Probability Sun is a 2001 science fiction novel by American writer Nancy Kress, a sequel to her 2000 publication Probability Moon. It was followed in 2002 by Probability Space, which won the John W. Campbell Memorial Award. The novel concerns a military expedition to the planet World, where aliens live who have developed a strange form of telepathy or collective unconscious, "shared reality", which causes piercing "head-pain" whenever "Worlders" attempt to hold strongly differing opinions. However, the expedition concerns a crash-landed alien artifact in the planet's crust which has uncharted powers, and may be the key to humanity winning a war against the "Fallers", a genocidal alien race. Source: Wikipedia (en)