contain equal amounts of matter and antimatter, most of it separated by vast oceans of space.