NASA - National Aeronautics and Space Administration

Was Einstein right? NASA is gathering proof 88 years later.

To test this claim in a way never done before, NASA launched the Gravity Probe B (GP-B) mission in April 2004. If all goes well with this satellite experiment, we should know by mid-2006 whether Einstein's theory of curved spacetime is a true description of our universe.


Einstein's theory, more formally known as the Theory of General Relativity, states that any mass, such as the Earth, warps space and time around it. And any object traveling near the Earth must follow this curve or warp in spacetime. The result is that the object falls toward the Earth. Newton claimed the force of gravity caused this effect; Einstein claimed it was the warp of spacetime around the Earth.

Image right: Artist concept of Gravity Probe B spacecraft in orbit around the Earth (NASA/MSFC)

A second, and even more remarkable, Einstein prediction is that the spinning Earth should twist spacetime around with it -- an effect called "frame-dragging." Any object in space near the Earth should respond to that twist of spacetime by turning with it.

This is the principle behind the Gravity Probe B mission -- watching an object in space to see if it responds to "frame-dragging" caused by the Earth's rotation.

Inside this 21-foot-tall satellite, now orbiting 400 miles above the Earth, are four spinning spheres, called gyroscopes. At the beginning of the experiment, a telescope on-board the satellite and the spin axes of the four gyroscopes were all pointed directly at a distant "guide" star, named IM Pegasi. Throughout the experiment, the spacecraft's telescope remains pointed at IM Pegasi, providing a constant reference line for measuring any turn in the pointing direction of the gyroscopes' spin axes. If Einstein's prediction is correct, the twisting spacetime near the Earth should cause the gyroscopes' spin axes to turn away from this reference line by the very tiny angle of 1/100,000th of a degree over the course of a year.

To measure this miniscule turn, Gravity Probe B created four near-perfect spheres. Each gyroscope is less than three 10-millionths-of-an-inch from perfect roundness. They are so close to perfect, in fact, that they are listed in the 2005 edition of the Guinness Book of World Records as roundest objects ever made in the world. The spheres spin in near-isolation: in a near-vacuum, near absolute zero, suspended inside a housing, protected from any magnetic, electric or solar forces.

In fact, most of this experiment uses one-of-a-kind record-setting equipment. Gravity Probe B will be the longest space mission kept near absolute zero -- 450°F below zero for 12 months. It can sense any change in the gyroscopes' position to within one 10-millionth of a degree. It controls the motion of the satellite with thrusters that produce forces softer than a human breath.

"It's been a long, amazing road to get to this point," said Rex Geveden, deputy director of NASA's Marshall Space Flight Center in Huntsville, Ala., which manages the Gravity Probe B program for NASA. "When Gravity Probe B was first proposed more than 40 years ago, the technology required for this experiment did not yet exist. At least nine new technologies had to be invented and perfected."

If Gravity Probe B's results are consistent with Einstein's theory, it will help solidify our understanding of phenomena like black holes and quasar jets. If, however, its results are not consistent with the predictions of General Relativity, it will require a significant revision of our theory of the fundamental structure of the universe.

Was Einstein right? After two billion spins of the gyroscopes and 7,000 orbits of the Earth, Gravity Probe B should know the answer.

For more information about Gravity Probe B, visit: http://www.gravityprobeb.com/

Contact:
Steve Roy, Marshall Space Flight Center
256.544.0034