What secrets hide past a black hole's edge

Several serious theories now suggest that what falls into a black hole does not end at the singularity. It comes out somewhere else.

The standard picture, from Einstein's general relativity, ends badly: anything past the event horizon is dragged to a point of infinite density where the equations stop making sense. But infinite density is a signal that the equations themselves have run out, not a description of reality. So physicists have spent decades looking for what happens when quantum mechanics is added back in.

Three candidates keep returning. The first is a *white hole* — the time-reverse of a black hole, spitting matter into a separate region of spacetime. The second is a *wormhole*, threading the interior of one black hole to the interior of another, possibly in a different universe. The third, from loop quantum gravity, replaces the singularity with a *quantum bounce*: matter compresses to a Planck-scale density, then explodes outward as a new region of spacetime.

None of this has been tested. All of it is consistent with the math. The event horizon may be less a wall than a turnstile, and we are on the wrong side to see what is on the other.