venerdì 9 settembre 2011

The "space compactor"

How many fascinating fantasy stories appeal to the physical concept of "black hole" to narrate adventures taking place in space-time tunnels, which violate the limits imposed by the speed of light to interstellar or time travelling, or permit to go into other dimensions. But in the collective black hole is above all that terrible monster, perhaps artificially created by reckless experiments, capable of sucking everything in it like a giant vacuum cleaner.

But what really is a black hole?

In general relativity black hole is defined as a region of space from which nothing, not even light, can escape. This can take place around a celestial body with extremely dense gravitational attraction so high as not to allow the removal of anything from its surface. This condition occurs when the escape velocity from its surface is greater than the speed of light. A heavenly body with this property is invisible and its presence could be detected only indirectly, through the effects of its intense gravitational field.
But how does rise a black hole?
At the core of a star at the end of its life cycle, after having consumed by nuclear fusion, 10% hydrogen turned into helium, the nuclear reactions stop. The gravitational force, which was previously in equilibrium with the pressure generated by nuclear fusion reactions, takes over and compresses the mass of the star towards its center. When the density becomes high enough triggers the nuclear fusion of helium, after which there is the production of lithium, nitrogen and other elements (up to oxygen and silicon). During this phase the star expands and contracts several times violently, expelling part of its mass. The smallest stars will stop at some point in the chain and off, slowly cooling and contracting, through the stage of white dwarf and over many millions of years become a sort of giant planet.If the core of the star exceeds a critical mass, called the Chandrasekhar limit of 1.4 times the solar mass, reactions can be up to the synthesis of the iron. The reaction that synthesizes the iron for the formation of heavier elements is endothermic, requires energy rather than emit, then the core of the star becomes an inert mass of iron and not presenting any nuclear reactions there is nothing that can oppose the gravitational collapse. At this point, the star undergoes a strong contraction that can cause a gigantic explosion, called supernova explosion of type II. During the explosion that remains of the star expels most of its mass, which is to be scattered around the universe ; what remains is a core extremely dense and massive. If its mass is small enough to allow the degeneracy pressure to counteract the force of gravity reaches equilibrium and forms a neutron star. If the mass exceeds three solar masses (Oppenheimer-Volkoff limit) there is nothing that can counter the gravitational force ; in addition, according to general relativity, the internal pressure is no longer carried out (in order to counteract the gravitational field), but becomes itself a source of the gravitational field, making inevitable infinite collapse.A true "space compactor."At this point the density of the dying star, now become a black hole, quickly reaches those values ​​to create a gravitational field so intense that it does not allow anything to escape its attraction, even the light: it has an infinite curvature of spacetime, which can give birth to passages within black holes in rotation. Some scientists have speculated that it, at least in theory, you can travel into the past, since the tunnels connecting two different regions of spacetime.Because of their characteristics, the black holes can not be "seen" directly, but their presence can be assumed due to the effects of gravitational attraction on matter in exercising nearby and on the light radiation passing nearby or "falling" down towards the hole. In astronomy, an increasing disk is a structure formed by material falling in a gravitational field source. The conservation of angular momentum requires that when an extended cloud of material collapses inward, any small rotation at the beginning should thrive. The centrifugal force causes the rotating cloud collapses into a disk and tidal forces tend to align the rotation of the disk with the rotation of the gravitational source in the center. The friction between the particles of the disk generates heat and dissipates the orbital angular momentum, causing the material falls toward the center of the disk in slow spirals until it impacts against the central body. The most spectacular increasing disks found in nature are those of active galactic nucleus: while the matter spirals into a supermassive central black hole, the strong gravity gradient leads to the development of a strong heat. The increasing disk of a black hole is hot enough to emit X-rays and temperatures of millions of degrees, just before crossing the event horizon.In fact a black hole is not completely black: it emits particles, in an amount inversely proportional to its mass, leading to a kind of evaporation. This phenomenon, demonstrated for the first time in 1974 by physicist Stephen Hawking, is known as Hawking radiation and is the basis of the thermodynamics of blacks holes. The Hawking radiation is thermal radiation believed emitted from black holes due quantum effects. One can understand the physical process by imagining particle-antiparticle radiation emitted just beyond the event horizon.An event horizon is, in the popular, a concept linked to black holes, and a prediction of general relativity. According to this theory, space and time form a single complex with four full size (called space-time). In the case of a Schwarzschild black hole, the event horizon is created when, in a self-gravitating body, the "matter" (term used here to identify the mass and energy, which according to general relativity, are the same thing) is so concentrated that the escape velocity should be equal or even superior to that of light. As definition given by Roger Penrose a black hole event horizon is a particular area of ​space-time that separates the seats from which signals can escape from those from which no signal can escape. In a much more general sense, if by "event" refers to a phenomenon (particular state of an observable physical reality), identified by the four space-time coordinates, an "event horizon" can be defined as a region of space-time beyond which shall stop to be possible to observe the phenomenon. In the case of black Schwarzschild holes, the event horizon is a spherical surface surrounding a singularity at the center of the sphere ; the latter is a point at which the density would be infinite and the laws of physics, according to the theory of general relativity, lose their meaning. The singularity may not be necessary, according to some theories of quantum gravity (loop quantum gravity), which require space-time as an entity endowed with a physical reality, and not just merely a mathematical concept, divided into discrete elements with diameter equal to Planck length. In other words, the space-time would have, according to this theory, an active, not passive, physically active role and its intimate structure would consist of real "atoms" that would form a dense network in constant evolution. Under normal conditions do not perceive the atomic structure of spacetime, which appear continuous mathematician and the universe would be described by general relativity ; but at the size of distances in the order of the Planck length things would change radically and quantum and gravitational effects take on comparable intensity. It would be like if the space took on a "physical personality" and interacted with its energy (mass) in an active way. Many results are only for speculative or hypothetical, given that, at the time no one has ever seen "up close" a black hole (they are often surrounded by increasing disks or dense matter halos). There also should be noted that the inside of a black hole can not leave any information that can say anything about its inner structure, or at least that there isn’t a theory well-established and supported by observational data.

Returning, then, to the Hawcking’s radiation, this does not come directly from the black hole itself, but rather is the result of virtual particles - always in pairs born in cosmic void - come to life due to the gravitational pull of the black hole. To be more precise, the quantum fluctuations of the vacuum causes the appearance of particle-antiparticle pairs near the event horizon of the heavenly body. A particle of the pair falls into the black hole, while the other managed to escape in the external universe. To respect the principle of conservation of the total energy, the particle that has fallen into the black hole must have negative energy (relative to an observer who is far away from the black hole). Due to this process the black hole loses mass and an outside observer it would appear that the hole has just emitted a particle itself.

Remains unsolved mathematical problem of the "singularity" that forbids to reconcile General Relativity and Quantum Mechanics.

Moreover, if you can theorize a mathematical singularity can not exist a physical singularity, despite the compactor that want to force the infinitely large space to become infinitely small.

Stefano Gusman.

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