I find a small black hole floating in space. It's very small, say 0.001 femtometer diameter. But it weighs about 300 thousand tonnes. It is also quite hot at about 400 trillion degrees and altogether it radiates about 4000 Terawatts of power. Apparently it will take about 70 years to evaporate. [ ref. https://space.geometrian.com/calcs/black-hole-params.php ]

I plan on maintaining the black hole's power-level and position using four cannons arranged in a tetrahedron around it. I'll put them a few kilometers away and protect them from the radiation. It seems like shooting a few cannon balls every day should do the trick?


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The smallest atom we know is about 1 Ångström = 0.1 nanometer ($1\cdot 10^{-10}m$) in scale, a proton's size is a bit less than 2 femtometers ($2\cdot 10^{-15}m$). I.e. your canons are extremely hard put to even hit the black hole.

Then, there is the problem of the Hawking radiation coming out of the black hole. 4 Petawatts ($4\cdot 10^{15}W$) coming out of a 1 attometer ($1\cdot 10^{-18}m$) is an insane amount of flux. So, if you shoot protons at your black hole, all they will hit is the radiation that's coming out of the black hole, and be deflected by it.

Bottom line: It's plain impossible to feed such a black hole. All you can do is run away from it. Far, far away. Board a spaceship and put at least star between yourself and the black hole. Because if you are still around when it evaporates, you'll evaporate as well.

  • $\begingroup$ Good point! If I happen to come across any small black holes, I will definitely give them a wide berth. Would this work if I used 1 solar mass and I chucked planets at it instead of cannonballs? $\endgroup$
    – Roger Wood
    Sep 26, 2020 at 22:12
  • $\begingroup$ @RogerWood A stellar BH has a diameter in the range of tens of kilometers. Easy enough to hit with asteroids and such. However, it's black. Really black. You won't get any appreciable amount of Hawking radiation out of it. That said, if you really toss a planet at it, most of it will miss the BH, but be accelerated towards it, collide with itself behind the BH, and heat up. I'm not sure about the extend of this, but I definitely do not want to be on that planet - I have a hunch that this might just cause the planet to explode... $\endgroup$ Sep 27, 2020 at 8:12
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    $\begingroup$ I figured maybe a 3000K source might be useful, i.e. sunlight - easy to turn into electric power. But if I understand correctly, a 3000K black hole generates only about one microwatt of power - despite weighing about 100x Mt. Everest. Black holes don't seem to be very useful at any size :-( $\endgroup$
    – Roger Wood
    Sep 28, 2020 at 6:56
  • $\begingroup$ "if you shoot protons..." – sorry, but where do you see anything about protons in the original question? I think the author meant cannons firing with conventional metal balls. Also about energy, at the distance of 500 km, the radiation would be similar to what we have on Earth from the Sun (~1.3kW/sq.m). According to the radiation rate, the black hole will lose ~44 g per second, which is a pretty doable rate of matter supply. $\endgroup$
    – greatvovan
    Mar 2, 2021 at 1:22
  • $\begingroup$ I think such an enterprise would be impossible but on different reason. I am concerned about actually "feeding" the BH. Taking into account its radiation, it would evaporate anything approaching it and probably push away the debris as its gravitational pull is negligible. $\endgroup$
    – greatvovan
    Mar 2, 2021 at 1:23

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