How is it possible for time to change inside a black hole?
A. In general relativity, time and space are a set of variables that can be used to parameterize the geometry of space-time and the kinds of geodesics that are possible. But they are not the only kinds of variables that form a set of four coordinates that “span” the dimensionality of space-time. In probing the mathematics of black holes, physicists have discovered other sets of coordinates that are even better. For example, the event horizon appears in the mathematics as a “coordinate singularity” if you use the coordinate set (x, y, z, t) or (t, r, theta, phi), but if you use the “Kruskal-Szekeres” coordinates, it vanishes completely. There is only one true singularity in a non-rotating Schwartzschild black hole solution, and that is the one at r=0, at the event horizon, the curvature of space is non-infinite. That means that coordinate singularities are not real singularities and can be mathematically transformed away. Now, if you study what happens to the Kruskal-Szekeres coordinate
