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Extremal Black Holes Are Possible
For decades, a black hole that has as much spin or charge as it can given it’s mass was considered mathematically impossible. A new proof reveals otherwise.
To understand the universe, scientists look to its outliers. “You always want to know about the extreme cases—the special cases that lie at the edge,” said Carsten Gundlach, a mathematical physicist at the University of Southampton.
Black holes are the enigmatic extremes of the cosmos. Within them, matter is packed so tightly that, according to Einstein’s general theory of relativity, nothing can escape. For decades, physicists and mathematicians have used them to probe the limits of their ideas about gravity, space, and time.
But even black holes have edge cases—and those cases have their own insights to give. Black holes rotate in space. As matter falls into them, they start to spin faster; if that matter has charge, they also become electrically charged. In principle, a black hole can reach a point where it has as much charge or spin as it possibly can, given its mass. Such a black hole is called “extremal”—the extreme of the extremes.
These black holes have some bizarre properties. In particular, the socalled surface gravity at the boundary, or event horizon, of such a black hole is zero. “It is a black hole whose surface doesn’t attract things anymore,” Gundlach said. But if you were to nudge a particle slightly toward the black hole’s center, it would be unable to escape.