One of the most mind-bending aspects of Albert Einstein's Theory of Relativity is time dilation. Time moves more slowly for a person in motion compared to a person at rest. This effect also applies in gravitational fields. Someone closer to a gravitationally-dense body like a black hole would be subject to a slowdown in time compared to others farther away. Mind you, life wouldn't proceed in 'slow motion' for these people. Everything would seem normal within their own frames of reference. The relative differences would only be noticed when everybody meets up.
In our own existence, we really don't have to concern ourselves with time dilation. Relatively speaking, humans move much too slowly to ever notice it. However, physicists finely tuned into the tinest ticks of time have observed dilation in a very human activity: international air travel.
Forty years ago this October, physicist Joseph C. Hafele and astronomer Richard E. Keating bought tickets for themselves as well as four highly-precise Hewlett-Packard atomic clocks to take two commercial airplane trips around the world, one heading east and one heading west. Their mission? Test Einstein's theory.
After the trips, they compared the times on the atomic clocks in the airplanes with the time of atomic clocks at the United States Naval Observatory. If relativity was correct, the clocks heading east should have been behind those on the ground, while the ones traveling west should have been ahead.
And that's what Hafele and Keating found! As astrophysicist Heino Falcke explained in his recent book Light in the Darkness:
The clocks that flew east – that is, with the Earth's rotation and with a slight difference in velocity relative to the clock on the ground – were 60 nanoseconds behind after the flight. The clocks on the flight traveling west – counter to the Earth's rotation and with a large difference in velocity relative to the clock on the ground – were a full 270 nanoseconds ahead of the clock in the laboratory.
Numerous additional tests conducted over the ensuing decades with different atomic clocks all confirmed the results seen on the original flight.
A fun takeaway: Yes, you are 'time-traveling' relative to individuals one the ground when you take long-distance flights, particularly on those traveling west. But you'll never notice. To add a mere second to your age relative to friends and family on the ground, you'd have to fly west around the globe 3.7 million times!