"Long before it's in the papers"
January 28, 2015


A “theory of everything” is said to solve its first real-world problem

July 8, 2009
Courtesy University of Leiden
and World Science staff

For the first time, re­search­ers say they have solved a real-world prob­lem us­ing a very ab­stract “the­ory of ever­ything” that of­ten has been crit­i­cized as un­test­a­ble.

Now, the sci­en­tists claim, the crit­ics may have to re­think their po­si­tion.

The sci­en­tists at­tempted to use the con­tro­ver­sial doc­trine, known as string the­o­ry, to ex­plain an as­pect of super-conducti­vity—a phe­nom­e­non in which elec­tric cur­rent zooms through an ob­ject with­out meet­ing any of the nor­mal re­sist­ance.

‘AdS/CFT’ cor­resp­ond­ence that relates a gra­vity-de­ter­mined world in a higher dimension to 'quan­tum-cri­ti­cal' worlds formed, for ex­am­ple, by elec­trons in a lower-dim­en­sion­al world on the 'out­side' of the first world. (Cour­tesy Sci­ence)

String the­o­ry is a bid to re­solve al­most all the mys­ter­ies of phys­ics at a b­low by bridg­ing the gap be­tween the two most suc­cess­ful the­o­ries of the 20th cen­tu­ry, gen­er­al rel­a­ti­vity and quan­tum me­chan­ics. Each has been suc­cess­ful at ex­plaining how the un­iverse be­haves over vast dis­tances and in ti­ny spaces, re­spec­tive­ly. But they con­flict in some ways; both can’t be right.

String the­o­ry claims all the par­t­i­cles of na­ture are ac­tu­ally dif­fer­ent vibra­t­ions of un­seen, ti­ny loops called “strings.” The the­o­ry math­e­mat­ic­ally fixes the ma­jor in­con­sis­ten­cies be­tween the oth­er two. In the pro­cess, if it’s cor­rect, it would show the un­der­ly­ing un­ity of na­ture’s forc­es. 

But it only works if the strings have sev­er­al ex­tra di­men­sions in which to vi­brate be­yond the di­men­sions we see. Dif­fer­ent ver­sions of string the­o­ry pro­pose 10 or 26 di­men­sions, some of which are in­vis­i­ble be­cause they are rolled up in­to ti­ny balls.

Sci­en­tists at the Un­ivers­ity of Lei­den in the Neth­er­lands used the math­e­mat­ics of string the­o­ry to un­der­stand so-called high-tem­per­a­ture su­per­con­duc­tiv­ity. The ef­fort­less shoot­ing of cur­rent through “su­per­con­duct­ing” ma­te­ri­als was once be­lieved to oc­cur only at tem­per­a­tures so ab­surdly cold as to make prac­ti­cal ap­plica­t­ions of the phe­nom­e­non un­like­ly. But more and more ex­am­ples are com­ing up where it al­so oc­curs at high­er tem­per­a­tures, ac­cord­ing to the Lei­den phys­i­cists.

Elec­trons, the sub­a­tom­ic par­t­i­cles that car­ry elec­tric cur­rent, can form a spe­cial kind of state, a so-called quan­tum crit­i­cal state, that plays a role in this high-tem­per­a­ture super-conducti­vity. 

“It has al­ways been as­sumed that once you un­der­stand this quan­tum-crit­i­cal state, you can al­so un­der­stand high tem­per­a­ture super-conducti­vity. But, al­though the ex­pe­ri­ments pro­duced a lot of in­forma­t­ion, we had­n’t the faintest idea of how to de­scribe this phe­nom­e­non,” said Lei­den phys­i­cist Jan Za­a­nen.

String the­o­ry now of­fers a so­lu­tion, he added. “This is su­perb. I have nev­er ex­perienced such eu­pho­ria,” he re­marked, ex­plaining that the num­bers fit so pre­cisely that he was as­ton­ished. The find­ing is re­ported this week in the re­search jour­nal Sci­ence.

Za­a­nen de­scribes the quan­tum-crit­i­cal state as a “quan­tum soup,” where­by the elec­trons form a col­lec­tive in­de­pend­ent of dis­tances, and show the same be­hav­iour at tiny scales or at the roughly hu­man scale. 

Be­cause of Za­a­nen’s in­ter­est in string the­o­ry, he and string the­o­reti­cist Koen­raad Schalm be­came ac­quaint­ed af­ter Schalm’s ar­ri­val at Lei­den Un­ivers­ity. Za­a­nen had an un­solved prob­lem and Schalm was an ex­pert in the field of string the­o­ry. Their com­mon in­ter­est brought them to­geth­er, and they de­cid­ed to work jointly. 

The pair used the as­pect of string the­o­ry known as Ad­S/CFT cor­re­spond­ence. This al­lows situa­t­ions in a large, so-called rel­a­tivistic, world to be trans­lated in­to a de­scrip­tion at mi­nus­cule, so-called quan­tum phys­ics lev­el. This cor­re­spond­ence bridg­es the gap be­tween these two dif­fer­ent worlds. By ap­ply­ing the cor­re­spond­ence to the the­o­ret­i­cal situa­t­ion where a black hole vi­brates when an elec­tron falls in­to it, they ar­rived at a de­scrip­tion of elec­trons that move in and out of a quan­tum-crit­i­cal state. 

This is the first time a cal­cula­t­ion based on string the­o­ry has been pub­lished in Sci­ence, even though the the­o­ry is widely known, Za­a­nen said.

“There have al­ways been a lot of ex­pecta­t­ions sur­round­ing string the­o­ry,” Za­a­nen ex­plained, hav­ing him­self stud­ied the the­o­ry to sat­is­fy his own cu­ri­os­ity. “String the­o­ry is of­ten seen as a child of Ein­stein that aims to de­vise a rev­o­lu­tion­ary and com­pre­hen­sive the­o­ry, a kind of ‘the­o­ry of ever­ything.’ Ten years ago, re­search­ers even said: ‘Give us two weeks and we’ll be able to tell you where the Big Bang came from.’ The prob­lem of string the­o­ry was that, in spite of its ex­cel­lent maths, it was nev­er able to make a con­crete link with the phys­i­cal real­ity—the world around us.”

“Ad­S/CFT cor­re­spond­ence now ex­plains things that col­leagues who have been bea­ver­ing away for ages were un­able to re­solve, in spite of their enor­mous ef­forts,” he went on. “There are a lot of things that can be done with it. We don’t fully un­der­stand it yet, but I see it as a gate­way to much more.”

* * *

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For the first time, researchers say they have solved a real-world problem using a very abstract “theory of everything” that often has been criticized as untestable. Now, the scientists claim, the critics may have to rethink their position. The scientists attempted to use the controversial doctrine, known as string theory, to explain an aspect of super-conductivity—a phenomenon in which electric current zooms through an object without meeting any of the normal resistance. String theory is a bid to resolve almost all the mysteries of physics at a blow by bridging the gap between the two most successful theories of the 20th century, general relativity and quantum mechanics. Each has been successful at explaining how the universe behaves over vast distances and in tiny spaces, respectively. But they conflict in some ways; both can’t be right. String theory claims all the particles of nature are actually different vibrations of unseen, tiny loops called “strings.” The theory mathematically fixes the major inconsistencies between the other two. In the process, if it’s correct, it would show the underlying unity of nature’s forces. But it only works if the strings have several extra dimensions in which to vibrate beyond the dimensions we see. Different versions of string theory propose 10 or 26 dimensions, some of which are invisible because they are rolled up into tiny balls. Scientists at the University of Leiden in the Netherlands used the mathematics of string theory to understand so-called high-temperature superconductivity. The effortless shooting of current through “superconducting” materials was once believed to occur only at temperatures so absurdly cold as to make practical applications of the phenomenon unlikely. But more and more examples are coming up where it also occurs at higher temperatures, according to the Leiden physicists. Electrons, the subatomic particles that carry electric current, can form a special kind of state, a so-called quantum critical state, that plays a role in this high-temperature super-conductivity. “It has always been assumed that once you understand this quantum-critical state, you can also understand high temperature super-conductivity. But, although the experiments produced a lot of information, we hadn’t the faintest idea of how to describe this phenomenon,” said Leiden physicist Jan Zaanen. String theory now offers a solution, he added. “This is superb. I have never experienced such euphoria,” he remarked, explaining that the numbers fit so precisely that he was astonished. The finding is reported this week in the research journal Science. Zaanen describes the quantum-critical state as a “quantum soup,” whereby the electrons form a collective independent of distances, where the electrons exhibit the same behaviour at small quantum mechanical scale or at macroscopic human scale. Because of Zaanen’s interest in string theory, he and string theoreticist Koenraad Schalm became acquainted after Schalm’s arrival at Leiden University. Zaanen had an unsolved problem and Schalm was an expert in the field of string theory. Their common interest brought them together, and they decided to work jointly on the research. The pair used the aspect of string theory known as AdS/CFT correspondence. This allows situations in a large, so-called relativistic, world to be translated into a description at minuscule, so-called quantum physics level. This correspondence bridges the gap between these two different worlds. By applying the correspondence to the theoretical situation where a black hole vibrates when an electron falls into it, they arrived at a description of electrons that move in and out of a quantum-critical state. This is the first time a calculation based on string theory has been published in Science, even though the theory is widely known, Zaanen said. “There have always been a lot of expectations surrounding string theory,” Zaanen explained, having himself studied the theory to satisfy his own curiosity. “String theory is often seen as a child of Einstein that aims to devise a revolutionary and comprehensive theory, a kind of ‘theory of everything’. Ten years ago, researchers even said: ‘Give us two weeks and we’ll be able to tell you where the Big Bang came from.’ The problem of string theory was that, in spite of its excellent maths, it was never able to make a concrete link with the physical reality—the world around us.” “AdS/CFT correspondence now explains things that colleagues who have been beavering away for ages were unable to resolve, in spite of their enormous efforts,” he went on. “There are a lot of things that can be done with it. We don’t fully understand it yet, but I see it as a gateway to much more.”