Time travel. It’s often used by science fiction novels, television shows and movies as either the central plot device, like in “Back to the Future,” or the fundamental property of the universe.

“Star Trek” created the “chroniton” particle to account for the existence of time-alerting effects. Is it possible or just the stuff of human imagination? Does a viable theory exist, or is time science just sound and fury? Here’s what we know so far.

Space-time: A Primer

Before tackling the question of time travel, it’s important to establish a foundation. What conditions of the universe make it possible to theorize about circumventing time in the first place?

First is the concept of space-time. In 1909, Hermann Minkowski published “Time and Space (translated in The Monist) which suggested that space-time had four dimensions: The three familiar physical dimensions and time itself. Soon after, Albert Einstein added to the space-time discussion by positing that perception of time can be altered depending on the speed of the observer. National Geographic details his “Lightning Strikes a Moving Train” thought experiment: If lightning simultaneously strikes the front and back of a train moving at 2000 mph, an observer outside the train will see the strikes happen at the same time. An observer on the train, however, will perceive the rear strike to happen slightly after the front strike because the train is rapidly moving forward.

Of Arrows and Entropy

Everything in the universe is traveling through time, but not at exactly the same rate. As NASA pointed out, we’re all going at the same rate: 1 hour per hour, but depending on your location — on a plane, on the top of Mount Everest or deep underground — your perception of time differs.

Space.com speculated that time — coined “the arrow of time” for its forward movement — bears a striking similarity to the second law of thermodynamics, which states that entropy (or chaos) only increases. That is, entropy only flows in one direction.

Any theory of time travel, therefore, must answer two critical questions: Can we speed gain control of time to make it go faster? Can we change its direction?

Theoretically Speaking

Einstein’s theory of relativity explains that as objects approach the speed of light, time slows compared to measurements taken by a stationary observer. Scientific American offered the scenario of a ship continuously accelerating at a speed equal to Earth’s gravity, which would take it to the center of the galaxy and back in just 40 years. Thanks to the time dilation effect at near-light-speed, however, 60,000 years would have passed on Earth.

Going back in time, meanwhile, is more complex. In theory, it should be possible to curve space-time in such a way that it creates closed timelike curves — paths through space that are so tightly curved that they return to their starting point in time, allowing travel to the past. Unfortunately, as noted by the Internet Encyclopedia of Philosophy, naturally occurring closed timelike curves are unlikely since the universe is only expanding, not rotating.

Tech Talk

Professor Gary T. Horowitz of the University of California at Santa Barbara told Scientific American that the time-dilation effect of high velocity, which “‘has been overwhelmingly supported by experimental tests, applies to all types of clocks, including biological aging.'”

Build a ship fast enough, and time travel becomes a real possibility. The caveat? Reaching speeds where time dilation becomes meaningful requires both massive amounts of energy (a planetary mass or more) and continuous acceleration.

As for traveling into the past, scientists have suggested the use of wormholes: tunnels in space that connect distant points by bending spacetime. According to Futurism “wormholes” have been developed which seemingly allow the teleportation of magnetic fields — while this isn’t on the order of moving people or even particles across space-time, it’s a step in the right direction for manipulating the forces necessary to test any theory of time travel.

Is Time Travel Possible?

The short answer here is … sort of. As spacecraft and engine technology pushes travelers closer to the speed of light, this could effectively reduce the rate at which days, hours and years go by, pushing astronauts into the future.

Going back, meanwhile, is more complicated. While general relativity allows travel to the past in theory, we haven’t found a practical method. In other words, don’t bank on the ability to go back and alter the past anytime soon — but don’t count it out just yet.