Keep­ing time just the begin­ning of what new atomic clock can do

It’s pre­cise enough to de­tect dark mat­ter.

The Times-Tribune - - HEALTH & SCIENCE - BY DEB­O­RAH NETBURN LOS AN­GE­LES TIMES (TNS)

Sci­en­tists have in­vented a new clock that keeps time more pre­cisely than any that have come be­fore.

The clock is so ac­cu­rate that it won’t gain or lose more than one sec­ond in 14 bil­lion years — roughly the age of the cos­mos. Its tick­ing rate is so sta­ble that it varies by only 0.000000000000000032 per­cent over the course of a sin­gle day.

That level of ex­ac­ti­tude is not re­ally nec­es­sary for those of us who rely on clocks to get us to a doc­tor’s ap­point­ment on time, or to know when to meet up with friends.

But keep­ing time is just the begin­ning. This new clock is so ex­act that it could be used to de­tect dark mat­ter, mea­sure the grav­i­ta­tional waves that rip­ple across the uni­verse, and de­ter­mine the ex­act shape of Earth’s grav­i­ta­tional field with un­prece­dented pre­ci­sion.

In­deed, these hy­per-ac­cu­rate clocks can help sci­en­tists bet­ter probe the mys­ter­ies of the cos­mos, ex­perts said.

“It turns out that if you have all these dig­its of pre­ci­sion for mak­ing a mea­sure­ment, it can give you a mi­cro­scope onto our very uni­verse,” said physi­cist An­drew Lud­low of the Na­tional In­sti­tute of Stan­dards and Tech­nol­ogy in Boul­der, Colorado. Lud­low led the work that pro­duced the new clock, which was de­scribed this week in the jour­nal Na­ture.

Since the 1960s, time has been mea­sured by so-called atomic clocks that use the nat­u­ral os­cil­la­tions of a ce­sium atom as a pen­du­lum. Think of it as a watch with a hand that ticks just over 9 bil­lion times per sec­ond.

The op­ti­cal lat­tice clock Lud­low and his col­leagues de­vel­oped mea­sures the much faster os­cil­la­tions of a yt­ter­bium atom. Its atomic pen­du­lum swings about 10,000 times faster, at a speed of 500 tril­lion times per sec­ond.

“Ce­sium is a beau­ti­ful atomic sys­tem, but we have reached the ba­sic lim­its of how good it can be,” Lud­low said. “Yt­ter­bium can break down time into much finer in­ter­vals, en­hanc­ing the pre­ci­sion with which you can mea­sure it.”

Op­ti­cal lat­tice clocks have been around for only 15 years, and they are still in the de­vel­op­ment stage, Lud­low said. Sci­en­tists con­tinue to tin­ker with them, grad­u­ally in­creas­ing their ac­cu­racy with each new ad­just­ment.

Most of the improvements in the lat­est it­er­a­tion are due to a new heat shield that Lud­low’s group de­vel­oped a few years ago. It pro­tects the yt­ter­bium atoms from the ef­fects of heat and elec­tric fields, which can in­ter­fere with their nat­u­ral os­cil­la­tions.

“We want to be sure that when we are mea­sur­ing the tick­ing rate of the atom, we are mea­sur­ing the rate Mother Na­ture gave it, and that it is not per­turbed or shifted due to an en­vi­ron­men­tal ef­fect,” he said.

With so many os­cil­la­tions, the yt­ter­bium clock can de­tect shifts in the grav­i­ta­tional field of our planet with un­prece­dented pre­ci­sion, Lud­low and his coau­thors wrote in Na­ture.

As Ein­stein’s the­ory of gen­eral rel­a­tiv­ity pre­dicts, time moves dif­fer­ently de­pend­ing on where you are in a grav­ity field.

A clock on top of a tall moun­tain — far from Earth’s cen­ter — will tick a tiny bit faster than a clock at the base of that same moun­tain.

It’s not a me­chan­i­cal er­ror. Time ac­tu­ally passes faster at the top of that moun­tain.

Most clocks aren’t ac­cu­rate enough to reg­is­ter that ex­tremely sub­tle dif­fer­ence. Af­ter all, in 10 years, two clocks that are 1,000 me­ters apart in al­ti­tude will be off by just 31-mil­lionths of a sec­ond.

Sci­en­tists have al­ready demon­strated that it is pos­si­ble to mea­sure dif­fer­ences in Earth’s grav­i­ta­tional field by com­par­ing the tick rate of two op­ti­cal lat­tice clocks in dif­fer­ent lo­ca­tions. How­ever, un­til now those same grav­ity maps could be made just as ac­cu­rately us­ing other, cheaper tech­niques.

The new clock can de­tect changes in just 1 cen­time­ter of el­e­va­tion, a mea­sure­ment far more pre­cise than was pre­vi­ously pos­si­ble, Lud­low said.

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