Albert Einstein published two papers between 1905 and 1915 theorizing about the relationships between space, time, mass and energy. His two theories were called general relativity and special relativity. In a nutshell, he theorized that time slows down if you move faster, and slows down when you are closer to Earth, where the gravitational pull is stronger. To prove Einstein’s theories, physicist Joseph C. Hafele and astronomer Richard E. Keating set up a simple experiment with commercial airliners in the 1970s. They booked flights that circled the world east, then west. Their idea was to use the speed of airliners over a great distance to determine whether time was slowing down or speeding up, consistent with what Einstein had theorized. When the two men concluded the experiment, Einstein was proven right.
In order to measure time as accurately as possible, they needed a type of clock that could measure at a much more precise level than normal clocks, but perfect timekeeping is slightly more complicated than one might expect. The only type of clock that was suitable was an atomic clock, and Hafele and Keating took four of them for the flights. Another atomic clock remaining at the United States Naval Observatory (USNO) served as a control and would be compared to the readings of the four clocks that accompanied the two men.
The results that proved Einstein’s theories right
The clocks were expected to lose about 40 ± 23 nanoseconds on the eastward flights and gain 275 ± 21 nanoseconds on the westward journey, if Einstein’s theories were correct. This is slightly counterintuitive, as both trips allowed the planes to fly above the ground at similar speeds. This is because speed only matters in relation to space, rather than in relation to the ground. Since the Earth is already spinning at about 1,040 mph, this means it acts like a giant conveyor belt for the plane, which ends up moving much faster relative to space than the plane traveling west in the opposite direction.
After traveling east around the world, time records were taken and compared to the control clock remaining at the USNO. The clocks that accompanied the men lost 59 nanoseconds on the journey east. The two men then headed west again and returned to perform the same control comparisons. What they found was that the clocks gained 273 nanoseconds during the journey west. Although there were variations in the exact amount of time lost or gained compared to what was expected, the results proved that Einstein was right.
How do atomic clocks work?
The experiment conducted by Hafele and Keating relied heavily on the accuracy of the clocks they used to measure time differences. They used atomic clocks because of their precision, but how does it actually work? Imagine a pendulum and how each swing takes the same amount of time. By counting the number of oscillations, you are effectively counting time, and this is essentially how normal clocks work. The problem with normal clocks is that they are subject to slight drifts in accuracy caused by temperature differences, wear and manufacturing defects. This is where atoms are a much better measure of time. Even NASA relies heavily on atomic clocks for all of its space missions because of their accuracy.
Each cesium atom is identical and by exposing them to microwave radiation, the electrons orbiting these atoms will switch between two energy states. This change occurs at its maximum rate when the microwave frequency is slightly above 9 billion hertz. Atomic clocks use this transition as a reference and adjust the frequency of the microwave oscillator until the number of atoms changing state peaks. In a cesium clock, the microwave oscillator plays the same role as a pendulum or quartz crystal in more traditional clocks. Modern atomic clocks are so precise and consistent that if they had been running since the Big Bang 13.8 billion years ago, they would have drifted by less than a second.
