An inertial frame, in which the laws of physics take on their simplest, because there are no fictitious forces.如果地球引力不影响时间和空间，那么回转仪自转轴将一直指向初始方向。实际观测结果是，受地球引力拖拽，回转仪自转轴方向发生了可测量的细微偏移拽，从而证实了爱因斯坦的理论

An inertial frame is a frame of reference that has a constant velocity with respect to
the distant stars, i.e., it is moving in a straight line at a constant speed, or it is standing
still. It is a non-accelerating frame, in which the laws of physics take on their simplest
forms, because there are no fictitious forces.
A non-inertial frame is a frame of reference that is accelerating. In a non-inertial frame
the motion of objects is affected by fictitious forces (e.g, centrifugal & coriolis force).
An invariant is a property or quantity that remains unchanged under some transformation
of the frame of reference (i.e., charge of an electron, Planck’s constant, any scalar)
Covariance is the invariance of the physical laws or equations under some transformation
of the frame of reference.


What is a "fictitious force"?

July 9, 2007

California Institute of Technology theoretical physicist and 2004 Nobel laureate David Politzer helps shed some light on these mysterious influences. The forces you feel in a moving car—those that push you back into your seat when the driver steps on the gas or throw you side to side when the car makes sharp turns—are everyday examples of fictitious forces. In general, these influences arise for no reason other than that the natural frame of reference for a given situation is itself accelerating. The term "fictitious force" has a precise meaning within Newtonian mechanics—in fact, it's always proportional to the mass of the object on which it acts. An elegant example of these types of apparent influences is the fictitious Coriolis force, which is responsible for the stately precession (or circular rotation) of a carefully suspended pendulum's plane of swing. If such a pendulum were suspended directly above the North Pole, it would appear to rotate 360 degrees every day. If you viewed this pendulum from a stationary point in outer space, however, it would appear to swing in a single, fixed plane while the Earth turned under it. From the outer space perspective, there is no sideways force (that is, perpendicular to the plane of swing) deflecting the sway of the pendulum. That is why the somewhat pejorative term "fictitious" is attached to this force. Likewise in the car, there simply is no real force pushing you back into your seat, your senses notwithstanding.

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Nevertheless, analyzing a situation in terms of fictitious forces may, in fact, be the most effective way to understand what is actually going on. Take a stirred cup of tea, a charming example of a consequence of the Coriolis force. If a few tea leaves are present in the cup, they end up in a pile at the center of the bottom surface (and not along the edge, as one might expect, as a result of the also fictitious centrifugal force). If you imagine yourself rotating around in sync with the stirred fluid, most of the fluid would appear to be at rest while the cup would be counter-rotating around you. That rotating cup drags some adjacent fluid along with it. Meanwhile, near the bottom, the Coriolis force on that dragged fluid pushes it toward the center of the cup, carrying the leaves along with it. With general relativity, Einstein managed to blur forever the distinction between real and fictitious forces. General relativity is his theory of gravity, and gravity is certainly the paradigmatic example of a "real" force. The cornerstone of Einstein's theory, however, is the proposition that gravity is itself a fictitious force (or, rather, that it is indistinguishable from a fictitious force). Now, some 90 years later, we have innumerable and daily confirmations that his theory appears to be correct.

引力探测器B证明爱因斯坦是对的

百对贵族 2015-02-20 14:10 131 0 字体： 小 中 大

美探测器证实广义相对论两项关键预测。

（图片提供：Bob Kahn/引力探测器B/斯坦福大学）

本报讯 物理学家于5月4日宣布，美国宇航局（NASA）的一部耗资7.6亿美元的探测器终于成功证明了爱因斯坦的引力理论，或者说广义相对论，而这一构想已经提出了50年之久。引力探测器B完成的测量与广义相对论的两种效应的理论预测相一致——广义相对论认为，当质量使时空产生弯曲时便会产生引力。“爱因斯坦万岁！”加利福尼亚州帕洛阿尔托市斯坦福大学的物理学家Francis Everitt这样说道。Everitt于当天在NASA华盛顿总部召开的新闻发布会上报告了这一结果。

经过几十年的发展，引力探测器B从2004 年开始，在17个月的时间里从南极到北极环绕地球运行，同时利用陀螺仪测量广义相对论的两个观点。其中一个便是测地线效应，是指引力场的时空曲率对处于其中的具有自旋角动量的测试质量的运动状态所产生的影响，这种影响造成了测试质量的自旋角动量在引力场内沿测地线的进动。结果显示，地球周长略短于地球半径的2π倍。

探测器还证实了参考系拖拽效应，这是指一种处于转动状态的质量会对其周围的时空产生拖拽的现象。Everitt解释说，这就像旋转的地球被沉浸于蜂蜜中一样。他说：“当地球旋转，它将拖拽周围的蜂蜜。同样，地球也应对时空产生拖拽效应。”

据介绍，引力探测器B的主要装备是4个超高精度的回转仪。当引力探测器B在距离地球约640公里的极地轨道上开始运转时，4个回转仪自转轴同时对准遥远恒星——IMPegasi。如果地球引力不影响时间和空间，那么回转仪自转轴将一直指向初始方向。实际观测结果是，受地球引力拖拽，回转仪自转轴方向发生了可测量的细微偏移拽，从而证实了爱因斯坦的理论。

据介绍，引力探测器B是NASA在2004年4月20日发射的一颗科学探测卫星。这个任务的计划是测量地球周围的时空曲率，以及相关的动量张量（描述物质的分布及运动的张量），从而对爱因斯坦的广义相对论的正确性和精确性进行检验。卫星的飞行持续到2005年，任务从2008年5月进入到了数据分析阶段。引力探测器B的研发历史可追溯到20世纪60年代，是NASA历史上研发时间最长的计划。

Everitt认为：“引力探测器B证实了爱因斯坦的两项最深奥的预测，对天体物理学研究具有深远意义。”