KA
← All notes

My first paper: stealing energy from spinning black holes

By

· 2 min read · physics

This was my first paper, and I still remember how exciting it was to see it go up on arXiv. It was later published in Universe, with Vishva Patel, Parth Bambhaniya, and Prof. Pankaj S. Joshi.

The Penrose process, simply

A spinning black hole drags space around with it. Just outside the horizon there is a region, called the ergoregion, where this dragging is so strong that nothing can stay still; everything is forced to rotate with the black hole.

In 1969, Roger Penrose noticed something surprising. Imagine throwing a rock into the ergoregion, and the rock splits in two. One piece falls into the black hole in a special way that effectively carries negative energy. The other piece flies back out carrying more energy than the original rock had. The extra energy comes from the black hole's spin, which slows down a tiny bit. You have taken energy out of a black hole.

Why it still matters

The original idea is not very efficient, but its descendants are central to black hole astrophysics:

  • The magnetic Penrose process adds magnetic fields and can be far more efficient. It is one way to explain how black holes power extremely energetic particles.
  • The Blandford-Znajek mechanism, an electromagnetic cousin of the same idea, is a leading explanation for the huge jets we see shooting out of galaxies with spinning black holes at their centres.
  • The collisional Penrose process looks at particles smashing together near a black hole, which can reach very high energies.

What we asked

Some theorists have proposed objects that look almost exactly like spinning (Kerr) black holes from the outside, but avoid the infinitely dense point at the centre. We call them black hole mimickers. We asked: if you run the Penrose process around them, does anything change?

For one family (Simpson-Visser spacetimes), the answer was no: the efficiency matched a normal Kerr black hole. For another family (conformally transformed spacetimes), the efficiency was clearly higher. That difference could, in principle, help tell a mimicker apart from a real black hole.

Looking back, this paper taught me how research actually works: a lot of patient calculation, a lot of plots that don't work the first time, and the quiet joy when they finally do.