"A really interesting and cool thing for astronomers to talk about...but you might want to pray that not one of 'em ever comes within a million trillion miles of the Earth."
Unfortunately, there is a rather large one not one sixth of that distance away (a million trillion miles is actually rather large - 170 kly - approximately double the size of our galaxy)
Yes - I took the grandparent comment's "descr to a non-physicist" as "describe to someone who really isn't into math or hard sciences". Those folks will hear "million trillion" as "a really Really REALLY REALLY big number". Not as "1 x 10^18".
I guess, but "light year" is easy to say, easy to understand, and much less likely to mislead. And most black holes would probably be safe at that distance.
Though the issue of small black holes is worth bringing up. If you scrunch up one cubic kilometer of stone into a black hole, that's heavy to be stable for trillions of years, but light enough to pass between the Earth and the moon without causing serious problems. And we really have no idea what the smallest existing black hole is.
Maybe? But if (1) it's a few solar masses, and (2) the https://en.wikipedia.org/wiki/Oort_cloud exists - "from 2,000 to 200,000 AU" - then the black hole could send a lot of planetesimals flying off in random directions. Quite of few of those could be moving toward Earth's neighborhood, some at very high speed. Humans might soon join the dinosaurs.
Astronomical distances are vast, million trillion miles too far, that's over a hundred thousand light years. There are known stellar-mass black holes within just 2000 light years of the Earth. Heck, there might be a primordial black hole in the inner Oort cloud and not only would it not destroy earth, we'd have (are having) trouble detecting it.
