# Kerr black hole horizons and infinite redshift surfaces

In the Kerr black holes we have infinite-redshift surfaces (where a infalling body is still according to the asymptotic observer) and event horizons (the escape velocity becomes greater than the speed of light), as shown in this image:

The surfaces of infinite redshift are obtained by the vanishing of $g_{tt}$ and the event horizons are the surfaces in which the r = const geodesics become null

Could a particle enter the outer surface of infinite redshift and then escape before entering the outer event horizon? How this would be seen by the asymptotic observer, since when the particle crosses the redshift surface it appears frozen from outside?

I understand how it works in Schwarzschild case, but Kerr black hole has an (outer) event horizon and infinite redshift surface. In Schwarzschild they are both the same (you seem frozen by an outside observer when you cross the event horizon) but this no longer happens in Kerr, so I'm interested in differences between Schwarzshild and Kerr

• The infinite redshift only applies to objects that have no angular velocity. – Peter Shor May 20 '16 at 12:33

This is indeed possible! With a risk of overstatement, this fact is extremely important for astrophysics because it turns out that by dipping in and out of the ergogregion (the region between the surface of infinite redshift and the event horizon) one may extract quite a bit of energy and angular momentum from a rotating black hole in a process known as the Penrose Process. In fact, the upper limit on this extraction is 29% of the black hole's mass, assuming that it starts off rotating almost as fast as possible (near-extremal) and ends up not rotating at all (so that it becomes Schwarzschild). If you plug in the numbers for some astrophysical black holes that are known to be rotating very fast, just to get a feel for the energies we're talking about, a fun back-of-the-envelope calculation can show that a black hole like GRS 1915+105 can release enough energy to blow up (in the sense of the death star blowing up Alderaan) $10^6$ of our Suns!!