So far, however, warped spacetime is keeping the events a secret.
Image: black hole artist's conception/Wikimedia
Black holes aren’t eternal. Like everything else in the cosmos, they have a lifespan: birth, life, death. In this sense, the galaxy-scale infinity represented by a black hole is something of a magic trick. While, yes, there is that mystical event horizon, past which information ceases to be retrievable to outside observers, the horizon also leaks. Black holes emit a slow dribble of radiation—known as Hawking radiation, in a reference to the astrophysicist who predicted the phenomenon—which will eventually drain the black hole completely, leaving only the mundanely observable.
Or, rather, it will leave the mundanely observable space that used to house a black hole. The innards of the black hole, even after the black hole’s death via slow trickle, remain lost. This is the “information paradox” of a black hole—information goes in, but the radiation that gets returned encodes nothing about what entered.
This violates one of the fundamental principles of quantum mechanics, which says that, given a complete description of a system, it should be possible to determine earlier states of that system. In a black hole, however, it would appear that all of the myriad different states that fall into it become just one single state, and all information is lost.
There are a wide range of different theories resolving the paradox, and Hawking himself has conceded that information is not really lost in a black hole. A quantum tunneling explanation, in which particles are allowed to traverse some otherwise insurmountable boundary (like an event horizon), seems particularly appealing. But what if we’re just wrong, generally, about black holes? A paper posted last week to the arXiv preprint server (via Nature) describes a scenario in which black holes blink (or burst) out of existence just as soon as they form, becoming “white holes” in which all of the captured stuff/information is barfed back out into the universe.
The authors, Hal Haggard and Carlo Rovelli, call it “black hole fireworks,” except we, as observers, don’t really see fireworks because space-time is so warped/stretched in the region surrounding a black hole that the process appears to take billions or trillions of years. The black holes formed in the very early universe might just now be “popping,” and sending detectable material our way, in the form of high-energy cosmic rays.
What astronomers currently interpret as supernova explosions might in reality be black holes erupting into white holes.
What astronomers currently interpret as supernova explosions might in reality be black holes erupting into white holes. The ultimate and so-far obscured end result: "A distant observer sees a dimming, frozen star that reemerges, bouncing out after a very long time, determined by the star’s mass and Planck’s constant,” the authors write. In that husk, one might have some access to what came before it.
The Planck constant, for reference, is basically when reality shrinks until it can’t shrink anymore, leaving forces and matter in quantized, discrete forms; that is, energy that may seem to be continuous is really delivered in indivisible “lumps,” like photons. If one were to take a photon and bomb it with another photon in an attempt to compress it to an even smaller size than this reality-defining constant, the result would be a black hole, albeit a very small one. The concept scales upward (in multiples of the smallest division/quanta): take a bunch of matter and energy together and bomb them with more energy, it’s possible to pack it all into a size so small that it’s beyond the predictions of physics: no lengths or sizes or shapes.
What if there’s a Planck scale for space-time itself? That is, what if you just keep compressing time further and further? Would it hit some quantum limit? Possibly. This is the basis for the Haggard/Rovelli paper—space-time in this theory is made up of very tiny loops, which would also account for gravitational effects at quantum scales, or quantum gravity. At our level of reality, those loops come together into something reasonably sensible, but if you were to shrink way down to their size, it might just cease to be possible to shrink anymore and still exist. And so, with a forever shrinking black hole, it might hit this limit at some point, and its entire black holeness would be questioned.
“The current tentative quantum gravity theories, such as loops and strings, are not sufficiently understood to convincingly predict what happens in the small radius region, so we are quite in the dark,” the authors lament. “What ultimately happens to gravitationally collapsing matter?”
What happens in the space-time quantum loop-based theory is that all of these no-longer-shrinkable loops begin to exert outward pressure as the black hole continues its collapse. This pressure is the result of an accumulation of the aforementioned quantum tunneling. Instead of just a fizz of particles tunneling past the black hole’s event horizon, you have an entire population of space-time loops pushing outward through it at once, as an accumulation built up over time.
Quantum tunneling, very simplified.
The result: a white hole. Or at least it’s a white hole for someone close by. For us, the space-time warp of a black hole keeps us, well, in the dark. “Importantly, the process is very long seen from the outside, but is very short for a local observer at a small radius,” the paper states.
As for Hawking radiation, the authors aren’t quite ready to chuck it either. “While the theoretical evidence for Hawking radiation is strong,” Haggard and Rovelli write, “we do not think that the theoretical evidence for the assumption that the energy of a collapsed star is going to be entirely carried away by Hawking radiation is equally strong. After all, what other physical system do we know where a dissipative phenomenon carries away all of the energy of the system?”
The authors note that their theory is rather uniquely concerned with the infinite guts of a black hole; that is, what happens to a massive object when its radius reaches perfect “0.” “Hawking radiation regards the horizon and its exterior: it has no major eﬀect on what happens inside the black hole,” they write. Meanwhile, the big “bounce” predicted by Haggard and Rovelli’s en masse tunneling has everything to do with it, leaving Hawking radiation to exist as a polite correction to the real forces.
Finally, “What does a white hole have to do with the real universe?” the authors ask. “But further reﬂection shows that this is reasonable: if quantum gravity corrects the singularity yielding a region where the classical Einstein equations and the standard energy conditions do not hold, then the process of formation of a black hole does not end in a singularity but continues into the future.
“Whatever emerges from such a region is then something that, if continued from the future backwards, would equally end in a past singularity,” the paper continues. “Therefore it must be a white hole. A white hole solution does not describe something completely unphysical as often declared: instead it is possible that it simply describes the portion of spacetime that emerges from quantum regions, in the same manner in which a black hole solution describes the portion of spacetime that evolves into a quantum region.”