Split Infinity

Split Infinity is a novel by Piers Anthony that moves between fantasy and science fiction, published in 1981. Its two worlds are Proton, which is technological and decadent, and Phaze, which lies beyond a curtain and is ruled by magic.

Stile is a serf on Proton and a master Gamesman, which is a combination the world he lives in does not make room for. Somebody is trying to destroy him, and his only way out is the curtain, which he finds through a robot that appears to care about what happens to him.

Phaze turns out to be no escape, because somebody there has already killed his alternate self and he is due to be the next victim. On Proton his fate comes down to winning the great games, and the novel runs the two worlds side by side so that each puts a different kind of pressure on the same man.

The curtain is the device that ties the two settings together, and Stile cannot leave either world behind, because his survival in one depends on what happens in the other. The book is the first of a sequence about him.