Alternative Timelines

Alternative timelines:

exist and even coexist in several works by Poul Anderson, including his Old Phoenix Sequence;

can exist but must be prevented and are often discussed in his Time Patrol Series;

are "emulated" (consciously simulated) by an inorganic intelligence in his novel, Genesis.

Whichever of these theoretical bases is used, similar imaginative processes are involved in conceiving of any alternative timeline. A certain kind of question has to be asked:

Are our myths and fictions realities elsewhere?
Could physical laws differ?
What would result if events had occurred differently at a particular historical turning point?

Alternative timelines are either fantastic (gods exist; magic works; Shakespeare's plays were true) or historical-speculative (what if Hannibal had sacked Rome, if either the Pope or the Emperor had decisively won the medieval church-state conflict or if the conciliar movement had reformed the church pre-Luther?).

The Time Patrol is historical-speculative. Time Patrolmen can either find themselves in an alternative timeline or just discuss what would have happened if the Mongols had conquered North America or if Cyrus the Great had not completed his reign.

Anderson's "Operation..." series begins as fantasy. Magic works. Then we are told that the apparent magic results from a scientific discovery in 1901. Thus, the status of this alternative timeline changes from fantastic to historical-speculative.