The most likely answer is that the past doesn't exist. We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well. If

The most likely answer is that the past doesn't exist. We're probably riding a "wave" of spacetime and if we are then the rest of the universe is as well. If that is the case, when a moment passes it ceases to exist in the same way that a ripple in a puddle of water does. If you wanted to go backwards in time, you would have to take the whole universe with you.