# Destroy black holes information

## The immortality of the black holes

What is left when a black hole evaporates?

When a star with at least eight times the solar mass (Tolman-Oppenheimer-Volkoff limit) dies in a final explosion, the gravitational pull of its own mass gives it a fascinating fate: it turns into a black hole. Its gravity is so great that not even light can escape it. What once disappears behind the so-called event horizon never comes to light again.

Black holes were predicted as early as the eighteenth century. Until the 1960s, however, it was considered a mathematical curiosity, a special solution to Einstein's equations of general relativity for point masses. However, the black hole does take up a certain extent in space. Even with a solar mass, the event horizon extends into space with a radius of a few kilometers, in the form of an ellipsoid of revolution (a body spanned by a rotating ellipse).

Black holes can only be observed through their effect, for example as part of multiple systems or through the gravitational lens effect. Since no information about the event horizon penetrates to the outside, only guesses can be made about the interior of a black hole. This is particularly difficult for physicists because this is where quantum physics and general relativity reach their limits. The singularity, which must form the core of a black hole, eludes complete calculations with today's means.

In 1975 Stephen Hawking and Jacob Bekenstein showed that black holes are constantly losing mass. This is because empty space is not really empty either: quantum fluctuations constantly create combinations of particles and antiparticles in a vacuum. Under normal circumstances, these almost immediately annihilate each other again, thereby settling the energy debt that they have taken on from the universe through their existence. However, if they arise on the edge of the event horizon of a black hole, it will occasionally happen that one of the two partners is just created inside the death zone, the other outside. If the black hole swallows one of the particles, it absorbs its negative energy like a poison pill and consequently loses mass, while the other particle escapes into the universe as so-called Hawking radiation.

The fact that black holes therefore have a limited lifespan is not the real problem for physicists. After all, a specimen with solar mass needs about 10^{67} Years until it is wiped out that way. The exciting question is rather: What happens to the information that gets into a black hole - and that is assumed to be indestructible? If a black hole were immortal, the physicists could comfort themselves with the fact that information that once got behind the event horizon is irretrievably lost, but still exists around the singularity. In order to be able to resolve this paradox, physicists presumably need the "theory of everything". Until this is worked out, you have to make do with these scenarios:

- The information is irretrievably lost. Unfortunately, this end violates the principle of information preservation. On the other hand, the mathematician Roger Penrose in his cosmology assumes the destruction of information in a black hole.
- Black holes are information incontinent. Information is constantly escaping from them. Unfortunately, this contradicts current calculations for macroscopic black holes, which agree well with reality.
- When the black hole dies, the information gathered escapes all at once. This scenario can be easily reconciled with current physics - except for the moment shortly before the violent death. Then a very, very small black hole would have to contain a huge amount of information, which contradicts current knowledge about the maximum information density.
- The final stage of a black hole is no greater than the smallest length in the universe, the Planck length. It contains all the information ever collected. The physicists would then not have to think of a way, as in scenario 3, through which the information escapes. However, the information density would be infinite.
- The information remains in a daughter universe that splits off from our universe. A nice idea, but unfortunately there is still no elaborated physical theory for it.
- The information is retained in temporal correlations instead of in spatial dimensions. This notion can be worked out well with current physics, but it contradicts the understanding of nature as an entity that evolves over time.
- The information encoded in the black hole in three-dimensional space is also stored on its two-dimensional boundary surface. This "holographic principle" developed by the physicist Juan Malcadena assumes that our universe consists of both a three-dimensional structure and a 2D element. In the 3D universe, strings, gravity and black holes follow the theory of relativity; in the flat part, elementary particles and their fields are based on the laws of quantum physics. Each piece of information from one part is also encoded in the other, but the other is inaccessible to a member of one structure. Information that evaporates with the black hole would not be lost at all, but would be preserved in the 2D structure of the universe.
- A so-called firewall is created at the event horizon. It is assumed that the particles emerging from nowhere are entangled with one another. If either partner falls into the black hole, the entanglement breaks and energy is released. The information remains in the form of an entanglement of all particles in this firewall. However, this scenario contradicts the general theory of relativity, according to which all gravitational fields are fundamentally the same. Why should something different happen when an object falls into a black hole than when a ball falls to the ground?
- The firewall around the black hole, which contradicts general relativity, could be avoided if the black hole itself and all particles of its Hawking radiation were connected by wormholes, abbreviations in spacetime. The idea published in June enables particles and antiparticles to remain entangled. As a result of the entanglement, not only is the information retained: it could theoretically also be retrieved.
- The evaporation of the black hole will come to an end at some point. The physicist George Ellis from Cambridge University developed this idea in a fairly fresh paper, which, however, still lacks hard calculations. A black hole distorts spacetime due to its gravitation. But that is not the only thing: The cosmic background radiation, under the influence of the gravitational field, also creates singularities shortly behind the event horizon, which change the structure of spacetime. If the virtual particles of the Hawking radiation now arise in the vicinity of such a distortion, both slide behind the event horizon. The more a black hole shrinks, the more likely this process becomes - until at some point it occurs more often than the opposite case and the mass of the black hole stabilizes. When exactly this is the case, Ellis cannot say - but regardless of this, the black hole would not evaporate completely, but would asymptotically strive towards a certain mass and thereby receive the information inside.

What would be so bad about it if information were lost in a black hole? Haven't we all forgotten something without the universe collapsing? The problem: information corresponds to order, a loss of information to disorder or entropy. In thermodynamics, an increase in entropy corresponds to an increase in temperature. If black holes were to destroy information, they would have to heat up, and in a short time by trillions of degrees.

Matthias Matting's eBook *The new biography of the universe*, which describes the development and structure of our universe in detail, is available from Amazon (Mobi, DRM-free), from iTunes (for iPad) and from Beam-eBooks (ePub, PDF, DRM-free).

(*Matthias Matting*)

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