In the late 1960s, a group of physicists posited that black holes were similar to elementary particles: miniscule points identifiable by their mass, spin, and force charge, just like matter particles. However, this is where the math breaks down: The tiny size and spin indicate that quantum mechanics are required, but the enormous mass requires relativity, which is how the two theories are most incompatible. String theory offers a route for moving past this obstacle, though the reasons are convoluted.

First, Greene returns to the idea of loops of string circling around a two-dimensional curled space, as discussed in Chapter 11. Physicists know from the equation that the loop of string protects a collapsing two-dimensional space from catastrophe. Now theorists asked what would happen if a three-dimensional sphere within a six-dimensional Calabi-Yau space collapsed. As a one-dimensional loop of string cannot surround it, early indications suggested that such a collapse would “yield a cataclysmic result” and “the workings of the universe would grind to a halt if such a collapse were to occur” (323).

However, as discussed in Chapter 12, newer versions of string theory (and M-theory) argue that along with one-dimensional strings, two-, three-, and even higher dimensional branes exist.