100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

Was Einstein Right? 219milliarcsecond per year frame dragging or Lense-Thirring effect caused bythe rotation of the Earth; its secondary goal was to measure to six parts in10 5 the larger 6.6 arcsecond per year geodetic precession caused by spacecurvature [Eq. (8)].A complementary test of the Lense-Thirring precession, albeit withabout 10 times lower accuracy than the GPB goal, was reported by Ciufoliniand Pavlis 53 . This experiment measured the precession of the orbitalplanes of two Earth-orbiting laser-ranged satellites called LAGEOS, usingup-to-date models of the gravitational field of the Earth in an attempt tosubtract the dominant Newtonian precession with sufficient accuracy toyield a measurement of the relativistic effect.4. The Binary PulsarThe binary pulsar PSR 1913+16, discovered in 1974 by Joseph Taylor andRussell Hulse, provided important new tests of general relativity, specificallyof gravitational radiation and of strong-field gravity. Through precisetiming of the pulsar “clock”, the important orbital parameters of the systemcould be measured with exquisite precision. These included non-relativistic“Keplerian” parameters, such as the eccentricity e, and the orbital period(at a chosen epoch) P b , as well as a set of relativistic “post-Keplerian” parameters.The first PK parameter, 〈 ˙ω〉, is the mean rate of advance of periastron,the analogue of Mercury’s perihelion shift. The second, denoted γ ′ isthe effect of special relativistic time-dilation and the gravitational redshifton the observed phase or arrival time of pulses, resulting from the pulsar’sorbital motion and the gravitational potential of its companion. The third,P˙b , is the rate of decrease of the orbital period; this is taken to be the resultof gravitational radiation damping (apart from a small correction due togalactic differential rotation). Two other parameters, s and r, are relatedto the Shapiro time delay of the pulsar signal if the orbital inclination issuch that the signal passes in the vicinity of the companion; s is a directmeasure of the orbital inclination sin i. According to GR, the first threepost-Keplerian effects depend only on e and P b , which are known, and onthe two stellar masses which are unknown. By combining the observationsof PSR 1913+16 with the GR predictions, one obtains both a measurementof the two masses, and a test of GR, since the system is overdetermined.The results are 54m 1 = 1.4414 ± 0.0002M ⊙ , m 2 = 1.3867 ± 0.0002M ⊙ ,P˙bGR / P ˙ b OBS = 1.0013 ± 0.0021 . (9)