You need to have zero mass to travel in time, and to do this you need to have signficant scientific breakthroughs in antigravitics to produce a super conductive propulsion unit to create a antigravitic field strong enough to remove the effects of gravity. This would theoretically allow you to travel at or near the speed of light in a vacuum eg. space, but i doubt it would be possible because there would be small dust particles etc. which would cause resistance. Another theorey involves the universe beinb folded in half, but thats fairly impossible if you ask me.
Also during the second world war the nazis were doing secret experiments in bunkers below the ground to find the affects of some odd radiation (can't remember the name) on time, and it was found that they could slow down time by fractions of milliseconds, also a result of this radiation was the death of scientists.
This is mostly from a book called the hunt for zero point by nick cook a journalist for janes.