The general relativistic prediction of the Lense-Thirring effect has been confirmed to a precision of less than 10%. The result was obtained by measurements of the position of two satellites over a period of more than 10 years.

In 1918, soon after Einstein published his general theory of relativity, Austrian physicists Joseph Lense and Hans Thirring predicted that a rotating massive body would pull on the surrounding space-time. This frame-dragging or Lense-Thirring effect is one of the "gravitomagnetic" phenomena that are absent in Newtonian gravity. They result from the motion of matter (matter currents) and are the gravitational analogues of magnetic fields arising from moving charges (electric currents). The spinning Earth, containing a large amount of matter in motion, is the source of a very weak gravitomagnetic force.

The Moon is too far away to be significantly affected by frame-dragging due to the Earth's spin, so it fell to artificial satellites LAGEOS and LAGEOS 2 (for Laser Geodynamics Satellites) to detect the Lense-Thirring effect. They were launched, respectively, in 1976 by NASA and in 1992 from the space shuttle as a joint project of NASA and the Italian Space Agency (ASI). Their instantaneous position can be ascertained by measuring the time it takes light to travel between a point on Earth and the satellite. This is achieved by sending short laser pulses from 50 Earth-based stations, and detecting them as they are sent back by retro-reflectors that cover the spherical satellites.

The analysis of millions of these laser-ranging measurements over a period of nearly 11 years has been published in Nature by I Ciufolini of INFN/Lecce and E C Pavlis of Maryland. The frame-dragging effect of the Earth causes the orbit of the satellites to precess by just 33 ms of arc per year, corresponding to less than 2 m per year at a distance of about 12,000 km, the semi-major axis of the LAGEOS satellites' orbit. The difficulty of measuring this tiny effect is complicated by the non-spherical gravitational field of the Earth, which has to be corrected for. Indeed, the Lense-Thirring effect could only be detected thanks to the precise knowledge of the ripples in the Earth's gravity field, measured in particular by the two satellites of NASA's Gravity Recovery and Climate Experiment (GRACE).

This first accurate measurement of frame-dragging confirms Einstein's predictions to within 10%. What remains for NASA's Gravity Probe B spacecraft launched this year is to confirm and refine this measurement to a precision of 1% using its orbiting gyroscopes (CERN Courier June 2004 p13). The confirmation of this effect is important, because it is one of the last predictions of general relativity to be measured, and plays an important role in the accretion disc around compact spinning objects such as neutron stars and black holes. In particular, the Lense-Thirring effect may contribute to the alignment of jets in active galactic nuclei and quasars.

Further reading

I Ciufolini and E C Pavlis 2004 Nature 431 958.

Compiled by Marc Türler, INTEGRAL Science Data Centre and Geneva Observatory