Black Holes

I was watching a (terrible) movie last night that "starred" a black hole, which was accidentally created or summoned by scientists in St. Louis, apparently so the filmmakers could trash the Gateway Arch.

First of all, the "black hole" in the movie had a creature come out of it and was actually a wormhole.

But the way they portrayed a black hole hitting Earth was, well, wrong. Egregiously wrong. Terribly wrong.

So, what would actually happen if a black hole hit earth.

First, it depends on the size of the black hole. A micro black hole, which are hypothesized to exist as part of the creation of the universe?

We. Wouldn't. Even. Notice.

The PBH would zip into or through the Earth as if it wasn't there. The energy produced would be about a tonne of TNT...spread through the area. The only way we'd know is if we noticed the radiation it left behind. The lab room in which it was created would become "hot," but probably not dangerously so.

A slightly larger black hole would also pass straight through the Earth, and it would ring the Earth's crust like a bell, creating about a magnitude 3 or 4 quake...across the entire planet. We'd notice, but it probably wouldn't do a huge amount of damage.

It would take a fairly large black hole to do a lot of damage, and then it would have to stick around.

So, what about creating a black hole in the lab? It wouldn't be moving fast enough to pass right through.

First of all, it would take a lot of energy to create a black hole that would not immediately evaporate. Black holes have a lifespan. Very small black holes, such as might appear in a particle detector, have a predicted lifespan so short we can't measure it.

And even if, by some means, we did manage to do it and the black hole fell to the center of the Earth and started eating it? First of all, it would probably be expelled from the Earth by momentum anyway. And even if it did, it would take longer...far longer...than the, uh, lifetime of the Sun. In other words, Earth would be destroyed by other means.

Now, for what happened in the movie to happen you would need a fairly large black hole that had an orbital velocity perfect to keep it at the Earth's surface, geostationary. And even then it would grow in three directions.

And for a black hole to be a threat to Earth it would have to weigh billions of tonnes, which mass has to come from somewhere.

In other words, least plausible physics almost ever and if anyone tells you to worry about black holes being created in labs? Don't.

(Note. If you search for "black hole created in lab" you'll find examples, but these aren't black holes, they're called "dumb holes" which use sound not light and are created as a safe way to study black holes).