Black holes could be a cosmological engine. When their magnetic fields disconnect and reconnect, they can accelerate plasma particles near the event horizon 鈥� the point beyond which nothing can escape a black hole鈥檚 gravitational pull. The finding could allow astronomers to better estimate the mass and spin of black holes.
We expect all black holes to spin, because they are formed out of a collection of matter that is also spinning before it condenses. And around a spinning black hole there are magnetic fields, which are dragged along with the black hole鈥檚 angular momentum and can influence the direction of matter falling towards the event horizon. These provide the power to the engine.
Luca Comisso at Columbia University in New York analysed the effect of those magnetic fields on plasma particles near the event horizon. Some particles are accelerated by the breaking and rejoining of magnetic field lines, and others are decelerated, thus acquiring 鈥渘egative energy鈥�.
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A black hole can drag space-time 鈥� the very fabric of space itself 鈥� along with it, flinging some particles away while others pass the event horizon and eventually fall into the singularity at the heart of the black hole.
When the black hole finally swallows these decelerated plasma particles and the accelerated particles escape the vicinity of its event horizon, the black hole鈥檚 energy decreases.
鈥淚t is because of this strong rotation of the space-time very close to the event horizon of the black hole that particles can take a negative energy value,鈥� says Comisso. 鈥淚t鈥檚 similar to a person eating candies with 鈥榥egative calories鈥� and losing weight.鈥�
Previous research has suggested that energy could be sapped from a black hole as oppositely charged particle pairs split at the event horizon and only the negatively charged partner is absorbed. Another proposed mechanism of energy extraction, suggested by Roger Blandford at Stanford University in California, suggests that this can occur solely as result of magnetic fields.
The new mechanism goes further, considering the interaction of these particles and fields, resulting in a more significant amount of energy leaking from black holes.
Blandford says this is an interesting idea, and could help us understand the full picture of how black holes lose energy. 鈥淭wo important questions are, will this be competitive with the direct [energy] extraction by electromagnetic fields and how does it lead to the formation of the jets that are commonly seen from around black holes?鈥�
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