Hafele–Keating Experiment (1971): The Real Experiment That Measured Time Dilation

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What Was the Hafele–Keating Experiment?

The Hafele–Keating experiment was a real physics experiment conducted in 1971 to test relativistic predictions using portable atomic clocks.

The basic idea was surprisingly simple:

Take extremely accurate clocks.

Put some of them on airplanes.

Fly them around the Earth.

Leave other reference clocks at the U.S. Naval Observatory.

Then compare the clocks after the journey.

According to relativity, the traveling clocks should not necessarily show exactly the same elapsed time as clocks that stayed on the ground.

The expected difference was incredibly small—measured in nanoseconds, or billionths of a second.

At the time, however, atomic clocks were accurate enough for scientists to attempt the measurement. (NIST)

Who Were Hafele and Keating?

Joseph C. Hafele was a physicist associated with Washington University in St. Louis.

Richard E. Keating worked in the Time Service Division of the U.S. Naval Observatory in Washington, D.C.

Together they designed an unusual experiment that turned a commercial airline journey into a test of Einstein's theories.

The work was not an attempt to search for paranormal time travel. It was a precision measurement experiment involving atomic clocks, aircraft motion, Earth's rotation and gravity.

Their results were eventually published in the journal Science in July 1972. (PubMed)

Why Did Scientists Need Atomic Clocks?

Ordinary mechanical clocks were nowhere near accurate enough for this experiment.

The predicted effect was measured in billionths of a second.

Atomic clocks provided a much more stable reference because they use the natural frequency associated with atoms rather than a mechanical pendulum or balance wheel.

The clocks used by Hafele and Keating were cesium-beam atomic clocks. Contemporary documentation identifies the instruments as portable cesium clocks, and later U.S. Naval Observatory material identifies the equipment photographed with the scientists as Hewlett-Packard HP-5061A cesium clocks. (CnMOC)

The Basic Idea Behind the Experiment

The experiment involved two different relativistic effects.

The first comes from motion.

The second comes from gravity.

These effects can act in different directions depending on the aircraft's speed, altitude and direction of travel.

This is what made the experiment particularly interesting.

Motion and Special Relativity

Einstein's special relativity predicts that a clock moving at high speed relative to another reference frame accumulates less elapsed time than a clock at rest in that frame.

This effect is called kinematic time dilation.

Commercial aircraft are nowhere near the speed of light, so the effect is extraordinarily small.

But an atomic clock can be precise enough to detect it.

Gravity and General Relativity

Einstein's general relativity predicts another effect.

A clock located higher in a gravitational field, where gravitational potential is different, runs at a slightly different rate from one deeper in the gravitational field.

An aircraft flying several kilometers above Earth's surface is therefore experiencing a slightly different gravitational environment from a clock sitting at the Naval Observatory.

This effect tends to make the airborne clock run faster relative to a clock at lower altitude.

The actual experiment therefore had to account for both gravitational and motion-related effects. (NIST)

Why Did They Fly in Both Directions?

This is one of the most important parts of the experiment.

Earth itself rotates.

Consequently, an aircraft flying eastward and an aircraft flying westward do not have exactly the same velocity relative to an Earth-centered reference frame.

The aircraft's own velocity is combined with Earth's rotational motion.

Flying eastward generally increases the aircraft's speed relative to an Earth-centered inertial frame.

Flying westward generally decreases it.

Because the motion-related relativistic effect depends on speed, changing the direction of travel changes the predicted time difference.

The gravitational contribution from flying at aircraft altitude, meanwhile, acts in the other direction.

This produced different predictions for the eastward and westward journeys. (NIST)

The Four Atomic Clocks

Four cesium-beam atomic clocks were used.

Their serial numbers were:

Clock 120
Clock 361
Clock 408
Clock 447

The clocks were transported together during the experiment and compared with the reference time scale maintained at the U.S. Naval Observatory.

The original published data show that the individual clocks did not all behave identically. Their measured results varied somewhat, which is important when understanding the experiment rather than simply looking at its final average. (ITP3 Download)

The First Round-the-World Journey

The first journey was made eastward.

According to documentation based on the experiment's records, the eastward trip began on October 4, 1971, at approximately 19:30 UTC.

The journey lasted approximately 65.4 hours, of which about 41.2 hours were actually spent in flight.

The clocks traveled on regularly scheduled commercial flights rather than aboard a specially constructed research aircraft.

The route involved multiple stops and aircraft changes.

The historical route records describe stops including locations such as London, Frankfurt, Istanbul, Beirut, Tehran, New Delhi, Bangkok, Hong Kong and Tokyo, before continuing across the Pacific and returning to the United States.

The precise route matters because the scientists used the actual flight paths, speeds and altitudes when calculating the relativistic prediction. (ResearchGate)

The Second Journey

The clocks were later flown around the world in the opposite direction.

The westward journey began on October 13, 1971, at approximately 19:40 UTC.

It lasted approximately 80.3 hours, including about 48.6 hours in flight.

Again, the clocks traveled aboard regularly scheduled commercial aircraft.

The aircraft did not simply fly continuously around the planet. There were landings, takeoffs, waiting periods and changes of aircraft.

That made the experiment more complicated than a simple "clock on a plane versus clock on the ground" demonstration.

The scientists therefore reconstructed the relevant flight conditions and calculated the expected relativistic effect from the actual journeys. (ResearchGate)

What Did Einstein's Theory Predict?

After taking the actual flight paths into account, Hafele and Keating calculated the expected differences relative to the clocks at the U.S. Naval Observatory.

For the eastward journey, relativity predicted that the traveling clocks would lose:

40 ± 23 nanoseconds

For the westward journey, relativity predicted that the traveling clocks would gain:

275 ± 21 nanoseconds

The plus-or-minus values represent the estimated uncertainty of the prediction. (PubMed)

What Did the Clocks Actually Show?

The measured results were:

In other words:

Eastward: the flying clocks ended up approximately 59 nanoseconds behind the reference clocks.

Westward: the flying clocks ended up approximately 273 nanoseconds ahead of the reference clocks.

The results were close to the theoretical predictions within the experiment's uncertainty. (PubMed)

What Does 59 Nanoseconds Actually Mean?

A nanosecond is one billionth of a second.

So:

59 nanoseconds = 0.000000059 seconds

And:

273 nanoseconds = 0.000000273 seconds

To a human being, these differences are completely imperceptible.

A person on one airplane would not suddenly notice their watch behaving differently from another person's watch.

The significance was that scientists could measure the difference with precision instruments.

That is what makes the experiment remarkable.

It was not a dramatic difference in everyday time.

It was a tiny difference that could nevertheless be detected experimentally.

Breaking Down the Prediction

The published analysis separated the predicted effects into gravitational and motion-related components.

For the eastward trip, the approximate predicted contributions were:

Gravitational effect: +144 ± 14 nanoseconds
Kinematic effect: −184 ± 18 nanoseconds
Combined prediction: −40 ± 23 nanoseconds

For the westward trip:

Gravitational effect: +179 ± 18 nanoseconds
Kinematic effect: +96 ± 10 nanoseconds
Combined prediction: +275 ± 21 nanoseconds

The two effects therefore partially canceled in the eastward case but reinforced one another in the westward case. (Wikipedia)

What Happened to Each Clock?

The published observations showed the following adjusted results:

The four clocks did not produce identical numbers.

That is normal for a precision experiment involving equipment operating under real-world conditions.

The final published result was based on the analysis of the ensemble of clocks rather than assuming that every individual instrument behaved identically. (ResearchGate)

The Clocks Were Not Perfect

This is an important detail that is sometimes omitted from simplified descriptions of the experiment.

The original paper reported that some of the clocks experienced changes in their rates during the experiment.

During the eastward journey:

Clock 120 changed rate three times.
Clock 361 changed rate three times.
Clock 408 changed rate twice.
Clock 447 changed rate once.

During the westward journey:

Clock 120 changed rate once.
Clock 361 changed rate four times.
Clocks 408 and 447 showed no significant rate changes according to the published analysis.

The researchers attempted to account for these changes when determining the relativistic result. (ITP3 Download) 

Why Were These Rate Changes Important?

An atomic clock is extremely precise, but "extremely precise" does not mean "perfect."

The experiment took place aboard commercial aircraft rather than inside a carefully controlled laboratory.

The clocks were subjected to transportation, aircraft operation, changes in altitude and the practical conditions of a long international journey.

The scientists therefore had to distinguish genuine changes relevant to the experiment from changes in clock behavior.

The published analysis used a method described as a correlated rate-change procedure to identify and account for rate changes in individual clocks. (ITP3 Download)

Did the Experiment Prove That Time Is Real?

Not exactly.

The experiment did not prove the existence of "time" itself in a philosophical sense.

What it demonstrated was much more specific:

Clocks following different physical paths can accumulate different amounts of elapsed time, and the differences can agree with relativistic calculations.

This is a measurable physical effect.

The experiment therefore provided an empirical test of relativistic predictions concerning clocks in motion and at different gravitational potentials.

Did the Experiment Prove Time Travel?

No.

This is one of the biggest misconceptions surrounding the experiment.

The Hafele–Keating experiment did not demonstrate a machine capable of transporting people through time.

It did not create a portal.

It did not send anything into the past.

It did not cause anyone to skip years or decades.

The difference was only measured in nanoseconds.

However, the experiment does demonstrate something that sounds strange when expressed in everyday language:

Two clocks can follow different paths and later disagree about how much time has elapsed.

That is a real consequence of relativity.

Is This the Same as the "Twin Paradox"?

The experiment is closely related to the physics behind the famous twin paradox, but it should not be described as literally sending one twin around Earth.

The twin paradox is a thought experiment involving two observers who follow different paths through spacetime and later compare their elapsed time.

Hafele and Keating replaced the hypothetical twins with actual precision clocks.

The experiment therefore provided a real-world clock comparison rather than a purely theoretical thought experiment.

Their 1972 paper specifically described the result in relation to the famous clock paradox. (PubMed)

What Did People Think About the Experiment?

The experiment attracted attention because it transformed an abstract idea from Einstein's relativity into something that could be demonstrated using an ordinary commercial flight.

Contemporary newspaper coverage presented the experiment as a practical test of Einstein's claim that time measured by clocks can depend on motion.

One contemporary 1971 newspaper report described Hafele and Keating preparing to carry atomic clocks on a commercial flight around the world and compare them with the observatory's master clock. (The Gateway to Oklahoma History)

The public-facing idea was fascinating:

Could two clocks that took different journeys actually disagree when they met again?

The answer was yes—but only by an extraordinarily small amount.

Why Did the Direction Matter So Much?

This is perhaps the strangest part of the case.

If you simply think:

"Flying around the world should make the clock move faster, so why would direction matter?"

the answer lies in Earth's rotation.

The aircraft does not move relative to a stationary Earth.

Earth itself is rotating.

An eastbound aircraft moves in roughly the same direction as Earth's rotation, while a westbound aircraft moves against it.

That changes the aircraft's velocity relative to an Earth-centered inertial frame.

The relativistic velocity contribution therefore changes substantially between the two journeys.

The gravitational contribution from flying at altitude also has to be included.

The final observed difference is therefore the result of several relativistic contributions acting together. (NIST)

Was There Anything Mysterious About It?

There is a legitimate sense in which the experiment is surprising.

Human intuition normally assumes that time passes at the same rate everywhere.

Relativity says that this is not universally true.

A clock's elapsed time depends on its path through spacetime.

The Hafele–Keating experiment provided a tangible demonstration of that counterintuitive idea.

But there is an important distinction:

The physics is unusual, but the experiment itself is not evidence of anything supernatural.

Everything involved—atomic clocks, aircraft motion, Earth's rotation, gravity and relativistic corrections—belongs to established physics.

Were There Alternative Interpretations?

Yes, although they should be divided into two very different categories.

There are scientific discussions about how the experiment should be analyzed, and there are much more radical claims that reject conventional relativity.

These should not be treated as equally supported.

Conventional Relativity Interpretation

The standard interpretation is that the measured eastward and westward differences arise from the combination of:

Special-relativistic time dilation from motion.
General-relativistic gravitational time dilation.
Earth's rotation.
The actual flight paths and altitudes.
The behavior and calibration of the clocks.

The measured results agree with the predictions within the experiment's uncertainty.

This remains the mainstream scientific interpretation. (NIST)

Sagnac-Effect Interpretation

Another legitimate part of the physics concerns the Sagnac effect, which is associated with rotating reference systems.

NIST material discussing the experiment notes that the around-the-world clock experiment also demonstrated the Sagnac effect associated with Earth's rotation.

Modern precision timing systems continue to take Earth's rotation into account. (NIST)

This is not a competing explanation that replaces relativity. It is part of the broader relativistic treatment of clocks and signals in a rotating Earth-based system.

Criticism of the Data Analysis

Over the years, some authors have criticized aspects of the original analysis, particularly the treatment of changes in clock rates and the adjustments made to the measurements.

Such criticisms exist in the literature, but they do not constitute evidence that the experiment was fraudulent or that relativity was disproved.

The original researchers explicitly documented clock-rate changes and their analytical treatment.

Later critiques can be useful when studying the experiment in depth, but they should be distinguished from the peer-reviewed original measurements and the much broader body of later tests of relativistic time dilation. (ITP3 Download)

Did Anyone Discover a Problem With Einstein's Theory?

The Hafele–Keating measurements did not reveal a confirmed failure of Einstein's relativity.

The measured results were close to the relativistic predictions.

More importantly, later experiments have tested the same general effects with increasingly accurate clocks and different experimental arrangements.

NIST notes that later aircraft and rocket experiments confirmed time dilation with greater precision. (NIST)

Therefore, the experiment is historically important not because it overturned physics, but because it helped establish that relativistic clock effects could be measured under real-world conditions.

What Happened After the Experiment?

The experiment became part of the growing experimental evidence supporting relativistic timekeeping.

As atomic clocks improved, scientists were able to measure much smaller gravitational and motion-related effects.

Eventually, these effects became technologically important.

One of the most famous applications is the Global Positioning System.

GPS satellites carry extremely accurate clocks.

Because the satellites move at high speed and operate far above Earth's surface, both special and general relativistic effects affect their clock rates.

Those corrections have to be accounted for if GPS is to provide accurate positioning and timing.

The U.S. Naval Observatory has explained that the relativistic effects first measured by Hafele and Keating have become routine considerations in modern GPS timekeeping. (CnMOC)

From a 1971 Airplane to Modern GPS

This is one of the most important legacies of the experiment.

In 1971, scientists had to physically carry atomic clocks around the world.

Today, precision clocks operate aboard satellites and in laboratories, and relativistic corrections are built into systems used for navigation, communications and scientific measurement.

The underlying idea is the same:

Motion and gravity affect the rate at which clocks accumulate elapsed time.

The difference is that modern instruments can measure these effects with vastly greater precision.

Why Is the Hafele–Keating Experiment Still Important?

The experiment is important for several reasons.

It tested relativity using real clocks rather than only mathematical calculations.

It used ordinary commercial aircraft, showing that relativistic effects could be measured outside a specialized laboratory.

It tested the effect in both directions around Earth.

It demonstrated that the predicted difference was not simply an abstract theoretical possibility.

And it helped connect Einstein's theories to the practical science of precision timekeeping.

The Most Important Numbers

For quick reference:

Experiment: Hafele–Keating experiment

Year of flights: 1971

Publication: 1972

Scientists: Joseph C. Hafele and Richard E. Keating

Clocks: Four cesium-beam atomic clocks

Reference: U.S. Naval Observatory atomic time scale

Eastward prediction: −40 ± 23 nanoseconds

Eastward measurement: −59 ± 10 nanoseconds

Westward prediction: +275 ± 21 nanoseconds

Westward measurement: +273 ± 7 nanoseconds

Main physical effects: special-relativistic motion effects + general-relativistic gravitational effects + Earth's rotation

Result: Consistent with relativistic predictions within the experiment's uncertainty. (PubMed)

What Remains Unusual About the Case?

There is no unresolved paranormal mystery here.

The interesting mystery is a scientific one:

Why should two clocks experience different amounts of elapsed time simply because they followed different journeys?

The answer is supplied by relativity.

Spacetime does not behave exactly like the simple universal clock assumed by everyday experience.

The Hafele–Keating experiment turned that strange theoretical prediction into a measurable difference on four physical clocks.

What the Evidence Actually Shows

The evidence supports several conclusions:

Four cesium atomic clocks were flown around Earth during October 1971.
The clocks traveled once eastward and once westward.
Reference clocks remained at the U.S. Naval Observatory.
Relativity predicted different results for the two directions.
The eastward clocks measured a loss of approximately 59 nanoseconds relative to the reference.
The westward clocks measured a gain of approximately 273 nanoseconds.
The measurements were broadly consistent with the relativistic predictions.
The experiment involved real-world clock-rate variations that had to be analyzed.
Later precision experiments further tested relativistic time dilation.
Modern satellite navigation systems must account for relativistic clock effects. (NIST) 

What the Evidence Does Not Show

The experiment does not show:

A time machine.
Human time travel.
Travel into the past.
A wormhole.
A supernatural alteration of time.
Evidence of an unknown force controlling time.
Evidence that Einstein's relativity has been disproved.

The difference measured was extraordinarily small, but scientifically meaningful.

Conclusion

The Hafele–Keating experiment is one of the rare cases where an idea that sounds almost impossible in everyday life was tested using an ordinary object: a clock.

In 1971, four cesium atomic clocks traveled around the world aboard commercial aircraft. Some went east, others west, while reference clocks remained at the U.S. Naval Observatory.

When the clocks were compared, they did not all show exactly the same elapsed time.

The difference was measured in billionths of a second—but it was in the direction and approximate magnitude predicted by relativity.

The experiment did not reveal a mysterious portal or prove that humans can travel through time. Its importance is more subtle.

It demonstrated that elapsed time is not completely independent of motion and gravity.

Today, that same principle is not merely a historical curiosity. Relativistic corrections are an essential part of modern precision timekeeping and satellite navigation.

What once required scientists to fly atomic clocks around the world is now part of everyday high-precision technology.

The mystery was not that the clocks disagreed.

The mystery was that Einstein had predicted they would.

And the clocks did.

Reliable Sources, Original Research & Real Images

For your website, these are the safer sources I would recommend linking rather than random UFO/paranormal websites.

NIST — Putting Einstein to the Test With the World's Most Accurate Clocks A modern explanation from the U.S. National Institute of Standards and Technology, including historical context and the connection to modern precision clocks. NIST article

U.S. Naval Observatory / U.S. Navy — Hafele–Keating Anniversary Material Contains historical information about the experiment and an archival photograph of Hafele, Keating and their atomic-clock equipment. U.S. Naval Observatory / U.S. Navy material

PubMed — Original 1972 Prediction Paper Bibliographic record for Hafele and Keating's original Science paper, Around-the-World Atomic Clocks: Predicted Relativistic Time Gains. PubMed: Predicted Relativistic Time Gains

PubMed — Original 1972 Observed Results Paper Bibliographic record for the original paper reporting the measured eastward and westward results. PubMed: Observed Relativistic Time Gains

CiNii Research — Original Science Paper Record Provides bibliographic information and the abstract for the 1972 observed-results paper. CiNii Research record

NIST — Around-the-World Relativistic Sagnac Experiment Useful for explaining Earth's rotation, the Sagnac effect and the connection between the Hafele–Keating experiment and modern precision timing. NIST technical document

Oklahoma Historical Society — Contemporary 1971 Newspaper Archive A useful historical source showing how the experiment was reported around the time of the original flights. Gateway to Oklahoma History newspaper archive

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Editorial note for your site: this case is especially suitable for an evidence-based science section because the underlying experiment, published measurements, and historical documentation are available from scientific and government sources. Keeping the article focused on the actual experiment rather than presenting it as supernatural "time travel" will also make the page substantially more accurate and advertiser-friendly. (NIST)

JourneyRelativistic predictionMeasured resultEastward−40 ± 23 nanoseconds−59 ± 10 nanosecondsWestward+275 ± 21 nanoseconds+273 ± 7 nanosecondsClockEastward resultWestward result120−57 ns+277 ns361−74 ns+284 ns408−55 ns+266 ns447−51 ns+266 nsAverage−59 ± 10 ns+273 ± 7 ns

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