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Imagine a spaceship constructed for merfolk and it is completely filled with oxygenated water instead of air.

Since they can swim around effortlessly in the water, do they even need artificial gravity similar to their natural habitat on Earth?

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    $\begingroup$ Are you trying to decide if artificial gravity is needed, or are you trying to justify there being an artificial gravity device on your spaceship because you need it for some other plot purpose? $\endgroup$ – Henry Taylor Oct 13 '17 at 5:34
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    $\begingroup$ @HenryTaylor:the former $\endgroup$ – user6760 Oct 13 '17 at 5:38
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    $\begingroup$ Apparently merfolk would have it even worst than humans. smithsonianmag.com/smart-news/… $\endgroup$ – SilverCookies Oct 13 '17 at 8:00
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    $\begingroup$ In relation to gravity, there's no difference between falling to the ground or sinking to the bottom of the ocean. Merfolk experience gravity as much as we do. It just so happens that merfolk are more buoyant in water than humans are buoyant in air, therefore exacerbating gravity's effect on human buoyancy in air (compared to merfolk buoyancy in water). At face value, I doubt there will be much difference in space (in regards to physics) whether the atmosphere of the ship is made up out of a liquid or a gas, as long as it's one homogenous atmosphere. $\endgroup$ – Flater Oct 13 '17 at 13:28
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    $\begingroup$ Is it even possible to get a ship full of water into space? $\endgroup$ – Azor Ahai -him- Oct 13 '17 at 18:48
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Artificial gravity will make it much easier to efficiently and evenly oxygenate the water.

Lack of gravity means lack of convection. Lower density gasses or fluids just sit where there are. That's a reason why fires in space are spherical and automatically extinguish themselves - since there is no "up", smoke does not raise high and away from fire, and doesn't cause currents drawing in fresh air with unspent oxygen.

Similarly, if you want to oxygenate water, you won't be able to simply make air bubbles pass through water from bottom to top, increasing diffusion by increasing contact surface, like those devices used in aquariums do. Pumping oxygen in will create one large bubble next to exhaust forcing a lot of creativity in making it work - first you need to stir water next to oxygen source to make it mix with water, later you need to stir entire room to spread oxygenated water and yet later you need to somehow extract carbon dioxide out of water. Easiest way I can imagine would be to use exhausts, intakes and pumps to create artificial current in entire room, filtering and oxygenating outside of room, before returning it to internal cycle (you still need similar system to filter water anyway, but not as much as for oxygenation).

Compare this to ease of lining floor with tiny oxygen exhaust, ceiling with oxygen/carbon-dioxide intakes and letting artificial gravity do the work.

Obviously, you need pumps and filters to close the cycle - unabsorbed oxygen gathered by ceiling intakes is pumped back into floor exhausts while carbon dioxide is recycled before returning to loop. However, if you already posses artificial gravity, then this is at least one of the reasons to use it: oxygenating water without artificial gravity will require much more plumbing and with much higher throughput.

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    $\begingroup$ Is the lack of convection not solved by the merfolk swimming in the water? Or is that convection orders of magnitude too small for sufficient oxygenation? How about adding water pumps, while still not having artificial gravity? Secondly, is the necessity of convection for oxygenation not built on the assumption that there's air (with oxygen) at the top? In the spaceship's water, there is no top layer exposed to air, and the water would have to be oxygenated in a different way. $\endgroup$ – Flater Oct 13 '17 at 10:21
  • $\begingroup$ (ignore the second remark, I briefly forgot that you were focusing on the even distribution rather than the oxygenation itself) $\endgroup$ – Flater Oct 13 '17 at 10:32
  • $\begingroup$ @Flater Honestly, I would rather consider a spacesuit for waterfolk. Pumps work better to maintain a relatively small area, and circulation is easier. It is not a must to have it made from cloth and plastics, it can be even organic, just like the one for Independence day's aliens. $\endgroup$ – Sonic Oct 13 '17 at 12:59
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    $\begingroup$ @Sonic We're talking about race of which all people live in water (OP refers to it as a merfolk spaceship, i.e. built by merfolk exclusively for merfolk). It's easier to maintain one big tank than it is to maintain several smaller ones. Plus it makes it possible for them to not have to wear a suit. Humans do it too: spaceships have a breathable atmosphere, humans aren't required to wear a space suit at all times. It would be incredibly inconvenient if you were not able to leave your suit during your extended stay in a spaceship. $\endgroup$ – Flater Oct 13 '17 at 13:14
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    $\begingroup$ @Sonic: I agree about the mass of water (the weight is irrelevant in a weightless environment, but I know that's pedantic). However, the inconvenience of always being bound to your suit for long distance traveling may outweigh the issue of an added fuel cost. If you're talking about long distance space travel in scifi, fuel costs are usually not a big problem anymore (relative to the fuel economy needs of current day spacecraft). A water suit may be acceptable for a spaceship built for both humans and merfolk (as a matter of compromise), but for a merfolk-only ship it seems silly. $\endgroup$ – Flater Oct 13 '17 at 13:43
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If a mermaid has a swim bladder, a lack of gravity will make it harder to move up and down.

This article has more info on how it works, but basically by increasing and decreasing the internal pressure of this bladder, a fish can easily change vertical elevation and maintain its current depth without expending a lot of energy.

If there's no gravity, there is no downward pull to make going down work. Buyoancy is defined as the upward force opposing the weight of the immersed object. With no gravity acting on you, you're weightless and have no buyoancy. That means that going up wouldn't work either.

If a mermaid had this trait, she could not change her position in vertical space with her swim bladder. She could be disoriented and need special training to learn how to travel differently. Handrails could be installed to make changing elevation easier.

Mermaid astronauts coming home might experience something akin to a swim bladder disorder, and have trouble staying level or moving vertically once they get back to their planet.

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    $\begingroup$ "If a mermaid had this trait, she could not change her position in vertical space with her swim bladder." There is no vertical space in a weightless environment. So long as merfolk are able to swim (= propel themselves forward), they can still move in any direction they want. And even if they couldn't swim (= propel themselves) either; humans are able to cope with this change in environment (using the walls and fixed handles to propel themselves), why would merfolk not be able to do the same thing? $\endgroup$ – Flater Oct 13 '17 at 13:21
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    $\begingroup$ Regardless of the lack of vertical space in an aircraft, a mermaid with this trait still would be accustomed to life in the sea where traveling up and down in the sea vertically was the norm. I didn't say they couldn't move in other ways, just that it would be different that what they're used to. Also you are correct in bringing up how human use walls and fixed handles to move about. That was the inspiration behind installing handrails. $\endgroup$ – Lot-Of-Malarkey Oct 13 '17 at 15:11
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    $\begingroup$ Your answer seems to suggest that merfolk intelligence is similar to that of an animal (notably hindered by not using its usual movement method), but they are intelligent humanoids. If merfolk are capable of space flight (building a space craft, traveling through space, ...), it's not logical for them to be stumped by having to move around using a different method. Sure, they will need to get used to it because it's different, but life aboard a spaceship will likely be different from life back home in many ways; and moving around is something you'd get used to really quickly. $\endgroup$ – Flater Oct 13 '17 at 16:10
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    $\begingroup$ I didnt mean to imply that they would be totally hindered. I think they could definitely adjust, i just mentioned training since real life astronauts do actually go through gravity training. They could get used to moving around, but some mermaids may sefinitely adjust quicker than others. $\endgroup$ – Lot-Of-Malarkey Oct 13 '17 at 18:25
  • $\begingroup$ If theres any way to make this clearer feel free to reccomend changes. $\endgroup$ – Lot-Of-Malarkey Oct 13 '17 at 18:26
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They would need it for the same reason, we thin-atmosphere-breathers need gravity. Yes, it is very nice that gravity helps us keep our boots on the ground so that we have an easier time stopping once we are in motion; but the real value of gravity is that it makes our equilibrium work.

Ask anyone who has ever suffered from chronic vertigo or sea sickness. Knowing which way is down is vital to having a happy life and journey!

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    $\begingroup$ I can't find any sources, but I believe it doesn't take long to adopt to orienting and moving in a microgravity environment even for humans. I would guess a day or so. And for creatures used to moving underwater, especially if they have experience with swimming in caves (where they wouldn't be able to tell up from down without bubbles), the difference is smaller. In fact, one of the largest "swimming" pools in the world is used for astronaut training because space and underwater are similar. $\endgroup$ – Arthur Oct 13 '17 at 10:30
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    $\begingroup$ I cannot prove it, but I would suspect that even fish living in caves know what Up and Down are without seeing the bubbles. Same for Deep Sea dwellers. Underwater is like space for us because WE are used to non-floaty-environments. $\endgroup$ – Layna Oct 13 '17 at 11:07
  • $\begingroup$ @Layna: Very true, fish - generally - orient themselves as you would expect, with dorsal fin on the top and ventral on the bottom. This could be a function of air-sacks along the spine to help keep them oriented. There is even a fish who have evolved so the stop of it's head is transparent, so it can look upwards for prey. Occams razor tells us that orienting it's self to face upwards would be simpler, there is some other pressure keeping the fish oriented flat in the water Swim baldders $\endgroup$ – Binary Worrier Oct 13 '17 at 11:58
  • $\begingroup$ @Arthur: As I understand it, the main idea of underwater space training is that you can simulate lower/higher gravity environments by changing the buoyancy of the diving suit. It's not so much that underwater environments and space environments are supposedly very close to eachother (underwater = higher pressure, in space = lower pressure, quite the opposite). You could accomplish a similar "buoyancy" in air, by attaching a balloon (or weights) to the person's suit, but this is simply more unwieldy due to having things externally attached to your suit (+the balloons would need to be quite big) $\endgroup$ – Flater Oct 13 '17 at 13:36
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    $\begingroup$ @BinaryWorrier: No worries. I understood your intended message, but Occam's razor actually tells us the opposite of what you mentioned, hence my reply :) $\endgroup$ – Flater Oct 13 '17 at 14:15
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Artificial gravity is expensive (you need a really long tether (which will hit space junk and dust, and will create disorienting Coriolis forces on-board) or magic), and a pump moving oxygenated water is cheap, so I'd laugh if I read that merfolk needed artificial gravity do deal with respiration.

As far as controlling one's location with a ballast goes, that's a much smaller adjustment than humans made to zero-gravity. Unless your merfolk are less physically adept than humans or bats or dogs or monkeys, that shouldn't be an issue.

I see no convincing reason to use artificial gravity for merfolk except the same reason we always see humans with artificial gravity in movies - it's prohibitively expensive to make TV and feature films in zero gravity situations.

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