If a black hole eats a book, what happens to the information? The latest work from a team of physicists says that in the distant future, the black hole eventually spits out the book's full contents. Even a black hole can't destroy information.
For decades this question has crystallised the conflict between quantum mechanics and Einstein's theory of gravity, called general relativity. Quantum mechanics insists that the information contained in quantum states must be preserved – not just the book's words, but the complete description of every atom and particle. So looking at any system today, you can in theory work out its state yesterday.
But Einstein seems to say that information can be destroyed. Anything that falls into a black hole is doomed to oblivion, because according to general relativity it will meet a "singularity" in space-time. At the singularity, gravity becomes infinite, and all structure is destroyed.
The singularity can be regarded as an edge to the universe, so whatever hits it simply ceases to exist. "The space-time of general relativity comes to an end at the singularity," says theoretical physicist Abhay Ashtekar of Pennsylvania State University in University Park, US.
Physicists believe that there is a resolution to this paradox in a quantum-mechanical theory of gravity. Although no fully-functioning theory of quantum gravity exists, it is thought that some form of quantum fuzziness in space-time should smooth out the singularity, saving information from destruction.
Now Ashtekar and colleagues Victor Taveras and Madhavan Vadararajan at Pennsylvania State have put that idea on a firmer footing. They set up quantum equations for the space-time geometry of a black hole, but in a "flatland" universe with just one space and one time dimension. "The equations are similar, and fortunately also much simpler," Ashtekar told New Scientist.
He and his team have traced the quantum state of their simplified black hole as it forms and evolves. In their model, there is no singularity, no edge to space-time, so all the information is preserved.
Eventually the black hole will slowly evaporate in a process called Hawking radiation, and the information will re-emerge. By collecting and analysing that radiation it would be possible in theory to find out what went into the black hole, and even to read any books that fell in.
"If we know the details of quantum gravity, then theoretically we will be able to run the movie backwards and say exactly how the black hole formed," says Ashtekar.
In practice, there would be a few snags. For any reasonable-sized black hole, Hawking radiation is so weak that it will take an immense amount of time to evaporate, vastly longer than the current age of the universe. And although the information would be there in principle, decoding it is liable to be unimaginably complicated.
The new work improves on some previous calculations, says quantum gravity researcher Steve Giddings of the University of California, Santa Barbara, US. But he doesn't think it settles the question. "I am not yet convinced that they have shown that the information comes out," he told New Scientist.
Journal reference: Physical Review Letters (forthcoming)
Cosmology - Keep up with the latest ideas in our special report.
By Bobby Taylor
Fri May 16 17:33:44 BST 2008
If we can see back in time 14 billion years then the universe is 28 billion years old because by the time the light got here it had another 14 billion years to expand, right? thank you......bobby taylorBy Mick Malkemus
Fri May 16 19:54:42 BST 2008
Our central location in the visible universe is 14 billion years old. If someone on the edge looks at us, they see 14 billion years in our direction, and also 14 billion years in a direction we cannot see, they cannot see the side of the universe we can see opposite them.By Charles
Mon May 19 06:46:47 BST 2008
According to David L Wiltshire, the time that has elapsed since the big bang varies according to where it is measured - because the universe is anisotropic on scales up to a few hundred million light years, there are "walls" and "voids" where matter respectively is very thin and relatively dense. In these (he reckons) time will have elapsed by several billion years more (in the void) or less (near a galaxy), and this difference will keep growing.By Graham Wilson
Sat May 17 09:19:22 BST 2008
Plus the time it took for the matter to actually get 14B light years away from the centre - somewhat more than 14B years.By Atum
Sat May 17 10:56:00 BST 2008
The matter hasnt travelled 14B light years though, the universe has expanded in this time so the distance between us and the other side of the universe has increased as the space between us has expanded.By Bpa
Sun May 18 22:25:11 BST 2008
Or perhaps the space has not expanded, but the speed of light has slowed down with everything else slowing down relative to it, as it is the upper limit of speed. There would be no way to distinguish between this happening and the space expanding, because we are measuring the amount of space by the time it takes the light to travel it. ...By David S
Fri May 16 18:32:12 BST 2008
If quantum fluctuations smear out the singularity at a black hole won't they also smear out the singularity at the start of the big bang?By Kieran
Fri May 16 20:35:05 BST 2008
You cant compare the start of the big band to now, the laws of physics were not or only just starting to be "passed" as suchBy Theon
Fri May 16 22:15:49 BST 2008
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20:00 03 July 2008
