Saturday, August 04, 2007

Is Reality a misunderstanding?

April 29, 2007Special to World Science


Sev­er­al phys­i­cists say they’ve con­firmed strange pre­dic­tions of mod­ern phys­ics that clash with our most bas­ic no­tions of real­i­ty and even sug­gest—some sci­en­tists and phi­loso­phers say—that real­i­ty is­n’t there when we’re not look­ing.

The pre­dic­tions have lurked with­in quan­tum me­chan­ics, the sci­ence of the small­est things, since the field emerged in the 1920s; but not all phys­i­cists ac­cept­ed them. They were un­dis­put­edly con­sist­ent with ex­pe­ri­ments, but ex­pe­ri­ments might not re­veal eve­ry­thing.

New tests—de­signed more specif­i­cally than be­fore to probe the real­i­ty ques­tion—have yielded un­set­tling re­sults, say re­search­ers who pub­lished the find­ings in the April 19 is­sue of the re­search jour­nal Na­ture. One of their col­leagues called the find­ings in­tri­guing but in­con­clu­sive.


The background

Quan­tum phys­i­cists have long not­ed that sub­a­tom­ic par­t­i­cles seem to move ran­dom­ly. For in­stance, one can meas­ure a par­t­i­cle’s lo­ca­tion at a giv­en mo­ment, but can’t know ex­act­ly where it would be just be­fore or af­ter.

Phys­i­cists de­ter­mined that the ran­dom­ness was­n’t just an ap­pear­ance due to our ig­no­rance of the de­tails of the mo­tion, but an in­es­cap­a­ble prop­er­ty of the par­t­i­cles them­selves.

Rath­er per­sua­sive ev­i­dence for this lay in math. Par­t­i­cles, for rea­sons no one quite knows, some­times act like waves. When they come to­geth­er, they cre­ate the same types of com­plex pat­terns that ap­pear when wa­ter rip­ples from dif­fer­ent di­rec­tions over­lap.

But a par­t­i­cle, be­ing at least some­what con­fined in space, nor­mal­ly acts on­ly as a small “wave pack­et”—a clus­ter of a few rip­ples in suc­ces­sion—un­like fa­mil­iar waves, in which doz­ens or thou­sands pa­rade along.

It turns out there is a math­e­mat­i­cal way to rep­re­sent a wave pack­et; but you must start by rep­re­senting an in­fi­nite­ly re­peat­ing wave, which is a sim­pler for­mu­la. Adding up many such de­pic­tions, if you choose them prop­er­ly, gives the packet.

Yet there’s a catch: each of these com­po­nents must have a slight­ly dif­fer­ent wave speed. Thus, the com­plete pack­et has no clear-cut speed. Nor, con­se­quent­ly, does the par­t­i­cle.

The previous experiments

Pre­cise­ly in line with such math, ex­pe­ri­ments find that par­t­i­cle speed is some­what ran­dom, though the ran­dom­ness fol­lows rules that again mir­ror the equa­tions. When you meas­ure speed, you do get a num­ber, but that won’t tell you the speed a mo­ment be­fore or af­ter. In es­sence, phys­i­cists con­clud­ed, the par­t­i­cle has no de­fined ve­loc­i­ty un­til you meas­ure it. Si­m­i­lar con­sid­er­a­tions turned out to hold for its lo­ca­tion, spin and oth­er prop­er­ties.

The im­pli­ca­tions were huge: the ran­dom­ness im­plied that key prop­er­ties of these ob­jects, per­haps the ob­jects them­selves, might not ex­ist un­less we are watch­ing. “No el­e­men­ta­ry phe­nom­e­non is a phe­nom­e­non un­til it is an ob­served phe­nom­e­non,” the cel­e­brat­ed Prince­ton Uni­ver­si­ty phys­i­cist John Wheel­er put it.

Still, human-made math­e­mat­i­cal mod­els don’t nec­es­sar­i­ly re­flect ul­ti­mate truth, even if they do match ex­pe­ri­men­tal re­sults bril­liant­ly. And those tests them­selves might miss some­thing. Sci­en­tists in­clud­ing Ein­stein balked at the ran­dom­ness idea—“God does not play dice,” he fa­mous­ly fumed—and the con­se­quent col­lapse of cher­ished as­sump­tions. The great phys­i­cist joined oth­ers in pro­pos­ing that there ex­ist some yet-unknown fac­tors, or “hid­den vari­ables,” that in­flu­ence par­t­i­cle prop­er­ties, mak­ing these look ran­dom with­out tru­ly be­ing so.

Phys­i­cists in due course de­signed ex­pe­ri­ments to test for hid­den vari­ables. In 1964 John Bell de­vised such a test. He ex­ploited a cu­ri­ous phe­nom­e­non called “en­tan­gle­ment,” in which know­ing some­thing about one par­t­i­cle some­times tells you a cor­re­spond­ing prop­er­ty of anoth­er, no mat­ter the dis­tance be­tween them.

An ex­am­ple oc­curs when cer­tain par­t­i­cles de­cay, or break up, in­to two pho­tons—par­t­i­cles of light. These fly off in op­po­site di­rec­tions and have the same po­lar­i­za­tion, or amount by which the wave is tilted in space. De­tec­tors called po­lar­iz­ers can meas­ure this at­trib­ute. Po­lar­iz­ers are like ti­ny fences with slits. If the slits are tilted the same way as the wave, it goes through; if op­po­sitely, it does­n’t; if some­where in be­tween, it may or may not pass.

If you meas­ure the two op­po­sitely-flying pho­tons with po­lar­iz­ers tilted the same way, you get the same re­sult for both. But if one of the po­lar­iz­ers is tilted a bit, you will get oc­ca­sion­al dis­a­gree­ments be­tween the re­sults.

What if you al­so tilt the sec­ond po­lar­izer by the same amount, but the op­po­site way? You might get twice as many dis­a­gree­ments, Bell rea­soned. But you might al­so get less than that, be­cause some po­ten­tial dis­agree­ments could can­cel each oth­er out. For ex­am­ple: two pho­tons might be blocked where­as orig­i­nal­ly they both would have pas­sed, so two de­vi­a­tions from the orig­i­nal re­sult lead to an agree­ment.

All this fol­lows from log­ic. It al­so de­pends on cer­tain rea­son­a­ble as­sump­tions, in­clud­ing that the par­t­i­cles have a real po­lar­i­za­tion wheth­er it’s meas­ured or not.

But Bell, in an ar­gu­ment known as Bell’s The­o­rem, showed that quan­tum me­chan­ics pre­dicts anoth­er out­come, im­ply­ing this “real­i­ty” as­sump­tion might be wrong. Quan­tum me­chan­ics claims that the num­ber of dis­a­gree­ments be­tween the re­sults when both po­lar­iz­ers are op­po­sitely tilt­ed—com­pared to one be­ing tilt­ed—can be more than twice as many. And ex­pe­ri­ments have borne this out.

The rea­sons why have to do with yet anoth­er odd pre­dic­tion of quan­tum me­chan­ics. Once you de­tect the pho­ton as ei­ther hav­ing crossed the po­lar­izer or not, then it’s ei­ther po­lar­ized ex­act­ly in the di­rec­tion of the in­stru­ment, or the op­po­site way, re­spec­tive­ly. It can’t be po­lar­ized at any oth­er an­gle. And its “twin” must be iden­ti­cal­ly po­lar­ized. All this puts ad­di­tion­al con­s­t­raints on the sys­tem such that the num­ber of dis­a­gree­ments can rise com­pared to the “log­ical” re­sult.

Past ex­pe­ri­ments have con­firmed the seem­ingly non­sen­si­cal out­come. Yet this alone this does­n’t dis­prove the “real­i­ty” hy­poth­e­sis, re­search­ers say. There’s one oth­er pos­si­bil­i­ty, which is that the par­t­i­cles are some­how in­stan­ta­ne­ously com­mu­ni­cat­ing, like telepaths.

The new experiment

The new ex­pe­ri­ment was de­signed to side­step this loop­hole: it was set up so that even al­low­ing for in­stan­ta­ne­ous com­mu­ni­ca­tion could­n’t ex­plain the “non­sen­si­cal” out­come, at least not eas­i­ly. One would al­so have to drop the no­tion that pho­tons have a def­i­nite po­lar­i­za­tion in­de­pend­ent of any meas­urement.

The work, by Si­mon Groe­blacher and col­leagues at the Aus­tri­an Acad­e­my of Sci­ences’ In­sti­tute for Quan­tum Op­tics and Quan­tum In­for­ma­tion in Vi­en­na, was based not on Bell’s The­o­rem, but on a re­lat­ed the­o­rem more re­cent­ly de­vel­oped by An­tho­ny Leg­gett at the Uni­ver­si­ty of Il­li­nois at Urbana-Champaign.

Full ex­pe­ri­ments based on Leggett’s con­cept re­quired an­a­lyz­ing pho­ton-waves that are po­lar­ized “el­lip­ti­cally,” which means a wave’s tilt changes con­stant­ly. One can de­tect this by sup­ple­ment­ing the po­lar­izer with a strip of ma­te­ri­al that’s bi­re­frin­gent, mean­ing it bends light dif­fer­ently de­pend­ing on its di­rec­tion.

The re­sults in­deed dis­proved that pho­tons have a def­i­nite, in­de­pend­ently ex­isting po­lar­i­za­tion, Markus As­pelmeyer, a mem­ber of the re­search team, wrote in an e­mail. The find­ings thus spell trou­ble for one “plau­si­ble no­tion of real­ism,” he added, though oth­ers could con­ceiv­a­bly sur­vive.

Not eve­ry­one is con­vinced. “The con­clu­sion one draws is more a ques­tion of taste than log­ic,” wrote Alain As­pect, who con­ducted the first con­clu­sive tests of Bell’s The­o­rem, in a com­men­tary in the same is­sue of the jour­nal. As­pect, of the École Poly­tech­nique in Pa­lai­s­eau, France, ar­gued that the find­ings can still be ex­plained by claim­ing cer­tain forms of in­s­tan­t­a­ne­ous com­mu­ni­ca­tion. But he con­ced­ed that he too is in­clined to re­nounce as­pects of real­ism in­stead. Such ex­pe­ri­ments, and the re­sulting de­bates, “al­low us to look deeper in­to the great mys­ter­ies of quan­tum me­chan­ics,” he added

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