Time is one of the most familiar parts of everyday life — and one of the strangest ideas in physics. Einstein showed that time does not pass at the same rate for everyone, while modern physics connects it with space, gravity and the evolution of the universe.
But could we actually travel through time? And what would happen if we could change the past? Explore the science of time and spacetime, the possibilities of time travel, famous paradoxes, and the ideas that have inspired some of science fiction’s most fascinating stories.
What Is Time?
Time seems simple because we experience it constantly, yet physics shows that it is far stranger than everyday intuition suggests. It is tied to change, motion, gravity and the structure of the universe itself.
Questions such as whether time had a beginning, why it appears to move only forward, whether it passes at the same rate everywhere, and whether it is ultimately fundamental are still central to modern physics.
Our broader guide explores these questions and provides the best starting point for understanding time before moving into spacetime, relativity and time travel.
→ 7 Mysteries of Time Explained
Explore the biggest questions about the nature of time, from the Big Bang and time’s arrow to relativity and quantum physics.
Time, Spacetime & Relativity
Einstein’s theories of relativity changed our understanding of time by showing that it is not separate from space. Instead, space and time form a single framework known as spacetime.
The passage of time can differ depending on speed and gravity. A clock moving very quickly, or sitting in a stronger gravitational field, will not tick at exactly the same rate as another clock elsewhere.
This is not just theoretical. Relativistic time differences have been measured experimentally and must even be accounted for in technologies such as GPS.
Our guide to the fourth dimension explores how spacetime works, why time behaves differently from an ordinary spatial dimension, and how relativity reshaped the way physicists think about the universe.
→ Is Time the Fourth Dimension?
Discover how space and time combine into spacetime, and what Einstein’s relativity really means for the passage of time.
Can We Travel Through Time?
Time travel is not entirely fictional. Relativity allows a form of travel into the future because time passes more slowly for someone moving at extremely high speed or experiencing stronger gravity. In principle, a traveler could experience less time than people who remained behind.
Traveling backward in time is much more problematic. Ideas involving wormholes, closed timelike curves and other unusual spacetime geometries appear in theoretical physics, but none provides a demonstrated method for traveling into the past.
Backward time travel would also create profound problems involving cause and effect. Could you change events that have already happened? Could an object or piece of information exist without ever having been created?
Those questions lead directly to some of the most fascinating ideas in the subject: time-travel paradoxes.
→ Can We Really Travel Back in Time?
Explore what modern physics says about wormholes, closed timelike curves, causality and the possibility of visiting the past.
Time-Travel Paradoxes
If travel into the past were possible, cause and effect could become extraordinarily complicated. Time-travel paradoxes explore what might happen when events in the present become entangled with events in their own past.
The grandfather paradox asks what happens if a time traveler changes the past in a way that prevents their own journey from ever taking place. The bootstrap paradox involves objects, information or even people caught in causal loops with no identifiable point of origin. The predestination paradox, meanwhile, describes situations in which attempts to change history actually become part of the events that were always destined to occur.
These ideas are useful thought experiments about causality as well as some of the foundations of time-travel fiction.
→ 5 Bizarre Paradoxes of Time Travel Explained
Explore the major time-travel paradoxes and the different problems they create for cause and effect.
Explore individually:
Grandfather Paradox
Bootstrap Paradox
Predestination Paradox
Time Travel in Movies & Science Fiction
Time travel has fascinated science-fiction writers and filmmakers for generations. Some stories use machines or wormholes to send characters into another era, while others trap them inside repeating time loops or construct elaborate causal chains in which past and future events depend upon one another.
The best stories often turn genuine questions about time and causality into puzzles. Movies such as Predestination, Primer, Timecrimes and Triangle explore ideas closely related to the same bootstrap, predestination and causal-loop paradoxes considered by physicists and philosophers.
Whether you want a complex time-travel puzzle, a repeating-loop movie or something rather more entertaining than a lesson in relativity, AstronomyTrek has plenty to explore.
→ 20 Best Time Travel Movies of All Time
Find your next movie with our spoiler-light guide to some of the greatest time-travel films ever made.
→ 10 Best Time-Loop Movies
From Groundhog Day to Edge of Tomorrow, explore movies built around repeating days, events and timelines.
Popular Time-Travel Movie Explanations
Some time-travel movies are designed to leave you wondering exactly what happened. Their timelines may involve multiple versions of the same character, repeating events, causal loops or information passing between past and future.
AstronomyTrek’s movie explainers unravel some of the more complicated examples, including Predestination, Triangle, Primer, Timecrimes and The Endless, looking at how their timelines work and which time-travel ideas lie behind them.
More Time-Travel Ideas to Explore
Time travel is also a rich source of comedy, strange thought experiments, memorable quotations and stories that play more loosely with the science. Once you have explored the physics and paradoxes, there are plenty of lighter routes through the subject.
For something less serious, explore our 10 Best Time Travel Comedy Films, or browse interesting quotes about time travel from scientists, writers and thinkers.