Frequently Asked Questions About Time Travel: What Physics Allows, What It Forbids, and What Has Been Measured
By Body Stats Editorial Team · 2 October 2026
Answers to frequently asked questions about time travel: what relativity allows, how GPS and atomic clocks prove time dilation, and why the past stays shut.
Frequently Asked Questions About Time Travel: What Physics Allows, What It Forbids, and What Has Been Measured
The GPS receiver in your phone corrects for a form of time travel every day. Satellite clocks run fast by about 38.6 microseconds per day relative to ground clocks, according to Ohio State University's Department of Astronomy. This guide answers the frequently asked questions about time travel: what is proven, what is theoretical, and what is fiction.
By the Body Stats Editorial Team.
Contents- The short answer: is time travel real?
- Travel to the future: proven and measured
- The evidence table: every major time dilation test
- Travel to the past: what theory permits
- Paradoxes and how physicists handle them
- A five-step checklist for judging time travel claims
- Common mistakes in time travel explanations
- Time travel FAQ
Key facts
1905Special Relativity was published by Albert Einstein in 1905, predicting that time passes more slowly for objects in motion relative to a stationary observer, which underlies modern discussions of time dilation. Source: Kronecker Wallis (2025)38.6 microseconds/dayGPS satellite atomic clocks gain approximately 38.6 microseconds per day compared to clocks on the ground due to the combined effects of special and general relativistic time dilation. Source: Ohio State University, Department of Astronomy (2017)In the 1971 Hafele-Keating experiment, four cesium-beam atomic clocks were flown eastward and westward around the world on commercial jets and compared to stationary clocks, with the resulting time differences consistent with predictions of both special and general relativity.Source: National Institute of Standards and Technology (NIST) (2026)33 centimetersIn a 2010 NIST experiment, two aluminum-ion optical atomic clocks placed at heights differing by about 33 centimeters (roughly one foot) ticked at measurably different rates due to gravitational time dilation. Source: National Institute of Standards and Technology (NIST) (2010)1 millimeterIn 2022, JILA physicists measured gravitational time dilation between two atomic clocks separated by a vertical distance of just one millimeter, the smallest scale at which the effect had been measured to that point. Source: National Institute of Standards and Technology (NIST) / JILA (February 2022)5 millisecondsAfter NASA astronaut Scott Kelly's 340-day mission on the International Space Station, orbiting at roughly 17,500 mph, relativistic time dilation left him about 5 milliseconds younger than his Earth-bound identical twin brother Mark. Source: Space.com (July 2016)The short answer: is time travel real?
Yes, in one direction. Moving fast or sitting deeper in a gravity field makes your clock run slower than someone else's, so you arrive in their future having aged less. That effect, time dilation, is measured routinely. Travel into the past has never been observed and remains purely theoretical.
Most frequently asked questions about time travel blur two very different ideas. The first is relative aging: two clocks start together, follow different paths through spacetime, and disagree when reunited. Albert Einstein's special relativity, published in 1905, predicted this for clocks in relative motion, as summarised by Kronecker Wallis. General relativity added that gravity slows clocks too.
The second idea is returning to an earlier moment, which would require a closed timelike curve: a path through spacetime that loops back on itself. Equations of general relativity contain such solutions on paper, but every known version needs conditions, such as exotic negative energy, that no experiment has produced at usable scale.
- Tier 1, Measured: time dilation from speed and gravity. Confirmed by clocks, particles and satellites.
- Tier 2, Mathematically permitted: wormholes and rotating-spacetime solutions. Consistent with equations, unbuilt and possibly unbuildable.
- Tier 3, Fictional: machines that jump to a chosen date. No physical model supports them.
Travel to the future: proven and measured
Forward time travel is an experimentally confirmed fact. Astronauts, aircraft passengers and fast-moving particles all age measurably differently from stationary observers. The effect is tiny at human speeds and enormous near light speed, but its existence is not in dispute among physicists.
How much younger did Scott Kelly come back?
About 5 milliseconds. After a 340-day mission on the International Space Station orbiting at roughly 17,500 mph, NASA astronaut Scott Kelly returned about 5 milliseconds younger than his identical twin Mark, Space.com reported. The orbital speed slowed his clock; the weaker gravity in orbit partially offset that, and speed won.
Why does GPS depend on time dilation?
GPS calculates position from signal timing, so clock errors become distance errors. Satellite clocks gain about 38.6 microseconds daily from combined special and general relativistic effects, and without correction position fixes would become false after only about two minutes, per Ohio State's astronomy department. Every working navigation fix is indirect evidence that relativity is right.
Does height change how fast you age?
Yes, and the effect is now measurable over tiny distances. In 2010, NIST showed that two aluminum-ion clocks about 33 centimeters apart in height ticked at different rates (NIST). In 2022, JILA physicists measured the effect across a vertical separation of just one millimeter (NIST/JILA). Your head ages slightly faster than your feet.
The evidence table: every major time dilation test
Time dilation has been confirmed by at least four independent method types: particle decay, flying clocks, stationary clocks at different heights, and orbital systems. The table below, compiled by the Body Stats Editorial Team, puts the key results side by side so the scale of each effect is comparable.
| Experiment | Method | Key result | Source |
|---|---|---|---|
| Frisch-Smith, Mount Washington (1963) | Cosmic-ray muons at 0.995c | Decay about nine times slower than rest-frame prediction | Mount Washington Observatory |
| CERN storage ring (1966) | Muons at 99.7% of light speed | Twelve-fold lifetime increase, within 2% of prediction | arXiv review |
| Hafele-Keating (1971) | Four cesium clocks flown east and west on commercial jets | Differences consistent with special and general relativity | NIST |
| CERN Muon Storage Ring | Muons at gamma = 29.33 | Lifetimes 64.419 µs (+) and 64.368 µs (−), fractional error 2×10⁻³ | Nature (1977) |
| NIST aluminum clocks (2010) | Optical clocks at different heights | Rate difference across about 33 cm | NIST |
| JILA clocks (2022) | Atomic clocks, vertical separation | Effect measured over 1 mm | NIST/JILA |
| Scott Kelly, ISS | 340 days in orbit at ~17,500 mph | About 5 ms younger than his twin | Space.com |
The pattern across the table matters more than any single row. Particle experiments test speed-based dilation with huge effects; clock experiments test gravitational dilation with tiny effects at extreme precision. Different teams, decades and techniques converge on the same theory, which is why answers to frequently asked questions about time travel can state forward time travel as settled.
Travel to the past: what theory permits
Backward time travel is not ruled out by general relativity's equations, but it is unsupported by any observation. Proposed routes include traversable wormholes and rapidly rotating spacetimes, and each requires ingredients such as large amounts of negative energy that physics does not know how to supply or stabilise.
The main theoretical routes researchers discuss:
- Traversable wormholes: a tunnel between two regions of spacetime; if one mouth is time-dilated relative to the other, the pair could connect different times. Keeping the throat open needs exotic matter.
- Rotating universes and cylinders: solutions where spacetime itself is dragged so strongly that paths loop in time. Our universe shows no sign of the required global rotation.
- Cosmic strings: hypothetical defects whose motion could, in principle, tilt light cones enough to form loops. None has been detected.
Many physicists suspect a deeper rule, often called chronology protection, under which quantum effects would destroy any time machine as it formed. That conjecture is unproven, so the honest position is: past travel is unforbidden by classical equations and unsupported by evidence.
Paradoxes and how physicists handle them
Time travel paradoxes are logical contradictions that appear if the past can be changed. Physicists offer three responses: past travel is impossible, history is self-consistent so changes cannot happen, or the traveller enters a separate branch of reality. None has experimental support because no past travel exists to test.
- Grandfather paradox: preventing your own birth removes the person who prevented it. Self-consistency answers that events conspire to stop the attempt.
- Bootstrap paradox: information or an object loops with no origin. It is not contradictory, only uncomfortable, which is why it troubles philosophers more than physicists.
- Twin paradox: not a real paradox. The travelling twin accelerates and changes reference frames, which breaks the symmetry; the Scott Kelly data shows the result directly.
Separating the twin paradox from the other two is the single most useful distinction in frequently asked questions about time travel. The twin case is measured physics; the grandfather case is a thought experiment about a technology no one has built.
A five-step checklist for judging time travel claims
To evaluate any headline about time travel, check direction, mechanism, size of effect, source type and whether results were replicated. This Body Stats checklist sorts most viral claims within a minute and works for news stories, videos and textbook explanations alike.
- Direction: future (measured) or past (theoretical)?
- Mechanism: speed, gravity, or an unnamed device? Unnamed devices are Tier 3.
- Magnitude: does it give a number with units, like milliseconds or microseconds? Vague claims rarely survive scrutiny.
- Source: is it a research body, peer-reviewed journal or official measurement standard?
- Replication: has another method confirmed it? Time dilation passes; past travel has nothing to replicate.
Common mistakes in time travel explanations
The most common error is treating time dilation as a clock malfunction rather than a real difference in elapsed time. Others include ignoring gravity, assuming faster-than-light travel is required for any time travel, and presenting wormholes as discovered objects.
- "The clocks are just wrong." Unstable muons, which have no gears, also live longer when fast, as the Mount Washington and CERN results show.
- "Only speed matters." GPS needs both corrections; gravity alone separates clocks 1 mm apart.
- "You need to beat light." Forward travel happens at any speed; it simply grows sharply near light speed.
- "Wormholes exist." They are solutions to equations, not observed objects.
Readers who want shorter, general answers about how Body Stats researches and reviews topics can browse the site's general reader questions page.
Time travel FAQ
These are the frequently asked questions about time travel the Body Stats Editorial Team sees most, each answered in its first sentence with supporting evidence after.
Has anyone actually travelled in time?
Yes, into the future by small amounts. Scott Kelly returned from 340 days on the ISS about 5 milliseconds younger than his twin, according to Space.com.
Can we travel back in time?
No method is known. General relativity allows looping spacetime paths mathematically, but they require exotic negative energy or cosmic structures never observed.
Is time dilation real or an illusion?
Time dilation is real. Fast muons at CERN with gamma = 29.33 lived as long as special relativity predicts, to a fractional error of 2×10⁻³, reported in Nature.
Do airline passengers time travel?
Yes, by fractions too small to notice. The Hafele-Keating experiment flew cesium clocks around the world and found differences consistent with relativity, as NIST describes.
Why would GPS fail without relativity?
Uncorrected satellite clocks drift about 38.6 microseconds per day, which would make position fixes false within about two minutes, per Ohio State.
Who first predicted time dilation?
Albert Einstein, in his 1905 special relativity, predicted that moving clocks run slow relative to a stationary observer.