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Comment by austincheney | Hacker News Reader

Could a black hole orbit our tiny yellow dwarf sun? Even if it were tiny for a black hole it would be hard to imagine that it would have less mass than our sun. If it had more mass than our sun wouldn’t our system be a binary system, and if so it would suggest a black hole is not really a satellite of the sun.

How small can a black hole be and still qualify as a black hole? I guess a better question is what is the least massive black hole observed and what is the smallest current theories allow?

I have no idea what I’m talking about so all of that is probably wrong. I am just not able to wrap my head around it.

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Any gravitating body whose size is less than its Schwarzschild radius becomes a black hole. As the article remarks (can’t quote it exactly because I’m using a client that has an irksome modal interface) some of the ideas floated is that this black hole could be about fifteen centimeters across, and have the a mass of a very large planet.

Science fiction fodder notwithstanding, from a suitable distance this kind of body would behave exactly like any other gravitationally interacting body, and would perfectly obey the usual laws of motion (Einstein’s General Relativity if you’re insisting on investigating it up close or in minute detail, classical Newtonian dynamics if you’re observing on a broader scale). It has mass, it has inertia, and it would be interacting gravitationally as the textbook “product of masses, reciprocal of square of distance” law. It would be a big, dark, featureless bowling ball with the mass comparable to a gas giant.

> Even if it were tiny for a black hole it would be hard to imagine that it would have less mass than our sun.

Planet Nine has a mass five to ten times that of Earth; that's explicitly mentioned in the article.

Such a black hole can't form from the gravitational collapse of a star, but it possibly could have formed in the early universe. That's what the article means by "primordial black hole".

> How small can a black hole be and still qualify as a black hole?

The theoretical lower limit to the mass of a black hole is the Planck mass, which is tiny (about 10^-5 grams).

We have never observed a black hole smaller than several solar masses, so we've never observed one that wasn't formed from the gravitational collapse of a star. Finding one as small as five to ten times the mass of the Earth, if it exists, would be basically impossible if it weren't hypothesized to be somewhere within our own solar system, and even then, as the article says, it's very difficult. So it's quite possible, from that point of view that there could be primordial black holes of that size out there that we haven't seen. The question is how likely theoretically is it that such black holes could have formed in the early universe, and the range of estimates on that is quite wide, since there's a lot we don't understand about the early universe and about how such holes might have formed.

What would it be like to be around one of those Planck scale black hole? Could they be around is right now?

They would pass right through you and you'd never notice.

If Hawking radiation exists, as seems likely, any primordial black hole of mass less than about 10^11 kg would have already evaporated into nothingness at present time. Indeed a Planck mass black hole would evaporated in Planck timescales, on the order of 10^-44 seconds.

There were theoretical reasons to suspect that the Large Hadron Collider, by ramming particles together with sufficient force, could create microscopic black holes with approximately the mass of an atom. This turned out not to occur, but yes, at least in theory black holes can be very small indeed.

"Theoretical" is overselling it, you need novel nonstandard physics to get black holes out of the LHC. Indeed standard physics cannot predict LHC black holes because the Standard Model and general relativity are famously incompatible at these scales. Moreover, if Hawking radiation exists, as is now widely believed, the lifetime of a hypothetical atom-mass black hole would be much, much less than a Planck time.

> How small can a black hole be and still qualify as a black hole?

There's no theoretical lower bound on the mass of a black hole. Once matter has ceased to repel other matter, it will continue to fall inward. A violent impact could generate the forces necessary on a small scale to create micro black holes. Black holes do seem to slowly lose energy though, so small ones would not live very long[0]:

[0]: https://en.wikipedia.org/wiki/Micro_black_hole#Hawking_radia...

> so small ones would not live very long

Actually, this is completely unknown. While Hawking radiation is a well-established effect in the semiclassical limit (read: when gravity is weak), we don't really know what happens when the Schwarzschild radius reaches scales where tidal effects become noticeable already near the event horizon and, eventually, quantum gravitational effects (supposedly) take over.

In fact, this whole question lies at the heart of the black hole information paradox. Do black holes evaporate completely? Or will evaporation stop at some point, leaving behind a black hole remnant?

> A violent impact could generate the forces necessary on a small scale to create micro black holes.

If there are extra dimensions that make it especially easy to create a black hole.

Otherwise, you're looking at a Planck mass of 22 micrograms as a lower bound. It's effectively impossible for a violent impact to cram that much matter/energy into a single subatomic point.

The important differentiator for this black hole is that it would be a captured primordial black hole. It's basically impossible for such a black hole to form in all natural circumstances except the big bang.

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