Lorentzian wormholes known as Schwarzschild wormholes or Einstein-Rosen bridges are bridges between areas of space that can be modeled as vacuum solutions to the Einstein field equations by combining models of a black hole and a white hole. This solution was discovered by Albert Einstein and his colleague Nathan Rosen, who first published the result in 1935. However, in 1962 John A. Wheeler and Robert W. Fuller published a paper showing that this type of wormhole is unstable, and that it will pinch off instantly as soon as it forms, preventing even light from making it through.
The Einstein-Rosen Bridge. A safe
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"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 space time. At first, the bridge was considered a mathematical oddity, but nothing more. It was essential for the internal consistency of the Schwarzschild solution to Einstein's equations, which was the first relativistic solution involving black holes. However, the wormhole could not be traversed because the center of a black hole is a singularity, a point of infinite space time curvature, where the gravity would also be infinite and all matter would be crushed to its most fundamental constituents. Additionally, travel through the wormhole would require motion faster than the speed of light, a physical impossibility. For these reasons, Einstein-Rosen bridges were quickly forgotten despite other later solutions that included them. They were assumed to be mathematical oddities that had no bearing on physical reality.
In 1963, Roy Kerr devised the famous Kerr solution to Einstein's equations, a more realistic description of black holes than the original Schwarzschild solution. Kerr assumed the star that would form the black hole to be rotating and found that it would not eventually collapse to a point, but rather to a ring. When approaching the ride from the side, gravity and space time curvature are both still infinite, so matter is again inevitably destroyed. However, traveling through the ring would result in large but finite gravity. An object that does so and avoids being crushed by the still-formidable gravity can enter the Einstein-Rosen bridge and gain access to the mirror universe."