A Dyson sphere around a black hole

The whole concept of a Dyson sphere is kinda idiotic. Any civilization that is capable to build one, it is probably able to work out fusion energy very efficiently.

There is no point to go and harness energy around a star or a black hole, when you can just produce it locally with a lot less resources/waste and materials. The sun itself is actually very inefficient in producing energy.

There is no need to harness the sun million of km away, when you can recreate it in your home planet. The only way to produce a dyson like of sphere, is to tame an over-heated sun, and reflect away un-needed energy. But there is no point to build one to just harness it.

It makes great sci-fi stories, but that's about it. Scientifically, it just doesn't make sense.

Because to power a type 2 civilization you need around 4 * 10^26W (as the paper states). Over a year that is around 10^31 kWh worth of energy. Assuming fusion can transform 1% of the input mass in energy you need ~ 1.4 * 10^19 kg per year. To put that number into perspective mount Everest weights 2.7×10^14 kg [1] so thousands Everests worth of mass.

That is a lot of mass to extract and transport to the power stations (accelerate, decelerate). So it just makes sense to only needing to build the facilities to collect the power from existing sources (stars, black holes) without the logistics of transporting the fuel.

Moreover, the space around a star or black hole is real estate that would go unused otherwise, while asteroids, moons, ... are more likely usable by such an advanced civilization.

[1] https://www.quora.com/What-would-the-estimated-weight-of-Mou... (I took the highest estimate in the first answer)

Any idea if there’s enough construction material in any given planetary system to construct a Dyson sphere around its star?

Seems like it would require, at minimum, several planets worth of raw material.

> Seems like it would require, at minimum, several planets worth of raw material.

You could mine it from asteroids, which have plenty of base metals. There's already a speculative market for investors wishing to mine them. The only thing is; the technology for transporting it around doesn't exist yet. (We can land probes on them however for recon, but that's about it so far!).