# Some astral energy extraction methods

Some notes on harvesting mass/energy at stellar scales from Anders Sandberg's fantastic book, Grand Futures. All errors are my own, any insight is from him.

Harvesting dark energy:  
1) Tying galaxies together: Anchor big rope to galaxies as they get pulled apart by dark matter. Build up elastic potential energy which can be harvested. Issue: inefficient. You get many orders of magnitude less energy than the mass-energy of the rope. Needs negative energy density matter to give better efficiency. OR some way of transmitting tension across cosmological distances. Not clear (to me) how you anchor rope to the galaxies.   
 

2) Letting particles expand apart: again, very tiny amounts of energy compared to mass energy of particles. So small, it isn't clear (to me) if it's a net return.

Dark matter:   
Hawking radiation: takes a long time to see any returns. Very poor efficiency for black holes past 10^11 kg. Past that point, it is just neutrinos which are hard to capture. You can chuck in dark matter which isn't very usable and get interacting stuff back out.     

Neutrino capture:  
Lots of neutrinos running around, especially if you use hawking radiation to capture mass energy of black holes. So you might want to make use of them. But neutrinos are very weakly interacting, so you need dense matter to absorb their energy/convert them to something else. Incredibly dense. To stop one neutrino with lead you need 1 lightyear of matter, with a white dwarf you need an astronomical unit, and for a neutron star (10^17 kg/m^3 density, 10km radium)  you need 340 meters of matter. So neutrino capture is feasible, but it's hard to make matter that dense move stably about the black holes which produce the neutrinos you'd like to capture. Not clear whether this scheme is cost-effective.

Some methods of extracting energy from ordinary matter using black holes.  
  

Accretion discs: chuck in matter to orbit black hole, get very hot due to friction from angular velocity gradients, and radiate energy. Half must be absorbed by the black hole, and perhaps more. As the light then leaves the gravity well, it loses energy. At most, 5% of the energy remains for stationary black holes, 43% for extremely rotating black holes. Very useful for converting matter we can interact with into energy. Not the most efficient, but you don't *need* black holes to have angular momentum to do this, which is perhaps useful. Neat fact: there's a theory accretions discs may have allowed supermassive black holes may have formed the cores of early, supermassive stars. Matter falls into the black hole, some forms an accretion disc and exerts pressure on matter above it, slowing the rate at which the black hole feeds and produces heat. This, in theory, forms an absolutely massive star. 

Penrose process: Extracts energy from angular momentum of black hole, a fair bit of which resides outside the event horizon in the form of frame-dragging spacetime. Have to drop in matter which gains energy, splits into new particles, some of which continue to fall in and others fall out. So not useful for dark-matter, which doesn't transition into ordinary matter. Has 20% efficiency at upper limits for the Penrose process, but Penrose like processes can get you >10x returns on mass-energy. But you need to use up the angular momentum of the black hole, which is boundedly large for a given mass. But you can get up to 50% for extremal charged black holes, and 29% for extremal rotating black holes. So this is good as long as you've got lots of spinning/charged black holes. Big black holes tend to spin reasonably fast, thankfully.  
 

Black Hole Bombs: Another interesting way of extracting energy from black holes are superradiant instabilities, i.e. making the black hole into a bomb. You use light to extract angular momentum from the blackhole, kinda like the Penrose process, and get energy out. With a bunch of mirrors, you can keep reflecting the light back in and repeat the process. This can produce huge amounts of energy quickly, on the order of gamma ray bursts for stellar mass black holes. Or if you want it to be quicker, you can get 1% of the blackholes mass energy out in 13 seconds. How to collect this is unclear.