Millisecond Pulsar PSR 1937+21: A Highly Stable Clock.

The stable rotation and sharp radio pulses of PSR 1937+21 make this pulsar a clock whose long-term frequency stability approaches and may exceed that of the best atomic clocks. Improvements in measurement techniques now permit pulse arrival times to be determined in 1 hour at the Arecibo radio telescope with uncertainties of about 300 nanoseconds relative to atomic time. Measurements taken approximately every 2 weeks since November 1982 yield estimates of fractional frequency stability that continue to improve with increasing averaging time. The pulsar's frequency stability is at least as good as 6 x 10(-14) for averaging times longer than 4 months, and over the longest intervals the measurements appear to be limited by the stability of the reference atomic docks. The data yield a firm upper limit of 7 x 10(-36) gram per cubic centimeter for the energy density of a cosmic background of gravitational radiation at frequencies of about 0.23 cycle per year. This limit corresponds to approximately 4 x 10(-7) of the density required to close the universe.