I understand the Kerr metric has an off-diagonal term between the rotation and the time degrees-of-freedom? That a test mass falling straight down toward a large rotating mass from infinity will begin to pick up angular momentum? Is that what’s called “frame dragging”? Did the Gravity Probe B verify that effect? Finally my main question: what is the large mass rotating with respect to? Before someone says “the distant stars”, remember that the Kerr metric is a MODEL of an otherwise empty universe. I don’t think there are any “distant stars” in the MODEL. I am not that familiar with this metric yet, but I assume that like the Schwarzschild it asymptotically goes to Minkowski at infinity?