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According to Wikipedia and Stephen Hawking's A Brief History of Time, micro-black holes are expected to evaporate quickly due to Hawking radiation, releasing a burst of final emission equivalent to the energy of millions of hydrogen bombs (over several trillion kilotons of energy).

An advanced civilization is in war with another advanced civilization, and is hoping to destroy the latter. The offending civilization (let's call it A) is developing a black hole gun, capable of launching 500-metric-ton black holes. However, they do not have the technology to prevent their black holes from evaporating. Therefore, the projectile must be formed in-situ and immediately fired (kind of like "there are no black hole refrigerators").

This gun can fire, at maximum load and power, black holes at 0.99c. Civilization A has worked out the times for a black hole to evaporate given the time and time dilation. They plan on launching a black hole at their enemy planet in four Earth days. Anything longer and their enemy will annihilate their space fleet. They still need clarification on whether or not to continue the project or ditch it. They ask: "Would this be a viable plan or would if blow up in our faces?"

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    $\begingroup$ if they can fire 500 tons at .99c, it would be much easier and just. as deadly to fire a lump of rock. $\endgroup$
    – ths
    Jan 5, 2021 at 15:10
  • $\begingroup$ @ths black holes are harder to vaporize than a 500 ton rock. $\endgroup$
    – WarpPrime
    Jan 5, 2021 at 15:13
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    $\begingroup$ at .99c that's really irrelevant. a cloud of vapour at that speed will be just as deadly. also how do you hit something coming at you at that speed? $\endgroup$
    – ths
    Jan 5, 2021 at 16:49
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    $\begingroup$ How do you even see anything coming at 0.99 c before it hits you. $\endgroup$
    – NomadMaker
    Jan 5, 2021 at 21:28

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According to this calculator a 500 metric tons black hole would have a lifetime of about 10 seconds, which for the black hole traveling at 0.99c would be 1.4 times longer.

Considering that the black hole would emit 340000 Megaton/s peaked somewhere in the gamma rays, I hope they have a suicide squad firing it, because they will be fried by the emission of the bullet while it travels toward the target.

If you play with the calculator you will find that increasing the mass of the black hole reduces its luminosity. With a mass of 16017 metric tons it would explode in 4 days but it would still emit 331 Megatons/s. If you can't block Hawking radiation until it reaches a safe distance from the firing ship, you won't have any non suicidal usage.

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  • $\begingroup$ Would a robot operating it be better? Or maybe an automated weapon fired from remote control as a separate, unmanned gunship $\endgroup$
    – WarpPrime
    Jan 5, 2021 at 15:17
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    $\begingroup$ @fasterthanlight, you would have to handwave how to bullet can be guided in any direction while being a continuously exploding large nuke $\endgroup$
    – L.Dutch ♦
    Jan 5, 2021 at 15:24

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