Temporal Mutability -- AKA The Sliding Scale Of How Easy It Is For Time Travelers To Change The Past, And Why. Time Travel is one of the richest concepts in Speculative Fiction; altering the past is easily one of the richest Time Travel plots. Could you go back and save your brother from that fatal car crash? Could you punch your boss in the face, then go back and stop yourself? Could you prevent World War II by going back to 1930 and killing Adolf Hitler? Nope. These settings tend to fall into one of the following categories (arranged here from least changeable to most changeable):