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THEY are the most fearsome objects in the universe. They swallow and destroy everything that crosses their path. Everyone knows that falling into a black hole spells doom. Or does it? In the past few years, cracks have started to appear in the conventional picture. Researchers on the quest for a more complete understanding of our universe are finding that black holes are not so black, and perhaps not holes either. Furious debates are raging over what black holes contain and even whether they deserve the name.

The term “black hole” was coined in the 1960s by physicist John Wheeler to describe what happens when matter is piled into an infinitely dense point in space-time. When a star runs out of nuclear fuel, for example, the waste that remains collapses in on itself, fast and hard. The gravitational attraction of this matter can overwhelm its natural tendency to repel itself. If the star is big enough, the result will be a singularity. Around the singularity lies an event horizon, a point of no return. Light cannot escape once it passes beyond this boundary, and the eventual fate of everything within it is to be crushed into the singularity.

But this picture always contained the seeds of its own destruction. In 1975 Stephen Hawking at the University of Cambridge calculated that black holes would slowly but inexorably evaporate. According to the laws of quantum mechanics, pairs of “virtual” particles and antiparticles continually bubble up in empty space. Hawking showed that the gravitational energy of the black hole could be lent to virtual particles near the event horizon. These could then become real, and escape, carrying away positive energy in the form of “Hawking radiation”. Over time, the black hole will bleed away into outer space.

This led to a problem dubbed the information paradox. While relativity seems to suggest that information about matter falling into a black hole would be lost, quantum mechanics seemed to be suggesting it would eventually escape. Hawking claimed the random nature of Hawking radiation meant that while energy could escape, information could not. But last summer, he changed his mind (91av, 17 July 2004, p 11). His reversal was just one part of a larger movement to rethink the rules that govern black holes.

Much of the impetus for this rethink comes from string theory, our best attempt to unify general relativity and quantum mechanics. Now 20 years old, string theory posits that space-time, and everything in it, is composed of vibrating strings so small we will be lucky ever to get evidence of their existence. Its big appeal is the promise that it could unite general relativity and quantum mechanics, because one type of string carries the force of gravity, while the vibration of the strings is random, as predicted by quantum mechanics.

String theory was first applied to black holes in the mid-1990s. Andrew Strominger and Cumrun Vafa of Harvard University began to work on the information paradox by imagining what the inside of a black hole might be like. The researchers found that string theory would allow them to build highly dense little structures from strings and other objects in string theory, some of which had more than three dimensions. These structures worked just like black holes: their gravitational pull prevents light escaping from them.

“The number of ways strings can be arranged in black holes is amazingly large”

Strominger and Vafa counted how many ways the strings in these black holes could be arranged, and found this was amazingly large.

The calculation was heralded as a huge validation for string theory. In the 1970s, Hawking and Jacob Bekenstein, then at Princeton University, had calculated the entropy of a black hole using quantum mechanics. The entropy of an object is roughly a measure of the amount of information it can contain. In particular, it measures the number of different ways the parts making up an object can be arranged. It just so happened that the number of ways that Strominger and Vafa calculated that strings could be arranged in a black hole exactly matched the entropy calculated by Hawking and Bekenstein.

Fuzzballs

But this did not tell physicists how those strings were arranged. Over the past year, Samir Mathur of Ohio State University and his colleagues have begun to look at what string configurations there could be in black holes. They found that the strings would always connect together to form a large, very floppy string, which would be much larger than a point-size singularity.

Mathur’s group calculated the total physical sizes of several stringy black holes, which they prefer to call “fuzzballs” or “stringy stars”. To his surprise, they found they were the same size as the event horizon is in traditional theory. “It is changing our picture of the black hole interior,” says Mathur. “It would really mean the picture of the round hole with a black dot in the centre is wrong.”

Mathur’s fuzzball does away with the idea of the event horizon as a sharp boundary. In the traditional view, the event horizon is a well-defined limit. Objects passing particular points in space at particular moments in time are guaranteed to end up being pulverised at the black hole singularity. In the fuzzball picture, the event horizon is a frothing mass of strings, not a sharp boundary.

The fuzzball picture also challenges the idea that a black hole destroys information. In Mathur’s description, there is no singularity. The mass of strings reaches all the way to the fuzzy event horizon. This means information can be stored in the strings and imprinted on outgoing Hawking radiation.

So what happens to the information that falls into a black hole? Imagine pouring cream into black coffee. Drop the coffee and cream into the old-style black hole and they will go to the singularity and be lost. You will never see the results of the mixture. But drop your coffee and cream onto a Mathur fuzzball and information about the cream-coffee mixture will be encoded into string vibrations. Hawking radiation that comes out can carry detailed information about what happened to each particle of cream and every particle of coffee. “There’s no information problem. It’s like any other ball of cotton,” says Mathur. This picture is very preliminary, cautions Vafa. Mathur has not yet calculated exactly how his model applies to large black holes or understood how a black hole evolves over time.

Gary Horowitz of the University of California, Santa Barbara, and Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey, also recently proposed that information can get out of a black hole. But unlike Mathur, they believe that black holes do contain a singularity at their heart. They suggested that information might escape by means of quantum teleportation. This allows the state of one particle to be instantly teleported to another. So Horowitz and Maldacena suggested that information could pass from matter hitting the singularity to outgoing Hawking radiation.

“The most information any black hole would possibly retain is just half a bit – everything else will eventually escape”

But to make their calculation work, they had to assume that infalling matter and outgoing radiation would not collide with each other. If they did, this could disrupt the teleportation process. Quantum information theorists Daniel Gottesman of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and John Preskill of the California Institute of Technology in Pasadena say such disruption could occur very easily.

That seems to raise a problem for Horowitz and Maldacena. But last summer, Seth Lloyd of the Massachusetts Institute of Technology worked out that all such disruptions would actually cancel each other out. Then Lloyd calculated that the most information a black hole would possibly retain permanently was just half a bit – everything else will eventually escape. This applies to all black holes, whether they are supermassive ones at the heart of a galaxy (see “Giants of the universe”) or mini black holes created in a particle accelerator (see “Baby black holes”).

But Gottesman and Preskill have a second criticism that might be more fatal to the teleportation picture. They showed that the effect could allow faster-than-light communication, which is taboo in relativity. The teleportation calculation relies on the assumption that every piece of matter inside a black hole has the same quantum state. Although quantum mechanics allows one particle to have an instantaneous effect on the quantum state of another, this cannot be used to communicate. For example, if one person, Alice, measures the quantum state of a particle that is linked with a particle held by her friend, Bob, the effect of this measurement will be instantaneously communicated to Bob’s, but there is no way to use this to communicate faster than light, because Alice needs to tell Bob what kind of measurement process she carried out on her particle, before he can decode the meaning of the change he sees. That information has to travel to Bob in the normal way.

Black hole communication

However, if Alice throws her particle into a black hole, the researchers found the measurement will be immediately constrained to the quantum state of the black hole. This would have an effect on Bob’s particle that he could determine without needing the extra information from Alice. Gottesman concludes that the teleportation idea cannot work very well. Indeed, he wonders if the framing of the information paradox is wrong in a way that is not yet understood. “My own guess is somehow we’re asking a stupid question,” he says.

The scenario also bothers quantum-gravity theorist Ted Jacobson of the University of Maryland, who still believes that the information that falls into black holes is lost forever to those outside the black hole. He finds the teleportation picture particularly unconvincing. “I put it in the category of desperate attempts to make information come out,” he says. And even the researchers themselves aren’t sure they are right. “We suggested one possibility,” says Horowitz, but he admits it doesn’t have a good basis in string theory yet. “So I can’t say we are confident this is the right picture.”

Jacobson argues that the connection between the outside and inside of a black hole is so complicated in string theory that no one can be sure they have ruled out the possibility of information leaking out of our space-time. People may be simply assuming the conclusion that they want for their own reasons. “I see no problem with letting the darn stuff fall down the drain. Why are people so afraid of the singularity?”

The problem, says Vafa, is that the concept of information could be very subtle in string theory, and not yet well-defined. “Information loss is a critical question, but our understanding of black holes is too primitive.”

So whether information can escape from black holes or is destroyed remains a topic of intense debate. But there might turn out to be a third option. One competing theory to string theory is called loop quantum gravity, pioneered by, among others, Lee Smolin of the Perimeter Institute in Waterloo, Canada. It proposes that space-time is constructed of loops even smaller than strings. Joining loops together creates a mesh of nodes and branches called a spin network. The advantage of this model is that space-time itself can be built out of these networks instead of having to be assumed, as it is in string theory.

Abhay Ashtekar of Pennsylvania State University in Pittsburgh and Martin Bojowald of the Max Planck Institute for Gravitational Physics in Golm, Germany, have studied a model of a black hole created using spin networks. They found the equations that describe space-time continue to apply in an orderly way even at the singularity itself. This is very different to the conventional picture, in which the equations of physics break down when space-time collapses. It means that information that reaches the singularity could survive there, encoded in the spin networks. As far as Ashtekar and Bojowald can tell, the information trapped in a black hole will be unable to escape via Hawking radiation. Wait long enough, however, and it will survive, eventually rejoining the rest of the universe when the black hole evaporates.

So whatever the theory that eventually supersedes relativity, it seems a good possibility that black holes may be just a little less dramatic than we thought. After all, who’s afraid of a big ball of string?