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President Obama may not have visited Mars, but humans from the future could be visiting us.

Perhaps you missed the news that President Barack Obama once visited Mars. According to a pair of retired chrononauts -- former time travelers for the Defense Department -- Obama was teleported to the Red Planet during his stint at the Central Intelligence Agency in the 1980s.

OK, the tale is ridiculous and nutty -- a splendid example of what Wired magazine calls "Tinfoil Tuesday" -- but it gained so much currency in the weirder corners of the blogosphere that the White House, two years ago this month, actually issued a denial. It comes to mind in the wake of the recent confession by two researchers that their careful study of Internet searches and social-network postings has failed to turn up any evidence of time travelers among us.

There is one main difference between the Obama-to-Mars story and the time-travelers story, however: There is reason to take the second one seriously.

The paper, by physicists Robert J. Nemiroff and Teresa Wilson, has drawn the fascinated attention of economists, legal scholars, mainstream journalists and, of course, techies galore. The authors describe their hunt for such "prescient" information as postings about events before they occurred. Of course, the selection of the events matters. Nemiroff and Wilson chose "Comet ISON" and "Pope Francis," on the theory that neither had been heard of before late 2012, but both were of sufficient importance that some future society might have heard of them.

Since the publication 14 years ago of the pathbreaking article "Worm Holes, Time Machines, and the Weak Energy Condition," a growing number of scientists have accepted that time travel might be at least theoretically possible. Once upon a time, researchers worried about such problems as the grandfather paradox, in which you go back in time and accidentally kill him before your father is born. Nowadays, physicists are busily hypothesizing ways to defeat the paradox -- for example, by building probabilistic time machines based on quantum theory.

Science-fiction lovers have been on board at least since 1895, when H.G. Wells published "The Time Machine." And time travel continues to excite us. But alas, enthusiasm is not enough. Nemiroff and Wilson's sad conclusion: "No time travelers were discovered."

This doesn't mean, they hasten to add, that there have been no time travelers. Perhaps some physical law, such as the Chronology Protection Conjecture hypothesized by Stephen Hawking, makes it "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." Or "it may be physically impossible for us to find such information." Or maybe, as the authors candidly admit, their study simply searched for the wrong evidence.

That last possibility is intriguing.

I'm not saying I believe in time travelers. But suppose they've been here. Assuming that no physical law prevents us from discovering their presence, what should we be looking for?

This is where we leave the realm of science and enter the legal imagination. For lawyers, life is one long process of considering what can go wrong. Even time travelers can get in trouble. We shouldn't be looking for their Google searches and Facebook posts; we should be looking for their distress signals.

Unsurprisingly, science fiction writers have trod this precise ground. Perhaps the best-known example is Isaac Asimov's 1955 novella "The End of Eternity," in which a time traveler from the 95th century named Cooper finds himself trapped in the 1930s. In order to get a message to his colleagues in the far future, he starts a business and takes out a magazine advertisement in which the slogan abbreviates to "ATOM" and the logo is shaped like a mushroom cloud.

The point is that nobody in the '30s would ever have seen an atomic explosion. But Cooper's colleagues, patiently searching ancient magazines, are able to figure out where (or rather when) he is. "It caught my eye instantly," says the man who discovers the hidden message. Why? Because of its "sheer anachronism."

The mushroom cloud is useful because it is iconic for everyone in the post-atomic generation and meaningless to anyone before. So maybe we won't be able to detect the presence of time travelers until we are able to work out a proper set of iconic images and then scan old but surviving documents (they'd have to be surviving documents) for their presence.

Nemiroff and Wilson consider one other explanation for their failure to find evidence of time travel: Maybe visitors from the future don't want to be found. I actually like that one best.

Consider the situation: You're the product of a future far more technologically advanced than ours. If humans have survived and thrived, the chances are that the future is also more ethically advanced. You show up in the early 21st century, eager to discover what it was really like, and you find our polarized politics, our seeming indifference to mass slaughters around the world, our growing worship of nothing but the self. Ask yourself: Would you cast off your disguise and share your true identity with us? Or might you go hunting for a more deserving era?

These questions help explain why even a project that found no evidence of time travelers deservedly excites our attention. This is science at its best: not inventing a wondrous future, but forcing us to think afresh about our present. Technology can build us new toys, but it can also challenge us to decide whether we want to be new people. And this surely is a project more vital than most of what we fight about.

And who knows? When we're ready, maybe we won't have to go looking for the time travelers. Maybe they'll come looking for us.

(Stephen L. Carter is a Bloomberg View columnist and a professor of law at Yale University. He is the author of "The Violence of Peace: America's Wars in the Age of Obama" and the novel "The Impeachment of Abraham Lincoln." Follow him on Twitter at @StepCarter.)

This column does not necessarily reflect the opinion of Bloomberg View's editorial board or Bloomberg LP, its owners and investors.