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Electrons boast near-perfect roundness, physicists report

May 26, 2011
Courtesy of Imperial College London
and World Science staff

Elec­trons—the elec­trical charge-carry­ing parts of atoms—are balls of near-per­fect round­ness, phys­i­cists have an­nounced af­ter a more than decade-long ex­pe­ri­ment.

Their re­sults indicate that the elec­tron de­vi­ates from per­fect spher­i­city, or round­ness, by less than a hun­dred sep­til­lionths of a mil­li­me­ter, or a mil­li­me­ter di­vid­ed by one with 25 ze­roes af­ter it. In other words, if this ti­ny par­t­i­cle were mag­ni­fied to the size of the so­lar sys­tem, it would still look round to with­in a hair’s width, said the sci­en­tists, who re­ported their find­ings May 25 in the re­search jour­nal Na­ture.

“We’ve been able to im­prove our knowl­edge of one of the bas­ic build­ing blocks of mat­ter. It’s been a very dif­fi­cult mea­sure­ment… but this knowl­edge will let us im­prove our the­o­ries of fun­da­men­tal physics,” said Jony Hud­son of Im­pe­ri­al Col­lege Lon­don, co-author of the stu­dy.

The researchers are now plan­ning to meas­ure the elec­tron’s shape even more pre­cise­ly. The re­sults are thought to be im­por­tant in the study of an­ti­mat­ter, an elu­sive sub­stance that acts the same way as or­di­nary ma­te­ri­als, ex­cept it has an op­po­site elec­tri­cal charge. For ex­am­ple, the an­ti­mat­ter ver­sion of the elec­tron is the pos­i­tron, which has pos­i­tive elec­tri­cal charge in place of the elec­tron’s neg­a­tive.

Un­der­stand­ing the elec­tron’s shape might help re­search­ers un­der­stand how pos­i­trons be­have and how an­ti­mat­ter and mat­ter may dif­fer. Hud­son and col­leagues stud­ied elec­trons in mo­le­cules called yt­ter­bi­um flu­o­ride. Us­ing a pre­cise la­ser, they meas­ured the mo­tion of these elec­trons. If they weren’t per­fectly round, then like an un­bal­anced spin­ning top, their mo­tion would show a dis­tinc­tive wob­ble, dis­tort­ing the molecule’s over­all shape, the re­search­ers said. No such wob­ble was found. While physi­cists have gener­ally as­sumed elec­trons are round, they could have been, say, egg-shaped, Hud­son and col­leagues say.

The group, at Im­pe­ri­al Col­lege’s Cen­tre for Cold Mat­ter, has been stu­dying elec­trons as a way to in­ves­t­i­gate a mys­tery in­volv­ing an­ti­mat­ter. Ac­cept­ed laws of phys­ics say the Big Bang, an explosion-like event that cre­at­ed our un­iverse, birthed as much an­ti­mat­ter as or­di­nary mat­ter. But since an­ti­mat­ter was first the­o­rized by No­bel Prize-winning sci­ent­ist Paul Di­rac in 1928, it has only turned up in min­ute amounts.

“Phys­i­cists just do not know what hap­pened to all the an­ti­mat­ter,” said Ed­ward Hinds, co-author of the re­search and head of the Cen­tre for Cold Mat­ter. “But this re­search can help us to con­firm or rule out some of the pos­si­ble ex­plana­t­ions.” The re­search­ers hope to ex­plain the lack of an­ti­mat­ter by search­ing for any pre­vi­ously un­known dif­ferences be­tween mat­ter and an­ti­mat­ter. Had they found that elec­trons, which phys­i­cists es­ti­mate to be about six tril­lionths of a mil­li­me­ter wide, aren’t round, this could have point­ed to one pos­si­ble dif­ference.

An­ti­mat­ter is al­so stud­ied in ti­ny quan­ti­ties in the Large Had­ron Col­lider, a par­t­i­cle smash­er in Switz­er­land, where phys­i­cists hope to un­der­stand what hap­pened in the mo­ments fol­low­ing the Big Bang and to con­firm some cur­rently un­prov­en fun­da­men­tal the­o­ries of phys­ics. Know­ing wheth­er elec­trons are round tests these the­o­ries, as well as oth­ers that even that huge mach­ine can’t test, said Hinds and col­leagues.

They are now de­vel­op­ing meth­ods to cool their mo­le­cules to ex­tremely low tem­per­a­tures, and con­trol their ex­act mo­tion, so they can study the em­bed­ded elec­trons’ be­hav­ior in un­prec­e­dent­ed de­tail. They say the same tech­nol­o­gy could al­so be used to con­trol chem­i­cal re­ac­tions and to un­der­stand the be­hav­ior of sys­tems too com­plex to sim­u­late with com­put­ers.


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Electrons—the components of atoms that carry electrical charge—are balls of near-perfect roundness, physicists have announced after a more than decade-long experiment. Their results suggest that the electron deviates from perfect sphericity, or roundness, by less than a hundred septillionths of a millimeter, or a millimeter divided by one with 25 zeroes after it. This means that if this tiny particle were magnified to the size of the solar system, it would still look round to within a hair’s width, said the the scientists with Imperial College London, who reported their findings May 25 in the research journal Nature. “We’ve been able to improve our knowledge of one of the basic building blocks of matter. It’s been a very difficult measurement… but this knowledge will let us improve our theories of fundamental physics,” said Jony Hudson, co-author of the study. The group is now planning to measure the electron’s shape even more precisely. The results are thought to be important in the study of antimatter, an elusive substance that acts the same way as ordinary materials, except it has an opposite electrical charge. For example, the antimatter version of the electron is the positron, which has positive electrical charge instead of negative as the electron does. Understanding the electron’s shape might help researchers understand how positrons behave and how antimatter and matter may differ. Hudson and colleagues studied electrons in molecules called Ytterbium Fluoride. Using a precise laser, they measured the motion of these electrons. If they weren’t perfectly round, then like an unbalanced spinning top, their motion would show a distinctive wobble, distorting the molecule’s overall shape, the researchers said. No such wobble was found. Hudson and colleagues, at Imperial College’s Centre for Cold Matter, have been studying electrons as a way to investigate a mystery involving antimatter. Currently accepted laws of physics say the Big Bang, the explosion-like event that created our universe, created as much antimatter as ordinary matter. But since antimatter was first theorized by Nobel Prize-winning scientist Paul Dirac in 1928, it has only turned up in minute amounts from sources such as cosmic rays and some radioactive substances. “Physicists just do not know what happened to all the antimatter,” said Edward Hinds, co-author of the research and head of the Centre for Cold Matter. “But this research can help us to confirm or rule out some of the possible explanations.” The researchers hope to explain the lack of antimatter by searching for any tiny previously unknown differences between the behavior of matter and antimatter. Had they found that electrons, which physicists estimate to be about six trillionths of a millimeter wide, aren’t round, this could have pointed to one possible difference. Antimatter is also studied in tiny quantities in the Large Hadron Collider, a particle smasher in Switzerland, where physicists hope to understand what happened in the moments following the Big Bang and to confirm some currently unproven fundamental theories of physics. Knowing whether electrons are round or egg-shaped tests these same fundamental theories, as well as other theories that even the Large Hadron Collider can’t test. Hinds and colleagues are now developing methods to cool their molecules to extremely low temperatures, and control their exact motion, so they can study the embedded electrons’ behavior in unprecedented detail. They say the same technology could also be used to control chemical reactions and to understand the behavior of systems too complex to simulate with computers.