Figure 1. The spacetime geometry of the Schwarzschild black hole solution can be depicted in different ways. In this representation, ingoing light rays always travel along ingoing lines heading toward the top and left at 45°; outgoing light rays asymptotically approach 45° lines at large radius r. Massive particles, with their slower speeds, must travel within the light cones (blue) between outgoing and ingoing light rays, as illustrated by the red path. No light ray can escape to infinity from inside the vertical dotted line, the horizon located at the mass-dependent Schwarzschild radius R(M). Instead, any trajectory beginning inside the horizon is pulled to a central point, the singularity at r = 0, where spacetime curvature becomes infinite.