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4:40 pm
Thu August 22, 2013

The World's Most Precise Clock Could Prove Einstein Wrong

Originally published on Thu August 22, 2013 6:00 pm

What a makes a good clock? Andrew Ludlow, a physicist at the National Institute of Standards and Technology, says one of the most important criteria is stability.

"If you could imagine a grandfather clock and see the pendulum swinging back and forth, ideally that pendulum would swing back and forth very uniformly," Ludlow says. "Each swing would take exactly the same amount of time."

That's stability. But what if something perturbs the system, like a mischievous toddler?

"Imagine that toddler shaking the grandfather clock itself — that oscillation period could vary quite a bit," Ludlow says. "How much that ticking rate varies determines the precision with which you can measure the evolution of time."

Ludlow is a clockmaker, but his clocks don't have pendulums or gears. They are atomic clocks that rely on what Ludlow calls "the natural internal ticking of the atom."

Every atom of a given element has its own characteristic resonant frequency. The speed of that vibration is very consistent and very fast — there are quadrillions of "ticks" every second. Atomic-clock makers use the regularity of these vibrations to keep time with extreme accuracy.

Toddlers can't mess up these clocks, but there's still a little instability. Atoms move around, and that makes their vibrations slightly harder to measure. So Ludlow and his team used a lattice of lasers to trap the atoms and then cool them down. With the atoms frozen in place, the scientists could more accurately measure their vibration.

Ludlow's clock is 10 times more accurate than the last model. It's the most precise atomic clock ever built.

"Obviously getting to a meeting on time doesn't require this type of precision," Ludlow says. "But believe it or not, there's a number of both scientific and technical applications."

Better atomic clocks will facilitate more precise GPS and faster telecommunication networks. And some physicists are excited about another application: testing Einstein's Theory of Relativity.

"Today many scientists believe that the theory of relativity is incompatible with other physical theories," Ludlow says.

Einstein predicted that certain physical properties, like the strength of the interaction between photons and electrons, or the ratio of the mass of electrons and protons, should never change. But competing theories say that those "fundamental constants" might actually fluctuate and such changes would slightly influence the ticking speed of atomic clocks.

"As clocks become better and better, they become more and more useful tools to explore this possible variation," Ludlow says.

Einstein also predicted that clocks in different gravitational fields would tick at different speeds. For example, a clock in Boulder, Colo., which is a mile above sea level, would feel a slightly weaker gravitational pull than a clock at sea level in Washington, D.C. As a result, it would tick just a bit faster — and after 200,000 years it would be a full second ahead.

That's not much of an effect, but it's big enough for most atomic clocks to measure. And Ludlow's clock can register the change in gravity across a single inch of elevation. That kind of sensitivity will allow scientists to test Einstein's theories with greater precision in the real world.

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

AUDIE CORNISH, HOST:

Scientists are always looking for a more accurate answer to the question: What time is it? A new atomic clock described in this week's Science magazine splits the second into quadrillions of pieces. But this ultra precise clock isn't just for telling time.

As NPR's Adam Cole reports, it could prove Einstein wrong.

ADAM COLE, BYLINE: What a makes a good clock? Andrew Ludlow, a physicist at the National Institute of Standards and Technology, says one of the most important criteria is stability.

ANDREW LUDLOW: If you could imagine a grandfather clock, you know, ideally that pendulum would swing back and forth very uniformly in a tick, tick, tick...

(SOUNDBITE OF TICKING)

COLE: That's stability. But what if you introduce some agent of chaos, like a mischievous toddler?

LUDLOW: Imagine that toddler shaking the grandfather clock itself. Tick, tick-tick, tick...

(SOUNDBITE OF TICKING)

LUDLOW: How much that ticking rate varies determines the precision with which you can measure the evolution of time.

COLE: Ludlow is a clockmaker but his clocks don't rely on pendulums.

LUDLOW: The atomic clock is looking at the very natural internal ticking of the atom.

COLE: Let's say you hit a cesium atom with a laser, it will vibrate with a certain frequency. Those ticks are very consistent and very fast - there are quadrillions every second. Atomic clock-makers use the regularity of these vibrations to keep time with extreme, extreme accuracy.

Toddlers can't mess up these clocks but there's still a little instability. Atoms move around and that makes their vibrations slightly harder to measure. So Ludlow and his team used a lattice of lasers to trap the atoms and then cool them - nearly reaching absolute zero. With the atoms frozen in place, scientists can more accurately measure their vibration.

Ludlow's clock builds on previous designs and it doesn't look anything like grandfather clock. It looks more a table at a mad scientist's garage sale. But the mass of tubes and wires is the most precise time-keeper ever built.

LUDLOW: Obviously getting to a meeting on time doesn't require this type of precision.

COLE: But lots of technology does. The new clock has the potential to improve GPS and telecommunication networks. And some physicists are excited about a different application: testing the Einstein's Theory of Relativity.

LUDLOW: Today many scientists believe that the Theory of Relativity is incompatible with other physical theories.

COLE: Einstein predicted that certain physical properties - like the strength of the interaction between photons and electrons, or the ratio of the mass of electrons and protons - should never change. But competing theories say that those fundamental constants might actually fluctuate. And such changes would slightly change the ticking speed of atomic clocks.

LUDLOW: And as clocks become better and better, they become more and more useful tools to explore this possible variation.

COLE: Einstein also predicted that clocks in different gravitational fields would tick at different speeds. Take two identical clocks.

LUDLOW: Let's have one of them be up here in Boulder, Colorado, where we're at a relatively high elevation.

COLE: A mile above sea level, where gravity is just a tiny bit weaker. And the other clock will be here with me in Washington, D.C.

LUDLOW: Down around sea level.

COLE: My clock will tick slightly slower. And after 200,000 years, Ludlow's clock would be a second faster than mine.

LUDLOW: So again, not a huge effect.

COLE: But it's big enough for most atomic clocks to measure. And Ludlow's clock can register the change in gravity across a single inch of elevation. That kind of sensitivity will allow scientists to test Einstein's theories with greater precision in the real world.

Adam Cole, NPR News. Transcript provided by NPR, Copyright NPR.