Famed Cambridge University physicist Stephen Hawking has finally come around to accepting what many of his colleagues have thought for decades: black holes preserve information about the material that falls into them. In a lecture delivered on July 21st at a physics conference in Dublin, Ireland, Hawking outlined his reasoning and conceded defeat in a bet he made in 1997 with Caltech physicist John Preskill. Hawking bought a baseball encyclopedia for Preskill and had it shipped across the Atlantic.
Although many physicists agree with Hawking's conclusion, they don’t necessarily buy his argument. Preskill himself says he does not understand it, and Caltech physicist Kip Thorne, who sided with Hawking in the bet, is not ready to concede defeat. Mainstream media outlets have reported this story largely because of Hawking's celebrity status, not because his Dublin lecture broke new ground. "I think it is a bit overhyped," says physicist Greg Landsberg (Brown University).
What, exactly, is going on here?
In 1974 Hawking published a landmark paper titled "Black Holes Ain't So Black." Contrary to the assumption of other scientists, Hawking showed that due to quantum effects, black holes slowly radiate matter into the surrounding space — a nearly infinitesimal trickle of particles later termed "Hawking radiation." At the very end of a black hole's life, as its size has been whittled down to that of an atomic nucleus, it evaporates much more rapidly in a flood of Hawking radiation.
Most physicists quickly embraced Hawking's "black holes ain't black" idea, but Hawking himself noted a problem that has since come to be known as the "information paradox." Hawking's calculations indicated that the radiation streaming from a black hole would be featureless and seemingly random, and thus would carry no information about the material that originally formed the black hole or that later fell into it. In other words, black holes would not preserve any record of the material they swallow.
But that conclusion violated a central tenet of quantum mechanics, the extraordinarily successful theory that explains the interactions of matter and energy at subatomic scales. In quantum mechanics, it should always be possible theoretically to trace back the initial conditions of a physical system to its origin. Because this tenet plays such an important role in physical law, and has been experimentally corroborated many times, most physicists have long thought that black holes must somehow retain a memory of the material from which they formed, even if it might be impossible in practice to extract that information. As Landsberg says, "It is a basic principle of quantum mechanics that information can’t be destroyed."
A few experts in general relativity, such as Hawking and Thorne, argued that the extreme gravitational forces in a black hole would literally scrunch the information out of existence. In the view of these relativists, all of the matter in a black hole falls to the center, forming a point of zero volume and infinite density known as a singularity. The infinite gravity at the singularity destroys all information. The outer boundary of the black hole, known as the event horizon, is the region from within which light cannot escape. According to Hawking, Thorne, and others, the region between the singularity and the event horizon was empty space.
Physicists have long tried to find a way out of the information paradox. In his Dublin lecture, Hawking used a concept known as imaginary time, in which the three known dimensions of space and one of time are instead modeled as four dimensions of space, with time becoming one of them. Hawking argued that in imaginary time, black holes preserve information. But many physicists do not think the paradox can be resolved this way. "We should stick with real time, not imaginary time," says physicist Samir Mathur (Ohio State University). "The original problem gets sidetracked when we go to imaginary time."
Earlier this year, Mathur and his colleagues used string theory to show how black holes can indeed preserve information. In the March 1st issue of Nuclear Physics B, they modeled black holes as large balls of tangled strings — tiny fundamental strands of energy that constitute all matter and energy in the universe. In string theory, the universe is a symphony of strings vibrating in 10 dimensions, and the mode of each string's vibration (like the note on a violin) determines whether it is an electron, quark, photon, or some other type of particle.
Mathur's calculations show that when black holes are modeled in string theory, the singularity disappears — a comforting realization for physicists who have a strong distaste for any infinite quantity. In addition, black holes have no empty space; the strings are extremely compressed, but they fill the entire space from the singularity out to the outer boundary. And because a black hole made of strings lacks a perfect outer boundary, it behaves like any other scrunched-up ball of matter. Just as matter and energy can escape a compact, high-density object like a neutron star, matter and energy can gradually escape a black hole despite the intense gravitational forces. "This solves the information paradox, because black holes radiate energy like any other body," says Mathur.
In the relativist view, two black holes with the same mass and spin are identical. But in the string theory view, no two black holes are exactly alike, because they form from different material. "We find many different kinds of black holes, with many different internal states," says Mathur.
Mathur points to these calculations as one more indicator that string theorists are on the right track in developing a quantum theory of gravity — a "theory of everything" that reconciles the two seemingly disparate pillars of 20th-century physics: quantum mechanics and general relativity. Amazingly, both string theory and general relativity predict the exact same diameter for a black hole of any given mass — even though the two calculations are entirely independent. In addition, the calculated radiation from a black hole made of strings exactly matches the properties that quantum mechanics predicts for the Hawking radiation.
"String theory is so miraculous that whenever it comes up to a problem, it always gives us the right numbers," says Mathur. It is for reasons like this, and the internal self-consistency of string theory, that its adherents feel confident they are hot on the trail of nature's deepest secrets.
"The black hole information paradox is crucial," says Mathur. "If we can't resolve it, quantum mechanics and general relativity cannot be put together."
If some of the extra dimensions in string theory are larger than a certain size, physicists might soon be able to test these ideas in the laboratory. Starting around 2007, the Large Hadron Collider (LHC) in Switzerland will smash subatomic particles into one another at extremely high energies. Some theorists have hypothesized that if the extra dimensions are as large as a few millimeters in diameter, these particle collisions will create miniature black holes, which will almost instantaneously evaporate in bursts of Hawking radiation. By analyzing the particles in the Hawking radiation, physicists might be able to determine the fundamental nature of black holes and whether the Hawking radiation carries information about the material that formed them. "I bet if mini-black holes are produced at the LHC or future colliders, we will be able to study Hawking radiation to death and answer all of these impending questions," says Landsberg.
But many physicists, Mathur included, remain skeptical that the extra dimensions in string theory are large enough to enable black holes to be created at the LHC. If so, it might be centuries before experiments shed light on the information paradox.