Lightest Neutron Star Pair Ever Found Confirms Einstein's General Relativity to Within 1%

A paper published in Physical Review Letters reports the discovery of PSR J1856-0039, the lightest known binary neutron star system, found using China's Five-hundred-metre Aperture Spherical radio Telescope (FAST) — the world's largest single-dish radio telescope. The system lies roughly 4,200 to 6,500 light-years from Earth in the Milky Way. The visible pulsar weighs about 1.30 solar masses and its companion approximately 1.19 solar masses, making both among the lightest neutron stars ever detected and close to the theoretical minimum for these objects. They complete one orbit every 2 hours and 22 minutes — the second-shortest orbital period ever recorded for a double neutron star — and will spiral together and merge in about 82 million years. Researchers measured three relativistic effects Einstein's theory predicts: the orbit shrinks by roughly 9 feet per year as the pair radiates gravitational waves; the elliptical orbit itself rotates (precesses) at 17.6 degrees per year; and time dilation causes the pulsar's clock to vary by about half a millisecond. All three measurements agree with general relativity to within approximately one percent, delivering one of the most precise tests of Einstein's theory in an extreme-gravity environment to date.