Black holes have an information problem. According to the laws of quantum mechanics, information about the state of a closed system cannot be destroyed, but black holes seem to obliterate it. Researchers have been trying for decades to solve this problem, called the black hole information paradox, and now one team claims to have finally figured it out.
The black hole information paradox was born in the 1970s when Stephen Hawking calculated that black holes should slowly evaporate by emitting random particles in what is now called Hawking radiation. This suggests that any information-containing matter that falls into a black hole would be destroyed as the black hole eventually shrinks to nothing. The problem there is that the laws of quantum mechanics require that if you know the state of any closed system at one time, you should be able to聽work out its state forward or backward in time 鈥� but if Hawking radiation is indeed random, that becomes impossible.
at the University of Sussex in the UK and his colleagues claim that they have solved the problem using a framework called quantum field theory. 鈥淲e have redone the calculation that Hawking did in the 1970s, but we have taken into account quantum gravity,鈥� says Calmet. 鈥淭he black hole information paradox is solved now, and we understand the physics of it.鈥�
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In earlier work, the researchers found that when they applied quantum mechanical corrections to calculations of stars evolving into black holes, the black holes鈥� gravitational fields would preserve information about what fell in. Now, they claim to have worked out what happens to that information as the black hole evaporates.
This problem is difficult to solve because of the way the information is distributed as it leaks out. 鈥淚magine your information is made of Lego pieces, and then you鈥檝e got a black hole made of Lego, and then over the course of the age of the universe, piece by piece, individual Lego pieces come out,鈥� says at King鈥檚 College London. 鈥淵ou put a huge amount of information in, and it comes out again so slowly, and in such small pieces, that you have to dig really deep into the theory鈥� to figure out how the information that fell in relates to what is coming out, he says.
Calmet and his team calculated that the gravitational field of the black hole should very slightly modify the energy spectrum of the Hawking radiation that emerges. 鈥淚t鈥檚 a tiny effect, but it means that the spectrum contains information,鈥� he says. Traditionally, Hawking radiation is thought to be random, so any order in its spectrum could allow information to leak out of the black hole as it evaporates and be preserved.
However, some other researchers in the field aren鈥檛 satisfied yet. 鈥淭his doesn鈥檛 fix the problem,鈥� says at the Massachusetts Institute of Technology.聽The objections largely boil down to the idea that, ironically, this theory does not have enough information about how exactly the information is preserved, particularly when the black hole evaporates completely. 鈥淚 don鈥檛 think it鈥檚 gotten any traction in the community, because I don鈥檛 think it鈥檚 precise enough,鈥� says Lambert.
鈥淭o resolve the black hole information problem requires a major change in our understanding of how space and time emerge from the theory of quantum gravity 鈥� I don鈥檛 think you鈥檙e going to get it by using just the standard laws of physics and quantum field theory,鈥� says Harlow.
Testing this work will be difficult because Hawking radiation is such a minuscule effect. 鈥淧ractically, honestly, it鈥檚 not measurable 鈥� we鈥檝e never seen Hawking radiation from a real black hole,鈥� says Calmet. 鈥淗awking radiation will never be measured in astrophysics, I think, but there are ways to build analogues to black holes where you can model Hawking radiation.鈥� Perhaps these analogues will provide resolution, he says.
Physics Letters B
