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Black hole 'hair' could be detected using ripples in space-time

An illustration of a black hole.
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The information locked inside black holes could be detected by feeling their 'hair,' new research suggests.

Black holes are celestial objects with such massive gravity that not even light can escape their clutches once it crosses the event horizon, or point-of-no-return. The event horizons of black holes lock secrets deep within them — secrets that could completely revolutionize our understanding of physics. 

Unfortunately, for decades many scientists thought whatever information falls into a black hole might be lost forever. But new research suggests that ripples in space-time, or gravitational waves may carry a faint whisper of this hidden information by revealing the presence of wispy "hairs" on a black hole's surface. 

Related: Warped physics: 10 effects of faster-than-light travel 

A hairy question?

As far as we understand them (which, admittedly, is not very much), black holes are suspiciously simple objects. Regardless of what falls in, whether stars, clouds of gas and dust, or your worst enemies, black holes can be described by three and only three simple numbers: charge, mass and spin. 

That means that if you had two black holes of the exact same size, exact same electric charge, and spinning at exactly the same rate, you wouldn't be able to tell them apart. The reason this is suspicious is that something had to happen to all that juicy information that fell into those two black holes. Did it get destroyed? Lost below the event horizon? Stuck in some inaccessible portion of the universe?

The simplest solution is the theorem, first coined by the American physicist John Wheeler, that "black holes have no hair" — they have no extra information encoded in them or on them. Just their mass, electric charge and spin. Everything else is simply destroyed (somehow) beyond the event horizon, locked away from the universe forever and ever.

A paradox of information

But in 1974, Stephen Hawking proposed a revolutionary idea: black holes aren't inescapable cosmic vacuum cleaners; rather, subatomic particles might flee black holes through an exotic quantum process, which would result in the release of radiation from their surfaces. Over time, this Hawking radiation, as it is called, would cause black holes to slowly lose energy (and therefore mass). Eventually, after eons of gradually losing energy, the black holes would evaporate entirely.

This is all fine and dandy, except for the pesky no-hair idea. If black holes can evaporate, what happens to all the information that fell into them?

As far as we know, Hawking radiation doesn't carry any information away with it. And we really, really don't think that information can be created or destroyed in this universe (it's certainly possible, but would make a bunch of known physics pretty wonky, which would violate observations and experiments).

Related: Stephen Hawking's most far-out ideas about black holes

And hence, the black hole information paradox. Information goes into a black hole, the black hole disappears, and we don't know what happens to the information.

To fix this paradox, either we need to fix what we know about black holes or fix what we know about Hawking radiation. Or both.

Maybe the information gets locked deep inside the black hole, near the singularity, and evaporation stops just before that point, leaving behind a tiny little ball chock full of information.

Or maybe black holes aren't entirely hairless. Maybe, just maybe, they maintain the information of anything that's fallen into them on their surfaces, contained in something called the "stretched horizon", a surface just above the event horizon containing quantum mechanical information. As black holes dissolve, the Hawking radiation carries away the information contained in the stretched horizon, solving the paradox and preserving our reality as we know it.

Great idea, but how do we test it?

Related: The 18 biggest unsolved mysteries in physics

Ripples in space-time

A new study, published June 22 to the arXiv database (but not yet peer reviewed), suggests one way to find these silky strands: a gravitational wave detection.

When black holes merge, they release a fury of gravitational waves that ripple throughout the cosmos. Despite the incredible energies of these collisions, the gravitational waves from these cosmic smashups are exceptionally weak. By the time these waves wash over Earth, they're barely capable of nudging individual atoms.

But we have LIGO — the Laser Interferometer Gravitational-Wave Observatory, a globe-spanning observatory — which can detect those subtle motions through the tiny changes in how long it takes light to travel from far-flung detectors. LIGO has observed the aftermath of dozens of potential black hole collisions throughout the universe, which even led to a Nobel Prize award in 2017. So far, those observations are consistent with the "no-hair theorem," suggesting there is no extra information encoded on the surfaces of black holes.

But there's still a chance. There could be "soft hair" on the black holes — just a little bit of information, structured in a way that is challenging to detect.

Of course physicists want to test this idea, because if we could demonstrate that black holes have hair, we would not only solve a major riddle in modern physics, but likely pave the way toward a better understanding of quantum gravity, or the theory that would reconcile general relativity, which governs the universe on a large scale, with quantum mechanics, which describes reality on the tiniest scales.Now comes the real hard work of science: connecting neat ideas to actual observation. The new arXiv paper suggests a way to find these soft hairs. The new study authors, Lawrence Crowell of the Alpha Institute for Advanced Studies in Budapest, Hungary and Christian Corda, a physicist at Istanbul University in Turkey, discovered that during the merging process, normally-quiet hairs can get excited, so to speak. In this energized state,, these hairs would intertwine with the outgoing gravitational radiation, altering those waves in subtle ways.

Those changes to the gravitational waves can't be detected yet, but future versions of LIGO might have the sensitivity to do it. And then we might be able to finally tell whether black holes are hairy or not.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.

Originally published in Live Science.

Paul Sutter

Paul M. Sutter is a research professor in astrophysics at  SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including  "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.