The physics world is abuzz this week with news that Stephen Hawking has solved the famous black hole information paradox—and that he has even discovered “a way to escape from a black hole.” The giddy announcements are somewhat premature, however—this paradox looks like it has staying power.
Hawking, a physicist at the University of Cambridge, first uncovered the conundrum in the 1970s when he predicted that black holes—supposedly inescapable gravitational pits—actually leak light, called Hawking radiation. Over time a black hole can theoretically emit so much radiation that it completely evaporates. That outcome, however, presents a problem because it seems to suggest that black holes destroy information—a definite nonstarter according to the theory of quantum mechanics.
Black holes, like everything else, should preserve a quantum mechanical record of their formation. A black hole may arise, for example, from the death of a large star that has run out of fuel for nuclear fusion and collapsed under its own gravity. According to quantum mechanics, the black hole should store the information about the star that gave birth to it as well as any matter that has fallen in since. But if the black hole someday evaporates, it would seem that information would be destroyed.
Physicists have tried to find a way for the information to escape the black hole’s demise via the Hawking radiation. The problem with this scenario, however, is that black holes appear to have no way to impart information to this radiation. Black holes, in fact, are very simple objects according to the theory of general relativity, which first predicted their existence. They have only three properties: mass, charge and angular momentum; other than those quantities, they have no characteristics, no other details—in physicists’ vernacular, they have “no hair.”
Hawking unveiled a potential “answer” to the information-loss paradox—a way to give black holes hair—during a presentation given at the KTH Royal Institute of Technology in Stockholm on August 25: “I propose that the information is stored not in the interior of the black hole as one might expect but on its boundary, the event horizon,” he said. The event horizon is the theoretical border of a black hole, a spherical “point of no return” for incoming matter. Hawking further suggested that the information resides in so-called “supertranslations” on the event horizon, which are imprints that would cause a shift in the position or the timing of the particles that are emitted via Hawking radiation. These supertranslations would be formed by the particles of the dead star and any other matter that fell into the black hole when they first crossed the event horizon. Hawking admitted that the information would not be readily retrievable but maintained that it at least would not be destroyed, thereby resolving the paradox. “The information about the ingoing particles is returned but in a chaotically useless form,” he said. “For all practical purposes the information is lost.”
A “greater state of confusion”
Most physicists say it is too early to know whether Hawking’s idea is a real step forward. His presentation was brief; he and two collaborators—Cambridge physicist Malcolm Perry and Andrew Strominger of Harvard University—plan to publish a paper in coming months detailing their idea further. “I think [the idea] has promise,” says Sabine Hossenfelder, a physicist at the Nordic Institute for Theoretical Physics who attended the talk. “But so far it is not a full solution.”
Hawking described the basics behind his idea that supertranslations can encode information. “That may be,” Hossenfelder adds, “but it is somewhat unclear right now how this happens and how efficiently it happens. Also, the mechanism they have to store information actually allows them to store too much information!”
And supertranslations are hardly the only solution on the table. In recent years physicists have come up with a host of ideas to solve—or further complicate—the information-loss paradox. “To be completely honest I must say that [the paradox] is in an even bigger confusion now than it has ever been before,” observes physicist Ulf Danielsson of Sweden’s Uppsala University, who was in attendance for the presentation. “With Hawking saying that he has solved the information paradox, to me that means now there’s another ingredient that is coming in, and the question is: Will this actually resolve anything or just leave us in an even greater state of confusion? I’m not really sure.”
Whatever happens to Hawking’s scenario, the topic will continue to be a hot-button issue in physics. The question is not just an arcane consideration about black holes—it is deeply tied to larger mysteries about the nature and origin of the universe. And to answer the question physicists will probably need not just a better understanding of black holes but a full theory of quantum gravity—a theory that has so far been missing.
Black holes are perplexing objects in part because they invoke two different theories of nature—quantum mechanics, which governs the subatomic world, and general relativity, which describes gravity and reigns on large cosmic scales. Yet the two theories are fundamentally incompatible. What physicists need is a way to describe gravity according to quantum rules. By invoking both quantum mechanics and relativity, the information-loss paradox “gives us a chance to focus what we know and what we don’t know and to try to work out the implications of different hypotheses about quantum gravity,” says physicist Lee Smolin of the Perimeter Institute for Theoretical Physics in Ontario.
Smolin and Hossenfelder recently collaborated on a review paper that summarized all the various possible solutions to the information-loss puzzle and concluded that they mostly fall into six categories, each taking a different tack to resolve the apparent paradox. One possibility is that information really is destroyed—perhaps that prohibition of quantum mechanics is wrong. Another is that inside a black hole a new region of spacetime forms a sort of baby universe, in which information is preserved. Other solutions involve theoretical objects called “white holes”—the opposite of black holes, in which the flow of time is reversed and nothing can fall in, only out (information included). Then there is the chance that black holes never quite evaporate—they only shrink down to incredibly small sizes, thereby preserving the information. Or perhaps information is somehow copied from inside a black hole to outside, so that when the black hole is destroyed the outside copy remains. And finally there are proposals in which information is encoded on a black hole’s horizon in various ways—Hawking’s idea falls into this category. “I think the real situation is unfortunately that we have a puzzle and we have several ways out and we just don’t know enough,” Smolin says. “It might even be that in nature there are different kinds of black holes and some resolve the puzzle in one way and others resolve it in another.”
However the solution turns out, it may affect not just black holes but also a theoretically related event—the big bang. The small, dense state of black holes is very similar to the presumed situation of our universe at its birth, and many of the same physical considerations apply. In both cases the mathematics currently predict a “singularity”—a point of spacetime that is infinitely dense and infinitely small. Some physicists say these infinities are proof that the equations are wrong whereas others maintain that the singularity is a physical reality. If the resolution of the information-loss paradox comes from a quantum theory of gravity that eliminates the singularity, it could imply a different origin for our universe. “Is there still a first moment of time,” Smolin asks, “or does the singularity get eliminated and turn into a bounce so that there was an era of the universe before the big bang?”