Your head is older than your feet, and we have measured it

Time is not one thing that happens to all of us at once. It runs a little faster at your head than at your feet, and that is not a figure of speech.

A clock higher up, where Earth’s pull is a shade weaker, ticks faster than a clock lower down. Einstein worked this out a century ago, and for most of that century it stayed the stuff of black holes and fast rockets, far from anything you could feel. The startling part is recent: we can now watch it happen across the height of a single person.

In 2010, physicists at NIST, the U.S. national measurement lab, built two of the most precise clocks ever made, each keeping time on the vibration of a single aluminum ion, and raised one of them about a foot above the other. The higher clock ran faster. Not on a mountain, not in orbit. One foot, the rise of a single stair, was enough to pull time measurably out of step. They wrote up the result under a plain title: your head is older than your feet.

The effect is almost nothing. Over a whole human life, that foot of height adds up to roughly ninety billionths of a second. You will never feel it, never catch your scalp pulling ahead of your soles. But almost nothing is not nothing, and the universe keeps its books to the last decimal.

And sometimes it matters. The phone in your pocket leans on this. GPS satellites orbit high enough that their clocks gain about thirty-eight millionths of a second every day over clocks on the ground. If the engineers did not quietly subtract that drift, the maps we trust would be wrong by ten kilometers within a day. We correct for the bending of time every single time we find our way home.

Here is the part I keep turning over. There is no master clock. No single “now” sweeping through the universe, gathering all of us into the same instant. Time keeps its own rate in every place, at every height, at every speed, and each of us is always drifting on a current that is ours alone. We picture time as one river carrying everyone downstream together. It is really countless separate threads, no two exactly the same, and even you have never quite shared a moment with your own feet.