13. The Hafele-Keating Experiment
In 1971, physicists Hafele and Keating tested Einstein’s predictions by flying atomic clocks around the world on commercial airplanes. When compared with clocks left on the ground, the airborne clocks showed measurable time differences—just as relativity predicted. This groundbreaking experiment provided concrete, real-world evidence that motion and gravity can truly alter the passage of time. Details on Wikipedia.