= Gravity Probe B
{c}
<Gravity Probe B> was a space experiment that measured the slow drift of orbiting gyroscope axes relative to a distant guide star to test <general relativity>. It targeted <geodetic precession> from motion through curved spacetime and <frame dragging> associated with Earth's rotation.
The apparatus used four nearly spherical gyroscopes operating at cryogenic temperature, a telescope tracking the guide star and superconducting magnetic readout of spin orientation. Drag-free spacecraft control reduced unwanted forces. A crucial challenge was separating the tiny relativistic drift from classical torques, including electrostatic surface-patch effects and changing gyroscope orientation.<Gravity Probe B: Final Results of a Space Experiment to Test General Relativity by C. W. F. Everitt and others>[Experiment description and modeling of classical torques]{ref}
The 2011 final analysis reported drift components $-6601.8\pm18.3$ and $-37.2\pm7.2$ milliarcseconds per year, compared with predicted geodetic and frame-dragging values $-6606.1$ and $-39.2$. The signs depend on the stated sky-coordinate convention. Dividing uncertainty by the measured magnitude shows why the experiment tested the much larger geodetic effect more precisely than the smaller frame-dragging effect.<Gravity Probe B: Final Results of a Space Experiment to Test General Relativity by C. W. F. Everitt and others>[Abstract, final combined drift rates]{ref}
Back to article page
