Eine Reise durch die Zeit
Eine Reise durch die Zeit (“A journey through time”) by H.G. Tannhaus — the Clockmaker. He wrote the book after he had already seen it. Ulrich showed him the book in his workshop in 1953, but the Clockmaker did not yet recognize his older self on the back.
The book contains a theory of time travel. Einstein-Rosen bridges, wormholes: Time is not linear; the past, the present, and the future are interwoven.
In 1986, Helge gives the book to Claudia when she takes over as manager of the nuclear power station. As an old man, he has it on his nightstand in the nursing home. Ulrich finds it in 2019 when he is looking for Helge. He takes the book with him and follows the old man into the cave — arriving in the year 1953, where he shows it to the Clockmaker. The book is thus its own inspiration.