Like a commuter studying Plato on the subway, a far-flung satellite has accomplished something unusually productive while cruising to work. On its way to Saturn, NASA's Cassini-Huygens spacecraft has tested Einstein's theory of gravity to unprecedented precision.
In 1915, Einstein posited that gravity is nothing other than the bending of space and time. Matter and energy warp space and time, and a freely falling object travels along the nearest equivalent to a straight line in the curved spacetime terrain. Known as general relativity, this theory has become a pillar of modern physics. Even everyday technologies, such as the Global Positioning System, rely on its predictions. Yet nobody had tested the accuracy of those predictions to better than a tenth of a percent.
Now, radio signals to and from Cassini have validated Einstein's theory to 50 times greater precision, report astrophysicist Bruno Bertotti of the University of Pavia, physicist Luciano Iess of the University of Rome-La Sapienza, and engineer Paolo Tortora of the University of Bologna in Flori. They studied the signals when Cassini was on the other side of the sun from Earth. When radio waves beamed from Earth passed through the spacetime dimple created by the sun, their frequencies shifted. The team could tell the size of the shift because Cassini responded with waves whose frequencies depended on those it received. To Einstein's credit, the shifts agreed with the spacetime-bending predictions of general relativity to within 20 parts per million, the researchers report in the 25 September issue of Nature.
The new results are a shot in the arm for gravity research, says Ronald Hellings, a physicist at Montana State University in Bozeman: "We've been waiting 30 years for a breakthrough like this." Most physicists believe that general relativity cannot be the ultimate theory of gravity, as it cannot be reconciled directly with quantum mechanics and does not account for the evolution of the universe. So high-precision tests of general relativity might reveal new forces and particles beyond gravity, says Thibault Damour, a theoretical physicist at the Institut des Hautes Études Scientifiques in Bures-sur-Yvette, France. Whatever Einstein may have missed might reveal itself in even more precise experiments already planned for other satellites, he says.
A one-page introduction to curved spacetime
The Cassini-Huygens home page
The Gravity Probe B home page
Future European Space Agency gravity missions
A thorough nontechnical primer on relativity