The German title of the book is Auf der anderen Seite der Tür, it was first published in 1992. I should warn you though: I just finished it and found out it's supposed to be continued in another part of Oceans of Time, but I can't seem to find the follow up (in any language)*. Thanks to the featured library in this book it still offers a valuable list of books about science, but the story does have kind of an open ending.
So not to leave you hanging, I also recommend her complete scifi set Tigeraugen and Der Roboter vom Flohmarkt :-)


*found an update from 2009(!): apparently she's still working on that book, there is however a picture book to accompany part 1, called Dichtbij ver van hier.