I am looking for a book I read back in 1987. All I remember is that a person from earth (maybe a scientist?) connected to someone via his mind who was a prince in another world in the galaxy. When they exchanged minds and entered each other's bodies the guy from earth found out that the prince's world was at war, and everyone on his planet thought that he was actually the prince. He couldn't get back to exchange minds. I can't remember the end of the book.