In theoretical physics, an extremal black hole is a black hole with the maxium possible mass that can be compatible with a given charge and angular momentum. In other words, this is the smallest possible black hole that can exist while rotating at a given fixed constant speed.
The concept of an extremal black hole is theoretical and none have thus far been observed in nature. However, many theories are based on their existence.
In supersymmetric theories, extremal black holes are often supersymmetric: they are invariant under several supercharges. This is a consequence of the BPS bound. Such black holes are stable and emit no Hawking radiation. Their black hole entropy can be calculated in string theory.
It has been suggested by Sean Carroll that the entropy of an extremal black hole is equal to zero. Carroll explains the lack of entropy by creating a separate dimension for the black hole to exist within.
The hypothetical black hole electron is defined as an extremal black hole.
Famous quotes containing the words black and/or hole:
“The lore of our fathers is a fabric of sentences.... It is a pale gray lore, black with fact and white with convention. But I have found no substantial reasons for concluding that there are any quite black threads in it, or any white ones.”
—Willard Van Orman Quine (b. 1908)
“But the surface of the Earth was meant for man. He wasn’t meant to live in a hole in the ground.”
—Edward L. Bernds (b. 1911)