Well as long as we're writing scifi... The way I think about time (also probably wrong and not nuanced enough) is through the energy of a system (entropy?). Es

Well as long as we're writing scifi... The way I think about time (also probably wrong and not nuanced enough) is through the energy of a system (entropy?). Essentially if the whole universe operates on a tick based update system, then moving from one state to the next is all about how various particles interact with each other. And this in general would be particles moving from a higher entropy state to a lower entropy state. So effectively to go back in time you'd need a lot of energy to undo all the chemical reactions / potential to kinetic energy conversions etc. that caused us to go from one state to the next. So even if it was possible to undo the appropriate state changes using some magic machine, there wouldn't be energy to do it Now what's really mind blowing is that if such a machine was operating on you, you might still be conscious in a potentially horrifying way. The conclusions of your thoughts come before the thoughs that triggered them, and then they melt away into the thoughts that will trigger them...