A Spin through Space-Time

A long-planned test of Einstein's theory is poised for takeoff

A satellite designed to test one of the more twisted predictions of Albert Einstein’s general theory of relativity is finally at its launch site after 40 years of preparation. The probe will look for evidence of a gravitational effect known as frame dragging. Just as a dipper drags honey along as it twirls in a honey jar, any spinning body in space, including Earth, ought to drag some space-time along with it. That was Einstein’s prediction, anyway. The effect has never been convincingly observed.

FLYING THERMOS. Stacked inside Gravity Probe B’s insulated tank of liquid helium, seen between the solar panels in this photo, are four gyroscopes and a telescope that can peer through the top hatch. GP-B Project
ROUNDNESS CHAMP. Nothing on Earth rivals the perfect roundness of the orb that each gyroscope on Gravity Probe B houses inside a quartz compartment, shown here in two pieces. GP-B Project

That’s partly because Earth’s tweaking of space should barely register on even the most sensitive instruments. Yet the effects of frame dragging may prove enormous in deep space where spinning, ultradense concentrations of mass known as supermassive black holes may torque space-time vigorously enough to create the enormously powerful jets of matter and energy known as quasars (SN: 4/5/03, p. 214: Cosmic Blowout: Black holes spew as much as they consume).

Many relativity experts are enthusiastic about the prospects for Gravity Probe B (GP-B), as the spacecraft is known. Gathering hard evidence that “space is not the fixed fabric we think of” would be a “stunning achievement,” says Clifford M. Will of Washington University in St. Louis, a gravitational physicist who served on a NASA-convened review panel that endorsed the mission’s science goals last spring.

He adds, “It’s the kind of result that will be written in physics textbooks for years to come.”

No one can say what the probe will find. Its measurements might confirm Einstein’s prediction, or it might find discrepancies. Such anomalies could provide crucial clues for a model of the universe that might ultimately succeed relativity.

Other researchers, who are less sanguine about the mission, say that its scientific value has declined drastically during its long period of development. Those critics argue that the mission’s estimated $700 million cost would have been better spent elsewhere.

Says physicist Kenneth Nordtvedt of Northwest Analysis in Bozeman, Mont., “The survival of GP-B through several decades . . . reveals to me how dysfunctional NASA has been in planning their strategy in this field of fundamental science.”

Let’s twist again

In his 1916 general theory of relativity, Einstein proposed that massive bodies cause space-time to curve. What’s more, he showed that gravity, which appears as an attraction between massive objects, is actually a manifestation of that curvature of space-time (SN: 12/21&28/02, p. 394: Getting Warped).

Two years after Einstein unveiled his general theory of relativity, Austrian physicists Joseph Lense and Hans Thirring deduced from it that space-time would become twisted in the vicinity of a rotating body (SN: 11/15/97, p. 308).

Then, in the late 1950s, Stanford University physicist Leonard I. Schiff and George W. Pugh of the Defense Department independently proposed detecting Earth’s frame dragging by sending an extremely stable gyroscope into an orbit that crosses the planet’s poles. If Earth were indeed twisting space-time, the gyroscope’s axis of rotation would tilt.

A gyroscope is a spinning object, usually a wheel, mounted in a frame that can swivel freely. The wheel’s spin produces inertia that keeps the wheel’s spin axis pointed in a fixed direction, so gyroscopes have long served as stable references for compasses and navigation systems.

The spacecraft, now in a prelaunch facility at Vandenberg Air Force Base near Lompoc, Calif., remains true to the original concept. In essence, the 3.5-ton, 7-meter-high satellite is a quartet of gyroscopes surrounded by much ancillary equipment. Some of that equipment keeps the gyroscopes ultracold–a requirement for high precision. Other features, such as a telescope and finely tuned thrusters, enable the spacecraft to stay exactly oriented on a distant star.

The satellite’s fixed orientation is intended as a reference against which to compare the gyroscopes’ orientations. Frame dragging is expected to make each gyroscope’s spin axis drift just 42 milliarc-seconds per year in the direction of Earth’s rotation. That’s hardly more than ten millionths of a degree.

To create gyroscopes sensitive enough to register such minute rotations, the GP-B team has crafted niobium-coated, solid quartz spheres the size of ping-pong balls (SN: 3/3/90, p. 143). Nowhere do these silvery orbs deviate by more than 40 atoms from perfect sphericity. In each gyroscope, one of these balls will spin at 10,000 revolutions per minute while floating weightless within a chamber.

Besides the subtle drift, or precession, due to frame dragging, the mission will also be looking for another, more readily detectable effect predicted by the general theory of relativity. Known as geodetic precession, this effect is expected to shift the gyroscopes’ spin axes by more than 150 times as much as frame dragging does.

However, in this case, the gyroscopes’ axes should swing in the direction of the satellite’s polar orbit around Earth, perpendicular to the direction of the frame-dragging effect. Einstein’s theory predicts that such gyroscopes will undergo geodetic precession merely because space-time is curved in the planet’s vicinity.

This effect would show up even if Earth were not spinning.

It’s all relative

Although geodetic precession seems huge compared with frame dragging, both effects are minuscule. GP-B is expected to measure frame dragging to an accuracy of 0.1 percent and geodetic precession to 0.0006 percent, or 6 parts per million.

At least for frame dragging, GP-B’s expected accuracy is not really a coup, says Nordtvedt. By bouncing laser pulses off reflectors on the lunar surface so as to precisely monitor the Earth-moon separation, he and other researchers claim to have already confirmed to an accuracy of 0.1 percent that Earth’s frame dragging matches the predictions of general relativity.

Using the distance measurements, Nordtvedt’s team calculated the strength of the so-called gravitomagnetic field. Gravitomagnetism is a relativistic addition to the ordinary Newtonian version of gravity, which doesn’t take the motions of bodies into account. It’s the gravitomagnetic field that causes frame dragging, Nordtvedt says.

As welcome as measurements of the gravitomagnetic field are, they’re only “indirect evidence” of frame dragging, contends Stanford University physicist C. W. Francis Everitt, who has led the GP-B project for decades.

“GP-B will provide a direct measurement,” noted the review panel that last April endorsed the mission. “No other laboratory or space experiment, current or near term, has the capability to measure this effect to comparable precision.”

Indeed, much of the long development of GP-B has gone to creating the gyroscopes and other technological wonders essential to making such fine measurements.

Whatever the scientific merits of GP-B, “everyone universally acknowledges that this is . . . a beautiful instrument technologically,” says NASA’s Michael H. Salamon, who oversees the project for the agency’s Office of Space Science in Washington, D.C.

Most likely, the mission will provide an anticlimactic result–a confirmation of the frame dragging that most physicists already accept. After all, general relativity has so far withstood all tests that scientists have thrown at it.

Moreover, no current theory predicts a value for frame dragging that differs from general relativity’s prediction enough for this mission to discern.

More thrilling would be a result that deviates from the predictions. But convincing gravity specialists of the result’s validity would be difficult, unless the GP-B team could rule out all sources of instrument error, says gravitation theorist M. Coleman Miller of the University of Maryland at College Park, who is not associated with the mission.

GP-B actually stands a better chance of finding a flaw in Einstein’s prediction of geodetic precession. From that result, mission scientists expect to compute a parameter known as gamma, which specifies how strongly mass distorts space-time. So far, none of scientists’ many measurements of gamma, using telescope observations and space probes, has deviated from Einstein’s predictions. However, GP-B’s accuracy is expected to surpass them all.

“Some people view this part of GP-B as the more important test,” Will says. That’s because some theories related to string theory (SN: 3/23/02, p. 187: The Black Hole Next Door)–a model in which infinitesimal, stringlike entities are the basic components of the universe rather than the pointlike particles favored today–predict a different value of gamma than general relativity does.

Grand finale

For GP-B to have reached the launch site is nothing short of a miracle to many people. No mission in the history of NASA has gone through such a long gestation period–or, perhaps, such a rocky one.

For most of its first 30 years, the project was a relatively minor technology-development effort on which NASA spent roughly $20 million. Then, in the early 1990s, the agency elevated the project to mission status and started pumping some $50 million per year into the effort.

During its long life, the project has surmounted one technical or political hurdle after another (SN: 6/10/95, p. 367). NASA has canceled and then reinstated the mission seven times. For his part, Everitt has campaigned tirelessly for the project in Congress and elsewhere.

In the past few years, the project has been particularly troubled. Originally scheduled to take off in December 1999, the mission missed that opportunity because a gyroscope malfunctioned, among other problems. Since then, GP-B has missed four rescheduled launch dates, in part because of technical problems. Those delays added $166 million to the mission cost, creating rancor among astronomers and other gravitation researchers whose projects were cut or postponed to keep GP-B alive.

One failure had a silver lining, recalls Everitt. The spacecraft flunked a crucial test in December 2002, leading to a threat last spring by NASA to cancel the project yet again. In that trial and a follow-up test that the spacecraft passed, Everitt and his colleagues found that their measurement accuracy would be “about a factor of 10 better than we originally thought,” he says.

He attributes the windfall to “the extreme care of the people building the apparatus.” Without that accuracy boost, the mission’s forthcoming geodetic-precession measurement would have already been eclipsed by a recently published result.

In the Sept. 25 Nature, Bruno Bertotti of the University of Pavia in Italy and his colleagues report a new measure of gamma made by sending radio signals to the Saturn-bound spacecraft Cassini at a time when the sun was between it and Earth (SN: 10/11/03, p. 238: Available to subscribers at Cassini confirms Einstein’s theory). The new finding agrees with Einstein’s predicted value for gamma to an accuracy of 23 parts per million. GP-B is expected to measure gamma 2.5 to 6 times as precisely.

Under close scrutiny from NASA officials, the GP-B team has completed all major technical and procedural requirements to meet its current launch date of Dec. 6. There’s no spare probe, so for Everitt and others who have worked on the project

for much of their careers, a crash of the launch vehicle would be devastating.

Everitt puts it this way: “Suppose you were driving down the freeway and a 10-wheeler truck hit you. Would you be upset?”

Assuming the launch goes well, the pace of the mission will be intense. As soon as the spacecraft is aloft, a clock will begin ticking. After decades of buildup, the mission will have only 16 to 18 months to check the equipment’s functions, take data for 13 months, and then carry out postexperiment validations of the instrumentation. After these months, the liquid helium that chills the instruments and serves as exhaust gas for the thrusters will be spent. Then, whatever the future of Einstein’s space-time models, GP-B’s long-awaited test of them will quietly come to an end.


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