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Planetary science

Planetary science

Mapping orbits within black holes

20 Apr 2011 Tushna Commissariat
Artist's impression of a black hole

The words “black hole” generally bring to mind destruction and an end to all ends. No-one – in fact or fiction – has considered the possibility of stable habitats existing within black holes. But that is precisely what physicist Vyacheslav I Dokuchaev of the Russian Academy of Sciences, Moscow, is suggesting in his new paper, “Is there life in black holes?”. Published in the Journal of Cosmology and Astroparticle Physics, Dokuchaev suggests that certain types of black hole contain stable orbits for photons within their interior that might even allow planets to survive.

Essentially, a black hole is a place where gravitational forces are so extreme that everything is sucked into them – including light. They have outer boundaries, known as event horizons, beyond which nothing can escape because matter starts moving at faster-than-light speeds. But charged, rotating black holes – known as “Kerr–Newman black holes” – exhibit an unexpected twist. They have not only an outer event horizon but also an inner horizon, called a “Cauchy” horizon. At this Cauchy horizon, because of the centrifugal forces involved, particles slow down back to the speed of light.

The final frontier

Since the 1960s, researchers have determined stable orbits for photons inside these charged, rotating black holes. In his new paper, Dokuchaev has looked at stable circular orbits as well as spherical, non-equatorial orbits for photons at the inner boundary. He concludes that there is no reason that larger bodies, such as planets, could not do the same. He even suggests that entire advanced civilizations could live inside this particular subset of black holes, on planets that orbit stably inside the hole – using the naked singularity as a source of energy. They would forever be shielded from the outside and not sucked into the singularity itself, he says.

In theory it should be possible to use the singularity as an energy source explains Andrew Hamilton, an astrophysicist at the University of Colorado in the US who has also calculated the orbits at the inner horizon inside these black holes. “A rotating black hole acts like a giant flywheel. A civilization can tap the rotational energy of the black hole by playing clever games of orbital billiards, something first pointed out by Roger Penrose,” he says.

However, Hamilton believes that, in reality, the situation is implausible. Inflation at the inner horizon would cause space–time to collapse, not to mention disturbances created by the high energy density at such a location, from massive amounts of matter falling into the black hole. On the whole, none of these circumstances would make for habitable conditions. Dokuchaev himself acknowledges these problems in his paper, but does not provide a solution.

Paradoxes and information losses

Even if a planet and then a civilization were to form inside these black holes, it would be almost impossible to discover them because all information is lost going into or coming out of a black hole. Although new theories state that information from the interior of black holes is encoded in the Hawking radiation emitted from them, this information could quite possibly be scrambled.

Arthur I Miller, a physicist and author of several popular-science books, believes that it is pointless to look at any possibility of life inside black holes, stable orbits notwithstanding. “It is, indeed, extreme science fiction to imagine the existence of worlds in them. Surely it would be a ‘crushing experience’ living inside a black hole?” he says.

So, while most scientists will agree that looking for life inside black holes is a futile venture, the sad truth is that we will never know if the real-estate market is missing out on a great new platform.

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