Between Universes

Between stars in the Cloud Universe, there are multiple navigational hazards and lethal radiation whereas, between universes, there is the tap room of the Old Phoenix! - at least in our imaginations. Also in imagination, John Rolfe steps through a shimmering surface on the wall of his basement onto a parallel Earth in SM Stirling's Conquistador.

We envisage two kinds of ways to travel to other worlds:

by traversing interplanetary or interstellar space;

by stepping through a wardrobe, painting or mirror, falling down a rabbit hole or entering an inn that is not usually there (etc).

Unfortunately, the easier way is also impossible. As I understand it, any theory that allows for parallel universes also disallows any travel between them. However, human beings have always imagined more than they experienced and now we learn that the universe that we experience is stranger than we can imagine.