The Truth Is Still Out There
The Truth Is Still Out There
By Paul Ginsparg
Stephen Hawking's recent concession that black holes do not irretrievably eradicate information after all has garnered much attention. It is refreshing to see the public focused, if just for a moment, on an important conundrum that has fascinated theoretical physicists for three decades, and prompted much conceptual progress. The scientific issues, however, remain much less settled than Dr. Hawking's celebrated wager on the question. He most recently pronounced: "If you jump into a black hole, your mass energy will be returned to our universe, but in a mangled form, which contains information about what you were like but in an unrecognizable state."
To appreciate why this is different from awakening after any night's sleep requires a brief reprise of 20th century physics. Einstein's theory of general relativity in 1915 was the culmination of centuries of classical physics, and Einstein and others soon found solutions to his gravitational field equations. One of these solutions was later termed a "black hole" in the 1960's, since it describes the gravitational field produced by an object so dense that nothing can escape, not even light. Indeed, the existence of black holes is inferred only through their gravitational effects on other astronomical bodies. Stars recently detected orbiting very close to the center of our own Milky Way galaxy, for example, suggest the existence of a supermassive black hole at the center, almost three million times the mass of our sun and about five million miles in radius. If the mass of the entire Earth were compressed into a black hole, it would be a little ball only a third of an inch in radius. Fortunately, the Earth is in no imminent danger of collapse because of the electrostatic repulsion of its constituent atoms.
Quantum mechanics, which describes the behavior of very small objects like atoms, blossomed a decade after general relativity, and the two are notoriously difficult to reconcile. Thirty years ago, Dr. Hawking published a calculation incorporating some quantum mechanical effects into black hole physics, and showed that matter or energy could leak from a black hole. While surprising, this was not paradoxical since there are examples of processes forbidden by classical physics but allowed by quantum mechanics. Shortly afterward, however, Dr. Hawking articulated a more shocking consequence of his calculation.
One of the central tenets of relativity theory, termed causality, is that nothing, not even information, can travel faster than the speed of light. This means that as long as a black hole exists, no information about objects that had fallen into it can ever emerge. Therefore, according to Dr. Hawking's original calculation, the radiation emitted quantum mechanically from a black hole is generic, in the sense that it conveys no information. Further, if a black hole were permitted to evaporate entirely, then the information content of any objects previously ingested by it would vanish from the universe, without a trace. By contrast, if you throw your diary into a fireplace, then the information contained therein could be reconstructed, at least in principle, from subtle properties of the resulting smoke and flames. A permanent loss of information because of black hole evaporation, on the other hand, is in contradiction with one of the central tenets of quantum mechanics, termed unitarity, which permits tracking information flow in all such processes and forbids its disappearance.
In the early 1980's, I was fortunate to attend some of Dr. Hawking's lectures in which he speculated on ways to modify quantum mechanics to accommodate this potential loss of information. He stimulated much debate among quantum field theorists, who in turn enjoyed working to rebut his arguments. The black hole information paradox thereby emerged as an important catalyst toward further theoretical progress in reconciling gravitational and quantum effects. Despite many new ideas and progress on other fronts, no definitive resolution emerged.
Near the end of a small meeting I attended in 1993, the question of "What happens to information that falls into a black hole?" arose, and a democratic method was chosen to address it. The vote proceeded more or less along party lines, with the general relativists firm in their adherence to causality, and the quantum field theorists equally adamant in their faith in unitarity. Of the 77 participants, 25 voted for the category "It's lost;'' and 39, a slight majority, voted for "It comes out,'' (that it re-emerges). Seven voted that the black hole would not evaporate entirely, and the remaining six voted for an unspecified "Something else." I voted with the majority, anticipating progress and hoping that one of us would soon perform a calculation to help Dr. Hawking and the relativists see the light. But with the question still unresolved four years later, three of the protagonists eschewed the old political duel-to-the-death methodology for a variety of practical reasons, settling instead on a simple wager whose unsatisfying outcome was announced last month.
It once was that important scientific results were presented to the general public only after they were subjected to peer review and accepted for publication in an edited journal. Some professional journals, particularly in medicine, still refuse to publish results that have already been announced via press release. Since the early 1990's, articles in many fields have nonetheless been publicly available in "prepublication" form through organized Internet repositories, a type of instant communication frequently concurrent with peer review.
The recent "resolution" of the information puzzle, however, has neither supporting publication nor calculation, peer-reviewed or otherwise. While a press release may be sufficient in some realms of human endeavor, one of the joys of scientific research is that it is subject to more objective measures of progress. It is possible that some new revolutionary mechanism to avoid information loss will yet emerge from the latest spectacle. But without even a hint yet as to what might have been missing from Dr. Hawking's original calculation, it is more likely that theoretical physicists will continue to view the information paradox as a profound puzzle whose resolution will provide clues to understanding the basic laws of physics.
String theory, a parallel quantum gravitational effort over the past 30 years, offers many tantalizing hints toward possible resolution of the puzzle. Perhaps some of its ideas have subconsciously persuaded Dr. Hawking to join the quantum conservation of information camp. But should we ultimately be more inclined to trust Dr. Hawking's past youthful intuition? Physicists, particularly eminent British ones, have a historical tendency to stray in excessively speculative directions in their later years. A bigger surprise may yet await us, and those who voted for "Something else'' may prove the most prescient. Perhaps the real winners of this bet will be some middle-school students who, inspired by the current hoopla, will help provide a more substantive answer a decade from now.
Paul Ginsparg, professor of physics and information science at Cornell University, was named aMacArthur fellow in 2002.
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