The double-double pays off

In the two years since the discovery of the double pulsar, the international research team has confirmed four gravitational effects predicted by Einstein’s general theory of relativity: Orbital decay The speeding neutron stars cast off orbital energy in the form of gravitational waves, causing them to gradually spiral into one another over millions of years. Measurements show the orbits shrinking by about seven millimetres a day, in line with the theory’s predictions. Periastron advance The path of Mercury around the sun shifts sideways slightly with each orbit, an effect Einstein predicted was increased by the gravitational distortion of space-time near the Sun. The same phenomenon takes place with the double pulsar. But the close distances, concentrated masses and highspeed orbits mean much greater distortion there, with a relative shift 140,000 times greater. The periastron is the line connecting the two pulsars at their closest approach. Shapiro delay A relativistic quirk overlooked by Einstein but predicted by American astrophysicist Irwin Shapiro. When one pulsar moves behind the other, its signals must manoeuvre through the patch of space-time warped by the companion star. This longer path draws out the travel time to Earth by about 90 microseconds. Time dilation General relativity predicts that a clock embedded in a strong gravitational field runs slower as seen by a distant observer, a bizarre phenomenon already measured by comparing atomic clocks in airplanes and on the Earth’s surface. Again, the intense gravitational field around the double pulsar produces a much larger effect. Geodetic precession Support for this predicted effect is still not yet rock-solid, but the evidence is building. Space-time distortion should cause a wobble in the axis around which each pulsar spins.