Hafele-Keating: Time on a Plane

In 1971, two physicists—Joseph C. Hafele and Richard E. Keating—set out to prove that Einstein's theories of relativity weren't just mathematical curiosities, but physical realities. They did this using four atomic clocks and a few commercial airline tickets.

The Method

The team flew the atomic clocks twice around the world on commercial flights—once eastward and once westward. They then compared the time on the traveling clocks to a reference clock that stayed at the U.S. Naval Observatory.

The Predictions

According to relativity, the clocks should have been affected by two things:

• Velocity (Special Relativity): The clocks moving relative to the Earth's surface should tick slower.
• Gravity (General Relativity): The clocks at high altitude (where gravity is weaker) should tick faster.

The Results

The experiment was a resounding success. When the clocks returned, they were out of sync with the ground clock by exactly the amount predicted by Einstein’s equations (within a margin of error of less than 10%). The eastward-flying clocks lost time, while the westward-flying clocks actually gained time relative to the ground.

Why It Matters

This was the first time that relativity had been measured using macroscopic objects (clocks) in a terrestrial environment. It proved that time is a malleable dimension that responds to speed and gravity in the "real world," not just in deep space.

Conclusion

The Hafele-Keating experiment took relativity out of the chalkboard and into the sky. On the Epoch Clock, we see a digital representation of time, but this experiment reminds us that every second is a journey through a warped and wonderful universe.