The prospect of revisiting the past or jumping to the future has captured people's imaginations for centuries. Our contemporary understanding of time travel is a marriage of theoretical physics and science fiction.
Hours of research by our editors, distilled into minutes of clarity.
These hypothetical distortions in the fabric of spacetime may connect vast distances across or even between universes. While science fiction often portrays them as traversable tunnels, the energy and matter required to keep a wormhole open defy current physical understanding.
Time travel has fascinated humans for centuries. It appears in myths, literature, and science fiction. One of the earliest examples comes from the 400 BCE Hindu epic Mahabharata, describing time dilation long before Einstein’s Theory of Relativity. By the 19th century, stories introduced machines enabling time travel—setting the stage for modern sci-fi.
The laws of physics, as currently understood, don’t categorically reject the possibility of time travel. But, scientists are skeptical, especially when it comes to traveling to the past. Hopping between past, present and future would require the world to have some wacky yet-to-be-discovered physical properties like negative mass, negative energy, and infinite density. Time travel also introduces philosophical questions about free will, causality, and chronology.
The equations of special relativity require these objects to possess imaginary mass, and they would travel faster the more energy is taken away from them. The existence of tachyons would necessitate an unknown form of matter and lead to paradoxes involving information that can hypothetically travel backward in time.
When you look up at a constellation with your naked eye, you’re looking thousands of years into the past. The stars are so far away that it takes their light that long to reach us. Today’s most powerful telescopes, such as the James Webb Space Telescope, can see billions of years into the past. So, as this article suggests, looking through a telescope (or even up at the stars with your own eye) could be considered a form of time travel.
While the author discounts the possibility of traveling to the past, it lays out five hypothetical modes of catapulting into the future: speed, gravity, suspended animation, wormholes, and light. Each has some scientific basis, but none are realistic for humans. Read the article to learn where they offer promise and where they fall short.
This 8-minute video explores how time travel influences the narrative arc of several movies, books and video games, from Harry Potter to A Christmas Carol. (Warning: it contains spoilers!) J.K. Rowling and Charles Dickens set myriad parameters as they’re worldbuilding to avoid paradoxes and create quasi-logical timelines. By drawing diagrams as he’s speaking, the video breaks down how time works in some of popular culture’s most beloved fictional universes.
According to quantum physics, traveling back in time to kill one's grandfather would not cause a paradox because time travel would bring one to a new timeline, where the person killed is not one's grandfather. Alternatively, causality would not be violated because the universe would prevent the murder from occurring.
In 2009, Hawking hosted a reception but only sent out invitations afterward, hoping that time travelers from the future would attend and prove time travel possible. He later joked it was "experimental evidence" that time travel doesn’t exist.
This grandfather paradox – the notion that changing the past may alter the present conditions that motivated the time traveler in the first place – instills doubt in time travel. But, as this video explains, subatomic particles regularly do different things in parallel. So, what if the universe exists in two states: one where your grandfather is alive and another where he’s dead? Intrigued? Confused? Let the animations in this 2-minute video help you wrap your mind around the possibility.
