A black hole forms when a massive star exhausts its nuclear fuel and its core collapses under gravity. If the remaining mass exceeds roughly 3 times the mass of our Sun, nothing — not even neutron degeneracy pressure — can halt the collapse.
The result is a singularity: a point of infinite density, wrapped inside an event horizon.
The Event Horizon The event horizon is not a physical surface. It is a mathematical boundary — the radius at which the escape velocity exceeds the speed of light. Cross it and no signal, no matter, no information can ever return.
For a non-rotating black hole (Schwarzschild), this radius is: r_s = 2GM / c²
For the Sun, that radius would be about 3 km. For Earth, just 9 mm.
Time Dilation at the Edge Time slows near strong gravitational fields — a prediction of General Relativity confirmed by GPS satellites and precision clocks. Near a black hole, an outside observer would see a falling astronaut freeze and redden at the horizon — never quite crossing it. The astronaut, however, would cross freely, feeling nothing unusual.
Hawking Radiation In 1974, Stephen Hawking showed that black holes are not entirely black. Quantum effects near the event horizon cause black holes to slowly emit thermal radiation, losing mass over astronomical timescales. A stellar black hole would take longer than the age of the universe to evaporate — but a primordial micro black hole could pop out of existence in a flash of gamma rays.
The Information Paradox If all information about infalling matter is destroyed — which Hawking's original calculation implied — it violates a core principle of quantum mechanics. Decades later, this paradox remains one of the deepest open problems in physics, with competing resolutions involving firewalls, fuzzballs, and holography.
Black holes are not endpoints. They are windows into physics we don't yet fully understand.