Imagine that you want to make something disappear—that unfortunate photograph of you in the sombrero, or the ill-advised iPhone video from your bachelor party, or that part of your seventh-grade diary describing your dream honeymoon with Kirk Cameron. Whatever it is, you want it gone.
You could shred it, but a motivated blackmailer could still piece it back together. You could burn it, but the laws of physics still promise that the information could be reassembled. So you decide to turn to the ultimate destruction: You launch that mortifying evidence right into a black hole and breathe a sigh of relief that now, finally, it is gone for good.
But is it really?
That question is at the heart of a problem that’s been called the black hole information paradox, and some theorists believe that it reveals a deep crack in the foundation of physics as we know it.
The pull of gravity inside a black hole is so strong that nothing can escape its grip. No one can fish your diary out from beyond the lip of the black hole; gravity’s pull on the fishing line would overpower your reel every time. And if a tech-savvy enemy tried to remotely link up with your iPhone and retrieve that embarrassing video, even the electromagnetic waves from your phone would still be trapped inside the black hole. To escape, they would somehow have to travel faster than the speed of light—and that is strictly forbidden by Einstein’s relativity.
But there is a hitch. Quantum mechanics has an equally strong rule that prohibits the loss of information. This principle, called unitarity, is intimately linked with other unbreakable laws of physics, like conservation of energy. “Conservation of information is what holds the world together,” says Steve Giddings, a physicist at the University of California, Santa Barbara.
To emphasize just how important information conservation is, Stanford physicist Leonard Susskind calls it the “minus-first” law of physics—“minus-first because I think it comes before everything else,” said Susskind in an online discussion sponsored by the Kavli Foundation. “If it’s true [that information conservation is violated], we go back to minus-first base.”
Now, you might argue that the information in the black hole isn’t truly lost. It’s just locked up and inaccessible. Theorists contented themselves with this view until 1975, when Stephen Hawking drew a revolutionary conclusion about black holes: Given enough time, a black hole will dematerialize, radiating away through a process we now call Hawking evaporation. And, according to Hawking’s accounting, that radiation would be random, revealing nothing of the black hole’s contents.
As Leonard Susskind wrote in “The Black Hole War,” his 2008 book on the problem of black holes and information loss, “The possibility of hiding information in a vault would hardly be a cause for alarm, but what if when the door was shut, the vault evaporated right in front of your eyes? That’s exactly what Hawking predicted would happen to the black hole.”
There is another possibility, though: Maybe this evaporation isn’t complete. Maybe it leaves behind a tiny ember that contains an enormously compressed version of all the information that ever fell into the black hole. “But this leads to some pretty crazy conclusions, too” says Giddings—specifically, you need to find a way for a single particle to take on infinitely many forms which, Giddings says, “results in equally disastrous consequences.”
This left physicists stuck between a rock and a hard place: Either information could be lost, or somehow something could escape from a black hole. A central tenet of quantum mechanics was pitted against the cornerstone of relativity. One theory, it seemed, had to give.
The debate went public in 1997, when Stephen Hawking and theoretical physicist Kip Thorne made a bet with John Preskill of Caltech that it would ultimately be shown that information was truly lost inside black holes. At stake: one encyclopedia of the winner’s choice, from “which information can be recovered at will.”
At the same time, and out of the media spotlight, string theorists were exploring a remarkable duality in their equations. They found that if you take a mathematical description of a system and add an extra spatial dimension and a negative curvature, you have something that looks very much like quantum fields in a three-dimensional universe without gravity. This observation sounds esoteric, but it gave mathematical chops to an idea called the holographic principle, which puts forward that all the information in our three (spatial) dimensional universe can be “stored” on a two-dimensional surface. In the context of the black hole information paradox, this suggested that information about the stuff in the black hole could somehow be encoded on the surface of the event horizon.
Still, the encyclopedia remained unclaimed as the bet dragged on, until 2004, when Hawking announced that he had changed his mind and was ready to concede. (Preskill’s prize: a baseball encyclopedia.) “My views have evolved,” he told Nature News. He published his results the following year.
To some theorists, Hawking’s concession came many years too late. “Stephen Hawking was like one of those unfortunate soldiers who wander in the jungle for years, not knowing that the hostilities have ended,” wrote Susskind in “The Black Hole War.” But others found Hawking’s explanation unsatisfying and, moreover, were irked at the public perception that one man’s change of heart had truly settled a debate that was still actively raging in the physics community. Still, there was a growing, if uneasy, consensus that this mathematical expression of the holographic principle, dubbed the anti-de Sitter/conformal field theory, or AdS/CFT, duality, pointed the way to a solution to the information problem, even if the roadmap was incomplete.
But now, we’ve hit another roadblock. A team of physicists out of the University of California, Santa Barbara has shown that if information thrown into the black hole is conserved, one of two other “unbreakable” rules of physics must give out. The first rule is that, once you’re a reasonable distance away from a black hole, the laws of physics work as usual. The second is that someone falling into a black hole would experience nothing “special” at the event horizon.
It’s that second rule, the Santa Barbara team argues, that is the weakest link in the chain. To protect the remaining postulate, they are willing to accept that something shocking might exist at the horizon: a “firewall” of broiling radiation that burns all that passes into a fiery crisp. Do they really believe that these firewalls exist? Probably not. But the possibility is enough to light a new fire (pardon the pun) under theorists.
Paradoxes and problems, after all, can drive great new discoveries. And, as science writer Jennifer Ouelette writes, “It comes at a time when theorists are hungry for a new challenge,” thanks to the maddening neatness of the Higgs result and physicists’ inability to spot cracks in the armor of the Standard Model.
So, back to that embarrassing thing you wanted to dump into the nearest black hole. Will it really be lost and gone forever? Will it burn up in a firewall, or be rewritten on a quantum screen at the edge of the universe? With the jury still out, perhaps you’ll do better just to hide it under the mattress.
Author's picks for further reading
arXiv: Black Holes: Complementarity or Firewalls?
The 2012 paper that laid out the firewall problem.
Cosmic Variance: Joe Polchinski on Black Holes, Complementarity, and Firewalls
Go inside the information paradox with physicist Joe Polchinski, one of the authors of the paper that sparked the firewall controversy.
John Preskill: On Hawking’s Concession
The winner of the famous bet on the information problem and the value of scientific wagers.
Physics Today: Black holes, quantum information, and the foundations of physics
In this comprehensive essay, Steve Giddings outlines the information problem and several proposed solutions.