frame-dragging

Frame-dragging is a subtle prediction of general relativity: a rotating mass drags the surrounding spacetime around with it, like a spinning ball in honey twisting the honey near it. In Newton's gravity only the amount of mass matters, but in Einstein's theory a body's spin also reshapes the geometry, twisting space and time in the direction of rotation. The faster and more massive the spinning body, the stronger the twist.

One striking consequence is that the very definition of 'not rotating' becomes local. Far from the spinning mass, the distant stars set the standard for no rotation. Close in, spacetime is being swirled, so a gyroscope that points steadily at a far star will slowly turn as seen from outside, simply because the space it sits in is itself being carried around.

The effect is extraordinarily tiny near the Earth, but it has been measured. NASA's Gravity Probe B flew ultra-precise gyroscopes in orbit and detected the Earth dragging spacetime by about 37 milliarcseconds per year, a tiny fraction of a degree, in agreement with the theory's prediction of roughly 39. Frame-dragging is far more dramatic near rapidly spinning black holes, where spacetime near the horizon is whirled around so forcefully that nothing can stay still against the rotation.