Laser measurements confirm Einstein's general theory of relativity.
One of the last untested predictions of general relativity has been confirmed by the first reasonably accurate measurement of how the rotating Earth warps the fabric of space.
The experiment, carried out for virtually no cost with Earth-based laser range-finders, scoops Gravity Probe B, the US$700 million orbiting craft launched in April to test exactly the same effect. However, the Gravity Probe B team has questioned whether the result is really quite as accurate as it seems.
The space warp is a consequence of Einstein's general theory of relativity, which describes gravity as a curvature in space produced by objects sitting in it. It also implies that a rotating mass will drag space around it like a spinning top placed in treacle - an effect known as the Lense-Thirring effect, or more commonly as 'frame-dragging'. The effect becomes important in understanding extreme situations like spinning quasars, and the rotation of jets of gas around black holes.
The effect was first predicted by Austrian physicists Joseph Lense and Hans Thirring in 1918, but until now scientists haven't had sufficiently accurate instruments to measure its tiny perturbations in the fabric of our Universe.
Ignazio Ciufolini at the University of Lecce, Italy and Erricos Pavlis at the University of Maryland in Baltimore charted the path of two NASA satellites, LAGEOS and LAGEOS 2, over 11 years with laser range-finders with the precision of a few millimetres. The effect dragged the satellite's orbits out of position by about 2 metres each year, the researchers report in this week's Nature1.
The researchers say that their result is 99% of the value predicted by relativity, with an error of up to 10%. "Their result is the first reasonably accurate measurement of frame-dragging," comments Neil Ashby, a physicist from the University of Colorado, Boulder.
But some scientists remain unconvinced that the measurements are as accurate as the Italian researchers claim. "One of the difficulties is extracting the frame-dragging effect from the huge gravitational effect of the Earth," says Clifford Will, a physicist at Washington University in St. Louis, Missouri, who chairs NASA's Science Advisory Committee for Gravity Probe B.
If the Earth were perfectly symmetrical, frame-dragging would be easy to measure. But the lumpy Earth generates an uneven gravity field, Will points out, which moves the satellites about far more than frame-dragging.
To tease the two effects apart, Ciufolini and Pavlis used a map of the Earth's gravity field provided by a NASA mission called GRACE, launched in March 2002. This relies on two satellites orbiting Earth about 220 kilometres apart, measuring the tiny changes in that distance as they pass through different parts of the Earth's gravity field. Ciufolini's previous attempt2 at measuring frame-dragging was less than 20% accurate, because it did not have the benefit of the GRACE gravity model.
"The laser-ranging method can deliver the accuracy, but it is still uncertain if the GRACE gravity models are good enough," says John Ries, a physicist at the University of Texas, Austin. Will adds that the Gravity Probe B team is also sceptical, and thinks that Ciufolini may have drastically underestimated his errors.
Either way, the Gravity Probe B experiment is expected to deliver a measurement of frame-dragging with 1% accuracy very soon. "I admire the people that have worked for 40 years on this experiment. It's certainly worthwhile," says Ciufolini.
Physicists did not expect either of these experiments to overturn relativity, but insist that confirmation was still essential. "There could be an effect," says Will. "It hasn't been measured, so we have to measure it." But he concedes: "It gets tricky when it costs so much."
The last major prediction of general relativity requiring confirmation is the existence of gravity waves. The LIGO experiment, run by the California Institute of Technology and the Massachusetts Institute of Technology, is already searching for these on Earth, while NASA's LISA probes are expected to launch in 2010.
CiufoliniI. & PavilsE. C. Nature, 431. 958 - 960 (2004).
CiufoliniI. & PavilsE. C. ChieppaF.Fernandes-VieiraE.Perez-MercaderJ.Science, 279. 2100 - 2103 (1998).
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Peplow, M. Spinning Earth twists space. Nature (2004). https://doi.org/10.1038/news041018-11