A List of Time Dilation Experiments
Collected by Ed Lake
(Page created on Sept. 3, 2018)
(Last updated: March 14, 2020)

If anyone knows of any other time dilation experiments that should be on this list,
or if you see any errors on this web page,

my email address

Click on the item to go to a description of the experiment.

1.  Hafele-Keating
2.  NIST Optical Clocks and Relativity
3.  Geodesy and Metrology experiment (measuring altitude by time difference)
4.  Muon experiments
5.  University of Maryland
6.  Japanese Mitaka to Norikura
7.  Briatore and Leschiutta
8.  National Physical Laboratory - 1996
9.  Van Baak - 2005
10. National Physical Laboratory - 2010
11. Van Baak - 2016
12. Tokyo Skytree - 2020

13.  Ives-Stillwell

----------------------- Details -----------------

1. Hafele-Keating

The Hafele-Keating experiments confirmed both Gravitational Time Dilation (GTD) and Velocity Time Dilation (VTD).

In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They wrote two papers about what they did.  The first paper explained the purpose of the experiment and showed how they did the calculations using data gathered during the trips.  The second paper described the actual clock data from the trips and how it agreed with the calculations and Einstein's theories of General and Special Relativity.

Hafele and Keating flew twice around the world, first eastward, then westward, and they compared the four atomic clocks they carried with them against a clock that remained at the United States Naval Observatory. When reunited, the two sets of clocks that were transported around the globe were found to disagree with the Naval Observatory clock, and the differences were fully consistent with the predictions of special and general relativity and with the calculations Hafele and Keating did of their actual data.

Hafele and Keating on a plane
Atomic clock used by Hafele and Keating
Joseph C. Hafele (seated) and Richard E. Keating aboard one of their flights.
One of the atomic clocks used in the experiments.

Their paper: Around-the-World Atomic Clocks: Predicted Relativistic Time Gains
Their paper: Around-the-World Atomic Clocks: Observed Relativistic Time Gains

Four cesium beam clocks flown around the world on commercial jet flights during October 1971, once eastward and once westward, recorded directionally dependent time differences which are in good agreement with predictions of conventional relativity theory. Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59 +/- 10 nanoseconds during the eastward trip and gained 273 +/- 7 nanoseconds during the westward trip, where the errors are the corresponding standard deviations. These results provide an unambiguous empirical resolution of the famous clock "paradox" with macroscopic clocks.
Wikipedia: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment

The purpose of the experiment as described in their "Predicted" paper:
One of the most enduring scientific debates of this century is the relativistic clock "paradox" (1) or problem (2), which stemmed originally from an alleged logical inconsistency in predicted time differences between traveling and reference clocks after a round trip.  This seemingly endless theoretical debate, which has flared up recently with renewed vigor (2, 3), begs for a convincing empirical resolution with macroscopic clocks. A simple and direct experimental test of the clock problem with portable atomic clocks is now possible because of the unprecedented ability achieved with these clocks (4).
Their conclusion is described at the end of their "Observed" paper:
In conclusion, we have shown that the effects of travel on the time recording behavior of macroscopic clocks are in reasonable accord with predictions of the conventional theory of relativity, and that they can be observed in a straightforward and unambiguous manner with relatively inexpensive commercial jet flights and commercially available cesium beam clocks. In fact, the experiments were so successful that it is not unrealistic to consider improved versions designed to investigate aspects of the theory that were ignored in the predicted relativistic time differences (1). In any event, there seems to be little basis for further arguments about whether clocks will indicate the same time after a round trip, for we find that they do not.
Another key paper written by Hafele only: http://www.dtic.mil/dtic/tr/fulltext/u2/a489971.pdf

I used that paper to plot the routes they flew.  The red arrows below show the 12 flights they took when they flew eastward
from from Washington's Dulles airport to London, to Frankfurt, to Istanbul, to Beirut, to Tehran, to New Delhi, to Bangkok,
to Hong Kong, to Tokyo, to Honolulu, to Los Angeles, to Dallas, and then back to Washington.

The black arrows show the 13 flights they took when they flew westward from Washington to Los Angeles, to Honolulu,
to Guam, to Okinawa, to Taipei, to Hong Kong, to Bangkok, to Bombay, to Tel Aviv, to Athens, to Rome, the Paris,
to Shannon Ireland, to Boston, and then back to Washington.

                                                          and Keating
                                                          flight routes

A quote from the Hafele paper:
In 1905 Einstein laid a radical new basis for the concepts of space and time. Though Newton's absolute time had proved adequate for most practical purposes, Einstein produced convincing arguments against it. Absolute time contains an element of mystery which is incompatible with precisely defined scientific quantities. Consequently, Einstein defined a new empirical basis for time by accepting a definition which states, in effect, that "time is that which is indicated by a clock," and then proceeded to develop his relativity theories on that basis. Einstein's relativity has proved to be completely compatible with all relevant observations; in fact, no definitive test ever performed has disproved it. The results of our flying clock experiments, at least at the present state of analysis, offer no exceptions.

2. NIST Optical Clocks and Relativity

The NIST Optical Clocks and Relativity experiment only involved Gravitational Time Dilation.  They lifted one aluminum ion atomic clock approximately 1 foot higher than another atomic clock and they were able to measure the fact that the higher clock ran faster than the lower clock.  It did so at the rate anticipated based upon Einstein's equations.

The NIST scientific paper: Optical Clocks and Relativity

Another consequence of Einstein’s theory is that clocks run more slowly near massive objects. In the range of speeds and length scales encountered in our daily life, relativistic effects are extremely small. For example, if two identical clocks are separated vertically by 1 km near the surface of Earth, the higher clock emits about three more second-ticks than the lower one in a million years.

An NIST article about the experiment: NIST Pair of Aluminum Atomic Clocks Reveal Einstein's Relativity at a Personal Scale

Scientists have known for decades that time passes faster at higher elevations—a curious aspect of Einstein's theories of relativity that previously has been measured by comparing clocks on the Earth's surface and a high-flying rocket.

Now, physicists at the National Institute of Standards and Technology (NIST) have measured this effect at a more down-to-earth scale of 33 centimeters, or about 1 foot, demonstrating, for instance, that you age faster when you stand a couple of steps higher on a staircase.

comparing clocks

3. Geodesy and Metrology experiment (measuring altitude by time difference)

The Geodesy and Metrology experiment performed in 2018 only measured Gravitational Time Dilation.

The Nature article: Geodesy and Metrology with a transportable optical clock
Arxiv.org version: Geodesy and metrology with a transportable optical clock

Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors.
Newsweek article: Portable Atomic Clocks can Measure the Height of a Mountain
“Time passes with different speeds depending how far you are away from large masses,” Christian Lisdat, co-author of the study and physicist of Germany’s National Meteorology Institute, told Newsweek. That large mass is Earth.

“Time really changes,” he added, explaining that “if you go up, time passes more quickly.” The gravity further above sea level at the top of a mountain is weaker, so time literally moves slightly faster. The difference, however, is tiny.

The Washington Post: Scientists take an atomic clock on the road and use it to measure the height of a mountain

Scientists have described a major step forward in using time to determine height above sea level. For the first time, they took an optical atomic clock out of the lab. Their liberated device was brought into the French Alps.

By comparing the tick rate of the portable atomic clock on a mountain with a similar clock in a lab in Torino, Italy, the researchers showed that the altitude difference between the two locations was about 1,000 meters, or 3,280 feet. Their work was published in Nature Physics.

The Los Angeles Times: Scientists take an atomic clock on the road and use it to measure the height of a mountain

According to Einstein's theory of relativity, time moves differently depending on where you are in a gravity field.

For example, a clock on top of a tall mountain — far from the center of the Earth — will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger.

It's not a mechanical error. Time itself actually passes faster at the top of the mountain.

That means your friend who lives in the Rockies is aging just a tiny bit faster than your friend who lives on the beach in Malibu.

"Your body and your biological experience exist in the real time of whatever place you are in," said Christian Lisdat, a physicist at Germany's National Metrology Institute who worked on the study. "And that is no different than clocks."

Most clocks aren't accurate enough to register the difference in the speed of time at different altitudes. After all, in 10 years, two clocks that are 1,000 meters apart from each other in height will be off by just 31-millionths of a second, Agnew said.

4. Muon experiments

The muon experiments only measure Velocity Time Dilation.  Essentially, they prove that a fast moving particle lives longer than a slow moving particle.  Time ticks slower for the fast moving particle, so a human observer sees it live longer.   Muons are created when cosmic ray photons from distant sources hit atoms in the upper atmosphere.  A muon quickly decays and should not exist long enough to reach the surface of the earth.  But they do, because of time dilation.         
Web page: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muon.html
Wikipedia: https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation

5.  University of Maryland

University of Maryland experiments performed between September 1975 and January 1976 involved putting three atomic clocks aboard a slow-moving aircraft and flying them around in circles for 15 hours at an altitude of 10 kilometers (32,808 feet).  The time difference was measured by direct clock comparison at the ground before and after the flight, as well as during the flight by laser pulses of 0.1 ns duration.

Article: The Maryland Experiment

The paper: Introduction to some fundamental concepts of General Relativity and to their required use in some modern timekeeping systems
The Maryland Experiment is described in detail on pages 708 to 724.

The concept of proper time in relativity is really central to the whole subject. The proper time is the ordinary time actually kept by a clock, its own time, or, in German, eigenzeit. The high stability that has been achieved by the time keeping community with modern atomic clocks allows the effects of motion and gravity to be actually measured, with results in agreement with Einstein's predictions. Einstein's ideas are no longer just a matter of great scientific interest, actually forming the basis of the view of the universe that we now have from modern astronomy, but also a matter of practical engineering concern,
Quote from page 703:
A clock will run faster the higher it is, and it will run slower the faster it moves.  The primary curvature for slow speeds and weak gravitational fields is the curvature of time, not the curvature of space, as you read in so many of the popular books.

Part of the above paper: Relativity and Clocks

Another paper: Proper Time Experiments in Gravitational Fields with Atomic Clocks, Aircraft, and Laser Light Pulses
(I haven't been able to find a free on-line copy of it.)

6.  Japanese Mitaka to Norikura

Between 1975 and 1977, Japanese scientists carried a commercial cesium clock back and forth from the National Astronomical Observatory of Japan in Mitaka, at 58 m (190 ft) above sea level, to Norikura corona station, at 2,876 m (9,436 ft) above sea level.

Article: The Detection of Gravitational Red Shift by Transportation of Atomic Clocks

According to the theory of general relativity, the gravitational red shift causes a clock placed at a lower altitude, where gravity is strong, to run more slowly than a clock placed at a higher altitude, where gravity is weaker.

Article: Relativisitic Effects in Time and Frequency Standards

It can be shown that as the gravitational potential decreases - that is, as its absolute value increases with respect to the infinitely remote zero potential point of gravity in Equation (11) - time passes more slowly.  This phenomenon is known as the gravitational red shift.

7.  Briatore and Leschiutta

In 1976, Briatore and Leschiutta compared the rates of two cesium clocks, one in Turin 250 m (820 ft) above sea level, the other at Plateau Rosa 3,500 m (11,500 ft) above sea level.

The paper: Evidence for the Earth Gravitational Shift by Direct Atomic-Time-Scale Comparison

Quote from page 220:
Now we present the results of our measurements of the altitude effect carried out on the basis of the height difference between the CNR cosmic-ray laboratory at Plateau Rosa, 3500 m, and Turin, 250 m above s.l., for which the equivalence principle predicts the relative time variation -- AU/ez~ At]t = 3.54.10 -18, corresponding to the gain Attl = 30.6 ns/d of the clock at mountain altitudes.

8.  National Physical Laboratory - 1996

In 1996, the National Physical Laboratory repeated the Hafele-Keating experiment by flying back and forth between London and Washington D.C.

Article: Demonstrating Relativity by Flying Atomic Clocks

On return to NPL the travelling clock was predicted to have gained 39.8 ns, including an additional geometric factor. This compared remarkably well with a measured gain of 39.0 ns. We estimated the uncertainty due to clock instabilities and noise to be around ±2 ns. This short flying clock experiment therefore provided a clear demonstration of relativistic effects.
The combined flight times of 14 hours and mean height in excess of 10 km resulted in a predicted clock gain of 53 ns. This followed the principle that a clock in a weaker gravitational field (higher altitude) will run faster.

The effect of the aircraft’s speed relative to the Earth’s surface resulted in a predicted clock loss of 16.1 ns.  This followed the principle that a moving clock runs slow.

9.  Van Baak - 2005

In 2005, van Baak measured the gravitational time dilation of a weekend at the top of Mt. Rainier using two ensembles of three HP 5071A cesium beam clocks.

Article: Clocks, Kids, and General Relativity on Mt Rainier


According to Einstein, fast-moving clocks run slow (special relativity), and high-elevation clocks run fast (general relativity). Clocks that run fast gain time, so given our high elevation and how long we stayed, the prediction was that these clocks would gain about 22 nanoseconds. This, not because the clocks were moving (they were in a parked minivan), but simply because the clocks experienced a lower gravitational field by being 5400 feet above sea level for two days.

Of course, the predicted effect is incredibly small, but with clocks accurate enough, elevations high enough, the stay long enough, and time interval counters precise enough, the effect becomes measurable. The goal of this fun experiment was to measure, or at least to demonstrate, relativistic time dilation using equipment I had at home.

10.  National Physical Laboratory - 2010

In June 2010, the National Physical Laboratory again repeated the Hafele-Keating experiment, this time around the globe (London - Los Angeles - Auckland - Hong Kong - London).

Article: http://www.npl.co.uk/news/time-flies

In June 2010 one of NPL's atomic clocks was flown [westward] around the world as part of a rare experiment to test Einstein's theories of Relativity. The results demonstrate that Einstein's theories are correct, as NPL was able to measure a clear time-shift of 230 ± 20 nanoseconds between the two clocks involved in the experiment. This agrees with the time-shift predicted by Einstein.
General Relativity effects are caused by the altitude of the flying clock - space time near the surface of the Earth is more steeply curved than at the height of the aircraft, so the airborne clock (and everything else on the aircraft) is travelling through space-time that is slightly less 'stretched' than it is at the Earth's surface. This stretching of space-time is what makes time run slower on the ground relative to on the aircraft.

11. Van Baak - 2016

In 2016, van Baak repeated his experiment on Mt. Lemmon for the television show Genius by Stephen Hawking.

Article: Project GREAT 2016a -- Hawking, Einstein, and Time Dilation on Mt Lemmon

Conceptually, the experiment is very simple. We take one accurate clock the top of the mountain and we take another accurate clock to a hotel at the base of the mountain and let them sit there for a day. Then we bring the clocks together again and compare. If time dilation is false then the clocks should still agree. If time dilation is real then we would expect the clock that was at the hotel to be a little behind the clock that was at the summit.
gravitational time dilation

atomic clocks in a car

12. Tokyo Skytree - 2020

Scientists use the Tokyo Skytree to test Einstein’s theory of general relativity

In another verification of the validity of Einstein’s theory of general relativity, published in Nature Photonics, scientists from
the RIKEN Center for Advanced Photonics and Cluster for Pioneering Research, with colleagues, have used two finely tuned
optical lattice clocks, one at the base and one on the 450-meter observatory floor of Tokyo Skytree, to make new ultraprecise
measurements of the time dilation effect predicted by Einstein’s theory of general relativity.
Link: https://www.riken.jp/en/news_pubs/research_news/pr/2020/20200407_2/index.html

Another article about it:

An excellent YouTube video about it:


The actual article is titled "Test of general relativity by a pair of
transportable optical lattice clocks"
It's at his link:  https://www.nature.com/articles/s41566-020-0619-8


13. Ives-Stillwell

Their 1938 paper: http://www.conspiracyoflight.com/Ives/HerbertIves1938a.pdf
Their 1941 paper: http://www.conspiracyoflight.com/Ives/Herbert_Ives-Rate_of_a_Moving_Clock_II.pdf
Publisher site: https://www.osapublishing.org/josa/abstract.cfm?uri=josa-28-7-215
Wikipedia: https://en.wikipedia.org/wiki/Ives%E2%80%93Stilwell_experiment

Article: Ives–Stilwell Experiment Fundamentally Flawed