Wormhole other end [duplicate]

Question

How do we know what lies at the other end of a wormhole (another universe, a place 50 miles away, a place very far away) from its metric equation? Why is it said that there lies another universe at the other end of a Schwartzchild Wormhole (Einstein Rosen Bridge)?

Answer 1

Solutions in general relativity are just solutions to the field equations: they do not have to correspond to anything ever found in reality.

Some solutions are apparently decent approximations of things we see, like the Schwarzschild and Friedman metrics. Even those are often best seen as approximations: the solar system is dominated by the Sun's gravity, but there is a bit from the planets making it deviate from pure Schwarzschild in a way that is extremely hard to put into analytic form. Physicists often make implicit guesses (sometimes with very good motivations) about the nature of these other solutions that ought to exist.

Wormhole solutions are potentially in the first category. The standard methods require potentially unphysical fields to exist. They can be studied and are not too out there compared to some other solutions.

Still, the physical interpretation of the places "on the other side" remains undefined. Saying it is "another universe" is a shorthand for "spacetime consists of two equal spherically symmetric parts glued together along a sphere in the wormhole throat" but does not say anything about there actually being another universe in reality.

Saying the wormhole leads to another location in the same universe assumes that the mathematical solution is actually close to a hypothetical solution where the wormhole connects at another spot. As far as I know, nobody has ever formally or numerically calculated such a solution. But just as the solar system is well approximated by a sun-centred Schwarzschild metric it is not crazy to think that two wormhole openings far away from each other would be a valid solution.

So the short answer is that we do know what is behind the wormhole in a model because we say so and it is reasonably consistent with that model. What we do not know, and matters a great deal, is whether such models are anything close to what can exist in our universe.