also available in book form:
Pulsars as Physics Laboratories, eds. R.D. Blandford, A. Hewish, A.G. Lyne and L. Mestel (Oxford University Press, Oxford; ISBN 0-19-853983-5), 39-75 (1993) QB843.P8P83
Theoretical Astrophysics, 130-33 California Institute of Technology, Pasadena, California 91125
As clocks, pulsars rival the best atomic clocks on earth. Though the rest-frame `tick' rate (period P) of any given pulsar is unknown, the rest-frame rates of change of the periods are known to be very small. Therefore when they are observed to be large, one is quite certain that the rate of changes must be due to changing Doppler shifts: first period derivative to acceleration, second period derivative to jerk, and periodic shifts to orbiting companion stars or planets. The first two give otherwise unobtainable information on the density and masses of the stellar remnants in the cores of globular clusters. The orbits of binary pulsars provide a test of the theory of the evolution of red giant stars, and in globular clusters provide the first direct evidence for the three and four-body encounters which are believed to determine the dynamical evolution of globular clusters. The orbits of binary pulsars in our own galaxy also show evidence for the fluctuations which the fluctuation-dissipation theorem implies should occur during the dissipative tidal circularization of orbits. And Newtonian dynamical effects may soon add irrefutable confirmation to recent observations suggesting that some pulsars are surrounded by planetary systems similar to our own. There may not be life on their planets, but pulsars certainly breathe new life into the study of Newtonian dynamical systems.