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. But is time travel possible, or the stuff of human imagination? Does a viable theory of time travel 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. Stephen Hawking noted that space-time is four-dimensional; events occurring in space-time can be labeled with three positional dimensions and one time dimension. However, Albert Einstein posited that the perceived time identifier of events can be altered depending on the speed of the observer. This suggests that objects traveling at high rates of speed experience time differently than those standing still.
Of Arrows and Entropy
Everything in the universe is traveling through time. As NASA pointed out, we’re all going at the same rate: 1 hour per hour. But can this rate be changed? Increased? Decreased?
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 up time and make it go faster? And, can we change its direction?
Time travel to the future is not necessarily a figment of science fiction.
Einstein’s theory of relativity explains that as objects approach the speed of light, the speed of time slows relative to outside observers. 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; creating them is nearly impossible due to the need for negative energy, which is in short supply.
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, researchers have already created a wormhole that allows magnetic fields to traverse distances almost instantaneously, but it is not sustainable enough to transport people.
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 time 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.
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