It has somthing to do with the "mass effect". It is explained in game. Basically the mass effect field not only makes it possible to travel FTL but it also space and time into a "pretzel" which basically loops time and space which in effect prevents time dilation. Here is a link to a physicist who can explain it better:
https://youtu.be/iwj8deuPjNE
I believe they have hired the guy to come up with a way to travel FTL.