If I travelled 100 lightyears away?

Utter nonsense. How fast are you proposing to travel to 100 light-years distance ? Even supposing that you somehow travelled at the speed of light, when you arrived, and turned around to look at Earth, you would only see what had happened since you left ! The light showing your departure would have been travelling "alongside" you, so it would have gone past you by then ! And any light showing your previous activity on Earth would be in advance of that ! You would have to travel at several times light-speed to catch up and overtake the light of your past experience. Even if it is theoretical, it is theoretical balderdash.