A month after Stephen Hawking and his colleagues published a paper on black holes, physicists are still struggling to come to a consensus. Some hail his latest work as a fresh way to solve the black hole puzzle; others are unsure of her authority. The former support the preprint's claim that it provides a promising way to solve the mystery of the so-called black hole information paradox that Hawking deduced over 40 years ago.
“I think there is a general excitement that we can look at familiar things in a different way, we will break the deadlock,” says Andrew Strominger, a physicist at Harvard University in Cambridge, co-author of one of the latest work. Strominger presented the results of his work on January 18, 2016 at the University of Cambridge, where Hawking is based.
Many are not convinced that this approach can solve the paradox, although they admit that it illuminates various problems in physics. In the mid-70s, Hawking discovered that black holes are not completely black, but emit a little radiation. According to quantum physics, from quantum fluctuations just beyond the event horizon - the point of no return of a black hole - pairs of particles should arise. Some of these particles leave the black hole's gravity, but carry away some of its mass, resulting in the black hole slowly contracting and eventually disappearing.
In a paper published in 1976, Hawking indicated that the escaping particles - now known as Hawking radiation - would have completely random properties. As a result, when the black hole disappears, the information stored in it will be lost to the Universe. But this result does not fit with the laws of physics, according to which information, like energy, is conserved, which gives rise to a paradox. "This work has caused more sleepless nights for theoretical physicists than any other work in history," Strominger recalled.
It was a mistake, he explained, to ignore the potential of empty space to carry information. In his work, along with Hawking and third co-author Malcolm Perry, also from the University of Cambridge, he turns to soft particles. These are low-energy versions of photons, hypothetical particles known as gravitons, and other particles. Until recently, they were mainly used for calculations in particle physics. But the authors note that the vacuum in which the black hole is located does not have to be devoid of particles - only energy - and therefore soft particles can be present in it in a zero energy state.
Everything that falls into a black hole, they continue, leaves an imprint - an imprint - on these particles. “If you are in a vacuum and breathe in - suppose you are breathing in a lot of soft gravitons,” Strominger says. After this disturbance, the vacuum around the black hole changes, and the information is saved in the end.
The paper goes on to propose a mechanism for transferring this information into the black hole - which theoretically resolves the paradox. To do this, the authors calculated how to decode the data in a quantum description of the event horizon, known as the "hair of a black hole."
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Tricky transition
Nevertheless, the work is far from complete. Abhay Ashtekara, who studies gravity at the University of Pennsylvania at University Park, says he finds the authors' way of transmitting information into a black hole ("soft hair") unconvincing. And the authors admit that they do not yet know how this information could subsequently be transmitted with Hawking radiation, and this is a necessary next step.
Stephen Avery, a theoretical physicist at Brown University in Providence, Rhode Island, is skeptical about the possibility of this approach solving the paradox, but he definitely believes that it will expand the meaning of soft particles. He notes that Strominger discovered that soft particles reveal subtle symmetries of known forces of nature, "some of which are known to us and some of which are new."
Other physicists are more optimistic about the prospects of this method in solving the information paradox. Sabine Hossenfelder of the Institute for Advanced Study in Germany says the results from "soft hair", coupled with her own research, could resolve controversies surrounding black holes like the firewall problem. You see, there is the question of whether the event horizon can become extremely hot due to Hawking radiation. This contradicts Einstein's general relativity, according to which an observer falling through the horizon would not notice sudden changes in the environment.
“If the vacuum has different states,” says Hossenfelder, “then you can transmit information into radiation without putting any energy on the horizon. Therefore, there will be no firewall."