ABSTRACT
The Schwarzschild and Kerr metrics discussed here inform several important aspects of the cosmic spacetime, notably the role played by the gravitational horizon. Schwarzschild’s metric cannot handle rapidly rotating objects (and is only an approximation for slowly spinning ones) because a rotating source of gravity impacts the spacetime around it in unexpected and challenging ways. Generally, light rays emitted anywhere in the spacetime follow geodesics outward from their point of origin, because these are the most ‘direct’ paths they can follow. The important new ingredient was the additional assumption of shear-free conditions everywhere in the spacetime. The phenomenon is commonly interpreted to mean that the spacetime itself swirls around the spinning object with a speed decreasing with distance from the center. The ‘frame dragging’ forces everything within that spacetime into co-rotation with the source of gravity, even if within that frame these objects are completely stationary.
