English: Shown here over the Hawaiian island of Oahu is a cross section of a common (albeit non-spinning and therefore perfectly spherical) classic black hole comprising an event horizon and the singularity inside. The black hole is precisely 40 kilometers in diameter, which corresponds to 6.8 solar masses (6.8 M☉), which is an unremarkable size for a “stellar-mass” black hole (one that is neither the product of mergers, nor a super-massive black hole at the centers of galaxies). Black holes in the 5–9 M☉ range are commonly detected electromagnetically (not via gravity-wave observatories).

Under conventional black hole theory, the shell’s outer surface as depicted here—the event horizon—demarcates not a physical entity, but a boundary where the laws of physics abruptly change. An event horizon is the border at which spacetime has been warped by gravity to such an extent that the escape velocity—the upwards velocity an entity must achieve to escape a gravity well—precisely equals the speed of light and even infalling photons (quanta of light) cannot escape. Classic black hole theory further holds that at the center of a black hole is a singularity, which is a zero-dimensional, zero-volume point in which all of a black hole's mass—the source of gravity—exists at infinite density.

This schematic also has utility in treatises on fuzzball theory, which is derived from superstring theory, and is advanced by its proponents as a true quantum mechanical description of black holes. Under fuzzball theory, the shell’s outer surface depicted in this illustration is an actual physical entity. Fuzzball theory also dispenses with the singularity at the heart of a black hole by positing that the entire region within the black hole's event horizon is actually an extended object: a ball of strings, which are bundles of energy vibrating in complex ways. String theory holds that vibrating strings are the ultimate building blocks of matter and light—the most extreme form of degenerate matter.