Black holes and their boundaries: from equilibrium properties to how dynamical black holes interact with their environment

当月球绕地球运行时，它的引力场会引起潮汐。当潮汐隆起绕着地球仪流动时，水和陆地之间的摩擦减缓了地球的自转。与此同时，凸起的引力场会拉回月球，因此地球的旋转角动量会转化为月球轨道的角动量。与任何涉及摩擦的问题一样，能量被耗散，这种引力耦合的净效应是月球以每年近4厘米的速度从地球后退。这个过程很好地展示了我们如何在经典物理学中进行推理。几何形状（在这种情况下是潮汐隆起的形状和到月球的轨道距离）与引力场相互作用，并驱动系统周围的能量和角动量转移。一旦我们了解了力和能量与动量的流动，我们就觉得我们理解了这个系统。

As the moon orbits the Earth, its gravitational field raises the tides. As the tidal bulges flow around the globe, friction between the water and land slows the Earth's rotation. At the same time, the gravitational field of the bulges pulls back on the moon and so terran rotational angular momentum is transformed into lunar orbital angular momentum. As in any problem involving friction, energy is dissipated and a net effect of this gravitational coupling is that the moon recedes from the Earth at a rate of almost 4cm/year. This process nicely demonstrates how we reason in classical physics. The geometry (in this case the shape of the tidal bulge and orbital distance to the moon) interacts with the gravitational field and drives a transfer of energy and angular momentum around the system. Once we know about the forces and the flows of energy and momentum, we feel that we understand the system.