Maybe we can't figure out a way to go back in time because it'd cause so many paradoxes, of which the outcomes are so unpredictable and uncomprehendable to our minds that we can't comprehend a way to go back in time.
Something either exists or doesn't. If we can launch people to the moon and let them walk around on it from the raw materials on this planet, there isn't anything we aren't eventually capable of understanding beyond death.
Someday we may even figure out what life means.
Since 500000000000000000000000000000000000000000000000000 years from now (actual figure) our universe will experience its 'heat death', I've always figured our purpose is to figure out what lies beyond it.
And if we can't? The purpose of life will just have been to enjoy what little time we had and to burn up.
That said, if it was possible to travel back in our time (concept of parallel streams aside) we would develop a method. And if we'd developed a method we'd already know because let's face it:
If in the future, such a method was deisgned, it's not going to be employed by terrorists or some vengeful little snot. It would be a project of, I dare say, global proportion. In order to test said method they would have to send subjects back and the only way to accurately test this would be sending something back with a method to come back. The idea of this being pulled off is proposterous since such a device could hardly be as simplified as a wristwatch.
If someone was sent back, they would not only have to go back in time, but also stay there, and what purpose would that hold unless they made it public.
Not only that, but simply the concept of travel itself is in and of itself a paradox. If the body is being rearranged, there must be something to receive it on the other end. Since such devices don't exist in our time, nothing could receive it, and if such devices don't exist at any time in the past, they could never travel to the past until there was such a device to receive it.
Now provided we wait X years and a receptor is created. That would mean it would only be possible to travel to that time and no further. BUT even that is a paradox in and of itself because it would have to be active and 'tuned' (whatever that could mean) to receive an individual. This information would be impossible to convey to the past because there's no travelling to the past to let anyone know UNLESS the whatever is already 'tuned' for THAT person. It's a vicious circle of going nowhere.
Forward is a yes. Back? Nope. (parallel universes maybe but I wouldn't know)