Without correcting for it, GPS systems would not work. General relativity has passed every test on Earth. Much of what we know about the universe - for example, the existence of dark matter, which seems to swaddle and shape the galaxies, and of dark energy, which seems to be speeding up the expansion of the universe - comes from presuming that Einstein’s General Theory of Relativity, which describes gravity as the warping of space-time geometry - is correct over cosmic distances. That last explanation would have been big news indeed. Was there an unknown planet or asteroid out there tugging on the spacecraft? Was it drag from interplanetary gas or dust? Something weird about the spacecraft? Or was something wrong in our calculation of gravity out there in the dark? In the 1980s it became apparent that a mysterious force was slowing them down a little more than should have been expected from gravity of the Sun and planets. The story starts with the Pioneer 10 and 11 space probes, which went past Jupiter and Saturn in the late 1970s and now are on their way out of the solar system. Now scientists from NASA’s Jet Propulsion Laboratory have reported that they have explained one of the great mysteries of the space age, one that loomed for 30 years as a threat to the credibility of Einsteinian gravity. In the spring physicists had to withdraw a sensational report that the subatomic particles known as neutrinos were going faster than light, Einstein’s cosmic speed limit they discovered they had plugged in a cable wrong. It’s been a bad year to bet against Albert Einstein.
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