For most airline passengers, a long-haul flight means crossing time zones, resetting a watch and dealing with jet lag. But in 1971, two scientists boarded commercial aircraft with something far more unusual than ordinary luggage: highly precise atomic clocks.
Their goal was extraordinary. They wanted to determine whether Albert Einstein was right that time itself does not pass at exactly the same rate for everyone.
The experiment became known as the Hafele–Keating experiment, named after physicist Joseph C. Hafele and astronomer Richard E. Keating. Four cesium-beam atomic clocks were flown around the world aboard regularly scheduled commercial flights. When those clocks returned, they were compared with reference clocks that had remained at the U.S. Naval Observatory.
They did not agree.
The differences were tiny, measured in billionths of a second, but they were close to what Einstein’s theories predicted. The original results were later published in Science, and the observed measurements can still be examined through the PubMed record for the Hafele–Keating experiment.
Einstein Had Already Changed the Meaning of Time
Before Einstein, it was natural to imagine time as a universal clock ticking identically everywhere. Whether a person was standing still, riding a train or looking at a distant star, one second seemed as though it should always represent the same amount of elapsed time.
Einstein’s theories challenged that assumption.
Special relativity, introduced in 1905, showed that the passage of time depends partly on relative motion. In simplified terms, a clock moving at high speed relative to another clock can accumulate less elapsed time.
General relativity went further. Gravity also affects the rate at which time passes. A clock deeper in a gravitational field ticks more slowly than a comparable clock at a higher gravitational potential.
These effects are incredibly small under everyday conditions, which explains why nobody notices their watch behaving differently after climbing a staircase or taking a flight. Yet sufficiently accurate clocks can detect them. NIST’s explanation of testing Einstein with atomic clocks describes how portable atomic clocks finally gave researchers the precision needed to investigate effects that had previously been extremely difficult to measure.
The question was no longer simply whether Einstein’s mathematics looked convincing. Could researchers actually put clocks on airplanes and watch relativity happen?
Four Atomic Clocks Took a Very Unusual Trip
In October 1971, Hafele and Keating took four cesium-beam atomic clocks on regularly scheduled commercial jet flights.
They flew around Earth in both directions.
That detail mattered enormously.
Earth itself rotates eastward. An aircraft traveling east therefore has a different motion relative to a non-rotating Earth-centered reference frame than one traveling west. Meanwhile, both aircraft spend substantial time thousands of metres above sea level, where Earth’s gravitational potential differs from that experienced by clocks remaining on the ground.
This meant two relativistic effects were operating simultaneously.
Motion produced the time-dilation effect associated with special relativity, while altitude produced the gravitational effect associated with general relativity.
The experiment was therefore much more interesting than simply asking whether an airplane clock would run slow. The final result depended on the combination of velocity, direction, altitude and the rotation of Earth.
According to the researchers’ published prediction, the eastward clocks were expected to lose approximately 40 ± 23 nanoseconds relative to the reference clocks, while the westward clocks were expected to gain approximately 275 ± 21 nanoseconds. The details of those calculations are preserved in the researchers’ original predicted relativistic time gains paper.
Then came the measurement that mattered.
The Clocks Came Back Showing Different Times
After completing the journeys, the airborne atomic clocks were compared with the atomic time scale maintained at the U.S. Naval Observatory.
The eastward clocks had lost 59 ± 10 nanoseconds.
The westward clocks had gained 273 ± 7 nanoseconds.
A nanosecond is one billionth of a second. To a person catching a flight, such a difference is meaningless. To a physicist testing relativity, it is exactly the kind of difference that matters.
More importantly, the direction and magnitude of the measured changes agreed with conventional relativity within the experimental uncertainties. The researchers described the results in their 1972 Science paper, whose abstract is available through PubMed’s record of the observed relativistic time gains.
The clocks had followed different paths through spacetime, and when they were brought back together, they had accumulated slightly different amounts of elapsed time.
That is the remarkable part of the experiment.
It was not merely that one clock was poorly synchronized with another. The differences depended on the direction and conditions of travel in a way relativity predicted.
Why Did East and West Produce Different Results?
At first, it might seem strange that traveling east and traveling west could produce opposite results.
The explanation comes from combining special and general relativity.
An aircraft’s speed contributes to special-relativistic time dilation. Generally, greater speed relative to the chosen reference frame means less proper time accumulates.
Altitude pushes in another direction. Because an aircraft is farther from Earth’s centre than a clock at sea level, it experiences a slightly different gravitational potential. Under general relativity, that tends to make the elevated clock accumulate more time relative to the lower clock.
The final reading therefore depends on the balance between these effects.
Earth’s rotation makes direction important as well. NIST’s modern account of the experiment explains why the eastward and westward flights produced different clock readings and how both special and general relativity were involved. The NIST overview of the 1971 atomic-clock flights also notes that subsequent aircraft and rocket experiments tested time dilation with greater precision.
The commercial aircraft were not traveling anywhere remotely close to the speed of light. Atomic clocks were simply accurate enough to reveal relativistic effects even at ordinary jet speeds.
Did the 1971 Experiment Really “Prove” Einstein?
Calling the experiment absolute proof of all of Einstein’s theories would go too far.
Science generally does not prove broad physical theories in the mathematical sense. Instead, a theory makes testable predictions, experiments attempt to falsify those predictions, and confidence increases when observations repeatedly agree with them.
The Hafele–Keating experiment was an important direct, macroscopic test of relativistic time effects using clocks transported around Earth. It did not single-handedly establish everything contained in special or general relativity.
What made it powerful was how tangible the experiment was.
Einstein’s ideas are often associated with objects moving near light speed, distant galaxies or extreme environments around black holes. Hafele and Keating instead took relativity aboard ordinary commercial aircraft.
The clocks returned showing that the amount of elapsed time depended on their journeys.
Time Dilation Is No Longer Just an Exotic Experiment
The importance of relativity extends far beyond a famous experiment from the 1970s.
Modern precision timekeeping routinely has to consider relativistic effects. Atomic clocks can now detect extraordinarily small differences associated with gravitational potential, allowing researchers to investigate relativity at scales that would have seemed almost unimaginable during Einstein’s lifetime.
NIST explains that today’s most accurate clocks are sensitive enough for scientists to compare how time passes at extremely small height differences. Its research into modern atomic-clock tests of Einstein’s relativity demonstrates how dramatically experimental precision has advanced since portable atomic clocks first made tests such as Hafele and Keating’s possible.
This changes the way the 1971 experiment should be viewed.
It was not simply an entertaining story about scientists taking expensive clocks on vacation. It represented a transition from relativity as something people might associate primarily with equations and astronomical phenomena to something measurable using technology operating within everyday human environments.
A Commercial Flight Revealed Something Profound About Reality
A passenger sitting aboard one of those 1971 flights would have noticed nothing unusual. Meals were served, destinations passed beneath the aircraft, and ordinary watches appeared to work exactly as expected.
Yet the atomic clocks were recording something profound.
Their journeys through different velocities and gravitational conditions caused them to accumulate measurably different amounts of time.
Einstein had predicted decades earlier that time was not the rigid, universal background human intuition suggested. By 1971, atomic-clock technology had become precise enough for scientists to carry that prediction onto commercial aircraft and test it directly.
When the clocks returned home, they disagreed by only fractions of a microsecond.
But within those tiny differences was a remarkable message: two clocks can leave together, travel through the world differently, reunite—and genuinely have experienced different amounts of elapsed time.
That is why the Hafele–Keating experiment remains one of the most memorable demonstrations of relativity. It transformed an idea that sounds almost like science fiction into something that could be measured after buying airline tickets and carrying atomic clocks around the planet.
