Time travel in past and future Why it is not even theoretically possible to travel in past but possible to travel in future ?
The thing about time travel is it's hard to define. Technically we're constantly traveling into the future. 
Now, if you went at near-light speed for a long time and then stopped and came back, you would have the illusion of having time travelled into the future because time essentially went on without you. This is entirely physically possible, though not feasible of course.
But past time travel, what does that mean? Does it mean rewinding all time for everything? Or one entity aging backwards? Or time goes backwards while you remain the same age, like the reverse of the future travel?
In any case, unlike the future travel, there is no way, with our current model of physics, to do any of them. 
And I must reiterate, future travel by the means I described is only an illusion. You didn't move though time, time just went on without you. Obviously, this won't happen the other way.