Squeeze Earth to nine millimetres and it would become a black hole
Any lump of matter crushed small enough would turn into a black hole. The threshold, called the Schwarzschild radius, grows with mass: about 3 kilometres for the Sun and a mere 9 millimetres for Earth, roughly the size of a marble. Neither will ever get there, because their gravity is far too weak to overcome the pressures holding atoms apart.
That radius marks an event horizon, a boundary beyond which nothing can send a signal to an outside observer. The idea is older than relativity. In 1784 John Michell suggested that a compact enough star might have gravity so strong that light could not escape it. Wolfgang Rindler coined the term event horizon in the 1950s, and in 1958 David Finkelstein used general relativity to define a black hole's horizon as a surface beyond which no event can influence the outside at all. Stephen Hawking later argued that collapse produces only apparent horizons, a position that led him to question whether black holes in the strict sense exist.
Popular images mislead. Black holes do not hoover up their surroundings; like any mass, they only capture material that strays close. Nor can anyone watch matter fall in. A distant observer sees an infalling object slow down and grow ever redder, never quite reaching the edge. What telescopes actually detect are accretion disks, where gas whirls so fast that friction makes it blaze, sometimes launching jets along the spin axis. Collapsing stars need roughly three solar masses to get past the degeneracy pressures that halt smaller remnants.
Crossing is less dramatic than it sounds. The horizon is not a wall that destroys things; a traveller would pass it in a finite time by their own clock, with nothing special marking the spot. Inside, though, every possible path bends further into the hole, so moving inward becomes as unavoidable as moving forward in time. Oddly, the horizon's existence depends on the entire future of the universe, so no measurement in a finite region could ever prove one is present.
Horizons are not unique to black holes. In a universe whose expansion outpaces light, some regions can never be seen, however long anyone waits. A constantly accelerating observer in empty space would also find a boundary behind them that light can never cross.
Source: Event horizon
