DAVID GREENE, host:

Most of us have, at some point, had the experience of time seeming to slow down - just ask a third-grader waiting for that final bell on the last day of school. But Einstein's theory of relativity says time really can slow down. And now, scientists have measured these time changes in our everyday lives. NPR's Joe Palca has our report.

JOE PALCA: Before we talk about the new results, we first have to talk about the nature of time. It's hard think of time as something that varies.

Dr. SEAN CARROLL (Theoretical Physicist, Caltech): It's just so counterintuitive to our everyday lives.

PALCA: Sean Carroll is a theoretical physicist at Caltech in Pasadena.

Dr. CARROLL: The miracle is that Einstein's way of thinking about it wasn't forced by some dramatic experiment that anyone did. It was really, just pure thought.

PALCA: The pure thought lead Einstein to the conclusion that light, not time, was the constant. It doesn't matter if you're standing still or traveling on a rocket ship, if you turn on a light and measure its speed, it will always be the same. But if that's true, it leads you to the inevitable conclusion that time has to be flexible.

Dr. CARROLL: And this is something that people have difficulty wrapping their minds around, because it doesn't show up in any obvious way in our everyday lives. It's a very, very subtle effect until you're flying close to a black hole, or moving close to the speed of light.

PALCA: To demonstrate this effect, I've asked my colleague David Greene to take a little trip on the NPR space ship. And unlike the clunkers that NASA flies, NPR's special space ship can travel close to the speed of light. I asked David to take a clock with him, and record a greeting when he was about 86 percent of the speed of light.

GREENE: Hi, Joe. How - you doing?

PALCA: That's how David would sound if I were standing next to him on the rocket ship. But if I were able to listen to him from here on Earth, this is how he would sound.

(Soundbite of ticking clock)

GREENE: Hi, Joe. How - you doing?

PALCA: To me, his time would have slowed down. That's what part of the theory of relativity predicts. But Einstein realized there was more to it.

Dr. CARROLL: He realized that not only does the amount of time you experience depend on how fast you move and how much you accelerate, it also depends on your gravitational field - how deeply you're imbedded in the gravitational field.

PALCA: The closer you are to a massive gravitational field like the Earth, the more the slowing of time. So time moves faster for someone sitting on the top of a flagpole compared to someone sitting on the ground, because the top of the flagpole is farther from the mass of the Earth and therefore, experiences less pull from gravity.

And now, finally, we're ready to talk about the new results. Using extremely accurate clocks, scientists at the National Institute of Standards and Technology in Boulder were able to measure these tiny changes in time caused by movement and gravity - not in imaginary space ships, but in their lab in Colorado.

Dr. CARROLL: What these guys were able to do was just measure that to absolutely unprecedented accuracy and precision. So, they're really driving home the fact that a clock moves faster on the second floor of your house than on the first floor.

PALCA: Physicist James Chin-wen Chou is one of those guys. As he reports in the journal Science, a clock moving at about 20 miles per hour ticks a tiny fraction of a second slower than a stationary clock - a really tiny fraction.

POST-BROADCAST CORRECTION: Dr. Chin-Wen Chou meant to say 15 zeros, and then some number after 15 zeros.

Dr. CHIN-WEN CHOU: Zero-point-zero and then there would be 17 zero, and then some number after the 17 zero.

PALCA: James Chin-wen Chou is as interested in designing extremely precise clocks as he is in relativity. His latest clock uses the vibrations of an aluminum atom missing an electron to measure time. And as Einstein's work suggests, getting time exactly right is trickier than you might think.

By the way, David Greene made it safely back to Earth. Thanks for helping out, David.

GREENE: It's a pleasure, Joe. Thanks for the trip.

PALCA: Joe Palca, NPR News, Washington.

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