What Is Spaghettification? Tidal Forces Near a Black Hole

Fall feet-first toward a black hole and gravity pulls harder on your feet than on your head. Stretched along the line of fall and squeezed from the sides, you would end up long and thin. That is spaghettification, and our tee Spaghetta is wearing it. The physics is ordinary tidal gravity pushed to an extreme. This post builds on our explainer Do Black Holes Evaporate?.

Where does the word come from?

NASA notes that the image was made famous by Stephen Hawking’s book A Brief History of Time (1988), in which a fictional astronaut crossing a black hole’s event horizon is “stretched like spaghetti” (NASA Science, “Too Close”^[1]). The physics behind it is older: it is the same tidal effect that raises ocean tides on Earth, where the Moon pulls the near side of the planet harder than the far side (NASA^[1]).

What is a tidal force?

Gravity weakens with distance. Newton’s law gives the acceleration toward a mass M at distance r as

g(r) = GM / r²

where G = 6.674 30 × 10⁻¹¹ m³ kg⁻¹ s⁻² is the gravitational constant (NIST CODATA 2022^[2]). An object of length ℓ pointing toward the mass has its near end at r and its far end at r + ℓ. If ℓ is much smaller than r, the difference in pull between the two ends is

Δa ≈ 2GMℓ / r³

That stretching acceleration falls off as 1/r³, much faster than gravity itself. Sideways, the pulls on the left and right edges both point toward the centre of the mass, so they converge and squeeze the object inward. Stretch along, squeeze across: spaghetti (Pinochet 2022^[3]). Near a black hole this Newtonian estimate is only a guide — a full treatment needs general relativity — but it gets the size and the trend right, which is all we use it for here (Pinochet 2022^[3]).

Why is a small black hole more dangerous at the horizon?

The event horizon of a non-rotating black hole sits at the Schwarzschild radius

r_s = 2GM / c²

where c = 299 792 458 m/s. For one solar mass that is about 3 km (NASA Imagine the Universe^[4]). Put r = r_s into the tidal formula and the stretching at the horizon becomes

Δa_horizon ≈ ℓc⁶ / (4G²M²)

It falls as 1/M². Bigger black holes are gentler at their edges. Here are our own worked numbers for a 2 m tall person, using G and c from CODATA and the Sun’s mass of 1.9884 × 10³⁰ kg from NASA’s Sun fact sheet (NASA NSSDCA^[5]):

• A 10-solar-mass black hole: r_s ≈ 30 km. At the horizon, Δa ≈ 2 × 10⁸ m/s², about twenty million times Earth’s surface gravity. The stretch already reaches about ten times Earth’s gravity nearly 4,000 km out. You would be torn apart long before you reached the horizon.
• A 4-million-solar-mass black hole like the one at the centre of our galaxy: r_s ≈ 1.2 × 10⁷ km, about 0.08 times the Earth–Sun distance. At the horizon, Δa ≈ 1 × 10⁻³ m/s² across your body — far too small to feel. You would cross the horizon intact; spaghettification would come later, inside, where no one outside could see it.

The mass of our galaxy’s central object comes from two decades of tracking stars around it: the 2020 Nobel Prize press release describes the groups led by Reinhard Genzel and Andrea Ghez finding around four million solar masses packed into a region no larger than our solar system (Nobel 2020 press release^[6]). What happens to infalling matter deep inside a horizon, near the predicted singularity, is beyond what any observation can test. That part is open.

Can we see spaghettification happen?

Not to astronauts, but to stars. When a star passes close enough to a massive black hole, tides overwhelm the star’s own gravity and rip it into a stream of gas: a tidal disruption event (TDE). In 1975 J. G. Hills proposed that the tidal breakup of stars passing within the Roche limit of a black hole could help power quasars and Seyfert nuclei (Hills 1975, Nature^[7]). In 1988 Martin Rees worked out what such an event would look like: some debris ejected at high speed, the rest swallowed, causing a bright flare lasting at most a few years, compatible with black holes of 10⁶–10⁸ solar masses in nearby galaxies (Rees 1988, Nature^[8]).

Those flares are now observed routinely. AT2019qiz, at 66 megaparsecs the closest optical TDE known when it was studied, was fitted as a black hole of about a million solar masses disrupting a star of about one solar mass, with an outflow dominating the early light (Nicholl et al. 2020^[9]). In 2025 NASA reported AT2024tvd, a TDE whose million-solar-mass black hole sits offset from its galaxy’s centre, and described the infalling star as stretched or “spaghettified” by tidal forces (NASA, AT2024tvd^[10]).

Why the biggest black holes swallow stars whole

A rough estimate shows why the disruption radius matters. Set the tidal stretch across a star of mass M_* and radius R_* equal to the star’s own surface gravity, 2GMR_*/r³ ≈ GM_*/R_*², and you get r_t ≈ R_*(2M/M_*)^1/3. This grows only as M^1/3, while the horizon grows as M. On our numbers, for a Sun-like star the two become comparable near 10⁸ solar masses; above that, the star crosses the horizon before tides can shred it, and there is no visible flare. That is an order-of-magnitude estimate, consistent with the upper end of Rees’s mass range, not a sharp cut-off.

Spaghettification myths

• “Black holes suck things in.” At a distance, a black hole’s gravity is the same as any object of the same mass. Trouble starts only close in.
• “You’d be spaghettified at the horizon of every black hole.” Only small ones. For supermassive black holes the horizon tides are tiny.
• “Spaghettification has been seen in a person or spacecraft.” No. It has been inferred for stars, from TDE light.

In the shop

Spaghetta is the stretch, Eventide Prophet guards the horizon, and the Stephen Hawking sticker honours the man who made the word famous. Browse the rest of Relativity & Cosmos, and for what horizons do to information, read What Is the Black Hole Information Paradox?.

FAQ

Would spaghettification hurt?

Near a stellar-mass black hole the stretching would rise from noticeable to lethal over a short fall. It is not survivable.

Does the Moon spaghettify Earth?

In a mild sense, yes: the same tidal stretching raises the ocean tides. It is enormously weaker than near a small black hole, because the Moon is light and far away.

Is the formula exact?

No. Δa ≈ 2GMℓ/r³ is the Newtonian estimate for an object much smaller than its distance from the mass. Near a horizon, general relativity is needed for precise numbers.

References

1. NASA Science (2023), “What Happens When Something Gets ‘Too Close’ to a Black Hole?”. https://science.nasa.gov/universe/what-happens-when-something-gets-too-close-to-a-black-hole/
2. NIST CODATA 2022, Newtonian constant of gravitation. https://physics.nist.gov/cgi-bin/cuu/Value?bg
3. J. Pinochet (2022), “The little robot, black holes, and spaghettification,” Phys. Educ. 57, 045008 (arXiv:2203.04759). https://arxiv.org/abs/2203.04759
4. NASA GSFC Imagine the Universe, “Black Holes”. https://imagine.gsfc.nasa.gov/science/objects/black_holes1.html
5. NASA NSSDCA, Sun Fact Sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html
6. NobelPrize.org, Press release: The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/press-release/
7. J. G. Hills (1975), “Possible power source of Seyfert galaxies and QSOs,” Nature 254, 295. https://www.nature.com/articles/254295a0
8. M. J. Rees (1988), “Tidal disruption of stars by black holes of 10⁶–10⁸ solar masses in nearby galaxies,” Nature 333, 523. https://www.nature.com/articles/333523a0
9. M. Nicholl, T. Wevers, S. R. Oates et al. (2020), “An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz,” MNRAS 499, 482 (arXiv:2006.02454). https://arxiv.org/abs/2006.02454
10. NASA Science, “NASA’s Hubble Pinpoints Roaming Massive Black Hole” (AT2024tvd). https://science.nasa.gov/missions/hubble/nasas-hubble-pinpoints-roaming-massive-black-hole/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.