# How old are people from several worlds? [closed]

In an universe with several worlds, how would people from different planets define their age in a way that everyone use the same basis ?

• Different species have different lifespan, like humans up to 80 something, elves up to 300 something, ...
• The planets are not from the same system or galaxy, in fact people don't know where their planets are located in the universe, or how far they are from each other, so you can not use for example the sun or another common celestial body
• Fantasy world, medieval like, so there is no scientific way to explain age

If possible, I'd like to find an idea that is not "they pick a main planet and apply their standard"

• You are in a medival technology setting but people travel between stars? Societies that have star travel technology just use something physics based, like the second. Medival societies live on only one planet so measuring time with reference to that planet works fine. – quarague Feb 20 '20 at 10:47
• @quarague They are travelling through magic portals – ClmentM Feb 20 '20 at 10:50
• They pick two planets and use the arithmetic mean of the duration of the years? Or the geometric mean? Or they pick an arbitrary interval, for example based on the luminosity cycle of Algol? Or they use as basis for time measurement the beatings between two slightly out of tune organ pipes of defined length in a suitably chosen atmosphere? (Oh, and if they have the problem of comparing their ages then it directly follows that they actually come into contact, and therefore they do see the same celestial bodies...) – AlexP Feb 20 '20 at 10:50
• Vernor Vinge solved this years ago. Time is measured in megaseconds and gigaseconds. – John O Feb 20 '20 at 14:31
• @Zxyrra: My comment was a polite explanation of why I voted to close as opinion based. If a 5 line comment can give four different answers plus a frame challenge, the question needs work to provide focus and define a way to choose one best answer. – AlexP Feb 20 '20 at 15:43

If possible, I'd like to find an idea that is not "they pick a main planet and apply their standard"

They likely wouldn't. It's hard enough for us on one planet to use a standard, try to apply it across species and different planets. In fact, let me illustrate this with an example from the real world: ask somebody from the USA how tall they are. Now ask somebody from, say France. They'll tell you quite different numbers even if the two people are the same size. Ask them how much they weigh, and you'd get different numbers. Ask them what is the temperature outside and again - two different numbers.

Different locations on the same planet use different measurements for the fundamentally the same things. Oh, and time isn't actually different.

We at least agree about minutes, hours and days but ask our American and French friends to write the date and compare them. Different numbers again. Different notation, too. Go across the world and everybody will be noting the dates in different ways - some separate them with / others with . third with just a space. Some will prefer Roman numerals for months, others words. It's not like we don't have a standard for this, either:

If you still think "oh, it's just the dates, right", then you'd want to have to listen to this video detailing even more date/time issues.

Bonus fact: did you know that the ISO 8601 standard also covers week numbers and a week can't belong to more than one year? Now you do. As a result the ISO year/week is not necessarily the same as the calendar week. Sometimes the last day or two of one year might fall into the next one. Or the first day or two of the new year might be counted towards the last one.

All this to say that we'd likely handle the different dates the same way: not very well. Each planet can have its own time counting, likely using their own units. You'd be able to convert between them - say, 5 [planet A years] might equal 7.5 [planet B years] or 3.5 of [planet C years]. Therefore the natives from these would likely just keep using the units they are used to and likely just convert if they need to converse with somebody else.

Cross-world traveller from planet A would still count their age in [planet A years] which has an advantage that they can return to planet A and can just celebrate their birthday on the same date as every other year.

• Voting you up just for referencing Tom Scott's epic time zone rant. That's one of his best videos, which is saying something. – Monty Harder Feb 20 '20 at 19:12
• @MontyHarder it hits too close to home. My favourite thing that he neglected to mention is how some people seem to not understand GMT. It's supposed to be similar to UTC - it's a timezone where the offset is always zero, so you can measure all other timezones in reference to it. Except some people believe that since the G stands for Greenwich and that's in the UK, therefore GMT is the timezone that UK uses. So, in the summer GMT is actually moving an hour ahead. The problem comes when these people then write a library that uses this behaviour. – VLAZ Feb 20 '20 at 19:28
• In Weber's Honorverse, people continue to use the Terran year for the most part, but I believe the official units of the Star Kingdom are Manticoran years (e.g. for assessing taxes, terms of office...), and folks from Sphinx also keep track of their age in Sphinxian years. Then you have the relatives on Gryphon, Medusa... and once the Talbott Quadrant comes into play, oh boy... The books mention at some point that time conversion software is popular. – Matthew Feb 20 '20 at 19:29
• @Matthew As the series went on, Weber started using T-years more and more, since Manticore had ceased to be a "neobarb" backwater and was directly involved with the Solarians. I think it's reasonable that humans would use T-years as their interstellar standard. – Monty Harder Feb 20 '20 at 19:35
• @MontyHarder, that may or may not reflect in-universe practice, however. It's honestly hard for me to imagine living on a planet and not using a local calendar (Weber even made a point of calling out Grayson's perversity at doing so). I suspect it's just as likely that Weber just let that bit of world-building fade into the background, while keeping the "T-year" designation to remind us that it's there. – Matthew Feb 20 '20 at 19:41

As a percentage or fraction of your typical lifespan

This works best if elves etc. have proportionately longer childhoods. A typical person on Earth lives to 80 Earth-years and I am 40, so my age is 50% or "one half-life". On planet Zod a human lifespan is 30 Zod-years, so an elf lives to about 113 Zod-years. An elf aged 150 Earth-years (aka 56 Zod-years) is also aged 50%, "one half-life" and may be biologically the same age even if the elf was born long before me.

If species age proportionately, then everyone reaches adulthood at approximately "one quarter-life". Longer spans of time might use a human lifespan as a unit of measurement.

Incorporate this very issue into your story

Of course time keeping was an important issue in the higher societies of wise men, that's why they discussed it, and the Time Keepers where formed. A small detachment of them exist in every "civilized" world, where they mantain a pulsating magical orb that is syncronized with every other orb. By counting the repetitive pulses (each every x time, as your story needs) they can keep track of passage of time in a standarized way. As always, not everyone uses this system, but wizards and scholars prefer it for obvious reasons.

You can set a universal time standard, that's planet-independent, and can be applied to faraway galaxies that aren't able to keep track of the rotations of a distant planet, by using universal physics time-based constants to keep track of time.

The way we do it, currently, is by creating microwave pulses whose frequency is a rational fraction of the caesium-133 isotope. Caesium-133 has a hyperfine energy excitation in its ground state whose energy difference is equal to that of a photon of light whose frequency is exactly 9 192 631 770 oscillations per second. (It used to be extremely close to that integer number, but since then we have redefined the second so that it is an exact integer.) So, when tuning a microwave signal to the exact frequency needed to excite Cs-133 in this particular transition, the signal can be tuned to 1/9192631770th its original frequency, and we take its pulses to be "seconds".

In principle, instructions for this procedure can be given to a distant galaxy that cannot keep track of Earth's rotations, and they would have the exact same unit of "seconds" that we do. And if we don't want to define longer stretches of time in an anthropomorphic sense, we can use nice round numbers instead. For example, a "day" can be 100 000 seconds instead of 86 400, and a "year" can be 50 million seconds, instead of 31.536 million seconds.

Another way to do use universal physics that's not locally dependent is to describe everything in terms of large multiples of Planck times. One Planck time one unitless value of time in "natural units", where universal constants $$c$$, $$G$$, and $$\hbar$$ are set to 1. (The above units are the speed of light, universal gravitational constant, and reduced Planck constant, respectively.) Physicists work in natural units all the time, as a convenient way of saving them a lot of time writing symbols. When that happens, time, distance, etc. are basically unitless numbers.

Again, conventions can be made for talking about large portions of time. For example, we can define the "day" to be $$10^{48}$$ Planck times. (Clearly each species would make a name for it; they wouldn't say "I'm leaving for my trip, I'll be back in $$6 \times 10^{48}$$ Planck times.")

The advantage to Planck Time, over Cs-133, is that the latter is a far more arbitrary choice of measuring time than the former. The constants $$c$$, $$G$$, and $$\hbar$$ are universal, and far more broadly descriptive of general physics than an extremely specific multiple of an extremely specific atomic energy transition. If we ever made first contact with an advanced alien species, and started talking about durations with them, we would most likely talk about elapsed durations in terms of Planck times.

I would keep in mind that it's convenient to talk about time in terms of your planet's rotational cycles. Especially if you're biologically tuned to sync your sleep cycles with the rotations. That's how we mentally keep track of time in order of days. If you had an event set for 8 days from now, but one "day" was, say, 0.7 rotational cycles of your planet, it would be mentally confusing to track when the event is, and you'd have to do the math. So while the above standards can be adopted as a universal standard for communication with people from other planets, people from the same planet would still probably talk about time in terms of their own rotational cycles.

I assume all relevant species know the concept of time and have a way to keep track of their time or else they would not even know how old they are compared to other members of their species.

All you need now is a way to mathematically convert the unit of time the humans use to the unit of time the elves use and then they can tell each other how old they are. This may get more complicated if one of the measurements is not linear like when the elves live on a planet which doesn't have a constant year length so to know their age in human years you have to take into account the different elf year lengths.

But in general it's like measuring in meters compared to measuring in feet. You just have to find the right conversion factor.

According to a comment posted by OP, travel between worlds is by magic portal. These portals could all experience a fluctuation, or oscillation, that happens at a very regular interval (say once every earth year). Every portal in the universe experiences this fluctuation at the same rate, based off of some yet to be discovered fundamental principle of the universe. This fluctuation can then be used to count the passage of time.