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. Setting The Probability trilogy takes place in a galaxy that has been colonized by humans. This was made possible by the space tunnels, a network of FTL warp gates that were created by a now-lost progenitor race. Humanity is not united under a common government and political system; instead, the various governments in the Solar System and beyond have united as the "Solar Alliance Defense Network" in light of the war against the Fallers. The Terrans have also discovered a number of alien races, most of them vastly similar in body format, living conditions and even DNA, leading to the hypothesis that the aforementioned progenitor race seeded the galaxy with sentient life, which then evolved according to the conditions on each planet. Of the known alien races, humanity is the only one that has reached space. Humanity's understanding of the space tunnels is very limited, but several peculiar traits have been discovered. For one: if Ship A enters Tunnel 1, exits Tunnel 2 and then turns around and enters Tunnel 2 again, it will emerge from Tunnel 1 again... Unless Ship B emerges from Tunnel 2 in the interim, at which point Ship A will instead emerge from wherever Ship B entered. (The single tunnel leading to World is #438, which gives an idea of how carefully passage through heavily used tunnels must be coordinated.) For two, objects can only enter the Tunnel if they are below a certain mass, about 100,000 tons; anything larger will actually fit into the aperture, but will collapse and explode. The threshold of what the tunnel can handle is determined by the object's Schwarzschild radius. Finally, nobody knows how the tunnels work. At all. Macro-level quantum entanglement has been proposed, but it is so far out