Quantum bounce could make black holes explode

Jul 29, 2014

By Ron Cowen


Black holes might end their lives by transforming into their exact opposite — ‘white holes’ that explosively pour all the material they ever swallowed into space, say two physicists. The suggestion, based on a speculative quantum theory of gravity, could solve a long-standing conundrum about whether black holes destroy information.

The theory suggests that the transition from black hole to white hole would take place right after the initial formation of the black hole, but because gravity dilates time, outside observers would see the black hole lasting billions or trillions of years or more, depending on its size. If the authors are correct, tiny black holes that formed during the very early history of the Universe would now be ready to pop off like firecrackers and might be detected as high-energy cosmic rays or other radiation. In fact, they say, their work could imply that some of the dramatic flares commonly considered to be supernova explosions could in fact be the dying throes of tiny black holes that formed shortly after the Big Bang.

Albert Einstein’s general theory of relativity predicts that when a dying star collapses under its own weight, it can reach a stage at which the collapse is irreversible and no known force of nature can stop it. This is the formation of a black hole: a spherical surface, known as the event horizon, appears, shrouding the star inside from outside observers while it continues to collapse, because nothing — not even light or any other sort of information — can escape the event horizon.

Because dense matter curves space, ‘classical’ general relativity predicts that the star inside will continue to shrink into what is known as a singularity, a region where matter is infinitely dense and space is infinitely curved. In such situations, the known laws of physics cease to be useful.

Many physicists, however, believe that at some stage in this process, quantum-gravity effects should take over, arresting the collapse and avoiding the infinities.

In a loop

One of the leading approaches to merging quantum theory and gravity, pioneered by, among others, theoretical physicist Carlo Rovelli of Aix-Marseille University in France, posits that it is not just gravity but space-time itself that is quantized, woven from tiny, individual loops that cannot be subdivided any further. The loops in this ‘loop quantum gravity’ — a theoretical attempt that has yet to find experimental support — would be so tiny that to any observer space-time looks smooth and continuous. In the new work1, Rovelli and his Aix-Marseille colleague Hal Haggard have calculated that the loop structure would halt the collapse of a black hole.

The collapsing star would reach a stage at which its inside can shrink no further, because the loops cannot be compressed into anything smaller, and in fact they would exert an outward pressure that theorists call a quantum bounce, transforming a black hole into a white hole. Rather than being shrouded by a true, eternal event horizon, the event would be concealed by a temporary ‘apparent horizon’, says Rovelli. (Theoretical physicist Stephen Hawking of the University of Cambridge, UK, has recently suggested that true event horizons would be incompatible with quantum physics.)

Other loop-quantum theorists have made similar calculations for cases in which it is not just a star that is collapsing but an entire universe23. They found that the universe could bounce back, and suggested that our own Universe’s Big Bang could in fact have been such a ‘big bounce’. Rovelli and Haggard have now shown that the quantum bounce does not require an entire universe to collapse at once. “We think this is a possible picture,” says Rovelli. “We have found that the [transformation] process can be completely contained in a limited region of space-time. Everything outside behaves following the classical Einstein equations.”

6 comments on “Quantum bounce could make black holes explode

  • I think you give him too much credit, Chopra don’ need no stinkin’ mathematics. He regularly talks about ideas like quantum physics that he has no figgin clue about at all and the math never bothered him there. I think Sagan is right, Depak just hasn’t gotten around to this one yet.

    Report abuse

  • So I have a dumb question for the physics people: how do theories like this reconcile with String theory? My understanding is that quantum gravity is in competition with String theory is that true? And if that’s true is it possible the actual answer could take a bit from quantum gravity and a bit from String theory or are the two models so in conflict that accepting one means you have to reject the other?

    Report abuse

  • PERSON Jul 31, 2014 at 4:36 am

    I wonder if this could explain how ring galaxies are formed.

    Probably not. Rings around planets (Saturn etc). circularised orbits (Asteroids, accretion disks) around stars, and spiral galaxies all have gravity pulling matter into approximate circles.

    Shock-waves are also important in galaxies, because they disturb invisible matter causing it to accrete into radiant stars which then become visible.

    Report abuse

Leave a Reply

View our comment policy.