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“Silitonga had shown that the smallest change imaginable would eventually have global repercussions. For a hypothetical time traveler who wanted to prevent Hitler’s rise to power, the minimal intervention wasn’t smothering the baby Adolf in his crib; all that was needed was to travel back to a month before his conception and disturb an oxygen molecule. Not only would this replace Adolf with a sibling, it would replace everyone his age or younger. By 1920 that would have composed half of the world’s population.”