From Halopedia, the Halo wiki
- A group of researchers headed by Tobias Shaw and Wallace Fujikawa secretly developed the Shaw-Fujikawa Slipspace Drive, a practical means of propelling spacecraft across vast interstellar distances. This new engine allowed ships to tunnel into "the Slipstream" (also called "Slipspace"). Slipspace is a domain with alternate physical laws, allowing faster-than-light travel without relativistic side effects. Faster-than-light travel is not instantaneous; "short" jumps routinely take up to two months, and "long" jumps can last six months or more. The SFTE generated a resonance field, which when coupled with the unusual physics of the Slipstream, allowed for dramatically shorter transit times between stars; however, scientists noted an odd "flexibility" to temporal flow while inside the Slipstream. Though no human scientist is sure why travel time between stars is not constant, many theorize that there are "eddies" or "currents" within the Slipstream. There is generally a five to ten percent variance in travel times between stars. This temporal inconsistency has given military tacticians and strategists fits-hampering many coordinated attacks.