Using the continent-wide Very Long Baseline Array, scientists have made a precise measurement of the curvature of space caused by the Sun's gravity.

"Measuring the curvature of space caused by gravity is one of the most sensitive ways to learn how Einstein’s theory of General Relativity relates to quantum physics," says Sergei Kopeikin of the University of Missouri. "Uniting gravity theory with quantum theory is a major goal of twenty-first century physics, and these astronomical measurements are a key to understanding the relationship between the two."

Albert Einstein published his theory of General Relativity in 1916, which included the idea that the gravity of a massive object such as the Sun was strong enough to produce curvature in nearby space, altering the path of light or radio waves passing near the object. The effect was first tested during a solar eclipse in 1919, and numerous measurements have been made since.

Physicists describe the space curvature and gravitational light-bending as a parameter called 'gamma' and Einstein's theory holds that gamma should equal exactly 1.0. "Even a value that differs by one part in a million from 1.0 would have major ramifications for the goal of uniting gravity theory and quantum theory, and thus in predicting the phenomena in high-gravity regions near black holes," says Kopeikin.

Kopeikin and colleagues used the National Science Foundation's Very Long Baseline Array (VLBA), which ranges from Hawaii to the Virgin Islands, to measure the bending of light caused by the Sun's gravity as it passed nearly in front of four distant quasars – galaxies with supermassive black holes at their cores. True to theory, the Sun's gravitational tug caused subtle changes in the apparent positions of the quasars because it deflected the radio waves coming from the more-distant objects.

The result was a measured value of gamma of 0.9998 +/- 0.0003, in excellent agreement with Einstein’s prediction of 1.0.

"With more observations like ours, in addition to complementary measurements such as those made with NASA's Cassini spacecraft, we can improve the accuracy of this measurement by at least a factor of four, to provide the best measurement ever of gamma," says Edward Fomalont of the National Radio Astronomy Observatory (NRAO). "Since gamma is a fundamental parameter of gravitational theories, its measurement using different observational methods is crucial to obtain a value that is supported by the physics community."

The results are reported in the 10 July issue of the Astrophysical Journal.