loop quantum gravity Loop quantum gravity (LQG) is a theory of quantum gravity, which aims to merge quantum mechanics and general relativity, incorporating matter of the Standard Model into the framework established for the pure quantum gravity case. It is an attem ...

theory, a Planck star is a hypothetical

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

, theorized as a compact, exotic star, that exists within a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

's event horizon, created when the

energy density In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or . Often only the ''useful'' or extract ...

of a collapsing star reaches the Planck energy density. Under these conditions, assuming gravity and spacetime are quantized, a repulsive "force" arises from Heisenberg's uncertainty principle. 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

, not the Planck length, and starts taking effect far earlier than might be expected. This repulsive "force" is strong enough to stop the star's collapse 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, there is adequate room for all the information captured inside a

to be encoded in the star; thus, avoiding information loss. While it might 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 extreme gravity of such an object slow down time for the Planck star to a similarly extreme degree. Seen from outside the star's Schwarzschild radius, the rebound from a Planck star takes approximately fourteen billion years, such that even primordial black holes are only now starting to rebound from an outside perspective. Furthermore, the emission of Hawking radiation can be calculated to correspond to the timescale of gravitational effects on time, such that the event horizon that "forms" a black hole evaporates as the rebound proceeds.

Carlo Rovelli Carlo Rovelli (born May 3, 1956) is an Italian theoretical physicist and writer who has worked in Italy, the United States and, since 2000, in France. He is also currently a Distinguished Visiting Research Chair at the Perimeter Institute, and c ...

and Francesca Vidotto, who first proposed the existence of Planck stars, theorized in 2014 that Planck stars form inside black holes as a solution to the black hole firewall and the black hole information paradox. Confirmation of emissions from rebounding black holes could prove

. Recent work demonstrates that Planck stars may exist inside

s as part of a cycle between black and white holes. A somewhat analogous object theorized under

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...

is the

fuzzball Fuzzball may refer to: * Fuzzball (sport), a variation of baseball similar to stickball * Fuzzball (string theory), an alternative quantum description of black holes * Fuzzball router, the first modern routers on the Internet * ''Fuzzball'', a car ...

, which similarly eliminates the singularity within a black hole and accounts for a way to preserve the quantum information that falls into a black hole's event horizon.

References

{{Portal bar, Astronomy, Spaceflight, Outer space, Solar System Star types Hypothetical stars Black holes

See also

References