Kerr black holes

A spinning black hole

Astrophysical objects possess angular momentum, so Kerr's solution is more realistic than the static Schwarzschild black hole. The rotation creates an ergosphere in which no observer can remain motionless with respect to infinity. Space-time is dragged around the axis and the horizons take on a more complex structure.

The ideal mathematical extension contains an outer horizon, an inner or Cauchy horizon, and a ring-shaped singularity. Beyond certain regions, closed time-type trajectories appear. On paper, a future line could return to previous temporal coordinates.

The ideal map is not a physical route

Popular representations turn the ring into a portal. That reading ignores that the relevant region is inside the black hole and that the exact solution assumes an eternal, isolated and perfectly stationary object. A real black hole is formed by collapse, receives matter and radiation and is subject to perturbations.

The instability of the internal horizon

The infalling radiation can undergo an enormous blue shift near the Cauchy horizon. Its effective energy grows and alters the geometry, a process associated with mass inflation. That's why the smooth ideal Kerr extension may not survive in a physical black hole. Furthermore, near the singularity the classical equations are no longer reliable and a complete theory of quantum gravity.

What is supported

• The existence of rotating black holes is supported by astrophysical observations.
• Frame creep is a real prediction of relativity.
• The ideal Kerr metric is an exact solution of Einstein's equations.
• No region with closed time curves or a transition to another time has been observed.

The scientific value of the model consists in subjecting causality to extreme conditions. It forces us to distinguish between the external observable part, the internal mathematical extension and that which can survive perturbations and quantum effects.

Connections

The rotation also supports the cilindro de Tipler and Gödel's universe. All three models show that spacetime entrainment is a centerpiece of acausal solutions, but none amount to a technology for travel.