>>8 that's not traveling in time though, observing the past is already theoretically possible through wormholes, as one could travel deep into space and observe the light from earth reach them. So if they traveled x light years through a wormhole they would see earth from x years ago.
>>1 there are different concepts of time travel, i.e. whether you'd alter the future when traveling backwards or if you'd return to a future where your effects on the past don't exist, and even upon traveling back in time, they might not have made themselves obviously from the future. Also, time travel could be possible but it is just the case that no one ever figured out how it worked, so never traveled back in time using such technology.