Science fiction books, movies, and television shows get a lot of mileage out of driving their characters through space and time. In some cases, the hero manages to travel through both with the benefit of a Time and Relative Dimensions in Space (TARDIS) machine. You probably know that space travel is already a reality. But did you also know time travel is possible?
To save space (not time) I will focus on leaps into the future.
Before I get science-y on you, I want to point out you are always traveling through time and space. If you jog a distance of five miles at an average speed of five miles per hour, then you have moved five miles through space and one hour through time.
I know this is obvious. But I want to point out that space and time are so intertwined in the physics of the universe that they must always be considered together. Physicists came up with a creative name for this unification: spacetime. Punchy, right? The unification is also sometimes referred to as the space-time continuum.
Albert Einstein went deeper by connecting space and time to the speed of light. In the vacuum of space, the speed of light is about 300,000 kilometers/second (about 671,080,888 miles/hour). This is the same speed from all points of view. It is absolute. If you are zipping along in your starship at 100,000 kilometers/second to chase down an enemy’s signal which contains the plans to your Death Planet, it will still be traveling ahead of you at a speed of 300,000 kilometers/second. If you speed up to 250,000 kilometers/second, guess what. The signal will still be moving at a speed of 300,000 kilometers/second away from you. I repeat: the speed of light is absolute from all points of view. In your case, the point of view is from your ship.
From this fact, Einstein deduced that time is flexible, relatively speaking. As we travel through spacetime, we take our frame of reference—clocks—with us. On earth, our personal clock is mostly in sync with everyone else’s because of our slow speeds relative to each other. Now, if we board a starship and accelerate from our home planet at a great speed, our clocks will begin to differ from those we left behind. With this in mind, the easiest way to understand special relativity is to understand: the faster we travel through space, the slower our journey through time becomes. In other words, as our speed increases, time bends to conserve the speed of light. This time bending is called time dilation. If this weren’t true, then the speed of light wouldn’t be absolute from all points of view. Since we know it is, time dilation must also be true.
Within spacetime, distances may also be relative. Take Tabby’s Star, for instance. It is about 1,500 light years from earth tucked away in the Cygnus constellation. What makes Tabby’s star so intriguing is that it flickers odd patterns. The drops in light are too significant to be due to a passing planet. What is going on out there? According to our earthly clocks, it took the light from Tabby’s star 1,500 years to reach us. Yes, the light shining into our telescopes right now started its journey while the Franks were battling the Visigoths. Maybe you’re thinking that this makes this star too far for us to explore during our lifetimes. Well, not necessarily. If you flew there in a starship capable of travelling at 99.99 percent the speed of light, according to the clocks for those aboard the starship the distance is only 21 light years, not the 1,500 light years for those homebodies waiting for you on earth. It’s not so far anymore. The distance has contracted. I’m not suggesting that it is possible to propel a ship that fast (without a warp drive), only that distance within this framework is as flexible as time.
As you can imagine time travel is a great scientific tool for the science fiction toolbox. When the science in fiction is accurate, characters traveling at high velocities through space should be experiencing (suffering from, really) time dilation. Two award winning science fiction classics that use relativistic time travel as a plot device are Forever War by Joe Haldeman and Orson Scott Card’s Ender’s Game.
In Forever War, time dilation makes itself known when troopers travel in fast ships to military encounters on different planets. Each battle takes our hero centuries farther from the earth he knows. When eventually the protagonist returns home, he is so socially displaced that his language is archaic and his heterosexuality is repulsive. Imagine two races fighting each other hindered by time dilation, unaware of the enemy’s stage of development when they next engage.
Ender’s Game also revolves around a relativistic war. The humans have charmingly named their alien enemies “buggers.” A secondary character, a hero from an earlier confrontation with the buggers, has been stashed away in the spaceship (eighty years before the book’s setting) and sent on journey at near light speed. He will return, nearly unaged, to earth when the humans have their fleet ready for the final battle against the buggers.
Both books offer realistic depictions of the science of time travel, at least into the future. I’ll leave you with this: science also allows for time travel to the past. So, why haven’t we met any time travelers from the future? There is a science answer. If we ever meet again somewhere in time and space, I’ll tell you.
David Siegel Bernstein is the author of BLOCKBUSTER SCIENCE: THE REAL SCIENCE IN SCIENCE FICTION , which comes out October 10, 2017. He has written two science fiction novelettes and many short stories for various science fiction magazines and anthologies. David works as a managing consultant for BLDS LLC, where he applies mathematical and statistical modeling to discrimination and civil rights matters. He previously worked at LECG and the Center for Forensic Economic Studies in Philadelphia.
Featured still from "Doctor Who" via The BBC