In 1915, Albert Einstein published his General Theory of Relativity, which revealed something extraordinary: time does not flow at the same rate everywhere. Gravity warps the fabric of space-time itself — and the stronger gravity's pull, the slower time passes.
Your feet are slightly closer to Earth's center of mass than your head. This means gravity pulls on your feet just a tiny bit harder. As a result, your feet experience time passing fractionally slower than your head — every single second of every day.
Where g is gravitational acceleration (9.8 m/s²), h is height difference, c is the speed of light, and t is elapsed time. The difference is tiny — but it is absolutely real and has been confirmed by atomic clocks.
In 2010, physicists at NIST used optical atomic clocks so precise they confirmed this effect across just 33 centimeters of height difference. This app shows you your personal piece of that experiment, happening right now.
The higher you are above sea level, the weaker Earth's gravitational pull — so time actually moves faster at altitude. GPS satellites must constantly correct their onboard clocks by 38 microseconds per day or navigation would drift miles off course.