Harnessing Energy from Black Holes?
Whether you’re a physicist or not, you’ve probably heard of a black hole. Despite the work of some of the greatest minds like Stephen Hawking, they’re still shrouded by cloud of mystery. However, we have learned quite a few things about them, and now there are talks that a hypothetical super-advanced civilisation (possibly humanity in a few centuries), could harness energy from a black hole. Sounds interesting, let’s talk about it.
Refresher
Most black holes are formed from the remnants of very large stars called Red Supergiants. As these stars expand due to the huge amounts of energy released during nuclear fusion at the core, they eventually get so large that there is too much matter to hold together and the outer layers are blasted off in a huge explosion, resulting in a supernova (which has to be one of the coolest looking things in all of physics).
All the matter in the core is under such a large pressure that cannot support itself, leading to it collapsing into a black hole. This point, known as a singularity, is so dense meaning the gravitational force around it is so strong nothing can escape… not even light. The size of this black hole grows massively over the course of hundreds of millions of years as it swallows up matter around it. But remember that the force due to gravity decreases (as an inverse square law) with distance, meaning the further you are from the singularity, the weaker the force. There is a boundary where on the outer side light can pass, but on the inner side, light is sucked into the middle. This oneway boundary is known as the event horizon.
Below is the first ever real image taken of a black hole taken in April 2019.
Why black holes?
Remember Einstein’s E = mc²? This equation showed that anything with mass holds a huge amount of energy. In fact, a 5kg cat, if entirely converted to energy, could power the whole of Norway for a year. The problem is that it’s incredibly difficult to efficient convert mass into energy. There are three main options for getting this energy: chemical reactions, nuclear reactions, and gravitational reactions.
Chemical reactions are absolutely terrible for converting mass into energy. Such little of the mass is converted to energy that we don’t even associate chemical reactions as mass-energy conversions, they might as well be two different things.
As explained by minute physics ( https://www.youtube.com/watch?v=t-O-Qdh7VvQ), the reaction of a ballon of 0.4g of hydrogen and 3.2g of oxygen gives a nice explosion, but only releases 47kJ of energy, which corresponds to 0.000000005g, meaning the efficiency is about 0.000000001% efficiency.
Nuclear reactions are better, but still not great on an absolute scale. A common nuclear fission reaction, where uranium-235 splits into krypton and barium, has a mass defect on the order of 10^-30 kg, meaning only about 0.08% of the uranium’s mass is converted into energy. Nuclear fusion is better, where the fusion of hydrogen into helium (like in the sun) converts about 0.7% of the hydrogen’s mass into energy. We often hear on the news about how incredible fusion will be at generating energy efficiently. Relative to chemical, this is very true, and for humans it will be more energy than we know what to do with. But I find it interesting that this is still not even 1% efficient. This is where black holes come in.
Harnessing energy from a black hole
Black holes are likely the best way to convert mass into energy in the universe (aside from antimatter annihilation, which is 100% efficient — but there is almost no antimatter in the universe so this is not possible). While it is true that nothing can escape a black hole once inside, the efficiency of black holes comes from what happens to matter as it falls towards them, before passing the event horizon.
Think on Earth: if you climb up a mountain, you are doing work and thus gaining gravitational potential energy. Then, when you jump off the mountain (with a bungee jump I hope!) that gravitational potential energy is transferred to your kinetic energy as you speed up. Now scale this up to a meteor heading towards Earth. As it falls very fast, it collides with the air particles in the atmosphere, heating up and thus releasing infrared radiation (energy). On this scale it’s pretty useless, but a black hole has a MUCH larger gravitational force, and so things will accelerate to ridiculous speeds (in theory up to a kinetic energy that is half its mass energy). But if it falls straight into the black hole, that energy is no longer accessible. So the key is to make an object spiral around a black hole, colliding with other matter on the way, heating up and radiating its energy for as long as possible. For a theoretical non-rotating blackhole, the ideal spiral radius is about 3 times the radius of the event horizon. In doing this, about 6% of the mass can be converted to energy… now we’re talking! But that’s not as good as it gets. Black holes are rotating. And because of their huge mass and gravitational forces, they create a region of space above the event horizon called the ‘ergosphere’.
This volume of space-time is rotating along with the black hole, meaning that even higher kinetic energies can be achieved, and it turns out that efficiencies as high as 42% can be achieved! That means 2 and a half cats could power Norway. Pretty cool! There are various other methods of extracting energy from a black hole. Watch this video for more:
Originally published at http://thephysicsfootprint.wordpress.com on January 17, 2022.
