The information locked in black holes could be discovered by feeling their ‘hair’, new research suggests.
Black holes are celestial objects with such a massive gravitational force that not even light can escape their claws once it crosses the event horizon, or point-of-no-return. The horizons of the events of black holes open up secrets deep within them – secrets that could completely revolutionize our understanding of physics.
Unfortunately, many scientists thought for decades that information falling into a black hole would be lost forever. But new research suggests that ripples in space-time, or gravity waves can whisper a faint whisper of this hidden information by opening the presence of wiffy “hairs” on the surface of a black hole.
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A hairy question?
As far as we understand them (which, admittedly, is not entirely), black holes are suspiciously simple objects. No matter what falls into, or stars, clouds of gas and dust, if your worst enemies, black holes can be described by three and only three simple numbers: charge, mass and spin.
That means if you had two black holes of the exact size, exactly the same electrical charge, and spinning at exactly the same speed, you would not be able to tell them apart. The reason this is suspicious is that something had to happen with all that juicy information that fell into those two black holes. Was it destroyed? Lost below the event horizon? Stuck in some inaccessible part of the universe?
The simplest solution is the statement, first coined by the American physicist John Wheeler, that “black holes have no hair” – they have no additional information encoded in them or on it. Just her mass, electric charge and spin. Everything else is simply destroyed (in one way or another) outside the event horizon, forever and forever cut off from the universe.
A paradox of information
But in 1974, Stephen Hawking proposed a revolutionary idea: black holes are inevitable cosmic vacuum cleaners; rather, subatomic particles can escape 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 gradually lose energy (and thus mass). Eventually, after centuries of slowly losing energy, the black holes would completely evaporate.
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 falls into them?
As far as we know, Hawking radiation does not carry any information. And we really, really do not think that information can be created or destroyed in this universe (it is certainly possible, but would make a forest of known physics quite shaky, which observations and experiments conflict with).
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And therefore, the paradox for information about black hole. Information goes into a black hole, the black hole disappears, and we do not know what happens to the information.
To solve this paradox, we need to repair what we know about black holes or repair what we know about Hawking radiation. Or both.
Maybe the information is trapped deep in the black hole, close to the singularity, and evaporation stops just before that point, leaving a tiny little bump full of information behind.
Or maybe black holes are not completely hairless. Maybe, just maybe, they keep the information of everything that has fallen into them on their surfaces, contained in something called the “extended horizon”, a surface just above the event horizon with quantum mechanical information. As black holes dissolve, the Hawking radiation carries the information contained in the stretched horizon, resolving the paradox and preserving our reality as we know it.
Great idea, but how do we test it?
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Ripples in space-time
A new study, published June 22 on the arXiv database (but not yet peer reviewed), suggests one way to find these silk strings: a detection of gravitational wave.
As black holes fuse, they create a rage from gravitational waves rippling through the cosmos. Despite the immense energy of these collisions, the gravitational forces of these cosmic smashups are exceptionally weak. By the time these waves are spreading across the earth, they are hardly individual atoms.
But we have LIGO – the Laser Interferometer Gravitational-Wave Observatory, an astronomical observatory – that can detect these subtle movements due to the small changes in how long the light takes to travel from distant detectors. LIGO has observed the aftermath of dozens of potential collisions in black holes through the entire universe, which itself late to a Nobel Prize in 2017. To date, these observations are consistent with the “no-hire statement”, suggesting that no additional information has been coded on the surfaces of black holes.
But there is still a chance. There could be “soft hair” on the black holes – just a little information, structured in a way that is challenging to trace.
Physicists want physicists to challenge this idea, because if we could show that black holes have hair, we would not only solve a great mystery in modern physics, but probably pave the way for a better understanding of quantum gravity, or the theory that it would reconcile general relativity, which governs the universe on a large scale, with quantum mechanics, which describes reality on the smallest scales. Now comes the really hard work of science: combining clever ideas with real observation. The new arXiv paper suggests a way to find these soft hairs. The new authors of the study, Lawrence Crowell of the Alpha Institute for Advanced Studies in Budapest, Hungary, and Christian Corda, a physicist at the University of Istanbul in Turkey, discovered that during the merger process, normal hair can become normal, in order to say. In this energy state, these hairs would intertwine with the outgoing gravitational force, and change those waves in subtle ways.
Those changes to the gravitational waves can not yet be detected, but future versions of LIGO may have the sensitivity to do so. And then maybe we can finally tell if 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 en Space Radio, and author of Your place in the universe.
Originally published in Live Science.
