If time travel were to become possible in the future, visitors from 2110 would probably already be walking among us - so where are they?
This simple piece of logic is often used by sceptics of time travel - which has always belonged more in the world of science-fiction than science fact - to disprove the idea. But evidence has recently been found that could make uncomfortable viewing for the sceptics, and it was unearthed in the most unlikely of places.
Jaw-dropping footage of a woman who appears to be chatting on a mobile phone has been discovered in an out-take from the 1928 Charlie Chaplin film, The Circus.
The five-second cameo appearance shows a heavy-set older-woman, dressed in a long, dark cloak, pointy leather shoes and a black hat, apparently using the anachronistic device in public. During the shot, she is seen walking from one side of the screen to the other, holding her left hand to her ear with the knuckles wrapped around something resembling a phone. Her lips also move the entire time she is on screen, most visibly at the end, when she turns to face the camera.
"I was kind of stumped at what I saw. I kept winding it back and playing it and winding it back and playing it," said George Clarke, from Northern Ireland, who first discovered the clip and uploaded it to YouTube. "Right now, the only conclusion I can come to ... is it's a time traveller."
Clarke screened the scene to about 100 people during the film festival in Belfast and claims nobody could give him an explanation.
"It is not an ear trumpet, it's not an AM/FM radio," he insisted.
The Circus also played at the Abu Dhabi Film Festival this year, but the deleted scene was not screened. Perhaps the cover-up has already begun.