Taylor in the region did not discover a pulsar associated with 4C21.53. With the lack of success in finding a pulsar in the region, other explanations for the scintillation were explored, including suggestion of entirely new classes of objects. After realizing in 1982 that previous searches for a pulsar in the region of 4C21.53 were not sensitive to periods short enough to produce the observed scintillation, Don Backer initiated a search in the area that would be sensitive to a wide range of pulse periods and dispersion measures, including very short periods. The initial search plan was to sample at a rate of 500 Hz, which would have been insufficiently fast to detect a pulsar spinning at 642 Hz. To simplify the search apparatus, Backer's then student, Shri Kulkarni, sampled as quickly as was possible, and time averaged the signal over a period of 0.4 milliseconds, thus effectively sampling at 2500 Hz. As a result, Backer et al. determined in November 1982 that the source was a pulsar rotating every 1.558 milliseconds, a rate far beyond anything that astronomers studying pulsars had expected. Characteristics Age and spin down rate When Backer et al. reported their finding in November 1982, they found that the rotation period of PSR B1937+21 was increasing at a rate of 3 seconds per second. Pulsars are expected to slow over time, as the energy that they emit is ultimately drawn from the rotational energy of the pulsar. Using the initially observed values for the period and spin down rate, and assuming a minimum period of 0.5 milliseconds for pulsars, the maximum age for PSR B1937+21 was found to be about 750 million years old. The estimate of the minimum possible period is obtained from the centrifugal break-up limit, which is the rotational period at which the centrifugal force and the self-gravity of the pulsar are equal. The value of the minimum rotational period depends upon the neutron star equation of state, with different models giving values between 0.3 and 1 millisecond, which corresponds to a rotation frequency of 1-3 kilohertz. There may be mechanisms such as gravitational radiation which keep the pulsar from reaching this absolute limit, but pulsars can spin no faster. An age of no more than 750 million years for the PSR B1937+21 was at odds with the observations of the region in other wavelengths. No optical supernova remnant, nor bright x-ray source,