Einstein was right, again

It's been a bad year to bet against Albert Einstein. 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. 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.

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. 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. 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?

That last explanation would have been big news indeed. 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. General relativity has passed every test on Earth. Without it GPS systems would not work. But some theorists have suggested that if gravity behaved differently over large distances from what Einstein thought, it would relieve astronomers of the embarrassing need to posit that 96 per cent of the universe consists of various kinds of unknown dark stuff. A similar, but larger, kind of deviation from Einsteinian theory could explain the Pioneer anomaly, as it is called. Pioneers 10 and 11 were launched in 1972 and 1973, respectively, and are now both about 10 billion miles out. They were last heard from in 2003, when the radio signal from Pioneer 10 got too weak to be detected. In 1998, when John D. Anderson of the Jet Propulsion Laboratory and his colleagues discovered that the spacecrafts were running a little late on their timetable to eternity, it seemed as if general relativity might be up for grabs—allowing the news media to ask their favourite science question: Was Einstein wrong? There was talk of a special deep space probe whose only mission would be to track its own movements.

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