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. This is the final part of a four-part series on Gravity Probe B.
What is the outcome and lessons of the GP-B experiment? According to Francis Everett of Stanford, a principal investigator on GP-B, “The background effects are going to be five or six orders of magnitude smaller [than the Einstein effects the experiment is looking for], so it will be an extraordinarily strong test.”
But the “bottom line” was this: the results from the analysis GP-B data were inconclusive and disappointing. It’s not that the probe proved or disproved the theories it set out to validate, or that frame dragging was or wasn’t observed. It’s just that irreducible error sources – some anticipated, and others not – meant there was too much uncertainty in the data, preventing researchers from developing any conclusions with a high degree of confidence.
What was the source of the error? In retrospect, the scientists concluded that one extremely subtle source of error in the gyros – as was everything associated with GP-B – had not been fully grasped. The niobium outer layer of the gyroscope rotors still retained a small electric polarity. As the gyros spun at their high rpm, these polarizations appeared as tiny magnetic fields on the rotors’ surface.
The GP-B team had anticipated this difficulty to some extent. However, it still evaded assessment, and it became the error source, the one that added unresolvable data noise to the project. These magnetic fields added a tiny but still significant wobble to their spin, in addition to the several other wobbles which were due to basic physics and so had been modeled and factored into the data analysis correction algorithms.
While this latest wobble could also be modeling and then subtracted to some extent, the modeling could not be precise enough for the experiment. The result was that the SNR of the data was marginal, and extensive calculations would have to be done on the terabytes of data to attempt to, just maybe, extract the true frame-dragging data. But the level of confidence in the resultant data was only about 1%, to use a gross simplification of the situation.
Despite an upbeat but cautious report on the probe’s results by the GP-B project leaders, claims of success and frame-dragging confirmation (met with skepticism) and even some multi-million-dollar private funds and grants to extend it, the NASA cost/benefit/risk analysis concluded it was not worth continuing. The lack of clear results and the presence of irreducible errors versus the cost of any continuation were critical factors. Note that the GP-B project was huge in both cost and personnel involving hundreds of researchers ranging from experienced, very senior academics down to Ph.D. candidates and post-doctoral fellows, and even some college students. GP-B completed its data collection operations and was decommissioned in December 2010.
Regardless of the results, the entire experiment has pushed the technological and experimental boundaries of instrumentation precision, as well as some more seemingly mundane challenges. Gravity Probe B has inspired advances in gyro fabrication, suspension, spin, and readout; near-perfect elimination of interfering magnetic fields; telescope pointing and control; large-scale Dewar technology; design simulation of every aspect, including modeling and control of supercooled helium sloshing and boil-off, and component fabrication, calibration, and test. For more physics, technical details, and insight.
For example, in the 1960s, when the project was first conceived, no one had flown a large quantity of liquid helium in space (helium liquefies at about 4 K, or −269 °C). Adequate Dewar flasks did not exist, and the sloshing of the superfluid in a non-gravity environment (called ullage) would have caused many problems. But in 1970, a graduate student at Stanford University developed a porous plug that uses a unique characteristic of the supercooled fluid called fountain pressure to control the helium boil-off (Figure 1). This innovative was flown successfully before GP-B on major scientific experiments, including the highly successful COBE (Cosmic Background Explorer), which studied the back-body radiation at 3K, which pervades the universe and is generally presumed to be a remnant of the Big Bang.
“Competition” and GP-B
The most serious competition GP-B’s frame-dragging “confirmation” came from the LAGEOS experiment, in which laser ranging accurately tracked the paths of two laser geodynamics satellites orbiting the earth. The first published result from LAGEOS, launched in 1976, cited an error in its measurements of 20 to 30% due to issues with modeling o the Earth’s gravitational field. (LAGEOS is a truly unusual satellite: at about 400 kilograms, it is totally passive, with no onboard sensors or electronics and no moving parts but and covered with 426 retroreflectors, so it looks like a giant golf ball.) However, the subsequent GRACE geodesy mission (launched in 2002) provided significantly improved Earth-gravity models, which were used in analysis of the LAGEOS data and yielded tests with an uncertainty of approximately 10%.
In addition, the project faced “competition” from the Laser Interferometer Gravitational-wave Observatory (LIGO) search for the gravity waves also predicted by Einstein. Although detection of these waves is not the same as verification of the frame-dragging phenomenon, they do have somewhat of a relationship, and it seemed to many as if two somewhat-related projects were excessive. Further, LIGO is Earth-based based and thus amenable to upgrades, modifications, repairs, and other flexibilities which the orbiting GP-B probe simply could not offer. In fact, the LIGO project was extremely successful, and conclusively detected these waves beginning in 2015. The work earned a Nobel prize in Physics for the lead researchers, and world-wide admiration and awe. There are many readable references about the LIGO project. I like the two listed in the references.
Was GP-B a success, failure, both, or neither? It’s not a question with a simple answer. While it didn’t succeed in answering the frame-dragging question, it did advance may other types of analysis, technologies, and techniques. Perhaps the GP-B team overreached, or perhaps they just couldn’t anticipate everything as well as they needed to – after all, it’s easy to look back and say what they should have done or not done. There is no doubt that GP-B and the thousands who worked on it in one way or another devised some amazing scientific and engineering innovations with almost inconceivable precision across multiple disciplines. Just as the I. I. Rabi’s experimental determination of the magnetic moment of atomic nuclei in the 1930s became the basis for the development of magnetic resonance imaging (MRI) in the 1970s, there’s no way to know now which, if any, of their advances may lead to unrelated advances.
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