TIME: Almanac 1993/TIME Almanac 1993.iso/time/051490/0514990.000

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                                                                                SPECIAL BOOK EXCERPT, Page 64

Why He Ranks as a World-Class Scientist


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.