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17th of December 2021
Stricter tests of General Relativity
by Kramer et al.
In 2003, a double pulsar system that offers a perfect opportunity to study general relativity under extreme conditions was discovered using the Parkes radio telescope, "Murriyang". A pulsar is 500,000 times heavier than the Earth, yet only 20 kilometres across. In the double pulsar system, PSR J0737–3039A/B, one ultra-dense "neutron star" spins 50 times a second, blasting out an intense beam of radio waves that our telescopes register as a faint blip every time it sweeps over Earth. There are more than 3,000 pulsars known in our Galaxy, but this one is unique because it whirls in an orbit around a similarly extreme companion star every 2.5 hours.

Sixteen years of observations of the two pulsars has enabled precision tests of theories of gravity for strongly self-gravitating bodies and also revealed new relativistic effects that have been expected but have now been observed for the first time. These include effects of light propagation in strong gravitational fields which are currently not testable by any other method. In particular, the team observed the effects of retardation and aberrational light bending that allow determination of the spin direction of the pulsar!

While Einstein's theory of General Relativity has passed all tests to date, other theories of gravity that diverge in extreme conditions have been proposed. The team demonstrate the utility of the double pulsar for tests of alternative theories of gravity by focusing on two specific examples, and also discuss some implications of the observations for studies of the interstellar medium and models for the formation of the double pulsar system. More details are given in this CSIRO blog by Dick Manchester and Adam Deller.

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