Furthermore, general relativity has brought us nearer to an understanding of the observational fact that the local inertial compass is fixed relative to the most distant cosmic objects, but there is surely desire for still deeper understanding." Thus does direct detection of frame-dragging by Gravity Probe B gain new importance: it will shine experimental light on what has heretofore been a theoretical mystery, namely the origin of inertia. For some, this is perhaps the most beautiful and profound manifestation of spin in Einstein's spacetime: it binds us here to the universe out there, in such a way that you, standing at night under the stars on a planet known as earth, cannot turn so much as around without feeling a tug from the rest of the universe.
Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from.