Two key predictions of Albert Einstein's general theory of relativity have been confirmed by NASA's Gravity Probe B mission, scientists announced this week.
"We've completed this landmark experiment testing Einstein's universe, and Einstein survives," principal investigator Francis Everitt, of Stanford University in California, said during a press briefing.
Launched in 2004, the Gravity Probe B mission used four ultraprecise gyroscopes—devices used to measure orientation—housed in a satellite to measure two aspects of Einstein's theory about gravity.
The first is the geodetic effect, which is the warping of space and time—or spacetime—around a gravitational body, such as a planet.
One common way to visualize the geodetic effect is to think of Earth as a bowling ball and spacetime as a trampoline. Earth's gravity warps spacetime the same way a bowling ball weighs down the middle of a trampoline.
The second effect of gravity tested by Gravity Probe B is frame dragging, which is the amount that a spinning object pulls the fabric of spacetime along with it.
Doing What Einstein Thought Impossible
To conduct these tests, Gravity Probe B used a device called a star tracker to keep one end pointed at a single star, IM Pegasi, while in a polar orbit 400 miles (644 kilometers) above Earth.
If we lived in a universe that behaved as envisioned by Isaac Newton—in which the geodetic effect and frame dragging don't occur—then the gyroscopes would stay aligned with the star forever.
In Einstein's universe, however, the direction of the spin axis of Gravity Probe B's gyroscopes should gradually change due to the mass and rotation of Earth.
"Imagine the Earth is immersed in honey, and you can imagine the honey would be dragged around and [an object in the honey] would also be dragged around," Everitt said. "That's what happens in the gyroscope."
Sifting through the data, the team found evidence of an angular change in the gyroscopes' orientation of about 6,600 milliarcseconds over the course of a year.
A milliarcsecond, Everitt explained, "is the width of a human hair seen at the distance of 10 miles [16 kilometers]. It really is a rather small angle, and this is the accuracy which Gravity Probe B had to achieve."
The change is so small, in fact, that Einstein didn't think measuring it was even possible.
In his 1953 book The Meaning of Relativity, Einstein wrote that frame-dragging effects "are actually present according to our theory, although their magnitude is so small that confirmation of them by laboratory experiments is not to be thought of."
But now, "thanks to NASA," Everitt said, "we've done more than think about them. We've actually measured them."
Gravity Findings to Unravel Distant Mysteries?
Although the results are only now being released, the Gravity Probe B satellite has completed its work, and it was decommissioned in December 2010.
Funded since 1963, Gravity Probe B is one of the longest running projects in NASA history. Scientists had the idea for the experiment before the required technology—such as the star tracker and gyroscopes—even existed.
The probe's predecessor, Gravity Probe A, was launched in 1976 and also confirmed a key aspect of Einstein's general theory of relativity, namely that a clock on Earth will run slower than one aboard an orbiting spacecraft. (Related: "Einstein's Relativity Affects Aging on Earth [Slightly].")
While it's been widely accepted that the geodetic effect and frame dragging occur, it was important to confirm them with experiments, physicist Clifford Will, of Washington University in St. Louis, said at the press conference.
"While the result in this case does support Einstein, it didn't have to," he said.
In addition, the findings, detailed online in the journal Physical Review Letters, may help scientists understand some of the most cataclysmic events in the universe.
"Measuring the frame-dragging effect caused by the Earth's rotation has implications beyond our planet," said Will, who was not involved in the Gravity Probe B project.
For example, he said, frame dragging likely plays a role in triggering energetic bursts from quasars, very distant galaxies that have actively feeding—and rapidly rotating—supermassive black holes at their hearts.