Time for the Stars, Robert A Heinlein, Charles Scribner's Sons, 1956, 244 pp
Earth is over-populated, families are only allowed three children and the Long Range Foundation is a non-profit with the goal of finding planets that could support human life so some of Earth's population can be exported. The Bartlett family had decided to pay the tax for having an extra child but the fourth birth produced twins. With the tax and two extra mouths to feed, money was tight.
Pat and Tom, the twins, are smart, inseparable but competitive (they share a girlfriend but she likes Pat better) and as it turns out, can communicate telepathically. The Long Range Foundation's plan includes sets of telepathic twins, one to stay on earth and receive reports of the planet finding mission from the other out in space. Telepathy is instant and overcomes the problem of communicating between two points that are light years apart. After some brotherly tricks and one upmanship, Pat and Tom get accepted.
Clever ideas, no? And they open the door to a story that includes all kinds of Einsteinian data about time and relativity, not to mention that while the twin in space ages slowly, his telepath on earth must eventually be replaced because of the discrepancy in time. This is the opposite to the way time behaves in the "multiverse," where children go to Narnia, for example, live for decades and come back to find that no time at all has passed.
I liked the book though the science got a bit over my head and the author did too much preaching. I forgive him because he is such a good storyteller. I think this is the first science fiction to take up the question of time differences scientifically; at least it is a first in my chronological reading system.