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BurningFrog

A second is still originally defined as 1/86400 of an Earth day.

That doesn't make it unusable as a cross galactic time unit, and I think the same goes for years and hours.

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Seconds are now (in SI) defined as calculated from behavior of cesium-133 atoms.

https://en.wikipedia.org/wiki/Caesium_standard

You can also define "days" and "years" in terms of that SI definition.

I don't think that helps with the original concern.

You can, yes. But having it all stem from some fundamental constant value any civilization can handle permits translation between civilizations.

"Our dates start x trillion rotations of pulsar y ago and our unit is defined as z wiggles of cesium" is a starting point.

9,192,631,770 is clearly a sensible number and not something that's blatantly chosen to match some arbitrary pre-existing geocentric standard like 10,000,000,000 would have been.

It's retrofitted to what we already defined as a second, sure.

But you can tell an alien species our units are expressed in multiples of that, and they can translate it into how theirs works. (Vinge, for example, has space-faring humans talk about "megaseconds" and "gigaseconds" rather than days/years.)

> "megaseconds" and "gigaseconds" rather than days/years.

More like weeks and decades. Arranging to meet someone in a megasecond is like meeting them on the weekend; a megasecond is ~11.5 Earth days. A kilosecond is short enough to be used for moment–to–moment planning. They’re about a quarter of an hour each so they’re good for scheduling work tasks, scheduling time to meet people for a meal, etc etc.

Gigaseconds are more rarely used, since each one is ~32 Earth years.

Diaspora by Greg Egan has some fun with this too. The main character is a software emulation; called a citizen rather than a flesher. Most emulations live at a subjective rate 800× faster than the flesher normal. The second chapter is three flesher days after the first but 256 megatau, or ~8 years, have passed for the main characters. The fourth chapter is two thirds of a teratau later, over 20k subjective years. For the fleshers of Earth a mere 21 years have passed. The main character has actually forgotten the events of the third chapter; one of his friends brings it up and he has to search his archived memories to figure out what his friend is talking about.

Unfortunately, the Second is measured for purposes of our timekeeping standards at sea-level on Earth which is ~1PPB slower than it would be in free space, as opposed to having a correction factor built into our time standards and so, for example, interplanetary ping times would be slightly shorter (in UTC/TIA nanoseconds) than expected.

A much more precious clock is used by USA to guide nuclear missiles without GPS. (nucleus of Thorium 229 controlled by a high-precision UV laser?)

That clock hasn't actually been built yet and it wouldn't be useful for guiding nuclear missiles.

https://en.wikipedia.org/wiki/Nuclear_clock

I mean, than nucleus is much heavier and much smaller than electron, so it will be much less affected by external forces. We may see no difference between sea level and space based Thorium-229 clocks, or difference will be much smaller.

ICBMs can be aimed as accurately as they need to be with current inertial navigation technology.

Case-in-point, you are mistaken. The duration of a day changes due to many things, both logically and also physically due to the nature of Earth. Also just because you can call a second a second doesn't mean that is helpful making datetime software usable or easy on a different planet.

I think he's speaking historically. Obviously now a second is a fundamental SI unit defined in terms of physics experiments, but the origin of it was as the amount of time that was 1/3600th of an hour of which there are 24 in the day.

Similar to how almost-pi-squared meters-per-second shows up in the constant for gravitational acceleration near Earth's surface because the meter was originally "the length of pendulum that ticks once a second" and there's a pi in the pendulum motion equation.

(... it's not exactly pi-squared because the French yanked it around a bit before settling into the modern number based on light in a vacuum and cesium atoms).

The issue is that planetary locales will each have their own days and years (and possibly hours), so it would be confusing to adopt that same nomenclature for an interplanetary/interstellar time unit. And since the latter will be inconsistent with local time systems anyway, it’s easier to just have it use powers of ten. At least until we meet aliens that may prefer a different base.

I was 100% thinking of use by humans living on other worlds. Pretty sure Mars will use seconds and hours. Handling dates will awkward whatever they decide on.

Currently, a Mars days is called "sol", FWIW.

If we find other species out there I won't speculate on how they think about time.

But this makes no sense, humans can't just change their circadian rhythm to match an arbitrary daylight cycle, and clocks aren't necessarily reconfigurable. And with a good enough artificial lighting you don't need to depend on star. Daylight is just weather, it has nothing to do with how calendar works.

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