I like Willk's idea of targeting something by boomerang-ing almost luminal neutrons in the gravitational field of the Moon. But that's not gonna happen without making Moon a lot denser.

Question: can we help a fellow WB-er to collapse the Moon into a blackhole, in situ, with the extra restriction that Earth's life should continue unimpeded (e.g. Earth is not to be bathed in deadly radiation or be ripped apart by strong gravity gradients)?

How would you do it? Bonus point if it can be done on reasonable time scales (history ages rather than geological eras).

You are allowed to assume technologies and exotic form of matter that don't violate the laws of physics

Note: something on the line of "beam the Moon away, Scotty, to the 'We shrink it for you, wholesale' guys, then bring it back with the tax invoice" is in poor taste

  • $\begingroup$ It's funny your question was inspired by an answer to my question, since my question was inspired by another question about plasma weapons. :) $\endgroup$
    – DWKraus
    Oct 29, 2021 at 18:14
  • $\begingroup$ @DWKraus that's the proper way to build worlds, yes. $\endgroup$ Oct 29, 2021 at 21:29

3 Answers 3


I am afraid it can't be done.

What would be needed to "blackholify" the Moon? Well, it would require something capable of shrinking the Moon into a dense sphere of radius 0.01 cm, starting from its current 1737 km. That's a lot of clenching!

And you also want

with the extra restriction that Earth's life should continue unimpeded

That's a pity, because large explosions is how we manage to compress matter to conditions like those existing in the core of the stars, just ask any H bomb expert. We want to go past those levels of compression, and explosions have this tendency to release a lot of energy, which in this case would end up impeding life on Earth.

The other option, adding mass and let gravity do its work, won't do because of the constraint you have added: long before you have reached the black hole condition you would have either flung Earth away from the solar system or used it as additional mass to your black hole project.

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    $\begingroup$ I'm afraid you are right, based on the energetic "feeling of guts" considerations. The gentlest collapse still needs to dissipate the gravitational energy between negligible to 4.12231E20m/s^2 of the entire 7.34E22kg of the Moon. $\endgroup$ Oct 29, 2021 at 4:07
  • $\begingroup$ Do you think my sketch on the idea (using a seed BH as exotic matter) that I added to the question makes sense? (maybe I should move it as a proto-answer and develop it when I have time, but I prefer to let others a chance to run/rubbish or find derivative approaches) $\endgroup$ Oct 29, 2021 at 9:40
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    $\begingroup$ @AdrianColomitchi, you can't edit a question in a way that invalidates existing answers. $\endgroup$
    – L.Dutch
    Oct 29, 2021 at 10:05
  • $\begingroup$ Ok, thanks, I'll move that in a proto-answer on my own $\endgroup$ Oct 29, 2021 at 10:07

Not without a large addition of mass or large reduction in size.

There's a reason why black holes lie either on the cosmic scale or the subatomic. They either need gigantic masses like stars or the tiny world of the quantum realm - where to my knowledge, they do not last very long.

One would have to add an immense amount of mass to the Moon, and I mean immense. If you added all the mass in the solar system together, all you'd have at the end is a bigger sun.

No, unless you are Gru from Despicable Me, thinking that shrinking the Moon to fit in your hand is a good idea (which I think would still only make it more like a neutron star), you are not going to get a black hole from the Moon.

I will caveat however, that this is with our present understanding of physics. If you set a sci fi world at some point far into the future, you could perhaps do a more plausible-ish handwave by couching your Moon to black hole transformation in an explanation that it's done by some new discovery about black holes, maybe some kind of antimatter reaction done in a specific way, etc.

  • $\begingroup$ Define "large" - 10^18kg may be huge in human terms, but a tiny fraction of the Moon's mass. What if that mass is a nano-blackhole? $\endgroup$ Oct 29, 2021 at 9:37
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    $\begingroup$ I'd categorize Alcubierre drive and the kind of matter is supposed to be using as a valid approach to the answer (on the "allowed to assume technologies and exotic form of matter that don't violate the laws of physics" exception) even if nobody know how such matter can be produced or is even possible. $\endgroup$ Oct 29, 2021 at 10:16
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    $\begingroup$ @AdrianColomitchi adding a tiny black hole to the moon will eventually consume it, yes. But the last millisecond or so, when 99% of the collapse occurs, will be so energetic that we need to calculate if the SUN will survive, much less Earth or good grief anything alive on the planet. Actually, both life and the planet itself will likely fall apart due to the gravitational pulse alone. And that's ignoring the hugemongous high gamma-to-cosmic radiation from the last infalling moon mass. MAkign black holes is not a gentle process. $\endgroup$
    – PcMan
    Oct 29, 2021 at 14:33
  • $\begingroup$ @PcMan that's an assertion that is likely true, except for the 99% in the last millisecond. $\endgroup$ Oct 29, 2021 at 21:27
  • $\begingroup$ @AdrianColomitchi look up the studies(theories, really) about solid matter being absorbed by a black hole. it collapsed inward, but very little enters the black hole. Instead it forms a hyperdense region of degenerate matter only a few meters across, surrounding the microscopic black hole but very little falling in due to radiation pressure. Then, it reaches a critical point and the whole shebang drops in, causing a most peculiar ripple of gravitation and radiation. Vaguely comparable to the final collapse of the core of a supernova. (very vaguely though) $\endgroup$
    – PcMan
    Oct 30, 2021 at 6:54

Seeding with a microscopic black hole that isn't perturbing significantly the life on Earth at the moment of seeding is something that's allowed - as one form of exotic matter.

Based on the Hawking's radiation calculator a $10^{18}kg$ blackhole has a radius comparable with the dimension of a gate of transistors used in the present computer chips, is only 0.1ppb 13.6ppm of Moon's mass and emits 0.3mW (with a peak in far UV). Setting such a nano BH on the surface of the Moon will cause it to sink into the Moon, shot past the center and get to the other side, gobbling whatever intersects the trajectory.

Not sure what happens with the gravitational energy that results from the matter in the tiny accretion sphere collapsing in the BH and the amount of time required for the Moon to gradually fall inside the BH.

Note: if someone wants to play with the idea (improve or rubbish it, or go on a tangent), by all means, go for it.

How about an estimation on the radius of the accretion sphere ($R_{a}$) for a nano-blackhole (nBH) on the Moon?

As a model, at what distance from the nBH an average atom will fall under gravity instead of keeping itself still bound in the crystal it belongs? (right! that assumes the Moon is mono-crystaline, but it will put a lower bound to the radius of damage). I'll assume a binding energy/atom of $E_b = 10 eV$ (graphene/diamond has the energy around 7.9eV (pdf warning), so we are in range) and the average atomic mass of the matter on the Moon at 22

$$E_g=\gamma \cdot \frac{M_{nBH} \cdot M_{avg}}{R_{a}} = E_B$$

$$R_{a_{0}} = \gamma \cdot \frac{M_{nBH} \cdot M_{avg}}{E_b} = 3.85\cdot 10^{-11} \cdot \frac{10^{18} \cdot (22 \cdot 1.66 \cdot 10^{-26})}{10 \cdot 1.6\cdot 10^{-19}} m = 0.88 m$$

Hmmm, not to bad. A macroscopic damage, but far from an immediate collapse of the whole Moon. There may be some chances, at least in the very few moments after the start.

At the very end, when most of the Moon "monocrystal" is inside the nBH, the same accretion radius is $R_{a_{1}} = 3.85\cdot 10^{-11} \cdot \frac{7.35 \cdot 10^{22} \cdot (22 \cdot 1.66 \cdot 10^{-26})}{10 \cdot 1.6\cdot 10^{-19}} m = 64589 m$

Again, not a total immediate havoc, there may be some chances for a gentle blockholeyfication of the Moon

What about that energy cause by the matter collapsing in the nBH?

  • $\begingroup$ Since the Moon weights around $10^{23} kg$, it is far, far more than 0.1ppb - more like 10 ppm... $\endgroup$ Oct 29, 2021 at 10:22
  • $\begingroup$ @RadovanGarabík thanks $\endgroup$ Oct 29, 2021 at 10:29
  • $\begingroup$ I had understood that black holes small enough to give off noticeable Hawking radiation (0.3 mW is quite visible, though with peak in UV it'll be more like a tiny black light LED than a laser pointer spot) have too small an event horizon to take in mass faster than they lose it via the radiation -- meaning it'll just orbit inside the Moon, heating the rock a tiny bit as it goes, getting hotter over time until the radiation rate is high enough to "explode" with exponential final evaporation. $\endgroup$
    – Zeiss Ikon
    Oct 29, 2021 at 13:25
  • $\begingroup$ @ZeissIkon 0.3W is a mass loss rate equivalent of... what?... Δm = P/c^2 kg/s = 3e-4/9e16 = 0.33e-20kg/s. Meanwhile, the event horizon has a diameter of about 3nm and a far larger accretion sphere. I should have used Bondi accretion but was too lazy to find the speed of sound in Moon's soil - in any case, at 1e18kg mass, the accretion sphere is macroscopic already. Note: that's not a lone BH, feeding on cosmic microwave background. $\endgroup$ Oct 29, 2021 at 13:38
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    $\begingroup$ @ZeissIkon yes, indeed. I'm a bit on the TGIF late night side of tired (actually it's already Saturday for about an hour), the energy fluxes due to gravitational collapse can wait for me to have my sleep. It may turn out that there's no gentle solution. $\endgroup$ Oct 29, 2021 at 14:10

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