This is going to probably be purely speculation, so I don’t think it belongs in GQ.
How big would the smallest black hole be? Pea sized? Basketball? Moon? Can they even be quantified that way?
What would happen if the smallest sustainable black hole collided with the earth? I guess it would be very much like a meteor impact, but then what?
I read a very fun sci-fi book once about a small black whole that got ‘loose’. Basically, it was artificially created on earth and was dropped. It started to ‘orbit’ back and forth through the center of the earth.
If it where possible to artificially create a small black hole on earth, and it got loose, what do you think would happen?
I’m not sure if there is a lower limit on the size of black holes. If so, it is very small, much smaller than a pea.
As to your other questions, you mention the “smallest sustainable black hole.” Under current theory no black hole is sustainable*, as they all evaporate at various rates. However, the smaller the black hole is, the faster it evaporates, so if a very small black hole got “loose” it would evaporate away very quickly.
*As for large black holes, they should, I would think, be sustainable for all intents and purposes, until the temperature of the background radiation (about 3 degrees Kelvin) drops below the temperature of the black hole. Up until that time the black hole should be taking in more energy than it loses.
Interesting read. I guess I’ll change the question to "What would be the smallest Schwarzschild radius/event horizon. I’m guessing it’s going to be quite large.