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 so-called 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.
In 1973, the prominent physicists Stephen Hawking, John Bardeen and Brandon Carter asserted that extremal black holes can’t exist in the real world — that there is simply no plausible way that they can form. Nevertheless, for the past 50 years, extremal black holes have served as useful models in theoretical physics. “They have nice symmetries that make it easier to calculate things,” said Gaurav Khanna of the University of Rhode Island, and this allows physicists to test theories about the mysterious relationship between quantum mechanics and gravity.
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