If you «to Google», Stephen Hawking is among the most famous living physicists. His most famous work concerns the information paradox of a black hole. If you are interested in physics, it is of course well known. To Hawking's black holes were not paradoxical. Yeah, if you threw the book into a black hole, you would not be able to read it. Because anything that crosses the event horizon of a black hole, is inaccessible from the outside. Event horizon — it is a closed surface, which can leave even the inside light. There is no way to retrieve information from a black hole; books no more. It's sad, but not particularly upsetting to physicists. Information from the book already don't extract, but there is nothing paradoxical.

And then came Stephen Hawking. In 1974, he showed that black holes emit radiation, and this radiation carries no information. It is completely random, with the exception of the distribution of particles depending on the energy which is the Planck spectrum with temperature inversely proportional to black hole mass. If the black hole radiates particles, it loses mass, shrinks and becomes hotter. After enough time and enough izlechim particles, the black hole will disappear and you will not be able to get the information included in it. The black hole has evaporated, the books inside are no more. Where's the information?

You can shrug and say, "Well, to hell with her gone and all. Don't we lose information?". No, we're not. At least not in principle. In practice, we permanently lose information, that's true. If you burn the book, you will not be able to read what was in it. However, from a fundamental point of view, all information of a book is still contained in the smoke and ashes.

Because the laws of nature, as far as we know our best physicists, you can scroll forward and back — each unique initial state corresponds to a unique end state. There are no two initial States that end up in the same nal state. The story burned your books will be totally different backwards. If you could very, very carefully to collect the smoke and ashes in a proper way, you could burn the book backwards and fix it, having collected literally from the ashes. It's an extremely unlikely process, and it is unlikely you will ever witness it in practice. But, in principle, it is possible.

Not Only with black holes. What would a black hole no matter, it will not be important when you look at the last page. In the end you still have only thermal radiation, which — in honor of its discoverer, called "Hawking radiation". This is a paradox: the process of evaporation of the black hole cannot rotate backwards. It is irreversible. And it is very sad for physicists, because it declares loudly: you do not understand the laws of nature.

The Loss of information in black hole paradoxical because it indicates an internal inconsistency in our theories. When we «marry» the General theory of relativity to quantum field theories of the standard model, and Hawking has done this in its calculations, the results are no longer compatible with quantum theory. On a fundamental level, every interaction involving particles must be reversible. But since the evaporation process is irreversible black hole, Hawking showed that these two theories, alas, can not be combined.

It would Seem that this is a contradiction, obviously follows from the fact that the irreversible evaporation was obtained without taking into account the quantum properties of space and time. For this we would need a quantum theory of gravity, which we don't have. Therefore, most physicists believe that quantum gravity will eliminate the paradox — while they just don't know how it works.

The complexity of the charges quantum gravity, however, is that on the horizon there is nothing interesting — it is a Kingdom that works perfectly General theory of relativity. Because the force of quantum gravity should depend on the curvature of space-time, but the curvature at the horizon of a black hole is inversely dependent on the mass of the black hole. That is, the larger the black hole, the smaller will be the expected quantum gravitational effects at the horizon.

Quantum gravitational effects will become noticeable only when the black hole reaches the Planck mass, about 10 micrograms. When the black hole shrinks to this size, the information may be released by quantum gravity. But depending on what formed the black hole, it at that time may be stuck arbitrarily large amount of information. And when there is only the ground Strap, to extract so much information with so little power for its decoding will be difficult.

Over the past forty years to solve this riddle tried the brightest minds of the planet. It may seem strange that such a distant problem attracts so much attention, but physicists have good reason. The evaporation of black holes — the best studied case of interaction between quantum theory and gravity, which means that this can be the key to creating a correct theory of quantum gravity. The solution to this paradox could be a breakthrough and, no doubt, will lead to a fundamentally new understanding of nature.

Until now, most attempts to resolve the information loss in a black hole fell into one of four broad categories, each with their own pros and cons.

**1. Information previously released.** Information begins to leak long before the black hole reaches Planck mass. Now this option is the most popular. But it is still unclear how this information is encoded in the radiation and how bypasses the calculations of Hawking.

The Advantage of this solution lies in its compatibility with what we know about the thermodynamics of black holes. The disadvantage is that for this to work is inevitable a kind of non-locality — spooky action at a distance. Even worse, it was recently announced that if the information will leak before the black holes will be surrounded by high-energy barrier: a wall of fire (firewall). If firewall exists, it would mean that would be violated by the underlying General theory of relativity the principle of equivalence. Not very nice.

**2. The information is stored or released at a later date.** In this case, the information remains in the black hole, while quantum-gravitational effects become strong, when the black hole reaches Planck mass. Then the information will either be released with the remaining energy either remains in her balance.

The Advantage of this option is that it requires no modifications to General relativity or quantum theory in those places where we wouldn't want that. They break exactly where you ought to break down when the curvature of space-time becomes too large. The disadvantage is that it leads to another paradox: in the weak background field may still receive the pair of black holes that is constantly around us. Theoretical backup to this argument is, but it is very weak.

**3. Information is destroyed.** Proponents of this approach just agree that the information is lost, falling into a black hole. But this option has long been considered a gross violation of the law of conservation of energy and leads to a heap of inconsistencies. In recent years, however, there are loopholes, pointing to the possibility of the conservation of energy in the loss of information, and the option raised. Though he is not very popular.

But as with the first option to check the possibility of data destruction, you need to modify quantum theory. You will need this modification, which will not conflict with any other experiments that verified and confirmed quantum mechanics. But since quantum mechanics is the most experimentally proven science, will make it more difficult.

**4. Black hole no.** the Black hole is never formed, and the information never crossed the horizon. This decision emerged early and POPs up now, but never found a wide circle of supporters. The advantage of it is that it bypasses the calculations of Hawking. The disadvantage is that it requires major deviations in General relativity, in regimes of low curvature and it is difficult to reconcile with precise tests of gravity.

There are several other decisions not yet included in any of the categories, but we won't today affect them. In fact, there is not one good review on this subject — probably because one thought on compile it throws in horror and shock. Literature very much. The loss of information in black hole probably been the most talked about paradox of modernity. And will remain so.

The Temperature of black holes that we can observe today, is too small for us to be able to catch. Thus, in the foreseeable future no one will be able to measure what happens to the information that crosses the horizon. Ten years later, the problem will probably remain unsolved.

Hawking recently celebrated its 75th anniversary, which is already a remarkable achievement. 50 years ago, the doctors declared him dead early, but he stubbornly clung to life. The paradox of information loss in a black hole may be even more resistant. Perhaps he will outlive us all.

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