" Kerr found that a spinning black hole would not collapse into a pointlike star, as Schwarzschild assumed, but would collapse into a spinning ring. Anyone unfortunate enough to hit the ring would perish; but someone falling into the ring would not die, but would actually fall through. But instead of winding up on the other side of the ring, he or she would pass through the Einstein-Rosen Bridge and wind up in another universe. In other words, the spinning black hole is the rim of Alice's Looking Glass.

If he or she were to move around the spinning ring a second time, he or she would enter yet another universe. In fact, repeated entry into the spinning ring would put a person in different parallel universes, much like hitting the "up" button on an elevator. In principle, there could be an infinite number of universes, each stacked on top of each other. "Pass through this magic ring and-presto!-you're in a completely different universe where radius and mass are negative!" Kerr wrote.

There is an important catch, however. Black holes are examples of "nontransversable wormholes"; that is, passing through the event horizon is a one-way trip. Once you pass through the event horizon and the Kerr ring, you cannot go backward through the ring and out through the event horizon. "

― , Physics of the Impossible