Ul­tra-Cold Atoms Har­nessed to Hunt For Dark En­ergy

Science Illustrated - - TECHNOLOGY/ABSOLUTE ZERO -

Just 0.0000000001 de­grees above ab­so­lute zero. That is how deeply an atom cloud will be chilled, in a new ISS ex­per­i­ment. In a state of weight­less­ness, the atoms re­main for 10 sec­onds, so physi­cists can study them and per­haps solve one of the ma­jor astronomy mys­ter­ies.

In 1995, when three US physi­cists cooled a gas of atoms to a few bil­lionths of a de­gree above ab­so­lute zero, pro­duc­ing the first Bose-Einstein con­den­sate, it was a sen­sa­tion. Still, the physi­cists were not per­fectly con­tent. They dreamed of car­ry­ing out the ex­per­i­ment at the in­ter­na­tional Space Sta­tion, ISS, be­cause in a state of weight­less­ness, an ul­tra-cold gas cloud will live much longer than on Earth, where sci­en­tists barely have time to study it. Back then, the vi­sion was pure science fic­tion, as the cool­ing re­quired large, heavy lasers, which could nei­ther be launched nor fit­ted into the cramped space sta­tion.

Now, things are dif­fer­ent. The large lasers of the past have been packed into a small chip, and the en­tire ex­per­i­ment set-up has been fit­ted into a box the size of a mi­crowave oven. Named the Cold Atom Lab­o­ra­tory (CAL), the box will now be launched and mounted out­side the space sta­tion by a team of NASA sci­en­tists, and sub­se­quently, the ex­per­i­ments can be re­mote- con­trolled from Earth via ra­dio sig­nals.

Physi­cists through­out the world have al­ready lined up to make ex­per­i­ments in the dis­tant lab, as the ul­tra-cold atoms are al­most com­pletely un­ex­plored. In a state of weight­less­ness, the gas will live for 10-20 sec­onds, so physi­cists have time to study and ma­nip­u­late the cold atoms. Ac­cord­ing to sci­en­tists, the ul­tra- cold atoms can be con­verted into ex­tremely sen­si­tive sen­sors that can mea­sure the strength of grav­ity with un­prece­dented ac­cu­racy, al­low­ing the sci­en­tists to mea­sure the ex­tent of ice cap melt­ing. Per­haps, the sen­sors can even find the un­known par­ti­cles that pro­duce the re­pel­lent black en­ergy which makes the ex­pan­sion of the uni­verse ac­cel­er­ate.

Cold atoms con­verted into wave

Close to ab­so­lute zero, atoms be­have very dif­fer­ently than they usu­ally do. Un­der nor­mal cir­cum­stances, ac­cord­ing to the laws of quantum me­chan­ics, atoms are both waves and par­ti­cles at the same time, but close to ab­so­lute zero, the atoms lose their in­di­vid­ual iden­ti­ties as par­ti­cles, in­stead be­com­ing a col­lec­tive wave. The state, which is pro­duced in gases at tem­per­a­tures of a few bil­lionths of a de­grees above ab­so­lute zero, is known as a BoseEin­stein con­den­sate, as the phe­nom­e­non was pre­dicted in 1924 by physi­cists Satyen­dra Nath Bose and Al­bert Einstein.

For years, sci­en­tists have dreamt of study­ing the ul­tra-cold atoms, but so far, Earth’s grav­ity has pre­vented it. Only 10 mil­lisec­onds af­ter the pro­duc­tion of a Bose-Einstein con­den­sate, the atoms fall to the bot­tom of the ex­per­i­men­tal cham­ber, where the cham­ber wall heats the atoms, mak­ing the col­lec­tive quantum state cease. How­ever, the state of weight­less­ness of the space sta­tion will al­low sci­en­tists time to make

mea­sure­ments. More­over, the state can be used to cool the atoms to even lower tem­per­a­tures than on Earth. In the space lab, the Bose-Einstein con­den­sate is pro­duced by laser cool­ing, while the gas is cap­tured in a mag­netic field by ul­tra-cold atoms. When the mag­netic field is "switched off", the gas spreads in the vac­uum cham­ber.

The spread cools the atoms even more, as the longer the dis­tance in be­tween atoms, the more rarely they col­lide, and the colder they get. The prin­ci­ple is the same as when a spray bot­tle becomes ice-cold af­ter the gas has been re­leased and the pres­sure in­side the con­tainer has been re­duced. The longer the cloud de­vel­ops, the colder the atoms.

Physi­cists hope to beat the cold record on Earth of 50 bil­lionths of a de­gree above ab­so­lute zero, which was set by dump­ing a ver­sion of the ex­per­i­ment down a 146-m-high tower in Ger­many. The free fall pro­duced five sec­onds of weight­less­ness, dur­ing which the gas ex­panded and was cooled, be­fore the ex­per­i­men­tal cham­ber landed in plas­tic balls at the bot­tom of the tower.

In the space lab, the gas will ex­pand and cool for 10-20 sec­onds, and the atoms will be­come even colder. If the cold record is beaten, the atom cloud at the space sta­tion will be the coolest spot in the uni­verse – about 100 mil­lion times colder than empty space, in which the tem­per­a­ture is 2.725 de­grees above ab­so­lute zero any­where.

The space ex­per­i­ments also in­volve another ma­jor ad­van­tage: whereas sci­en­tists can only make three ex­per­i­ments a day in the tower, CAL will be avail­able to physi­cists 24/7, and the re­mote-con­trolled ex­per­i­ments can be car­ried out over and over again from any­where on Earth.

Highly ac­cu­rate mea­sure­ments

First, physi­cists aim to study the prop­er­ties of Bose-Einstein condensates and ma­nip­u­late the col­lec­tive quantum wave, into which the cloud of ul­tra-cold atoms is con­verted, in or­der to see, if they can shake the wave or make it pro­duce a cir­cle.

When an im­proved ver­sion of the lab is launched no later than in 2021, it will be pos­si­ble to carry out atomic in­ter­fer­om­e­try, by which a laser beam splits the quantum wave in two, one half mov­ing a lit­tle fur­ther away from Earth than the other, caus­ing a tiny dif­fer­ence of the weak grav­ity ac­cel­er­a­tion from Earth, which the top and bot­tom waves are sub­jected to. When the two quantum waves are re­united, they pro­duce a grooved pat­tern, that can be used to cal­cu­late the tiny dif­fer­ence and the strength of the grav­ity ac­cel­er­a­tion very ac­cu­rately.

The ul­tra-cold atomic in­ter­fer­om­e­ter can make the world’s most ac­cu­rate grav­ity mea­sure­ments and is able to mea­sure small changes in Earth’ field of grav­ity, as they are pro­duced. Apart from ob­serv­ing melt­ing ice caps, sci­en­tists will also be able to say to which ex­tent the ground wa­ter de­posits of an area are emp­tied of wa­ter for drink­ing and ir­ri­ga­tion pur­poses.

In­tense search for dark en­ergy

An atomic in­ter­fer­om­e­ter will be sen­si­tive to all types of fields and so, it could also re­veal the phys­i­cal mech­a­nism be­hind dark en­ergy, which, ac­cord­ing to as­tronomers, makes the uni­verse ex­pand ever faster. The key is that the de­tec­tor is lo­cated in empty space, where the par­ti­cles’ re­jec­tion can­not be out-com­peted by the at­trac­tion of grav­ity.

The dom­i­nant the­ory in­volves that dark en­ergy orig­i­nates from empty space and has a constant strength – i.e. any given vol­ume of empty space al­ways con­tains the same quan­tity of dark en­ergy. A com­pet­ing the­ory says that dark en­ergy is me­di­ated by par­ti­cles that change their masses and forces de­pend­ing on their lo­ca­tion. Near com­pact masses such as Earth, the par­ti­cles have so much mass that not even CERN’s LHC ac­cel­er­a­tor can pro­duce them. But at the ISS, a highly sen­si­tive atomic in­ter­fer­om­e­ter might be able to de­tect the force fields pro­duced by the par­ti­cles. So, sci­en­tists hope that the ul­tra-cold atoms in space can soon solve the ma­jor mys­tery of astronomy once and for all, ex­plain­ing dark en­ergy.

D SCOTT CLARK

An atom chip from the Amer­i­can com­pany ColdQuanta is the heart of the space sta­tion ex­per­i­ment. The Bose-Einstein con­den­sate is pro­duced at the cen­tre of the chip.

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