Questions raised:
1. Can you leave him in 600 A.D. a bunch of times to make a Robo army?
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1. Robo Army
Inquiry
During Chrono Trigger, Robo is left in 600 A.D. for four hundred years, after which he is picked up by the party. Can the party go back in time and leave him for another four hundred years several times, resulting in several Robos existing from 600-1000 A.D. at once?
Answer
While Chrono Trigger does not explore this possibility of its own accord, the theory of Time Bastard can provide an answer. As you know, Time Bastard is a reflexive action by the universe to protect conservation of energy by preventing the duplication of beings through time travel (for more information, see its entry). You could conceivably leave Robo behind several times and achieve an unlimited number of Robos in 600 A.D. at once. However, let's say you try to take them to the future to do battle; you have about 5 Robos. Now, let's say Robo's age the first time around was 300 years old. Now, as soon as you leave the 5th Robo in 600 A.D., you take them to the future to fight. You have 5 versions of Robo in different stages of his personal history; one copy is 300 years old, one is 700 years old, one is 1100 years old, one is 1500 years old, and one is 1900 years old. Now, let's say they fight a really long time in the future -- on the scale of 400 years. After that time elapsed, we would only have one Robo. Time Bastard would take effect for all the other copies. Imagine it like this:
Robo originally goes to Fiona's forest at age 300. He ages 400 years to age 700, and travels back in time. He ages to 1100 years, then travels back in time, and so on.
Since he traveled back in time after 400 years passed, these copies of Robo would similarly disappear after 400 years passed in the future (or any era you took them to). This is because these copies of Robo originally traveled in time four hundred years after arriving in 600 A.D., which is the instant you finally rounded them up to do battle. Only one Robo would remain after 400 years of fighting -- and he'd be 2300 years old by then (1900 when you brought him there). This is the most current version of Robo, and the one protected by Time Traveler's Immunity.