Hafele

The Hafele experiment, conducted in 1971 by physicist Joseph Hafele, demonstrated the effects of Einstein’s theory of relativity on time. Researchers flew atomic clocks aboard airplanes around the world and compared them to identical clocks kept on the ground. Due to the effects of both special and general relativity, the clocks aboard the planes experienced slightly different time flows—some running faster, others slower—based on their speeds and the Earth's gravity. This experiment provided real-world proof that time is relative, confirming scientific predictions about how motion and gravity influence the passage of time.