by SETH FLETCHER
Artist’s representation of a black hole IMAGE/Wikipedia
In the mid 1970s, Stephen Hawking made a string of unnerving discoveries about black holes—that they could evaporate, even explode, and destroy all information about what had fallen in. Physicists spent the next 40 years sorting through the wreckage. Then last year, at a conference in Stockholm, Hawking said that he and some collaborators were close to a solution to the so-called black-hole information paradox. Details, however, would have to wait.
Now the details are here—at least some of them. This week Hawking, the University of Cambridge physicist Malcolm J. Perry, and the Harvard University string theorist Andrew Strominger posted a paper online in which the authors claim to make real progress toward solving the black-hole information paradox. Despite the inviting title—“Soft Hair on Black Holes”—the paper is mercilessly technical, so I asked Strominger to walk me through it. An edited transcript of our conversation follows.
Seth Fletcher: Physicists are comfortable with all sorts of insane-sounding ideas, but the idea that black holes destroy information is not one of them. Why is this something that they cannot abide?
Andrew Strominger: Black holes destroying information means that the world is not deterministic. That is, the present doesn’t predict the future perfectly, and it also can’t be used to reconstruct the past. That’s sort of the essence of what a physical law is. Going way back to Galileo or earlier, the idea of a physical law is that you start out with bodies in some state of motion and interacting, and you use the physical laws to determine either where they will be in the future or where they must have come from. So it’s a very big thing if black holes destroy information. It’s a very big thing to say that we cannot use physical laws in the way that we’ve been accustomed to for thousands of years to describe the world around us.
Now just because it’s a very big thing doesn’t mean that it’s impossible. In a way, the history of physics is the history of learning that things that we thought had to be true weren’t true. We used to think that space and time were absolute. We used to think the Earth is the center of the universe. All of these things seemed completely obvious and well defined. And one by one they went by the wayside. That could happen to determinism, too. The very fact that the universe has a beginning seems to be in contradiction with determinism, because if you have nothing and then there’s something, that’s not deterministic. So determinism should be on the table. And indeed when Hawking first came out with his argument [that black holes destroyed information], it seemed like such a good argument that many or even most of the people who listened to it believed that determinism was over.
But three things happened that have changed that. The first is that you can’t just throw up your hands and say we can’t describe the universe. You need some kind of alternative—some sort of probabilistic laws or something. And Hawking and other people put out some formalism that enables you to have probabilistic laws, and so on, but it was rather quickly shown to be internally self-inconsistent.
The second thing was that experimentally it’s not plausible to say that determinism breaks down only when you make a big black hole and let it collapse because according to quantum mechanics and the uncertainty principle, you would have little black holes popping in and out of the vacuum. And so you would have to violate determinism everywhere. And the experimental bounds on that are truly extraordinary. So experimentally there are very serious consequences if there are even teeny, tiny violations of determinism.
SF: What are some of those consequences?
AS: In order to say that a symmetry implies a conservation law, you need determinism. Otherwise [symmetries] only imply conservation laws on the average. So electric charge would only have to be conserved on the average. Or energy would only have to be conserved on the average. And the experimental bounds on energy conservation are extraordinary. If you added terms to the laws of physics that violated determinism in some form, they would have to have fantastically small coefficients, one part in 101,000 or something.
So [the black-hole information paradox] is experimentally a problem and it’s theoretically a problem. Those are the first two things. The third thing was string theory. I would say up until the 1990s, the community was kind of split 50-50. But then Cumrun Vafa and I showed that certain string-theoretic black holes were capable of storing the requisite information, and they apparently also have some method of letting the information go in and out. And the fact that that worked—I mean, people had been trying for 25 years to reproduce this Bekenstein-Hawking area entropy law, or in other words, to derive the information content of a black hole from first principles. And nobody had been able to do it. And then we did it with complete accuracy. All the numbers, everything worked perfectly. And it had to be some kind of clue to something. It couldn’t just be an accident.
Scientific American for more