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Could self-moving objects explain away “dark matter”?

March 20, 2007
Special to World Science  

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 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. 
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.

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.

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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 certain 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 pervades the universe. This would provide sources of gravitational pull which, added to the visible sources, would account for the oddities. Some physicists consider dark matter a proven fact. But a minority demur, unsettled by the fact that dark matter has never been detected, even though it have to outweigh regular matter by fivefold. A number of these skeptics have developed alternative theories that account for the mysterious motions through slight alterations to the traditional laws of gravity and motion, as developed by Isaac Newton in the 1600s and elaborated by Albert Einstein in the last century. These 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 accelerations, 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 the Earth is spinning, rotating and orbiting in various ways, its motion overall would preclude tests of the idea. But Ignatiev calculated that twice yearly, there would be two points on the surface where all such motions would 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 be in the north of 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 designed to measure exotic type of 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 would these involve observations of locations in space where the near-zero accelerations 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.