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


“Standard model” safe as physicists can’t find misbehaving light particles

June 29, 2010
Special to World Science  

Phys­i­cists say they have con­firmed to a near cer­tain­ty that par­t­i­cles of light obey a bas­ic dis­tinc­tion be­tween two types of build­ing blocks of the uni­verse.

This dis­tinc­tion, be­tween par­t­i­cles called “bosons” and “fermions,” has long been as­sumed, but nev­er quite prov­en by phys­i­cists. It is tied in turn to many as­sump­tions be­hind the “stan­dard mod­el,” a work­ing mod­el of real­ity that un­der­pins main­stream phys­ics.

A beam of hot bar­i­um at­oms ex­its an ov­en and passes through a col­li­ma­tor be­fore hit­ting over­lap­ping la­ser beams. If pho­tons some­times act like fermions, eve­ry once in a while a bar­i­um at­om would ab­sorb two pho­tons and sub­se­quent­ly emit a flash of light. (Cred­it: Da­mon En­glish/UC Berke­ley)

If the boson-fermion di­chot­o­my proved wrong, “the con­se­quenc­es would be far-reach­ing, af­fect­ing our as­sump­tions about the struc­ture of space-time and even caus­al­ity it­self,” said phys­i­cist Da­mon Eng­lish of the Uni­vers­ity of Cal­i­for­nia, Berke­ley, one of the in­ves­ti­ga­tors in the stu­dy. 

One pos­si­ble con­se­quence of this break­down would be the pos­si­bil­ity of “re­ceiv­ing a flash of light be­fore it was emit­ted,” he added.

The re­search­ers set up an ex­pe­ri­ment in which the build­ing blocks of light—which are boson­s—were giv­en some 10 bil­lion op­por­tun­i­ties to act in a way char­ac­ter­is­tic of fermions, but ap­par­ently nev­er did.

Bosons are spe­cial in that they can all oc­cu­py an iden­ti­cal “quan­tum state,” a set of char­ac­ter­is­tics that in­cludes loca­t­ion. As a re­sult, for in­stance, any num­ber of par­t­i­cles of light can be in the same space. Fermions can’t do this—which is why, since most or­di­nary at­oms con­sist of fermions, it seems ob­vi­ous to most peo­ple that two ob­jects can’t be in the same place at once.

Eng­lish and col­leagues bom­barded bar­i­um at­oms with par­t­i­cles of light, called pho­tons, from two iden­ti­cal la­ser beams. They looked for signs that the bar­i­um had ab­sorbed two pho­tons of the same en­er­gy at once. 

Gen­er­ally speak­ing, events of this sort are com­mon. The in­com­ing light would kick one of the at­om’s elec­trons, or elec­tric­ally charged par­t­i­cles, in­to a higher-en­er­gy state. 

But the spe­cif­ic va­ri­e­ty of elec­tron en­er­gy jump be­ing sought in this ex­pe­ri­ment was one that can only be trig­gered by bosons, not fermions.

The rea­sons for this come down to math, in­clud­ing a con­sid­era­t­ion of the “spin” of par­t­i­cles, Eng­lish said. Many fun­da­men­tal par­t­i­cles be­have in some ways as though they were spin­ning, al­though the ex­act rela­t­ion­ship of this “spin” to or­di­nary spin, such as that of a top, is hard to pin down.

The elec­tron en­er­gy jump in ques­tion would have tak­en the at­om from hav­ing a spin of ze­ro to a spin of one, ac­cord­ing to con­ven­tion­al phys­ics. This means that the ar­riv­ing pho­tons must al­so have had a com­bined spin of one. This is math­e­mat­ic­ally impos­si­ble for equal-en­er­gy pho­tons, un­less they fol­low the equa­t­ions that de­scribe fermions, Eng­lish ex­plained.

Con­sist­ent with these ideas, the re­quired en­er­gy jump nev­er hap­pened, Eng­lish said—though no phys­i­cal prin­ci­ple ex­cept the boson-fermion dis­tinc­tion should pre­vent it. The en­er­gy jump would have been de­tected when the at­oms emit a par­tic­u­lar col­or of flu­o­res­cent light, the re­search­ers not­ed.

Si­m­i­lar con­clu­sions about the boson-fermion dis­tinc­tion were reached by a 1999 ex­pe­ri­ment by sci­en­tists in­clud­ing the prin­ci­pal in­ves­ti­ga­tor in the new stu­dy, Dmitry Bud­ker of the Berke­ley Na­tional Lab­o­r­a­to­ry in Berke­ley. But the new tests im­proved the pre­ci­sion of the re­sults by about 3,000 times, the sci­en­tists said, thus con­strain­ing the chance of bosons act­ing like fermions to less than one in a hun­dred bil­lion in the sys­tem tested. The find­ings are pub­lished in the June 25 is­sue of the jour­nal Phys­i­cal Re­view Let­ters.

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Physicists say they have confirmed to a near certainty that particles of light obey a basic distinction between two types of building blocks of the universe. This distinction between particles called “bosons” and “fermions”—long assumed but never quite proven by physicists—is tied in turn to many assumptions behind the “standard model,” a working model of reality that underpins mainstream physics. If the boson-fermion dichotomy proved wrong, “the consequences would be far-reaching, affecting our assumptions about the structure of spacetime and even causality itself,” said physicist Damon English of the University of California, Berkeley, one of the investigators in the study. One possible consequence of this breakdown would be the possibility of “receiving a flash of light before it was emitted,” he added. The researchers set up an experiment in which the building blocks of light—which are bosons—were given some 10 billion opportunities to act in a way characteristic of fermions, but apparently never did. Bosons are special in that they can all occupy an identical so-called quantum state, a set of characteristics that includes location. As a result, for instance, any number of particles of light can be in the same space. Fermions can’t do this. And that is why, since most ordinary atoms consist of fermions, it seems obvious to most people that two objects can’t be in the same place at once. English and colleagues bombarded barium atoms with particles of light, called photons, from two identical laser beams. They looked for signs that the barium had absorbed two photons of the same energy at once. Generally speaking, events of this sort are common. The incoming light would kick one of the atom’s electrons, or electrically charged particles, into a higher-energy state. But the specific variety of electron energy jump being sought in this experiment was one that can only be triggered by bosons, not fermions. The reasons for this come down to math, including a consideration of the “spin” of particles, English said. Many fundamental particles behave in some ways as though they were spinning, although the exact relationship of this “spin” to ordinary spin, such as that of a top, is hard to pin down. The electron energy jump in question would have taken the atom from having a spin of zero to a spin of one, according to conventional physics. This means that the arriving photons must also have had a combined spin of one. This is mathematically impossible for equal-energy photons, unless they follow the equations that describe fermions, English explained. Consistent with these ideas, the required energy jump never happened, English said—though no physical principle except the boson-fermion distinction should prevent it. The energy jump would have been detected when the atoms emit a particular color of fluorescent light, the researchers noted. Similar conclusions about the boson-fermion distinction were reached by a 1999 experiment by scientists including the principal investigator in the new study, Dmitry Budker of the Berkeley National Laboratory in Berkeley. But the new tests improved the precision of the results by about 3,000 times, the scientists said, thus constraining the chance of bosons acting like fermions to less than one in a hundred billion in the system tested. The findings are published in the June 25 issue of the journal Physical Review Letters.