Timeshift — D.R. Evans

Four hundred years in the future, two superpowers have carved up the galaxy and share an uneasy truce. But the Union launches a clandestine mission to kill the Empire's leader, setting in motion a train of events that leads to a revelation that will forever change the balance of power. In response, the Empire unleashes the ultimate weapon, propelling a Union spy into a timestream populated by only one other person: a young physicist from a backward planet he calls Earth, who is desperate to find a way to return to normal time.