So here's a weird funny theory. What if what some people perceive as ghosts are actually time travelers from the future. And they have technology that make them invisible to the people of the past (or present) but some people are immune to whatever technology makes them invisible, and they decided to call them ghosts. And the creepy ghosts are just assholes (most likely teens in the future) who have that technology and are bored and like to bother the people of the past (or present). That, would be funny. Does that seem viable to you?