system
Given the following post, respond as if you're commenting on Reddit:
human
Why is frame-dragging limited by the speed of light?

One of the explanations I've seen for why black holes can't spin faster than c (at the event horizon) is that this would cause objects to be frame-dragged (frame-drug?) at superluminal speeds as seen by a distant observer. But frame-dragging is the warping of space, and as we see with the cosmological expansion of space and theoretical technologies like an Alcubierre drive, space itself isn't prohibited from moving faster than light relative to other areas of space and carrying matter along with it.
gpt
That's not right. One of the solutions of the Einstein field equations is the Kerr black hole. That is characterized by a zone, called the "ergosphere" where objects can't sand still because space itself is rotating at superluminal speeds. Object inside the ergosphere would appear as moving faster than light for distant observers.  


The reason why the black hole can't rotate faster is a mathematical one: the equation produces no meaningful solutions if the rotation rate is above a certain limit .