Which statement is true about energy extraction from rotating black holes?

• A. Energy extraction occurs via processes in the ergosphere that tap rotational energy.
• B. Energy can only be extracted after crossing the event horizon.
• C. Energy extraction is impossible for rotating black holes.
• D. Only Hawking radiation can extract energy.

Answer: (A)

Rotating black holes have an ergosphere, a region outside the event horizon where spacetime is dragged so strongly that nothing can stay stationary. This dragging enables processes that tap the hole’s rotational energy. The classic idea is the Penrose process: a particle or infalling matter enters the ergosphere and splits into two pieces. One piece falls into the black hole with negative energy (as seen from infinity), while the other escapes to infinity with more energy than the original particle had. The missing energy effectively comes from the black hole’s spin, so rotational energy is extracted.

In the real universe, magnetic fields threading the ergosphere can also siphon off energy from the spin, via mechanisms like Blandford-Znajek, powering relativistic jets. The essential point is that you don’t need to cross the event horizon to extract energy; the ergosphere outside the horizon is the region where rotational energy transfer can occur. Hawking radiation, while a true quantum process, is negligible for astrophysical black holes and is not the main way energy is extracted in these systems.