THIS is truly a fascinating article written so all can understand. From Space Daily.....
A 1971 experiment flew four atomic clocks around the world on commercial airliners — first heading east, then heading west — and when the clocks were brought home and compared with stationary clocks at the U.S. Naval Observatory, they were measurably out of sync, in the first direct demonstration that time itself moves at slightly different rates depending on how fast you are traveling, exactly as Einstein had predicted half a century earlier
The experiment began with a back-of-the-envelope calculation. According to the Wikipedia reference on the Hafele-Keating experiment, Joseph C. Hafele, a physicist at Washington University in St. Louis, was preparing notes for a physics lecture in 1969 when he worked out that an atomic clock placed on a commercial airliner should be precise enough to detect the relativistic time dilation effects that Albert Einstein had predicted more than half a century earlier. Hafele spent the following year unsuccessfully trying to obtain funding for the experiment, until he gave a talk on the topic in 1970 and was approached afterward by Richard E. Keating, an astronomer at the US Naval Observatory in Washington who worked with atomic clocks professionally. Together, Hafele and Keating obtained $8,000 in funding from the Office of Naval Research — one of the cheapest tests of general relativity ever conducted — and arranged for one of the most famous experiments in 20th-century physics.
Of the $8,000 budget, $7,600 was spent on round-the-world airline tickets. The two men needed seats for themselves and seats for their instruments: four HP 5061A cesium-beam atomic clocks, each about the size of a large suitcase. The clocks needed their own seats because they were too large and too sensitive to be stowed in cargo. The ticket booking forms accordingly listed “Mr. Clock” as the passenger in two seats on each flight. On 4 October 1971, Hafele, Keating, and four atomic clocks boarded commercial flights heading east from Washington and began an eastward circumnavigation of the Earth. The trip lasted 65.4 hours of which 41.2 hours were spent actually in flight. The clocks were returned to the Naval Observatory and compared with the stationary reference clocks. They were then flown again, this time westward, from 13 to 17 October 1971, in a journey that lasted 80.3 hours. After this second trip, they were returned to the Naval Observatory and compared a second time.
What the clocks showed
The published results in Science, in July 1972, were striking. According to the original Hafele-Keating prediction paper, special relativity and general relativity, taken together, predicted that the clocks should have lost approximately 40 nanoseconds during the eastward trip and gained approximately 275 nanoseconds during the westward trip, relative to the stationary clocks at the Naval Observatory. The actual measurements showed losses of 59 nanoseconds on the eastward leg and gains of 273 nanoseconds on the westward leg. The eastward result agreed with the prediction within experimental uncertainty; the westward result agreed almost exactly. The clocks that had travelled around the world had run at measurably different rates than the clocks that had stayed in Washington, and the differences were of the magnitude and direction that Einstein’s theory predicted.
The asymmetry between the eastward and westward results — losing 59 nanoseconds in one direction and gaining 273 in the other — is one of the more counterintuitive aspects of the experiment. Both flights covered roughly the same distance at roughly the same speed at roughly the same altitude. If only the velocity of the aircraft mattered, the two flights should have produced identical effects. But the relevant velocity for relativistic calculations is not the velocity of the plane relative to the ground; it is the velocity of the plane relative to a non-rotating reference frame centred on the Earth. The Earth itself rotates eastward at approximately 1,600 kilometres per hour at the equator. An aircraft flying east adds its velocity to the Earth’s rotational velocity, producing a larger total velocity in the non-rotating frame. An aircraft flying west subtracts, producing a smaller velocity. The eastward clocks therefore experienced larger time dilation due to special relativity than the westward clocks, and this effect was what produced the directional asymmetry in the measurements.
Two effects, not one
The Hafele-Keating experiment did not test only special relativity. It tested both special and general relativity simultaneously, because each of the two theories predicts a different time-dilation effect, and the experimental setup made both effects observable. Special relativity, the 1905 theory, predicts that clocks moving faster relative to an inertial reference frame run slower than clocks at rest. This is the velocity-based effect responsible for the directional asymmetry. General relativity, the 1915 theory, predicts that clocks at higher gravitational potential — farther from a massive object — run faster than clocks at lower potential. Aircraft at 35,000 feet are slightly farther from the centre of the Earth than ground-based clocks, and therefore should run slightly faster.
The two effects partially cancel each other and partially add, depending on direction. According to the HyperPhysics reference on the experiment, on the eastward flight, the special relativity effect dominated: the clocks ran slower because they were moving fastest in the non-rotating frame. On the westward flight, the general relativity effect dominated: the gravitational speed-up from being at altitude was larger than the velocity-based slowdown, because the westward velocity in the non-rotating frame was relatively small. The combined predictions of both theories matched the observed results to within about 10 percent on the eastward trip and within about 3 percent on the westward trip — confirming both Einstein’s 1905 theory and his 1915 theory in a single experimental run with macroscopic, manufactured devices that could be inspected before and after by anyone.
Why this mattered
Relativity had been experimentally confirmed before 1971. The Ives-Stilwell experiment in 1938 had tested special relativistic time dilation using hydrogen ion beams. The Rossi-Hall experiment in 1941 had confirmed the predicted lifetimes of cosmic-ray muons. Eclipses observed in 1919 by Arthur Eddington had confirmed general relativity’s prediction of light deflection by gravity. By the late 1960s, the basic correctness of both theories was accepted in mainstream physics. But all of the confirmations to date had been either indirect, requiring particle accelerators or astronomical observations, or extremely small-scale, involving individual particles rather than ordinary objects. The Hafele-Keating experiment was the first time that macroscopic, manufactured devices — clocks of the kind that any laboratory could in principle build and operate — had been demonstrated to run at measurably different rates under conditions consistent with relativistic predictions.
The practical implications of Hafele-Keating became apparent two decades later when the Global Positioning System was deployed for civilian use. According to a 2021 commemorative document published by the US Naval Observatory on the 50th anniversary of the experiment, GPS satellites carry onboard atomic clocks that must account for both special and general relativistic effects to remain synchronised with ground-based reference clocks. The satellites’ clocks run approximately 7 microseconds per day slow due to special relativity (because they are moving at roughly 14,000 kilometres per hour relative to ground stations) and approximately 45 microseconds per day fast due to general relativity (because they are 20,000 kilometres above the Earth’s surface, in lower gravitational potential). The net effect is approximately 38 microseconds per day faster than ground clocks. Without correcting for both effects, GPS positions would drift by approximately 10 kilometres per day, and the system would be useless for navigation within hours of being switched on.
The experiment that fit on an airline ticket
The Hafele-Keating experiment remains a kind of touchstone in the experimental physics community, for reasons that go beyond its specific scientific contribution. It demonstrated that one of the most fundamental and counterintuitive predictions in physics — that time itself runs at different rates depending on motion and gravity — could be confirmed using off-the-shelf commercial technology, on regularly scheduled airline flights, for less than the cost of a single graduate-student stipend. The atomic clocks involved were standard laboratory equipment of the era. The flights were ordinary commercial round-the-world routes. The reference clocks were already operating at the Naval Observatory. The whole experiment was, in a sense, an exercise in seeing what was already possible if someone bothered to look.
Joseph Hafele eventually left academic physics and spent the latter part of his career teaching at a private school. Richard Keating remained at the Naval Observatory for the rest of his working life. Their joint experiment has been reproduced many times since 1971, with progressively higher-precision atomic clocks and progressively more demanding experimental conditions. The basic result has been confirmed every time. Time runs at different rates depending on how fast you are travelling and where you are sitting in a gravitational field. The effects are tiny by the standards of human experience — nanoseconds per round-the-world trip — but they are large enough to detect, large enough to matter for satellite navigation, and large enough to confirm, with macroscopic instruments built by human beings, that Einstein’s account of how time works is correct in the literal physical sense.
