Time Travel

Of all the topics explored in science fiction, time travel is probably the least realistic, with modern scientists generally agreeing that travelling through time is impossible.

But stories about time travel are not supposed to be realistic. Just like other branches of the genre, they are usually used as metaphors and thought experiments, which challenge audiences to consider philosophical themes.

Free will versus determinism is a common debate in time travel narratives. If we had the power to travel back in time, would we be able to change anything? Or are our paths already chosen for us, whether we like those paths or not?

When people think about time travel, they might not think of ghosts. But some literary critics have argued that the ghost stories written in the 19th century were an early form of time travel fiction.

In these stories, people in the present (the living) interact with people in the past (the dead). These interactions break the rules of time, and could be classified, loosely, as time travel.

In one of the world’s most famous ghost stories – A Christmas Carol – the time travel is actually explicit. In the novel, ghosts take Ebenezer Scrooge to visit the past and the future.

Scrooge’s future looks dark and bleak, and this inspires him to become a better person in the present. In this story, the future is not pre-determined – by changing his behavior, Scrooge is able to choose a different path.

Apart from ghost stories, the earliest work of time travel fiction was H.G. Wells' groundbreaking novel: The Time Machine. This iconic story, published in 1895, included the first example of a time travelling machine – an idea adopted by hundreds of writers in the years since.

In the story, a time traveller visits the future, and finds a society where humans have evolved into two branches: the Morlocks and the Eloi. The Morlocks are brutal and industrious, living deep underground, while the Eloi are lazy and child-like, lounging about in the sun.

The story was a commentary on Victorian society, where class distinctions were becoming more and more pronounced. The Morlocks were a possible future for the downtrodden working classes, while the Eloi were a possible future for wealthy, pampered elites.

In many stories about time travel, the writers explore the dangers of changing the timeline. The Back to the Future film series is full of examples, with actions in the past creating ripple effects that reshape the entire future.

Sometimes, these ripple effects create mind-bending paradoxes. The classic example is the grandfather paradox: if a time traveller went back and killed their grandfather, they would never be born in the future. But if the time traveller was never born, who killed the grandfather?

To make sure not to cause any ripple effects, characters will sometimes visit time periods under strict instructions not to change anything. This is often referred to as time tourism – the time travellers must only observe.

Instead of imagining a changing timeline, some science fiction writers have explored the idea that the timeline is completely fixed. Even if someone travels to the past, they cannot change the future.

Connie Willis’ Oxford Time Travel series is about historians at a future Oxford University, who travel back in time to study historical eras firsthand. These historians might feel the urge to intervene – maybe they think they can stop a war – but their actions have no effect.

In some stories, the characters’ actions might cause the future they thought they were trying to avoid. In their effort to stop the war, they actually ended up starting it. This raises questions about free will: if the future is fixed, do our choices matter?

Instead of a changeable timeline, or a fixed timeline, some writers have imagined a branching timeline: if we go back in time, and make a change, a parallel universe will split off from the first, and take a completely different path.

The original timeline will still exist, but the second timeline runs parallel to it. In other words, we cannot change the future, but we can create another path. It is an interesting thought: what if there are billions of timelines out there, each with a different version of ourselves moving down a different path?

In the last few years, this idea was explored in the Marvel Cinematic Universe. The characters travelled back in time, changed events, and created a branching multiverse with lots of different timelines.

Some writers have used the concept of branching, parallel timelines to explore alternate histories: how would the world have unfolded differently if a moment had gone another way?

In the 1930s, Murray Leinster wrote Sidewise in Time. In this story, characters from other timelines begin to arrive in our own, like a group of legionaries from an alternate history where the Roman Empire never fell.

In the 1960s, Philip K. Dick's The Man in the High Castle imagined an alternate history where Axis powers won World War II, and the United States is divided up under Nazi and Japanese rule. It is a dark, dystopian vision, which reminds its reader what almost came to pass.

In the real world, time travel does exist. According to the theory of general relativity, time slows down or speeds up depending on an object's speed and its position within a gravitational field, in a concept known as time dilation.

It is an observable principle. Due to the combination of their high-speed orbit and distance from Earth's mass, astronauts aboard the International Space Station (ISS) experience time dilation.

While their high speed would cause time to slow down, being farther from Earth's gravitational mass has the opposite effect, causing time to speed up. When these effects are combined, astronauts on the ISS age slightly faster, but the difference is minuscule.

In theory, if an object moved close enough to the speed of light, significant time dilation would occur. Entire centuries could pass in one location while only days elapsed in another. Such an object would seem to be traveling into the future from the perspective of a stationary observer.

Gravity also results in time dilation. If a spaceship were to approach a black hole, time would slow significantly for the astronauts on board – an idea explored in Christopher Nolan’s Interstellar.

Time dilation makes it theoretically possible for people to travel to the future. And according to the theory of general relativity, an object would start to move backward through time if it traveled faster than the speed of light.

Most physicists think this level of speed is physically impossible, but some people disagree. Ronald Mallett, a theoretical physicist and science fiction fanatic, has devoted his life to time travel research. He aims to travel back in time and save his father’s life – he died when Mallett was 10.

But there is one piece of evidence working against Mallett. If backward time travel was theoretically possible, surely someone in the future would have invented it, and traveled back in time to meet us?