Free, "green" energy is part of the dreams of many people: cars running on water, producing no pollutants, electricity from vacuum energy, etc. However, there is a scientifically sound scheme for "free" energy that may perhaps come true one day.

We just have to find a reachable, spinning black hole with a "hospitable" environment around its equator. Then we can "colonize" that black hole by building a static, circular, torus-like tube around its equator, large enough to contain cities, land for livestock and crops, lakes, etc.

To provide power to this stationary "planet", we can use Sir Roger Penrose's black hole energy extraction process. This rather neat process can actually get rid of the planet's garbage in a completely clean way as well! Here's how it works.

The trucks of the garbage disposal company are fitted with specially designed containers that, when full, can be dumped straight down towards the black hole. Because the black hole spins, the containers do not fall into the hole, but are dragged around it by a process called "frame dragging".

The containers do not go into orbit around the black hole. The spinning black hole gives them a slingshot that sends them back to the planet with some kinetic energy "robbed" from the black hole's spin momentum.

But this is not all. By ejecting the garbage from the container into the black hole at the correct point during the slingshot, an enormous amount of extra energy can be added to the container in the form of extra speed. The empty container recoils outward from the ejection point with an enormous velocity.

This is not just the Newtonian effect "for every action there is an equal and opposite reaction" - it is a relativistic effect according to Einstein's famous equation E=mc^2 (Energy equals mass times the square of the speed of light).

At the planet's power station, the high-speed container is caught in a machine that converts most of the momentum of the empty container into rotational momentum of a giant flywheel. This flywheel drives the generators that feed the planet's electricity grid.

Think about some of the benefits:

(i) Garbage completely out of sight, forever hidden inside a black hole. No odors, no pollution, no potential disease-causing bacteria...

(ii) Even at a very conservative 1% overall efficiency, the energy output of this process can be astonishing. A single kilogram of garbage can produce enough electrical energy to power a futuristic, power-hungry household for more than a century!

On the reverse side of the coin, we may soon have the planet's "green party" raising at least three issues:

(i) We are polluting our black hole's insides and who knows what the consequences might be?

(ii) We are "burning" our black hole's mass and hence slowly destroying it (if the green party can figure this one out!)

(iii) Why do citizens have to pay for free electricity being produced from their own garbage?

The process described above is not only a theory - a number of observed stars that have apparently been flung out of their host galaxies at incredible speeds, are attributed to this process.

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