Stories or accounts of slipping into another dimension or time are often considered as experiences related to the paranormal or speculative fiction. While they can be intriguing to contemplate, there is no scientific evidence to support the existence of such events or phenomena.
If your grandfather claimed to have witnessed something unusual, it could be attributed to various factors such as misperception, optical illusions, vivid imagination, or even a story exaggerated over time. Human perception can sometimes be fallible, and our memories are subject to distortions.
If you have a keen interest in speculative topics like alternate dimensions or time travel, there is a rich world of literature, movies, and creative works that explore these concepts. However, it's important to distinguish between fictional narratives and real-world experiences.