How was gravitational time dilation proven on a human scale?

In 2010, a NIST team used two ultra-precise atomic clocks separated by about 33 centimeters of height, and the raised clock ran faster, exactly as relativity predicts. In 2022, JILA physicists measured the effect across a single millimeter.

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Do Your Feet Really Age Slower Than Your Head?

Q: Do your feet really age slower than your head?

Yes. Gravity slows time, so a clock closer to Earth's center ticks slightly slower. Your feet sit closer to the planet than your head, so they age very slightly slower — about 90 billionths of a second over a 79-year lifetime, per foot of height.

Q: Does gravitational time dilation matter in everyday life?

You will never feel it, but GPS satellites have to correct for this exact effect every day, or their positions would drift off by miles.

Yes. Einstein's relativity says gravity slows time, and in 2010 physicists at NIST proved it on a human scale: two atomic clocks placed just one foot apart in height ticked at measurably different speeds — the lower one slower.

Quick facts

The effect: gravitational time dilation — time runs slower where gravity is stronger
The proof: NIST atomic clocks separated by about 33 centimeters (roughly one foot) of height, 2010
Follow-up: in 2022, JILA physicists measured the effect across a single millimeter
Size of the gap: about 90 billionths of a second over a 79-year lifetime, per foot of height
Practical impact on you: none you will ever feel — but it is happening right now

Einstein's strange prediction

General relativity says time is not a fixed river flowing the same everywhere. Mass bends space and time, so a clock deeper in a gravity well — closer to Earth's center — ticks a tiny bit slower than one farther away. Your feet sit closer to the planet than your head does. Which means, strictly and measurably, your feet are living very slightly in your head's past.

For most of a century this could only be tested with rockets and airplanes. Atomic clocks changed that.

The 2010 proof

In 2010, a NIST team led by James Chin-Wen Chou used two of the world's most precise atomic clocks — so stable they'd lose about a second in billions of years. They raised one clock just 33 centimeters above the other, and the raised clock ran faster, exactly as relativity predicts. In 2022, JILA researchers shrank the experiment further, detecting the tick-rate difference across one millimeter inside a single cloud of atoms.

So how much younger are your feet?

About 90 billionths of a second over an entire 79-year lifetime, for each foot of height difference. Nobody will ever notice it, no cream can sell it back to you, and your feet certainly don't feel younger. But "too small to matter" is not the same as "not real" — GPS satellites have to correct for this exact effect every day, or their positions would drift off by miles.

Watch it

See the short version in the LOE PRESSURE "Things You Didn't Know" series — "Your Feet Age Slower Than Your Head."

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Keep learning

→ Does your brain really run on 20 watts?

→ Does your body really glow in the dark?

Fascinating facts you didn't know. Come learn with me — no pressure.

Sources: NIST, "JILA Atomic Clocks Measure Einstein's General Relativity at Millimeter Scale" (2022) — https://www.nist.gov/news-events/news/2022/02/jila-atomic-clocks-measure-einsteins-general-relativity-millimeter-scale; Popular Science (2010), "Superaccurate Atomic Clocks Prove Your Head Ages Nanoseconds Faster than Your Feet" — https://www.popsci.com/science/article/2010-09/superaccurate-clocks-prove-your-head-older-your-feet/. Verified 2026-10-12 in week-facts-research.md.