The strange, other-worldly world of antimatter is taking shape in the minds of men. Last week Dr. Emilio Segrè of the University of California showed the first bubble-chamber picture of an anti-neutron—or rather, a place where an antineutron could be proved to have been.
The great bevatron at Berkeley creates antiprotons (protons with negative charges, in fair quantities. When they hit particles of ordinary matter—protons, neutrons, etc.—they generally annihilate themselves and their targets, both turning into weightless energy and neutrinos. About a fortnight ago an antiproton observed by Dr. Segrè and Dr. Wilson M. Powell behaved differently. It entered Dr. Segrè's bubble chamber, which is filled with liquid propane on the point of boiling, and made its normal, slightly curving trail of tiny bubbles (see cut). Suddenly the trail stopped, and a "star" of four diverging bubble trails appeared a few inches ahead.
Star of Suicides. Dr. Segrè has no doubt about what happened. The antiproton, he says, hit an ordinary, positively charged proton and reacted with it in such a way that the collision produced one ordinary neutron and one antineutron. These two particles differ only in their magnetic properties. Neither has any electric charge, and therefore they left no bubble trails. The neutron shot out of the picture undetected, but the antineutron hit a carbon atom in the propane and committed double suicide with one of its protons or neutrons. The atom disintegrated, leaving a star of bubble trails made by pi mesons. Anti-neutrons have been detected electronically, but this was the first time that one of them has shown up in a bubble picture where its behavior could be studied in some detail.
The study of antimatter, says Segrè, is a new branch of physics whose interests reach from atomic nuclei to the universe. Scientists can now create all the antiparticles they need to build up atoms of antimatter. They have anti-neutrons, antiprotons and antielectrons (positrons). Theoretically, it should be possible to put together antiatoms with negative (not positive) nuclei at their centers and positive (not negative) electrons revolving around them. Dr. Segrè says that this stunt is difficult and probably will not be accomplished for some time.
Anti-Gravity. Another long-range problem is to find out whether antiparticles have antigravity. Some theorists think that they do. repelling ordinary matter instead of attracting it in the normal way. Physicist Segrè thinks this unlikely, but he says that the question of anti-gravity cannot be answered conclusively without an actual experiment. One way would be to isolate anti-neutrons and observe whether they rise in the earth's gravitational field instead of falling as neutrons do. This experiment looks difficult, and Dr. Segrè fears that it may not be accomplished for another generation.
The biggest problem of all is whether the universe contains large quantities of antimatter. Since the two kinds of matter destroy each other in microseconds, there is no antimatter on earth, and probably none in the local Milky Way galaxy, but Segrè is not so sure about the distant galaxies scattered through space. They are so far apart that they would not normally bother each other, even if made of opposite kinds of matter.