Mac Keiser was the chief scientist of the Gravity Probe B program. He is now a Senior Research Scientist, Emeritus, at Stanford.

Spherical, electrostatically-supported gyroscopes have been used for navigation for more than 40 years but their accuracy is far from what was needed to measure the effects predicted by general relativity. Fortunately, the accuracy could be significantly improved by reducing the forces required to support the gyroscope, improving the sphericity of the rotor, and increasing the gyroscope spin speed. Operating in a satellite reduces the support forces by a factor of 10⁷, and using one of the gyroscopes as a drag-free sensor brings a further reduction of nearly 10⁴. Improving the sphericity of the rotor further reduces the support-dependent torques but leads to the additional questions about how to spin-up and measure the spin-axis orientation of the rotor. Using a superconducting readout that Continue reading →