Even before string theory, the idea existed that the geometry of the universe would allow for shorter paths between points. In fiction, this can be seen in stories such as Lewis Carroll’s Alice in Wonderland, and in science, it can be seen in the wormholes.
A wormhole is a shortcut to go from one location on a surface to another, just as a worm can dig through the center of an apple to get from one surface to another (thus the name). This concept arises from Einstein’s theory of general relativity, proposed years before string theory was conceived.
These traditional wormholes connect different regions in the same universe and have been exploited for many outlandish theoretical purposes, despite the fact that no one knows for sure whether they exist.
Similarly, it’s possible that in a brane world scenario, we are somehow touching or connected to another brane. If these branes overlap, it’s conceivable that there would be a way to travel from the space of one brane to the space of another brane.
(This is not the standard way that multiple branes interact in string theory. Much more common are the brane world scenarios where separate branes host difference pieces of the physics of our universe and then interact gravitationally.)
It’s unlikely that such a brane jump would take place merely by stepping into a mirror, but something as powerful as a wormhole might do the trick. It’s possible that a wormhole — generally hypothesized by general relativity as existing within rotating black holes and being notoriously unstable — might allow bits of matter or energy to slip from one universe into another parallel universe.
If such strange events occurred at points where different branes overlapped in the bulk (the greater space that contains all of the branes), it’s unclear whether they might provide a way to get matter and energy from one brane to another as well.
In fact, one possible resolution to the black hole information paradox that has long been considered by some is the idea that information that enters a black hole exits into a parallel universe by means of a wormhole at the center of the black hole.
Such ideas are obviously highly speculative, but mathematical models have shown it’s feasible that some sort of wormhole — if held open by a form of negative energy — could provide a means of connecting different portions of space.
If this is the case, then the arguments in favor of parallel universes are on our side, because given an infinite universe and infinite time, everything is bound to happen somewhere. In a universe where parallel universes exist, travel between them may be guaranteed.