There are lots of plausible ways to move rapidly into the future by traveling near the speed of light. There are no known ways to reverse this trick to visit times in the past. I know there have been several theoretical physicists who have advocated using cosmic strings and worm holes, but there is no evidence that cosmic strings and wormholes are real physical objects. Theory is strewn with perfectly good ideas that nature simply never bothered to take advantage of...such as tachyons.