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


Distant quasars could close loophole in quantum mechanics

Feb. 20, 2014
Courtesy of MIT
and World Science staff

Re­search­ers are pro­pos­ing an ex­pe­ri­ment that they say could fi­nally prove the uni­verse fol­lows the seem­ingly il­log­i­cal laws of prob­a­bil­ity from the field of quan­tum me­chan­ics.

These in­clude the no­tion that the meas­ure­ment of one par­t­i­cle can in­stantly af­fect an­oth­er, even if those “en­tan­gled” par­t­i­cles are at op­po­site ends of the uni­verse.

The only re­main­ing ob­jec­tion to that proof, the phys­i­cists said, is a far-fetched one, but it still has to be ruled out ex­pe­ri­men­tal­ly. It has to do with the idea the en­tan­gled-par­t­i­cle ex­pe­ri­ments are giv­ing skewed re­sults be­cause the phys­i­cists car­ry­ing them out lack free will.

In a pa­per pub­lished this week in the jour­nal Phys­i­cal Re­view Let­ters, re­search­ers from the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy de­scribed an ex­pe­ri­ment meant to close the last ma­jor loop­hole of “Bel­l’s in­equal­ity.” That is a 50-year-old the­o­rem that, if vi­o­lat­ed by ex­pe­ri­ments, would mean that our uni­verse is based not on the text­book laws of clas­si­cal phys­ics, but on the less-tangible prob­a­bil­i­ties of quan­tum me­chan­ics.

Such a quan­tum view would al­low for seem­ingly absurd phe­nom­e­na such as en­tan­gle­ment, in which the meas­ure­ment of one par­t­i­cle in­stantly af­fects an­oth­er, even if those en­tan­gled par­t­i­cles are at op­po­site ends of the uni­verse. Among oth­er things, en­tan­gle­ment—a quan­tum fea­ture Al­bert Ein­stein skep­tic­ally re­ferred to as “spooky ac­tion at a dis­tance”— seems to sug­gest that en­tan­gled par­t­i­cles can af­fect each oth­er in­stantly, faster than the speed of light.

In 1964, phys­i­cist John Bell took on this seem­ing dis­par­ity be­tween clas­si­cal phys­ics and quan­tum me­chan­ics, stat­ing that if the uni­verse is based on clas­si­cal phys­ics, the meas­ure­ment of one en­tan­gled par­t­i­cle should not af­fect the meas­ure­ment of the oth­er—a the­o­ry, known as lo­cal­ity, in which there is a lim­it to how cor­re­lat­ed two par­t­i­cles can be. Bell de­vised a math­e­mat­i­cal for­mu­la for lo­cal­ity, and pre­sented sce­nar­i­os that vi­o­lat­ed this for­mu­la, in­stead fol­low­ing pre­dic­tions of quan­tum me­chan­ics.

Since then, phys­i­cists have tested Bel­l’s the­o­rem by meas­ur­ing the prop­er­ties of en­tan­gled quan­tum par­t­i­cles in the lab­o­r­a­to­ry. Es­sen­tially all of these ex­pe­ri­ments have shown that such par­t­i­cles are cor­re­lat­ed more strongly than would be ex­pected un­der the laws of clas­si­cal phys­ics—findings that sup­port quan­tum me­chan­ics.

But sci­en­tists have al­so iden­ti­fied loop­holes in Bel­l’s the­o­rem. These sug­gest that while the out­comes of such ex­pe­ri­ments may ap­pear to sup­port the pre­dic­tions of quan­tum me­chan­ics, they may ac­tu­ally re­flect un­known “hid­den vari­ables” that give the il­lu­sion of a quan­tum out­come, but can still be ex­plained in clas­si­cal terms.

Though two ma­jor loop­holes have since been closed, a third re­mains, the re­search­ers said. Phys­i­cists re­fer to it as “set­ting in­de­pen­dence,” or more pro­voc­a­tive­, “free will.” This loop­hole pro­poses that a par­t­i­cle de­tec­tor’s set­tings may “con­spire” with events in the shared caus­al past of the de­tec­tors them­selves to de­ter­mine which prop­er­ties of the par­t­i­cle to mea­sure—a sce­nar­i­o that, how­ev­er far-fetched, im­plies that a phys­i­cist run­ning the ex­pe­ri­ment does not have com­plete free will in choos­ing each de­tec­tor’s set­ting. Such a sce­nar­i­o would re­sult in bi­ased meas­ure­ments, sug­gesting that two par­t­i­cles are cor­re­lat­ed more than they ac­tu­ally are.

“It sounds creepy, but peo­ple real­ized that’s a log­i­cal pos­si­bil­ity that has­n’t been closed yet,” said MIT phys­i­cist Da­vid Kai­ser. “Be­fore we make the leap to say the equa­t­ions of quan­tum the­o­ry tell us the world is in­escapably cra­zy and bi­zarre, have we closed eve­ry con­ceiv­a­ble log­i­cal loop­hole, even if they may not seem plau­si­ble in the world we know to­day?”

Now Kai­ser, along with MIT post­doc­tor­al re­search­er An­drew Fried­man and Ja­son Gal­lic­chio of the Uni­vers­ity of Chi­ca­go, have pro­posed an ex­pe­ri­ment to close this third loop­hole by de­ter­min­ing a par­t­i­cle de­tec­tor’s set­tings us­ing some of the old­est light in the uni­verse: dis­tant qua­sars, or bright ga­lac­tic cores, which formed bil­lions of years ago.

The idea, es­sen­tial­ly, is that if two qua­sars on op­po­site sides of the sky are suf­fi­ciently dis­tant from each oth­er, they would have been out of caus­al con­tact since the Big Bang some 14 bil­lion years ago, with no pos­si­ble means of any third par­ty com­mu­nicating with both of them since the be­gin­ning of the uni­verse—an ide­al sce­nar­i­o for de­ter­min­ing each par­t­i­cle de­tec­tor’s set­tings.

As Kai­ser ex­plains it, an ex­pe­ri­ment would go some­thing like this: A lab­o­r­a­to­ry set­up would con­sist of a par­t­i­cle gen­er­a­tor, such as a ra­di­o­ac­t­ive at­om that spits out pairs of en­tan­gled par­t­i­cles. One de­tec­tor meas­ures a prop­er­ty of par­t­i­cle A, while an­oth­er de­tec­tor does the same for par­t­i­cle B. A split sec­ond af­ter the par­t­i­cles are gen­er­at­ed, but just be­fore the de­tec­tors are set, sci­en­tists would use tel­e­scop­ic ob­serva­t­ions of dis­tant qua­sars to de­ter­mine which prop­er­ties each de­tec­tor will meas­ure of a re­spec­tive par­t­i­cle. In oth­er words, qua­sar A de­ter­mines the set­tings to de­tect par­t­i­cle A, and qua­sar B sets the de­tec­tor for par­t­i­cle B.

The re­search­ers rea­son that since each de­tec­tor’s set­ting is de­ter­mined by sources that have had no commu­nica­t­ion or shared histo­ry since the be­gin­ning of the uni­verse, it would be vir­tu­ally impos­si­ble for these de­tec­tors to “con­spire” with an­ything in their shared past to give a bi­ased meas­ure­ment; the ex­pe­ri­men­tal set­up could there­fore close the “free will” loop­hole. If, af­ter mul­ti­ple meas­ure­ments with this ex­pe­ri­men­tal set­up, sci­en­tists found that the meas­ure­ments of the par­t­i­cles were cor­re­lat­ed more than pre­dicted by the laws of clas­si­cal phys­ics, Kai­ser said, then the uni­verse as we see it must be based in­stead on quan­tum me­chan­ics.

“I think it’s fair to say this [loop­hole] is the fi­nal fron­tier, log­ic­ally speak­ing, that stands be­tween this enor­mously im­pres­sive ac­cu­mu­lat­ed ex­pe­ri­men­tal ev­i­dence and the in­ter­preta­t­ion of that ev­i­dence say­ing the world is gov­erned by quan­tum me­chan­ics,” Kai­ser said.

Now that the re­search­ers have put forth an ex­pe­ri­men­tal ap­proach, they hope that oth­ers will per­form ac­tu­al ex­pe­ri­ments, us­ing ob­serva­t­ions of dis­tant qua­sars.

“At first, we did­n’t know if our set­up would re­quire con­stella­t­ions of fu­tur­is­tic space satel­lites, or 1,000-meter tele­scopes on the dark side of the moon,” Fried­man said. “So we were nat­u­rally de­light­ed when we dis­cov­ered, much to our sur­prise, that our ex­pe­ri­ment was both fea­si­ble in the real world with pre­s­ent tech­nol­o­gy, and in­ter­est­ing enough to our ex­pe­ri­men­talist col­la­bo­ra­tors who ac­tu­ally want to make it hap­pen in the next few years.”

Adds Kai­ser, “We’ve said, ‘Let’s go for broke—let’s use the histo­ry of the cos­mos since the Big Bang, darn it.’ And it is very ex­cit­ing that it’s ac­tu­ally fea­si­ble.”

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Researchers are proposing an experiment that they say could finally prove the universe follows the seemingly illogical laws of probability from the field of quantum mechanics. These include the notion that the measurement of one particle can instantly affect another, even if those “entangled” particles are at opposite ends of the universe. The only remaining logical objection to that proof, the physicists said, is a rather nonsensical one, but it still has to be ruled out experimentally. It has to do with the idea the entangled-particle experiments are giving skewed results because the physicists carrying them out lack free will. In the paper, published this week in the journal Physical Review Letters, researchers from the Massachusetts Institute of Technology described an experiment meant to close the last major loophole of “Bell’s inequality.” That is a 50-year-old theorem that, if violated by experiments, would mean that our universe is based not on the textbook laws of classical physics, but on the less-tangible probabilities of quantum mechanics. Such a quantum view would allow for seemingly counterintuitive phenomena such as entanglement, in which the measurement of one particle instantly affects another, even if those entangled particles are at opposite ends of the universe. Among other things, entanglement — a quantum feature Albert Einstein skeptically referred to as “spooky action at a distance”— seems to suggest that entangled particles can affect each other instantly, faster than the speed of light. In 1964, physicist John Bell took on this seeming disparity between classical physics and quantum mechanics, stating that if the universe is based on classical physics, the measurement of one entangled particle should not affect the measurement of the other — a theory, known as locality, in which there is a limit to how correlated two particles can be. Bell devised a mathematical formula for locality, and presented scenarios that violated this formula, instead following predictions of quantum mechanics. Since then, physicists have tested Bell’s theorem by measuring the properties of entangled quantum particles in the laboratory. Essentially all of these experiments have shown that such particles are correlated more strongly than would be expected under the laws of classical physics — findings that support quantum mechanics. But scientists have also identified loopholes in Bell’s theorem. These suggest that while the outcomes of such experiments may appear to support the predictions of quantum mechanics, they may actually reflect unknown “hidden variables” that give the illusion of a quantum outcome, but can still be explained in classical terms. Though two major loopholes have since been closed, a third remains, the MIT researchers said. Physicists refer to it as “setting independence,” or more provocatively, “free will.” This loophole proposes that a particle detector’s settings may “conspire” with events in the shared causal past of the detectors themselves to determine which properties of the particle to measure — a scenario that, however far-fetched, implies that a physicist running the experiment does not have complete free will in choosing each detector’s setting. Such a scenario would result in biased measurements, suggesting that two particles are correlated more than they actually are, and giving more weight to quantum mechanics than classical physics. “It sounds creepy, but people realized that’s a logical possibility that hasn’t been closed yet,” said MIT physicist David Kaiser. “Before we make the leap to say the equations of quantum theory tell us the world is inescapably crazy and bizarre, have we closed every conceivable logical loophole, even if they may not seem plausible in the world we know today?” Now Kaiser, along with MIT postdoctoral researcher Andrew Friedman and Jason Gallicchio of the University of Chicago, have proposed an experiment to close this third loophole by determining a particle detector’s settings using some of the oldest light in the universe: distant quasars, or bright galactic cores, which formed billions of years ago. The idea, essentially, is that if two quasars on opposite sides of the sky are sufficiently distant from each other, they would have been out of causal contact since the Big Bang some 14 billion years ago, with no possible means of any third party communicating with both of them since the beginning of the universe — an ideal scenario for determining each particle detector’s settings. As Kaiser explains it, an experiment would go something like this: A laboratory setup would consist of a particle generator, such as a radioactive atom that spits out pairs of entangled particles. One detector measures a property of particle A, while another detector does the same for particle B. A split second after the particles are generated, but just before the detectors are set, scientists would use telescopic observations of distant quasars to determine which properties each detector will measure of a respective particle. In other words, quasar A determines the settings to detect particle A, and quasar B sets the detector for particle B. The researchers reason that since each detector’s setting is determined by sources that have had no communication or shared history since the beginning of the universe, it would be virtually impossible for these detectors to “conspire” with anything in their shared past to give a biased measurement; the experimental setup could therefore close the “free will” loophole. If, after multiple measurements with this experimental setup, scientists found that the measurements of the particles were correlated more than predicted by the laws of classical physics, Kaiser said, then the universe as we see it must be based instead on quantum mechanics. “I think it’s fair to say this [loophole] is the final frontier, logically speaking, that stands between this enormously impressive accumulated experimental evidence and the interpretation of that evidence saying the world is governed by quantum mechanics,” Kaiser said. Now that the researchers have put forth an experimental approach, they hope that others will perform actual experiments, using observations of distant quasars. “At first, we didn’t know if our setup would require constellations of futuristic space satellites, or 1,000-meter telescopes on the dark side of the moon,” Friedman said. “So we were naturally delighted when we discovered, much to our surprise, that our experiment was both feasible in the real world with present technology, and interesting enough to our experimentalist collaborators who actually want to make it happen in the next few years.” Adds Kaiser, “We’ve said, ‘Let’s go for broke — let’s use the history of the cosmos since the Big Bang, darn it.’ And it is very exciting that it’s actually feasible.”