Robert Heinlein wrote a book about this. Twin boys, one on Earth and one on an interstellar spaceship, communicate instantaneously, thus ignoring the speed of light.
Dune novel prequels covered that aspect as well. Two twin boys, one became a Navigator, the other a guerilla fighter to Is to free it of the Tlilaxu.
The Russians were heavy into instantaneous communication over long distance using telepathy.
Supposedly they had pairs of people that could communicate this way, and tested it by having one on a submarine and the other at the Institute testing the process.
Twins have been studied extensively. How does a twin “just know” his/her twin is in trouble, or happy, or whatever, even though they are on opposites sides of the planet and have not been in communication for months?
Spooky action at a distance, indeed.
Heinlein also wrote the best time-travel yarn ever: The Door Into Summer.