In 1969 the Soviet Union used a device called “Tokamak-3” (a Russian abbreviation for their phrase for “electric-magnetic”) to keep a supply of hydrogen-2, a millionth as dense as air, in place while heating it to tens of millions of degrees for a hundredth of a second. A little denser, a little hotter, a little longer—and controlled fusion might become possible.[5] BEYOND FUSION Antimatter Is there anything that lies beyond fusion? When hydrogen undergoes fusion and becomes helium, only 0.7% of the original mass of the hydrogen is converted to energy. Is it possible to take a quantity of mass and convert all of it, every bit, to energy? Surely that would be the ultimate energy source. Mass for mass, that would deliver 140 times as much energy as hydrogen fusion would; it would be as far beyond hydrogen fusion as hydrogen fusion is beyond uranium fission. And, as a matter of fact, total annihilation of matter is conceivable under some circumstances. In 1928 the English physicist Paul Adrien Maurice Dirac (1902- ) presented a treatment of the electron’s properties that made it appear as though there ought also to exist a particle exactly like the electron in every respect except that it would be opposite in charge. It would carry a positive electric charge exactly as large as the electron’s negative one. If the electron is a particle, this suggested positively charged twin would be an “antiparticle”. (The prefix comes from a Greek word meaning “opposite”.) [Illustration: _P. A. M. Dirac_] [Illustration: _The first picture of the positron (left) was taken in a Wilson cloud chamber. On the right is C. D. Anderson, the discoverer of the positron._] The proton is _not_ the electron’s antiparticle. Though a proton carries the necessary positive charge that is exactly as large as the negative charge of the electron, the proton has a much larger mass than the electron has. Dirac’s theory required that the antiparticle have the same mass as the particle to which it corresponded. In 1932 C. D. Anderson was studying the impact of cosmic particles on lead. In the process, he discovered signs of a particle that left tracks exactly like those of an electron, but tracks that curved the wrong way in a magnetic field. This was a sure sign that it had an electric charge opposite to that of the electron. He had, in short, discovered the electron’s antiparticle and this came to be called the “positron”. Positrons were soon detected elsewhere too. Some radioactive isotopes, formed in the laboratory by the Joliot-Curies and by others, were found to emit positive beta particles—positrons rather than electrons. When an ordinary beta particle, or electron, was emitted from a nucleus, a neutron within the nucleus was converted to a proton. When a positive beta particle, a positron, was emitted, the reverse happened—a proton was converted to a neutron.