Black holes (full version)

Black holes

Black holes are of one of the most famous astronomical objects from science. Even though they were studied for decades, we know very least about the nature of the stellar devastators. Black holes first came from theoretical field, being characterized as general solutions of the general theory of relativity by the astronomer Karl Schwarzschild in 1916; only after several decades of studies, black holes were, in general, accepted as real objects. Those objects can only be studied indirectly, that is, analyzing the effects they cause in the medium they are in.

Stellar black holes form when massive stars (of at least five solar masses) collapse under their own gravity, compressing a huge amount of matter into a tiny point called singularity. At that point, science as we know stops to work because of such extreme conditions and then the singularity becomes one of the greatest science mysteries.

We know from general relativity that gravity is given by the presence of matter/energy which cause the deformation of the space-time fabric. The more matter we have in a small region of space the greater the bend in the space-time fabric and so the greater the attraction felt by a body nearby; because singularity is, approximately, a point with a density which tends to infinite, the space around bends in such a way that nothing, not even light, can scape when heading towards a black hole. The reason why we can’t see directly a black hole is explained by the fact that light gets trapped inside it forever. The event horizon is a spherical region spaced by a certain value (given by the Schwarzschild radium) from the singularity, from which nothing can avoid to be “eaten” by the starving space object (think about the event horizon as an oyster and the singularity as the pearl which we can´t see). When matter is added to the black hole, its mass and radium are increased.

Another consequence from general relativity is the temporal dilatation. As mentioned, black holes cause a big deformation in the fabric of space time and so time itself, for something near the black hole, will pass much slower in comparison to someone who is out of the influence it. {The movie Interstellar (which was produced with the help of the astrophysicist and winner of Nobel Prize Kip Thorne) explores this idea in a marvelous way (the movie’s black hole served as a model to many studies made since the move was out).

Black holes are not rare objects; we can say (in a non-rigorous estimative) there are over 200 million stellar black holes in the Milky Way, with the most famous being the proposed Sagittarius A*, supermassive black hole of 4.31 × 10^6 solar masses, being in the middle of our galaxy. In most spiral and elliptical galaxies, we find galactic nucleus, neighborhood of supermassive black holes.

In a general and not rigorous way, black holes can be defined as any point where density tends to infinite; that way, if Earth was compressed to just the right size, it would turn into a black hole of approximately the size of a pea.

Even though we can’t obtain information of objects which go beyond the event horizon, we can get three and only three information of any black hole, and those are: mass, angular momentum and electric charge.

In binary systems with massive stars it is common for a star to become a black hole, eating its neighbor along time. Such a feat generates an accretion disk (wrap of matter orbiting a central body), which accelerates towards the black hole, releasing orbital energy in form of electromagnetic radiation (in general, in the frequency of x-rays).

As said, nothing comes out of a black hole because of its immense gravity. However, Stephen Hawking discovered that black holes emit a form of radiation, called Hawking radiation. Hawking radiation uses the concepts of quantum vacuum fluctuations, which says that even the vacuum is not totally empty; there are particles (called virtual particles) which arise in pairs (a particle and its antiparticle) out of nothing, for a short time, and then annihilate (hence there is no violation of the law of energy conservation long-term). Fluctuations happen all the time throughout the cosmos, and when they happen near the event horizon, one of the particles is "swallowed" and lost forever while the other is out; as both separate, they are free from annihilation, becoming real particles. In order for there to be no violation of energy conservation, the energy required to “pay” the debt of energy from vacuum quantum fluctuations come at the expense of the mass of the black hole (which decreases). Thus, the outside particle can escape from the vicinity of the black hole. Over billions of years, this continuous process that occurs on a quantum scale becomes significant so that the whole black hole can "disappear" because of the radiation emitted over this time. The fact that the curious objects "evaporate" brings to light one of the great paradoxes of science, discussed by many scientists, the so-called information paradox.

Despite all the mystery involving these curious objects and the evident difficult in studying them, science has shown itself to be powerful, being able to provide information about this matter in the midst of so many obstacles. The great physicist Stephen Hawking dedicated most of his life to the understanding of black holes, taking humanity steps ahead and enlarging our knowledge about the functioning of the cosmos.

Reference material: A brief history of time (Stephen Hawking's book)