Can information get back out of black holes?

A new theory predicts that not everything is swallowed up forever.

Maybe physics doesn't break down inside a black hole after all. Credit: NASA

If you were sucked into a black hole, you wouldn't stand a chance. But new calculations suggest that some things might survive travelling to the heart of the Universe's darkest objects.

'Quantum information' could make it through a black hole, says a group of theorists at Pennsylvania State University. If their calculation holds water, it would solve an important problem for quantum mechanics — and make the behaviour of black holes easier to predict.

Black holes have a dastardly reputation for devouring everything they come across. Anything that travels beyond a hole's 'event horizon' — the boundary of the region where gravity is so strong that not even light can escape — will eventually fall into its centre.

And at the black hole's centre lies the 'singularity', a single point where mass becomes infinite and the laws of gravity break down.

A central problem

Singularities pose a problem for quantum mechanics, according to Abhay Ashtekar, who is based at the University Park campus of the Pennsylvania State University. A basic principle of quantum theory is that a system's present-day state can be used to figure out what it looked like in the past. In other words, a theorist can use equations to 'rewind' a group of particles and see how they got where they are.

Singularities destroy this option, because once information goes in, it can't come out, Ashtekar explains. It may sound like a largely academic problem, but it could have real consequences, particularly because some theories predict that tiny black holes might be created by the supermassive energies that will be generated at the Large Hadron Collider, a giant particle accelerator approaching completion at CERN near Geneva, Switzerland.

These 'mini black holes' will evaporate before they grow too large, but even their momentary existence could destroy valuable information and confound physicists' efforts to discover new particles. "This soaking-up of information casts a long shadow," Ashtekar says.

To get around the problem, Ashtekar and his colleagues rearranged space and time (but thankfully only in theory). Einstein's theory of gravity views space-time as continuous and unbroken, like a sheet of paper. Ashtekar's team took the analogy further still. Just as a sheet of paper is made up of atoms, they theorized that space-time itself might be made of billions of building blocks. Like the paper, space-time would seem to be continuous from afar — but up close, one would be able to see its individual units.

In and out

With this assumption in hand, the team recalculated what the centre of a black hole might look like. To simplify their equations, they used only two dimensions instead of the three dimensions of space and one of time that exist in our Universe. In a two-dimensional system, they found that the singularity vanished and was replaced by a bizarre region where quantum fluctuations ran wild.

Space-time in that part of the hole would become so unpredictable that all conventional ideas of cause and effect would break down. "Classical intuition fails in that region, but quantum mechanics is definitely happy," says Ashtekar, who will report the results in the 20 May issue of Physical Review Letters1.

If black holes behave in the way Ashtekar predicts, information will never be lost and quantum mechanics will continue to function, even in the extreme environment beyond the event horizon.

'One foot in reality'

It's an interesting idea that seems to have "at least one foot in reality", comments Bill Unruh, a gravitational theorist at the University of British Columbia in Vancouver, Canada. Importantly, Ashtekar's visualization predicts other properties of a hole, he says.

But Unruh adds: "The whole calculation rests on some very particular properties of two-dimensional space-time." It's far from clear whether it could be carried over into the four dimensions in which we live, Unruh says.

"The theory is still in its infancy," Ashtekar admits. But he adds that he is confident it will work in a four-dimensional world. And if it does, he says, it will bring physicists closer to understanding how quantum mechanics and gravity interact.


  1. 1

    Ashtekar, A., Taveras, V. & Varadarajan, M. Phys. Rev. Lett. doi:xxxx (in the press). [note: please provide doi, couldn't check publication date]

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Brumfiel, G. Can information get back out of black holes?. Nature (2008) doi:10.1038/news.2008.833

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