Theoretical physicists Andrew Hamilton from the University of Colorado and Tyler McMaken from the University of Maine have shown that the singularity inside a non-rotating Schwarzschild black hole is not a point of infinite density, as commonly believed, but a flat three-dimensional surface. The work has been accepted for publication in the journal Physical Review D.

How the understanding of the singularity has evolved

As early as the 1930s, the singularity of a simple black hole was pictured as a point to which all matter collapses. In the 1960s, for rotating Kerr black holes, it was found that centrifugal forces turn the singularity into a ring. However, for Schwarzschild black holes, the point model remained a convenient and intuitive picture.

What changed with the new study

Back in 1998, Hamilton, while creating visualizations of falling into a black hole for students, noticed that instead of a point, a surface appeared, but he was unable to explain the effect at the time. The breakthrough came by tracing the trajectories of two observers falling into the same black hole from opposite sides.

If the singularity were a point, both observers should end up at the same location. However, the curvature of space-time prevents light or any other signal from traveling between them: they lose causal contact forever. This means they cannot converge at a single point. The contradiction disappears if the singularity is a flat three-dimensional spacelike surface, which each observer reaches at their own point.

What this means for physics

According to the authors, a similar conclusion applies to rotating Kerr black holes: their ring singularity, under realistic collapse, also turns into a spacelike surface at the inner horizon.

Hamilton noted that quantum gravity is a central question in theoretical physics, and it is precisely on the singular surfaces of black holes that it occurs in our Universe. If the Schwarzschild singularity is a surface rather than a point, this changes the mathematical starting point for a future theory that unifies general relativity with quantum mechanics.

In brief

Physicists have shown that the singularity inside a Schwarzschild black hole is not a point of infinite density but a flat three-dimensional surface. This conclusion was reached by analyzing the trajectories of observers falling into the black hole from different sides. The new picture changes the understanding of where quantum gravity manifests itself and could influence future theories. The work has been accepted for publication in the journal Physical Review D.