Number 147 October 13, 1993

THE PHYSICS NOBEL PRIZE GOES TO JOSEPH TAYLOR AND RUSSELL HULSE,

both of Princeton University, for their discovery of the first binary pulsar and for subsequent studies leading to a verification of the theory of general relativity for a system outside our solar system. Using the 300-m Arecibo radio telescope in Puerto Rico, Taylor and Hulse in 1974 monitored the beacon-like emissions of the pulsar PSR 1913+16 and inferred that the pulsar---believed to be a rapidly spinning neutron star---was accompanied by a nearby comparably-massive (1.5 solar masses) and compact (20-km diameter) companion object.

A great deal has been learned from the pulsar's RADIO BURSTS, which arrive at Earth about 17 TIMES A SECOND with a regularity that rivals that of the best ATOMIC CLOCKS. For example, a Doppler effect evident in the pulse sequence provides the information needed to work out the orbit parameters for the system.

Furthermore, by recording the pulses over a multi-year period, general-relativistic properties of the binary system could be extracted. In particular, a very slight inward spiralling of the two partners causes their mutual orbit to speed up and close in. According to Taylor, this phenomenon, which shows up as a DECREASE IN THE ORBITAL PERIOD OF ABOUT 75 MSEC PER YEAR, comes about because the system is losing energy (about 10**32 ergs/sec) via the emission of GRAVITATIONAL WAVES. (Equivalently, the advance of the system's periastron is 4.2 degrees per year; by contrast the advance of the periastron for the planet Mercury is only 43 arc-sec per century.) Because the observed decrease in the period so closely matches the value predicted by Einstein's theory of general relativity, many astronomers regard these observations as being important (albeit indirect) evidence for the existence of gravitational waves. (Taylor, 609-452-4368; Hulse, 609- 243-2418. See the Oct. 1981 issue of Scientific American.)

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