I just finished reading “Anathem” by Neal Stephenson. I thought it was quite interesting as a work of fiction, although the philosophical parts seemed a little shaggy at times.
In the book they talked about the theory that the ‘polycosm’ (multiverse) was a directed acyclic graph, so that information would flow from one cosmos to another without forming a loop. But the main event in the novel is the appearance of a spaceship from one cosmos (Urnud) that has been called by the people of another cosmos (Arbre). So doesn’t that make a loop? The people of Arbre send information to the people of Urnud, and the people of Urnud travel to Arbre. After this is made clear, the main characters don’t seem to care that their great theory has a big hole in it; they still keep referring to a ‘directed acyclic graph’.
Am I missing something? (Probably.)
It’s been a while since I read it, but I do not remember anything about the ship being ‘called’ by the people of the downstream universe. At least, not in the sense that they actually sent a message to the upstream universe.
Some of the people in the thousand-year tower were aware of their presence, and tried to communicate with it, but that was after the ship had already made the move downstream. I don’t think two-way communication between universes was ever mentioned.
As I understand it, the Urnud spaceship receives some kind of message from Arbre around the time of the Third Sack (about 1000 years before the novel starts); the novel doesn’t specify whether the message was intentional or accidental, or exactly what caused it. So according to the ‘complex Protism’ theory, that means that Arbre is ‘upstream’ from Urnud (information can only flow downstream from a ‘more Hylaean’ cosmos to a ‘less Hylaean’ cosmos). So far, so good.
But when the spaceship travels from Urnud to Arbre, isn’t that obviously a case of information flowing upstream? Wouldn’t that invalidate the idea of ‘complex Protism’?