The thing I always found a bit unnerving is that if they've left and are traveling at a speed close to the speed of light we'd never know until they arrive. If

The thing I always found a bit unnerving is that if they've left and are traveling at a speed close to the speed of light we'd never know until they arrive. If something is traveling at the speed of light towards us (whether a ship or a death star laser) we wouldn't know until it arrived. Now we've been sending radio signals for several decades you have to begin to question if there's a civilisation out there maybe 10-100 light years away that may now be aware of us and have sent something our way which may be about to arrive. There is an explanation for Fermi paradox that suggests maybe alien civilisations choose to be quiet or immediately wipe out other transmitting civilisations with speed of light weaponry. Why? Well, if there's no way to have prior warning of an attack traveling at the speed of light your only defence is some combination of remaining hidden and extreme hostility towards other civilisations.