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An Essay on the Prize in Astronomy 2007

The past fifty years represent a golden age for astronomy. Exciting new discoveries occurred across a wide range of phenomena. Our intellectual horizons expanded correspondingly. At the edge of that expansion, often creating mighty explosions of his own, one invariably finds Peter Goldreich.


Goldreich's career began auspiciously with his explanation, with Peale, of the role of resonances in the solar system. This understanding formed the basis for the later successful prediction that volcanoes would exist on Io. When the motion of Io was found to correlate with intense decametric radio emission from Jupiter, Goldreich and Lynden-Bell put forth an elegant explanation that relied on the presence of a million-ampere tube of electric current flowing between Io and Jupiter's magnetosphere, a prediction that was later confirmed by direct satellite imaging of the hot spot in Jupiter's atmosphere that is linked magnetically to Io.


In the mid-1960s, the origin of the beautiful spiral arms seen in disk galaxies became a hot topic of astrophysical research. In a classic paper with Lynden-Bell, Goldreich identified the swing amplifier as the dominant mechanism that causes self-gravitating, differentially-rotating, disks of gas and stars to spontaneously generate spiral instabilities.


With the stunning discovery of radio pulsars, many theorists focused on explanations that involved the emission of electromagnetic waves into a vacuum from a strongly magnetized, rapidly spinning, neutron star. With Julian, Goldreich pointed out that the immense electric fields associated with the rotation of a magnetized rotating neutron star would rip out electric charges from its surface and fill its magnetosphere with highly energetic particles. This picture has become the paradigm for all further theoretical development of pulsar emission mechanisms, as well as many semi-empirical attempts to interpret the observational data.


Intense emission in radio lines from the near-vacuum of interstellar space came also as a surprise. If the emission were thermal in origin, the sources would have equivalent temperatures in excess of a trillion degrees. The emission has to arise instead from maser activity, and it was Goldreich and his students who constructed the most comprehensive theory of maser pumping and amplification in interstellar and circumstellar gas.


Occultations of background stars by the planet Uranus showed that this giant planet is surrounded by an intricate set of narrow planetary rings. What keeps a ring confined to a narrow arc despite the disruption of continual collisions among the constituent particles? Goldreich and Tremaine proposed shepherding satellites, a prediction spectacularly confirmed by the Voyager satellite when it discovered two such bodies on either side of Saturn's F ring.


Related studies by Goldreich and Tremaine focused on the resonant gravitational interactions that occur in a rotating disk of material encircling a central body and nearby or embedded companions orbiting the same body. They foresaw that the back reaction of the spiral density wave produced in the disk would cause the companions to migrate in orbital location. Thus, when the discovery of extrasolar planets came a decade ago, theorists had ready-made explanations for the peculiar proximity of many such bodies to their host stars.


Among the most intractable of scientific problems is the nature of astrophysical turbulence. Rising to the challenge Goldreich produced the best theory of how the turbulence driven by solar convection can excite the rich spectrum of small-amplitude solar oscillations that provide such fruitful probes of the interior of the Sun. He has also studied how magnetohydronamic turbulence can explain the properties of interstellar radio scintillation and other transient phenomena observed in the interstellar medium.


The variety, depth, breadth, and innovativeness of Goldreich's most important contributions to theoretical astrophysics are truly staggering. He is universally admired as one of the most influential scientists in modern astronomy. Everything he touches turns to gold. For his lifetime achievements in theoretical astrophysics and planetary sciences, he is a most worthy recipient of the Shaw Prize in Astronomy for 2007.

Astronomy Selection Committee
The Shaw Prize

11 September 2007, Hong Kong