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The most precise test yet of one of Albert Einstein鈥檚 ideas about gravity has once again shown he was right. The finding means that physicists may need experiments that are even more accurate to figure out where his general theory of relativity breaks down and potential new forces and phenomena kick in.

For decades, physicists have looked for violations in general relativity in the hope of shedding light on phenomena missing from Einstein鈥檚 theory, like dark matter, and leading to a theory of gravity that includes quantum effects. 鈥淕eneral relativity is a very good theory that works very well, but it doesn鈥檛 explain all the observation in our universe,鈥� says at the French aerospace lab ONERA.

One area of focus is a pillar of general relativity called the weak equivalence principle. It states that all objects, regardless of their shape or what they are made of, fall with the same acceleration when gravity is the only force acting on them. The principle has been tested many times and has always held up.

To test it with even greater accuracy, Rodrigues and his colleagues launched a small satellite into space. On board were two devices known as electrostatic accelerometers that could measure how objects experienced gravity. Inside each device was a pair of cylinders 鈥� a smaller one nestled inside a large one. One accelerometer contained cylinders that were the same material and the other had cylinders of two different materials. The weak equivalence principle says gravity should act the same on both cylinders regardless of these differences. However, if for some reason it didn鈥檛, the electrostatic accelerometer would be able to pick it up.

The experiment was in an orbit 710 kilometres above the Earth for two-and-a-half years, but it didn鈥檛 detect any gravitational differences. The set-up was sensitive enough to detect changes as small as a hundredth of a trillionth of a per cent, making it the most precise measurement of the weak equivalence principle so far.

Rodrigues says that the new measurement is a hundred times more precise than previous tests, most of which were on Earth. With the satellite, there weren鈥檛 disturbances like people walking in nearby rooms or even the imperceptible shaking of lab buildings that could disrupt the experiment.

at the French National Centre for Scientific Research, who didn鈥檛 participate in the mission, says that the results will help justify future attempts to test fundamental physics theories in space. The project involved controlling a satellite extremely precisely and developing data analysis techniques that account for even the smallest disturbances such as friction between the satellite and space dust. This means it could become a foundation for ten or a hundred-times more precise satellite-based experiments in the future, he says. He and collaborators are planning one such mission that would use a quantum device instead of an electrostatic accelerometer.

Physicists will continue looking for violations of the equivalence principle, says at the University of Washington in Seattle. 鈥淭here鈥檚 more to physics than we understand, that鈥檚 pretty clear. And this is a pretty likely place to look [for something new].鈥�