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Stephen Hawking's latest scientific article was published by physicists who have worked with the late cosmologist throughout his career to understand what happens in information when objects fall into black holes.
This work, which addresses what theoretical physicists call "the paradox of information," was completed a few days before Hawking's death in March. It has now been written by colleagues at Cambridge and Harvard Universities and posted online.
Malcolm Perry, professor of theoretical physics in Cambridge and co-author of the paper, Black Hole Entropy and Soft Hair, said the information paradox was "at the center of Hawking's life" for more than 40 years.
The origins of the puzzle go back to Albert Einstein. In 1915, Einstein published his theory of general relativity, a tour de force that describes how gravity derives from the material effects of matter that inflects space-time, and explains why planets surround the sun. But Einstein's theory has also made important predictions about black holes, namely that a black hole can only be completely defined by three characteristics: its mass, its charge and its rotation.
Nearly 60 years later, Hawking added to the picture. He argued that black holes also have a temperature. And because hot objects lose heat in space, the ultimate fate of a black hole is to evaporate. But that poses a problem. The rules of the quantum world require that information is never lost. So what happens to all the information contained in an object – the nature of the atoms of the moon, for example – when it falls into a black hole?
"The problem is that if you throw something into a black hole, it seems like it's going away," Perry said. "How could the information contained in this object be recovered if the black hole disappears on its own?"
In the last article, Hawking and his colleagues show how at least some information can be preserved. Launch an object in a black hole and the temperature of the latter must change. The same goes for a property called entropy, a measure of the internal disorder of an object, which awakens as it rises in temperature.
Physicists, including Sasha Haco at Cambridge and Andrew Strominger at Harvard, have shown that the entropy of a black hole can be recorded by photons surrounding the horizon of events of the black hole, at which point light can not escape the light. 'intense gravitational attraction. They call this burst of photons "soft hair".
"This document shows that" soft hair "can explain entropy," said Perry. "It tells you that soft hair really does a good job."
This is not, however, the end of the information paradox. "We do not know that Hawking's entropy represents everything you could possibly throw at a black hole, so that's really a step forward," said Perry. "We think it's a good step forward, but there is still a lot of work to be done."
A few days before Hawking's death, Perry was at Harvard to work on paper with Strominger. He was not aware of Hawking's health condition and called the physicist to keep him informed. It was perhaps the last scientific exchange Hawking had had. "It was very difficult for Stephen to communicate and I was put on a speaker to explain where we had arrived. When I explained it, he simply made a huge smile. I told him we had something. He knew the end result.
Among the unknowns that Perry and his colleagues must now explore, there is the way the information associated with entropy is stored physically in soft hair and how this information comes out of a black hole when It evaporates.
"If I throw something inside, does all the information on what is stored at the black hole's horizon?" Said Perry. "This is what is needed to solve the information paradox. If only half, or 99%, is not enough, you have not solved the problem of the information paradox.
"It's a step forward, but it's certainly not the whole answer. We have a little less headache than before, but there are definitely some troubling issues. "
Marika Taylor, a professor of theoretical physics at the University of Southampton and a former Hawking student, said, "Understanding the microscopic origin of this entropy – what are the underlying quantum states of entropy? – has been one of the great challenges of the last 40 years.
"This article proposes a way to understand the entropy of astrophysical black holes based on the symmetries of the event horizon. Authors must make several non-trivial assumptions. The next step will be to show that these assumptions are valid. "
Juan Maldacena, theoretical physicist at the Princeton Institute of Advanced Studies, Einstein's alma mater, said, "Hawking discovered that black holes had a temperature. For ordinary objects, we understand the temperature as due to the movement of the microscopic constituents of the system. For example, the temperature of the air is due to the movement of the molecules: the faster they move, the hotter it is.
"For black holes, it's hard to know what these components are and if they can be associated with the horizon of a black hole. In some physical systems with special symmetries, the thermal properties can be calculated as a function of these symmetries. This article shows that near the horizon of the black hole, we have one of these special symmetries. "
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