What region around a rotating black hole enables energy extraction via frame dragging?

• A. The event horizon.
• B. The accretion disk.
• C. The photon sphere.
• D. The ergosphere.

Answer: (D)

Frame dragging from a spinning black hole twists spacetime enough to create the ergosphere, a zone outside the event horizon where no observer can remain at rest relative to distant stars. In this region, the hole’s rotation can be tapped for energy because the energy of particles measured at infinity can be negative there, allowing one part of a split particle to fall in with negative energy while another escapes with greater energy. Magnetic-field–driven processes also exploit this region to pull rotational energy from the hole. The event horizon is simply the boundary you can’t cross outward; nothing inside can contribute to energy extraction that escapes to infinity. The accretion disk is matter orbiting the hole, and the photon sphere is where photons can orbit, but neither is the zone that enables energy extraction from frame dragging. Therefore, the ergosphere is the region that enables energy extraction via frame dragging.