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Comment by dctoedt | Hacker News Reader

Stupid question, posed but apparently not answered in TFA: What if a small black hole approached a person, or even came into contact? Would we see it as, say, someone's arm suddenly falling off for no apparent reason? Or would the size of the black hole be so infinitesimal that for practical purposes it wouldn't matter?

FTA: "'I think someone asked me what would happen if a primordial black hole passed through a human body,' recalls Tung, who did a quick pencil-and-paper calculation to find that if such a black hole zinged within 1 meter of a person, the force of the black hole would push the person 6 meters, or about 20 feet away in a single second. Tung also found that the odds were astronomically unlikely that a primordial black hole would pass anywhere near a person on Earth."

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Wouldn’t a black hole that small evaporate pretty quickly?

Iirc a black hole with the mass of an asteroid (whose event horizon would be very small indeed) would have a hawking temperature smaller than the cosmic microwave radiation so there wouldn't be any net evaporation.

Hmm what mass of black hole is needed to rip your arm off? Probably more than an asteroid I’m guessing?

Probably a fairly significant multiple of earth's mass. What multiple of your arm's weight would be required to pull it off? (and tidal forces are generally less than the absolute gravitational pull)

The heavier the black hole, the lower the hawking temperature.

Only the tiniest black holes would have hawking temperature high enough to evaporate given the presence of the CMB

I asked o1-preview a couple times, it gave me:

*Answer:*

Approximately 27 trillion kilograms—the black hole would need about 2.7 × 10¹³ kg of mass to exert tidal forces strong enough at 1 meter to tear off an arm. - This radius is much smaller than an atomic nucleus, so at a distance of 1 meter, the person is far outside the event horizon.

No, the black hole with a mass of approximately \( 2.7 \times 10^{13} \) kilograms would not evaporate quickly due to Hawking radiation. Even though this mass is relatively small compared to stellar black holes, such a black hole would have an extremely long evaporation time—*on the order of \( 5 \times 10^{16} \) years, which is about 50 quadrillion years*. This duration is vastly longer than the current age of the universe, which is approximately \( 13.8 \times 10^{9} \) years (13.8 billion years).

Here's another answer to the Fermi Paradox, that while life is normal and abundant, there are lots of random possibilities for destruction such as a primordial black hole coming by.

The apparent rarity of intelligent life might be a statistical matter of there being lots of opportunities for such life to arise, but also lots of hazards of random destruction.

... on reflection, while this might be intellectually satisfying, it's not particularly nice to contemplate.

That sounds like a variation of the Great Filter[0]

[0]: https://en.wikipedia.org/wiki/Great_Filter?wprov=sfla1

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