What is Binary pulsar?
A '''binary pulsar''' is a [[pulsar]] with a [[binary star|binary companion]], often a [[white dwarf]] or [[neutron star]]. In at least one case, the double pulsar PSR J0737-3039, the companion star is another pulsar as well. Binary pulsars are one of the few objects which allow physicists to test [[general relativity]] in the case of a strong [[gravitational field]]. Although the binary companion to the pulsar is usually difficult or impossible to observe, the timing of the pulses from the pulsar can be measured with extraordinary accuracy by [[radio telescope]]s. Binary pulsar timing has indirectly confirmed the existence of gravitational radiation and verified [[Albert Einstein|Einstein]]'s [[general theory of relativity]]. == Relativity == Two objects orbiting do not do so in absolutely [[circular]] paths. the paths are virtually always [[ellipse|elliptical]]. So twice a circuit they are closest, and twice a circuit they are furthest away. This is obvious for the [[Earth]] and [[Sun]], but the idea applies much more widely. When the two bodies are close, the gravitational field is stronger, and the passage of time is slowed. With pulsars, the time between pulses (or ticks) is lengthened. As the pulsar clock travels more slowly through the weakest part of the field it regains time. This is a relativistic time delay. It is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around its companion, and what is actually observed.<ref>Weisberg J.M. Taylor J.H. and Fowler L.A. 1981. Gravitational waves from an orbiting pulsar. ''Scientific American'', October.</ref> Binary pulsars are one of the few tools scientists have to detect evidence of [[gravitational waves]]. Einstein’s theory of general relativity predicts that two neutron stars would emit gravitational waves as they orbit a common center of mass, which would carry away orbital energy, and cause the two stars to draw closer together. As the two stellar bodies draw closer to one another, often o