Frame dragging is a prediction of general relativity that says a rotating mass does not just curve spacetime, it drags it around in the direction of its spin, like a ball spinning in honey pulls the honey around with it. Nearby objects and even light get subtly swept along in that rotation.
The effect is extremely small for anything the size of the Earth, but it is real and it has been measured. NASA’s Gravity Probe B satellite confirmed it in 2011 by detecting a tiny, predicted drift in the orientation of onboard gyroscopes caused by the Earth’s rotating mass.
Frame dragging becomes far more dramatic around a rotating black hole, where it creates a region called the ergosphere just outside the event horizon, in which spacetime is dragged around so violently that nothing, not even light, can stay still relative to a distant observer.
Beyond Gravity Probe B, frame dragging shows up in astrophysical settings far more dramatic than a satellite’s gyroscopes. Around a rapidly spinning black hole, the effect can twist and warp the disk of gas swirling inward, forcing the inner portion of that disk to precess, or wobble, in a way that leaves a distinct signature astronomers can look for in X-ray observations. Binary pulsar systems, pairs of extremely dense, rapidly spinning neutron stars orbiting each other, offer another natural laboratory, since their orbits shift over time in ways that match frame dragging predictions to a striking degree of precision, giving physicists one more confirmation that general relativity holds up even in some of the most extreme environments the universe has to offer.
The effect is also called the Lense-Thirring effect, after the two Austrian physicists who first derived it mathematically from Einstein’s equations in 1918, decades before any experiment existed sensitive enough to measure something so small. It took nearly a century of technological progress before Gravity Probe B could finally put that century-old prediction to a direct test.