The current Underwater tag challenge gave me an idea: What if the whole universe were actually liquid?
To elaborate: How can a liquid universe come to life?
Ultimately, I want to have a universe where the space between planets and stars is filled with liquid. I would like that universe to violate as few of the laws of physics as possible, though if violations are necessary, I'd like to know which ones.
So: how, plausibly, could such a universe come to life?
Your idea greatly resembles aether theories. As such, they are very well consistent with the understanding of physics (and were absolute mainstream) up to the Michelson–Morley experiment of 1887 which strongly suggested the absence of aether; but even after that the Lorenz aether theory successfully incorporated some special relativity phenomena although based on the aether framework.
However, in the above aether theories aether is, to quote Huygens, “an omnipresent, perfectly elastic medium having zero density” which makes it quite different from the liquid-as-we-know-it. On the other hand, it clearly might allow for a world consistent with pre-20th-century physics and maybe even able to incorporate some of non-relativistic quantum mechanics. A description of the electromagnetic waves as transmitted by the aether allows for a non-contradictory physics where some of the non-everyday physics effects like stellar aberration would have been different (cf. aether drag hypothesis with references to the Stokes model).
For Average Joe, it is probable that "liquid" means "I can swim in it". That means that you can actually push the liquid away to make yourself move, by conservation of momentum.
So your liquid needs to have a mass. That yields several problems :
If the liquid have a mass, it is attracted by other massive objects such as star. In order to still fill the whole universe, and not just be aggregated around stars (or at a larger scale around galaxies), it needs to be incompressible. At the same time your liquid must dilate as the same rate as the universe to always fill it (but I will not address this problem in my answer).
Planets moving through the liquid will push on it. Therefore, a planet is slow down by the liquid except if both travel at the same speed. That implies currents in the liquid. That kind of system could be stable only if there is no friction, otherwise the currents will slow down, and thus the planets. It would leads to the planets simply falling into their stars (and stars falling on the centre of their galaxies). So, your liquid needs to be frictionless.
A partial conclusion is that you need a massive, incompressible and frictionless liquid. It does not looks like a big deal at first, but in fact it is.
The problem is for the liquid to be incompressible. When you push on a liquid in real world, you create a compression wave in it, and since the compression/decompression process travelling in the liquid takes time, the wave propagate at finite speed (the speed of sound in the medium).
However if the liquid is truly incompressible, the wave (i.e. information) will travel instantaneously, thus faster than light. Breaking relativity is probably not a good start.
It is possible that due to this, any push on the liquid will leads to the displacement of an infinite amount of mass (and therefore being impossible), but I have not yet figured it out in details.
In conclusion, to looks like a liquid for Average Joe, your interstellar liquid needs to breaks some laws of physics. Indeed, in our world a incompressible frictionless liquid is not possible. And this breaking leads to even greater breaking of physics, at least relativity, but it is hard to figure out exactly how much you will have to break.
For this to work, you're going to have to throw out gravity as we know it.
If the interstellar fluid repels other matter, but not itself, other matter will form bubbles within it. Think what happens if you drip oil onto the surface of water - the oil forms round droplets, and if droplets come into contact with each-other, then they'll coalesce into a larger droplet.
Depending on the precise nature of the boundary effects, you may be able to create sufficient pressure in a sufficiently large droplet of hydrogen in order to achieve a fusion reaction, as well as providing a force similar to that of gravity on the surface of a world.
A universe of this sort would be quite different to our own. There would be no orbits as we know them; it is more likely that objects would move as a result of thermal convection in the interstellar fluid, and this movement could be rather chaotic.
A world may be in motion relative to a star by virtue of being caught in a convection cell, with planetary rotation induced by boundary effects between neighbouring convection cells. An interesting quirk of this is that day length could vary considerably depending on the position of the world within its convection current, and it might be ejected from its current to another one at some point in time.
The interstellar fluid may carry a significant amount of heat around a star, allowing for planets to be warm even on their dark side.
Interstellar travel would be slowed to a degree dependant on the viscosity of the interstellar fluid, though it would also provide a medium against which a ship could push using propellers. It should also be possible to sail the convection currents. With a combination of these two methods, interstellar travel should be possible.
If you had a universe where gravitons don't exist or mass doesn't make gravity, then you could have a completely liquid universe, without having to worry about pressure.
Though how such a universe could form in the first place is a bigger problem.
Ignoring that, it could be interesting.
Planets could be rocky lumps with molten cores to warm the liquid like deep sea vents, which would support life around them.
Having no gravity makes fluid dynamics problematic, since if the liquid got hot enough to boil, it would create a gas bubble that wouldn't go anywhere. This could allow for stars to operate to some degree, but not currents very easily.
If this liquid had a freezing point in the single digits kelvin then "interstellar" space would stay liquid. You could travel between planets if you some insulation or a heat source.
A universe in which mass does not cause space time curvature is what you would need to have a liquid universe as you describe. Your liquid universe would be similar to the idea of flatland but with three dimensions instead of two.
Also this universe would likely be closer to having thermal energy evenly spread through the universe making useful work more problematic as there would be less of a difference between the temperature of a life form or machine and the environment than in our universe.
