system
Given the following post, respond as if you're commenting on Reddit:
human
What causes black holes to have an upper limit to their rotational speed?

Wikipedia mentions the theoretical upper limit to how fast a black hole can rotate. What is the limiting factor?
gpt
The *intuitive* reason is that angular momentum has an associated rotational energy. This energy increases the mass of the black hole (through E = Mc^(2)).

Now, if you fix the total mass M, increasing the spin J then increases the fraction of M that is due to this rotational energy, and decreases the fraction of "normal", or "bare" mass (mass it would have if not rotating).

At a certain point, the bare mass becomes zero and M is entirely due to the rotation. This is an extremal Kerr hole. If you continue increasing J then the bare mass must become negative, which means the BH has become superextremal. Superextremal BHs are unstable and are impossible to create. Intuitively because negative mass is impossible.

Thus, for any given total mass, there is an upper bound on the spin which is given by the extremal black hole. Normal black holes satisfying the bound are called subextremal.