As has been recently reported online, (well, in some places, and confusingly in others), Stephen Hawking and his collaborators have a new theory (see also this brief, more easily understandable synopsis) about black holes that might solve one of the more serious problems with their existence. Basically, Hawking is trying to solve a problem he created. The problem, essentially, is that the quantum mechanical states of subatomic particles are supposed to be a conserved quantity that’s neither created nor destroyed, but one of Stephen Hawking’s most famous theories suggests that black holes destroy that information – once it falls into the black hole the information was just gone. So, either black holes violated one of the fundamental laws of physics, the law is not a fundamental law of physics, or our understanding of black holes is wrong.
Needless to say, the preferred solution is that our understanding of black holes is wrong; quantum-mechanical information CAN be preserved in and retrieved from a black hole. This theory is the latest attempt to explain exactly how it’s preserved.
When confronted with black holes, the physicist John Archibald Wheeler (although Wheeler says someone else coined the phrase) said “black holes have no hair”. Presumably he meant that in the sense that we have nothing to grasp to understand what they are. This is actually usually understood as “a black hole has three hairs”, because there ARE three observable things about black holes we can see from the outside. (If “hair” sounds funny to you, you’re not alone. Stephen Hawking and his collaborators decided to title one section of the paper “Quantum hair implants” because they have a sense of humor). We can determine the black hole’s mass, its angular momentum (how it spins), and its electrical charge, all based on their effect on matter swirling around the black hole (in its accretion disk, for example). But we can’t get the quantum information back.
I should clarify: when I say information, I mean things like quantum states – particle “spins” (which are not quite the angular momentum spin I mentioned earlier) and other stuff like that (not, say, whether the object is a planet, a gas cloud, or a cruise liner formerly piloted by Zapp Brannigan). Quantum states are conserved quantities elsewhere in the universe, but once they’re dumped into the black hole they disappear from view, and the universe could (in theory) end up with some SLIGHT but mathematically uncomfortable excess of, say, “spin +1/2” particles. That’s a problem.
This prompted some explanations like black holes forming wormholes that dump material out in some other time and place (like the wormhole in Star Trek: Deep Space Nine), or even creating another baby universe where this information was preserved (as seen in Carl Sagan’s Cosmos, or Disney’s creepy The Black Hole). Basically, rather than being destroyed, the material just moves somewhere else, and the system as a whole is now balanced if you include the extra universe, or assume the equation is ultimately balanced by some kind of time travel.
The problem with those theories is that Hawking himself (in the 1974 paper that he’s arguably most famous for) says that black holes slowly evaporate in a trickle of isotropic radiation. Yes, not even black holes will live forever. And they’re not completely black if they’re giving off tiny amounts of radiation.
Hawking Radiation, as it’s called, comes about because of one of the weirder parts of quantum mechanics: In the vacuum of space, there is a constant potential for “virtual” particles to be created and destroyed, leaving a net zero state. But what happens at the knife-edge of the event horizon? One particle could be pointed such that it will escape, while the other might plunge immediately into the black hole. They would not annihilate each other immediately and return to zero. Hawking’s theory states that this actually happens, however rarely. Rather than annihilate each other instantaneously, the “virtual” particles becomes real and one escapes. The energy (because mass and energy are the same thing, according to E=mc^2) is basically deducted from the black hole, making a (very) small energy drain on the black hole. That kind of energy loss wouldn’t really do anything to a black hole in billions (or even quintillions) of years, but if we’re talking about an infinite amount of time… at some point, the black hole runs out of energy and evaporates; its mass, charge, and angular momentum strewn across the universe as tons of charged particles.
This solves a number of problems with physics that I don’t completely understand and won’t try to explain here, but it leaves one crucial thing unsolved: those quantum-mechanical states. The problem is, this Hawking radiation neither knows nor cares about what’s going in inside the black hole. Even if we created a black hole and fed it nothing but spin +1/2 particles (keeping all the -1/2 particles for ourselves), the Hawking Radiation would come out as a 50/50 mix. Hawking Radiation destroys information.
The general response from physicists has been “well, that can’t be right”.
Theoretical Physicists, Hawking among them, have been pursuing the idea that our understanding of black holes is wrong. Over the past twenty years they’ve made a number of breakthroughs to the point that Stephen Hawking admitted he was wrong about Hawking Radiation destroying information, back in 2004. This new theory outlines a means of how Hawking Radiation does not destroy information, using another strange principle of quantum physics called the holographic principle.
When I say holograms, I’m not talking about the way Tupac or Michael Jackson have appeared recently; it’s also not a hologram in the sense of the holodeck on Star Trek. I’m talking about the eerie usually colorless 3-d scenes generally (in my experience) printed on some kind of gold foil, as on some credit cards as a security mark. National Geographic used them occasionally on magazine covers (December 1988, which was my first encounter with them), and they’re often found in science museum gift shops. Those are real holograms: The foil is etched with the interference patterns of the entire light field sweeping across a scene, such that when light falls on the 2d foil, the light is interfered with (and bounced back to your eye) exactly as if the object was still there. Put another way, holography is like looking at an object through a window, except that all information about what light should do while passing through the window, is recorded ON the window.
The basic point of the holographic principle in physics is that it forms a kind of limit on the amount of information that can be stored on a 2D surface. The biggest development in black hole information loss (and what prompted Hawking to admit that black holes could preserve information) was a proof that the black hole’s event horizon was exactly large enough to contain a holographic record of everything that was inside (as long as the recording spaces have sizes of planck lengths, which is thought to be the smallest physical length possible), AND that such a holographic surface exists (How? I admit: I have no idea. I don’t understand the math). As matter falls within the black hole, the black hole’s mass increases, and therefore the event horizon’s size increases… by exactly enough to maintain the space necessary for the perfect record of everything that fell in. The trick was, how do you get that information out? And that’s where this new theory comes in.
Hawking’s theory demonstrates a potential mechanism (which I do not understand) by which the escaping Hawking Radiation particles will interact with the hologram stored on the event horizon of the black hole. This interaction allows information about what’s inside to leak out, very slowly. The black hole will still decay into a shower of particles, but by the time it disappears, it will have leaked out everything about its interior anyway. An observer arbitrarily far away from the black hole gets to see the mass, charge, spin, AND all the quantum mechanical states. Physics is saved, black holes are saved, and we can all go home.