A black hole is a region in space where the high concentration of mass causes such a gravitational field that no material particle, not even the photons of light, can escape from a point called "event horizon" or border. The existence of black holes is supported by astronomical observations, in particular through the emission of x-ray binary stars and active galaxies.
A black hole is formed when a star of sufficient mass collapses and most of its mass squeezes into a small area of space, causing space time curvature at a point known as a "singularity". If the mass of a star is more than two times that of the Sun, there comes a time that not even neutrons can withstand gravity. The star collapses in different phases and finally becomes a black hole.
These observations have been explained by a variety of alternative theories, but the majority of scientists believe that black holes are the most likely explanation.
The dark side of a black hole
No one can predict what is behind a black hole event horizon, because light cannot reach us from this point. Although an explorer would be sent to the interior of a black hole, he never could communicate what he saw. Current theories predict that all matter existing inside a black hole is compressed in a single central point called singularity, but scientists do not know its functioning.
To properly understand the black hole requires unifying the theory of gravity with quantum mechanics, which is known as quantum gravity. This is one of the main unsolved problems in physics. Future studies of black holes may one day provide the solution to this mystery.
Einstein's theory of general relativity provides awesome characteristics for black holes. The central singularity might be a hypothetical bridge to another universe, which is also known as a wormhole. These mysterious wormholes might allow travel to other universes or even carry out time travel. But the real existence of these bridges and wormholes is purely speculative, since experimental and observational data are not available.
Is the Firewall Paradox a one way trip?
Let's imagine now that you undertake a trip that starts outside the event horizon of the black hole. As you approach, you see a dark circle surrounded by the night stars, whose measures have been distorted, as the light bends due to the gravity of the black hole.
As you start falling into the black hole you move faster and faster, accelerated by its gravity. Physicists have assumed that the gravitational forces will become extreme when you approach the point called the singularity. There, the gravitational pull will be much stronger on your feet than on your head by the action of tidal forces, and your body will stretch apart. Finally, if the black hole is small, your body will be completely torn apart before you reach the event horizon.
Your luck will be different if your fall into a supermassive black hole. In this case, your body is not altered, even when you cross the event horizon. But as soon as you reach the central singularity, your body will be compressed into a point of infinite density, and you will be melt with the black hole. Unfortunately, you will never return to relate your experience.
However, a new hypothesis can increase physics hopes. If the recent investigations are correct, as you cross the event horizon you will encounter a massive wall of fire that will incinerate you completely. Now you must be thinking why this is a better option, but it means that at least one of three appreciated notions in theoretical physics must be wrong.
Relativity says that if you fall into a black hole, your body will be gradually stretched by the increasingly strong gravitational forces. But last year, when Joseph Polchinski and his colleagues from the University of California explored the quantum implications of black holes, they found a problem. Black holes emit photons through what is known as Hawking radiation, and these photons are "linked" with the interior of the black hole and also among them. This breaks the rule of quantum mechanics which says that particles cannot be linked to two things at the same time.
Polchinski suggested last year that the linkage black hole-photon can be broken. This produces a wall of energy in the event horizon of the black hole that debunks the relativity since anyone who falls will burn out rather than becoming a spot. This is the birth of the black hole's "Firewall Paradox".
The solutions seemed to be multiple, but now Juan Maldacena, from the Institute for advanced study in Princeton and Leonard Susskind, from Stanford University in California, have presented an innovative theory: A new kind of linkage wormhole does not need to break. First, the two physicists showed that these tunnels in space-time, usually described by the mathematics of general relativity, also arise from quantum theory if two black holes are linked. It is as if the wormhole would be the physical manifestation of the linking.
Then they spread their idea into a single black hole and the Hawking radiation resulting in a new kind of wormhole. Fundamentally, they suggest that this wormhole, that links a black hole and the Hawking radiation, may not be a problem for quantum singularity in the way that it is with the normal linking. As a result, the wall of fire does not need to appear, while retaining the relativity.
Patrick Hayden, from McGill University in Montreal, Canada, said that the idea of pairs of linked black holes wormholes is unconvincing, but it takes more work for the case of the black hole and a photon. Polchinski, for its part, is cautiously optimistic, thinking that this new ideas are challenging but there is still much that needs to be filled.
Jets of plasma
In April 2008 Alan Marscher, leader of an international team of research from Boston University, published a study explaining that collimated jets of plasma are based on magnetic fields near the edge of the black holes. In specific areas of such magnetic fields the jets of plasma are oriented and accelerated to speeds close to C (speed of light). Such a process is comparable to the acceleration of particles to create a jet stream in a reactor. When the jets of plasma that are caused by a black hole are observable from Earth such a black hole falls into the category of blazar.
That a black hole "emits" radiation seems a contradiction, but this is explained: Every object (suppose a star) that is trapped by the gravitational pull of a black hole, before being fully "engulfed" is so strongly pressed by the tidal forces that a small part of its stuff comes out fired at speeds close to the light. This can be compared to the process when an orange is strongly squeezed and part of the orange pulp gets ejected in the form of juice jets. In the case of objects caught in a black hole, part of its mass comes out centrifugally triggered in the form of radiation outside the gravitational field of the singularity.
At the core of many galaxies, supermassive black holes expel powerful jets of particles. The most widely accepted theory says that the particles are accelerated by magnetic fields close to the hole, but confirm that the idea requires a difficult close look. Now using the resolution of the set of NRAO VLBA radio telescopes, astronomers have seen material behaving according to prediction.
The particles are accelerated at the innermost portion of the jet and everything supports the idea that twisted and spiral magnetic fields are throwing out material. Alan Marscher thinks that this is a major advance in the understanding of a remarkable process that occurs throughout the universe.