Stephen Hawking was wrong about black holes

"Even black holes have edges", writes the editor of the magazine Astronomy, Steve Nadis, in an article in the journal Quanta (republished from Wired):

Black holes spin through space. As matter falls into them, they begin to spin faster and faster. and if that material has a charge, they are also electrically charged.

In principle, a black hole can reach a point where it has as much charge or spin as it can, given its mass. Such a black hole is called an "extrema" - the edge of the edges. These black holes have some strange properties. Specifically, the so-called surface gravity at the boundary, or event horizon, of such a black hole is zero. "It's a black hole whose surface no longer attracts things," said Carsten Gundlach, a mathematical physicist at the University of Southampton. But if you pushed a particle slightly towards the center of the black hole, it couldn't escape.

In 1973, prominent physicists Stephen Hawking, James Bardeen and Brandon Carter argued that extreme black holes cannot exist in the real world because there is simply no way for them to form. However, for the past 50 years, extreme black holes have served as models in theoretical physics. "They have nice symmetries that make things easy to calculate," 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.

Now two mathematicians have proved Hawking and his colleagues wrong. New work by Christoph Kehle of the Massachusetts Institute of Technology and Ryan Unger of Stanford University and the University of California, Berkeley, shows that there is no known law of physics that prevents the formation of an extreme black hole.

Their mathematical proof is "beautiful, technically innovative and physically surprising," said Michalis Dafermos, a mathematician at Princeton University (and Kehle and Unger's PhD advisor).

It hints at a potentially richer and more diverse universe in which "extreme black holes could be out there astrophysically." That doesn't mean it's happening.

"Just because there's a mathematical solution that has nice properties doesn't necessarily mean that nature will use it," Khanna said. "But if we somehow find one, that would really make us think about what we're missing." Such a discovery, he noted, has the potential to raise "some pretty radical kinds of questions." Before Kehle and Unger's proof, we thought that extreme black holes could not exist.