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When I was younger I learned about time dilation and since my story was (and is) set post-planet I thought many hours about how to handle it. I came to the conclusion that I could just drop it and barely thought about it again. Now I can finally ask if this approach is justified.

As an author, of course I can just ignore physics and write what I want, but if that was my plan I wouldn't have given the problem so much time in the first place. So here are my thoughts:

  • In my setting for some reason almost all intelligent lifeforms are human-like (they have the same ancestor lifeform).
  • I recall that we humans react poorly to heavy gravitational changes.
  • Hence all inhabitated planets should have gravitation about 9-10 m/s².
  • Hence time dilation on inhabited planets is not practically noticable by the lifeforms.
  • Therefore I don't have to deal with time dilation in my stories unless there is a very specific event.

Are there any mistakes in my reasoning? What are your thoughts?

EDIT: The usual planetary travel is almost immediate teleportation, which is backed up by a magic-science-combination, including worm holes. The second-to-most travel relies purely on wormholes. There is no other way (known to me), e.g. you can't win a race against light if you too are using the same road.

Time dilation is a thing that happens all around us, just too small to notice. There is a time dilation effect between the netherlands and the himalaya because of the gravitational difference, but we ignore it. I feared the effect could become more notable between planets with greater gravitational difference. Also our galaxy spins, but around what exactly? I think a black hole and being nearer to it or too far away may also have time dilation side effects, so I'm keeping the interesting stuff happening on planets which are not too near to the center or rim of the galaxy.

EDIT: I think all answers are useful to me and I really would like to thank you all. I marked the most useful one as accepted, but the favourite of the community is my runner up ;-)

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  • $\begingroup$ Do you have some method of FTL travel to get between these planets? If you don't, then time dilation will always be relevant. $\endgroup$ – Z.Schroeder Nov 25 '16 at 22:32
  • $\begingroup$ @Z.Schroeder reading your comment again I notice I may not understood it. Can you elaborate? $\endgroup$ – Ayutac Nov 25 '16 at 22:41
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    $\begingroup$ Time dilation due to what exactly? Speeds approaching the speed of light, gravity, or something else? You're asking about whether you can ignore time dilation effects. So you need to tell us what things would happen that you think might cause such effects. $\endgroup$ – Nicol Bolas Nov 25 '16 at 23:01
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    $\begingroup$ If you teleport between planets, use wormholes or have FTL drives then that's fine. But normal interstellar distances absolutely CANNOT be traveled without experiencing some time dilation. Our closest neighboring star is over 4 light years away, meaning you'd have to travel at relativistic speed to get there within a normal human lifetime. And when you travel relativistically (between 1% and 99.999999999999999 etc.% of light speed), you will absolutely experience time dilation due to the effects of general relativity. $\endgroup$ – Z.Schroeder Nov 25 '16 at 23:03
  • $\begingroup$ Yeah, if you're not traveling at relativistic speeds or exploring black holes than time dilation isn't really an issue. $\endgroup$ – FirstLastname Nov 25 '16 at 23:03
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You absolutely can get away with ignoring it in a vast majority of cases. My preferred go-to-argument for this is Sanderson's First Law of Magic. I quote it so often I feel like I should probably bookmark the link, but it's so bloody useful I keep using it!

Sanderson’s First Law of Magics: An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.

Yes, you're talking about science fiction, but the argument is identical. So identical that, if you want, I can steal Arthur C. Clarke's argument for my own:

Any sufficiently advanced technology is indistinguishable from magic.

In the end, a book is for the reader. I don't care how much attention you gave to the science, or how little. If the reader enjoys it, you're winning, and that's all there is to it.

The only time you really have to make serious concern of time dilation is if you are using it to resolve conflict. If there was a potential conflict caused by the inability to travel quickly between planets, and you just hand-waved it away as "oh, those relativity concerns just don't matter," you could get in trouble. However, all you have to do is justify it enough so that the reader understands how you're resolving that conflict.

A priceless example shows up in Frank Herbert's Dune with the navigators. Dune includes faster than light travel to make the plot work. It originally does so via hand waving it away as "the navigators make it possible." However, Herbert weaves this concept so deeply into the politics of his novel that the reader feels the understand the FTL issues enough to permit Herbert to handwave them away. In fact, the reader almost feels privy to a secret, as one watches the politics surrounding this handwaving twist and turn, because it turns into several major plot points throughout the series.

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  • $\begingroup$ Thanks for the book recommondation. However, the scenario "inability to travel quickly between planets" exists. A group of pioneers travels through planets in the early days and their travel machine needs time and ressources to reload. In that time they need not to anger or offend the local folks, but mistakes happen. What would be the consequences you thought of in this case, assuming hese pioneer planets don't have space travel of any kind or just the kind we have today? $\endgroup$ – Ayutac Nov 26 '16 at 9:33
  • $\begingroup$ That's a strange one. When time travel is involved, I'd expect anything related to relativistic time dilation may play as small of a part as you please. Current physics predicts that you cannot build a time machine using any known methodology, so if you have a time machine, you've already broken free of the shackles of relativity. $\endgroup$ – Cort Ammon Nov 26 '16 at 15:51
  • $\begingroup$ Off hand, how much of an effect do you believe time dilation has when not traveling at significant fractions of the speed of light? $\endgroup$ – Cort Ammon Nov 26 '16 at 16:04
  • $\begingroup$ when did I mention time travel? "Travel machine" in the sense of portable wormhole creation machine. $\endgroup$ – Ayutac Nov 26 '16 at 18:47
  • $\begingroup$ @Ayutac Ahh, forgive me, I read "their travel machine" right near the word "time" as "time travel machine!" $\endgroup$ – Cort Ammon Nov 27 '16 at 2:01
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Your question is based on the following assumptions.

Hence all inhabitated planets should have gravitation about 9-10 m/s².

Hence time dilation on inhabited planets is not practically noticable by the lifeforms.

The time dilation in question is that experienced in the gravitational fields of inhabited planets. Note many of the other answers leapt to the erroneous conclusion that your question about time dilation related to relativistic space travel. It doesn't, so read the questions more carefully in future.

The time dilation in the gravitational fields of inhabited planets with 9.8 m/s/s will be negligible. Exactly as it is here on good old planet Earth. It can be ignored for all everyday purposes.

Now there may be time dilation to due to forms of space travel in your world, but that is an entirely different question. With a science-"magic" combination of FTL travel, it doesn't hurt to handwave away any unwanted relativistic effects like time dilation.

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    $\begingroup$ If you do feel compelled to mention gravitational relativistic effects for some reason, you could always have a non-techie ask a techie why navigational systems - or clocks - have such complex programming, at which point the techie briefly explains that they have to correct the nanoseconds due to relativistic effects. More than that should probably be as unnecessary as explaining this in things like GPS in real life. $\endgroup$ – Trevortni Nov 26 '16 at 3:03
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    $\begingroup$ @Trevortni Precisely. General relativity affects GPS, but no-one talks about it -- unless it's techies. Gravitational time dilation is only significant for high precise systems. So unless there's a good story reason to deal with it, it's best forgotten. $\endgroup$ – a4android Nov 26 '16 at 3:21
  • $\begingroup$ Thanks for reading carefully, but my question included me asking if the assumption about gravity and us is correct. Or will other gravities lead to DBZ-like strengthened lifeforms?Or should I put that into an extra question? $\endgroup$ – Ayutac Nov 26 '16 at 9:27
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    $\begingroup$ @Ayutac Tour assumption about gravity is correct & time dilation is insignificant. Comfort & being well adapted makes a lot of sense. Living on higher gravity planets is probably another question. $\endgroup$ – a4android Nov 26 '16 at 9:47
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    $\begingroup$ @TimB Only significant for machines with atomic clock precision. Real but extremely small. You could have fun devising a story where even microscopic time differences have big consequences. Though not for the faint hearted. $\endgroup$ – a4android Nov 26 '16 at 9:51
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¿How is the artificial gravity achieved? Because if it is done via some device or generator it would no be such a stretch to use the same technology to cause a counter-dilation to compensate. After all, gravity affects space-time, and humans living with a standard gravity across the board would also desire a standard time across the board.
I think it is possible.

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  • $\begingroup$ The gravitron is indeed something I put in the black box and this is a simple analogon. Haven't thought of it, many thanks. Still, do you think something like this is actually needed for interplanetary daily life? $\endgroup$ – Ayutac Nov 25 '16 at 23:12
  • $\begingroup$ Maybe not for daily life, but can be handy in case of emergency, such as a ship being adrift or a space-jump taking you to the wrong coordinates. In those cases a protocol that automatically adjusts the passing of time could be considered simply part of the life-support system. $\endgroup$ – ThreeLifes Nov 25 '16 at 23:19
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Learning to avoid something complicated to explain is a great way to keep your writing on point.

You can give a nod to it in the writing without going into detail, which tells the reader that you did know about it.

Yiur backstory and notes can have details that you keep in mind but don’t get elaborated on in the actual story.

See also this subject concerning FTL instant jumps and timekeeping, in detail.

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  • $\begingroup$ Thanks for the feedback, but the point wasn't really about explaining it to the readers, but more for my good conscience. $\endgroup$ – Ayutac Nov 26 '16 at 9:16

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