Imagine I had a sophisticated gun that is capable of producing a micro blackhole on contact with a target, that blackhole is designed to be completely evaporated as hawking radiation in exactly 1 second so I wonder would it devour the being and consume everything in it's path or create a powerful blasts that vaporize anything within range? Due to the science tag I will need to know the threshold for example the feeding rate overwhelm the glowing rate, thanks.

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    $\begingroup$ Black holes are a result of an object with extremely high mass producing a gravity well so deep that it forms an event horizon, across which even light cannot escape. The problem with your sophisticated gun is that it's bullet would have to contain (pre-firing) and transport (during-firing) that extremely high mass from the shooter to target. The amount of energy needed to just carry such a loaded gun, let alone fire it, puts it way of the scale in terms of portability and efficiency. If you have energy density capable of carrying and firing this gun, just throw the energy at your enemy. $\endgroup$ Aug 6, 2020 at 3:10
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    $\begingroup$ Worth reading The Hole Man by Larry Niven for if a somewhat larger black hole is used. $\endgroup$ Aug 6, 2020 at 4:29
  • $\begingroup$ wp tells me that an evaporating black hole which falls below $10^6$kg explodes with the power of $10^{15}$ kg of TNT $\endgroup$
    – Karl
    Aug 6, 2020 at 21:09
  • $\begingroup$ I'd give you a -1 if it weren't for the fact that Marvel handwaves this sort of thing all the time by saying the gadget uses vibranium (a.k.a. magic makes it possible) so I'll just say - not gonna work in a world without magic. $\endgroup$
    – Gwyn
    Aug 7, 2020 at 3:25
  • $\begingroup$ @Gwyn: no fair, I'm DC please look close at my avatar again. $\endgroup$
    – user6760
    Aug 7, 2020 at 3:38

1 Answer 1


I am NOT (not, not not) fluent in celestial mechanics, so if I'm wrong, let me know and I'll delete this answer. But...

Our favorite supervillain, Gru, has asked his most trusted scientist, Dr. Nefario, to build him a black hole gun!

From Wikipedia we find...

$$t_{ev} \approx 2.1\times10^{67}\left(\frac{M}{M_{\odot}}\right)^3\;\text{years} = 6.623\times10^{74}\left(\frac{M}{M_{\odot}}\right)^3\;\text{seconds}$$

where $M_{\odot}$ is 1 solar mass, or $2\times10^{30}$ kg.

This means that $\left(\frac{M}{M_{\odot}}\right)^3 = 1.51\times10^{-75}$ or $M =$ 229,577 kg, which might be the approximate mass of the small hill behind Gru's house.

The mass of the micro-black-hole is 229,557 kg. Gru can't lift the gun. But let's ignore that for a second.

Using a handy-dandy online gravity calculator we find that given the above mass of the black hole, Gru's mass of 100 kg (yeah, that's a little on the light side for Gru, but work with me here), the half-meter or so between Gru and the gun in his outstretched hand, the gravity is a paltry 6 mN. Which isn't surprising since the hill behind Gru's house isn't exactly sucking him into a dark and terrifying doom.

Ignoring all of the analysis that suggests the force of a cartridge explosion isn't evenly distributed between you and the bullet for a normal gun, let's look at the basics of Newton's 3rd law.

I'm assuming black holes aren't magic, which means that the hill-sized force needed to move that black hole forward such that it could be a threat to someone (F=mA, let's say 1,200 m/s, so 2.6 Mega-Newtons, that's a small nuclear explosion, isn't it?), moved Gru backward. Gru's either a thin, pink paste against the hill behind his house... or he's in low orbit, having ricocheted off the hill.

Yeah, yeah, yeah... but did Gru get the good bad guy?

The Schwarzschild radius of this black hole is...

$$r_s = \frac{2GM}{c^2}$$

or 3.4x10-22 meters, which looks sub-atomic to me.


An object so small that it could pass between atoms yet having the weight of a small hill passed harmlessly through your opponent.

Which Gru didn't know since he's still in low orbit from the gun's recoil.

With one small... glitch...

@Notovny pointed out one small problem with all of this. That black hole bullet wasn't quite as harmless as we might have supposed. Oh, it's subatomic all right...

But during its brief passage through one second of time, the evaporated power was about 4.93 million megatons of TNT.

The Tsar Bomba, the largest nuclear weapon detonated on Earth, was a meaningless 58 megatons or something around 1/100,000th of that power.

So... While Gru couldn't see what happened to the bad good guy from low orbit, what he could see was the entire opposite hemisphere burst into flame, burning with the holy glory of the eternal sun! All but cracking the Earth in half!

Which wouldn't be such a bad thing... if it hadn't pushed the darn planet TOWARD him...

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    $\begingroup$ Over the second that the 229,557 kg black hole evaporates, it will release the energy of 4.93 billion kilotons of TNT. $\endgroup$
    – notovny
    Aug 6, 2020 at 10:20
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    $\begingroup$ Merely ricocheting off the hill will not change a suborbit path into an orbital one. Also what Notovny said. Perfect answer otherwise. $\endgroup$ Aug 6, 2020 at 11:56
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    $\begingroup$ @notovny, I never even thought about how much energy would be released during the evaporation... Thanks for the detail! (Although it'll be later today before I can do anything about it.) So, while Gru's enemy won't be sucked into anything... the entire hemisphere is reduced to ash, the Earth is cracked almost in half, and it's shifted more-or-less the diameter of Jupiter out of its orbit! Maybe Gru gets to land after all (when the Earth catches up to him). Oh, and the bad guy gets a better-than-average suntan. $\endgroup$
    – JBH
    Aug 6, 2020 at 13:16
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    $\begingroup$ This is the most XKCD answer I could imagine and I love it. $\endgroup$
    – IT Alex
    Aug 6, 2020 at 14:21
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    $\begingroup$ I'd say we're well off from cracking the world in half. Even at 4.93 million megatons, we're still 25 times less than Wolfram Alpha's estimate on (119 million MT) on the Chicxulub Dinosaur-Killer impact, assuming it all goes to heat and kinetic energy. "Oceans will rise, cities will fall," and Earthlife will probably take a severe body blow, but the Planet's handled much worse. $\endgroup$
    – notovny
    Aug 7, 2020 at 21:05

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