During Thursday evening's Republican debate, Ben Carson referenced a particularly strange threat to the United States — an electromagnetic pulse (EMP) deployed by an enemy to wipe out America's electrical grid:

Now we have dirty bombs and we have cyber attacks and we have people who will be attacking our electrical grid. And you know, we have a whole variety of things that they can do and they can do these things simultaneously. And we have enemies who are obtaining nuclear weapons that they can explode in our exoatmosphere and destroy our electric grid. So think about a scenario like that. They explode the bomb; we have electromagnetic pulse.

Carson is referring to a common version of the EMP fear: a nuclear weapon detonated high up in the atmosphere, thus supposedly sending an electromagnetic blast that would destroy electronics.

During the undercard debate just prior, Rick Santorum also warned of the EMP threat:

The president of the United States has put Iran on a path to a nuclear weapon. And we have done nothing to do anything to harden our grid. There is actually a bill in Congress that would put money forward to try to put redundancy and harden our electric grid so it could actually survive an EMP. An EMP is a devastating explosion that sends a pulse that knocks out all electric, everything, everything that is connected to any kind — that is wired, that has a circuit board gets fried out. Everything is gone. Cars stop. Planes fall out of the sky.

This isn't just a fringe-candidate thing. Ted Cruz, one of the top-polling candidates, warned in July of an "EMP [that] could shut down the en­tire elec­tric­al grid on the East­ern Sea­board" that "could res­ult in tens of millions of Amer­ic­ans dy­ing."

So just how scary is an EMP? Not so much. The odds of an EMP actually hitting America are "roughly the same as terrorists deploying MegaMaid from Spaceballs to steal America’s oxygen," Matt Duss, the president of the Foundation for Middle East Peace, tweeted.

Duss's comment reflects the expert consensus. "I had the impression that nobody who was technically competent believed the scare stories about EMP," renowned physicist Freeman Dyson told the Project on Government Oversight in 2011 — when then-GOP presidential candidate Newt Gingrich was warning of the same thing.

This is because, as Patrick Disney explains in the Atlantic, the whole scenario relies on an American enemy detonating a nuke in the atmosphere to kick off an EMP. First of all, rogue states like Iran and North Korea don't have missiles that could reliably reach American shores (and Iran doesn't even have a nuke).

Second, if they had a nuke, why wouldn't they just nuke a city? It's far from obvious that a nuclear explosion actually would be able to produce an EMP, whereas it's fairly clear what a nuke would do if it were, say, dropped on New York City.

"A terrorist, after going through the trouble of acquiring a nuclear warhead and a missile capable of delivering it to America's shores, would be a fool to employ the ultimate weapon in such a cockamamie fashion," Disney writes.

So Carson, Santorum, and Cruz are hyping a threat that, more or less, doesn't exist.

Watch: The candidates were introduced in awkward fashion at the latest Republican debate