Four superconducting ping-pong balls floating in space have just confirmed two key predictions of Einstein’s general relativity, physicists announced in a press conference May 4.

“We have completed this landmark experiment testing Einstein’s universe, and Einstein survives,” said physicist Francis Everitt of Stanford University, the principal investigator on NASA’s Gravity Probe B mission.

The probe, which launched in 2004, was designed to test the effect Earth’s gravity has on the space-time around it. According to Einstein, the Earth warps its local space-time like a bowling ball sitting on a trampoline, a phenomenon called the geodetic effect. This effect means that a circle of fabric with the Earth’s circumference, about 24,900 miles, would be pulled into a shallow cone with a circumference 1.1 inches shorter.

The Earth also swirls the nearby space-time around with it as it rotates, like water spiraling around a drain, in an effect called frame-dragging.

“Picture the Earth immersed in honey, and you can imagine the honey would be dragged around with it,” Everitt said. “That’s what happens to space-time. Earth actually drags space and time around with it.”

Both effects are minuscule — Einstein himself wrote that “their magnitude is so small that confirmation of them by lab experiments is not to be thought of.” But Gravity Probe B measured them both. The results will be published in Physical Review Letters.

The spacecraft orbited the Earth for 17 months carrying four ping-pong ball sized gyroscopes. The gyroscopes were made of fused quartz spheres, which hold the Guinness Book record for “most spherical man-made object.” The spheres were covered in a soft metal called niobium and cooled to the temperature of liquid helium.

At that temperature, niobium becomes superconducting, which means that electrons can flow forever without losing energy. When the spheres are set spinning, the circling electrons give rise to a little magnetic pointer.

In Newton’s universe, that pointer would point in the same direction forever as the spacecraft circled the Earth. But in Einstein’s universe, where Earth twists and tugs the space-time around it, the gyroscopes’ pointer was sent atilt at a sliver-thin angle. The north-south tilt measured the geodetic effect, and the east-west tilt measured frame-dragging.

The pointer shifted by just 6,000 milliarcseconds — the width of a human hair as seen from 10 miles away — over the course of a year, Everitt said. Despite the difficulty in detecting such a small tilt, the physicists were able to confirm the geodetic effect to an accuracy of 0.28 percent, and frame-dragging to within 20 percent.

Because general relativity describes the large-scale structure of the universe, the Gravity Probe B results could help improve physicists’ understanding of cosmic phenomena from black holes to gamma-ray bursts, Everitt says.

Gravity Probe B is one of the longest-running NASA projects ever. It started in 1963, before men walked on the moon. It took five decades to develop the technologies to build gyroscopes sensitive enough to see gravitational effects. In the meantime, those technologies found homes in a host of other NASA Earth-observing satellites, plus the Cosmic Background Explorer satellite, which measured the cosmic microwave background and provided Nobel Prize-winning evidence for the big bang.

Physicist Clifford Will of Washington University in St. Louis, head of the external review board for Gravity Probe B, called the research team’s efforts “heroic” and stressed the importance of testing fundamental theories of nature, not just taking them for granted.

“It is popular lore that Einstein was right, but no such book is ever completely closed in science,” he said. “While the result in this case does support Einstein, it didn’t have to.”

Image: An artist’s rendition of the way the Earth warps space-time, called the geodetic effect. NASA/Gravity Probe B

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