> A key performance metric of computers is their energy dissipation. One contribution to dissipation is friction at the rotary joints in each logic gate. Due to the joint’s small frictional drag, mechanical computers constructed from them can, in principle, dissipate orders of magnitude less power than conventional semiconductor computers, while still operating at relatively high speeds.

This claim seems dubious, perhaps someone has more expertise to comment. The justification

> Operating this lock involves rotation at the joints by up to ∆θ≈1 rad. The model system analyzed in [11] is an excerpt of the links and joints shown in the closeup on the right of Figure 24. From [11, Eq. 2], this rotation dissipates bout 2.4×10−27J per rotary joint when operating at f = 100 MHz.

Seems akin to saying "We operate our microchip at 1 microvolt / 1 pico ampere at f=100MHz, giving 10-26J per operation." -- which seems like a silly aspiration without carefully analyzing noise and quantum mechanical constraints. (without which it would seem almost any computing device could operate at arbitrarily low power).