GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

y a mere 1.1 inches. You can see this effect (not to scale) by cutting out a pie-shaped wedge from the circle andthen closing the gap. The circumference of the resulting cone is slightly diminished, and the orientation of agyroscope will now shift as it moves around the circular edge of the cone, as shown in the right half ofFigure 2-7. This shifting orientation of a gyroscope’s spin axis as it moves through warped spacetime is theessence of the GP-B experiment.The other effect being measured by GP-B, known as “frame-dragging,” was postulated by Austrian physicistsJosef Lense and Hans Thirring two years after Einstein published his general theory of relativity. It states that asa celestial body spins on its axis, it drags local spacetime around with it, much like a spinning ball in bowl ofmolasses would drag around some of the molasses as it spins. This concept is illustrated in Figure 2-8.Figure 2-8. Illustration of frame-draggingParticularly intriguing, the frame-dragging measurement probes a new facet of general relativity—the way inwhich spacetime is dragged around by a rotating body. This novel effect closely parallels the way in which arotating electrically charged body generates magnetism. For this reason it is often referred to as the“gravitomagnetic effect,” and measuring it can be regarded as discovering a new force in nature, thegravitomagnetic force.2.1.5 The Significance of the GP-B ExperimentPhysics advances experimentally in two ways: 1) by measuring known effects with higher accuracy and 2) byinvestigating previously untested phenomena. The geodetic effect has previously been determined to ~1% incomplex studies of the Earth-Moon system around the Sun. GP-B aims to measure it to about 0.01%. Theframe-dragging effect, on the other hand, is so minuscule around a planet the size of our Earth that until GP-B,it has not been possible to measure this effect directly. However, the frame-dragging effect is of particularinterest to physicists and cosmologists who study black holes, because like the air in a hurling tornado, thehurling space around a black hole has enormous destructive potential.34 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission