How would we know if time travelers are visiting the present? You could look for clues — but the best indication might be found on the internet. Unfortunately, two researchers tried to look for clues to time travel online... and came up with nothing.
It's an unpublished study that appears at the Cornell University Library's arXiv pre-print archive, but it's interesting nonetheless. Similar to looking at old photos or newsreels for chronological inconsistencies, or the case where a man named John Titor told the Internet he came from the future, researchers Robert Nemiroff and Teresa Wilson searched the web for prescient information not previously available. Technically speaking, then, this wasn't a study delving into the physical plausibility of time travel; rather, the authors tried to prove time travel by uncovering anachronistic (i.e. chronologically impossible) data on the Internet.
"Technically, what was searched for here was not physical time travelers themselves, but rather informational traces left by them," write the authors.
For the study, the duo chose two terms that couldn't possibly have existed until recent times, namely "Pope Francis" and "Comet ISON." The current pope is the first in history to go by that name, and ISON was only recently discovered. Nemiroff and Wilson searched both Google and Twitter hashtags for mentions of these two terms over the last seven years. They also conducted log searches at NASA's Astronomy Picture of the Day (APOD) web site.
And because they're not stupid, the researchers also asked time travelers to respond actively to a request for prescient communication. To do so, they asked hypothetical time travellers to to respond with a communication including either the hashtagged term "#ICanChangeThePast2" or "#ICannotChangeThePast2" on or before 2013 August.
Unfortunately (or fortunately, depending on your opinion of time travel), Nemiroff and Wilson found absolutely no evidence of time travellers. But they admit this isn't definitive proof that time travel isn't possible. The researchers conclude their study with these words:
First, it may be physically impossible for time travelers to leave any lasting remnants of their stay in the past, including even non-corporeal informational remnants on the Internet. Next, it may be physically impossible for us to find such information as that would violate some yet-unknown law of physics, possibly similar to the Chronology Protection Conjecture . Furthermore, time travelers may not want to be found, and may be good at covering their tracks. Additionally, time travelers just may not have left the specific event tags that we were searching for. Finally, our searches were not comprehensive, so that even if time travelers left the exact event tags searched for here, we might have missed them due to human error, oversight, incompleteness of Internet catalogs and searches, or inaccurate content time tags.
Full props to Nemiroff and Wilson for a highly imaginative and original study.