A short story, decades old, translated to my native German, probably written by a female Canadian in French:
Aliens have contacted Earth and installed a space station with a portal to their home world. They do not accept tourists, but assign any interested traveller some task to perform there. For undisclosed reason the travel time is unpredictable: you may arrive back after a short stay and many years have passed on Earth, or you stay long and arrive on Earth visibly aged, but your friends claim only a short time has passed.
The protagonist has just seen his girlfriend leave for the alien world. Traveling as a pair is also not accepted, so he hopes she will return soon. In the meantime he talks to people returning, asking if someone has met his girlfriend, and should he also travel there.
The story drags on, without him finding his girlfriend, until he decides to end his wait, but we are not told if he gave up or will visit the alien world. Only then did I realize:
She had already returned, aged beyond recognition, giving him some advise and leaving him there.
