Planck star

A Planck star is a hypothetical astronomical object that is created when the energy density of a collapsing star reaches the Planck energy density. Under these conditions, assuming gravity and spacetime are quantized, there arises a repulsive 'force' derived from Heisenberg's uncertainty principle. Namely, if gravity and spacetime are quantized, the accumulation of mass-energy inside the Planck star cannot collapse beyond this limit because it violates the uncertainty principle for spacetime itself.

The key feature of this theoretical object is that this repulsion arises from the energy density, not the Planck length, and starts taking effect far earlier than might be expected. This repulsive 'force' is strong enough to stop the collapse of the star well before a singularity is formed, and indeed, well before the Planck scale for distance. Since a Planck star is calculated to be considerably larger than the Planck scale for distance, this means there is adequate room for all the information captured in the black hole to be encoded upon the star, thus avoiding information loss.

While it would be expected that such a repulsion would act very quickly to reverse the collapse of a star, it turns out that the relativistic effects of the very extreme gravity such an object generates slows down time for the Planck star to a similarly extreme degree. Seen from outside, the rebound from a Planck star of stellar mass takes longer than the timescale of the universe to date, such that stellar mass black holes seem to be stable to an external observer. Even more elegantly, the emission of Hawking radiation can be calculated to correspond handily to the timescale of the gravity effects on time, such that the event horizon that 'forms' a black hole naturally evaporates as the rebound proceeds.

In 2014 Carlo Rovelli and Francesca Vidotto proposed that there is a Planck star inside a black hole.^[1] This theory, if correct, would resolve the black hole firewall and black hole information paradox. This idea is based on loop quantum gravity.^[2]

References