While most physicists are convinced that Einstein's theory of gravity in terms of a curved space-time geometry is basically correct, it is always good to check. Now researchers in Italy have made a new, more stringent test of general relativity using measurements of the frequency shift of radio signals to and from the Cassini spacecraft as it passed on the opposite side of the Sun from Earth. Bruno Bertotti of the University of Pavia and colleagues measured the parameter γ, which is unity in general relativity and zero in Newton's theory, and therefore indicates the degree to which gravity can be explained purely in terms of geometry without any additional forces.
The team found that γ-1 = 2.1 ± 2.3 x 10-5, which is a dramatic improvement over previous measurements that gave results at the 0.1% level, and strongly supports the idea that gravity is of purely geometrical origin, at least over distance scales of the order of our solar system. The improved accuracy is due to technological improvements implemented on the spacecraft's communication system and at NASA's ground station at Goldstone, California. Meanwhile, Cassini, a joint NASA/ESA mission, continues its journey towards Saturn, which it will begin orbiting on 1 July 2004.
Further reading
B Bertotti et al. 2003 Nature 425 374.