European "Disco Ball" Probe to Test Einstein's Relativity

[Nishahameetha]

In the mid-2000s, after more than 40 years of development, the $800-million Stanford University-led Gravity Probe B detected frame dragging. This effect, predicted by Einstein's theory, is caused by Earth's rotation dragging the fabric of space and time along with it.

But because of a technical glitch, the NASA craft was able to measure frame dragging with an estimated accuracy of no better than 20 percent.

Scientists working with the new Italian probe, which cost $10 million (U.S.) to build, hope to improve on those readings.

"If we reach one percent [accuracy]—and I am fairly confident we will—we'll have an order of magnitude's improvement" over Gravity Probe B's measurement, said mission leader Ignazio Ciufolini, of the University of Salento in Lecce, Italy.

LARES, for Laser Relativity Satellite, lifted off from a spaceport in French Guiana at 7 a.m., local time. Now in orbit, the probe will provide data on frame dragging for several years.

"Achieving the LARES design sensitivity would allow an impressive confirmation of general relativity," said Alan Kostelecky, a theoretical physicist at Indiana University in Bloomington who's not on the LARES team.

"Disco Ball" to Help Validate Einstein

LARES is essentially a really heavy disco ball: The totally passive probe is a solid sphere of metal—a tungsten alloy—that packs 880 pounds (362 kilograms) in a ball just 14 inches (35.5 centimeters) wide.

The probe is covered with reflectors so that, as the craft orbits Earth, an international network of laser-ranging stations will be able to track its position with millimeter-scale precision.

The craft's orbit around our planet will be tilted at an angle to Earth's Equator. From calculations based on Einstein's theory, Ciufolini's team expects that frame dragging will make LARES's orbital plane slowly precess, or rotate, around Earth as the probe gets dragged along with our planet's distortion of space-time.

Over a year, this effect should add up to just a few tens of millionths of a degree, meaning that it would take about ten million years for the probe's orbital plane to complete one full twist around te planet due to frame dragging alone.

Still, the small shift should amount to about 13 feet (4 meters) of displacement a year, which the laser-ranging system should be able to measure to less than one percent accuracy.

Don't Stop Testing General Relativity

LARES's high density makes the probe less sensitive to atmospheric drag, which at about 900 miles (1,450 kilometers) above Earth's surface will be very small but not negligible, Ciufolini said.

The dense sphere will also be virtually unaffected by radiation pressure from sunlight.

Other effects, such as the fact that Earth is not a perfect sphere, will actually make the probe's orbit precess by much larger amounts—one full twist every three years or so.

But the researchers will use various data-analysis techniques, together with data from previous missions, to extract the frame-dragging measurement.

In the end, Einstein's theory will probably pass the test: Scientists believe that general relativity should eventually break down, but only at scales so small that quantum physics comes into play.

Still, in science there's never such a thing as absolute certainty, Ciufolini said.

"General relativity has been confirmed spectacularly by all experiments for nearly a hundred years now," he said. "But that does not mean that one should stop putting it to the test."

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