Over a century after it was first formulated, Einstein's theory of relativity has been proven right many times. But one of the most interesting predictions has been a bit of a mystery -- until now.

An orbiting satellite, conducting a joint experiment of NASA and Stanford University, has shown that the Earth's gravity warps the space around it and that it drags spacetime itself along as it spins. The effect was predicted by Einstein's theory, but at the time there was no way to test it.

In 1959 the first proposals were floated to use a satellite equipped with gyroscopes and pointed at a single, distant star. In 1963, NASA said it was interested, but it took another 41 years to get the satellite - called Gravity Probe B -- launched.

The satellite, pointed at the star, maintains the same orientation over the course of a year. If Einstein were wrong, the gyroscopes would stay pointed in the same direction. But they did not - they were off by a small amount.

The satellite also found a frame dragging effect, which was predicted by Austrian physicists Josef Lense and Hans Thirring in 1918, as a consequence of relativity. The theory of relativity states that any massive body bends the space around it. This is why things fall toward the Earth. (When Newton formulated his theory of gravity, he didn't have an explanation as to why objects should be attracted to one another, and his theory broke down when objects move at near the speed of light or are very massive).

Lense and Thirring predicted that massive objects would also drag spacetime itself, similar to the way a spinning body in a thick liquid might drag the liquid along.

Frame dragging is important because it might explain why some objects - particularly massive black holes and neutron stars - generate the kinds of energy that they do. The effect around the Earth is very small - only one part in a few trillion. (This is why Newtonian mechanics works as well as it does).

It also has some interesting consequences. An object that is not perfectly spherical orbiting another very massive one - a black hole, for example - would have one side dragged in the direction the massive object is rotating. The other end, further away, would be dragged in the direction opposite the massive body's rotation, spinning the orbiting body in a gravitational gears effect.

Another consequence is that to a distant observer, light beams near the spinning object would look as though they were travelling slower if headed in the anti-spinward direction, and faster when travelling spinward.

Gravity Probe B has also led to a number of advances in GPS and the technology used for measuring the cosmic background radiation, which confirmed one of the underpinnings for the Big Bang theory.

NASA NASA Black hole