In quantum field theory particles are said to travel backwards in time. I assume this is allowable over quantum distance scales.Over what kind of distance scale does such particle behaviour stop? And if it happened when the universe as a whole had a radius equal to the quantum distance scale, would a particle travelling backwards in time still be acceptable to theorists? What was the motivation for having particles travelling backwards in time in the first place? And how is this sort of time travel compatible with the fact that I always see a clock travelling forwards in time?