Do Events Inside Black Holes Happen?

Black holes aren’t “places you can’t see” so much as collections of events that, for observers who stay outside the event horizon, never get to be assigned a consistent “when.” That reframes the usual picture: the famous black disk in the sky is what remains after external observers delete entire occurrences from their self-consistent histories—an outcome of general relativity’s spacetime geometry, not a simple visibility problem.

The core thought experiment uses gravitational time dilation and the relativity of simultaneity. Far from a black hole, a pony orbiting experiences normal time, while a distant observer sees the pony’s processes run in slow motion. Then a monkey falls radially toward the black hole. From the monkey’s viewpoint, crossing the horizon is uneventful in the sense that the monkey continues forward in time. But from outside, the monkey’s rotation and progress slow, and the monkey appears to freeze at the horizon: not rotating, not advancing, and not aging past that moment. Even if the outside observers wait an infinite time, they never see the monkey’s crossing occur. The monkey’s claim and the outside observers’ records cannot be reconciled into a single consistent assignment of timing for the same events.

In this framework, a black hole is defined as the set of events that physically occur for observers located at those events, while the horizon is the boundary of the last events that can still be assigned a “when” by outside observers. The “event horizon” is therefore a surface in spacetime, not a spherical shell in space. For any particle that enters, there is always a last event on its world line that makes it into an outside observer’s movie of the universe at time infinity; the collection of those last events across all infalling objects forms the horizon.

Despite this event-based definition, black holes behave like objects with mass because the external spacetime geometry can match that of an ordinary spherical mass. Replacing the Sun with a spherical black hole of the same mass leaves the exterior geodesics unchanged. Earth would still orbit in the same way, even though it would eventually freeze in the strong-field region. This motivates the Schwarzschild radius: a spherical black hole of mass M has a radius at which the exterior spacetime is identical to that of a non-black spherical object of the same mass. The Schwarzschild radius scales with mass—about 3 kilometers for a solar-mass black hole, and just under 1 centimeter for Earth’s mass.

The episode then dismantles common misconceptions. Black holes don’t “suck” like vacuum cleaners; inside certain regions, geometry removes stable circular orbits, so freefall becomes inevitable without implying extra pulling beyond gravity’s geometric role. Black holes aren’t black because light can’t escape in a Newtonian sense; instead, external observers see extreme gravitational redshift. Light emitted near the horizon is stretched to undetectably low frequencies, so the region effectively becomes invisible. And “density” depends on definitions: larger black holes can have low average density, while tidal effects near the horizon weaken for higher masses.

Finally, the mass question remains philosophically slippery in idealized solutions: general relativity permits an eternal Schwarzschild black hole with no collapsing matter at all, raising the issue of what “mass” means when there’s no stuff behind the horizon. The episode closes by emphasizing that thinking of black holes as “things” carries interpretational subtleties, even within classical general relativity.