You also have the Grandfather Paradox. Which ponders the question that if you went back in time and killed your grandfather would you continue to exist or would you disappear the same moment he died. This is an intresting point, but does not state that time travel into the past is imposible. Time travel forward is quite easy all you need is speed. Granted we can't achieve a speed high enough to get very far into the future, but it is still possible. Going backwards how ever is a tricky beast to deal with. Some scientist suggest use of a worm hole, to move through space and time. While others suggest different uses of speed and physics. Time is not a solid constant that is unchangable, time and space are actually very flexible. The realm of time travel forward and reverse is not something of impossibilty, just something that needs some more advancements and time to work with.