> Could some well-placed solar cell on the watch do the trick (think calculators).
My Casio watch has two tiny solar panels built into its face. According to the manual, 5 minutes of sunlight each day (or a few hours of indoor lighting) is enough to compensate for normal use, and the battery will last 6 months even without any sunlight.
I've had the Casio for 9 years now. It still works fine, and I've never had to manually charge or replace the battery. The battery does get a bit low in the winter when the watch spends a lot of time covered by thick clothing, but that problem is easily fixed by leaving the watch in the sunlight for a few hours when spring comes.
Sure, it's not "smart", but it can display the time in several different timezones, act as a stopwatch, and even has a button that lights up the display at night (which will probably drain the battery faster). All running off of two solar panels the size of the nail on your pinky.
The energy requirements are magnitudes away, you could never do that for a "smart" device. By the way, we also have the much older self-winding mechanical watches which harvest body movement; those are pretty reliable and can also run indefinitely without maintenance.
That sounds overly pessimistic. Computing power per watt has improved by several orders of magnitude over the last couple of decades already. Combined with more energy-efficient displays (anyone wants to improve on e-ink?), who knows what we could pull off 5-10 years from now, even without a significant change in battery capacities?
Well, I opened a calculator to prove you wrong, and guess what?
300K * k * 1GHz = ~4pW
Or, in other words, you are right, we can in theory reduce the power consumption of our computers so much that a current watch will last for a lifetime.
Computing power is not the limiting factor. Far more relevant are the display, which has efficiency physically limited by its luminosity -- and we're less than a magnitude away from the limit.
Even disregarding that for e-ink, there's the radio, which is limited by the channel capacity, from which I'd say we're also less than a magnitude away for isotropic radiation.
My Casio watch has two tiny solar panels built into its face. According to the manual, 5 minutes of sunlight each day (or a few hours of indoor lighting) is enough to compensate for normal use, and the battery will last 6 months even without any sunlight.
I've had the Casio for 9 years now. It still works fine, and I've never had to manually charge or replace the battery. The battery does get a bit low in the winter when the watch spends a lot of time covered by thick clothing, but that problem is easily fixed by leaving the watch in the sunlight for a few hours when spring comes.
Sure, it's not "smart", but it can display the time in several different timezones, act as a stopwatch, and even has a button that lights up the display at night (which will probably drain the battery faster). All running off of two solar panels the size of the nail on your pinky.