Though he contributed to the theory, Einstein had questions about Quantum Mechanics. He famously debated Niels Bohr on this subject and later said:

"Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One. I am at all events convinced that He does not play dice."

I'm not really qualified to say but, at a guess, Einstein didn't feel that the Quantum guys had enough figured out to be saying they had a full fledged theory, as opposed to a hypothesis. Dad is suggesting that whether Einstein or Bohr were right there is still something beyond Einstein's theoretical conception of the universe ... and it is in that area that the guts of this story lurk.

At the time that Dad was writing it was common for people, particularly fiction writers, to discuss the concept of Alternate Realities or Parallel Worlds using terms like "Other Dimension" or "Fourth Dimension" ... as opposed to another SET of dimensions, or an infinite number of other sets of dimensions.

I like the idea that we all dwell in multiple realities all the time ... it's just that there is so little difference between the differing realities we never notice unless we wonder where that missing sock went, or why everyone started acting like they grew up in a totally different country.

Perhaps, places like the "other world" in THM lurks just a bit off to one side, needing just a bit too much energy for us to make the jump unless conditions are exactly right. More likely these "other worlds" or other sets of dimensions are reflections or related aspects to the reality we see every day, like the relationship a photographic negative has to both reality and the printed photo.

At one time I trained myself to meditate or self hypnotize to the point where I was hallucinating. I believe that my Dad could to this to a certain extent too. These were not random hallucinations (they always started the same way) and contained a certain amount of tactile sensation as well as things I could "see." Was I able to visit the dimension of the "code", the photo negative? Was I just activating my natural DMT (a hallucinatory chemical in our brains)? No idea, but it was convincing that there was something legitimate and consistent beyond or underlying this reality ,it just wasn't anything like another "world."

Take a look at the Lost Treasures version of THM. There's a good deal of background in it. I think that it and The Californios were both reactions to the realization that rockets were never going to take us to other worlds, the technology was just too limited. Science Fiction needed a different frontier.