The crosstime category is probably the second largest category in terms of usage, the other being the straightforward alternate history. It can be subdivided into involuntary and voluntary crosstime travel, although some novels are a blend of both. The mechanism for voluntary and in some cases the involuntary crosstime story involve some means of traveling through alternate timelines. This is accomplished either through natural or artificial means. Some stories posit naturally occurring rifts in time/space that allow being to travel through timelines or some inborn ability to do so. Other rely on purely artificial means.
Many of the involuntary crosstime travels begin with someone thrust, as in the words of Howard the Duck 'into a world he never made". Although some of these are purely by chance as in Two Hawks from Earth or Michael Moorcock's Oswald Bastable series, where the protaganist slipped through a naturally occuring space/time rift most of the involuntary crosstime travel tales have something to do with some form of artificial crosstime travel. Calvin Morrison, Edward Bear, Brion Bayard, Blake Walker and Mark Strang are all stranded in worlds that are different from their own by traveling through an device that crossed timelines. Calvin Morrison found himself in Pennsylvania in a far different ruled by Aryans in Lord Kalvan of Otherwhen by H. Beamer Piper courtesy of the an oversight by the Paratime police. Edward Bear was accidentally transported to the North American Confederacy through an experimental device in The Probability Broach by L. Neil Smith. Brion Bayard was kidnapped by members of a crosstime organization, in The Worlds of the Imperium by Keith Laumer. Blake Walker was also forced into an alternate timeline by a crosstime organization in the The Crossroads of Time by Andre Norton. In John Barnes' Patton's Spaceship While following a terrorist organization that killed his wife and mother, Mark Strang was stranded in the 1960s of a world dominated by the Nazis. In Sideslip Ron Archer was thrown into a world where World War II never occured through a a laboratory accident. In almost all of these situations the person is a fish out of water and must learn to live in his new home.
One odd subsect of the involuntary crosstime travel tale is the transfer to another world by death. In John Clagett's A World Unknown, Simon Ashe is transported to a world where Ancient Rome never died when he died in a plane crash. Eric Norden's The Ultimate Solution seems to be a straightforward alternate history about a Nazi dominated United States until it is revealed that one of the main characters had died and been transferred to this particular universe. In Michael Bishop's A Secret Ascension, upon his death science ficton writer Philip K. Dick peers into an alternate world where he was a successful main stream author which was also a world where Watergate never took place. In Gordon Eklund's All Time Possible when Tommy Bloom dies travels from an alternate world where America became fascist to one where America becomes a soviet type state.
Although it is more common in fantasy fiction and somewhat in time travel fiction, the idea of a consciousness traveling into another world or dimension has been used a few times in alternate histories. In The Wheel of If by L. Sprague de Camp, Alistar Park's consciousness is rotated into several alternate doppelgangers until he is stuck in a world where the Vikings had settle America. This transfer was done by a mechanical means by men who wanted to get rid of Park's doppelganger without actually killing him.
A great deal of the crosstime alternate history stories have to do with voluntary travel, usually through an organization that either polices the timelines or profits from trading among them. We have mentioned the Piper's Paratime series, Nortons Crosstime series and Laumer's Imperium series. Other such police type organizations involve Sam Merwin's Watcher organization which are the crosstime organization in his House of Many Worlds and Three Faces of Time. A subset of the policing crosstime organization is the organizations which war across timelines. This concept was probably introduced by Fritz Leiber in The Big Time and Changewar. This involved two time traveling factions that recruited soldiers from all eras and who changed history as part of their stragegies. This concept of warring across time and recruiting soliders from all eras was also part of Simon Hawke's Timewars series. The premise of this series was that in the future war had been changed to a sort of game where by soldiers achieved points by participating in historical battles. Time was believed to be impossible to change, however as the series progress the time line split and the two timelines fight against one another. Richard C. Meredith's Timeliner series portrayed the experiences of a solider recruited to fight across several timelines, believing that he was preventing a catastrophe that would have wiped out all of existence. Keith Laumer's Worlds of the Imperium was also often involved crosstime battles as the zero zero timeline often had to fight against invaders from other timelines. John Barne's Timeline Wars carried on in this vein.
Here are some stories, novels or series that use the ability to travel through alternate histories for profit. Larry Niven's All the Myriad Ways, which also shows how the very concept of alternates can have a serious psychological affect. Piper's Paratime series, Jack C. Chalker's G.O.D. series, Michael P. Kube-McDowell's Alternities, S. M. Stirling's Conquistador, Charles Stross Merchant Princes series and Harry Turtledove's Crosstime Traffic series.
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