TIME: Almanac 1995

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<text id=90TT1277> <title> May 14, 1990: He Ranks As A World-Class Scientist </title> <history> TIME--The Weekly Newsmagazine--1990 May 14, 1990 Sakharov Memoirs </history> <article> <source>Time Magazine</source> <hdr> SPECIAL BOOK EXCERPT, Page 64 Why He Ranks as a World-Class Scientist </hdr> <body> By Dennis Overbye [Dennis Overbye is the author of Lonely Hearts of the Cosmos, to be published this winter by Harper & Row.] Andrei Sakharov's greatest achievements lie buried in missile silos and the bays of Backfire bombers. But enough of his other research slipped past the walls of national security to suggest that he was a great physicist as well as a great man. Some of his work at the Installation concerned tapping the same terrible energy source that powers the hydrogen bomb--thermonuclear fusion--to provide an inexhaustible source of peaceful energy. Ordinary nuclear reactions produce energy from the splitting of atoms. In a thermonuclear reactor, the energy would come, as it does in the sun, from the fusing of hydrogen nuclei to form helium. Getting atoms to fuse, however, is much harder than getting them to split. To overcome the electrostatic repulsion between positively charged nuclei and bring them close enough to fuse, the hydrogen has to be squeezed to high densities and a temperature many times that at the center of the sun--about 100 million degrees. In bombs this trick is accomplished by setting off a nuclear explosion around a core of deuterium and tritium (heavy isotopes of hydrogen). That would not work very well in a reactor; what was needed was a "bottle" that could hold a 100-million-degree gas. In 1951 Sakharov and his mentor Igor Tamm proposed that a magnetic field could serve as the bottle. At the high temperatures required for fusion, atoms are stripped of their electrons, resulting in a gaseous mixture of charged particles known as a plasma. Since a magnetic field can bend the paths of charged particles, a properly designed field could force the hot plasma particles to travel around in a circle, never hitting the sides of the container. His idea became the basis for tokamaks, the doughnut-shaped magnetic chambers that most researchers believe are the best hope for fusion-power sources. Ten years later, Sakharov thought of blasting a small pellet of deuterium and tritium on all sides with a powerful laser beam to generate fusion. Today multibeam laser systems capable of delivering tens of trillions of watts are racing their tokamak cousins to achieve sustainable fusion reactions. As Sakharov's bomb work was winding down, he followed his friend Yakov Zeldovich into cosmology, and it was here that he made his other great mark. Sakharov's reputation would be secure if he had published only a single prophetic paper, which appeared in 1967. It addressed the question, Why is there matter in the universe? By then cosmologists were beginning to accept seriously the notion that the universe had come into being as an infinitely hot and dense burst of energy known as the Big Bang. According to the laws of relativity and quantum mechanics, elementary particles of matter, such as quarks and electrons, could spontaneously appear in such an intense energy field. But there was a hitch. For each type of elementary particle in nature there is an antimatter twin with identical mass but with opposite charge and spin. In a particle accelerator or any other arena, man-made or God-made, in which energy is transformed into matter, particles were created only in such matched pairs--a quark and an antiquark, say, or an electron and a positron. Their properties are precisely balanced so that they cancel each other and leave nature's balance sheet unviolated. This creation process is offset by destruction; when particle and antiparticle meet, they annihilate each other in a flash of radiation and revert back to energy. According to the most elegant theories, therefore, the Big Bang should have produced equal amounts of matter and antimatter. The primordial fireball would have been a dense roiling stew of radiation and elementary particles condensing out of the ambient energy, annihilating each other, recondensing, then colliding and disappearing all over again. As the universe expanded and cooled, it would stop producing particles, and the remaining matter and antimatter would kill each other off. The present-day universe should be empty. Yet the earth, the Milky Way galaxy and, as far as astronomers can tell, the rest of the visible universe are all made of matter. And except for the stray sparks created by cosmic rays and high-energy physics experiments, no antimatter is anywhere in sight. Where is it? Through the '60s this question gnawed at cosmologists. Some speculated that matter and antimatter had separated into different realms, but nobody could think of a realistic sorting mechanism. Others briefly considered the possibility that the universe had been born "cold" with a seed stock of matter in the form of hydrogen atoms. In his historic paper, Sakharov in effect turned the problem around. If the universe had started, as theory held, with equal quantities of matter and antimatter, what would be required to tilt the balance over time so that only matter existed today? This could happen, Sakharov said, if two conditions were met. First there had to be forces or processes operating at the extreme high energies of the early universe that could create matter or antimatter independently of each other, violating what had been presumed to be an ironclad law known as the conservation of baryon number, the hypothetical marker that distinguished matter from antimatter. The second condition was that particles and antiparticles form and decay at slightly different rates, an effect that had actually been recently observed in the decay of a strange particle called the K-meson. Sakharov showed that these two effects, along with the expansion and cooling of the universe, would combine in an intricate chain of reactions slightly favoring the production of matter and leading to a minuscule excess of matter. Only about one quark out of every billion that existed during the Big Bang would escape annihilation and survive to form the modern universe, but that was enough. From this trace of what had once existed would spring all the crystalline shapes and blazing stars and chains of galaxies. Sakharov's paper was a prescription for the formation of matter and also for the future direction of physics. At the time he wrote, no force that would create matter or antimatter independently of each other was known or contemplated. In the 1970s, when physicists started trying to construct the so-called Grand Unified Theories (GUTs) that united the electromagnetic, weak and strong nuclear interactions, the force that Sakharov had prophesied was a natural feature. By the end of the '70s, teams of physicists around the world were essentially retracing Sakharov's calculations in the light of more detailed theories in an attempt to explain the existence of matter. Another prediction of these GUTs was that protons, the presumed building blocks of ordinary matter, were unstable and should radioactively decay in about 10 30 (ten to the thirtyith) years--a span almost unimaginably longer than the 15 billion years or so since the Big Bang--finally redressing the imbalance that had been created so long ago. In the long run (if the universe lasted) matter would prove to be only a passing thought in the long history of time. So far there is no experimental evidence of proton decay. The case for grand unified theories so far rests with the universe, with evidence under our fingernails, and with the work Sakharov started 24 years ago. In cosmology, as in nuclear fusion, human rights and so many other fields, the world is still playing catch-up to him. </body> </article> </text>