Death by primordial black hole is hard to come by

The UniverseFridays

April 24, 2026

5 min read

Add Us On GoogleAdd SciAm

Death by primordial black hole is hard to come by

Subatomic black holes from ancient cosmic history could, in principle, make you have a very bad day. But chances are you’ll never encounter one

By Phil Plait edited by Lee Billings

What’s faster than a speeding bullet, smaller than an atom and as massive as an asteroid? If you answered “a primordial black hole,” you’re right. Now duck!

koyu/Getty Images

Stay connected to The Universe: Get email alerts for this weekly column by Phil Plait

Enter your email

I agree my information will be processed in accordance with the Scientific American and Springer Nature Limited Privacy Policy. We leverage third party services to both verify and deliver email. By providing your email address, you also consent to having the email address shared with third parties for those purposes.

Sign Up

Back in the early 2000s I gave a public talk called “Seven Ways a Black Hole Can Kill You.” Despite the rather macabre subject matter, it was actually a fun talk—real science disguised as a tongue-in-cheek series of cartoonish astrophysical antics. I covered the usual topics, including simply falling into a black hole and getting spaghettified or being too close to the gamma-ray burst released when a new black hole is formed.

Now, though, I wish I had covered getting hit by a subatomic asteroid-mass black hole moving at a million kilometers per hour that was born in the first moments after the big bang—and being blasted by the ensuing shock wave as the black hole carves a narrow corridor through your body.

I mean, obviously that’s what would happen. At least, that’s the conclusion of research published in 2025 in the International Journal of Modern Physics D. It’s a rather unusual topic for a professional paper—the title is “Gravitational Effects of a Small Primordial Black Hole Passing through the Human Body”—but there are some actual scientific conclusions about black holes and even dark matter that can be drawn from the fact that, thankfully, we haven’t ever seen such a grisly event occur.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

In fact, because some people fret about such things, let’s start off with a bit of good news: the odds of such an event happening at all, let alone to you or someone you know, are so (literally) astronomically long that they’re hard to express in a meaningful way. I’d estimate the chance of you dying this way is about the same as simultaneously winning the lottery and getting struck by both lightning and an asteroid during a shark attack while on a unicycle juggling aardvarks.

Feel better? Okay, let’s take a closer—but not too much closer, because yikes—look at what this is all about.

As I wrote in my March 27, 2026, The Universe column, primordial black holes, or PBHs, could have emerged in the first few infinitesimally thin slices of a second after the cosmos itself formed, when immense pressures and densities in the hot miasma could theoretically have compressed clumps of matter into volumes so small that the gravity of those clumps became huge, initiating a runaway collapse. Voilà, primordial black holes!

There’s no real lower limit to the mass of a PBH. But because of bizarre quantum mechanical effects very tiny black holes actually emit a type of radiation called Hawking radiation, causing them to evaporate away mass as they do so. Any PBH less than about a billion metric tons in mass would have wholly dissipated by now. Other theoretical considerations, such as any observable effects of these PBHs emitting radiation and subatomic particles as they evaporate, puts that limit closer to 100 billion metric tons.

That’s roughly the mass of a smallish rocky asteroid, perhaps four or five kilometers in diameter. An Earth impact from an asteroid like that would be very bad—not quite as bad as the one that wiped out the nonavian dinosaurs 66 million years ago but still not a really fun day.

The weird thing—and likely completely counter to your intuition—is that a PBH with that mass hitting Earth wouldn’t be anywhere near that devastating.

How can that be? It’s a black hole. It should tear the planet apart, right?

The reality, though, is that it’s very, very small black hole. A PBH with that mass would be far smaller than a hydrogen atom. Although the gravity of a black hole is intense—that’s its defining characteristic—the force drops off rapidly with distance. From a kilometer away, you’d barely feel it.

On the other