Travelling back in time is possible—theoretically, at least. According to Einstein’s theory, you could travel back in time by moving faster than the speed of light, as long as you could somehow have infinite mass. Since that probably won’t work, you could create “wormholes” between two points in space-time. (This would also be tough since humanity still hasn’t invented the technology to build a wormhole.) Or you could try bending space-time by plucking some “cosmic strings.” Two of these theoretical strings, which are thin streams of pure energy that are moving in opposite directions at very near the speed of light, could theoretically warp space-time enough to create a closed time-like curve—also known as a time machine.