July 4, 2014 | 3
Theoretical physicist Carlo Rovelli, one of the creators of loop quantum gravity, and his collaborator Hal Haggard have just come out with a new paper on black holes. Ever attuned to puns, Rovelli calls it the “fireworks” model, alluding to the firewall argument that has consumed black-hole theorists over the past two years. As if that weren’t enough to make you groan, Rovelli had to post the paper on American Independence Day.
Rovelli and Haggard propose that black holes pop off like firecrackers—very, very slow-motion firecrackers. These are not the black-hole explosions that Stephen Hawking talked about in the ’70s. A more apt comparison is a supernova. A star collapsing inward to form a black hole does not crunch itself all the way down to a singularity of infinite density, but instead reaches some maximum density and bounces back out. The process is rapid—just a few milliseconds for a hole with the mass of our sun, since the material implodes at nearly the speed of light. The black hole looks black because, from the outside, the explosion appears to take trillions upon trillions of years. The intense gravitational field stretches light waves by such an enormous amount that it seems as though time has almost stopped.
Here’s a slightly edited and rearranged version of the email exchange Rovelli and I had yesterday:
GM: What’s the basic problem that black holes pose and that you’re trying to solve?
CR: The way I view the basic problem with black holes is simple. It is the following: what happens at matter when it falls inside the hole and reaches its center? More precisely: suppose you were on a (very robust) spacecraft and went in, what would you see, what would happen to you, after the (quite short) time needed to reach the center? The reason why this is interesting is that our current established laws of physics do not tell us. It is funny: on the one hand, black holes are now normal objects in the sky; on the other, we have no idea what happens to the large amount of matter we actually see falling in them.
GM: How is your new idea related to the firewall argument?
CR: It is strictly related. In a sense, the black-hole firewall argument points out the fact that something curious must happen. Our new idea clarifies what is this “something curious.”
GM: How is the material able to escape the hole—doesn’t the intense gravity create an event horizon that traps it?
CR: The correct technical statement is this: intense gravity creates a horizon, but it is not an event horizon. It is locally like an horizon, but not globally. So, matter is trapped for a while, but not forever; it is called sometimes a “trapping” horizon. This was considered by many other authors in the past. We give a precise explanation of how this might happen. The trapping horizon remains there until quantum effects destroy it. There is no causality violation. The reason is that the quantum-gravity effects that destroy the horizon do not come from inside the horizon. They develop outside.
GM: Ah, so it’s a door that closes, then opens.
CR: Yes, exactly so!
GM: Usually people think information that falls in reemerges in the Hawking radiation, whereby the hole evaporates away. Yet I gather that you think the Hawking effect is only minor part of the story.
CR: Indeed we started off with a question: when in the world do we see a process where the energy goes totally into dissipation? Does not make sense physically. All the stuff that falls into the hole cannot be all transformed into grey Hawking radiation.
GM: But the Hawking effect does operate, to some degree. So does some information get lost, in which case the information paradox still occurs?
CR: I now think that it is completely possible that no information is lost. Things go in, things come out.
GM: How are you able to concoct such a model without having a theory of quantm gravity? How are you able to get away with ignoring what exactly happens at the singularity?
CR: There are many estimates of quantum effects that one can do even without a full quantum theory. Tunneling phenomena are a good example of this. And the process we study here is a version of quantum tunnelling: over the long ages of a black hole, the hole can tunnel into a white hole.
GM: How does quantum gravity nonetheless affect what’s happening?
CR: It is tunneling: a standard quantum phenomena, in which a particle can go where classically it could not go. Similarly, the black-hole metric can tunnel into a white-hole metric. We give explicitly the external metric of the process.
GM: But wouldn’t the horizon form long before quantum effects become important?
CR: This is precisely the beauty of the story: seen from the outside, the process takes long. But seen from within, the process is very short—the time it takes light to cross the star. A black hole is a fast process seen at superslow speed from far away.
GM: This sounds a bit like the old “remnant” theories, in which the black hole would have a hard nugget at its center able to store matter and information, either temporarily or permanently. In your model,the hole has to hold onto its information during the period while the door is closed. So how does this model avoid the problems that people have talked about for those theories?
CR: The problem with remnants was that they were supposed to be small, because they were what remains after the full Hawking evaporation. There is never anything small here. The white hole is as large as the black hole.
Photo courtesy of Carlo Rovelli