The Ringworld novels take place in Larry Niven’s own particular future history, Known Space. Like Isaac Asimov’s and Robert Heinlein’s future histories, it has been charted and time-lined, but unlike theirs, it includes a variety of sentient alien species and several million years of prehistory. According to this schema, humanity arose not simply because of evolution on Earth but also because the attempt of a race known as the Pak to establish a colony on Earth went awry. Human beings are the mutated remnants of the Pak breeder stage, which could not metamorphose into the protector stage because the necessary catalytic plant could not be grown on Earth.
At the time of the novels, the two most important alien races are the kzinti, a fierce race of large felinelike carnivores whom humans have beaten in a series of savage wars, and the puppeteers, so called because of their two heads, which resemble human hand puppets. The character of the puppeteers is the exact opposite of that of the kzinti: They are diffident herbivores who would rather flee than fight. The puppeteers, however, disappeared from Known Space some two hundred years before the novels open, because they discovered that the core of the Milky Way is exploding. Even though a deadly wave of radiation from it will reach Known Space in the distant future, the puppeteers are moving their entire race now.
In their migrations, they have discovered the Ringworld, an artificial ring of matter forming a band around its sun, at roughly Earth’s distance from Sol. Its land area is that of three million Earths. Because the civilization that built such an artifact must be immeasurably powerful, the puppeteers organize an expedition to examine the Ringworld more closely. The team...
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