Physics

Moving clocks tick slower

Moving clocks run slower because, under special relativity, time itself passes more slowly for anything in motion relative to you, a real difference, not an illusion or a signal delay. In 1971, atomic clocks flown around the world on airliners came back 59 nanoseconds behind clocks that stayed on the ground, matching Einstein's prediction.

Sources & further reading

HyperPhysics, Georgia State UniversityMuon Experiment in RelativityA university physics page explaining how time dilation lets short lived muons reach Earth's surface.American Journal of Physics, American Association of Physics TeachersMeasurement of the Relativistic Time Dilation Using Mu-MesonsThe classic 1963 experiment that directly measured time dilation using mountain top and sea level muon counts.Science, American Association for the Advancement of ScienceAround-the-World Atomic Clocks: Observed Relativistic Time GainsThe 1971 experiment that flew atomic clocks around the world on airliners to directly measure relativistic time differences.National Institute of Standards and TechnologyPutting Einstein to the TestA U.S. government explainer on why GPS satellite clocks are engineered around measured relativistic effects.NatureMeasurements of relativistic time dilatation for positive and negative muons in a circular orbitA CERN particle accelerator experiment confirming time dilation to high precision using circulating muons.Space.comWhy Astronaut Scott Kelly Aged Slightly Less Than His Twin BrotherA science news explainer on how orbital speed left astronaut Scott Kelly measurably younger than his Earth bound twin.