Thought Experiments


Do Time Travelers Tweet?

We live in four dimensions: three spatial dimensions plus a fourth dimension, time. Of these, time is the most fluid, the one that is most influenced by personal experience. It goes too fast, or not fast enough. Time gets to the very heart of what it means to be human since our experiences, which take place in time, are what shape our lives and decisions in the present and the future. And yet it is the only dimension in which we are restricted to moving in only one direction: forward.

Or are we? And if we can move backward in time, how would we prove it?

Augustinian Friar's Astrological Clock at the Clock Museum in Vienna. Credit: Curious Expeditions/Flickr, under a Creative Commons license.

These are the kinds of questions my research group and I were discussing one day over pizza and poker. Time travel: Literature and movies are full of references to it. The stories may be fictional, but much about the nature of time is still unknown, and science thrives in these gaps in knowledge: as we try to fill the gaps, we discover how many more there are. Yet for as long as scientists and philosophers have been contemplating time travel, no one seems any closer to having an answer.

Since Einstein’s Theory of Relativity, time travel to the future has stood on firm scientific footing. To slow down the rate at which time passes, all you must do is travel close to the speed of light. This phenomenon, called time dilation, is famously illustrated by the Twin Paradox, in which a space-traveling astronaut returns to Earth to find that her homebody twin sister has aged decades while she’s aged only a year. It is possible to test and confirm this effect even without the aid of a near-light-speed journey into interstellar space: You just need a pair of very precise clocks and an airplane. Indeed, time dilation has been tested by placing one such clock on a plane and leaving another on the ground. When the flying clock landed, it showed a time that was about 10-7 seconds behind the other clock.

That discrepancy includes the effect of a second, competing “time travel” phenomenon: time dilation due to gravity. Time slows down in the presence of a gravitational field, so a clock on an airplane, at the top of a mountain, or on the top floor of a skyscraper will run just a bit faster than a clock closer to the Earth’s surface. Even just a meter off the ground, time runs measurably faster.

While these are very small differences, they prove a point: Time travel is well within our grasp. Of course, here we are discussing extraordinary time travel. Technically, we all travel to the future every day at the rate of one second per second. We are interested in what happens when that number is greater or less than one.

But that’s only half of the story. Traveling back to the past is where things start to get interesting, and the science starts to become more speculative. Special Relativity allows travel into the past—but only if you move faster than the speed of light. Though this is traditionally forbidden, some theorists have found mathematical solutions in General Relativity that allow faster-than-light travel. The mathematician Kurt Gödel devised one such solution to Einstein’s field equations, and theorists have conjured scenarios in which time-travelers might use wormholes and black holes to exceed the universal speed limit. However, these solutions all involve what physicists call “closed time-like loops,” anomalies in space-time in which, instead of continuing into the future, one always returns to the same starting point in time. Most scientists consider these to be unphysical, meaning that they do not accurately describe reality and lead to philosophical and historical problems like the so-called Grandfather Paradox: What would happen if you went back in time and killed one of your grandparents before you or your parents were born? Would you still be alive to travel through time and commit the crime?

Finding the right search term was the first order of business. It had to be a word or phrase that was “born” on a specific date, before which it had never been used, and which would continue to be significant into the distant future. We decided on two such terms, one historical and one astronomical: “Pope Francis” and “Comet Ison.” We then began to methodically search for evidence of prescient mentions of these events in search engines and on social media sites. Many of our attempts to scour search engines were futile. Google searches yielded results that turned out to be ads on random pages and Google+ was generally unreliable. Facebook was too easily tampered with, as it allows users to pre-date their posts as far back as the day the account was opened.

Twitter, however, turned out to be a practical venue, though our search came up with nothing we could identify as a time travelers’ post. We also searched through the log files of a popular astronomy site ( to see if anyone had presciently searched for our terms, but to no avail. We decided to try a more direct appeal, and broadcast a request on a popular astronomy forum that any time-travelers in the audience transmit an email sent before we made the post, but no email arrived. Not that we were expecting one. We were really more interested in the process: As far as we know, no one has carried out a search this extensive, verifiable and reproducible.

So do our results mean that there are no time-travelers in our midst? Not necessarily. Our results verify the general consensus that time-travel does not exist, but do not rule out the possibility. We only searched for two terms, neither of which may hold any interest for time-travelers. Maybe they do not want to be discovered and have covered their tracks. Or perhaps it isn’t backwards time travel that is prohibited, but the discovery of such a phenomenon. Maybe some law of physics makes finding time travelers impossible.

Our exercise in time travel is over, but there are plenty of other ways one could look for evidence of time-travelers: uncanny lottery picks and eerily accurate March Madness brackets, just to name a few. It may sound like a ridiculous idea, but you’ll never find anything if you don’t look!

Go Deeper
Editor’s picks for further reading

Mail Online: Stephen Hawking: How to build a time machine
In this whimsical essay, Stephen Hawking describes how wormholes could be used as “time tunnels” to travel through time.

Relativity for the Questioning Mind
In this friendly introduction to Einstein’s special and general relativity, Oberlin College physicist Daniel Styer provides a rigorous but non-technical look at time dilation, the twin paradox, and more.

Scientific American: How does relativity theory resolve the Twin Paradox?
Ronald C. Lasky, a lecturer at Dartmouth College’s Thayer School of Engineering, explains.

Tell us what you think on Twitter, Facebook, or email.

Teresa Wilson

    Teresa Wilson is a physics Ph.D. candidate at Michigan Technological University, where she studies under Robert Nemiroff. She worked as a teacher before coming to graduate school and enjoys doing outreach to the local community. She has a bachelor’s in math from Gonzaga University.

    • cynthia

      Perhaps civilization will come to an end before the future generation can determine how to travel through time.

    • Fre20741

      all righty then – milk that grant for all it is worth

    • joe

      Question: In the twin sister scenario, which sister is considered the time traveler? Can it not be considered time travel to the past when the entire world meets an astronaut many years after she has left but seemingly only a year to her? Time is too subjective for acurate measurement.

    • HelloDave

      Don’t believe this, but it sure is a fun read for anyone interested in time travel…

    • Xander

      John Titor lives!

    • Time-Traveler

      Just a thought, consider Dr. Who. He can configure a phone that can call people in a specific era of time. So making a status update to Twitter would of course to appear in our timeline although he could be fluttering about in space and time. So that it seems obvious that if one does have a time machine, they might even have the technology that could contact people in certain parts of time. Therefore making it impossible to determine if a space traveler was visiting us now.

      Also if you are a time traveler and you understand that mentioning time specific events (especially future events depending on the time that you are in) could risk the fate of the universe, I imagine the time traveler would possibly refrain from using our internet and social media apps that could affect the time-space continuum.

      So, not finding anything could also mean that the time traveler is just smart enough not to mess up history by announcing “oh hey everyone, i am a time traveler! Let me leave a trail of time crumbs that could ruin time and space itself to let primitive people in the past know that time travel exists…”. Yea because that makes sense. I dunno, i am not a scientist….


      UNLESS YOU ==ONE== UNDERSTANDS CONSCIOUSNESS… Your understanding of Man’s Universe ==environmental reality== will always be flawed.


      2 understand time ==mankind’s life line consciousness environmental reality== understand that the Word and the Big Bang are two sides of the same =perspective= coin…

    • No one of consequence I hope

      Yes. Time travelers tweet. But we can’t toy with paradox by saying anything definitive. Our own individual future existences are at stake. Putting ourselves on the wrong side of a major branch point could mean that we have no future to return to. All futures exist, just not all individuals in their desired realities. Some futures are ones I would not care to return to even if a version of me existed there. This post is skirting the edge of disaster, but for me alone, not all travelers, or all futures. That’s the only reason I took the chance.

    • Thanks for the great idea for my next sci-fi anthology: “Time Twitter” 😉