An audacious and very costly orbiting science experiment set out to verify a subtlety of General Relativity by pushing the boundaries of science and engineering but ended with inconclusive and frustrating results.
On April 20, 2004, a Boeing Delta II rocket launched from Vandenberg Air Force Base carried Gravity Probe B into orbit and so initiated the next step in a nearly 50-year quest to verify a corollary of Einstein’s General Theory of Relativity. First conceived in 1960 and initially funded in 1964 but delayed for decades by financial, physics, and implementation challenges, this experiment embodied the ultimate in accurate measurement while identifying and then attempting to cancel or eliminate error sources. As with other beyond state-of-the-art engineering and science projects, GP-B (Gravity Probe B) is elegant in concept but brutal in details of its execution, and somewhat disappointing in its results.
We now accept Einstein’s Theory of Relativity as fact — and with good reason. Since he put forth the Special Theory in 1905 and followed it in 1916 with the General Theory, which links space, time, and gravity, both theories have been confirmed by innumerable tests, observations, and their ability to predict and explain large- and atomic-scale phenomena. Special relativity, summarized by the elegant formula E=mc2, is what the average person associates with relativity if they have any association at all.
But one subtlety of the General Theory, known as frame-dragging, had evaded direct experimental confirmation. The effect that the theory predicts is so subtle that only one observation, the Hulse-Taylor binary-pulsar discovery of the mid-1970s, has indirectly confirmed it.
What is frame-dragging? In simple terms, Einstein’s General Theory envisions the universe as composed of more than just “space,” but as space and time as a unified “fabric.” Any mass and its gravitational field will distort this time-space fabric and cause it to curve. Researchers have confirmed this effect, which is similar to what would happen if you put a basketball on a large, stretched-out net.
But it goes beyond that: just two years after the publication of the General Theory, physicists Josef Lense and Hans Thirring calculated that a rotating body in the space-time fabric would also slowly drag and twist the space-time around it. It would be like a spinning basketball grabbing and twisting some of the surrounding net as it spins (Figure 1). GP-B aimed to be the first direct experiment to confirm this frame-dragging effect (Figure 2).
An audacious project with precision almost beyond comprehension
The GP-B experiment has little resemblance to what goes on in a scientist’s lab. The entire unit was encased in a three-ton, 21-foot-long housing (Figure 3) and placed in a polar orbit at a 400-mile altitude. Typical of the experiment’s requirement for exceptional precision, the launch window each day for liftoff was just one second — if missed, the unit would be unable to achieve the precise polar orbit needed to align it with its guide star.
Many experienced partners contributed to the billion-dollar-plus GP-B project. Prime contractor Stanford University built the science instrument, working with the NASA Marshall Space Flight Center. Lockheed Martin Missiles and Space had a major subcontract, and the Harvard Smithsonian Astrophysical Observatory provided key guide-star measurements. (Note: Gravity Probe A was launched on June 18, 1976, for a planned 115-minute suborbital flight, solely to test some of the concepts and verify some critical data.)
The heart of GP-B is a set of four gyroscopes, but they have little resemblance to the crude ones sold as toys or even the precise ones used for missile guidance. The gyros are electrically spun spheres the size of ping-pong balls (with a 1.5-in. diameter), spinning in a vacuum at 10,000 rpm, housed in a nine-foot-long chamber.
The principle is deceptively simple: A perfect gyro stays pointed in the same direction in space. Any frame dragging distorts the space-time fabric and affects the gyro’s pointing. But this experiment poses two problems. First, imperfections in the gyro assembly cause it to drift. Second, the simple phrase “same direction” loses meaning in the context of the curvature of space-time, frame-dragging, and relativistic effects. To complicate the experiment, the geodetic effect of the gyros traveling through the space-time curvature is much larger than the frame-dragging effect.
Part 2 of this article will look at the objectives of GP-B and the plan for the experiment.
References – EE World Online
- “GPS, Part 1: Basic principles”
- “GPS, Part 2: Implementation”
- “Magnetic resonance imaging (MRI), Part 1: How it works”
- “MRI, Part 2: Historical development (and lawsuits)”
- “Gyroscopes, Part 1: Context and mechanical designs”
- “Gyroscopes, Part 2: Optical and MEMS implementations”
A project as extensive, expensive and recent as Gravity Probe B has an enormous set of documentation and references ranging from well-written articles to detailed Stanford NASA reports, as well as extensive images, photos, and graphics. Some of the references link, in turn, to other references. If you are interested in GP-B, there’s plenty to read and absorb, including a 600-page NASA final report posted online.
LIGO, LAGEOS, and GRACE missions
- LIGO Caltech, “2017 Nobel Prize in Physics Awarded to LIGO Founders”
- Photonics Media, “LIGO Continues Making Waves”
- NASA, “Now 40, NASA’s LAGEOS Set the Bar for Studies of Earth”
- NASA, “GRACE Mission Overview”
GP-B: Stanford University
- Stanford University, “Gravity Probe B: Testing Einstein’s Universe”
- Stanford University, ”Overview of the GP-B Mission”
- Stanford University, “The Extraordinary Technologies of GP-B”
- Stanford University, “Frequently Asked Questions”
- Stanford University, “Image Gallery”
- Stanford University, “Gravity Probe B Presentations” (organized list of various presentations)
- Stanford University, “Gravity Probe B Scientific Papers” (organized list of technical published papers)
- NASA, “ Science Results— NASA Final Report, December 2008”
- NASA, “NASA’s Gravity Probe B Confirms Two Einstein Space-Time Theories”
- NASA, “Post Flight Analysis–Final Report, March 2007” (600+ pages)
GP-B: other sources
- Aviation Week, “NASA Set to Test Einstein’s Theory,” April 12, 2004
- AAAS Science, “At Long Last, Gravity Probe B Satellite Proves Einstein Right,” May 4, 2011
- New Scientist, “Gravity Probe B scores ‘F’ in NASA review,” May 20, 2008
- APS Physics, “Viewpoint: Finally, results from Gravity Probe B,” May 31, 2011
- Sky & Telescope, “Gravity Probe B: Relatively Important?,” May 6, 2011
- IEEE Spectrum, “The Gravity Probe B Bailout,” October 1, 2008