Added 1 new A* page:Stephen Hawking, the theoretical physicist who has shaped much of the thinking on black holes in recent decades, has just put out a new paper, Information Preservation and Weather Forecasting for Black Holes, in which he posits that the idea we've had of event horizons being these nice smooth spheres of no escape around black hole singularities is incorrect--that they're more likely to be "chaotic," and likens their unpredictability to weather on Earth, in the sense that you can't predict the weather (super-accurately, I guess he means) more than a few days in advance.
The paper is just a couple pages long but extremely dense, even for an theoretical astrophysics paper. : o I'm not going to pretend I understand much of the details, but the raison d'etre appears to be the old black hole information paradox, which Hawking himself helped create in 1975 when he proposed the idea that black holes slowly radiate energy, dubbed "Hawking radiation." This would mean that any information--stuff--that goes into them comes out in a single form, so whatever they were before has been completely obliterated--which would violate the principle that...in quantum ways I don't really understand--information is not supposed to be destroyed. This led to all sorts of arguments over the years, and Hawking in 2004 conceded a bet and reversed his original stance, saying that "quantum perturbations of the event horizon could allow information to escape from a black hole, which would resolve the information paradox."
That explanation didn't satisfy everyone, and other theories were proposed, including the idea--which Hawking takes care to shoot down specifically in this latest paper--that a "firewall" of energy might surround the black hole, zapping incoming stuff and re-emitting its information losslessly, or something. It seems anyway that even Hawking has no longer been satisfied with his or other theories that tried to get around the information paradox, so now he's throwing out the whole event horizon notion that spawned it in the first place:
| "The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can't escape to infinity. There are however apparent horizons which persist for a period of time. This suggests that black holes should be redefined as metastable bound states of the gravitational field." |
Stuff doesn't fall in and get scrunched into a singular state in which its history is lost, he says, but rather it swirls around in this chaos of "apparent horizons" for a "period of time"--but ultimately it will radiate back out and still be unique little snowflakes, so its information doesn't get lost and we can all be happy. I think. The idea of nice neat event horizons was abstractly pleasing, but yeah I guess if you think about how things in our universe tend to work out, a messier picture of what black holes are actually like would seem more in keeping with our observations of other phenomena; Hawking is saying that our tidy picture of black holes, under which you supposedly could know all there was to know about a given black hole by putting either its mass or radius through Schwarzschild's simple equation, was far too simplistic--and in fact it is a reliance upon that approximately correct, but overly simplistic model "that is responsible for the information loss in gravitational collapse": if you don't smooth things out with an approximation, the information should emerge.
Hawking, I should point out, did not invent the idea of the event horizon in the first place--that came much earlier; Wikipedia says "In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon"--and Schwarzschild's equation, leading to the idea of a singularity in the first place, goes all the way back to 1915--and even centuries before Einstein's equations actually made approaching this stuff in a coherent way possible (actually the specific equation Hawking was referring to as being responsible for making people think information would be lost was the Kerr metric, discovered by Roy Kerr in 1963, that gave the geometric solution for a rotating black hole--Schwarzschild's had only been for a non-rotating body), some scientists had a notion that what we would come to know as black holes could exist: in 1783, John Michell wrote: "If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity." But back then there was no idea of how gravity could influence light, so those original "dark star" ideas didn't stick.
And it's just as well since now Hawking says we should throw a lot of this stuff out!
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