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The String Theory was the theory invented by Sam Beckett that inspired him to create Project Quantum Leap. He demonstrated his theory with a string. One end of the string represented birth, the other end represented death. Tie the ends together, and what you get is a loop. Ball the loop, and all the days of your life touch each other. Therefore, one could leap from one day to another in his/her own lifetime.
Sam originally got the idea for his String Theory from watching the 1950s television series Captain Galaxy, where he had sent a letter to the star of the show (Moe Stein) which was read aloud on the show. At this time, Sam was just over 4 years old.