Rel­a­tiv­ity cut down to size

Los Angeles Times - - Science File - Eryn Brown eryn.brown@latimes.com

Among the oft-re­peated pre­dic­tions of Al­bert Ein­stein’s fa­mous the­ory of rel­a­tiv­ity is that if a twin trav­els through the cos­mos on a high-speed rocket, when he re­turns to Earth he will be no­tice­ably younger than the twin who stayed home.

Now physi­cists have demon­strated that the same is true even if the trav­el­ing twin is merely driv­ing in a car about 20 mph. But in that case, when the twin gets home from the gro­cery store, he is only a tiny frac­tion of a nanosec­ond younger, ac­cord­ing to a re­port in Fri­day’s edi­tion of the jour­nal Sci­ence.

The re­verse is of­ten said to be true for a twin who spends time high on a moun­tain­top; gen­eral rel­a­tiv­ity pre­dicts that time passes more quickly at greater al­ti­tudes be­cause ob­jects don’t feel Earth’s grav­ity quite as strongly.

But the physi­cists found that a twin who lives just about a foot above sea level will age ever-so-slightly faster than a twin liv­ing at sea level.

“Dur­ing your daily life, you ex­pe­ri­ence rel­a­tiv­ity,” said James Chin-Wen Chou, a post­doc­toral re­searcher at the Na­tional In­sti­tute of Stan­dards and Technology in Boul­der, Colo., who led the ex­per­i­ments.

“This [makes] sci­ence res­onate with reg­u­lar peo­ple.”

In the past, the phe­nom­e­non known as time di­la­tion has been proved by study­ing what hap­pens to clocks strapped aboard jets streak­ing across the sky or perched on rock­ets miles above Earth.

But the rules of rel­a­tiv­ity are al­ways in force, even over the small­est dis­tances and at the slow­est speeds. The dif­fer­ence is that the ef­fect is in­finites­i­mally small.

In its paper, the team re­ported that the sec­ond hand of a clock po­si­tioned about two-thirds of a mile above an iden­ti­cal clock near Earth’s sur­face will speed up only enough to tick out three ex­tra sec­onds over the course of a mil­lion years.

Chou and his col­leagues were able to ob­serve an even smaller time di­la­tion — in clocks sep­a­rated by just a foot — be­cause they built a pre­cise time­piece.

Their atomic clock, which would fill a large din­ing ta­ble, works by cal­i­brat­ing the fre­quency of a laser to that of an alu­minum ion. The os­cil­la­tions of the laser are the equiv­a­lent of a tra­di­tional clock’s ticks, but they oc­cur far more rapidly — more than a mil­lion bil­lion times per sec­ond.

That’s about 100,000 times as fast as the tick rate of the mi­crowave-based atomic clocks that cur­rently set the time stan­dard in the United States.

“We are able to di­vide time into finer chunks,” Chou said.

In the el­e­va­tion ex­per­i­ment, the alu­minum-based atomic clocks let the re­searchers mea­sure a dif­fer­ence of ap­prox­i­mately 90 bil­lionths of a sec­ond over a hu­man’s 79-year life­time.

Chou said that three de­vel­op­ments al­lowed this pre­ci­sion: re­cent im­prove­ments in laser technology; the devel­op­ment of op­ti­cal fre­quency combs, which count a laser’s “ticks”; and ad­vances in quan­tum in­for­ma­tion sci­ence, which make it pos­si­ble to use the ex­tremely sta­ble alu­minum ion to cal­i­brate the laser.

Cal­tech pro­fes­sor Sean Car­roll, who stud­ies the­o­ret­i­cal physics and cos­mol­ogy, said the ob­ser­va­tions of time di­la­tion were “cool, but the un­der­ly­ing ad­vances in technology that let this hap­pen” were more sig­nif­i­cant. “I don’t know what the ap­pli­ca­tions will be,” he said, “but they will be cru­cial.”

If im­proved fur­ther, Chou said, the clock might be able to help sci­en­tists per­form a va­ri­ety of tasks, in­clud­ing mea­sur­ing the changes in Earth’s grav­i­ta­tional field at dif­fer­ent places on the planet.

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