How fast do black holes spin?

Black holes bend space-time so abruptly that even light finds it difficult to escape their gravitational pull. Nobody knows what happens at the center of a black hole because the known laws of physics break down at the singularity. This puts them among the strangest known objects in the universe.

Image: ESO, ESA/Hubble, M. Kornmesser (CC BY 4.0)

But, mathematically speaking, a black hole is a very simple object because it can be completely described using only three properties: mass (M), angular momentum or spin (J) and charge (Q).

Black holes form when an extremely heavy star (above 20 solar masses) collapses under its own weight. Since black holes are formed from stars that rotate, black holes must also rotate (conservation of angular momentum). In fact, since matter in a black hole is compressed into a very tiny region, it must rotate at extremely high speeds.

A high mass star going supernova (Image: NASA)

Also, since stars contain equal amounts of positive and negative charge, black holes forming from the collapse of stars are expected to be neutral i.e. Q = 0. Thus, a rotating black hole can now be described using only two properties: angular momentum (J) and mass (M). These two properties of a rotating black hole are related in the following manner:

Here, G = universal gravitational constant and c = speed of light

This shows that the maximum angular momentum of a black hole depends on its mass. A heavier black hole can rotate at a higher speed. As the speed of rotation of a black hole increases, its event horizon shrinks.

Supermassive black holes rotate at speeds very close to the speed of light (Image: NASA/ JPL-Caltech)

A black holes angular momentum can never exceed the value of GM²/c. If it exceeds this value, the event horizon of the black hole would shrink to such an extent that its singularity would be revealed to the outside world, which is forbidden by the laws of physics.

Most black holes observed in nature have spin rates much lower than
GM²/c but there are some black holes that spin at the maximum allowed speed.

Image: NASA/ CXC/ M. Weiss

For example, the black hole at the heart of the galaxy NGC 1365 is spinning at 84% of the speed of light. It has reached the maximum allowed speed limit, and cannot spin any faster without revealing its singularity.

Black Holes are very interesting. Don’t you think so?

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