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Comment by Konohamaru | Hacker News Reader

This is not scary. Yes the event horizon will expand the more matter falls into its gravity well, but otherwise a black hole acts like a normal planet orbiting the Sun.

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Is there a theoretical limit on the minimum mass of a black hole?

It's counter-intuitive to me that something could get dense enough to form a black hole and not have more mass than the Sun, or at least enough mass to steal Pluto.

Small black holes evaporate fairly quickly from Hawking radiation. The rate increases dramatically the smaller the mass. Since the only time primordial black holes would have formed was in the Big Bang there is a lower limit based on the black hole surviving 13.8 billion years. iirc the number is on the order of 10^12 kilograms.

Well since the mass of the Earth is ~ 5x10e24 seems we're still in the realm of possibility

Ted Kazinski was concerned that high energy particle physics would create a blackhole large enough to become self sustaining. If I remember correctly, high energy physics creates tiny black holes during particle collisions, but they "evaporate" faster than they can absorb mass.

So to answer your question with my limited understanding, yes, there's a lower limit to stable black holes. Beyond the critical mass, they simply evaporate in a matter of seconds.

> Beyond the critical mass, they simply evaporate in a matter of seconds.

For a sense of scale, a black hole with a mass of 2.2 * 10^5 kg will evaporate in roughly 1 second. A 1 kg black hole would evaporate in roughly 100 attoseconds.

"Evaporate", meaning "vaporize everything for miles around", right?

And for the 10^5 kg one, "vaporize the entire planet" if one is nearby?

Yeah, the event horizon the size of a Planck unit.

Again, unless you get super close, it acts identical to a regular planet of the same mass.

Do we actually have a theory that predicts a theoretical minimum size of a Planck length?

Black holes are an artifact of gravity and the theory of relativity. The planck unit comes up as a result of quantum mechanics. Without a working theory of quantum gravity, we do not have a description of what happens in scenerios where both theories are relevent.

There's an approximation that's expected to hold, semiclassical gravity, where both theories are relevant. This is the background against which calculations of Hawking radiation and black hole evaporation are made.

I don't think so. With the various colliders there was talk of black holes but they would be small and super short lived. The question may be more interesting if we talk about stable black holes at least by human time scales.

Schwarzschild radius and the Compton wavelength both combine to say that in theory, the minimum mass required for a quantum black hole is the Planck mass(ish). They might not be possible or they might require more energy than what we can produce today.

https://link.springer.com/article/10.1140/epjc/s10052-017-54...

This article looks interesting and will probably go into more detail if you are wanting to know more. I will read it tomorrow. Great question!

It's a little complicated... Hawking radiation establishes the minimum size of stable black hole in our universe. If the temperature of the event horizon is higher than the temperature of the CMB, than a black hole will lose mass by Hawking radiation, which raises is temperature further and so on. Using a knowledge of expansion of the universe (and thus the cooling of the CMB) plus the basic math of hawking radiation, you can pretty easilly calculate the minimum size of a black hole that you should see.

Yes, Tolman–Oppenheimer–Volkoff limit would estimate a star around 1.5-2.5 sun masses could result in a stellar black hole.

We don't know much about stars collapsing, and the mass wasted in the process to have a more accurate number at this point.

That's only true for black holes created from the collapse of stars. Primordial black holes would have been created in the early universe and be much much smaller.

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