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I recently read a sci-fi book, where some (very few) people can communicate telepathically, which happens to be instantaneous.
So, every spaceship has a crew member who telepathically communicates with his/her partner on Earth.
Would this lead to a cause/effect paradox? How and why?
More about the book, in case anyone is interested:
Heinlein: Time for the Stars - The book is narrated by Tom, a young man, who discovered he can telepathically communicate with his twin brother Pat. At age 16, they are hired as special communicators by Long Range Foundation. Tom leaves on a spaceship, while Pat stays on earth, and the pair serves as a communication link between Earth and the spaceship.
The rocket accelerates towards almost light speed. As the speed increases, the twins find it increasingly hard to communicate, because of time dilation. Back on Earth, Pat gets married, and has a daughter, while Tom is still a teenager...
ADDED: One thing that makes this different from generic FTL communication is that each pair of communicators started on Earth. Most are twin brothers/sisters, and there is also a set of triplets, where one stays on Earth, and the other two leave with two different spaceships into two different directions.
So the only way to make sense of this is to say that there is some distinguished frame of reference whose simultaneity dominates. For instance, perhaps some bond forms between Tom and Pat while they are both on Earth, and this bond is formed specifically in reference to the Earth's frame of reference. When Tom goes off into space, the bond retains this frame of reference. It's "set" on Earth's frame of reference, regardless of what Tom and Pat do.
There is definitely a special bond between each pair of communicators. They are either twins, or two people who spent a lot of time together to develop telepathic communication.
As the whole project originated on Earth, all these communicator bonds are "set" on Earth's frame of reference (whatever that means...)