Is time travel possible?
Into the future, yes. Einstein's relativity, confirmed by atomic clocks, GPS satellites and astronauts, shows that time runs slower when you move fast or sit in strong gravity, so you can arrive in the future faster than everyone else. Into the past, probably not: no known physics allows it without exotic matter that may not exist.
Key facts
| Travel to the future | Possible (time dilation), proven by experiments |
|---|---|
| Travel to the past | No known way; probably impossible |
| Proof | Hafele–Keating experiment (1971), GPS, astronauts |
| ISS astronauts age less by | About 0.007 s per 6 months |
| Theory behind it | Einstein's special (1905) and general (1915) relativity |
Time travel to the future: proven
Einstein's relativity says that time passes at different rates for different observers. Move fast, or sit deep in a gravitational field, and your clock runs slower than someone else's. That means you can travel into their future.
- Astronauts do it. On the International Space Station, moving at about 28,000 km/h, astronauts age slightly slower than people on the ground: about 0.007 seconds less over six months. Scott Kelly came back from almost a year in orbit about 5 milliseconds younger than he would have been.
- GPS depends on it. GPS satellite clocks gain about 38 microseconds a day relative to ground clocks (they run faster because gravity is weaker up there, and slower because they move fast). Without correcting for it, GPS would drift by about 10 km a day. See how GPS uses time.
- Clocks on airplanes. In 1971 Joseph Hafele and Richard Keating flew atomic clocks around the world and found exactly the time differences relativity predicted.
To jump far into the future, you would need to travel close to the speed of light. Spend 5 years on a ship at 99.5% of light speed and you would come home to find about 50 years had passed on Earth. The physics works; the engineering is far beyond us.
Time travel to the past: probably not
Some solutions to Einstein's equations do allow paths that loop back in time, called closed timelike curves: Kurt Gödel's rotating universe (1949), traversable wormholes and rotating cosmic strings. But they all need things that may not exist, such as matter with negative energy in large amounts.
Going back would also raise paradoxes, like the grandfather paradox. Stephen Hawking proposed a "chronology protection conjecture": the laws of physics probably prevent travel into the past. In 2009 he held a party for time travellers and sent the invitations only afterwards. Nobody came.
Want to feel time differently?
Read how relativity slows time, what spacetime is, or whether time is real at all.
Questions people ask
Is time travel possible?
Into the future, yes: relativity makes time pass more slowly for fast-moving or gravity-bound clocks, and astronauts and GPS satellites experience it every day. Into the past, there is no known way, and most physicists think it is impossible.
Has anyone time traveled?
In a small way, yes. Astronauts who spend months on the International Space Station return a few milliseconds younger than they would otherwise be. Cosmonaut Sergei Krikalev, after 803 days in orbit, is about 0.02 seconds in the "future".
Can you travel back in time?
Not with any known physics. Some solutions to general relativity allow it in theory, but they require exotic matter that may not exist, and Stephen Hawking argued that nature prevents it.
What is the grandfather paradox?
If you went back and prevented your grandfather from meeting your grandmother, you would never be born, so you could not have gone back. It is a classic argument that travel into the past leads to contradictions.
How fast do you need to go to travel in time?
Any speed slows your clock a little. To slow it a lot you need a large fraction of the speed of light: at 87% of light speed, time passes half as fast; at 99.5%, about ten times slower.
Last updated October 23, 2026.