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Let's first talk about what a Kerr black hole is, and then we'll address how one might (well, theoretically) be used as an intergalactic subway system. A Kerr black hole, loosely thought of as a "spinning black hole," is a kind of black hole that rotates on a central axis and has no electrical charge. It's thought that Kerr black holes are among the most common in the cosmos, because the dying stars from which they form have rotation before their collapse and subsequent "birth" as a black hole. This rotation is passed on to the Kerr black hole thanks to a principle of physics called angular momentum, which describes the momentum an object has because of its rotation [source: Encyclopedia of Science].
The mathematical underpinning for this type of black hole came from New Zealand physicist Roy Kerr's work on Albert Einstein's field equations that deal with gravity and the curving of spacetime. Kerr (one of the great benefits of doing significant work in physics on largescale objects is that you can end up with an entire black hole named after you) perfectly solved for Einstein's field equation of general relativity, and his solution used what we now call "Kerr black holes" [source: Absolute Astronomy].
And now for the fun part  time travel. A black hole is shaped like an ice cream cone, and its narrowed bottom point is known in physics as the singularity. Everything that gets sucked into the widest part of the black hole  the wider top of the ice cream cone, known as the event horizon  is pulled down to the singularity and destroyed. In a Kerr black hole type, a theoretical person entering from one of its two event horizons could avoid the singularity entirely and pass out through the black hole and into, perhaps, an entirely different universe. But before anyone gets too excited about black hole time travel, scientists will need to come up with a quantum theory of gravity so they can understand what effects might destabilize any attempt at black hole travel [source: Encyclopedia of Science].

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