Time Travel: Fact or Fiction? (in which I attempt to refute Stephen Hawking's silly notion that time travel is impossible because we're not chatting it up with our future selves)
Tonnies continues: "Other physicists are at work refuting the paradox of going back in time and killing your parents before you are born. If they're right, a time traveler from the future could interact with others, including his or her past self, so long as no action was taken that would endanger the traveler's own continued existence. It's difficult to visualize how this might work, although the idea makes logical sense. Maybe the best analogy would be a physical system that relies on a principle of least action, such as a ball rolling inexorably downhill."
He further notes: "The fascinating upshot of this is that there's a chance we're indeed being visited by advanced beings from our own future, but their interactions with us would be necessarily limited lest they doom themselves to nonexistence."
Tonnies also wonders if the many UFO sightings that have been reported for decades may not be due to the actions of aliens from the other side of the galaxy, but the result of time-traveling humans masquerading as ET to keep secret their real point of origin.
"If time travel is possible," says Tonnies, "the behavior of UFOs may be at least partially explained: formal contact with us would result in a causality violation of some sort, so they must remain content with maintaining their presence behind a curtain of subterfuge."