In October 1971, at an airport gate in Washington, a physicist named Joe Hafele boarded a commercial flight with a colleague, some luggage, and four atomic clocks strapped into their own paid seats. The tickets, the story goes, listed the passenger as Mr. Clock.

I found this story in January, the month I always end up thinking about time anyway, and I have been telling it to everyone who will sit still. Because it might be the best road trip in the history of science.

Hafele and the astronomer Richard Keating flew those cesium clocks all the way around the world, eastward, airport by airport, the clocks humming in their seats like well-behaved children. Then they did the whole loop again westward. Back in Washington, they walked their travelers into the Naval Observatory and compared them with the identical clocks that had stayed home.

The clocks disagreed.

Not by much, by nanoseconds. But exactly, and I mean bullseye exactly, by the amounts Einstein's equations had predicted half a century earlier, sight unseen. The clocks that flew had ticked at a different rate than the clocks that stayed. Nothing was broken. Nothing was miscounted. Time itself had passed differently for the travelers, because that is what time does. Motion changes its flow. Gravity changes its flow. The seats near the window lived a slightly different length of October than the desk in Washington did.

If your instinct says fine, lab curiosity, nanoseconds, who cares, then look at your phone. The GPS satellites over your head carry atomic clocks that run measurably fast relative to yours, gravity being thinner up there, and engineers correct for Einstein constantly, because without the correction your little blue dot would drift off the map within minutes. Every time you have ever navigated to a beach bar, relativity was quietly doing the math.

But here is the part of the story that I carried out of January like a lamp. When the clocks came home disagreeing, nobody asked which one was wrong. That question has no answer, and Einstein is the reason. He went looking for the master clock, the one true time by which all others could be judged, and proved that the universe does not contain one. Each clock measures the path it actually traveled, honestly, correctly, to the last decimal. The traveler's time is real. The homebody's time is real. The universe holds both and does not referee.

I walked the sea road thinking about the people I know who need Mr. Clock's ticket stub framed on their wall. The friend whose grief is taking years longer than the people around her think it should. The musician four years deep in an album while his classmates buy houses. Everyone doing the slow inner retraining this notebook keeps returning to, at a pace that looks wrong from every window except their own. Behind, ahead, too slow, too late, all of those words secretly assume a master clock. And the master clock is the one thing physics has formally abolished.

Your clock has been measuring your actual route, through your actual weather, your actual gravities. Heavy years bend time, literally, Einstein showed that too. Nothing was lost in yours. It was all, in the strictest scientific sense, your own true time.

A moving clock runs slow, and both clocks are right. Two things can differ and neither one is broken.

Somewhere a cesium atom is vibrating nine billion times a second in a museum drawer, having once flown around the world in its own seat, just to prove that to you.