# Something Strange Happens When You Follow Einstein's Math

## Black Holes

- Black holes prevent anything, even light, from escaping beyond the event horizon.
- The Schwarzschild metric describes the curvature of spacetime around a single spherically symmetric point mass and has two problem spots: a singularity at the center of the mass and another at the Schwarzschild radius.
- Black holes were initially doubted due to the massive amount of mass required to collapse into a tiny space.
- Oppenheimer and Snyder showed that for the heaviest stars, there is nothing to prevent them from collapsing indefinitely, suggesting the possibility of black holes.
- Einstein initially resisted the idea of black holes due to the apparent freezing of time at the event horizon.
- The singularity at the event horizon is an artifact of the chosen coordinate system and disappears under a different coordinate substitution.
- Space can be visualized as flowing into a black hole like a waterfall, with photons struggling to escape against the flow.
- Inside the event horizon, space falls faster than the speed of light, causing everything to fall into the singularity.

## White Holes and Wormholes

- A white hole is the time reverse of a black hole, where things are expelled outwards instead of falling in.
- The Schwarzschild black hole solution contains a second universe that has its own independent set of coordinates from our universe.
- An Einstein-Rosen Bridge is a hypothetical wormhole that connects two universes and could allow for travel between them.
- These wormholes are not stable in time and would collapse before anything could pass through them.

## Rotating Black Holes

- Spinning black holes have a more complex structure compared to non-rotating black holes, as described by the Kerr solution.
- The Kerr black hole consists of several layers, including the ergosphere, the outer horizon, and the inner event horizon.
- Inside the inner event horizon, there's a region where objects can move around freely and avoid the singularity.
- Beyond the inner horizon, there could potentially be a white hole that leads to another universe.
- By traveling through a rotating black hole and a white hole, it's theorized that one could access an infinite number of universes.

## Current Understanding

- According to our current understanding, white holes, traversable wormholes, and parallel universes are unlikely to exist, but there's always the possibility of future surprises.