Appeared in Analog Science Fact & Fiction, April 1961
"Lance," said Carolyn.
"You feel it too, don't you?"
"That there is danger involved. That something dreadfully, dreadfully wrong can happen to you while you're out there. No matter what the eggheads say about it." A paroxysm of sobs suddenly racked the girl's slender body. "Oh, darling, don't go!"
Fun fact: the above theme of the protagonist being entirely oblivious to everything around him and needing everything to be explained to him will continue throughout this story!
There are certain--tics, perhaps--in Golden Age-era science fiction. Epithets like "Space!" and "Unity!" because god forbid anyone encounter the word "fuck" in print, the rah-rah belief that the United States of the 1950s is the social model that will one day be projected over vast swaths of the galaxy, and at least in John W. Campbell's Analog, a focus on square-jawed white male engineers solving technical problems. Robert Donald Locke's "Next Door, Next World feels like a story that was written in the 1940s and features one of the thick-headedest protagonists I've ever tagged along with, and had I not been cooling my heels in a hospital waiting room I doubt I'd have bothered.
He experiences Weird Shit in hyperspace, of course, because what would hyperspace _be_ without that? He feels himself split apart, sees duplicates of his ship outside where there should be nothing at all, but returns to normal space feeling none the worse for wear and after a quick sightseeing expedition, makes the return hop back to Earth where his fiancée Carolyn is waiting for him--except, shock! When he lands, no one recognizes her name, not even her dad, his boss Colonel "Hard-Head" Sagen! Lance immediately jumps to the logical conclusion: everyone is pulling a complicated prank on him. After his experience is declared classified and he's put in a cell, he manages to escape--because of course it's easy as hell to steal a military guard's sidearm and effect escape with it--and goes to his girl's place, looking for answers from her mother.
Because of course the best place to go looking for answers after you've broken out of military custody is *the home of the military superior who placed you in custody*! Seeing as how Mrs. Sagen is, you know, intelligent, she alerts the military to Lance's presence--and while he escapes again, he does so fuming about her "double-cross," while I start to wonder if hyperspace pilots are chosen specifically for their expendability.
He returns to the base--because of course they won't look there--and finds that his friends don't recognize him, he remembers things that are no longer the case, and comes to the conclusion that everyone is lying to him. Finally, he's brought before a military psychiatrist and finds out the truth that was pretty apparent from the first word: he's slipped into a parallel universe, where his girlfriend was never born. Apparently this happens a lot.
It was at this point that I seriously began to doubt Cooper's bonafides--it's as if someone shaved the stupidest Watson and stuffed him into a starship. Despite being presented with a litany of examples of pilots who came back "off"--wearing the wrong uniform, a man with a mustache he couldn't possibly have grown so fast--and by his own admission being aware of the parallel worlds theory, he does not consider that he might actually be in a parallel world until he's practically being told his name is Homer Thompson. Nevertheless, driven only by a desire to see his girlfriend again, he takes the Colonel at gunpoint and finagles his way back into his hyperspace ship, and blasts off in the hopes of reaching his own world. Does he make it?
Cooper's thickheadedness over his predicament is, I think, another one of those artifacts of older science fiction. Science fiction is built around the projection of current trends into the future, but at the time this story was written, science fiction was still culturally marginal, and so a lot of Golden Age sf feels like they're set in worlds which do not themselves include a cultural legacy of science fiction. I mean, if I got shot through hyperspace and came back to find people I knew were gone and the world was just subtly askew, the notion of parallel universes would be on my theory plate thanks to things like Sliders or the Mirror Universe from Star Trek or actual scientific investigations toward whether a multiverse exists. Lance Cooper, being the resident of a '50s future, doesn't have that cultural background and so looks stupid for never even entertaining the notion.
As for the story itself, it's written in the stilted manner that's common for a lot of sf writing from that era. I mean, things like "Dad opined he'd have walloped the daylights out of me" - who the hell uses words like "opined" in casual conversation? It's no surprise Robert Donald Locke didn't leave a mark on the field--he's got only eleven credits on ISFDB, and this was in fact his last story.
If you're interested, Next Door, Next World is available for download on Project Gutenberg.
#2 - "In the Imagicon" (February 1966)
#1 - "Blitz Against Japan" (September 1942)