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Last Updated: Thursday, 21 October, 2004, 12:58 GMT 13:58 UK
Einstein's warp effect measured
Quasar, Nasa/CXC/A.Siemiginowska et al
Frame-dragging was first predicted in 1918
Two scientists appear to have beaten a $600m Nasa mission to be first to measure a phenomenon predicted by Einstein's theory of relativity.

"Frame-dragging" is the effect wherein a massive body like Earth drags space-time around with it as it spins.

Ignazio Ciufolini and Erricos Pavlis measured frame-dragging by studying the movements of two satellites in Earth orbit over a period of 11 years.

The results are published in the latest edition of the academic journal Nature.

In 1959, Leonard Schiff of Stanford University used Einstein's theory of general relativity to calculate that a gyroscope in polar orbit around Earth at 400 miles (643km) should go out of alignment by an angle of 42 milli-arcseconds per year.

One milli-arcsecond is one thousandth of one second of arc, which is itself a unit of an angle.

In April, Nasa launched Gravity Probe B, which carries precision gyroscopes to measure the effect on its one-year mission. The mission was first proposed in the 1960s, but financial and technical hitches delayed its launch until this year.

Robust analysis

Ciufolini, from the University of Lecce, Italy, and Erricos Pavlis from the Joint Center for Earth Systems Technology in Baltimore, US, analysed millions of laser range-finding signals that are reflected by the satellites Lageos and Lageos 2.

These reflected signals are normally used to map variations in the Earth's gravitational field.

But the researchers analysed them for evidence that the satellites' orbits were altered by frame-dragging, also known as the Lense-Thirring effect after the Austrian physicists who predicted it in 1918.

Ciufolini and Pavlis say their result is 99% of the value predicted by Einstein's theory, plus or minus 5%. This result has an uncertainty of about 10% say the scientists.

Commenting on the research, Neil Ashby of the University of Colorado, US, said the result was "the first reasonably accurate measurement of frame dragging."

He added: "Further analysis is anticipated as additional geodesy missions are undertaken to improve our knowledge of Earth's gravity field."

The same researchers reported preliminary findings in 1998, which were roundly criticised. But reaction to the latest measurements has been broadly positive. The new work is based on a new gravity map released last year.

'Einstein' probe heads into space
20 Apr 04  |  Science/Nature
Q&A: Testing Einstein's theories
19 Apr 04  |  Science/Nature

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