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.
Gravity Probe B was intended to measure the unmeasurable. Part 1 explored the principle. This part explores the plan of the experiment and instrumentation.
The experiment had to have six major characteristics:
- a drift-free gyroscope with absolute drift from nonrelativistic effects of less than 10–11 degrees/hour (one-millionth the drift of the best guidance gyros);
- a gyro readout to determine changes in spin angle to 0.1 milliarc-second;
- a stable reference, based on a telescope, to relate the gyro and its assembly to a guide star;
- a guide star in the right area of the sky with well-known optical and radio-emission characteristics;
- a way to separate the two relativistic factors of frame-dragging and geodetic effects; and
- a way to calibrate the entire assembly after launch to eliminate larger scale instrumentation errors that may obscure the signals that indicate the desired effects.
(Note: a milliarcsecond is 1/1000th of one of an arcsecond or 1/3,600,000th of a single degree. This tiny angle is approximately the width of a human hair seen from 10 miles away, the width of a quarter from 6,000 miles away, or the height of an astronaut on the Moon viewed from the Earth.)
The core of the GP-B is a 21-in.-long block of fused quartz, which is bonded to a quartz telescope and contains the four gyros (Figure 1). It also houses a proof mass—in this case, a quartz sphere the same size as the gyro rotors—that floats in a vacuum cavity at the center of mass of the spacecraft. The role of the proof mass is to follow the ideal gravitational orbit. Sensing its position in the cavity and making compensating adjustments to the satellite-based on position changes cancel out the geodetic effect.
The four gyro rotors are made of fused quartz, fabricated to an extreme level of material homogeneity, and then ground to the near-absolute sphericity (Figure 2). The spheres are round to within 40 atomic layers, which is proportionally equivalent to an Earth-sized sphere with surface height variations of only 16 feet. Four gyros provide redundancy, and by spinning two of the gyros in one direction and two in the opposite direction, the system can cancel some error sources.
The gyro and sensing
It’s one thing to have a virtually perfect gyro rotor, but that alone does not provide the necessary performance for this experiment. The designers had to solve three design puzzles: how to suspend these rotors in their cavity without disturbing their spin, how to get and keep the rotors spinning, and how to read out the spin direction of a perfect, unmarked sphere.
They solved the first problem by levitating the rotors with three pairs of saucer-shaped electrodes. The electric fields center the rotors to a few millionths of an inch. They did not perform the spinning up electrically, however. Instead, they directed a precise stream of helium gas, traveling at nearly Mach 1, at the rotors. It takes about half an hour for the rotor to reach full speed, and it loses less than 1% of this speed over 1000 years in the super-vacuum of the cavity.
But the third problem proved the most challenging. The designers used superconductivity as the basis of a noninterfering pointer readout based on the SQUID (superconducting quantum interference device) (Figure 3). When a superconductor spins, it generates a magnetic field due to the London moment (named after Fritz London, the physicist who predicted it). To read the rotor-spin direction, the designers coated the rotors with a superthin, precise layer of niobium (more on this later). The rotors generate a tiny electric field due to differences in electron motion in their lattice; this effect, the London moment, exactly aligns with the spin axis.
A thin, superconducting loop connected to a SQUID circles each of the rotors. When the rotor tilts, due to the anticipated frame-dragging, the London moment also tilts, changing the magnetic field through the loop. The SQUID-based London-moment readout can detect changes in magnetic field of 5×10–14 gauss, corresponding to a gyro tilt of 0.1 milliarc-second. By comparison, the Earth’s magnetic field is 13 orders of magnitude larger than this level of detection.
Part 3 looks at the rest of the instrument and probe design in more detail.
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