Rahul that it is impossible to treval back into time as that would lead to cycles of energy trevalling back and forth between different time zones till it destroys its self. Since treval back in time is not possible, most of these paradoxes are laid to rest. And that is probably we haven't seen a time trevaler from the future. However, it is possible to treval forward in time and that is a relatively simple endeavor based on relativistic mechanics. If you treval close to the speed of light you can sort of freeze time for yourself so that time around you passes really fast and you end up in a future time period. Another question that arises then is what is the value of future time treval for the present society. I personally think that it might be beneficial for the select few who treval forward in time, but as a society you don't gain anything from the knowledge these people acquire.