> its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice It wouldn't. The hatch itself could operate

> its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice It wouldn't. The hatch itself could operate with no net energy lost/gained, e.g. using a counterweight to cancel out the momentum. Any required calculations could be carried out with a reversible computer, e.g. a billiard ball computer. This way, once our computation is finished, we just can just run it in reverse to get back any energy that was spent (e.g. anything that needed charging during the computation, will get discharged on the way back; and likewise for any lifting, pushing, etc.). The only thing which requires energy is resetting the device. This cannot be done reversibly, since it has to 'forget' whether the hatch got opened or not; i.e. reset(opened) -> init and reset(closed) -> init, so we can't "undo" the 'reset' operation to recover any energy it used.