system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] What sort of propulsion were spacecraft using prior to the discovery of Arrakis?

Arrakis is the only planet on which one can harvest spice melange, which allows for the bending of space, thus permitting intergalactic travel. But before the planet was discovered, how were people crossing the light-milennia beween galaxies?
gpt
The Navigators aren't folding space, they're just... navigating. The ships use something called the Holtzman effect to travel, but the calculations necessary to guide the ship safely are impossibly complicated. The first starships just used computers, but since the Butlerian Jihad that's not an option. Guild Navigators obviate the need for calculations by being prescient, and are actually an improvement over the computers.