General relativity passes a new test

General relativity, Einstein’s theory of gravity, has passed every high-precision test in the solar system. But GR poses serious problems for quantization, cosmological inflation, and the much-desired unification of the fundamental forces. So theorists widely anticipate that the true macroscopic gravity theory will diverge significantly from GR in regions with much stronger gravitational fields than those of the solar system. Several attractive generalizations of GR suggest detectible effects in neutron stars above some critical mass. No such divergence has been seen. For several years, an international team centered at the Max Planck Institute for Radio Astronomy in Bonn, Germany, has been looking at the most promising neutron-star system yet found. It’s a relativistic binary pair with a white-dwarf star closely orbiting a radio pulsar (to its left in the artist’s impression) every two hours. The pulsar, twice as massive as the Sun, is the heaviest neutron star for which we have a good mass measurement. Such extreme binary orbits have been conjectured to lose energy by gravitational radiation much faster than GR predicts. But the team’s pulsar-timing observations to date show that the orbital periodicity is slowing at just about the 8 microseconds per year predicted by GR. That result already excludes important parameter-space regions of some prominent candidate theories. (J. Antoniadis et al., Science 340, 1233232, 2013 .)—Bertram Schwarzschild