# What happens at the center of a black hole?

From the December 2023 issue

All matter in a black hole is concentrated in a central point of infinite density and extremely small volume, which is the singularity.

Credit: Dotted Yeti/Shutterstock.

**Do black holes have centers? If so, what’s going on there?**

*Richard Levitsky**(Seal Beach, California).*

Einstein spent 10 years wrestling with three fundamental concepts in physics: acceleration, the special theory of relativity, and the force of gravity. This heroic effort culminated in 1915 with the general theory of relativity, a remarkable set of equations linking the curvature of space-time and the matter moving in it.

Although they are easy to write down, finding solutions to these equations has intrigued physicists and mathematicians ever since. To deal with any set of equations relating to a physical system that evolves over time, a good place to start is to make assumptions that simplify things. Just one month after Einstein’s publication, Karl Schwarzschild discovered one of the simplest solutions. His description of a compact, spherical mass that is symmetrical and time-invariant due to the curvature of the surrounding spacetime gave us what we now call a black hole.

Initially, this solution was rejected by many because it predicted an unusual condition: all the matter in a black hole is concentrated in a central point of infinite density and extremely small volume – the singularity. Singularities make physicists somewhat uncomfortable.

Once you’ve determined what’s at the center, what about the empty space immediately surrounding the singularity? Consider the trajectory of a spaceship falling into a black hole. From the perspective of a distant observer, the ship will accelerate and quickly reach the central singularity, but on the way it passes the point of no return: the event horizon. The closer to the central mass, the stronger the gravitational force; Inside the event horizon, the force is so strong — that is, spacetime is so curved — that no engine is strong enough to overcome gravity and push the ship outward. In fact, within this radius, even light cannot move fast enough to escape the singularity’s gravity. Just outside the event horizon, the ships are still on unusual paths, but at least they can return home.

The two problems that keep physicists busy at night are: 1) our understanding of quantum mechanics makes the idea of a singularity untenable, and 2) the event horizon prevents us from measuring what happens inside a black hole. These issues lead us to the cutting edge of theoretical physics: the connection between general relativity and the quantum world; Reconciling the event horizons of black holes with the laws of thermodynamics; Even speculation about wormholes, where the center of the black hole is a path rather than an unfortunate dead end.

*Mark Avara**Astrophysicist, Institute of Astronomy, Cambridge, University of Cambridge, United Kingdom*