Sean Carroll in UnDark:
There are two things going on, both of which are crucial to the operation of a pendulum clock. One is a little gizmo called an escapement, which turns the back-and-forth-rocking of the pendulum into the one-way ticking of the clock. Robert Hooke, a rival of Isaac Newton’s, invented the first escapement back in the seventeenth century. The clock hands are driven by an “escape wheel” with pointed teeth that are angled in a uniform direction. The pendulum, meanwhile, is connected to a two-armed piece called the “anchor.” As the anchor rocks back and forth, one of the arms first pushes the escape wheel in one direction, and then the other catches the teeth so that the wheel cannot move in the other. In this way, the oscillations of the pendulum become the uniform motion of the clock hands.
All of this sounds good, and would seem at first to be sufficient: the angling of the anchor arms and the teeth on the escape wheel provide a directionality to the motion of the clock. Except: where did entropy come in? How does the universal arrow of time governed by increasing entropy become related to the local arrow of time of this particular clock?
The answer resides in the seemingly innocent lifting and pushing of the anchor. It seems, by looking at the drawing of an escapement, that the wheel can obviously move in only one direction. But the underlying laws of physics assure us that if something can move in one direction, it can also move in the reversed direction. In this case, that would involve the anchor briefly lifting up, with the escape wheel swiftly and spontaneously moving backwards while it was lifted.
Why doesn’t that happen, and what does it have to do with entropy?