# Could Black Holes Give Birth to 'Planck Stars'?

Black holes are vexing objects. Not only do they defy our everyday understanding about how the Universe should work, they confound even the most complex mathematical models. Science writers describe them as “gravitational behemoths” that warp spacetime so much that even light cannot escape the surrounding boundary, known as the “event horizon.” But what lies inside? Well, that doesn’t matter, we argue, everyday physics does not apply once you cross the event horizon.

ANALYSIS: No Black Holes? More Like Grey Holes, Says Hawking

The horribly unsatisfactory explanation has been battered in recent months when a serious physics confrontation entered the public arena. What goes on inside a black hole’s event horizon has actually caused a theoretical conflagration and now, two theoretical physicists have proposed a new idea that may marry quantum mechanics with gravity, extinguishing the tricky “firewall” and finding a solution to the “information paradox.”

A Brief History of Burning Black Holes

In January, British physics superstar Stephen Hawking published a short paper declaring “there are no black holes.” Of course, Hawking wasn’t saying that black holes didn’t exist, but that the physics of the black hole’s event horizon needs some tweaking.

The root of this issue can be found in a 2012 research publication by Joseph Polchinski and his team at the University of California in Santa Barbara. When tackling the thorny problem of whether or not black holes destroy information, they found that if black holes truly do not destroy information (a standpoint that Hawking himself reluctantly advocates) and that information can escape from the evaporating black hole via Hawking radiation, there must be a raging inferno just inside the event horizon called the “firewall.”

And herein lies a paradox. If we view a black hole as an object governed by general relativity, should an unfortunate astronaut get dragged across the event horizon, they shouldn’t experience anything out of the ordinary (“no drama”); he or she will just drift on through, into the black hole, where, eventually, intense tidal forces gruesomely “spaghettify” them. But if we view black holes as objects governed by quantum mechanics, and they conserve information, that astronaut will immediately get incinerated by Polchinski’s firewall (the antithesis of “no drama”). The two theories are symptomatic of our growing unease with the compatibility of general relativity and quantum mechanics, and black holes have become the front line of this battle.

ANALYSIS: Hawking: Great Scientist, Bad Gambler

Hawking therefore came forward with a possible (unpublished) solution last month: perhaps the black hole’s event horizon isn’t the definite boundary that theoretical physicists think it is. Perhaps the event horizon should be replaced with an “apparent horizon,” a consequence of the chaotic mess of information inside the event horizon. This may stick a Band Aid over the problem, but in a new (unpublished) paper submitted to the arXiv preprint service, two theoretical physicists have come up with an alternative idea.

Not Such a Singularity

The conventional view of a black hole is that it is composed of two key components: the singularity and the event horizon. Everything else is just details. The event horizon is the distance from the singularity where gravitational forces are so strong that even light cannot escape. The singularity is an infinitely dense point where all the matter of the black hole is concentrated. However, the singularity assumes that there is no quantum structure that can compete with the inward forces created by gravity.

Spotted the problem here? Yes, once again the black hole has become a battleground for quantum mechanics and gravity.

Carlo Rovelli from the University of Toulon, France, and Francesca Vidotto from Radboud University in The Netherlands have found a potential solution to the “Firewall problem” by throwing out the concept of a singularity and replacing it with an extreme class of “star” — the Planck Star.

Rovelli and Vidotto looked at this problem from a different perspective. While working on models of a collapsing universe — i.e. the opposite to the Big Bang, known as the Big Crunch — they found that the fundamental quantum structure of the Universe prevents an infinitely dense singularity from forming. The collapse of the Universe therefore reaches a fundamental density, causing the universal collapse to rebound, or “bounce.” This bounce could spawn a cyclical universe where “Big Bounces” lead to cosmological inflation, then contraction and ultimately a Big Crunch… and the process starts all over again.

ANALYSIS: Did Some Black Holes Survive the Big Bang?

Say if a similar model can be used to describe a black hole?

A Planck Star Rises

If a massive star explodes as a supernova, creating a black hole in its wake, what if the superdense material that formed the black hole actually didn’t form a “singularity”? Sure, the material is unimaginably dense, but the object in the core of the black hole still has structure. Rovelli and Vidotto argue that the inward force of gravity is counteracted by the quantum structure of the Planck density.

If we were to zoom in, far beyond the size of quantum particles, it is theorized that we will reach a fundamental scale known as the Planck length. Should matter be compressed to these scales, rather than disappearing into an “infinitely dense” singularity — a solution that doesn’t make a whole lot of sense — perhaps the contraction stops at the Planck density, creating a “Planck Star” and the object rebounds, or “bounces.” From the perspective of the Planck Star, it will be a very short-lived affair; it’s collapse and bounce would occur rapidly. But to outside observers elsewhere in the Universe (i.e. us), as space-time surrounding the Planck Star is so extremely warped, time dilation makes the black hole (and the Planck Star it contains) seem static and unchanging.

Over time, as the black hole loses mass to Hawking Radiation and the Planck Star continues to expand after the rebound, the event horizon of the black hole will slowly contract, eventually reaching the surface of the Planck Star contained within. At this point, argue the researchers, all of the information the black hole ever consumed over its lifetime will be suddenly released to the Universe — solving the “information paradox.” What’s more, we should be able to detect this deluge of information.

ANALYSIS: Death By Black Hole Firewall Incineration It Shall Be

“(Planck Stars) produce a detectable signal, of quantum gravitational origin, around the 10-14cm wavelength,” they write. This signal could embody itself in cosmic rays of energies in the GeV range, a signal that can be easily detected by gamma-ray observatories.

Like Hawking’s “grey hole” paper, Rovelli and Vidotto’s work is currently not peer reviewed, so we are hearing arguments fresh from the theoretical community. But it would be interesting if we can link the emergence of Planck Stars from beyond black hole event horizons through the detection of gamma-rays from deep space, a field of study that has more than its fair share of mysterious signals.

Pre-print: “Planck stars,” Rovelli & Vidotto, 2014. arXiv:1401.6562 [gr-qc]