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same relativistic phenomenon, using not the Mössbauer effect but the precision of atomic clocks, was observed in 1971 by J. C. Hafele and Richard Keating through sensitive comparisons of the progress of time aboard airplanes. In this case, it is special relativity that is important owing to flight velocity, as well as general relativity owing to altitude, as the airplanes changed positions along the gravitational force. Even so, the importance of general relativity was not always recognized even long after this successful experiment. On June 23, 1977, the satellite NTS-2 was launched, the first to carry an atomic clock for experimental purposes. The atomic clock was constructed so as to correct for the relativistic changes in the flow of time. But the developers of the satellite were not fully convinced of the need for corrections for general relativity; instead, the clock was equipped with a gadget to shift the clock rate to the correct value if necessary. After about twenty days in space, the signals indeed showed a deviation in the clock rate compared to clocks on Earth, exactly as predicted by general relativity. In this case, fortunately, the mistake could be corrected by switching on the frequency shift.

Perhaps the most impressive confirmation of general relativity was achieved through observations of double pulsars. These are systems of two stars closely orbiting around each other, one of which (the pulsar) emits radiation at regular intervals. The reason for the emission is usually a rapidly rotating neutron star that, like a lighthouse, emits signals into space and to us. Depending on the position of the pulsar in the double system, signals are delayed at different rates, for they must traverse different routes to reach us. The orbit and possible changes of the system’s radius can thus be determined very precisely. In particular, general relativity predicts that gravitational waves are emitted during the orbiting process, causing the system to lose energy. The loss of energy implies that the two stars approach each other, which should be detectable by precisely measuring the orbit. The loss would be largest when the stars are closest. Each of them then sits more deeply in the gravitational field of its partner and general relativistic effects are stronger.

In 1974, Joseph Taylor and Russell Hulse identified a very close double pulsar consisting of two 20-kilometer-radius neutron stars orbiting around each other in just one-third of a day.5 Their closest distance from each other is a mere 700,000 kilometers, roughly the radius of the sun! This is an ideal test system for slight deviations in the orbit as they are predicted by general relativity, and indeed, observations still being made up to the present day agree exactly with the predictions. (This system is, by the way, also subject to an additional shift in the orbit just like Mercury’s, which does not involve a change of the radius as the energy loss due to gravitational waves does. Rather, one can visualize it as a slowly rotating oval whose shape shows the planet’s orbit and its shift. In the double pulsar, the angular shift is four degrees per year—far larger than Mercury’s—and can be used to estimate the neutron stars’ masses.) Since then, many more close double pulsars with a wide range of orbital properties have been discovered, allowing a large variety of observational tests.

One of the most recent experiments is Gravity Probe B—a satellite launched on April 20, 2004, to collect data for sixteen months in orbit. The idea for this mission was first conceived in 1959, but developing the required technology took a long and tedious road even under the skillful direction of Francis Everitt. The effects aimed at—namely, a “yanking” exerted on space-time in the vicinity of the rotating Earth—demand extremely precise gyroscopes. To avoid perturbations by too uneven a shape, which would prevent any measurement of such sensitive effects, the gyros had to contain the most perfect spheres ever constructed. Even the whole universe can offer few rivals; only some