I'm pretty sure travelling back in time is impossible. For one simple reason, their is no physical space for a person to travel back to. The physical space where the past was is clearly the present. The only possible way their could be a physical location for the past, is if every single moment was its own separate universe. Which basically means each previous moment is constantly happening again. Also the odds that every single person on the planet is doing the same thing again is highly unlikely, impossible even. So each variation is creating an alternate reality and each one of those realities has physical dimensions of each moment, which again is constantly replaying itself. Assuming the universe is created by a big bang, where is the big bang for these universes that are being created by each individual moment? Their probably isn't one unless somehow the universe is aware of each one of these moments that are going to happen and preprogrammed all these big bangs before they existed, which essentially means all the universes that are created by each moment would already exist when "the prime universe" was created. And the chances that any of that is happening is nearly nonexistent.