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Can a fish swim out of a sphere of water in a zero-gravity environment?

I am going to state some assumptions.

  1. We assume the sphere of water is not affected by any thermal properties of its zero-gravity environment

  2. The sphere of water is appreciably larger than the fish (at least 10x the volume of the fish)

  3. The fish starts at the center of the sphere and it can survive forever in the sphere

A version of this question has been dramatized in the movie Passengers.

Can the fish swim out of, assuming the fish is strong enough to break the surface tension, the sphere?

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    $\begingroup$ Please explain why you think that it could or couldn't. $\endgroup$
    – Bergi
    Commented Dec 14, 2023 at 6:24
  • $\begingroup$ The two arguments that keep coming up in my discussions are 1) it could not because the surface tension will act to enclose the inner volume of water and any sort of motion (fish flapping its fins) will just endlessly redistribute the water around the fish and it will never get out, and 2) it could because the fish and blob of water can be viewed as two individual masses that exert equal and opposite forces, so any attempt to swim will push it towards the surface of the blob $\endgroup$
    – Jonathan L.
    Commented Dec 14, 2023 at 16:00

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The experiment has been done. Zero gravity is not required. Fish are observed to jump out of bodies of water on Earth.

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    $\begingroup$ This is powerful. $\endgroup$
    – Jonathan L.
    Commented Dec 13, 2023 at 20:50
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    $\begingroup$ This does not answer the question at all, because in your case water is under gravity, so it "assists" fish with greater inner resistance forces, and also buoyancy forces assists fish too, because buoyancy is only due to pressure difference on body which arises because of same Earth gravity. In micro-gravity or zero gravity there would be no buoyancy and under "fast strokes" with fins, fish can easily disrupt the water bubble (depends on bubble size) which I would not call "swimming". Conclusion, it's not so clear to me as to you what would happen with swimming in weightless environment. $\endgroup$
    – Agnius Vasiliauskas
    Commented Dec 13, 2023 at 20:55
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    $\begingroup$ Most fish try to remain neutrally buoyant, so any lack of buoyancy has little impact on how the fish swims. There is no impact of gravity on surface tension. If the 'bubble' is much larger than the fish, it will swim as usual without issues. $\endgroup$
    – Jon Custer
    Commented Dec 13, 2023 at 21:32
  • $\begingroup$ It's also worth mentioning that there have been plenty of experiments that involved putting fish in space. None involving jumping out of water as far as I know, but basically it takes them a few days to get used to not knowing which way is up, and then they can swim just fine. $\endgroup$
    – N. Virgo
    Commented Dec 14, 2023 at 5:45
  • $\begingroup$ @JonCuster For maintaining neutral buoyancy fish uses Swim bladder which will not work in micro-gravity conditions, because there's no buoyancy ! However, to what I can agree that in that case (in zero gravity) fish doesn't needs Swim bladder organ at all, because "neutral buoyancy" would be maintained by zero weight itself (no gravity). So probably fish tries to accommodate in these few days to "rendered obsolete" swim bladder organ". $\endgroup$
    – Agnius Vasiliauskas
    Commented Dec 14, 2023 at 7:51
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Some fish species are famous for swimming in free-falling water.

a random video of salmon swimming up waterfalls

If the body of free falling water is in a standstill with respect to the observer, this may aid the observation, but it will be the same for the fish.

What is harder to see in videos like this is the core stream of falling water that is under the bursty droplets, where the fish systematically advances up. When the fish gets out of the core stream, it tries to swim in air with small percent of water mixed in. This does not work well (flying is like swimming but requires different surface to weight ratio) so we see the fish jumping out of the waterfall.

Fish swims by throwing water back. The process is not straightforward and involves a lot of transverse movement. The gravity is what forces water together against the fish movements that spray it around.

Without gravity and relying only on the surface tension to hold the water mass together, an energic fish will create something like a water droplet explosion that will leave the fish dry, but just like in a waterfall, the fish will move out of the initial water boundary.

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