The Einstein-Rosen Bridge

In 1916 Einstein first introduced his general theory of relativity, a theory which to this day remains the standard model for gravitation. Twenty years later, he and his long-time collaborator Nathan Rosen published a paper showing that implicit in the general relativity formalism is a curved-space structure that can join two distant regions of space-time through a tunnel-like curved spatial shortcut.

The Einstein-Rosen Bridge is based on generally relativity and work done by Schwarzschild in solving Einstein’s equations; one of the solutions to these equations was the prediction of black holes

The relativistic description of black holes requires wormholes at their centers. These wormholes, called Einstein-Rosen bridges after Einstein and his collaborator Nathan Rosen, seem to connect the center of a black hole with a mirror universe on the "other side" of spacetime