The reason why no one has found us is that i) With the likes of Simon Cowell, Spice Girls & other media hungry airheads on TV then any other civilisation will probably be thinking...What the F*** should we go there for? ii) Compared to other species we're about as ugly as they come and we scare other civilisations off. Or iii) We've got so much junk around our planet that no other space vehicle could get near us without having something crash into them.

The thing that always springs to mind when someone mentions Quantum Mechanics is Schrodinger's Cat and a playing card facing both up and down at the same time. Time travel that has no consequences, thats a bit of a paradox in itself, though if you did go back at least you wouldn't die in the past since it would be recorded and you'd know not to be in that place at that time.