The antiproton was as unlikely to last as long as the positron was. It was surrounded by myriads of proton-containing nuclei and in a tiny fraction of a second it would encounter one. The antiproton and the proton also underwent mutual annihilation, but having 1836 times the mass, they produced 1836 times the energy that was produced in the case of an electron and a positron. There was even an “antineutron”, a particle reported in 1956 by the Italian-American physicist Oreste Piccioni (1915- ) and his co-workers. Since the neutron has no charge, the antineutron has no charge either, and one might wonder how the antineutron would differ from the neutron then. Actually, both have a small magnetic field. In the neutron the magnetic field is pointed in one direction with reference to the neutron’s spin; in the antineutron it is pointed in the other. [Illustration: _Bubble chamber photograph of an antiproton annihilation._] In 1965 the American physicist Leon Max Lederman (1922- ) and his co-workers produced a combination of an antiproton and an antineutron that together formed an “antideuteron”, which is the nucleus of antihydrogen-2. This is good enough to demonstrate that if antiparticles existed by themselves without the interfering presence of ordinary particles, they could form “antimatter”, which would be precisely identical with ordinary matter in every way except for the fact that electric charges and magnetic fields would be turned around. If antimatter were available to us, and if we could control the manner in which it united with matter, we would have a source of energy much greater and, perhaps, simpler to produce than would be involved in hydrogen fusion. To be sure, there is no antimatter on earth, except for the submicroscopic amounts that are formed by the input of tremendous energies. Nor does anyone know of any conceivable way of forming antimatter at less energy than that produced by mutual annihilation, so that we might say that mankind can never make an energy profit out of it—except that with the memory of Rutherford’s prediction that nuclear energy of any kind could never be tapped, one hesitates to be pessimistic about anything. The Unknown Physical theory makes it seem that particles and antiparticles ought to exist in the universe in equal quantities. Yet on earth (and, we can be quite certain, in the rest of the solar system and even, very likely, in the rest of the galaxy) protons, neutrons, and electrons are common, while antiprotons, antineutrons, and positrons are exceedingly rare. Could it be that when the universe was first formed there were indeed equal quantities of particles and antiparticles but that they were somehow segregated, perhaps into galaxies and “antigalaxies”? If so, there might occasionally be collisions of a galaxy and an antigalaxy with the evolution of vast quantities of energy as mutual annihilation on a cosmic scale takes place.