This month, the Evil Engineer weighs up whether black holes could make for an unconventional source of renewable energy.
Dear Evil Engineer,
Since our friend in Moscow moved on Ukraine, energy markets have been in an irresistible state of chaos. Do-gooders and villains alike see this as an opportunity to realign markets in their interests – whether with a shift towards LNG, nuclear, or renewables.
I have been watching and waiting, wondering about how to make the most of this moment. Given the abundance of government incentives for sustainable energy projects in twin pursuit of decarbonisation and energy security, I ought to make the most of those.
However, I cannot risk compromising my brand by investing in wind turbines, solar farms and other benign, tried-and-tested infrastructure. It would be more brand-appropriate to harvest renewable energy from a black hole. Would that be possible?
A power-hungry villain
In theory, it is possible. In practice, merely accessing a suitable black hole would probably take tens of millions of years.
Energy can be harvested from a black hole via the Penrose process, which was laid out by mathematician and Nobel Laureate Sir Roger Penrose in 1969. The process concerns the ergosphere. This is the vicinity surrounding the event horizon – the boundary at which nothing can escape the black hole’s gravitational pull – in which spacetime is ‘dragged’ around in a sort of vortex by the aggressively spinning black hole. The term ‘ergosphere’ is derived from the Greek ergon (work) because it is possible, in theory, to extract energy from it.
Imagine a rocket being dragged around in a black hole’s ergosphere. Before it reaches the event horizon, it fires a propellant backwards (against the direction of movement). With some precise planning, the propellant can be sent on a negative-energy trajectory into the black hole, while the rocket is sent on a positive-energy trajectory out of the ergosphere. Conservation of energy requires that, as a negative-energy object falls into the black hole, an equal amount of positive energy should escape it – according to the calculations of fellow Nobel laureate Subrahmanyan Chandrasekhar, an object could exit the ergosphere with 20.7 per cent more energy than it had initially.
The Penrose process may seem ‘noncommonsensible’. However, a 2020 Nature Physics paper written by University of Glasgow researchers described a successful experimental analogue for the process. This found that the amplitude of sound waves could be increased by ‘borrowing’ energy from a rapidly spinning disc.
To be pedantic, a black hole is not a truly renewable source of energy. As energy is extracted from the black hole, its spinning slows very slightly and, eventually, it would come to a halt. Given the timescales over which this would happen, however, this is not such a concern. The practical engineering challenges are more concerning – Penrose himself acknowledged that these were of such a magnitude that only a very advanced civilisation would be able to tap into the power of a black hole.
Before you can start thinking about how to design this rocket and generate electricity from it, you need to find a black hole.
Unfortunately – or perhaps fortunately? – there are no black holes nearby. In January, it was announced that a dormant black hole had been discovered closer to Earth than any other. This black hole is just 1,600 light years away, in the constellation of Ophiuchus – it would take 30 million years to reach using current rocket technology. While it is hard to say what will happen next regarding geopolitics, it is probably safe to say that the current energy crisis will have passed by then.
So, how about creating a black hole closer to home? A black hole is created when a colossal star dies and its remnant core collapses under its own gravity – essentially when a tremendous amount of mass is crushed into a tiny region of space. Replicating such an astronomical phenomenon is impossible with the technology available in the foreseeable future, but it might just be possible to create miniature versions (micro black holes) here on Earth.
Creating a micro black hole would require energies of around 1016TeV to be condensed into a region of space on the order of the Planck length (1.6×10-35m). These energies are far beyond what even the Large Hadron Collider (LHC) can reach. However, in scenarios in which there are more than the familiar three dimensions of space – the extra dimensions being folded up and hidden – the strength of gravity could be many, many magnitudes stronger at the small scales relevant to micro black holes. Computer simulations show it could be possible, under those circumstances, to produce a micro black hole with energies in the range accessible by the LHC. It was actually suggested that micro black hole production could be an effect observed at the facility, although none have been detected so far.
This is all very exciting physics – but is it an opportunity for exciting engineering? Almost certainly not. Such small black holes would evaporate in a fraction of a second, making them useless for power generation.
It is theoretically possible to use a black hole as a power station. However, there is no practicable way to do this within a timescale comparable to human history, simply because of the great distance between Earth and the nearest known black hole. Might I interest you in fusion power instead? It is, as ever, 50 years away.
The Evil Engineer
PS: There is an alternative to the Penrose process, the Blandford-Znajek process, which also proposes using a black hole’s ergosphere to extract its energy. The powerful magnetic field in the ergosphere accelerates charged particles away from the black hole, producing energetic jets which could be used for power generation. Like the Penrose process, this rather depends on having a rotating black hole to hand.
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