I wrote briefly on this topic in the debate articles. It seems though that no one understands the topic well enough though because article number 1 didn't answer the question and article number 3 made it quite clear that he had no idea what he/she was talking about. Seeing as how no one else knows what to say here I will break it down for you as simply as I can and explain why it is possible. Before you can understand why time travel ONE WAY is feasible, though, you must understand Einstein's general theory of relativity.
Einstein had a problem for years with Newtons theory of gravity. This problem originated from the fact that his theory of special relativity states that no object can travel faster than the speed of light. According to Newton any change in the distance between to objects or the size of the object causing the gravitational influence, would be noticed immediately by an observer. This is impossible though because Newton said gravity was caused my some sort of medium, in which case the medium can't travel faster than the speed of light so the effects of a change COULD NOT be felt immediately. So Einstein set out to fix the problem. Summed up, both theories of relativity state that 1. Time is a fourth dimension which accompanies the three space dimensions we all know and love. 2. No object can travel faster than the speed of light. 3. Gravity is caused by the warping of spacetime in the presence of matter and/or energy 4. Accelerated motion and gravity are perceived to be the same thing (email me and I can explain this further). 5. Any object subjected to accelerated motion and/or gravity will pass through time slower and will also shrink in the direction of it's motion (or perpindicular towards the source of gravity).
Having stated the fact that time slows down under accelerated motion or the affects of gravity one can see why time travel to the future is feasible. For those people sitting there thinking, "how can time slow down?" here is a simple explanation. You have 3 space dimensions that we all know and love. As i walk forward I move through one dimension. If I stop and take a step to the left I move through another. The same thing happens if I stop and jump straight up. Einstein had a thought that the motion through all 4 dimensions of space and time will always add up to the speed of light. This means if I walk forward at 2 meters/second (a slow rate) than my velocity through the time dimension will be 299,999,998 meters/second because they add up to 300,000,000 meters/second (the speed of light). Lets say though I get in a space ship and fly straight at 250,000,000 meters/second, which is a significant fraction of light speed. While in the space ship my velocity through the time dimension will only be 50,000,000 meters/second, which is significantly slower. Therefore, if I fly away at a high speed and come back to Earth a "year" later, than much more than a year will have passed on Earth and I will have successfully traveled to the future, while only aging a year.
There are many theories about traveling back in time, such as black holes and wormholes but none have strong enough backing to be as solid of a theory as the one I have just laid out for you. It is believed that black holes warp time and space enough that matter sucked into one will be spat back out somewhere else. Where you might ask? Well who knows. Even if I jumped into a black hole to find out the tidal forces would rip me to shreds before I got close to the center. If you have any further questions or feel I should elaborate on something better feel free to email me.