"It's a very abstract thing," admits Wei Cui of the Massachusetts Institute of Technology, but the idea is that heavy objects distort space-time much as a bowling ball would distort a sheet supporting it.

Following his habit, Einstein started thinking about gravity with a "thought experiment" -- a mental series of "what-if" questions. Einstein asked himself "What if I were falling through space?" and realized that he wouldn't feel gravity, no matter how intense.

Consequently, from the point of view of a free-falling object, the laws of physics could be explained without gravity. But since the laws of physics must, according to Einstein's axioms, apply everywhere, then gravity must be an effect of something else.

Gravity -- not what it seems
Eventually Einstein concluded that gravity was an effect of the space-time through which objects fall. What is space-time? It's an abstraction that causes physicists, in desperation, to start babbling about sheets and bowling balls. When a massive object (the ball) distorts space-time, then other objects tend to fall toward the resulting "valley" of space-time.

Thus for gravity, Einstein substituted geometry.

One of the earlier proofs of relativity -- and the relevance of space-time -- occurred when scientists showed that the sun's gravity caused a tiny deviation in Mercury's orbit. Newtonian physics had been unable to explain the deviation.

But in 1918, even before that observation, Austrian physicists Joseph Lense and Hans Thirring expanded on Einstein's general relativity to predict that a spinning object would also drag space-time along with it. (This predicted phenomenon was dubbed the frame-dragging effect, because it dragged the entire frame of reference with it.)

And that's exactly what was found 79 years later in the new studies of black holes and neutrons stars. Both were massive spinning objects, and both did seem to drag space-time around with them. (An anonymous editor of our acquaintance claims that egocentric children are the ultimate massive objects, bending space-time with ease.)

Cui and the Italian researchers found frame dragging with the Rossi X-ray Timing Explorer. The satellite data showed that the orbiting matter was shifted in accordance with the prediction of Lense and Thirring.

May the new force be with you
So instead of thinking of gravity as just a depression in the "fabric" of space-time, we must think of it -- for spinning objects -- as a whirling depression -- kind of an upside-down tornado. Extremely massive objects, Cui says, "distort the shape -- the warp of space-time."

To The Why Files, the frame-dragging data mean that space is no longer flat, or even just warped. Now it is twisted as well. And as a matter of principle, The Why Files likes twisted.

The twisting, Cui says, seems to create a "gravetomagnetic force" which only exists when a massive object rotates. And since space-time affects matter, this gravetomagnetic force will cause other matter to move.

So if you fired a laser toward a black hole, you'd see the photons in its beam curve along the lines shown in the picture in our previous file. The same distortion would affect stuff near neutron stars and black holes, which "move around, not just because of gravity, but also because the spinning changes the shape of space," Cui says.

And it was these changes that proved, once again, that Einstein was right. "These are all results of his theory of general relativity, which described gravity," Cui says.

Einstein concluded that gravity was a property of space-time, not a separate force. In Cui's words, "You can take all the facts of gravity and explain them with a certain geometry of space-time."

So Einstein canceled gravity. Pretty neat -- we've always wanted to fly, too. But gravity is still out there. A couple of decades back, astronomers learned to exploit the immense gravity of ancient galaxies to make natural telescopes.

That followed another prediction of relativity -- that gravity bends light waves. That's the cool thing about relativity -- you can have your gravity and eat it too.