Black Holes
Region of Spacetime from which Nothing can Escape

Black Holes
Region of Spacetime from which Nothing can Escape

A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.

Black holes are the doorway to understanding the universe around us.

Black holes are one of the most destructive forces in the universe, capable of tearing a planet apart and swallowing an entire star.

Yet scientists now believe they could hold the key to answering the ultimate question - what was there before the Big Bang?

The trouble is that researching them is next to impossible. Black holes are by definition invisible and there's no scientific theory able to explain them.

Despite these obvious obstacles, Horizon meets the astronomers attempting to image a black hole for the very first time and the theoretical physicists getting ever closer to unlocking their mysteries.

It's a story that takes us into the heart of a black hole and to the very edge of what we think we know about the universe.

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations.

Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.

This led the general relativity community to dismiss all results to the contrary for many years.

However, a minority of relativists continued to contend that black holes were physical objects, and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Penrose proved that a singularity will form somewhere inside it.

Shortly afterwards, Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter.

The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.

The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite. For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.

In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution. The singular region can thus be thought of as having infinite density. Observers falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid being carried into the singularity, once they cross the event horizon.

The black hole information paradox is a phenomenon in astrophysics. It is when quantum mechanics and general relativity are put together.

This phenomenon states that the information a physical system has can be lost when it enters a black hole. The object that falls into a black hole, only keeps information about its spin, mass and charge.

However, the quantum mechanics says that information cannot be lost. This makes it paradox. It appears to contradict itself.

They can prolong the experience by accelerating away to slow their descent, but only up to a point; after attaining a certain ideal velocity, it is best to free fall the rest of the way.

When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole.

Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity.

Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.

The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.

It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.

It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes. The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.

This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions.

To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.