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Black holes, famously portrayed as dark destroyers of the Universe lurking amongst the cosmos, are a well-known phenomenon not just to space physicists but to anyone with even a vague interest in science. What happens to someone when they fall into a black hole? It's a question that has long been thought about, and yet the answer continues to perplex the greatest minds in science. With the nearest black hole over a thousand light years away from Earth it's unlikely somebody will fall into one anytime soon and tell us, so discussions surrounding black holes are obviously theoretical, but research continues to probe and uncover some of the biggest mysteries of our Universe.
If you were unlucky enough to find yourself heading helplessly into a black hole it has long been expected that you would cross the event horizon, the point of no return, after which you would quickly be spagettified (yes, that's the actual scientific name for this) and then crushed by the immense gravitational forces as you approach a spacetime singularity at its centre. Not much fun. It now seems though that an even worse fate may await you.
Forty years ago physicists Stephen Hawking and Jacob Bekenstein famously showed that black holes are in fact not really black - they radiate energy, known as Hawking radiation, which is comprised of photons and other subatomic particles. This radiation is emitted slowly back out into space, and eventually causes the black hole to evaporate away. At the time of this discovery Hawking believed this radiation was so random that it could not carry any information, so when the black hole eventually disappears the information contained within it must be lost.
A key principle of quantum mechanics though is that information cannot be destroyed, thereby directly contradicting the apparent nature of Hawking radiation... the black hole information paradox was established.
Hawking eventually changed his mind years later to say that black holes do not destroy information, persuaded by the work of Juan Maldacena which showed that quantum mechanics is valid at the surface of a black hole and that information is not lost as it evaporates. But the question remained unanswered... when a black hole dies, what happens to the information it sucked in if it can't ever escape or be destroyed?
This paradox has revealed a galactic struggle between general relativity and quantum mechanics over control of the Universe.
Information cannot just leak out of a black hole, since it would have travel faster than light, thereby breaking a fundamental law of physics. Perhaps then this Hawking radiation isn't so random as it was initially thought to be. If Hawking radiation does transmit information away from a slowly-evaporating black hole, as a recent theory implies it can, then the consequence is that a 'firewall' would be established at the event horizon. And you thought the impending stretching and squashing to infinity was going to be the worse part of your journey.
Throughout the Universe virtual particles pop into and out of existence, and if one of these entangled particles falls into the black hole, yet the other somehow escapes, we have trouble. The particles inside the event horizon would become incredibly energetic as they pass information to their paired particles on the outside, with the extreme buildup of energy creating a 'firewall' of intense heat at a temperature of up to 1032K that would instantly incinerate anything and everything that enters into the black hole. This though contradicts general relativity, which says that crossing the event horizon of a black hole should be pretty uneventful. Some physicists are thus skeptical of the idea, including theoretician Leonard Susskind, though he proposes that the firewall could signify the movement of the black hole's singularity. The absence of a singularity at the centre of a black hole could help to solve the problem of information loss. If black holes aren't the all-guzzling bottomless pits we thought they were, then information they take in doesn't have to be lost when they evaporate away - it may just be able to leak out through the firewall. The shaken belief that information can never be destroyed is stabilised. The firewall idea may sound rather unbelievable, but at the moment it's the best explanation for how information could escape from black holes as part of Hawking radiation.
Perhaps though if you do find yourself falling into a black hole, there may yet be hope of avoiding an unpleasant end.
The question of how a journey into a black hole ends is a similar problem to that of explaining how the Universe came into existence, which is widely thought to be from a 'big bang' event at a singularity point. A few years ago Ashtakar applied loop quantum gravity (LQG), a theory that combines general relativity with quantum mechanics and defines the Universe as a grid of 10-35m sized 'bits' of spacetime, to the task of explaining the beginning of the Universe. The findings were that simulating back in time the 'big bang' event was reached, however instead of an initial singularity point, a 'quantum bridge' was crossed into another older universe. Pullin and Gambini recently applied LQG on a much smaller scale to model an individual black hole. The simulation found that whilst the gravitational field still increases towards the centre of the black hole, instead of reaching infinity at a singularity point it eventually reduces. It's as if the black hole leads to another region of the Universe, or indeed a different Universe altogether. Whilst this finding is only for a simple model of a simple black hole, the theory could overturn the long established belief that singularities exist within real black holes. Things are suddenly looking slightly better for you. The removal of the singularity means that black holes could act as portals to other parts of our Universe or even to other universes... but we must be careful not to jump at such ideas and wander in to the realms of science fiction.
Decades later, Hawking's black hole information paradox is yet to be resolved. We still don't know what happens when you fall into a black hole. Perhaps your fate is spaghettification, perhaps a fiery death, or even to stumble out into a whole new Universe. Maybe none of these. Whatever happens, down the black hole lies the true nature of reality.
