After 40 years of planning and hundreds of millions of dollars, NASA announced last week the first results from the Gravity Probe B experiment, which was designed to measure how Earth warps the fabric of space-time. The results may have been scooped, though, by astronomers bouncing lasers off the moon decades ago.
NASA launched Gravity Probe B (GPB) in April 2004. The satellite was equipped with precision-engineered gyroscopes to measure two effects predicted by Einsteinâs general theory of relativity. In one, called the geodetic effect, Earthâs gravity dents space-time such that it should tilt each gyroscope by 0.0018 degrees over the course of a year. In a second, more subtle effect, called frame-dragging or gravitomagnetism, Earth drags space-time along with it as it rotates.
The GPB team only reported a measurement of the geodetic effect. Team leader Francis Everitt of Stanford University in California says that the team has seen âglimpsesâ of frame-dragging but is not yet ready to report a figure. âThe results arenât quite what weâd hoped for at this stage,â admits Bill Bencze, GPB programme manager.
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In contrast, it looks like decades of measuring the moonâs orbit using laser rangefinders â reflecting lasers off mirrors left on the moon by Apollo astronauts â have provided more than sufficient data for calculating frame-dragging. It took time for physicists to realise this, though. âThere was not a magic moment when the lunar laser-ranging measurement of gravitomagnetism was released,â says Kenneth Nordtvedt, a physicist who retired from Montana State University in Bozeman.
âDecades of measuring the moonâs orbit has provided more than sufficient data for calculating gravitomagnetismâ
Instead, in 1988, on the eve of his retirement, Nordtvedt published a paper pointing out that laser-ranging data could be used to calculate gravitomagnetism. Nordtvedtâs work was ignored until 2001, when Tom Murphy, a physicist at the University of California, San Diego, heard of it. He started looking at the way NASAâs Jet Propulsion Laboratory had been using the 35 years of lunar laser-ranging data and realised that an estimate of gravitomagnetism was implicit in the calculations of lunar orbits. âTurn off the gravitomagnetic term in the equations and you would not be able to fit the lunar orbit,â says Murphy.
His latest calculations, to be published in Physical Review Letters, show that gravitomagnetism causes the moonâs orbit to oscillate with an amplitude of about 6 metres. Laser rangefinders can detect the moonâs position to within a few centimetres, which allows researchers to calculate the effect of gravitomagnetism to within 0.1 per cent of that predicted by general relativity. In contrast, GPB is expected to achieve an accuracy of between 1 and 0.1 per cent.
Still, some see a reason to wait for GPBâs data. âThe moonâs orbit acts as a gyroscope,â says Clifford Will, an expert on general relativity at Washington University in St Louis, Missouri, âbut lunar ranging provides indirect measurements. GPB provides a direct measure thatâs unique.â
Murphy vehemently refutes this. âI can take the term from the moonâs orbit and apply it to a gyroscope. They are exactly the same,â he says. âThe project has been going so long that the scientific landscape has changed under their feet.â
Peter Aufmuth of the Albert Einstein Institute in Potsdam, Germany, still thinks that GPB is worth the money. Gravitomagnetism is so important to general relativity that you need as many tests of it as possible,â he says.
