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Photons and the Casimir Effect

Physicists extract light from seeming emptiness

Courtesy of Chalmers University of Technology and World Science staff: Phys­i­cists in Swe­den say they have man­aged to cre­ate light from vac­u­um, the clos­est thing to emp­ty space known to ex­ist. In find­ings pub­lished in the re­search jour­nal Na­ture, the sci­en­tists said they ver­i­fied an ef­fect pre­dicted over 40 years ago by cap­tur­ing some of the par­t­i­cles of light, or pho­tons, that con­stantly ap­pear and disap­pear in the vac­u­um.

A di­a­gram il­lus­trating how vir­tu­al pho­tons bounce off a “mir­ror” that vi­brates at a speed ap­proach­ing that of light. The round mir­ror in the pic­ture is a sym­bol, and un­der that is the quan­tum elec­tron­ic com­po­nent (re­ferred to as a SQUID), which acts as a mir­ror. This makes real pho­tons ap­pear in pairs, phys­i­cists say. (Cred­it: Phil­ip Krantz, Chal­mers U.)

A vac­u­um is a space de­void of atoms, the un­its that make up air, oth­er gas­es and fa­mil­iar ob­jects. That means a vac­u­um is the next best thing to a space truly emp­ty of an­y­thing at al­l—some­thing phys­i­cists say can’t ex­ist in na­ture as we know it, thanks to a phe­nom­e­non called the un­cer­tain­ty prin­ci­ple. This holds that noth­ing can be in a state that is pinned down with per­fect pre­ci­sion.

The un­cer­tain­ty prin­ci­ple en­sures that the vac­u­um teems with var­i­ous sub­a­tom­ic par­t­i­cles that flit in and out of ex­istence. They ap­pear for an in­stant and disap­pear again, the en­er­gy fu­el­ing their ex­istence “bor­rowed” from the void. Since their life is so fleet­ing, they are called vir­tu­al par­t­i­cles.

In the new work, Chris­to­pher Wil­son and col­leagues at Chal­mers Uni­vers­ity of Tech­nol­o­gy in Goth­en­burg, Swe­den said they coaxed pho­tons in­to leav­ing their “vir­tu­al” state and be­com­ing real pho­tons—measura­ble light. The phys­i­cist Ger­ald Moore pre­dicted in 1970 that this should hap­pen if vir­tu­al pho­tons bounce off a mir­ror mov­ing at nearly the speed of light, in a phe­nom­e­non called the dy­nam­i­cal Casimir Ef­fect.

“S­ince it’s not pos­si­ble to get a mir­ror to move fast enough, we’ve de­vel­oped anoth­er meth­od for achiev­ing the same ef­fect,” said Per Dels­ing, a phys­i­cist at Chal­mers. “In­stead of var­y­ing the phys­i­cal dis­tance to a mir­ror, we’ve var­ied the elec­tri­cal dis­tance to an elec­tri­cal short cir­cuit that acts as a mir­ror for mi­crowaves.” The “mir­ror” con­sists of a de­vice called a SQUID or su­per­con­duct­ing quan­tum in­ter­fer­ence de­vice, which is ex­tremely sen­si­tive to mag­net­ic fields. By chang­ing the di­rec­tion of a mag­net­ic field sev­er­al bil­lions of times a sec­ond the sci­en­tists said they made the “mir­ror” vi­brate at one-fourth the speed of light.

“The re­sult was that pho­tons ap­peared in pa­irs from the vac­u­um, which we were able to meas­ure in the form of mi­cro­wave radia­t­ion,” said Dels­ing. “We were al­so able to es­tab­lish that the radia­t­ion had pre­cisely the same prop­er­ties that quan­tum the­o­ry said it should have when pho­tons ap­pear in pa­irs in this way.” Quan­tum the­o­ry is the sci­ence of ex­tremely small par­t­i­cles.

Dur­ing the ex­pe­ri­ment, Dels­ing said, the “mir­ror” trans­ferred some of its en­er­gy of mo­tion to vir­tu­al pho­tons so they could ma­te­ri­al­ize. Göran Jo­hans­son, anoth­er phys­i­cist at Chal­mers, said oth­er par­t­i­cles might al­so be ex­tracted from a vac­u­um in prin­ci­ple, but pho­tons are eas­i­er. That’s be­cause the equi­val­ence of en­er­gy and mass, dis­cov­ered by Ein­stein, im­plies that photo­ns—being weigh­tless—can be stim­u­lated “out of their vir­tu­al state” with re­lative­ly little en­ergy. Obtain­ing chunk­i­er par­t­i­cles, such as elec­trons or pro­tons, which make up atoms, “would re­quire a lot more,” he added.

The sci­en­tists said the pho­tons that ap­pear in pa­irs in the ex­pe­ri­ment may be use­ful in the re­search field of quan­tum in­forma­t­ion, which in­cludes the de­vel­op­ment of su­per­fast “quan­tum” com­put­ers. But the main val­ue of the work, they said, is that it aids our un­der­stand­ing of bas­ic phys­i­cal con­cepts, such as vac­u­um fluctua­t­ions. Some sci­en­tists be­lieve these may have a con­nec­tion with “dark en­er­gy” which drives the ac­cel­er­at­ing ex­pan­sion of the uni­verse, a disco­very it­self rec­og­nized this year with a No­bel Prize in phys­ics.”

Learn more about Photons: Chi Energy and the Science of Biophotons