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The concept of "age" on Earth is based on the number of revolutions the planet has completed since one's birth. In a setting where interstellar colonies exists, of even in a setting where multiple colonies are established on planets which all have different revolution times around their sun, how would one speak about his age to an inhabitant of another planet? For example, a 32 year old human on planet Earth would be equivalent to ~380 years for a Jovian.

What standard measures could be applied to beings to easily understand the age of the other without requiring constant conversion into Earth revolutions? At the moment, my ideas are either:

  1. Physical and mental assessment test; or
  2. Length of telomeres

Would like comments or suggestions. Thanks

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    $\begingroup$ Physical and mental assessments can be affected by many other things over just age, as can telomere length (disease, depression, etc). Telomeres have been found to actually grow longer over time in some people, although for reasons not yet explained. $\endgroup$ – TheBloodyPoet Jul 24 '16 at 14:52
  • $\begingroup$ Universal coordinated time? Counting seconds? $\endgroup$ – Donald Hobson Aug 10 '16 at 11:47
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Use Earth standard years or some other standard unit.

Individuals could track their ages in standard years or perform conversions from local time units when wanting to compare. These time conversions factors could easily be programmed and computed similar to the way time zones and time changes are done by computers today.

Similar digital systems could also be used to track each individuals local times and ages relative to different locations caused by time dilation from travel at near light speed. This type of system would be necessary if there is any large scale travel at near light speeds, as calculating the physical ages of people (and equipment) would be much more complex than just measuring the time elapsed.

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Time is not measured on planetary orbits

Seconds as standardized by the SI system is defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom." Because this is related to physical properties of an element, it will be universally applied. Therefore members of the same species across different colonies will use some definition of a "Year" that is some integer multiple of a second. If you want to use a standard earth definition it would be 31,536,000 seconds.

This may be too difficult for an average person to remember, so whatever the center of your system of government is could have an annual event or holiday (like every Christmas Day on Earth). Then a person's age could be the amount of holidays these people have experienced. This will be easy to keep track of because the official government timekeepers will announce it every standardized period.

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    $\begingroup$ Worth noting that the concept of 'seconds' came into existence long after the concept of 'years' and 'days', and was specifically defined to fit into those pre-existing concepts. Units of time, as measured by humans, are indeed a construct of the Earth's rotation and orbit. $\endgroup$ – TheBloodyPoet Jul 24 '16 at 13:32
  • $\begingroup$ They originally were, but they have since been redefined to be based upon the second, which is based upon intrinsic properties of the universe, so they are no longer. $\endgroup$ – Jarred Allen Jul 24 '16 at 13:38
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    $\begingroup$ You have it the wrong way round. The SI definition is an after-the-fact definition, specifically chosen to match the already-defined period of 1/86400 of a day, just standardise it. To non-Earthlings, the SI standard 'second' would seem completely arbitrary, as would minutes and hours. $\endgroup$ – TheBloodyPoet Jul 24 '16 at 14:01
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There seem to be two types of age in the question. The first is how someone is in terms of how long they have lived or their age by the calendar. While the second age is their biological age or equivalents thereof. This would be a sort of gerontological clock.

For interstellar colonies, calendar age can be calculated both in terms of the colony planet's local calendar (or Colony Planet Years) and in Standard Earth Years. @knowads correctly suggested calendar age can be calculated in seconds. The second is a standard unit of time which will remain so irrespective of the length of the day on any given planet or the period of its orbit around its primary star.

A 32 year old (in Standard Earth years) is 32 times 31,536,000 seconds old or 1,009,152,000. This is, of course, a ridiculously exact figure. So expect persons communicating across the colonial empire to give their calendar ages in approximations. Like "On May 23, I turned 1,009,152,000 seconds or 32 Standard Earth years old." The person receiving this age can quickly estimate the age of their pal on another planet.

For biological age, it's necessary to make one assumption. Namely, that medical and biological science has advanced sufficiently that there exists a simple test for estimating how far along someone's lifespan any given person is gone. This doesn't mean when the person reaches the end of their estimated lifespan they drop dead immediately. This will always be a probability.

Assuming two people chatting via ansible (don't worry this is only to simplify the reasoning here) and they both have had the lifespan test. "I'm 32% of my lifespan long," she said. "Oh," he replied. "I've just reached 31% myself."

Percentages are one way to calculate an estimated passage along a person's lifespan. It could be done by the time on a clock face. Any suitable metaphor will do. If the test worked sufficiently well people could talk about how many more years they expected to live.

"I'm 400 now and I've only got another 800 years left." This will need to be translated into whatever calendar the speakers use to calculate age. This can be Standard Earth Years or their respective Local Colony Years.

How does the biological age test work? Perhaps by the length of telomeres or any other molecular biological clock that may be ticking off our lifespan. There might be ways of measuring the accumulated wear and tear on cellular structures. The biological sciences are simply too complex to predict where they will go or what they will discover. For this reason, it is simpler to assume that a biological age test will exist in the age of interstellar colonization and leave it at that.

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  • $\begingroup$ Maybe they'll just use SI prefixes on the second, just like on (almost) every other unit. A kilosecond is about 16.5 minutes, a megasecond about 11.5 days, and a gigasecond about 31.5 years. (BTW, you forgot to account for leap years). $\endgroup$ – celtschk Jul 24 '16 at 19:52
  • $\begingroup$ @celtschk, use of prefixes makes sense. That's why I said the figures in seconds was ridiculously exact. By basing the time intervals on a hypothetical Standard Year scale bypasses leap years. Accounting for leap years would occur when keeping track of dates on any given planet. The orbital periods for interstellar colonies would be every which way, so dates and calendars would be a mess. Working with standard units of time was, I hoped, a reasonable solution. $\endgroup$ – a4android Jul 25 '16 at 6:40
  • $\begingroup$ I mean, when defining a standard year, you'd base it at the actual length of a year, not at the length rounded to an integer (which is the reason we have leap years). An average Gregorian year has 365.2425 days; that's certainly a sufficiently close approximation to the full year, and it corresponds to 31556952 seconds. If you are going to be ridiculously exact, do it right. $\endgroup$ – celtschk Jul 25 '16 at 7:15
  • $\begingroup$ I was aiming at an arbitrary standard unit of time as an aid to communicating ages and timespans across interstellar distances. Not a calendar that you'd work out your birthdays by. if the Standard Earth year was based on the Gregorian year, then that would be a different number. Example: Charles Stross standardised 1 g acceleration at 10 m/s in IIRC "The Singular Sky". So I standardised Years on an arbitrary unit of time. I know it's only way of doing this, but I was aiming at something workable and comprehensible not perfect. $\endgroup$ – a4android Jul 26 '16 at 1:53
  • $\begingroup$ Something practical would be e.g. 32 million seconds. Yes, it's about five days longer than an Earth year, but then, 370 is a more "round" number than 365 anyway. $\endgroup$ – celtschk Jul 26 '16 at 6:58
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Since human concepts of time are relative to our existence, they would mean nothing to an alien species. You would have to pick a universal constant (or as close to it as you can) that any scientific species would understand.

Half lives are constant. The half life of the most abundant material in the universe, the two most stable isotopes of hydrogen, are 12.32 Earth years. Since the half life never changes, that would be an excellent measure of time, as you can quantify half life with any measurement of time.

I do not like your 2 points, as they have no completely relative and/or subjective.

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I can tell you the answer and provide you with peanuts on the way

This answer depends on whether or not you want hard science fiction or classic science fiction. As in one FTL travel is easy and the other not. If you go with the former, then going farther than your own solar system will take many years to arrive at the closest star. (Unless we get into ftl tech, but that gets a little... spacey :D) This means that on the way, passengers can simply be told what their age is in local years. (similar to how flights tell you local time) They can also set their new circadian rhythm and sense of time.

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There are a number of concepts here which coalesce into one measure here on earth: elapsed time, subjective elapsed time allowing for relativity and hibernation, calendars, planetary orbital and rotational periods, biological ageing, biologically programmed diurnal periods (esp. Sleep cycles)

I expect (absent alien influence) that on starship and for historical and financial record keeping, time will be measured in seconds since a universally agreed base date and reference frame. A gigasecond is about 30 years and will probably come to replace the century. A hundred thousand seconds may well replace the day on board starships. Note that our computers already record times in seconds and convert seconds since a base date into calendar dates when needed.

On a planet local days will need to be compatible with human physiology. We can adapt to 22 hour days or 28 hour days so on such a planet one day will be one rotation. On a 55 hour planet people will almost certainly be awake morning and evening with an eight hour "siesta" for sleep in the middle of the planet's day. Local calendars will reflect sleep cycles "days" and the seasons "years". Months, if they exist at all, will be quite arbitrary and local.

Subjective age of human beings may well retain earth-year numbers. "I'm about forty" would serve as a civilisation-wide description of (say) a person who was born 350 earth years ago, who has been in hibernation between stars for 300 years, contracted ten years by travel at relativistic speeds, even on a world where a local year is 2.7 earth years and a local day is 13 earth hours with humans sleeping only every other night. For these added complexities you would need a conversation or a CV.

Here on Earth lying about ones age is usually socially acceptable, with sympathy or mockery as the only punishment if you cannot convince. For employment or insurance, proof of bio-age is likely to be demanded. We are already starting to see anti-ageism legislation. As medicine advances we may see many more biological 60+ people who are "about 40" in most senses that matter. For employment, job performance or interview tests may soon be mandated to replace calendar-age discrimination.

Can there be a medically objective measure of ageing-related fitness? I doubt it. We all know of people whose bodies are disintegrating but whose minds are sharp as ever. Were they athletes? Or are they authors? I once read about a travel company that under new (young?) management forced retirement on all of its tour guides at 70. Its competitors could not believe their luck!

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People could use ruthenium half-lives as their standardized unit of measure for long durations of time (assuming that there are enough people away from the Earth for it to make sense to not just use Earth time and have them convert between Earth time and local time). This would be constant anywhere and independent of any astronomical measurements.

Also, if you are going with interstellar colonies in hard sci-fi, it would not be important to have one measure of time for all star systems. Communications would take years, so there is no reason to not use a unit of time that makes sense for that star system and convert bask and forth for the few communications.

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