Time travel

Can people travel into the future or the past?

Travel into the future is already happening in a very small way. Clocks flown on aircraft in 1971, satellite clocks that GPS corrects for, and optical clocks that detect height differences of less than a metre all show that time runs at different rates in different conditions. The effects are measured in nanoseconds and microseconds, so forward travel of any useful length is not started. Travel into the past has no experimental evidence. It rests on theoretical solutions of general relativity that need exotic conditions, and Hawking conjectured that physical law forbids them.

AI illustration · Illustrative scene, not a photograph of the specific device or experiment.
WHERE WE AREScience demonstrated

Far away. The science works, but nobody has built a working system yet.

The forward part is demonstrated. Time dilation from speed and gravity has been measured with clocks on aircraft, in satellite navigation and in optical clock experiments. Travel into the future by more than a tiny amount is not started, because the measured effects at speeds and heights that people can reach are nanoseconds to microseconds. Travel into the past has no experimental evidence. It is theoretical, and the stage for it is science-unproven.

The tech tree

Everything recorded on Vrulum that this future needs, oldest foundations at the top and the goal at the bottom. Lines show what each step requires.

What it takes: Time travel Unlocked Being researched Locked
  1. 2015Optical atomic clocksResearch and prototypes Being researched.
  1. 1971Clocks flown around the world on aircraftDone Unlocked.
  2. 1988Traversable wormhole that could serve as a time machineNot started Locked.
  3. 1992Closed timelike curves in general relativityNot started Locked.
  4. 2010Optical clocks sensing height differences of under a metreDone Unlocked.
  5. 2022Gravitational redshift across one millimetre of atomsDone Unlocked.
  6. Date not recordedSatellite navigation clocks corrected for relativityDone Unlocked.
  7. Date not recordedForward travel of a useful lengthNot started Locked.
  8. Date not recordedExperimental evidence of travel into the pastNot started Locked.
  1. GoalTime travelGoalRequires: Optical atomic clocks, Clocks flown around the world on aircraft, Satellite navigation clocks corrected for relativity, Optical clocks sensing height differences of under a metre, Gravitational redshift across one millimetre of atoms, Forward travel of a useful length, Closed timelike curves in general relativity, Traversable wormhole that could serve as a time machine, Experimental evidence of travel into the past. Locked.

The road so far

4 of 8 steps done.

  1. Done · Oct 1971Clocks flown around the world on aircraft

    In October 1971, Hafele and Keating flew four caesium-beam clocks on commercial airliners, once eastward and once westward, and compared them with reference clocks at the US Naval Observatory. Predicted offsets were about 40 ns slow (±23 ns) for the eastward flight and 275 ns fast (±21 ns) for the westward flight. The measured offsets were 59 ns slow (±10 ns) and 273 ns fast (±7 ns). The summary page's breakdown table gives the predicted westward figure as ±21 ns, which matches the prediction above. The eastward result differs most from prediction, but it lies within the stated errors. [1]

  2. DoneSatellite navigation clocks corrected for relativity

    A secondary summary of GPS states that special relativity slows satellite clocks by about 7 µs a day, while weaker gravity in orbit speeds them up by about 45 µs a day. The summary states the gravity figure without derivation. The net gain is about 38 µs a day. Without correction, the summary says, navigation errors would grow by more than 10 km a day. The summary does not cite the GPS interface specifications, so the figures should be checked against a primary GPS document before reuse. [2]

  3. Done · 24 Sept 2010Optical clocks sensing height differences of under a metre

    Chou and colleagues compared two optical atomic clocks through a 75 m length of optical fibre. The abstract reports time dilation from relative speeds of less than 10 m/s and from changes in height of less than 1 m. The abstract gives no uncertainty figures. [3]

  4. Done · 2022Gravitational redshift across one millimetre of atoms

    Bothwell and colleagues used a single millimetre-scale sample of ultracold strontium and measured a linear frequency gradient across it, which the paper relates to gravitational redshift. The abstract reports a fractional frequency measurement uncertainty of 7.6 × 10^-21. It does not state a height difference in millimetres. [4]

  5. Not startedForward travel of a useful length

    The measured offsets are a few nanoseconds to a few microseconds. No person or clock has yet been moved far enough or fast enough to show an offset that a traveller would notice. The sources reviewed contain no experiment that moves a person forward in time by a useful amount. [1]

  6. Not started · 15 Jul 1992Closed timelike curves in general relativity

    Some theoretical solutions of general relativity contain closed timelike curves, which would allow travel into the past. Hawking's 1992 paper calls chronology protection a conjecture. It says the back-reaction 'seems' would prevent such curves from appearing, and it states that the results strongly support the conjecture. It does not prove it. [5][6]

  7. Not started · 26 Sept 1988Traversable wormhole that could serve as a time machine

    Morris, Thorne and Yurtsever (1988) state that a wormhole for interstellar travel could be converted into a time machine if causality can be violated, and they ask whether field theory enforces an averaged version of the weak energy condition. No traversable wormhole has been built or observed, and the sources reviewed do not show that one can exist. [7]

  8. Not startedExperimental evidence of travel into the past

    No experiment has shown a closed timelike curve, a causal loop or any route to the past. The sources reviewed contain no experimental result for backward travel. [5]

Why it matters

Time dilation is part of the timing that GPS and other navigation systems rely on. Measuring it more precisely tests general relativity and could give geodesists and clock makers a way to detect small differences in height. Any claim of backward travel would also test how causality works, so it is worth stating clearly what has been shown and what has not.

What stands in the way

  • Travel into the past needs closed timelike curves or traversable wormholes, and neither has been observed or made.
  • Whether field theory allows the averaged weak energy condition to be broken, which decides whether a traversable wormhole can exist, is still an open question.
  • Hawking's chronology protection conjecture argues that physical law stops closed timelike curves from forming. It is a conjecture, not a proof.
  • Forward travel of a useful length needs a much larger speed or gravity difference than any measured so far. The measured effects are nanoseconds to microseconds.

Signals to watch

  • Optical clock comparisons that reach smaller uncertainties across height differences of a millimetre or less.
  • Clocks carried at speeds or altitudes large enough to show offsets that matter to a traveller.
  • Theoretical results that settle whether the averaged weak energy condition can be violated in quantum field theory.
  • Any reported closed timelike curve or causal loop that is confirmed by independent measurement. None has been reported in the sources reviewed.

Sources

  1. other
    Hafele and Keating experiment (HyperPhysics summary of Science 177, 166, 1972)
    J. C. Hafele, R. E. Keating. HyperPhysics, Georgia State University (secondary summary), 1972.
  2. other
    IAPS Article of the Month, March 2023 (relativistic effects on GPS)
    Raghav Sharma. International Association of Physics Students (IAPS), 1 Mar 2023.
  3. peer reviewed paper
    Optical Clocks and Relativity
    C. W. Chou, D. B. Hume, T. Rosenband et al.. Science 329, 1630-1633, 24 Sept 2010.
  4. peer reviewed paper
    Resolving the gravitational redshift within a millimeter atomic sample
    Tobias Bothwell, Colin J. Kennedy, Alexander Aeppli et al.. Nature 602, 420-424 (2022); arXiv:2109.12238, 2022.
  5. peer reviewed paper
    Chronology protection conjecture
    S. W. Hawking. Physical Review D, 15 Jul 1992.
  6. preprint
    From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
    Matt Visser. arXiv preprint hep-th/9202090; published as Phys. Rev. D 47, 554-565 (1993), Feb 1992.
  7. peer reviewed paper
    Wormholes, Time Machines, and the Weak Energy Condition
    Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever. Physical Review Letters, 26 Sept 1988.

Last reviewed 21 Oct 2026.