Time Travel:Need help with a theory

I am not sure if somebody said this yet, because I am to lazy to read. Anyways something can't travel back in time to before it was made. So, a time machine couldn't go back in time before it was turned on and a human could not travel back in time before it was born. This means that if time travel does exist in the future, we won't know intill the time machine is turned on.

If the Many Worlds Interpretation of Quantum Physics is correct, then going back in time would create another timeline. You could go back and blow up the planet and it wouldn't make a difference, because the moment you arrived in the past the timeline would have diverged.

It would solve most problems if time actually is linear, a single point of reference in which no future things have happened just yet. It does not collide with the fact that you can "travel" into future either, since it just means changing a reference of a single object (human) to make time appear slower on him/her. Then, you don't have any questions like "where are the time travellers?".