Imagine building a device that could send a fully functioning facsimile of your own self into the past. You would put right some of the things you got wrong, wouldn't you! Nothing could go wrong, could it? You couldn't make things worse, could you? You could just use it for sightseeing maybe. See first-hand history being made. You would be a bystander having no effect on history so no danger there then. Oh but wait, do all plans turn out right? What if it all goes wrong? What do you do? Go back again to put it right? This could get complicated and very messy. What if the device becomes smart, very smart and decides that it wants to go its own way? It would have no use for you anymore, would it? This could become sinister. It could pose real danger to your health and welfare. It could even start a war...