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Overview of Gravity Probe B
Posted: April 17, 2004

In 1916, in what has been called one of the most brilliant creations of the human mind, Albert Einstein formulated his general theory of relativity, which stands as one of the foundational theories of modern physics. The stuff of both hard science and science fiction, Einstein's theory weaves together space, time, and gravitation, and predicts such bizarre phenomena as black holes and the expanding Universe, yet it remains arguably the least tested of scientific theories. Now, 88 years later, a team from Stanford University, NASA, and Lockheed Martin is poised to launch the Gravity Probe B spacecraft to measure two of Einstein's oddest predicted effects.

The first of these effects, known as the geodetic or curved spacetime effect, postulates that any body in space warps or curves its local spacetime. This is Einstein's theory that gravity is not an attractive force between bodies, as Isaac Newton believed, but rather the product of bodies moving in curved spacetime. One way to visualize this effect is to think of local spacetime as a flat bedsheet and the Earth a bowling ball lying in the middle. The heavy ball warps or puts a dent in the bedsheet, so that a marble (another celestial body) moving along the bedsheet will be inexorably drawn down the warped slope towards the massive ball. The geodetic effect being measured by Gravity Probe B is the amount of the tiny angle by which the Earth is warping its local spacetime.

The other effect, known as "frame-dragging," was postulated by Austrian physicists Josef Lense and Hans Thurring two years after Einstein published his general theory of relativity. It states that as a celestial body spins on its axis, it drags local spacetime around with it, much like a spinning rubber ball in bowl of molasses drags around some of the molasses as it spins. Particularly intriguing, the frame-dragging measurement probes a new facet of general relativity - the way in which space and time are dragged around by a rotating body. This novel effect closely parallels the way in which a rotating electrically charged body generates magnetism. For this reason it is often referred to as the "gravitomagnetic effect," and measuring it can be regarded as discovering a new force in nature, the gravitomagnetic force.

The experiment aboard the Gravity Probe-B spacecraft is designed to measure these effects with unprecedented precision and accuracy. Basically, the spacecraft consists of a polar-orbiting satellite containing four ultra-precise spherical gyroscopes and a telescope - which is like saying that an aircraft carrier consists of a some sophisticated fighter planes and a floating runway. In other words, there's a whole lot more going on.

The gyroscopes must be maintained in a pristine environment, in which they can spin in a vacuum, unhindered by any external forces, magnetic disturbances from Earth, or disturbances from the satellite itself. At the beginning of the experiment, the telescope (and satellite) are aligned with a distant "guide" star. The gyroscopes are aligned with the telescope, so that initially, their spin axes also point to the guide star. The gyroscopes are spun up, and over the course of a year, while keeping the telescope (and satellite) aligned with the guide star, the gyroscopes' spin axes are monitored to detect any deflection or drift due to the geodetic and frame-dragging relativistic effects.

If the predictions based on Einstein's theory are correct, the gyroscopes' spin axes should slowly drift away from their initial guide star alignment - both in the satellite's orbital plane, due to the curvature of local spacetime, and perpendicular to the orbital frame due to the frame-dragging effect. While physicists believe that the effects of relativity - especially the frame-dragging effect - are enormous in the vicinity of black holes and other massive galactic bodies, around a small planet like Earth, these effects are barely noticeable. For example, the predicted angle of spin axis deflection due to the frame-dragging effect corresponds to the width of a human hair as seen from 10 miles away!

When Stanford physicist Leonard Schiff (and independently, George Pugh at the Pentagon) first proposed this experiment in 1960, America had just created NASA, launched its first satellite, and entered the space race. Landing men on the Moon was still 10 years away. At the time, this experiment seemed rather simple, but it has taken over four decades of scientific and technological advancement to create a space-borne laboratory and measurement instrument sophisticated and precise enough to quantify these minuscule relativistic effects.

Gravity Probe-B's measurement of the geodetic effect, the larger of the two, will be to an accuracy of 0.01%, which is far more accurate than any previous measurements, and will provide the most precise test ever of general relativity. The frame-dragging effect has never directly been measured before, but Gravity Probe-B is expected to determine its accuracy to within 1%.

At least nine new technologies had to be invented and perfected in order to carry out the Gravity Probe B experiment. The spherical gyroscopes have a stability more than a million times better than the best inertial navigation gyroscopes, and the magnetometers, called SQUIDs (Super-Conducting Quantum Interference Devices), that monitor the spin axis direction of the gyroscopes, can detect a change in spin axis alignment to an angle of approximately 1/40,000,000th of a degree. These advances were only possible through GP-B's unique combination of cryogenics, drag-free satellite technology and new manufacturing and measuring technologies.

Over its 40+ year lifespan, spin offs from the Gravity Probe B program have yielded many technological, commercial, and social benefits. The technological benefits include cryogenic products used in other NASA missions, Global Positioning Satellite (GPS) products used in aviation and agriculture, optical bonding and fused quartz technologies that have commercial applications, and photo diode detectors that have ramifications for digital camera improvements.

Less tangible, but perhaps most important, the Gravity Probe B program has had a profound effect on the lives and careers of numerous faculty and students - both graduate and undergraduate, and even high school students, at Stanford University and other educational institutions. Nearly 100 Ph.D. dissertations have been written on various aspects of this program, and GP-B alumni include the first woman astronaut, the CEO of a major aerospace company, professors at Harvard, Princeton, Stanford and elsewhere, and a recent Nobel Laureate in Physics.

Gravity is the most fundamental force in nature; it affects all of us all the time. But, gravity is still an enigma - we don't completely understand it. Einstein's 1916 general theory of relativity forever changed our notions of space and time, and it gave us a new way to think about gravity. If the Gravity Probe-B experiment corroborates the two predictions of general relativity, then we will have made the most precise measurement of the shape of local spacetime, and confirmed the mathematics of general relativity to a new standard of precision. If on the other hand, the results disagree with Einstein's theoretical predictions, then we may be faced with the challenge of constructing a whole new theory of the universe's structure and the motion of matter. Whatever the result, Gravity Probe B will provide us another glimpse into the sublime structure of our universe.