 Figure 4  is a spacetime diagram of a sphere of matter collapsing to form a black hole. The curvature of the spacetime is represented by the tilting of the light cones away from 45 degrees. Notice that the light cones tilt inwards more and more as one approaches the center of the black hole. The jagged line running vertically up the center of the diagram depicts the central singularity inside the black hole. As we emphasized in Section 1, this is not actually part of the spacetime, but might be thought of as the “place” where the structure of spacetime breaks down. Thus, one should not imagine the possibility of traveling through the singularity; this would be as nonsensical as something's leaving the diagram (i.e., the spacetime) altogether. 