Relatively speaking

Science fiction is known for bending the rules — and the laws of time are no different. But sometimes there is more science in the fiction than one might think.

Illustration by Alexis Lee

Illustration by Alexis Lee

What time is it? That depends.

In science fiction, time is often of the essence. Whether you’re travelling through time in the TARDIS with the Doctor, getting frozen in time in The Twilight Zone, or having your timeline stretched in Interstellar, fiction loves to play with time. But how can we play so freely with time, and how much science is in our fiction?

Time, as we commonly experience it, follows a strict set of rules. It only moves in one direction, causes occurring before effects, the swing of the bat before the hit. It’s uniform, with a second as I experience it taking the same amount of time to pass as one for you. Finally, it’s unchangeable: there is nothing we can do to stop, slow, or reverse its passage. Engaging our minds in a good book or a video game may allow us to ignore time for a while - but it will still keep passing.

Space takes time to traverse, and the mind-bogglingly large scale of space means that it takes an incredible amount of time. Our closest star, Alpha Centauri, is over 4 light-years away. Travelling at the speed of the Voyager space probes (58,536 km/h), it would take us 78,000 years to reach it. Human experiences and human lifespans are far too short for us to explore the stars as we have the oceans, at least with time being what it is.

If we look at it from a physics standpoint, however, time isn’t as fixed as it seems. There are cases and places in the universe where time can be stretched, squashed, or even stopped. Einstein developed two theories in the early 20th century that gave us two ways in which time varies, depending on where you are or what you’re doing: general and special relativity.

Special relativity simplified. If light travels at a constant speed, then a beam of light bouncing between a pair of moving mirrors (right) will take longer to bounce than between stationary mirrors (left). Michael Schmid/Wikimedia Commons (CC-BY-SA-3.0)

Special relativity simplified. If light travels at a constant speed, then a beam of light bouncing between a pair of moving mirrors (right) will take longer to bounce than between stationary mirrors (left). Michael Schmid/Wikimedia Commons (CC-BY-SA-3.0)

In general relativity, gravity itself controls your time perception. To understand this, we need to consider time as ‘fourth dimension’ in space, a concept known as ‘spacetime.’ Spacetime is what all matter passes through, and a strong gravitational field, such as that found near a large planet, will bend spacetime around itself. The closer you are to the source of gravity, the more spacetime is bent. This leads to an interesting effect: the greater the bending, the slower the local perception of time. A clock on top of Mount Everest will run ever so slightly faster than one at sea level, and the computer clocks on GPS satellites run even faster: so much so that we need to correct them to give accurate positions.

The stronger the gravitational field, the more difference there will be in the passage of time between two points. If you were to travel close to a black hole, this ‘dilation’ of time is so strong it can change a single day into several years! For science fiction writers, this is often a rich source of complications, and the drama practically writes itself.

In the Stargate SG-1 episode “A Matter of Time,” the ‘Stargate,’ a portal connecting two distant points in space, is opened up near a black hole. When the portal is frozen open, the time within the military base where the gate is kept begins to be dilated. Samantha Carter, far from the gate, watches Jack O’Neill — who is standing metres closer — appear to slow down to a near crawl, even whilst being sucked into the gate. Samantha uses this to her advantage, taking two weeks to prepare a rescue while Jack only experiences a few minutes.

In Interstellar, an even larger black hole is the key plot point of the movie. The enormous “Gargantua” dilates time so much that a single hour on the closest planet is equivalent to seven years of time on Earth. When a disaster prevents the protagonists from leaving the planet quickly, they find their companion on the spaceship has been waiting 23 years for their return. Later, 51 years pass before they escape Gargantua’s gravity. This ‘time-travel’ into the future provides the central dramatic tension: the main character’s daughter grows up and is near death when he returns to her.

 

World Science U on black holes and time dilation.

 

Special relativity also offers an option for slowing your pace: when you reach speeds comparable to the speed of light, time will slow down for you. This is largely due to the speed of light remaining unchanged no matter how fast you’re moving relative to it. To get an idea of how this works, think of a freeway. When you’re standing on an overpass, the cars below appear to be moving rapidly towards you. When you’re driving in the freeway, however, other cars move slowly relative to you, since you’re nearing their speed. Since light never changes speed, moving at close to its speed makes events appear to pass slowly.

Unlike general relativity, special relativity works both ways: when you ride a fast-moving ship, others see your time as passing slower, but you also see others’ time as passing slower. This is because all motion is relative: moving quickly past someone is the same as it is if they moved past you. There is no “fixed point” in the universe. The easiest (and least-mind blowing) way to think about this is by standing far away from someone: they appear small to you, and you to them.

This disparity in time can be used intentionally to send characters far into the future. In the original Planet of the Apes, astronauts believe they have crashed on a distant planet. They repeatedly try to escape from the strange ape society, only to discover that they have actually landed on Earth: the astronauts had travelled in a circle at near-lightspeed, while thousands of years passed outside their spaceship. The apes are the descendants of primates who took over the Earth, after humanity blew itself up in nuclear war.

Another application of this is to reduce (perceived) travel times across interstellar distances to within human lifetimes, allowing characters to travel to distant planets. In Joe Haldeman’s The Forever War, humanity uses this to send soldiers across the galaxy, to fight a war with an alien race. The trade-off is that when they return to Earth, over a million years have passed, and humanity has evolved into an unrecognisable form. Haldeman used this massive change as a metaphor for the alienation that soldiers, having fought in the Vietnam War, experienced upon their return to the United States.

 
If you stand above a freeway, the cars appear to move quickly relative to you. If you are driving on the freeway, the other cars move slowly relative to you, because your speed is near their speed. Edwin van Buuringen/Flickr (CC BY 2.0)

If you stand above a freeway, the cars appear to move quickly relative to you. If you are driving on the freeway, the other cars move slowly relative to you, because your speed is near their speed. Edwin van Buuringen/Flickr (CC BY 2.0)

 

But writers don’t always use physics so literally. Often, for the sake of the plot, relativity is quietly swept under the rug to make way for more fantastical physics. The common devices of  ‘hyperdrive,’ ‘warp speed,’ or ‘portals’ allow the stories to follow characters all over the universe, without such annoyances as ‘time dilation’ and ‘the speed of light’. Writers and readers usually make the distinction between ‘hard’ and ‘soft’ science fiction, with hard sci-fi paying attention to the laws of physics, while soft sci-fi plays with them.

A prime example of soft sci-fi is Doctor Who, of which the core plot device is the Doctor’s ability to travel through time in his TARDIS. Ancient history and the far future often collide dramatically, and cause and effect are largely ignored. Time in Doctor Who is fluid: 

“People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.”
- The Doctor, ‘Blink

But even within this fantastic universe, the plot generates its own “laws”: the TARDIS cannot visit the same time and place twice, The Doctor can’t interact with his own timeline, and some points in time are “fixed” and cannot be altered. But why limit the ability to travel to any time and place in the universe? We live our lives without the ability to travel through time, but we often wish we could — however, this ability would make us less human. By placing these rules upon the TARDIS, we can relate to the Doctor: for all his power, there still exist things that he cannot change, and events that he regrets.

Time can be bent, through realistic and unrealistic means, but it rarely makes sense to do away with its natural flow altogether. As a shared human experience, the passage of time links us to the characters. Using real physics to slow time can bring the limits of the human experience into sharp relief, as characters watch their loved ones age before them. Stopping time, or travelling into the past, can give characters a precious few more hours to say goodbye to those they’ve lost. And by speeding up time, events that happen on a cosmic scale — like the death of a star or the birth of a galaxy — can be experienced in a human lifetime.

Time never stops flowing. But if a story is told well enough, perhaps it can be paused for a little while.

Edited by Sara Nyhuis