"Long before it's in the papers"
January 27, 2015


Physicists report drawing light from seeming emptiness

Nov. 18, 2011
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 this week 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 Am­er­i­can 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.

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Physicists in Sweden say they have managed to create light from vacuum, the closest thing to empty space known to exist. In findings published this week 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 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 Per 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 as they’re weightless. Because energy and mass are equivalent, this means it doesn’t take much energy to stimulate the photons “out of their virtual state,” he explained. Getting chunkier particles out of a vacuum, such as electrons or protons, which make up atoms, “would require a lot more energy,” 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 increases 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.