Frederik Pohl 
The Day of the Boomer Dukes 

Just as medicine is not a science, but rather an art—a device, practised in a scientific manner, in its best manifestations—time-travel stories are not science fiction. Time-travel, however, has become acceptable to science fiction readers as a traditional device in stories than are otherwise admissible in the genre. Here, Frederik Pohl employs it to portray the amusingly catastrophic meeting of three societies…