"Long before it's in the papers"
May 05, 2011

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“Confirmed”: spinning Earth drags space along

May 5, 2011
Courtesy of Stanford University
and World Science staff

Re­search­ers say they have con­firmed two pre­dic­tions of Al­bert Ein­stein’s gen­er­al the­o­ry of rel­a­ti­vity, con­clud­ing a pro­ject that has spanned more than half a cen­tury.

The first pre­dic­tion is the ge­o­det­ic ef­fect, or the warp­ing of space and time around a body that ex­erts gravita­t­ional force. The sec­ond is frame-dra­gging, which is the amount a spin­ning ob­ject pulls space and time with it as it ro­tates.

“Imag­ine the Earth as if it were im­mersed in hon­ey. As the plan­et ro­tated its ax­is and or­bited the Sun, the hon­ey around it would warp and swirl, and it’s the same with space and time,” said Fran­cis Everitt, a phys­i­cist at Stan­ford Un­ivers­ity in Cal­i­for­nia and prin­ci­pal in­ves­ti­ga­tor for the proj­ect, dubbed Gra­vity Probe B.

Pre­vi­ous stud­ies had al­so backed up the pre­dic­tions, but less di­rect­ly, the re­search­ers said. They used four ultra-precise gy­ro­scopes housed in a sat­el­lite to meas­ure the ef­fects. A gy­ro­scope is a spin­ning wheel mount­ed in a frame that lets the wheel keep its ori­enta­t­ion in­de­pend­ently of the move­ment of the frame.

Af­ter 52 years of con­ceiv­ing, build­ing, test­ing and wait­ing, the test meas­ured both ef­fects with un­prec­e­dent­ed pre­ci­sion by point­ing at a star, IM Pe­gasi, while in a po­lar or­bit around Earth, the sci­en­tists ex­plained. A po­lar or­bit is one in which a sat­el­lite cir­cles Earth while go­ing over the North and South poles on each or­bit.

If gra­vity did­n’t af­fect space and time, the gy­ro­scopes would point in the same di­rec­tion for­ev­er while in or­bit, re­search­ers said—but fol­low­ing Ein­stein’s gen­er­al the­o­ry of rel­a­ti­vity, they un­der­went meas­ur­a­ble, ti­ny changes in the di­rec­tion of their spin. The find­ings ap­pear on­line in the re­search jour­nal Phys­i­cal Re­view Let­ters.

The test “con­firmed two of the most pro­found pre­dic­tions of Ein­stein’s un­iverse, hav­ing far-reach­ing im­plica­t­ions across as­t­ro­phys­ics re­search,” Everitt said. 

“The dec­ades of tech­no­log­i­cal in­nova­t­ion be­hind the mis­sion will have a last­ing lega­cy,” he added. Much of the tech­nol­o­gy needed to test Ein­stein’s the­o­ry had­n’t yet been in­vented in 1959 when Leon­ard Schiff, head of Stan­ford’s phys­ics de­part­ment, and George E. Pugh of the U.S. De­fense De­part­ment in­de­pend­ently pro­posed to ob­serve the be­hav­ior of a gy­ro­scope in an Earth-or­biting sat­el­lite with re­spect to a dis­tant star. To­ward that end, Schiff teamed up with Stan­ford col­leagues and sub­se­quent­ly, in 1962, re­cruited Everitt.

NASA joined the proj­ect in 1963 with the in­i­tial fund­ing to de­vel­op the req­ui­site equip­ment. For­ty-one years lat­er, the sat­el­lite was launched in­to or­bit. The proj­ect was soon be­set by prob­lems and dis­ap­point­ment when an un­ex­pected wob­ble in the gy­ro­scopes changed their ori­enta­t­ion and in­ter­fered with the da­ta. It took years for a team of sci­en­tists to sift through the mud­dy da­ta and sal­vage the in­forma­t­ion they needed.

De­spite the set­back, Gra­vity Probe B’s dec­ades of de­vel­opment led to ground­break­ing tech­nolo­gies to con­trol en­vi­ron­men­tal dis­tur­bances on space­craft, such as aer­o­dy­namic dra­g, mag­net­ic fields and ther­mal varia­t­ions, phys­i­cists said.

In­nova­t­ions en­abled by the proj­ect have al­so been used in the Glob­al Po­si­tion­ing Sys­tem, such as carrier-phase dif­fer­en­tial GPS, with its pre­ci­sion po­si­tion­ing that can al­low an air­plane to land un­aided. Ad­di­tion­al tech­nolo­gies were ap­plied to NASA’s Cos­mic Back­ground Ex­plor­er mis­sion, which de­ter­mined the un­iverse’s back­ground radia­t­ion. That meas­urement is the un­der­pin­ning of the “big bang the­o­ry” and led to a No­bel Prize for NASA’s John Math­er.

“The mis­sion re­sults will have a long term im­pact on the work of the­o­ret­i­cal phys­i­cists for years to come,” said Bill Danchi, sen­ior as­t­ro­phys­i­cist and pro­gram sci­ent­ist at NASA Head­quar­ters in Wash­ing­ton.

* * *

Researchers have confirmed two predictions of Albert Einstein’s general theory of relativity, concluding one of the space agency’s longest-running projects. The first prediction is the geodetic effect, or the warping of space and time around a body that exerts gravitational force. The second is frame-dragging, which is the amount a spinning object pulls space and time with it as it rotates. “Imagine the Earth as if it were immersed in honey. As the planet rotated its axis and orbited the Sun, the honey around it would warp and swirl, and it’s the same with space and time,” said Francis Everitt, a physicist at Stanford University in California and principal investigator for the project, dubbed Gravity Probe B. Previous studies have also supported these predictions, but less directly, the researchers said. They used four ultra-precise gyroscopes housed in a satellite to measure the effects. A gyroscope is a spinning wheel mounted in a frame that lets the wheel keep its orientation independently of the movement of the frame. After 52 years of conceiving, building, testing and waiting, the test has measured both effects with unprecedented precision by pointing at a star, IM Pegasi, while in a polar orbit around Earth, the scientists explained. A polar orbit is one in which a satellite circles Earth while going over the North and South poles on each orbit. If gravity didn’t affect space and time, the gyroscopes would point in the same direction forever while in orbit, researchers said—but following Einstein’s general theory of relativity, they underwent measurable, tiny changes in the direction of their spin. The findings appear online in the research journal Physical Review Letters. The test “confirmed two of the most profound predictions of Einstein’s universe, having far-reaching implications across astrophysics research,” Everitt said. “The decades of technological innovation behind the mission will have a lasting legacy,” he added. Much of the technology needed to test Einstein’s theory hadn’t yet been invented in 1959 when Leonard Schiff, head of Stanford’s physics department, and George E. Pugh of the U.S. Defense Department independently proposed to observe the behavior of a gyroscope in an Earth-orbiting satellite with respect to a distant star. Toward that end, Schiff teamed up with Stanford colleagues and subsequently, in 1962, recruited Everitt. NASA joined the project in 1963 with the initial funding to develop the requisite equipment. Forty-one years later, the satellite was launched into orbit. The project was soon beset by problems and disappointment when an unexpected wobble in the gyroscopes changed their orientation and interfered with the data. It took years for a team of scientists to sift through the muddy data and salvage the information they needed. Despite the setback, Gravity Probe B’s decades of development led to groundbreaking technologies to control environmental disturbances on spacecraft, such as aerodynamic drag, magnetic fields and thermal variations, physicists said. Innovations enabled by the project have also been used in the Global Positioning System, such as carrier-phase differential GPS, with its precision positioning that can allow an airplane to land unaided. Additional technologies were applied to NASA’s Cosmic Background Explorer mission, which determined the universe’s background radiation. That measurement is the underpinning of the “big bang theory” and led to a Nobel Prize for NASA’s John Mather. “The mission results will have a long term impact on the work of theoretical physicists for years to come,” said Bill Danchi, senior astrophysicist and program scientist at NASA Headquarters in Washington.