Time Travel is a very common theme in Science Fiction. This may be because everyone has something they’d like to do over, or would like to see instead of read about. Or go forward five years and know that everything will turn out alright. So throughout the centuries, writers have imagined ways to make time travel possible. And even if we can’t do it today, we may be able to in the future.

Time travel was seen as mere fantasy until Einstein’s Theory of Relativity. When Einstein realized that time was relative, it changed everything in terms of time travel. Stephen Hawking even tried to disprove time travel, but was unable to (this doesn’t mean it will never be disproven, but it’s a very impressive point in time travel’s favor). H.G. Wells’ Time Traveler argued that time is simply another dimension (the 4th), like up, down or forward in physical space, and since we can move in those, we should be able to move in time.

The Problem with H.G. Wells and Marty McFly

H.G. Wells’ Time Traveler creates a time machine that moves only in the 4th dimension, and travels millennia into the future, even to the end of Earth itself, and returns to Victorian England in time for dinner to tell his story. In Back to the Future, Marty and Doc travel to the past to fix something that wasn’t supposed to happen. Throughout the series they are nearly trapped in the past because of complications with the DeLorian (mostly getting it to 88 miles an hour). Unfortunately, in order to travel through time you would need to reach nearly the speed of light, something neither vehicle is capable of. According to the Theory of Relativity, time is not a constant. The faster you move, the slower time moves. Even astronauts experience this: in orbit they are traveling faster than the people on the surface of the planet, so when they return, they have aged a few microseconds less than everyone else.

Cause and Effect: The Grandfather Paradox and the Butterfly Effect

There are two opposite theories about the effects of time travelers in the past. The Grandfather Paradox states that if you go back in time and kill your grandfather (accidently or on purpose) you will cease to exist. Therefore, since you have already traveled back in time and do exist, you can’t kill your grandfather. This means that no matter what you do in the past, you cannot change the future (either you won’t be able to do it, or it was supposed to happen anyway). Another way to explain time travel to the past (without changing the future) is the many worlds theory. In Quantum Mechanics, the idea that parallel universes exist is plausible. In the instance of time travel, this would mean that if you went back in time and killed your grandfather, you would be thrown into a parallel universe; so essentially you didn’t kill your grandfather, you killed his parallel genetic duplicate.

On the other hand, the Butterfly Effect discusses the cause and effect of two seemingly unrelated events. The simplest explanation I’ve ever heard was in Jurassic Park: “A butterfly can flap its wings in Peking, and in Central Park you get rain instead of sunshine”. For time travel, this means that anything that you disrupt in the past could have profound consequences in the future (see “The Butterfly Effect” and “A Sound of Thunder”). This is why so many Science Fiction movies have time traveling “laws”. But it seems unlikely that this will actually happen.

In his book Physics of the Impossible, Michio Kaku argues that wormholes are the most logical way to travel through time. Using Einstein’s theory and the yet undiscovered Theory of Everything, time travel can happen once we understand the physics of an Event Horizon (in this case, at the edge of the wormhole). Wormhole time travel occurs in Contact by Carl Sagan. Ellie is not only transported to another star system, but she is returned to Earth within seconds, despite her visit on Vega lasting all day.The actual process would look very similar: “[Two] chambers would consist of two concentric spheres…by imploding the outer sphere, the two spheres would create…negative energy. Next, take the first chamber and send it into space at near might-speed velocities. Time slows down in that chamber, so the two clocks are no longer in synchronization. Time beats at different  rates inside the two chambers, which are connected by a wormhole…if you are in the second chamber, you can instantly pass through the wormhole…” (224).

In order for a wormhole to work as a time machine, you have to have an entrance point (in time and space) and an exit point. Unfortunately, this means that you would be unable to travel backward in time to a point before your time machine was created (the main “entrance” point). But if you went forward in time, you’d at least be able to get home (and it will be the same home you left-paradox free).

Time Travel, Terminator Style-You can’t take it with you

In the Terminator series, time travel was invented to go back in time to change the future. While we’ve seen that this wouldn’t really work, there are other problems with Terminator style travel. In the series when you go back in time, you can’t take anything with you (this is also true in The Time Traveler’s Wife). The end result is you 1) can’t take any cloths with you and 2) can’t get back to your time, unless you know how to create the time travel device. Since the ways in which physics can currently explain time travel (faster than light machine or wormhole) would allow anything within the boundaries of the device to travel, anything on you or your ship would go back in time.

While we may never see time travel in our lifetime, it is very possible that time travel will be very real for future generations: the science fiction of yesterday is the reality of tomorrow.