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

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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.