The problem is to start with teletransport. Thats the process of moving object from place to place. They can do it now by breaking down molecules and putting them back together in another place BUT, when it comes to our brain, our brain is soo complex and teletransport doesn't allow the brain to retain memory so basically if we could teletransport, we would have a brain of an infant. As soon as we can teletransport, we can thinkg about time travel.