String theorist Vijay Balasubramanian grapples with the garbage cans of the universe: black holes. His research on particle physics at the University of Pennsylvania and The City University of New York Graduate Center questions the apparent loss of information in the presence of black holes. This work is just one aspect of his impressively broad range of pursuits, which span from the structure of spacetime to the cognitive “sense of place” in the human mind. In the following edited interview, Balasubramanian tells Nautilus why black holes might not be the infinite information sink they’re made out to be. What is a black hole?
If you have enough matter concentrated in a small region of space, the matter curves space and time in such a way that nothing can escape. The possibility was originally discovered by Karl Schwarzschild while solving Einstein’s equations for the theory of general relativity in 1915. Inside a black hole, no matter which direction you try and accelerate, all futures point toward the singularity of the black hole, a region of space that is infinitely warped and has vanishing volume. So does that make black holes the ultimate garbage cans?
At the level of the classical theory of gravity, yes. In Einstein’s equation describing the theory, you get the strange property that once you get in you can’t get out. However, right at the singularity, where gravity would become infinitely strong, the classical theory of gravity breaks down. Is that where quantum theory becomes important?
Until the 1970s, physicists thought that quantum mechanical effects would only become important near the singularity. But around then, theoretical physicist Jacob Bekenstein demonstrated that a black hole carries entropy, in proportion to the area of the horizon. The horizon is the boundary that marks the division between a black hole and the rest of the universe. Around the same time, Stephen Hawking wrote a paper that suggested black holes also radiate, as if they are warm and have a temperature. That radiation is now called Hawking radiation. How is the radiation created?
In quantum theory, pairs of particles and antiparticles—like the electron and its partner, the positron—pop in and out of existence in the vacuum of space. Near the horizon, one of these particles would fall into the black hole and the other would stream into space, and it would appear that the black hole radiates. Because it’s radiating stuff, the black hole loses energy to the universe, and its mass decreases. Eventually the black hole evaporates completely. Big black holes, like the one at the center of our galaxy, take much longer than the age of the universe to evaporate; smaller ones take a shorter amount of time. But either way, you’re getting stuff out of the garbage can. So then black holes aren’t the repositories of the cosmos’ detritus after all?
Actually, they still are. According to the way Hawking envisioned it, the radiation is completely random—each photon or particle of light that the black hole emits comes out independently of every other particle, so there is no information about the black hole in the radiation. No matter how closely you look at the Hawking radiation, you can’t tell anything about what fell into the black hole. Information is lost. Does that pose a problem?
Hawking accepted that information loss is a necessary consequence of having black holes, but not many people are satisfied with that solution because it means giving up a central postulate of quantum mechanics—that information is always preserved. To give that principle up, we’d have to modify quantum theory rather extensively. And quantum theory, as it’s now formulated, works very well. Along with Einstein’s theory of gravity, it’s one of the two great discoveries in physics in the 20th century. So is there a resolution?
Possibly. Quantum theory describes the microscopic states of a particle. General relativity is a macroscopic description of space and time. Maybe when general relativity merges with quantum theory, the subatomic, quantum mechanical properties of spacetime will resolve the puzzle. Perhaps those subatomic details get lost in the coarse, macroscopic description of gravity from Einstein’s theory. Has there been any progress in marrying quantum theory and gravity?
Yes. In fact, encountering a paradox like this often leads to new solutions in physics. In the 1990s, scientists using string theory, which attempts to unify quantum theory with gravity, found something startling. They showed mathematically that a 3D universe containing a special class of black hole, governed only by gravity, is equivalent to a 2D universe in which gravity does not exist but all the particles and fields obey the laws of quantum mechanics. Since quantum theory always preserves information, information is never lost in the 2D universe. And because of the equivalence of the two universes, the 3D universe with black holes must also preserve information. But no one is sure exactly how this happens. What do you think the solution is?
It’s possible that the Hawking radiation does encode information about the black hole after all. In a recent paper, my colleagues and I consider the different internal, quantum states of a black hole. We suggest that the separation in energy between the different quantum states may be so tiny that it would take a very large, nearly infinite amount of time to analyze the Hawking radiation and determine the contents of the black hole. So in that sense, the black hole serves as a garbage can for extremely long periods of time. For a black hole weighing as much as the sun, this would be a period much longer than the current age of the universe. So, for all practical purposes this is forever. But eventually, before the universe ends, the black hole spills its contents, scrambled and recycled. |
