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Comment by sliken | Hacker News Reader

Even a moon mass black hole would survive. The temperature would be about 2.7 kelvin, same as the CMB is now. So any blackhole created at or after the big bang will have stayed the same or gained mass.

The threshold I could find for surviving since the big bang is around 10^11 kg which is quite a bit smaller than the moon. 10^11 kg is quite a bit compared to human scale, more than the mass of the Three Gorges Dam, but quite small on the astronomical scale (10^11 times smaller than the moon).

So while an earth mass black hole is small (9mm), it will last quite long indeed. The evaporation time is proportional to the mass^3.

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Something about your comment triggered a primal fear in me of tiny, immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide, it's still has a full gravity field the size of Earth's. If one's coming our way, we probably can't do much, but we'll see it coming.

Seems unlikely. It emits close to nothing, it's temperature is less than 2.7 kelvin, and we'd have to be spectacularly lucky to see a micro lensing event with such a small mass.

Hell we barely notice when a several km comet enters the solar system and I've read about fairly large asteroids getting closer to the earth than the moon before being noticed.

We are going to notice a 9mm black hole? Maybe if it punctured Jupiter first.

the fast moving mass of earth will distort orbits via gravity. that will be noticed

If you removed the earth, except for the moon the solar system would change very little.

Sure if the earth came between Jupiter and it's moons there might be some chaos, but the likelyhood of that is minimal. Even if it did happen we would notice, but the earth massed black hole would be very unlikely to make it into the inner solar system at that point.

The earth is a minor gravitational force in the solar system, most likely the effects would take significant time to notice... well after the nearest approach. Small gravitational tugs take many samples and significant time to notice. For example the outer solar system still doesn't add up... and we have no idea why.

This sounds like a super fun simulation to run. Do you have recommendations on simulation software?

I found these in a quick search. I have a couple 64 core machines available.

https://wwwmpa.mpa-garching.mpg.de/gadget/

https://rebound.readthedocs.io/en/latest/

http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html

The outer solar system is pretty well accounted for. For many years we thought it wasn't, and kept hunting for another (sufficiently-massive so not Pluto) planet beyond Neptune. The discrepancy turned out to be an overestimation of Neptune's mass, which was resolved by Voyager and subsequent observations. We've now ruled out anything of Neptune's mass or greater existing out to at least 100 AU and likely several multiples of that.

Ah, thanks for the update.

Probably not. It would be something that would likely have to be observed over multiple complete orbital cycles and basically somebody basically run into it by accident. By which, of course, the earth in this hypothetical, would be long dead.

A black hole with the mass of Earth out of sync with it surroundings will surely emit a lot on UV and X-rays. Space isn't completely empty, and the stuff on its path won't orbite it.

We would detect one entering our Solar System.

This comments, reminds me of the dual vector attack in the Death's End novel of Remembrance of Earth's Past trilogy from the writer Liu Cixin. Its was a small object fired at the solar system by a highly advance race, where 3d space was projected into a 2d plane so the whole solar system was destroyed.

Pretty big plot point! Don't want to give away the story

> immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide

Now imagine what a strong force an electron could provide :D

How do you suppose we would see it coming?

If it's not moving quickly, we would notice its gravitational interaction with the well-known dynamics of the various large objects already in our solar system. A 9mm black hole's gravitational influence would be fairly subtle though compared to a multiple Earth-mass black hole; the bowling ball sized black hole mentioned in the paper as an upper bound is not the kind of thing that could wander into the inner solar system unnoticed.

I read a fiction book about a tiny black hole getting caught in the planets gravity well then falling threw Earth back and forth just fucking stuff up every time. Scary stuff.

Do you remember its name?

Sounds familiar, could that be the Big Mistake in Hyperion or the Hole Man short story? https://en.wikipedia.org/wiki/The_Hole_Man https://hyperioncantos.fandom.com/wiki/Big_Mistake

Maybe ...

Earth by Brin

Hyperion Cantos by Simmons

Ilium by Simmons

Doomsday Effect by Thomas Wren

If I thought for a few more minutes I could probably come up with a half dozen examples (or expanded the definition to include eating the moon or mars, or included ones where it would be a massive spoiler)...

A gravitationally captured/trapped black hole eating earth is relatively common plot device in science fiction.

Plenty of titles at https://en.wikipedia.org/wiki/Black_holes_in_fiction

It would go right through Earth. There might be pretty big explosions though as it feeds on earth's mass.

Total (unqualified) spitballing, but wouldn't a black hole "bullet" that hit the Earth experience effectively 0 frictional losses? By definition it won't be pushing stuff out of the way, so it seems like an inbound black hole would punch right through the Earth. Local gravity is probably going to be quite nasty during the transit, but beyond that, it seems like a fairly survivable event.

Won't it pull some of the matter it passes through under the event horizon? The extremely curved spacetime will likely turn rock to dust in near vicinity. Then, as the mass grows, so would the gravitational pull, while the speed relative to the planet being punctured would lower due to conservation of momentum.

If the tiny bullet-class black hole makes it through a planet, it could gain significant additional mass, and leave quite an exit wound.

If a random PBH of 9mm traveled at near light speed through Earth, would it even pull anything into the event horizon that is not within close range of that 9mm diameter?

For example, it travels through Earth and then leaves a 9mm hole throughout the planet.

No it wouldn't, but I'm thinking it would be able to transfer some kinetic energy to the earth which would be a very very bad thing.

I'm not good at physics, but it seems to me if this 9mm object had the mass of earth, then at a distance of about 6400 km it would exert 1G, and at minimal distance, it would exert about a zillion Gs. So even assuming it didn't pull in matter per se, the gravity and tidal forces ought to destroy earth about as effectively as colliding with a regular earth sized planet.

An earth’s mass black hole has a ~1cm sized event horizon. It would suck up matter several times that, but it’s not enough to gain significant mass relative to it’s size on a single pass. This stays true as you scale things down.

Something in the 10^12 kg range is going to have an atomic scale event horizon so it’s not picking up significant mass as it shoots through the earth.

Well, do you know how we "see" black holes? The "body" of a black hole is nothing but darkness, but we see the radiation given off by the mass accelerating toward the black hole event horizon that can often reach significant fractions of c. Given the high mass density of a planet compared to space, a massive amount of hard radiation will be released.

Also the relative velocities of earth and the black hole is extremely important. Depending on the relative velocities, it's possible that the blackhole simply gets lodged in the earth's core and we're 100% dead, another 100% dead scenario is if it plunges in, out the other side, but then "falls back" for another pass and so on.

You should read Seveneves by Neal Stephenson. The plot of that book follows the events after a primordial black hole streaks through the Moon, causing it to implode. It's packed with detail, but interesting for anyone interested in space and engineering.

Hmmm, that's not what happened in the Seveneves I read... The reason for the moons destruction was never mentioned iirc. The 'Agent' was never revealed.

Good yarn though.

Truth.

I've been thinking that it was a misrouted Viking.

This whole topic and discussion makes me think about "The Nothing" that rips apart the world in the movie The Neverending Story. I had nightmares from the movie and The Nothing for weeks after watching the movie when I was around ten years old. 1984 might really be approaching, not the book, but The Neverending Story which was released then.

In some exotic models, dark matter acts like bullets that are always flying through you, but with a tiny chance could interact and leave something basically like a bullet wound: https://arxiv.org/abs/1907.06674

Calculator I used: https://www.omnicalculator.com/physics/black-hole-temperatur...

A black hole with 10^11 Kg seems to have a temperature of about 1.2 * 10^12 K. Even though it would be small, it would be emitting crazy amounts of all sorts of photons.

It's a black hole, that's not going to be throwing off black body radiation like that. It would be just hawking radiation plus whatever effects from matter in its accretion disk. That would still amount to pumping out 35.6 GW of hawking radiation, but being small isn't something that would inhibit it releasing energy, it's the other way around. So in other words, tiny black holes give off gobs of energy whereas larger ones do not.

Hawking radiation is indistinguishable from black body radiation.

I'm curious how an object this size with a planet's mass is not sweeping all the dust and rocks in its area as it moves through its orbit, like Jupiter cleans up the outer solar system. Even interstellar space has around 1e6 loose ions per cc. Is it radiating faster than it's eating?

Gravity doesn't care what size the object is, from far enough away. It would sweep the same area as any other density of planet with the same mass. For something at 5 earth masses, that's not a very big area.

Things can’t collide with a small object.

Almost none of the orbital sweeping effect comes from collisions. Even Jupiter is tiny, in the scope of space. The majority of orbital sweeping comes from repeated gravitational deflection.

Having a tiny radius will reduce the amount of of collisions, but it won't have a significant impact on orbital clearing.

The comment mentioned "eating" specifically, where I felt collisions would be important. You're right that for clearing the size of the object isn't very important.

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