An "instance in time" is simply the state of all energy at any one moment. I say "all energy" because our reality is comprised of 1) energy in many different f

An "instance in time" is simply the state of all energy at any one moment. I say "all energy" because our reality is comprised of 1) energy in many different forms and 2) space - the container of the energy. To travel back in time would be to reverse the course of all energy - from the perspective of a segment of the energy. Given the total amount of energy in our reality - that's not very likely.