A common theme in science fiction media, time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point in the future. But how much of the idea is based in reality? Is it possible to travel through time?

Is time travel possible?

According to NASA, time travel is possible, just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light, which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation, according to Live Science, is how one’s perception of time is different to another, depending on their motion or where they are. Hence, time being relative.

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories.

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve spacetime.