It has somthing to do with the "mass effect". I believe It is explained in game. Basically the mass effect field not only makes it possible to travel FTL but it also allows you to harness negative energy which in turn bends space and time into a "pretzel" which basically prevents time dilation. Here is a link to a physicist who can explain it better:
Dr. Michio Kaku is one of the top physicist in the world.
Here is some more info:
"Cosmic strings: Another theory for potential time travelers involves something called cosmic strings — narrow tubes of energy stretched across the entire length of the ever-expanding universe. These thin regions, left over from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them.Cosmic strings are either infinite or they’re in loops, with no ends, scientists say. The approach of two such strings parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory."
