[everettForth]

Thermodynamically speaking, a perfectly reversible computer which erases no bits (and creates no entropy) approaches zero watts per operation to run.

Kurzweil has pointed out that one can think of the 10^15 state changes per second going on inside a 1Kg rock with no outside energy input as a computation device.

Ok, so at 28 minutes, he's referring to some quantum mechanical lower bound to change a bit. I imagine the rock is using that really tiny amount of energy from the outside environment.