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This question was stimulated by the question How would an aquatic race develop computers?.

The answers obviously were based on the aquatic civilization being earth based, but what about a totally aquatic planet. Could evolution based on different requirements be quite different than evolution on Earth, and interpretations of physics also differ?

Seems to me that the usage of light, as opposed to electricity might be more advantageous in an aquatic environment, and we understand very little about bio-luminescence other than it is chemically based. Our understanding of physics is based on our knowledge. Is it not possible that another environment might create other forms of knowledge, and therefore different understandings of physics?

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    $\begingroup$ I can't quite tell what you're asking. Are you asking if the physics would be the same, or if the understanding of physics would be the same? $\endgroup$ – DonyorM Nov 8 '14 at 13:22
  • $\begingroup$ donyorm, I guess I am asking both, but was trying to then ask, is the physics dictated by immutable universal rules, or by our understanding of physics. For example, quantum theory is redefining or adding to our knowledge of physics; is it not possible that other discoveries on our alternate planet might also change or add to their ideas of physics? $\endgroup$ – fubar50 Nov 8 '14 at 13:49
  • $\begingroup$ I would pick one or the other, because they are different. I think you actually want the latter. And possibly, this would be a better question on Physics (I don't really know) $\endgroup$ – DonyorM Nov 8 '14 at 14:54
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    $\begingroup$ I would recommend changing the title to mention "interpretation of physics" to avoid confusion. "Would the physics change" is an unanswerable question. Either your universe has the same laws of motion as we have in ours, or it has a different set of laws. It has little to do with aquatic or not. However, the interpretation of physics is LOADED with cultural reasons to explore this mechanic or that, so I think that could be a very interesting world building question. It would be a psychological and social question more than a science question. $\endgroup$ – Cort Ammon Nov 8 '14 at 16:06
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    $\begingroup$ Honestly, I would prefer that you avoid talking about the other question about computer and just make the emphasis on physics. It's simpler that way. $\endgroup$ – Vincent Nov 8 '14 at 17:28
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Fluid Dynamics

I think the answer to this one is obvious. A nice theory of fluid dynamics is Navier-Stokes equations, for compressible and incompressible flow. They would have to develop the common mathematical tools, which is kinda independent of the environment where you are. So, this is a successful theory they would do.


Newton's Laws and Newtonian Gravity

Newton's laws can be derived from Navier-Stokes equations. These equations are encoded: momentum conservation, Newton's first and second laws, mass and energy conservation. As gravity only depends on the mass, and buoyancy depends only on the fluid displacement due to volume, it is possible to differentiate, and very likely a aquatic race would do so. Hence, a real theory of gravity like $F = mg$ would be developed. Also, gravity is predicted in Navier-Stokes equations, as a body force $\mathbf f$. Hence we can safely conclude gravity would be developed. And of course, buoyancy would be a well known consequence of Navier-Stokes equations in the static limit. They could develop optics and observe stars, and maybe figure out newtonian gravity as a whole: $$\mathbf F = -\frac{GMm}{r^2} \mathbf{\hat r}$$


Electromagnetism

Electric and magnetic fields exists in a lot of media, including water. There are 4 equations describing all electromagnetic phenomena in any kind of media: macroscopic Maxwell equations. Therefore, electromagnetism does work inside media, and they could use it. The only problem is the presence of ions in the water that could trigger ionic currents (that's why some electric devices do not work in water). However, this can be predicted using this equations, particularly with the density current $\mathbf J$. It is also possible to derive a wave equation inside water, hence demonstrating the possibility of generation of electromagnetic waves inside water. So, possible to do a radio. Of course, the speed of light $c_w$ in water would be: $$ c_w = \frac{1}{\sqrt{\epsilon_0\mu_0\epsilon_r\mu_r}} = \frac{c}{\sqrt{\epsilon_r\mu_r}} = \frac{c}{n} $$ Where, $c$ is the speed of light in vacuum, and $\epsilon_r$ the relative electric permittivity of water, and $\mu_r$ the relative magnetic permeability of water, $n$ is the refractive index of water. This would give for water: $c_w \approx 0.752c$. Just a curiosity.


Thermodynamics and Statistical Mechanics

Thermodynamics is famous of working anywhere. We can easily build thermodynamics of electromagnetism, thermodynamics of Newton's laws, thermodynamics of gas, and surely, thermodynamics inside water. As for statistical mechanics, it can be done by finding out microstates in water. An nice application here is to develop the diffusion equation to explain hydrothermal vents or other thermal phenomena under water. Also, this could be the key for developing astronaut clothing, or I might say, "groundnaut", for exploring non-water domains, or places where oxygen concentration in water is few (assuming they breath oxygen).


Quantum Mechanics

Quantum mechanics began when physicists realized that when we heat up a black body it emits electromagnetic waves according to Planck's law. A consequence of this law says energy is quantized. This is the first step for a quantum theory. Artificial local heating at large temperatures in order of 5000K is also possible using previous knowledge of electromagnetism or/and thermodynamics. So, it is possible to come up with quantum mechanics, then finally a quantized atomic model.


Special and General Relativity

It was discovered as a consequence of what frame of reference are electromagnetic waves. As an aquatic race, sound waves is present. So they could postulate, like we did, that electromagnetic waves travel in a media called aether, as sound waves travels in a water media. They could do the Michelson–Morley experiment below water to prove it wrong, and finally discover relativity, just like we did. As for general relativity, it is literally a generalization, as special relativity is only valid for inertial frames. General relativity is valid for any reference frame (including non-inertial frames). Only later Einstein noticed its close relation with gravity as curved spacetime.


Nuclear Physics, QCD and Weak force

Once relativity growing, they would know: $E = mc^2$, which is a very important equation in nuclear physics. Several nuclear reactors are built underwater, like ATR or RRR. For instance, pool-type reactors could be common. Curiosity: You can identify when a nuclear reactor is under water: if it is emitting Cherenkov radiation. Also, a nuclear theory could be the basis for QCD-Theory, which is the theory who explains the strong nuclear force, and the weak force theory.


QFT, Electroweak, Standard Model and String Theory

QFT, QCD, and the electroweak force theory could have being developed with the previous knowledge we have so far, on mathematical models, or experimental data: like nuclear physics. Joining all of it, we have the Standard Model and now it just needs to be combined with general relativity to come up with unification attempts, like string theory, or something nicer.

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8E&M might not be affected much at all. We spent much of our technological growth believing light traveled through a material (ether) anyways. There are major things I could see becoming dominating factors in how they learn science:

  • Water is very dense, and moves a lot. It also doesn't move in straight lines.
  • Water yields buoyancy.
  • Water is the "universal solvent"

Frames of Motion, and how they discover Newton's 1st law

Swimming in a 3 dimensional world, they are subject to motions that we never notice. The human brain is generally not wired to make sense of the coreolis effect (which occurs when moving while rotating), but in a world where the water moves you around constantly, these effects would be quick to learn.

They would be much faster to learn calculus and differential equations, as mentioned above, to try to grasp these issues. Newton's first law might be discovered as a corollary of measuring velocities in fluid flow, rather than a measurement of positions and times in air.

Because of how complex the movement of water is, I would expect their mathematics of topology and fields to explode dramatically faster than we do. I would expect every 8yr old aquatic denzin be intuitively more comfortable with manifold topologies than your average PhD mathematician.

Gravity would likely be tied in tightly with buoyancy. It is possible they could develop their entire system of equations of motions around bouyancy. While we think life exists at (1 gee), they might think of it existing centered around (0 buoyancy). The air above them would seem to inflict negative bouyancy.

Another interesting direction: when moving in water, chaotic flow is a big deal. If your flow goes chaotic, you slow down fast. It took us to the mid 1900's before Edward Lorentz discovered chaos theory. They would have easily picked it up many years before that. Because so many systems in the body are easier described by chaos theory than traditional models, their understanding of their anatomy and biology could be far superior.

An alternate viewpoint

Perhaps the most impressive change in how they approach physics and technology would be the most nuanced. Underwater, form and function are rarely separated. The cost of separating them, in terms of fluid dynamics, is too great. This could cause a society to focus more on harmony in movement. The entire march of science and physics could be done in order to be "in harmony with nature," while much of Western science focuses more on "triumphing over nature." Their entire approach to science and physics could be more Eastern in nature because of this.

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    $\begingroup$ I think this is basically the same answer I gave, except that it provides well-considered examples in all the places where I resorted to unfounded genralizations and hand-waving. $\endgroup$ – octern Nov 10 '14 at 6:24
  • $\begingroup$ I like this answer best, but it doesn't address something I feel is important (nor does any other answer). These beings should theoretically be able to make observations about the world above water. If they ever go near the surface, they could immediately observe physics outside of their buoyant fluid. They could make observations about objects on the surface, projectiles in the open air, and objects partially submerged. Being an underwater species doesn't mean all of their knowledge needs to be gained from underwater physics. $\endgroup$ – Web Head Nov 24 '14 at 6:30
  • $\begingroup$ That is very true, CreationEdge. They would have that ability. However, they would need a good reason to want to explore above-water physics before they did so (something along the needs for archer fish to hunt). Otherwise they would explore it in the late game, rather than early on, and their models would reflect it. When they finally start tackling above-water physics, it would be likely be an adaptation of their underwater rules, just as Relativity is an adaptation of Newtonian physics. Thus I would expect their modeling to be different (not wrong, just different). $\endgroup$ – Cort Ammon Nov 24 '14 at 16:08
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The hard part here would be that the medium this civilization lives in would be fairly disturbing. Ocean currents can fluctuate easily in certain spots, creating chaotic flows. Maelstroms and other disturbances would mess up the local environment, and rip currents could be devastating to creatures living near the surface. I'm not an expert in marine activity, but I do know that it would be hard for these creatures to avoid interacting with the medium around them.

That said, there are a lot of principles I would think they would figure out:

  • $F=ma$: These creatures would realize that if you apply a force on an object, it accelerates. It would be evident after a lot of experimentation.
  • Newton's third law: If you push on something, the object pushes on you. In still water, this would be incredibly obvious.
  • Gravity: Only very buoyant objects would not sink after some time.
  • Conservation of energy and momentum: I think this will follow after playing around with experiments resulting from studying the principles listed above.

Some things they would not figure out:

  • Newton's first law: An object in motion might not stay in motion; an object at rest might not stay at rest. Currents will interact strongly with the environment, moving objects around.
  • Electricity (and perhaps magnetism): In case you haven't noticed, electricity and water don't mix too well. Unless these creatures are electric eels, electricity won't be a major force (pun intended) in their lives. As for magnetism. . . How many magnetic materials can be found at the bottom of a body of water?

Figuring out more advanced principles would depend on their technological development.

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In a technological society physics would be physics. The route to a solid understanding of the universe would be different for an aquatic species, but if they live in our universe (or one with similar physics) then the rules of the game are the same for everyone.

If you're curious about a more topical approach rather than the fundamental laws of nature, then there would be some significant differences. They would likely have an innate understanding of fluid dynamics and solution chemistry that we do not. It's likely that their understanding of biology would far outstrip their chemistry or physics, since there are a number of extremely important biological topics that can be learned with fairly rudimentary tools and a quick mind.

They would have a disadvantage with some basic concepts like inertia, gas phase chemistry, momentum, and even one dimensional motion. To solve a one dimensional motion problem underwater you have to do differential equations - math that the bulk of our population never even comes close to learning. Air is sufficiently close to a vacuum that we teach 8 year olds how to do them. Metallurgy would be difficult or impossible, as would the development of glass (an incredibly useful substance for science.)

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The earliest attempts at codifying physical laws involved ideal equations that basically assumed bodies moving in a vacuum. This wasn't accurate, but it was close enough that we were able to start making decent predictions and gradually refine them. In water there's far too much resistance to ignore.

Similarly, air pressure / density varies at different points on the Earth, but not by so much that it makes approximations impossible. If your aquatic civilization is dealing with a variety of depths or salinities, there's no way they could get the idea that all objects in all situations move following the same laws. HDE mentioned the effects of currents, which could be even worse -- but those are obvious enough that I think they'd be easier to recognize and remove from one's calculations (analogously, you don't need a complex understanding of air pressure to realize that you shouldn't perform physics experiments in a tornado).

The same would be true for any discoveries in optics that involve modeling light as a ray that travels indefinitely in a straight line. You would have to average out so much interference in order to make this observation that it would probably never occur to anyone.

Newton made the great discovery that the same law that governs a falling apple also governs the motion of the planets, which gave his physics a grand, majestic feel and an air of Platonic (or Euclidian?) purity. Underwater physics might be more context-dependent and focused on how real-world practices need to be carried out with a careful understanding of one's environment. They might lag in discovering what we consider basic physical laws, but have an advanced understanding of more complex engineering principles.

On the other hand, perhaps there are physical laws that can be more clearly perceived in water than in air. Maybe their overarching, Newtonian insights would involve temperature conduction or wave propagation.

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It would definitely be understood differently.

Let's start with something easy: Archimedes law. Here we have our since forever, but because Archimedes loved to bath. In your society such law would probably be defined as speed of object falling to the ground.

Speaking of which. How did they discover gravity? On earth were have nice story about falling apple. But would be there trees? And also, the density of water makes this pretty hard to guess how the gravity works.

The biggest novelty to your civilisation would be anything "gas" connected. From having pure has available to the whole flying thing

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We understand bioluminescence quite well. I also don't see how this contributes to the question. Please clarify this.

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