Gravitational Effects of Small Primordial Black Hole Passing Through Human Body

Original Article ↗ Hacker News Discussion ↗

Intuition: huge mass, tiny size, weak effect on humans

  • A PBH with ~10^17 g is ~20,000× the mass of the Great Pyramid, yet its Schwarzschild radius is sub‑atomic (∼10^-13 m, smaller than a proton).
  • Key distinction: “massive” vs “big”. Macroscopic objects hurt via electromagnetic contact forces over milliseconds; a PBH interacts mainly via gravity and only where it passes.
  • Gravity near the PBH can be 10^3–10^5 g at meter scales, but the flyby speed (~100–200 km/s) makes interaction times microseconds, so total momentum transfer is small.
  • Damage is confined to a microscopic “bullet track” plus a gravitational shock wave; the paper’s cutoff mass (~1.4×10^17 g) marks where that starts to resemble a gunshot wound.

Hawking radiation and dose estimates

  • Commenters estimate Hawking power in the ~10–30 kW range for the relevant masses.
  • One thread wrongly claimed current CMB temperature suppresses Hawking emission; others correct: PBHs always radiate, but large ones gain more energy from the CMB than they lose.
  • For the assumed transit speed, total energy deposited inside a human is estimated at a few–100 J, corresponding to ~0.1–1+ Sv if spread through the body: probably non‑lethal but high enough to cause acute symptoms.

Interaction with matter: “like a neutrino, but not really”

  • Because the PBH is far smaller than atoms and moves fast, direct non‑gravitational hits on nuclei/electrons are extremely rare; most atoms are simply missed.
  • When it does “eat” something, it might preferentially swallow a nucleus or an electron, leaving ionized matter behind.
  • Some discussion of whether event horizons smaller than a proton can still capture protons (unclear in the thread).

Dark matter, frequency, and constraints

  • PBHs are discussed as a dark matter candidate in a certain mass window.
  • If dark matter were mostly PBHs of these masses, the expected number density would be tiny: more like a few per solar system per century, not a constant “neutrino‑like” flux.
  • The absence of unexplained PBH‑like injuries in humans is cited as (very weak) evidence against a high abundance. Others note such deaths, if any, would likely be misattributed.

Gravity, orbits, and capture

  • Gravitational impulse from a passing massive object depends on both field strength and interaction time; faster flybys produce smaller net deflections.
  • In planning spacecraft trajectories, such transient gravitational interactions (gravity assists) are indeed central.
  • Objects entering the solar system from interstellar space exceed solar escape velocity; without substantial “friction” (accretion), a PBH would not spiral into the Sun or Earth.

Stability and exotic variants

  • Hawking evaporation sets a lower surviving primordial mass scale (~10^15 g) for neutral, non‑rotating PBHs.
  • Charged/rapidly spinning (“extremal”) black holes could in principle have very low temperatures and be stable; this is noted as an active theoretical area, though highly speculative in this context.

Motivation, tone, and side notes

  • Some commenters question the practical value (“so unlikely to harm humans”), others defend the work as a playful way to constrain PBH dark matter and build intuition.
  • Several humorous asides: “new fear unlocked,” crib‑death and Alzheimer’s jokes, insurance claims about tiny black holes, and SF references (micro‑BH murder mysteries, BHs in Earth’s core, etc.).