The Schwarzschild radius [0], which defines the radius at which the escape velocity equals the speed of light, is given by 2GM/c^2. I don't know what the impact

The Schwarzschild radius [0], which defines the radius at which the escape velocity equals the speed of light, is given by 2GM/c^2. I don't know what the impact parameter is either, but given these two expressions, it sounds like b is dimensionless. I don't know how fast the radius of curvature drops off as a function of the Schwarzschild radius, but I'd imagine it's at least R^-1. So assuming a spherical cow^H^H^H oven with gravitational lensing dying out at ~100x the event horizon, we need a Schwarzschild radius the size of a tennis ball in order to still see the curvature a few metres out. The oven needs to weigh ~4x the Earth's mass for that. [0] https://en.wikipedia.org/wiki/Schwarzschild_radius https://en.wikipedia.org/wiki/Schwarzschild_radius