Editing General relativity (section)

==== Geodetic precession and frame-dragging ====
{{Main|Geodetic precession|Frame dragging}}
Several relativistic effects are directly related to the relativity of direction.<ref>{{Harvnb|Penrose|2004|loc=§&nbsp;14.5}}, {{Harvnb|Misner|Thorne|Wheeler|1973|loc=§&nbsp;11.4}}</ref> One is [[geodetic effect|geodetic precession]]: the axis direction of a [[gyroscope]] in free fall in curved spacetime will change when compared, for instance, with the direction of light received from distant stars—even though such a gyroscope represents the way of keeping a direction as stable as possible ("[[parallel transport]]").<ref>{{Harvnb|Weinberg|1972|loc=sec. 9.6}}, {{Harvnb|Ohanian|Ruffini|1994|loc=sec. 7.8}}</ref> For the Moon–Earth system, this effect has been measured with the help of [[lunar laser ranging]].<ref>{{Harvnb|Bertotti|Ciufolini|Bender|1987}}, {{Harvnb|Nordtvedt|2003}}</ref> More recently, it has been measured for test masses aboard the satellite [[Gravity Probe B]] to a precision of better than 0.3%.<ref>{{Harvnb|Kahn|2007}}</ref><ref>A mission description can be found in {{Harvnb|Everitt|Buchman|DeBra|Keiser|2001}}; a first post-flight evaluation is given in {{Harvnb|Everitt|Parkinson|Kahn|2007}}; further updates will be available on the mission website {{Harvnb|Kahn|1996–2012}}.</ref>

Near a rotating mass, there are gravitomagnetic or [[frame-dragging]] effects. A distant observer will determine that objects close to the mass get "dragged around". This is most extreme for [[Kerr solution|rotating black holes]] where, for any object entering a zone known as the [[ergosphere]], rotation is inevitable.<ref>{{Harvnb|Townsend|1997|loc=sec. 4.2.1}}, {{Harvnb|Ohanian|Ruffini|1994|pp=469–471}}</ref> Such effects can again be tested through their influence on the orientation of gyroscopes in free fall.<ref>{{Harvnb|Ohanian|Ruffini|1994|loc=sec. 4.7}}, {{Harvnb|Weinberg|1972|loc=sec. 9.7}}; for a more recent review, see {{Harvnb|Schäfer|2004}}</ref> Somewhat controversial tests have been performed using the [[LAGEOS]] satellites, confirming the relativistic prediction.<ref>{{Harvnb|Ciufolini|Pavlis|2004}}, {{Harvnb|Ciufolini|Pavlis|Peron|2006}}, {{Harvnb|Iorio|2009}}</ref> Also the [[Mars Global Surveyor]] probe around Mars has been used.<ref>{{Harvnb|Iorio|2006}}, {{Harvnb|Iorio|2010}}</ref>