Astronaut Scott Kelly relaxes inside a Soyuz spacecraft simulator during training for his yearlong mission on the International Space Station, 2015. His time in orbit left Kelly an extra 8.6 milliseconds younger than his twin brother and fellow astronaut, Mark. 

NASA/Bill Ingalls

It’s about Time

A recent book traces the many ways we travel through time.

James Gleick. Time Travel. Pantheon. 2016. 352 pp. $27.

Let’s start with what may be a surprising fact: time travel already exists, and I don’t just mean at the standard rate of one second per second we all experience. Einstein figured out that time slows down or speeds up depending on how fast you’re moving, and if you travel at the speed of light, time will stop, from your perspective at least. (Maybe. Probably. Things get a little wibbly-wobbly, timey-wimey when you go to extremes.) Gravity plays a role, too: the closer you are to a massive object, the slower the clocks tick.

The classic example of this principle is that one twin traveling in a spaceship at or near the speed of light ages more slowly than the twin back on Earth. Voila! Time travel. Sadly, we’re not quite up to speed yet: astronaut Scott Kelly spent more than a year in orbit and ended up only 8.6 milliseconds younger than his earthbound twin, Mark. It’s a start.

A more everyday example of time slowing down and speeding up is the Global Positioning System. The satellite that determines your location is moving much faster than your car, so the satellite’s clock registers time more slowly than your GPS (about 7 microseconds a day). But since Earth’s gravitational effects on the satellite are less than on your car, the satellite’s clock ticks slightly faster (by about 45 microseconds per day). So in total the satellite clock is faster than your GPS by about 38 microseconds per day. That may not seem like a lot, but if the designers didn’t think about this time difference and compensate for it, your GPS would fail in its navigational functions after about two minutes and you might end up driving into a lake.

Since the concept of time travel is sound, it is theoretically possible, right? Why then hasn’t anyone developed this technology? Why didn’t anyone attend the party for time travelers that Stephen Hawking threw in 2009 (invitations mailed afterward)? One possibility is that time is more static than we think, and so we couldn’t do anything really drastic to the past. (No killing Hitler.) Another is that even if a time machine were to exist, it might not be able to travel to a time before it was built, leading some to believe a time machine may exist at some point in the future, just not yet.

The first time machine as we know it was created by H. G. Wells. (Some other types of literary temporal jumps existed before Wells, like Rip Van Winkle sleeping himself into the future, but if we’re  looking for someone building a vehicle with the express intent of moving through time, we have to wait until 1895 and the publication of The Time Machine.) While such a machine was undoubtedly a pioneering idea, Wells didn’t come up with time travel completely out of the blue: the late 19th century was awash with new ideas about time.

Wells’s era coincides with accelerating scientific, technological, and cultural changes: ideas about biological evolution were becoming mainstream, while horses and carriages were giving way to steam engines and cars. Time was harnessed by the zoopraxiscope and kinetoscope, machines that made still images appear to be moving. A new science called archaeology made the past seem like a real place. This being the heyday of spiritualism, traveling through an invisible dimension didn’t seem quite as outlandish as it might once have. The word nostalgia changed meaning around this period, from an equivalent of homesickness to a longing for a past time. Wells would have picked up on these new ideas, and it’s no wonder they made their way into his work. An author is “a sort of lightning rod for an accumulation of atmospheric disturbances, a St. Sebastian absorbing in his tattered breast the arrows of the Zeitgeist,” according to Donald Barthelme (an author himself).

James Gleick explores these aspects of time travel and then some in his aptly named book. And it’s more than just the history of the idea of time travel and the science that could make it possible; it’s also an examination of time itself, how we perceive it and talk about it. For example, in most Western societies the future metaphorically lies before us and the past behind. Aymara speakers in the Andes consider the past in front of them (since they can see it) and the future behind them (since it can’t yet be seen). For Mandarin speakers the same word means both above and earlier, and ditto for below and next.

For Gleick time’s facets are endless, and he explores as many as he can. He ponders what exact length of time constitutes the present and how time capsules let us send messages to the future. He wonders if free will is possible if the future already exists and explores the sorts of verb tenses we’d need if we could shuttle back and forth through time. He considers what “outside of time” might look like, whether from the perspective of an immortal deity or Isaac Asimov’s imagination. He looks at the acceptance of the Gregorian calendar and International Date Line: the former led to hiccups in the flow of time, and the latter still does. Just last November a set of twins was born in North Carolina, but because the clocks “fell back” at the end of daylight savings that night, the firstborn’s birth certificate shows a later time than his younger sister’s certificate.

It’s amazing how much of this minor time bending we do in a single day. On my morning train ride I might be reading a book set in the far future. On my walk from the train station I immerse myself in a Philadelphia long since gone, passing Independence Hall and Benjamin Franklin’s house. At work I’m surrounded by CHF’s historical collections, a physical embodiment of the past. (What are antiquities, asked Francis Bacon, but “remnants of history which have casually escaped the shipwreck of time”?) I get home and turn on The Simpsons while I eat dinner, and maybe watch one of their time-travel episodes where Lisa grows up to be president or Homer’s shoddy toaster repair throws him back to the age of the dinosaurs. I head out to choir and practice a song written 500 years ago.

That’s just me, and just one day. I can’t even begin to imagine the hops through time everyone else is making. It’s a wonder any of us show up in the present at all.

These casual time jumps (not even full-bore, machine-assisted time travel) would have been incomprehensible to Westerners just over 200 years ago. Their thoughts about time weren’t the same as ours: “Historians studying our modern notion of progress have observed that it began to develop in the eighteenth century, along with our modern notion of history itself,” says Gleick. And we didn’t get time travel for another 100 years after that. Today, time jumps are so prevalent it’s like a fish trying to see water. It’s in our blood; we are natural-born citizens of Everytime. So whether your time machine of choice is a DeLorean or a museum, a Renaissance faire or a time capsule, a WABAC machine or your own memory, I wish you happy travels.