This is fairly simple. For time travel, we hardly know where to start. First we need to understand it, and the problem is that stuff like time is hard to test and experiment with.
Our first obstacle for time travel would be light speed travel, I think, and we're kinda close to that. When we have that, with relativity, we can "travel" into the future. But most physical objects don't like being accelerated that fast.
Actually, one could achieve time travel while remaining perfectly stationary in space. At light speed, an object is moving only through space and not in time, therefore does not experience any time: to a photon's perspective (if it could have one), it is everywhere its trajectory would take it, simultaneously. As a corollary, one could imagine an object that traverses only through time and not through space. However, our current best theoretical approach to specifically retrograde time travel is achieving a velocity faster than the natural velocity of light with respect to a vacuum. That doesn't mean it is the only way to time travel, or even the most feasible way.
We can do this, but no time travel?