If time travel is possible, where are the tourists from the future?’ Stephen Hawking asked in A Brief History Of Time.
Hawking also famously staged a party for time travelers, complete with champagne and a banner saying, 'Welcome time travelers.'
None showed up, in case you were wondering.
But there ARE certain unsettling signs which seem to suggest that time travelers are in our midst - from a woman on a mobile in a Charlie Chaplin film, to a Greek statue using a laptop.
Here are five things which 'prove' time travel exists - at least according to people on the internet.
Vladimir Putin, the immortal
Is Vladimir Putin a time traveler - or a magical, ageless being who has defended his native Russia through the ages?
UFO site Disclose.tv says, 'On social networks are circulating pictures from 1920. and in 1941. for which some people claim that they are pictures of Russian President Vladimir Putin.
'In fact, supporters of the thesis that Putin is almighty and immortal, have launched a story that their president is a mythical creature that resides on our planet for hundreds, if not thousands of years.'
But, like previous 'sightings' of celebrities such as Jay-Z 'in the past', this is just a case of hoaxers finding similar faces in old photos.
The Greek statue with the laptop
Hang on, is this an ancient Greek woman actually using a laptop - complete with USB ports?
That's what conspiracy theorists would have you believe.
The statue, ‘Grave Naiskos of an Enthroned Woman with an Attendant’ is in The J. Paul Getty Museum in Malibu, California.
YouTuber StillSpeakingOut says it, 'depicts an astonishing object which bears a striking resemblance with a modern laptop or some hand-held device.'
The gallery's own description is much more modest, describing it as a 'shallow chest'.