From H.G. Wells to "Bill and Ted's Excellent Adventure," the literary and cinematic history of time travel offers two lessons of overriding importance. The first: Watch your step, especially when going backward in time. Everything you do, or don't do, will have unintended consequences. You could end up killing your grandfather in childhood by accident. Twist cause and effect into a pretzel of paradox and you'll probably wish you hadn't.
Lesson two: Be wary of visitors from the future. This advice will be superfluous in the case of evil Schwartzeneggerian robots programed to kill , but it holds good more generally. Even with the best possible intentions, whatever time-travelers from the future say will mess with your sense of possessing free will. Without that, you might as well stay in bed in the morning.
Heedless of all this hard-won wisdom, Robert J. Nemiroff, a professor of physics at Michigan Technological Institute, spent a couple of months in late 2013 looking for signs of chrononauts among us. His paper "Searching the Internet for evidence of time travelers"  (coauthored with Marcia Goodrich, an editor of two Michigan Tech magazines) was posted at the scientific preprint repository arXiv on the day after Christmas. Its findings -- not to leave anyone in suspense -- were that chrononauts seem not to have left a digital footprint.
A reader pointed out the link a few days after the article appeared, and I set out to interview the author. The effort was complicated by the fact that Nemiroff was in transit to Washington to attend the American Astronomical Society meeting. We were able to talk by phone on Sunday morning – a day before he and his students discussed their search at a poster presentation.
The design and execution of Nemiroff's project are easily explained, but first a word about the state of time-travel research. It is focused, at this point, on speculative viability rather than engineering. Stephen J. Hawking is probably the best-known exponent of an argument against the possibility of time travel. But some of the more phantasmagoric entities in particle physics behave in ways suggesting that they move backward in time, albeit in unimaginably small fractions of a second. It is, in short, an open question. Two entries in online philosophical encyclopedias (here  and here ) provide rich overviews of the current state of the discussion.
With time travel, most experiments are thought experiments, but Nemiroff went in search of empirical evidence. "The question of time travel was bouncing around in my head," he told me. "If it were possible and had happened, how would you know?"
The topic came up this past summer during the weekly poker game among Nemiroff and some of his students. They started kicking around ideas, and an approach took shape. If time travelers had visited us, the best evidence would be references to events or developments well before they occurred. A book from 1967 mentioning President Obama, for example, would be pretty hard to explain on any other basis.
The next step was combing through enormous masses of text in search of the "informational traces" (as the paper calls them) left by presumed chrononauts. Nemiroff and his students came up with a number of events and names -- "Pope Francis," for one, since the current pontiff is the first ever to use that name -- and went looking for anachronistic references. The task would be impossible without search engines, of course, while hashtags and Google Trends made it easier to find needles in the haystack.
Or not find them, as it happened. It turns out Dr. Who has not been passing through, or at least not posting on Twitter.
Some commentators have responded, paraphrasing broadly, "Well, duh." But the paper itself points out that the project's design also covered another possibility: that "information itself could be sent back in time," rather than people. Indeed, the retro-transmission of data seems at least somewhat more credible than the idea of human time-jumper. It "would be a type of time travel that might not directly involve the backwards transport of a significant amount of energy or momentum," the paper notes.
"This might be considered, by some, a more palatable mode of backwards time travel than transferring significant amounts of matter or energy back in time, as the later might break, quite coarsely, local conservation of energy and momentum. For example, were the same person at different epochs to stand next to themselves, the energy tied into their own rest mass seems not to have been conserved. Similarly, instantaneous time travel to the same place on Earth might violate conservation of momentum, as the motion of the Earth around the Sun (etc.) might delegate a significant change in momentum for a corporeal object even over a time scale of minutes."
Passages like that make it difficult to calibrate how much tongue Nemiroff had in cheek when undertaking the project. So I asked him outright.
"The whole thing was somewhat whimsical," he said. At the same time, he considered it "a real research project," driven by the primal scientific feeling of curiosity. And the brainstorming required also had pedagogical value: "Students learned a lot about classical physics, about how time and special relativity works, while I learned more about social media. I’m 53. I don’t use hashtags that much and didn't know about Google Trends. So it was a matter of the history of physics and the hypermodern world colliding in a cool way." It also exemplified a basic principle Nemiroff learned from his mother: "She said that philosophers used to talk about how many teeth a horse had. When somebody counted them, science was born."
Nemiroff submitted the paper to three journals, each of which rejected it without even sending it out for review, so he decided to make it available through arXiv. The online repository, while not practicing the full-court peer-review process, does screen submissions to keep out the alchemists, perpetual-motion engineers, and suchlike. Acceptance of the paper by arXiv, like the poster session at the astronomers' meeting, is a sign that time travel remains a topic for serious scientific consideration. "It's not likely," he told me, "but you can’t point to laws that preclude it."
For that matter, his reported findings don't rule out the possibility of time-travelers among us. They might be very discreet about what they know. Besides, as the old saying goes, the absence of evidence is not evidence of absence.