And the related problems of Elvish aging. Certain characters have to be born in certain places and married in certain places, and if the Elvish aging rate is inappropriate for that then things fall apart.
There's a lot of ink spent answering this problem.
Middle earth is confirmed to have atoms and isotopes.
The Valar told the elves about them, but didn't explain what they are for, so elvish nuclear engineering is canon-compatible, but not implied by canon.
People may say that Elves in middle earth are psychic. This is not correct.
All thinking minds in middle earth may directly examine each other. The rise of language makes this a less practiced skill.
Anyone who does not wish to be read psychically may close their mind at will.