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The Black Hole Bomb - A theoretical infinite energy source

Energy is required for almost all activities. It is required to cook, to turn the lights on, and to keep your room cool over hot summer days. Over the IGCSE and IB physics course, even during KS3 science classes, you may have heard about different energy sources and how they are processed in fossil fuel stations, thermal power stations, nuclear power stations, hydroelectric dams, etc. Though these energy productions are effective and have many advantages, most of them play a role in enhancing the greenhouse effect and threatening our environment. To neglect this significant drawback, this article will address a theoretical energy extraction from outside our planet using the black hole via an artificial “black hole bomb”. 

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What is a black hole?

A black hole is a region of space having a gravitational field so strong that any matter or radiation - even light - cannot escape. It is the largest collection of pure energy in our whole universe as it traps all the energy of any object that moves close to it forever. 

 

There are two types of black holes: non-spinning and spinning black holes. We can only extract energy from the spinning black hole. 

 

How are black holes created?

To explain this, I will explain the life of stars:

 

Stage 1: The Nebulae

This is when gravity slowly pulls dust and gas particles together into clumps which get bigger. They are pulled together so tightly that the temperature and pressure increase, which stimulates the nuclear fusion reaction to begin. This causes a huge amount of heat and light energy to be released and protostar to be formed.

 

Stage 2: Main sequence (stable) star

Gravitational forces pull the particle together and inwards and radiation pressure from the nuclear reaction acting outwards are balanced. During this stage, stars release energy by converting hydrogen into helium.

 

Stage 3: Red giant

Eventually, the hydrogen nuclei run out and hydrogen fusion reactions stop. During this time, the gravitational force becomes larger and compresses the star. As the star shrinks, there is a large increase in temperature that is so high that fusion reactions between the helium nuclei begin. The energy released from these reactions causes the star to expand. The star swells and becomes cooler and redder. 

 

Stage 4: Death of a star

There are two paths from stage 3 depending on the size of the star. If the star is relatively small, it will become a red giant, but if it is large, it will become a red supergiant. Only the supergiants will turn into a blackhole whereas the red giant will turn into a white dwarf. The red supergiant will contract and become unstable as time passes and eventually explode releasing massive amounts of energy, dust and gas (the supernova). The supernova will leave behind either a neutron star or a black hole!

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Why black holes spin:

So, why do these black holes spin? 

The stars that the black holes are originally from were rotating before they died. This means that they had angular momentum. A key principle to momentum is that they are conserved. Hence, due to the conservation of momentum, the angular momentum the star had originally is passed down to all objects formed in the supernova. Additionally, as the star is exploded, the resulting particles will be smaller and lighter than the original star. Therefore, they will spin faster until the star collapses completely into a black hole. This is why the black holes spin extremely fast. 

This rapid spinning of black holes causes them to morph space and time themselves. This creates a new region of space-time called the Ergosphere, which envelops the blackhole. Unlike the horizons of the black hole, it is possible to enter and leave the ergosphere.  The spinning black holes transfer their own kinetic energy, in the form of rotation, to everything that enters the ergosphere. This is where the energy will be ‘stolen’ from in the black hole bomb theory.

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How to steal energy from the black hole

When an object drops into the ergosphere, the ring-ularity (ring with a thickness of zero and no surface, spinning extremely fast, containing all the mass of the black hole) forces energy onto the object.

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So how does this relate to gaining energy? If you send a rocket into the ergosphere and give some mass energy by dropping a part of the rocket, the ergosphere will return by giving some of its rotational energy (Newton’s third law!). This rotational energy allows the rocket to leave the ergosphere with much more energy the rocket had when it entered the ergosphere. 

The diagram on the left is a simplified version of this process.

In the future with advanced technology, it could be possible to harvest asteroids to drop them into the black hole instead of having to build new rockets every time and lose our finite resources. The mirrors can be made of metals in a large asteroid. Thus, black hole bombs are much more resource-efficient than firing the rocket. 

 

The Black Hole Bomb

Using the same theory, we could make a ‘black hole bomb’ to extract energy. 

 

The process is as follows:

  1. Set up massive mirrors with some windows that completely envelop a rapidly spinning black hole. 

  2. Shoot electromagnetic (EM) waves at the black hole through a window in the mirrors

  3. These waves will hit the black hole at light speed. Some of the waves will go into the event horizon (the centre of the black hole where nothing can escape from it) and disappear, but the majority will gain rotational energy from the ergosphere and get amplified. 

  4. These amplified EM waves will begin superradiant scattering (bounce around between mirror and black hole and get stronger)

  5. We can extract this amplified energy from the waves through the window

 

However, there are some dangers to this method. If the waves are not released often, it will continue to get stronger exponentially until the mirror shatters. This would cause a sudden release of large amounts of energy as big as a supernova, making it the largest explosion ever created. Furthermore, this is all in theory. Therefore, we cannot assume the practical difficulties involved in this process. 

 

Nonetheless, this is not science-fiction. In the very far future, this may be the only way to gain energy to sustain the dying universe. After all, the red dwarfs have cooled down, and all the white dwarfs transform into black dwarfs, the universe will turn dark forever and these rotating black holes might be the only source of energy in the entire universe. 

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By Jiwon Yoo (Year 13 Student at Bangkok Patana School)

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Bibliography

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Kurzgesagt - in a nutshell, “The Black Hole Bomb and Blck Hole Civilizations”, 2018, Available on : https://www.youtube.com/watch?v=ulCdoCfw-bY [Accessed 23 August 2021]

 

Wikepdia, “Rotating black hole” Available on: https://en.wikipedia.org/wiki/Rotating_black_hole#:~:text=A%20rotating%20black%20hole%20is,of%20its%20axes%20of%20symmetry. [Accessed 23 August 2021]

 

Wikepedia, “Ergosphere”, Available on: https://en.wikipedia.org/wiki/Ergosphere#:~:text=The%20ergosphere%20is%20a%20region,%2C%20which%20means%20%22work%22. [Accessed 23 August 2021]

National Schools’ Observatory, “Life of a Star”, Available on: https://www.schoolsobservatory.org/learn/astro/stars/cycle [Accessed 23 August 2021]

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