How black holes form according to general relativity

• In Einstein's theory, gravity is caused by the curvature of spacetime itself around massive objects
• When a massive star runs out of fuel, it collapses under its own gravity
• If the star is heavy enough, not even neutron degeneracy pressure can stop the collapse
• It collapses into a black hole - a region where spacetime curvature is so extreme that not even light can escape from inside the event horizon

Strange properties of black holes

• To an outside observer, an object falling into a black hole appears to slow down and freeze at the horizon, never quite entering
• But for the falling object itself, it crosses the horizon without anything unusual happening, then hits the singularity
• This is due to extreme time dilation near the black hole
• At the event horizon, space itself falls into the black hole faster than light, so nothing inside can escape

Rotating black holes and speculative consequences

• Real black holes must be rotating, since the stars they form from are always spinning
• Rotating black holes are stranger than static ones - they contain inner and outer horizons
• In theory, it might be possible to avoid the singularity inside a rotating black hole and emerge in a white hole
• This white hole could lead to other universes in a maximally extended spacetime
• However, these scenarios are highly speculative and probably unstable in realistic situations

The possibility (or not) of traversable wormholes

• Wormholes that you could travel through are technically allowed by general relativity
• But they require "exotic matter" with negative energy density to be stable
• Such exotic matter probably does not exist, so traversable wormholes are likely impossible