Researchers explore harvesting energy from black holes

First image of a black hole, actually the shadow of the black hole, at the centre of the galaxy Messier 87. Image: Event Horizon Telescope

New research identifies a mechanism and conditions for harvesting the free energy stored in spinning black holes.

Black holes, believed to form when massive stars collapse at the end of their lifetime, are arguably the most enigmatic objects in the universe. They comprise a region of spacetime where gravity is so strong that nothing can escape from it. Thus, one cannot see them directly and can only infer their presence, and their highly energetic nature, from the surrounding matter.

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The concept of extracting rotational energy from a black hole is a prediction of general relativity and was first realised in the late 1960s, but no viable mechanism for extraction has emerged.

Now new research from physicists Luca Comisso from Columbia Astrophysics Laboratory in New York and Felipe Asenjo from the Adolfo Ibaàƒ±ez University in Chile, suggests there is such a process, in essence of immersing the spinning black hole in an externally supplied magnetic field and plasma of charged particles. This would cause a rearrangement or ‘reconnection’ of the magnetic field lines with an emission of particles, some that fall into the back hole and others that would escape.

The research, which is published in the journal Physics Review D, identifies two key conditions for energy extraction. One is that the black hole must be fast spinning and the other that it should be massive. With both the necessary spin rate and the energy extraction potential scaling with the square of the mass, the optimum target black holes would be the most massive and fastest spinning.

The process also is found to be highly efficient compared with other models, with an energy efficiency that can reach 150%. It is not unlimited, however, as the power extraction can induce a significant spindown of the black hole and a loss of efficacy of the magnetic reconnection process.

In practice, the researchers suggest that the magnetic reconnection is likely to occur intermittently and the associated emission expected to display a bursty nature.

This is the theory anyway and while the research should lead to further understanding of high energy phenomena in the universe, harvesting energy from black holes will not take place in the foreseeable future. But science fiction has a way of becoming science fact and should not be ruled out for future civilisations.

In the meantime, we are just starting to harness the energy output of our sun and after several tens of years of research still have to effectively replicate the nuclear fusion reactions that power the sun and the other stars. A solution to this fusion conundrum would go a long way to meeting the energy challenges of the times and years ahead.

The black hole in M87 is estimated at just under 40 billion km across and with a mass of 6.5 billion times that of the sun. It is 520 quintillion km away.

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