h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You forgo the most important thing, the temporal prime directive. when going back in time one must not interfere with things.

In my case I observe but dont touch.

Considering that no one has yet travelled back in time you can't say that for sure. It would certainly get rid of some of the paradoxes, but that doesn't mean its true. It does seem the most logical (compared to the multiple historys theory, when you go back in time you create a seperate track of history), but some of the most important discoveries have not been logical. For instance the logical conclusion would have been that the Earth was flat and that everything goes around us (as it appears) and the round Earth Theory was far fetched. Look how that turned out.

Well, according to the Heisenberg Uncertainty Principle, even observing something can affect the thing being observed. Realistically speaking, any travel to the past will affect it somehow (unless you had already affected it, a la the "no free will" argument).