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Four decades ago, Stephen Hawking proposed that black holes could destroy information—a conclusion that is incompatible with standard laws of quantum physics. This idea started a controversy known as the “black hole information problem” that even now has not been resolved. A new study by Hawking himself and Malcom Perry, both at the University of Cambridge, and by Andrew Strominger at Harvard University shows that some of the assumptions that led to the information problem might be wrong [1]. Their results do not completely solve the problem, but point to a promising research direction that might lead to its long-awaited solution.

According to Einstein’s general theory of relativity, stationary black holes are completely determined by just three observable parameters: their mass, charge, and angular momentum. Almost none of the information about what fell into the black hole is visible from the outside. Physicist John Wheeler described this idea by saying that “black holes have no hair.”

In 1975, Hawking studied the behavior of quantum matter in the vicinity of a black hole and showed that black holes are not really black [2]. They emit nearly thermal radiation, just like a hot object radiates away heat. If no matter falls in, the energy lost through such “Hawking radiation” will cause the black hole to reduce its mass and eventually evaporate. This raises the question of what happens to all the information stored inside the black hole. Since the Hawking radiation comes from the surface of the black hole, which is determined by only a few parameters, Hawking argued that the information would be lost [3].

By the late 1990s, other developments in physics, most notably in string theory, convinced most researchers that all the information that falls into a black hole must come out when the black hole evaporates. How this might happen is still unclear. But one can start with a simpler question: What is wrong in Hawking’s original argument that information must be lost? The paper by Hawking, Perry, and Strominger provides a possible answer. They point out problems with two underlying assumptions that originally led Hawking to his conclusion. The first is that the vacuum in quantum gravity (the quantum state with the lowest possible energy) is unique, and the second is that black holes have no hair. Instead, they argue that there is an infinite family of degenerate vacua in the quantum theory, and that black holes can carry what the authors call “soft hair”—quantum hair associated with very-low-energy quanta.

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