4.2 Searching for White Holes

summary

There are two macroscopic cases for which Loop Quantum Gravity is relevant.

Humanity does not have any technology capable of inspecting the Planck scale at which loop quantum gravity takes place. The LHC can see at very small scales, but the distances it probes are still far larger than 10⁻³³cm
Quantum gravity is relevant when nature itself does something on the Planck scale which then gets amplified to levels that humans can observe. There are two main examples of this.
The first is the early universe, when the density of matter and energy was extremely large and the quantum effects of gravity could be apparent on the macroscopic scale.
The other objects in nature that can be analyzed for evidence of loop quantum gravity are black holes.

The structure of black hole singularities is presently unknown.

Black holes are most often formed by stars that undergo gravitational collapse. In general relativity, all of their matter falls into a single point and becomes an infinite-density singularity.
This singularity is surrounded by an event horizon, which is a volume of spacetime from which no light or matter can escape once it enters.
The exact structure of the singularity is unknown because the event horizon prevents any direct observation of the interior of the black hole.
For the most part, black holes can only be observed indirectly. The presence of a black hole can be inferred from things like the bending of light or the orbit of stars around unseen bodies.
Gravitational wave astronomy provides a method for the direct observation of black holes, but this still only allows for the measurement of a black hole’s macroscopic parameters. We still do not have a window into the event horizon.
General relativity provides no answers about the singularity since parameters like the curvature of spacetime become infinite, rendering them effectively meaningless.

A quantum theory of gravity is needed to explain the interior of black holes.

Quantum theory allows for events that are classically forbidden. One example is the tunneling of alpha particles out of a uranium nucleus – classically, a uranium nucleus is stable, but experimental evidence has shown the existence of a quantum tunneling effect that allows for uranium to radiate.
Classical black holes are also stable, but this does not mean that they will be quantum mechanically stable.
In a quantum framework it is likely that the gravitational collapse of a star will not result in an infinitely-dense point singularity. Instead, it would be compressed into a star with a size on the order of the Planck scale.
This Planck-sized star would achieve the maximal density of matter possible, but it would still have an actual value. Infinite density would be averted.

White holes are time-reversed black holes.

Quantum mechanics suggests that a repulsive force should cause the compressed matter of the Planck-sized star to “bounce” back out almost immediately after it forms.
This bounce would result in the formation of a white hole. White holes are, in fact, solutions to the Einstein field equations, so while they are not thought to exist by most scientists, they are theoretically possible.
White holes are time-reversed black holes because they only emit matter. Nothing can ever enter a white hole, just like nothing can ever escape a black hole.

A quantum theory of gravity is needed to explain the interior of black holes.

The bounce that turns a black hole into a white hole will be very quick. This seems to run counter to observations of black holes over the last few decades – they are quite stable and none have disappeared or transformed into white holes.
The reason that black holes are stable over such long time periods is that time passes more slowly in regions of high gravity. To a hypothetical observer sitting inside a black hole, the bounce will be almost immediate, but to an outside of observer it could take billions of years for the black hole to transition into a white hole.
The black holes we observe today will likely not transition into white holes for millions if not billions of years. However, black holes that formed during the extremely dense conditions of the early universe would be white holes today, producing observable signals (emitted light and matter).
Computations reveal that white holes should produce signals very similar to something called a fast radio burst.
Fast radio bursts (FRB) are brief, high-energy pulses of broadband electromagnetic radiation that were recently discovered (the first was observed in 2001). The sources of FRBs are currently not understood, but if FRBs are in fact the result of white holes, then they will provide macroscopic evidence in support of quantum gravitational effects.