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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.<p>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.
"<p>It had been thought impossible for such black holes to exist. However, new work now demonstrates that such black holes are indeed possible.<p>None have been found, however. Though this seems unsurprising. How would you detect one?