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

Such rigorous experimental verification is essential to furthering our understanding of the nature of ouruniverse, particularly about massive objects in space, such as black holes and quasars. As GP-B Physicist JohnMester puts it,“General relativity is our current theory of gravitation, and it has wide-ranging implications for ourunderstanding of the structure of the cosmos. At present, Einstein’s theory of gravitation lies outsidethe other three forces of nature (the strong force, the weak force and the electromagnetic force),which are explained within a unified framework called The Standard Model. Attempts to unify allfour forces of nature have eluded physicists from Einstein to the current day. Testing theories to highprecision will help define their range of validity or reveal where these theories break down.”Whatever the result, Gravity Probe B will provide us another glimpse into the sublime structure of our universe.2.1.4 The Geodetic and Frame-dragging EffectsGravity Probe B was designed to test, through a direct, controlled experiment, two predictions of Einstein’sgeneral theory of relativity. The first, known as the geodetic —or as it is sometimes called, deSitter— effect,measures the size of the very small angle by which our Earth warps its local spacetime. One way to visualize thiseffect is to think of local spacetime as a trampoline and the Earth a bowling ball lying in the middle. The heavyball warps or puts a dent in the trampoline, so that a marble (another celestial body) moving along thetrampoline surface will be inexorably drawn down the warped slope towards the massive ball.Another, and perhaps more useful way to visualize the geodetic warping of spacetime is the so-called “missinginch” shown in Figure 2-7 below.Figure 2-7. The Missing InchIf you were to draw a circle with the same diameter (D) as the Earth (~7,900 miles) in empty space, thecircumference of this circle, calculated using standard Euclidian geometry, would be πD (~24,000 miles).Furthermore, a gyroscope following this circular path in empty space would always point in the same direction,as illustrated in the left side of Figure 2-7. However, if you were then to slip the Earth inside this circle, Earth’smass warps the spacetime inside the circle into a shallow cone, thereby shrinking the circumference of the circleGravity Probe B — Post Flight Analysis • Final Report March 2007 33