4 ms·

This is mentioned in the article, it isn't a contradiction. You need both periodic phenomena and the generation of entropy for a clock to work. Consider what w

This is mentioned in the article, it isn't a contradiction. You need both periodic phenomena and the generation of entropy for a clock to work. Consider what would happen if we used the earth's rotation as a clock, but without generating entropy. We would have a system which would go back to its original state every day, each rotation. This means there would be no way to discern how many days have passed, as the system would be in the same configuration as it was at the beginning of the rotation. In order to encode how many days have passed, you need to increase the system's information, and by doing so you increase its entropy. The same problem applies to a pendulum clock: you can't have a pendulum clock as anything but an entropy-generating device, or else it will keep going back to its initial state without any changes, not measuring any passage of time at all. In the pendulum clock, the mainspring provides the entropy flow that allows it to time-keep and register the passing of time on the indicator. In the sundial, the flow of light from the sun into the sundial and into our eyes provides the flow. The light flow gets interrupted by the gnomon, casting a shadow (an irreversible process). In order to register that this has happened, you look at the sundial and take note of the time (another irreversible process). You can replace yourself with light-sensitive paper, which gets etched by the light (yet again another irreversible process). The other sources of entropy (like tidal friction, as well as atmospheric friction and friction between the layers inside the Earth) are irrelevant for the sundial's working, but could possibly be used to create timekeeping devices themselves. I'd say the main difference between the Sundial and the Pendulum clock's working is that in the pendulum clock, the energy to keep the pendulum oscillating and to register the passage of time are both provided by the same source (the mainspring), while for the Sundial there are two different sources of energy: sunlight provides the energy to register the passage of time, while the Earth's inertia is the only source of energy that keeps it rotating. We could make the two systems more similar by changing the pendulum clock so it's only a pendulum, creating a separate indicator consisting of a light shining onto a pendulum and a sheet of photosensitive paper slowly moving down and receiving the pendulum's shadow, registering the passage of time onto the paper without using the pendulum's energy for that.

dredmorbius 5y ago
We would have a system which would go back to its original state every day, each rotation. This means there would be no way to discern how many days have passed, as the system would be in the same configuration as it was at the beginning of the rotation. That's precisely what bodies in angular rotation or orbits do. "One day looks pretty much like another." For a sundial, the (apparent) movement of the shadow provides the marker of time throughout the day. It (mostly) doesn't indicate the passage of time greater than a day (not strictly true: see the annalema). But within any 24 hour period, you've got a timekeeping piece in which entropy plays no role in the measurement of the passage of time itself, but only as a signalling mechanism (casting a shadow). In order to complete the clocked-ness, you either need a second repeating phenomenon (e.g., Earth's rotation vs. Moon's orbit, Earth's rotation vs. Earth's orbit). In which case with an external recording mechanisms the passage of time can be noted. But the fundamental timekeeping mechaism, the phenomenon which is regularly occurring with time occurs without discernable entropic decay, once set in motion. The rotation or orbit is the escapement here. It's functioning without an entropic drive. Pointing out that an entropic flow might be used independently of a sundial does not explain how it relates to the sundial.
- Hayarotle 5y ago
  A sundial uses more than just angular rotation and orbits, though. You can't measure time with angular rotation alone. Using a sundial as a clock (by registering its cycles or by looking at it and remembering how many cycles it went through) inevitably increases the system's entropy. You don't need a second repeating phenomenon, just as you don't need a second pendulum in the pendulum clock. The pendulum might repeat its motion at a short interval, but is able to measure intervals longer than its period. If you have a sundial that works without increasing entropy, it doesn't work as a clock. Just as a perpetual motion pendulum alone wouldn't work as a clock. Using other orbits doesn't change that. You need the entropic flow to register the passage of time. In the case of the sundial, it can be done by recording its cycles. A sundial on a rotating planet isn't able to measure time within its cycle without increasing entropy any more than a pendulum is able to measure time within its period without increasing entropy. They're not able to do that. A Sundial without light and a recording device isn't a clock, just as a lone pendulum with no energy losses and no recording device isn't a clock.

dredmorbius 5y ago

We would have a system which would go back to its original state every day, each rotation. This means there would be no way to discern how many days have passed, as the system would be in the same configuration as it was at the beginning of the rotation. That's precisely what bodies in angular rotation or orbits do. "One day looks pretty much like another." For a sundial, the (apparent) movement of the shadow provides the marker of time throughout the day. It (mostly) doesn't indicate the passage of time greater than a day (not strictly true: see the annalema). But within any 24 hour period, you've got a timekeeping piece in which entropy plays no role in the measurement of the passage of time itself, but only as a signalling mechanism (casting a shadow). In order to complete the clocked-ness, you either need a second repeating phenomenon (e.g., Earth's rotation vs. Moon's orbit, Earth's rotation vs. Earth's orbit). In which case with an external recording mechanisms the passage of time can be noted. But the fundamental timekeeping mechaism, the phenomenon which is regularly occurring with time occurs without discernable entropic decay, once set in motion. The rotation or orbit is the escapement here. It's functioning without an entropic drive. Pointing out that an entropic flow might be used independently of a sundial does not explain how it relates to the sundial.

Hayarotle 5y ago
A sundial uses more than just angular rotation and orbits, though. You can't measure time with angular rotation alone. Using a sundial as a clock (by registering its cycles or by looking at it and remembering how many cycles it went through) inevitably increases the system's entropy. You don't need a second repeating phenomenon, just as you don't need a second pendulum in the pendulum clock. The pendulum might repeat its motion at a short interval, but is able to measure intervals longer than its period. If you have a sundial that works without increasing entropy, it doesn't work as a clock. Just as a perpetual motion pendulum alone wouldn't work as a clock. Using other orbits doesn't change that. You need the entropic flow to register the passage of time. In the case of the sundial, it can be done by recording its cycles. A sundial on a rotating planet isn't able to measure time within its cycle without increasing entropy any more than a pendulum is able to measure time within its period without increasing entropy. They're not able to do that. A Sundial without light and a recording device isn't a clock, just as a lone pendulum with no energy losses and no recording device isn't a clock.