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Einstein's relativity theory survives test in space
BY STEPHEN CLARK
SPACEFLIGHT NOW
Posted: 05 May 2011


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NASA released results Wednesday from a revolutionary satellite that probed Einstein's general theory of relativity, confirming the Earth's gravity and rotation warp space and time as predicted.


Artist's concept of Gravity Probe B, Earth and the warped web of space and time. Credit: NASA/Stanford University
 
Using fast-spinning gyroscopes in a satellite 400 miles above Earth, the Gravity Probe B mission measured how the tug of Earth's gravity and the planet's spin distorts space and time in our own cosmic neighborhood.

The findings have far-reaching implications because they verify two crucial tenets of Albert Einstein's general theory of relativity, the standard of modern astrophysics that guides research into black holes, active galactic nuclei and gamma ray bursts, the most powerful explosions in the universe.

More massive objects in the distant cosmos have more fundamental and obvious effects on space and time.

"There is a strong link between Gravity Probe B and some of the most energetic and cataclysmic events in the universe," said Clifford Will, a physics professor at Washington University in St. Louis.

Francis Everitt, Gravity Probe B's principal investigator from Stanford University, said the mission's results were nearly identical to what Einstein's theory predicted.

"We completed this landmark experiment testing of Einstein's universe, and Einstein survives," Everitt said Wednesday.

Earth bends space and time in two ways.

First, the planet's immense gravity field warps the space-time continuum. And secondly, Earth's rotation drags space and time along as the planet spins on its axis.

Earth's effect is so small it took custom-built gyroscopes, a specially-constructed glass telescope and nearly 50 years of development to produce the mission's indisputable results.

"Imagine the Earth as if it were immersed in honey. As the planet rotates, the honey around it would swirl, and it's the same with space and time," Everitt said. "GP-B confirmed two of the most profound predictions of Einstein's universe, having far-reaching implications across astrophysics research. Likewise, the decades of technological innovation behind the mission will have a lasting legacy on Earth and in space."

NASA started developing a relativity gyroscope experiment in 1963. Decades of innovation yielded technologies to control drag, magnetic fields and temperatures aboard the satellite, which had to be free of disturbances to ensure its results weren't polluted by other factors.

Scientists finally launched the spacecraft on a Delta 2 rocket in April 2004.

Thought to be the most perfectly round objects ever manufactured, Gravity Probe B's four niobium-coated quartz gyroscopes, each about the size of a ping pong ball, spun up to 5,000 rpm during the satellite's mission. An ultra-precise telescope made of fused quartz glass pointed at a guide star named IM Pegasi.

The star served as a reference point for Gravity Probe B's measurements, which counted on the satellite's gyroscopes to wobble like a top. If the rotating gyros oscillated, it would support Einstein's theory.

"Picture a gyroscope moving in orbit around the Earth and let's ask what happens," Everitt said. "If we live in (Isaac) Newton's universe where space-time is absolute, and you have a purely imaginary, perfect gryo, and you point it at a point in space, nothing will happen. Einstein's universe is different. In Einstein's universe, space and time are warped by gravity."

It turns out the axis of the spinning gyroscopes drifted, but just barely.


Sketch of the Gravity Probe B spacecraft. Credit: NASA/Stanford University
 
"Basically you have a rotating top, so you look at its precession or how much it wobbles," said Colleen Hartman, a senior NASA advisor and professor at George Washington University.

Gravity Probe B's science-gathering phase ended in late 2005, but researchers spent the next five years meticulously analyzing the mission's data, removing potential errors and tracking down inconsistencies in its measurements until they were satisfied the satellite upheld Einstein's theory.

Calibration efforts showed Gravity Probe B's gyroscopes were exhibiting signs of unpredicted torques, or spinning. After extensive analysis, scientists concluded the torques were due to a resonance between the rotating gyros and movements of the spacecraft. Everitt said the science team calibrated the science data to account for the unexpected torque.

In the end, researchers say Earth's gravity and rotation distorts space-time by about 1.1 inches in 25,000 miles, about what Einstein's theory forecasts.

Gravity Probe B measured the effect of Earth's gravity, also known as the geodetic effect, is responsible for a bend of about 6,601.8 milliarcseconds per year in space and time. That's less than two one-thousandths of a degree.

The "frame-dragging" effect, in which space-time is twisted by the Earth's rotation, accounts for about 37.2 milliarcseconds of warping each year, but the margin of error for that result is about 20 percent.

A milliarcsecond is the width of a human hair as seen from a distance of 10 miles.

The "amazingly sensitive" scientific instruments was immersed in 600 gallons of superfluid helium, a cryogenic liquid that cooled the experiment to nearly absolute zero, Everitt said. The helium naturally boiled off during the mission, limiting Gravity Probe B's science phase to 17 months.

The frigid temperatures allowed the niobium-covered gyros to become superconducting, where the rotor loses electrical resistance and its magnetic field aligns with a spin axis determined through the moment generated by a spinning superconductor.

The superconducting magnetic effect is known as the London moment.

"The mission results will have a long-term impact on the work of theoretical physicists," said Bill Danchi, senior astrophysicist and program scientist at NASA Headquarters in Washington. "Every future challenge to Einstein's theories of general relativity will have to seek more precise measurements than the remarkable work GP-B accomplished."

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