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The term "light year" is used a lot in futuristic writings that focus on space, but light years are defined based on the length of a single year on Earth. Would that not make the entire measurement completely irrelevant to the characters in the story, especially when a character is not from Earth, or long removed from it?

Of course, this is easily side-stepped by the likelihood that if you are worrying about this level of accuracy, you would also likely not have travel above the speed of light, making the entire point moot.

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    $\begingroup$ Aren’t you more worried by these futuristic people long removed from Earth speaking 20th or 21st century English? $\endgroup$ – Mike Scott Dec 27 '17 at 21:03
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    $\begingroup$ It's worth noting that the concept of a "light year" would arise on every planet upon which intelligence evolved... regardless the orbital period of the planet. In other words, the phrase is universal even if the measurement is not. Is the precision of the reference so important in your story that it matters to the reader? $\endgroup$ – JBH Dec 27 '17 at 21:28
  • $\begingroup$ @JBH I suppose one could plausibly make a plot point (in some kind of story) out of trying to communicate what it means. $\endgroup$ – a CVn Dec 27 '17 at 22:01
  • $\begingroup$ This would apply to every unit of measure, especially time. Second? Meter? They are all earth-based. But unless there is some common term that can be used throughout SciFi, identifying universal units, changing the units will only confuse the reader unless the author somehow translates them into earthly units. $\endgroup$ – Justin Thyme Dec 28 '17 at 1:53
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    $\begingroup$ Possible duplicate of What is the best unit of measure for the time portion of a non-earth-bound light"year"? $\endgroup$ – OldBunny2800 Dec 29 '17 at 13:42
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Actually, the "light year" is a unit of length that has its origins in Earth's movement around the Sun, but has since been detached from it.

The SI unit of length is the meter. It is defined as

the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second

The second, in turn, 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

By combining these two, we can define the meter as

the length of the path travelled by light in vacuum during the duration of (9 192 631 770 / 299 792 458) [which is approximately 30.663318988] periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom

This definition isn't very useful in everyday life, but it is very accurate and, more importantly in this case, has absolutely nothing whatsoever to do with Earth's size, or Earth's orbit around the Sun.

The International Astronomical Union goes on to define the astronomical unit as

a conventional unit of length equal to 149 597 870 700 m exactly

as well as, more importantly to your question, the day and century as

The astronomical unit of time is a time interval of one day (D) of 86400 seconds. An interval of 36525 days is one Julian century.

...which means that 100 years (one "century") is, by definition, equal to 36 525 * 86 400 seconds = 3 155 760 000 seconds. Remember that we already defined the second above.

One year is thus 1/100 of this value, corresponding by definition to 31 557 600 seconds, or approximately 2.90097e19 cesium-133 periods. Multiply this (a unit of time) with the speed of light (speed, which is distance over time), and what we end up with is the distance travelled by light in one of these years (a distance). Someone editing Wikipedia's page about the light year has looked up the exact value for us, and apparently by definition it is 9 460 730 472 580 800 meters exactly. This number is somewhat unwieldy, so we now choose to call this amount of distance "one light year".

Et voila, we have defined a year inspired by but eventually completely detached from the orbital period of the Earth around the Sun, and use this unit of duration plus the speed of light to define a unit of length. The unit of duration is arbitrary but defined, and the speed of light in vacuum is believed to be constant. The specific resultant value (the distance represented by this) has no immediate meaning; it just happens to be a somewhat useful unit for discussing moderate interstellar distances, and aligns with units that people in general are somewhat familiar with; there's no particular reason why the "year" couldn't be a purely scientific unit.

While it is certainly true that it's extremely unlikely that an alien civilization would come up with these exact contants (after all, there's nothing particularly universally special about 299 792 458 or cesium-133), with just a modicum of physics definitions knowledge, the definition of a light year could be translated into whatever some alien (or future humans; that would amount to about the same thing) culture uses, and we'd know that when we say "light year" and they say "przzqrtzfbn", we are talking about the same distance, even though we derive it differently. From there we can discuss amounts of distance in terms of multiples or fractions of this value.

The same, by the way, goes equally for all SI units, and everything derived from them.

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This is actually related more to politics than anything else. In many historical cases, measurements were set by the politically dominant power or by a generally-accepted "old imperial" power which had left a lasting influence on a region. For example, Greenwich Mean Time (the mean solar time at the Royal Observatory in Greenwich, London) was the universal global time standard for a long time. This is because the British Empire was the largest empire in human history and ended up controlling bureaucratic apparatuses all over the planet simultaneously. As such, they had to figure out a way to coordinate time for the entire empire, and do invented GMT.

In a scenario where Earth is not important or known, it is very likely that something very much like a light year would be used, since it is a logical unit of measurement. It would probably be called something similar to "light year" as well. It would (probably) just be the galactic standard light year as defined in Trantor years (to use Isaac Asimov's "galactic capital planet" from the Foundation series). Or perhaps a light year defined by OldDeadImperialCapitalPlanet years. Several successor government all using that as a standard because it makes commerce easier.

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  • $\begingroup$ Would that not then result in a different constant value of a light year than the one we have today? $\endgroup$ – Ambluj Dec 27 '17 at 20:45
  • $\begingroup$ Yeah, obviously a civilization which had no or little contact with us would have no or little overlap between how we measure things and how they do. We would have to learn what they meant when they said "light year" just like we would need to know what they mean when they use a unit of weight. Also: depending on how long a year is on whatever planet they use as their basis of measurement, a light year might not be trivial even in FTL terms. $\endgroup$ – JBiggs Dec 27 '17 at 20:54
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You can understand that earth people know what earth light years are and that when you are translating from alien language to earth language, you are expected to translate to earth units.

And on the point that why people would use earth light years even if they are not from earth. If earth is where people are originally from and they developed their units from that, they can continue to use it without having a logical basis for it. People still use horsepower as units of power even though almost nobody uses a horse to do work any more.

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    $\begingroup$ I remember a footnote in (I think it was) Jack Vance's "The Killing Machine" that patiently explained that all measurements and languages were translated for the convenience of the reader. When I first read it as a teen I thought it a pleasant way to get around an obvious problem. A similar solution was presented during the Tom Baker years of Dr. Who. His companions could always understand every alien. It was a gift from the Time Lords that he shared with them. $\endgroup$ – JBH Dec 27 '17 at 21:26
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    $\begingroup$ Ah, yes, "for the convenience of the reader" gives many authors free passes for this kind of stuff... and as a side effect, probably means their stories actually get read by normal people. $\endgroup$ – a CVn Dec 27 '17 at 22:36
  • $\begingroup$ I think "Battlefield Earth" also starts off with a preface that explains it is a translation into English, with all units being translated as well. $\endgroup$ – computercarguy Dec 28 '17 at 14:43
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I see some books handling currency by just not saying any numbers, for example:

He asked her how much the blaster costs, she told him a number that made him fume with rage.

Tell your reader what the distance means to the characters, not the actual distance:

He did some calculations "We don't have enough rations to make the trip home"

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Like any other civilization, an intergalactic civilization would need standards to remain viable as a civilization. If they send snail mail, the page should fit both into the printer and into the envelope. Several planets on, the envelope should fit into the mail slot in the door of the recipient. And it between, it has to fit various sorting machines. If they send email, there are different but no less complicated requirements.

It is natural for people living on a planet to use the day of that planet for many day-to-day purposes. The day gets divided into sub-units.

When I say "0800 local time" then I expect that it will still be dark in the winter, but already light in summer. I can deal with office hours starting at 0800 year-in, year-out. It would be quite vexing if work starts on 0800 on one day, 1800 the next day, 0400 the day after that, and so on. Sure, I could do the math, but without the help of a computer I'd probably get things wrong if I had to suggest a meeting time 28 days from now. The alternative of office hours starting in the morning only part of the time would be equally vexing, especially for outdoors work.

So I'd expect that everybody thinks both in planetary days and years and standard time units. Some people/professions would be more familiar with the planetary units, others routinely take the standard units.

As a writer, you can use that from time to time to show the intergalactic nature of your civilization.

  • "I'm 18 on my homeworld. Give me a beer!" "Not until you're 6570 standard days."
  • "The planet had an inconveniently short day, just over 14 standard hours. Most residents stuck to a 28.7-standard-hour cycle."
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  • $\begingroup$ A planet that gets dark around siesta time? Where do I sign up!! :) $\endgroup$ – Muuski Dec 28 '17 at 15:40
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    $\begingroup$ Mars may be the only place in the Solar System where humans use a new clock. Every other body has a cycle too long or too short for our comfort, or is so small that crossing the street is likely to take you to a new time zone. $\endgroup$ – Anton Sherwood Dec 29 '17 at 1:08
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    $\begingroup$ In some stories – Beam Piper's Space Viking comes to mind – space travelers habitually speak in hours rather than days. “I saw him about eight hundred hours ago.” $\endgroup$ – Anton Sherwood Dec 29 '17 at 1:09

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