Credit: ZME Science/Midjourney.

A tiny black hole zipping through your body sounds like the kind of death dreamed up by a pulp-era sci-fi writer. Which, fittingly enough, it was. In 1974, Larry Niven imagined a murder carried out by a microscopic black hole. Half a century later, a physicist has finally run the numbers.

Robert Scherrer, a professor at Vanderbilt University, lays out a stark scenario: what happens if a primordial black hole — the kind scientists think were forged in the first seconds after the Big Bang — slips through a human?

And if that possibility tells us anything useful about dark matter, that elusive cosmic ingredient whose invisible mass shapes every galaxy in the universe.

The answer is both terrifying and comforting. Terrifying because it turns out a black hole could, in theory, kill you in the most gruesome way imaginable. Comforting because the chances of that ever happening are practically zero.

The Gruesome Death You’ll Never Have to Meet

Primordial black holes — PBHs, for short — are hypothetical relics from the dawn of the universe, born less than a second after the Big Bang. Some might be smaller than an atom, others as heavy as thousands of Suns. For decades, physicists have wondered whether these compact leftovers could explain dark matter, the invisible mass that makes up most of the cosmos.

Scherrer’s paper dives into an unsettling thought experiment: if one of these objects plowed through a person, what would its gravity do to human flesh?

He considered two main effects: shock waves and tidal forces.

A black hole moving faster than sound would generate a supersonic shock wave, much like a bullet tears through air — or tissue in this case. “The shock wave propagating outward from the black hole trajectory destroys tissue along the way,” Scherrer writes.

Using a standard equation for energy transfer, he calculated that a black hole with a mass greater than about 1.4 × 10¹⁷ grams—roughly the mass of a small asteroid — could deposit enough energy to be lethal.

For comparison, Scherrer’s calculations put the black hole’s energy on par with that of a .22-caliber rifle bullet. Below that threshold, the black hole would zip through your body unnoticed. Above it, you’d experience a sudden, searing impact.

Gravity and Tidal Forces

At this point, it’s worth mentioning that while a black hole as massive as a small asteroid sounds like it should do more damage than a bullet, its Schwarzschild radius is less than a trillionth of a centimeter. Almost all of its mass-energy remains gravitationally self-contained. Only a minuscule fraction affects the material it passes through.

As it enters, the black hole doesn’t “touch” flesh in the usual sense. Instead, its gravity yanks at nearby atoms as it passes, like a super-dense needle dragging space itself. The tissue near the trajectory is first pulled inward, then blasted outward by the rebound of compressed material.

Then there’s the second effect: tidal forces. This is the same phenomenon that moves Earth’s oceans but on a microscopic, murderous scale. As a black hole passes, it pulls more strongly on one side of a cell than the other. Scherrer calculated that it would take a black hole of 7 × 10¹⁸ to 7 × 10¹⁹ grams to tear brain cells apart. That’s hundreds of times heavier than the lethal threshold for shock waves, which makes the latter the real danger.

Even so, the odds of such an encounter are vanishingly small. “The number density of primordial black holes with a mass above this cutoff is far too small to produce any observable effects on the human population,” Scherrer concludes.

Or, as he told Vanderbilt University’s press office: “A smaller primordial black hole could pass through you, and you wouldn’t even notice it… the density of these black holes is so low that such an encounter is essentially never going to happen.”

Cosmic Ghosts and the Mystery of Dark Matter

Portrait of Robert Scherrer. Credit: Vanderbilt University.

This strange research has a serious scientific purpose. Physicists have long speculated that primordial black holes could make up some or all of dark matter. But if these objects were common and deadly, we might have noticed by now.

Scherrer’s calculations put a new limit on their abundance. If black holes large enough to kill existed in significant numbers, there would be casualties. Since no one has ever been fatally struck by one, that helps rule out certain masses of black holes as dark matter candidates.

The study builds on earlier work Scherrer co-authored with Jagjit Singh Sidhu and Glenn Starkman of Case Western Reserve University, who analyzed macroscopic dark matter (MACROs) — large, composite clumps of dark matter particles that could also, in theory, blast through the human body.

“Given that no deaths by MACROs have been reported, limits can then be set on the properties of these particles,” Scherrer said.

Science Fiction vs Probable Facts

Science fiction writer Larry Niven imagined this scenario back in 1974 in a short story where a man is murdered by a miniature black hole. Scherrer remembered that tale when he began his research. “I wanted to see if this would be possible,” he said. His results show that, at least in principle, Niven’s cosmic assassin could exist — but would be an exceedingly rare one.

In reality, the Milky Way’s dark matter halo is vast, and if PBHs are part of it, their separation would be so immense that Earth might never see one. The odds of a person being struck in their lifetime are less than one in ten trillion.

The cosmos is full of threats, but this isn’t one of them. A black hole with the mass of an aircraft carrier could pass through your torso and you’d never know.

But somewhere in another universe — maybe one Larry Niven dreamed up — someone else might not be so lucky.

The findings were reported in the International Journal of Modern Physics D.