Could self-moving objects explain away 'dark matter'?
Apr 06,2007 00:00 by Bend_Weekly_News_Sources

A phys­i­cist has cal­cu­lat­ed that in four spe­cial places on Earth each year, for a thou­sandth of a sec­ond, ob­jects might be able to move slight­ly with no push of any sort.

The prop­o­si­tion stems from an un­u­su­al the­o­ry of mo­tion that some re­search­ers have de­vised to ex­plain move­ments of stars and galax­ies which, oth­er­wise, seem to vi­o­late the laws of grav­i­ty. For in­stance, stars ap­pear to or­bit the cen­ters of galax­ies faster than they should.

The pre­vail­ing ex­pla­na­tion for this is “dark mat­ter”: the idea that un­seen blobs of an un­i­den­ti­fied sub­s­tance per­vade the uni­verse. This would pro­vide sources of grav­i­ta­tion­al pull which, added to the vis­i­ble sources, ac­count for the odd­i­ties.

The red square marks a spot in Green­land at 79°50' North lat­i­tude, 56° West lon­gi­tude, where strange laws of mo­tion could take hold next year, ac­cord­ing to a stu­dy. 

Some phys­i­cists con­sid­er dark mat­ter a prov­en fact. But a mi­nor­i­ty dis­a­g­ree, un­set­tled by the fact that dark mat­ter has nev­er been found, even though it would have to out­weigh reg­u­lar mat­ter by five­fold.

A num­ber of these skep­tics have de­vel­oped al­ter­na­tive the­o­ries that ac­count for the mys­te­ri­ous mo­tions through slight chang­es to the tra­di­tion­al laws of gra­v­i­ty and mo­tion—prin­ci­ples de­vel­oped by Isaac New­ton in the 1600s and elab­o­rat­ed by Al­bert Ein­stein in the last cen­tu­ry.

The re­vised the­o­ries are known as mod­i­fied New­ton­ian dy­nam­ics, or MOND.

A com­mon ver­sion of MOND holds that one of the most venera­ble laws of mo­tion, called New­ton’s Sec­ond Law, must be re­vised to give dif­fer­ent re­sults at ex­treme­ly low ac­cel­er­a­tions. 

Tra­di­tion­ally, the law states simp­ly that an ob­jec­t’s ac­cel­er­a­tion is pro­por­tion­al to the force on it; in eve­ry­day lan­guage, the harder you push it, the faster it will move. But in the re­vised re­gime, at near-zero ac­cel­er­a­tion, ob­jects would move ac­cord­ing to a dif­fer­ent law.

The trick is how to de­fine near-zero ac­cel­er­a­tion. Ze­ro with re­spect to what? Eve­ry­thing moves with re­spect to some­thing else. 

In a new pa­per, Al­ex Ig­natiev of the The­o­ret­i­cal Phys­ics Re­search In­sti­tute in Mel­bourne, Aus­tral­ia, pro­poses that in our neigh­bor­hood of the uni­verse, the ef­fects of the re­vised law would be­come noticea­ble in places that are at near-zero ac­cel­er­a­tion with re­spect to the cen­ter of our gal­axy.

Be­cause Earth spins, rotates and or­bits in var­i­ous ways, its mo­tion over­all would pre­clude tests of the idea. But Ig­natiev cal­cu­lat­ed that twice year­ly, there are two points on the sur­face where all such mo­tions can­cel out, put­ting these spots mo­men­tar­i­ly at near-zero ac­cel­er­a­tion with re­spect to the gal­axy cen­ter.

The events would take place near the equi­nox dates, two days in the fall and spring when the day is the same length as the night. The lo­ca­tions would dif­fer year by year. On Sept. 22 of next year, they would lie in north­ern Green­land and across the globe in Ant­arc­ti­ca.

Ac­cord­ing to Ig­natiev’s cal­cu­la­tions, if this ver­sion of MOND is cor­rect, an ob­ject at that lo­ca­tion might brief­ly shift lo­ca­tions by one fifth of a tril­lionth of a mil­li­me­ter, be­fore re­turn­ing to its orig­i­nal place a frac­tion of a sec­ond lat­er. The event would be measura­ble by in­stru­ments known as grav­i­ta­tion­al wave de­tec­tors, he ar­gued, which are built to meas­ure an ex­ot­ic phe­no­me­non in­volv­ing rip­ples in grav­i­ty.

The ex­per­i­ment could be a “ma­jor step” in re­solv­ing the “the ‘MOND ver­sus dark mat­ter’ dilem­ma,” Ig­na­tiev wrote.

Oth­er ex­per­i­ments that could re­solve this are pos­si­ble in prin­ci­ple, he not­ed, but most of these in­volve ob­ser­va­tions of places in space where the near-zero ac­cel­er­a­tion would be un­mea­sur­a­ble in prac­tice. The new pro­pos­al de­scribes for the first time a test that could take place on Earth, mak­ing it doa­ble, he added. The study ap­pears in the March 9 is­sue of the re­search jour­nal Phys­i­cal Re­view Let­ters. A copy is also posted online
here.

Courtesy World Science