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The most common use of parallel universes in science fiction, when the concept is central to the story,
is as a backdrop and/or consequence of time travel. A seminal example of this idea is in Fritz Leiber’s novel, The Big Time where there’s a war across time between two alternate futures each side manipulating history to create a timeline that results into their own world. Time-travelers
in fiction often accidentally or deliberately create alternate histories, such as in The Guns of the South by Harry Turtledove where the Confederate Army is given the technology to produce AK-47 rifles and ends up winning the American Civil War. (However, Ward Moore reversed this staple of alternate history fiction in his Bring the Jubilee (1953), where an alternative world where the Confederate States of America won the Battle of Gettysburg and the American Civil War is destroyed after an historian and time traveller from the defeated United States of that world travels
back to the scene of the battle and inadvertently changes the result so that the North wins that battle.) The alternate history
novel 1632 by Eric Flint explicitly states, albeit briefly in a prologue, that the time travelers in the novel (an entire town
from West Virginia) have created a new and separate universe when they're transported into the midst of the Thirty Years War in 17th century Germany. (This sort of thing is known as an ISOT among alternate history fans, after S.M. Stirling's Island in the Sea of Time: an ISOT is when territory or a large group of people is transported back in time to another
historical period or place.[citation needed]
The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time," is often used to allow characters to pass through many different alternate histories,
all descendant from some common branch point. Often worlds that are similar to each other are considered closer to
each other in terms of this sidewise travel. For example, a universe where World War II ended differently would be “closer”
to us than one where Imperial China colonized the New World in the 15th century. H. Beam Piper used this concept, naming it "paratime" and writing a series of stories involving the Paratime
Police who regulated travel between these alternative realities as well as the technology to do so. Keith Laumer used the same concept of "sideways" time travel in his 1962 novel Worlds of the Imperium. More recently, Frederik Pohl used the idea in his novel The Coming of the Quantum Cats which is explicitly based on upon a human-scale reading of the "many worlds" interpretation of quantum mechanics, postulating that every historical event spawns a new universe for every possible
outcome
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