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There exists a Universe in which all of outer-space is filled with some form of human-breathable air. Theoretically, you could fly a Zeppelin up there and explore other worlds, with a sufficiently long travel time.

Nearly everything about physics, except for maybe some minor differences, are the same as in your universe, though. How can this be?

My question, then, to the scientists of this Universe:

What is/are the least intrusive physical law(s) that would need to change in order for Humans to be able to breathe in outer-space?

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    $\begingroup$ Due to the framing of the question, (i.e. "least intrusive"), answers that massively affect the way life on planets work are perfectly acceptable. :) $\endgroup$ – Onyz Jan 23 '18 at 16:13
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    $\begingroup$ humans that don't need to breath is far far more possible than breathable "space" that only violates the laws of thermodynamics. $\endgroup$ – John Jan 23 '18 at 16:50
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    $\begingroup$ @Pleiades: One atmosphere is about 100 kPa, or, more commonly 1000 hPa. $\endgroup$ – AlexP Jan 23 '18 at 16:58
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    $\begingroup$ "Outer space" full of atmosphere isn't outer space. It's just an extended atmosphere. $\endgroup$ – RonJohn Jan 23 '18 at 18:26
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    $\begingroup$ FWIW I have a pet theory that the Star Wars universe has a Permeable Aether. It explains SO much: Solo & Leia could go out in the space worm with just breathers (not even gloves!), Fighters fly like WW2 planes, sound in space, rebel helmets and more. As long as you have some special means for fast travel (eg between planets/systems) you could do similar! $\endgroup$ – aslum Jan 23 '18 at 20:32

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Most other answers have already made clear that gravity affects gas, that such an amount of gas would have disastrous effects in real world, and that the effects of tweaking gravity can be even worse.

Therefore, the least intrusive physical law change is to tweak the gas. Interplanetary gas must have some interesting properties:

  • In order not to collapse into planets nor form a black hole, it must be massless. At least it can't have gravitatory mass (gas atoms can't attract other masses by gravity). Being massless, that gas is less dense than any planet atmosphere and therefore it floats over planets instead of mixing with them.
  • Gas can't cause friction, just not to slow planets. You can get rid of viscosity by handwaving, but turbulence may still be a problem. Making it absolutely massless might solve the problem, but I'd prefer just to make the gas move in a perfect flow following the planets. In this system all orbits are nearly circular, and if comets fall into the system they are slowed because they aren't moving with the gas.
  • Airships (or airplanes) need to move through gas. That is, propellers need to work. That's the reason not to make the gas absolutely massless. It must retain its inertia and follow orbits although it can't exert gravitatory effects on other bodies. I'm afraid you need to overlook Newton's third law, here, but as long the gas keeps symmetrically distributed in the system planets will be fine.
  • Until now, you could fly a plane through the gas or keep alive just by using a breathing equipment. If you want to breathe that gas, it needs to be made of several elements that are chemically equivalent to the usual ones, although keeping their special properties. Most of the gas can be made of the equivalents of nitrogen and oxygen. In fact, people or animals breathing such a gas for some time may get some special oxygen incorporated in their body and get some of that special characteristics.
  • In addition, some components of that gas may react exothermically under special circumstances - like those inside a four stroke aviation engine. This way, you can get rid of the need of fuel for your interplanetary travellers, while preventing the whole system from exploding.

In summary, interplanetary gas should be:

  • Massless from gravity point of view, but with inertial mass.
  • Frictionless and orderly moving with the planets.
  • Made of special kind of atoms, chemically equivalent to usual ones.
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    $\begingroup$ If the question hadn't stated "human"-breathable, that kind of breathable aether could make a very suitable answer, since animals could be adapted to breathe aether even in their home planets. Since humans are involved, I see more intrusive handwaving human biology than handwaving oxygen-like gas viscosity. However, it's just a matter of opinion. $\endgroup$ – Pere Jan 24 '18 at 10:22
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    $\begingroup$ I think this is the answer that best answers my question so far, despite the difference in votes. Making the particles chemically equivalent to the kind that humans breathe is a nice work-around to not being able to modify human biology :) $\endgroup$ – Onyz Jan 24 '18 at 13:45
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    $\begingroup$ This is also what I would have answered, a gravity-less breathable superfluid is the least intrusive solution. it is also probably completely transparent, with no heat capacity and a pretty good insulator - that is, vacuum-like as far as heat and light is concerned. Breathing the stuff may not feel good, and may not be good for unmodified lungs. Also, the stuff is the best airship lifting gas possible, as it lifts as if it was vacuum. Incidentally, standard propellers won't work in a superfluid, physics.stackexchange.com/questions/311069/… $\endgroup$ – Eth Jan 24 '18 at 18:14
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    $\begingroup$ The similarity of this to dark matter is noteworthy - a substance that pervades much of the universe, but is (as we know it) (mostly) unaffected by the fundamental forces (except gravity). Oddly enough, this substance would be almost the opposite of dark matter - it would have to be affected by forces like electromagnetism and such to be breathable, but be unaffected by gravity. Since it's the opposite of dark matter in many ways, I recommend calling it 'light matter.' There are obviously still many problems, physically, but it's better than regular gas. $\endgroup$ – Blapor Jan 25 '18 at 21:33
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    $\begingroup$ @Onyz The top voted answer essentially says "physics doesn't work like that; you're going to have to handwave like mad." Inventing an impossible gas that sometimes acts like it has mass and sometimes doesn't, and further breaks or ignores additional fundamental laws of physics, very much qualifies as "handwaving like mad." From a hard science point of view, this is basically the same as saying "it's magic, deal with it." There's nothing incompatible about these two responses--Pere has just given you a concrete example of the mad handwaving that you could do. $\endgroup$ – GrandOpener Jan 26 '18 at 12:39
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You're going to need to handwave this universe something awful

  • Gravity is what keeps planets together. It's also what created them in the first place. Planets and stars form by pulling together all that breatheable gas. Solar systems can be thought of as cosmic vacuum cleaners. The star and the planets keep pretty much all the light-weight stuff "swept up" by attracting it into said star or planets.

In an effort to help you rationalize/explain your universe, if gravity operated not on the inverse-square law, but on (for example) an inverse-cubed law. Planets and stars would on average be smaller, but the force of gravity would decrease much more quickly with the distance from the mass. This would allow for bands of mass between planets (maybe, see below) and more mass between stellar systems.

However, the faster gravitic drop-off would mean planets would all orbit closer to their suns, so there might not be anything between them anyway. Worse, there's nothing you can do about planets sweeping the gas into themselves as they orbit.

Finally, the amount of mass you're talking about, even at its thinnest to keep it breatheable, would mean you couldn't see even your nearest planetary neighbor. Heck, you probably couldn't see your own moon. You might not even be able to see your own sun (if you can, it would be as a lightened smudge in the sky). It would be a very dark universe.

  • Next problem: solar winds. Those winds actually push things away, so while gravity is attracting, the solar winds are fanning. Which means there would be no breatheable air anywhere near the sun and possibly not for entire AU from the sun. OK, maybe you can see the sun after all since it's either vacuuming or sweeping the gas away immediately around itself. On top of this is the fact that human-breatheable air has oxygen ... stuf that a star can ignite. Your universe might exist for a second before all the stars ignite the air around them and the universe burns itself out in a cataclysm that makes the word "biblical" seem like a description for buttering toast.

No... I'm sorry, but there isn't a way of bending physics enough to justify the universe you seek. Your only option is to declare it so in the semblence of a beneficient god and move on.

On the other hand... if you just handwave this in the tradition of Jules Vern to have fun with the story (a "parallel universe where anything can happen"), then the interstellar atmosphere would spin around a star like a whirlpool. Eddies and currents would form between systems. Black holes would be like waterfalls and gyres. You're thinking atmosphere, but it's much more like the sea. Perhaps in your universe (where we're not worried at all about our own physics) you have something akin to pulsars or quasars that spew atmospheric gasses rather than radiation. Hate to be near one of those. And the concussive shock of a supernova is very, very real.

The story could be a lot of fun... but only if you ignore our reality completely.

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    $\begingroup$ Definitely agree here. If you're going to have breatheable atmo in-space...you can't bring physics into the equation. Just declare that your universe operates according to its own rules, rename space to...like...The Ethereum or something, and run with it. This is a case where trying to play by the rules is more trouble than it's worth $\endgroup$ – guildsbounty Jan 23 '18 at 16:51
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    $\begingroup$ An inverse-cube-law field is not conservative, which means that closed orbits are almost impossible without a net energy input. ("Almost" in the mathematical sense, that is, the set of possible closed orbits is of null measure.) $\endgroup$ – AlexP Jan 23 '18 at 17:02
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    $\begingroup$ Your last paragraph sounds seriously similar to treasure planet the movie $\endgroup$ – bendl Jan 23 '18 at 18:53
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    $\begingroup$ @bendl Treasure Planet was the first thing I thought of when I read this question :) $\endgroup$ – Ben Sutton Jan 23 '18 at 19:45
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    $\begingroup$ @Will, I mean it's impossible for the OP to get the universe he wants without an obscene amount of handwaving. "Handwaving" has many derivations. One is the "look over here!" distraction used by magicians. Another is the "I'm simply not going to talk about that" gesture you sometimes get with a person who really wants to talk about something else. Simply put, you can't bend the laws of physics to get there, you break them completely. Thus my admonition later in my answer to simply declare it to be so (handwave) and move on with the story. $\endgroup$ – JBH Jan 25 '18 at 0:54
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Almost everything related to gravity, just for starters.

lets give an example.

The sun is 4 light years from it's nearest neighbor.

But lets imagine something smaller than that, lets imagine a genie snaps it's fingers and suddenly we have a cube of air around the solar system 1 light year on a side all inside a big box to keep the air in.

All well and good, you fly around in the air etc....

Problem. Air has mass, a balloon full of air has a tiny bit of it's own gravity.

The mass of 1 cubic light year of air (a pressure equivalent to sea level) is 1.08×10^48 kg.That's a lot of mass.

Unfortunately the Schwarzschild Radius for a mass of 1.08×10^48 kg is 169,543 light years.

Unfortunately the entire milky way galaxy is only about 100,000 light years in diameter

So that 1 ly cube immediately becomes a black hole with an event horizon larger than the entire galaxy.

Everybody dies. enter image description here

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    $\begingroup$ A black hole that big? If any black hole is going to contain an alternate universe, it will be that one. Everyone inside might actually be OK (albeit unable to leave). $\endgroup$ – Brilliand Jan 23 '18 at 21:37
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    $\begingroup$ @Brilliand They won't. Being in black hole might be temporary non fatal but all spacetime path lead to singularity (i.e. point this solar system collapses to - people in it will undergo spaggetification). $\endgroup$ – Maciej Piechotka Jan 23 '18 at 23:45
  • $\begingroup$ This would make for a violently cool story. $\endgroup$ – Hankrecords Jan 24 '18 at 11:04
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    $\begingroup$ I am glad you pointed this out before we tried helping out your Universe by giving you a bunch of our extra air...! $\endgroup$ – Onyz Jan 24 '18 at 13:48
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All of them

Having this happen is just... is really not possible that I can think of without magic and hand-waving.

I think you would have to have magically appearing gas. Assuming that you are fine with having magically appearing gas appearing all over, not a lot I guess (sarcasm).
But seriously, you have to hand-wave everything, including absolutely massive volumes of gas and almost constant creation of said gas, not to mention a host of other problems.

Since you mention travelling with zeppelins though I think you are massively underestimating the volume/distance of space. Space is huge. Just our solar system would be FAR too massive for a zeppelin to traverse. Lets take a look at that.

Airship (Space) Travel

Lets assume that we are completely ignoring the gravitational pull of planets and we can just happily float off into space (spoilers: we cant). If we dont assume this you could never explore other places with a blimp.

According to Wikipedia and some general googling the fastest I can see a large blimp type airship going is 80 mph (~129 kph) although it seems they usually trundle along at more like 40-50mph, assuming there is not a headwind.
Lets be optimistic shall we? Lets say they can average 80mph without a tailwind. Using that figure lets see how long it would take to get to some nearby celestial bodies assuming they are in their closest position to earth (according to this calculator).

All calculations are approximate

  • Moon: (238,900 miles)
    • travel time: 125 days (0.34 years)
  • Venus (25,724,767 miles)
    • travel time: 13,398 days (36.71 years)
  • Mars (48,678,219 miles)
    • travel time: 25,353 days (69.46 years)

The travel speed of Apollo 11 to break earths orbit was about 25,200 mph or about 315 times faster than our assumed zeppelin speed. Unless you can get a blimp moving very very very fast (how???), it will likely never leave any planets orbit, and might even get stuck on some moons. Its not really worth it to talk about exploring any of the outer planets, let alone other solar systems knowing these things.

Solution?

You just break everything and anything and make your own rules for whatever the heck you want. You will be breaking everything but just fill the space inbetween planets with gas. Make the planets really close together so you can travel to them but somehow not crash into each other or pull on your zeppelin, probably because of its handwavium coating.

You have no way to make this work in a universe that resembles ours. You will have to break all of the rules as we know it and make things that way "because you say so".

Did you get this idea from Treasure Planet? Because that universe is suuuper hand-wavey and this sounds kind of like that with the zeppelins and breathable space

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    $\begingroup$ you could improve the answer by giving a short list of "everything". things like the fact you can't have planets orbiting with drag. $\endgroup$ – John Jan 23 '18 at 16:48
  • $\begingroup$ @John I feel like JBH's answer covers a nice number of the aspects that come along with filling the vacuum of space with a gas if more of those are wanted. I didn't think it necessary for the answer to be any more technical because as it stands OP will need to create their own rules for their universe, so explaining what would break in our universe isn't as important. If needed for the question, I will alter my answer to have more technical details. $\endgroup$ – BornToDoStuff Jan 23 '18 at 16:57
  • $\begingroup$ If you have a blimp with normal air drag, it won't ever be "in orbit". That means moving sideways fast enough to continuously miss the planet (which as you pointed out is one of the key problems with having air drag everywhere), not just being at a few hundred km up. what-if.xkcd.com/58 $\endgroup$ – Peter Cordes Jan 26 '18 at 3:36
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Aristotelian universe.

Aristotelian universe https://www.jw.org/en/publications/magazines/awake-no5-2016-october/aristotle-view-of-universe/

https://en.wikipedia.org/wiki/Geocentric_model

In the fully developed Aristotelian system, the spherical Earth is at the center of the universe, and all other heavenly bodies are attached to 47–55 transparent, rotating spheres surrounding the Earth, all concentric with it. (The number is so high because several spheres are needed for each planet.) These spheres, known as crystalline spheres, all moved at different uniform speeds to create the revolution of bodies around the Earth. They were composed of an incorruptible substance called aether. Aristotle believed that the moon was in the innermost sphere and therefore touches the realm of Earth, causing the dark spots (macula) and the ability to go through lunar phases. He further described his system by explaining the natural tendencies of the terrestrial elements: Earth, water, fire, air, as well as celestial aether. His system held that Earth was the heaviest element, with the strongest movement towards the center, thus water formed a layer surrounding the sphere of Earth. The tendency of air and fire, on the other hand, was to move upwards, away from the center, with fire being lighter than air. Beyond the layer of fire, were the solid spheres of aether in which the celestial bodies were embedded. They, themselves, were also entirely composed of aether

There are several nice things about using an Aristotelian system for the world you want.

  1. Aristotle was a keen observer. There is an explanation within the Aristotelian system for just about any physical phenomenon you can name. His system is internally consistent too.

  2. The tenets and terms you use do not have to be invented - they are all there and they will lend their medieval scholarly gravitas to your fiction endeavor.

  3. There is no reason air cannot extend out indefinitely although I am not sure what sort of resistance you will encounter when you traverse the spheres of Aether that carry the celestial bodies.

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    $\begingroup$ I've seen a lot of science neat fantasies that use some kind of variant on an Aristotelian cosmos. Is there a resource, print or online, that you would reccomend as a go-to for using Aristotle for world-building? $\endgroup$ – Random Jan 23 '18 at 21:20
  • $\begingroup$ @Era - I do not know one. But if you find one, link it up here! $\endgroup$ – Willk Jan 24 '18 at 1:20
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    $\begingroup$ You could start with the Wikipedia page on Aristotelian physics and go on from there. $\endgroup$ – Tacroy Jan 26 '18 at 20:23
  • $\begingroup$ Note that Ptolemy developed the system further, and was (accurately) used for 1500 years. (It's a clever idea, though. +1) $\endgroup$ – RonJohn Jan 27 '18 at 9:43
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This has been done in a smaller scale in The Integral Trees (1984) by Larry Niven

The story occurs around the fictional neutron star Levoy's Star (abbreviated "Voy"). The gas giant Goldblatt's World (abbreviated "Gold") orbits this star just outside its Roche limit and therefore its gravity is insufficient to keep its atmosphere, which is pulled loose into an independent orbit around Voy and forms a ring that is known as a gas torus. The gas torus is huge—one million kilometers thick—but most of it is too thin to be habitable. The central part of the Gas Torus, where the air is thicker, is known as the Smoke Ring. The Smoke Ring supports a wide variety of life.

No "ground" exists in the Smoke Ring; it consists entirely of sky. Furthermore, the Smoke Ring is in orbit and therefore in free fall: there is no "up" or "down". Most animals have trilateral symmetry that allows them to see in all directions. The majority of Smoke Ring animals have evolved to fly on at least an occasional basis—even the fish. The Smoke Ring contains numerous "ponds," globs of water of various sizes which float free like everything else. While there are aquatic and amphibious animals in the Smoke Ring that live the majority of their lives in such ponds, these animals may find their habitat unsuitable at any moment. Whether their home pond drifts too far out of the habitable center of the Smoke Ring and into the gas torus, becomes too large and breaks up due to tidal forces, or impacts a large object such as an integral tree, aquatic animals must be able to propel themselves through the air sometimes in order to find a new place to live.

Use the same gas torus, but make your humans smaller, which makes the gas torus "universe" bigger

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    $\begingroup$ Yep, came here looking for this. See also, Peter F Hamilton's Silfen Gas Halo. Rather than being a naturally occurring phenomenon ala Niven it's an artificially maintained structure by a hyper technological/mystical/evolved species. $\endgroup$ – Necoras Jan 23 '18 at 23:29
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Others have listed some good reasons why this would be difficult while still maintaining the laws of physics that your readers will be accustomed to.

An alternate approach would be changing the physiology of the inhabitants of the world. What if they didn't need to breathe at all in order to live? They might still need gas for other things (as a medium for communication, for example) but those might be solvable on their own (pressurized areas for speaking in? Telepathy?)

Now, that may make the universe's inhabitants too far removed from humanity for your tastes, but it is an option.

Another option would be to change what they need to breathe. If the air that they breathe has no mass, then gravity would be a non-issue. Maybe this mass-less breathed-medium exists in space and on planets, but on planets the (mass-having) gases accrue, and this accounts for the rest of the differences that atmosphere cause (eg, refraction of light).

EDIT - Regarding the assertion that this would be changing humanity rather than the laws of physics (which is not what the question asks about) - that's definitely true of some of the more farfetched pieces of what I propose above (eg telepathy). But the latter suggestion (of changing what the "humans" breathe) can fit fairly closely within the historical framework of ways that humans have understood ourselves and the environment - it's basically just positing the existence of aether planetside in addition to in outer space, and claiming that that's the part that humans actually breathe (similar to air on (our) earth - we breathe in a mixture of all sorts of stuff, including a bunch of nitrogen, but it's only the oxygen that we actually need).

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    $\begingroup$ Hello and welcome. We generally frown at answers that just tell OP not to do what he wanted to do with his world. This question is somewhat an exception due to its nature, but please keep it in mind when you will answer in future. If you have some time, please also see Help Center, especially sections about what is and what is not on topic. $\endgroup$ – Mołot Jan 23 '18 at 17:38
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    $\begingroup$ @Molot, in this case the question doesn't really admit another answer though. This seems constructive at least. $\endgroup$ – Clumsy cat Jan 23 '18 at 19:00
  • $\begingroup$ @Mołot I've added an edit which explains my thinking on this a bit further, though I agree with you that I structured my answer in a way that buries that a little bit. I can restructure it to surface that more explicitly if you think that would be valuable. $\endgroup$ – autophage Jan 23 '18 at 21:38
  • $\begingroup$ Glad to see it edited. And I'm glad I could honestly +1 it now :) $\endgroup$ – Mołot Jan 23 '18 at 22:12
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I can see two options that involve what I would consider to be roughly the same level of alteration to physics.

The first is to replace normal inverse-quadratic gravity with gravity that's much stronger, but follows an exponential Yukawa potential. This would result in large structures, like stars and planets, effectively being held together by surface tension, and gravity effectively disappearing once you get a certain fixed critical distance away from any individual body, regardless of its size. The surface gravity of any given body above a minimum critical size would be directly proportional, not to its total mass, but to the density of the surface materials--which means all rocky planets would have approximately the same surface gravity! Perfect for campy space opera. :)

The disadvantages of this system are twofold:

First, there are no orbits. That comes with a silver lining, though, because it means you don't have to worry about drag causing orbits to decay, and planets to fall into their suns. And you don't really need orbits; as long as a planet remains at a fixed distance from its sun, does it really matter if it's orbiting, or just sitting there, inert in space?

Second is that internal pressure is inversely proportional to radius. That means bigger stars actually have a harder time developing enough internal pressure to start fusion than smaller ones, and rocky planets might well have enough internal pressure to fuse iron! So, in addition to fiddling with gravity, you'll want to fiddle with weak and strong nuclear force interactions to make fusion of light elements easier, and fusion of heavy elements harder. I don't know whether or not supernova explosions would be feasible in this sort of universe for distributing heavy elements, but if not, that's OK: dying stars would eventually just cool off and become habitable rocky planets! (Because, remember, surface gravity doesn't depend on total mass--just the density of materials near the surface.) A side benefit of this complication is that hollow worlds become possible, and may even form naturally--if a planet becomes large enough, due to two smaller worlds crashing into each other or whatever, its internal pressure may go low enough to allow the expansion of dissolved gasses in the rock / magma, blowing bubbles in the interior. If you could tunnel into them, you could even walk on the interior surfaces, because gravity is a surface phenomenon, and the Newtonian shell theorem doesn't apply.

The second option is to try to keep gravity the same, and change other stuff to accommodate it.

The problem of all of the air simply collapsing into a black hole is relatively easily solvable; if the air fills the entire cosmos, then no particular patch of it will be able to collapse. The self-gravity of any particular chunk is balance by the gravity of all of the other chunks of space surrounding it. You just need a larger cosmological constant to ensure that space remains reasonably close to flat. So, no changes needed to gravity or any other laws yet.

You do, however, need to figure out how to deal with density variations in the cosmic air background. On the one hand, you don't want density fluctuations to run away and result in local collapses, because if they do, then the background gas will rapidly become segmented into dense stars, separated by space that's been emptied of air... just like out universe. But on the other hand, you do want local collapses of a sort, because you want stars and planets and so on.

That's something of a contradictory situation. The only way I can see to resolve it is to arrange for two different kinds of matter, which can pass through each other with minimal interaction, so that one can be allowed to collapse (forming stars and planets) while the other does not (forming your cosmic atmosphere).

We also need to deal with drag, because orbits are still important in this option. We can solve that problem by positing that whatever material forms the cosmic atmosphere is superfluid--or at least maintains a superfluid state in regions where planets orbit! It's OK if, e.g., stellar heat breaks downs superfluidity at close ranges, or if there are pockets of normal fluid elsewhere out in the galaxy. That way, a planet can move through the cosmic atmosphere with near-zero exchange of energy, and no significant drag.

You could manage by making the cosmic atmosphere out of some variety of dark matter--but that then raises the question of how and why people need to or can breathe it at all! Instead, I propose magnetic monopoles. This universe will have two parallel periodic tables: one of electric elements, like our own, and one of magnetic elements, whose fundamental particles may have other slight differences as well, besides just the nature of their inherent electromagnetic charge, so as to allow mag-hydrogen to condense into a superfluid state at relatively high temperatures.

Most of the time, mag-matter and electro-matter would be able to pass through each without noticing, since there would be no Fermi degeneracy interactions between, e.g., magnetons and electrons. However, just like normal electric atoms and molecules in our universe can have an innate magnetic dipole, magnetic atoms and molecules could also have innate electric dipoles, which allow specific types of mag-atoms and mag-molecules to make dipole bonds with specific types of electro-atoms and electro-molecules. There is thus a clear mechanism for explaining how and why planetary life could end up evolving to make use of cosmically-abundant mag-hydrogen (or some other simply mag-molecule) in its metabolism, despite the fact that most mag-matter passes through most electro-matter as if it weren't even there.

In either case, if the humans in these settings are supposed to be able to breathe the cosmic background air, for metabolic purposes, they would not be breathing oxygen--at least not the oxygen we know from our universe. You will probably also need to handwave minor tweaks to atomic physics to make the air much closer to being perfectly transparent than air is in our world, or sunlight would never make it all the way from a star to a planet! But tweaking biochemistry to give them an inverted hydrogen-breathing metabolism, or something involving exotic mag-molecules, is pretty tame compared to re-inventing basic physics. :)

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You could make air out of particles that are not gravitationally attracted to anything but still react electrochemically with it. Stars would still fuse hydrogen; that would no longer be a component of the air that people would breathe. Instead, people would breathe this new air, which can be used by them because it does experience electromagnetism. This new air could technically be burnt by stars, but that usually doesn't happen because solar winds and general radiation pressure push it away before it ever gets close.

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  • $\begingroup$ What if you did the opposite? The same "electrochemical air" you suggested, but it's generated by a star. Thus, the closer to get to the sun, the more "air" there is. This would explain why there is solar wind and pressure changes (don't forget that stars don't burn at equal rates across it's entire surface). Because different spots burn at different rates, there would be "air drafts" as pressures shift around to try to reach equllibrium. As you went further from the sun, it would be equivalent to climbing a mountain. The amount of air slowly diminishes until you approach a new star. $\endgroup$ – Ben Schwabe Jan 23 '18 at 18:59
  • $\begingroup$ The electrical storms from such a universe would explode galaxies. And what happens with electrons who want to swap over to this gravity-free matter? I guess we have to make them not interact with gravity. Solar systems might be swept clear by solar wind, but maybe not; galactic "air pressure" would be too large. We breathe by incorporating the air into our bodies; so everything would have some of this material in them. Ooo, you can make lift gas or even lift wood by concentrating this material in matter. $\endgroup$ – Yakk Jan 24 '18 at 16:37
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TL;DR: Swim through and breathe dark matter, but only when you want to.

I'm not sure how the physics pans out for this, but here goes.

I would posit another fundamental force, one that allows us to interact with the particles that permeate your space. This gives you complete freedom to determine how these particles behave. If you want them to be spread out pretty evenly across the universe, then they should rarely interact with anything, including each other (otherwise they will aggregate or disperse, neither of which is desirable). They will not interact with light, so could be called "dark". Unfortunately, if you want to fly around using this medium, you'll have to be able to interact with it somehow.

I recommend being able to "turn on" and "turn off" your ability to interact with your dark particles, via your new force. Activate the interaction force and swim/fly through your medium, like a zeppelin moving horizontally (since there's no "up"), then deactivate your interaction when you're done. Since these are your own creation, you can make this motion as powerful as you'd like.

But how can your humans breathe? Well, if these particles always existed, perhaps your humans have evolved to take advantage of them. Rather than using oxygen to break down sugars for the energy from the sugar bonds, the special mitochondria of your humans could use the dark particles instead, activating and deactivating the interactions as needed.

This solves the problem of breathing. There are still many other issues to space travel, like the temperature and the long distances, but those are left as an exercise to the reader. There are also many fun corollaries, like sinks and sources and currents. Maybe a comet that is "on" causes a stir in the dark particles, or maybe it is used by aliens to hitchhike from system to system.

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  • $\begingroup$ After reading all the above answers, I was thinking of exactly this! I would say it is by far the best answer - wish I could give it more upvotes! $\endgroup$ – Blapor Jan 25 '18 at 21:40
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How much of outer-space do you want them to be able to breathe? If it's a space too big to cross in a lifetime of human powered movement, like "air-swimming" with fans on your feet then James Jenkins already pointed you to The Integral Trees/Smoke Ring and Necoras the Silfin Gas Torus but there's another Larry Niven title worth checking out if you want something bigger; Bigger Than Worlds is a non-fiction guide to space habitats and well worth reading for anyone looking at building science fiction universes as all the objects (except possibly the ringworld, because structural strength issues) are possible based on current engineering, mathematics, and material science.

The object that is of interest in this case comes towards the end of the essay it's called the "Megasphere", a solid shell Dyson Sphere with the core of a galaxy at it's heart and a biosphere around the outside. There's basically no gravity in the biosphere so humans would have to be able to survive freefall or build either extremely massive (AKA planets) or spinning (like a ringworld) structures if we can't so we have gravity. The atmosphere of the Megasphere is Light-years deep so there's no shortage of space, and a society living on a planet orbiting the Megasphere, within the atmosphere, would never need to know what was either at the core or the edge of the huge atmospheric bubble in which they lived.

Do note that any orbit within the atmosphere of a Megasphere is a crash orbit, anything under a full atmosphere of drag is going to lose momentum, but the orbits can be so large, and the surface gravity is so low, that objects can stay aloft for spans greater than that of the total habitability zone forecast for planet Earth.

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I believe that with slight modifications to existing physics, it can be done. As other answers have deftly explained, gravity is your main issue, so, make breathable air be something either:

a) Massless or

b) Of a mass that does not experience gravity

If you want to float on it in an airship or move through it with propeller, option (a) gives you problems, but you add a complication to deal with in option b that cosmic bodies are experiencing drag. Perhaps again the whole universe is in motion and for hand-waved reasons, the drag of the air-streams doesn't remove energy from the system. Again, note with either option you can't make the a properly floating space blimp, so perhaps a third option presents itself:

c) Your breathable air, called 'atmosphere', has an anti-gravity counterpart called 'othresphere' or something that works opposite the way gravity does, as well havign no gravitational attraction to other othresphere particles (no inverse black holes, thank you very much). It doesn't descend all the way to the surface of planets because it works on the inverse of gravity. There is a background universe pressure that provides a base pressure for the 'othresphere' far away from masses. The pressure column drops as you rise in an atmosphere until you reach the "equilibrium zone" where the transition of airships from atmosphere to 'othresphere' is made. The pressure in this zone is the lowest it can be anywhere. Moving into this zone your escape from gravity becomes dependent on on engine power and aerodynamics, but just like that you've moved into breathable outer space.

How does this effect the physics of the universe/

Well, the large and weird cosmological bodies do even wierder things, but assuming your nearest problems are planets and stars, think of it this way.

Planetary drag in 'othresphere': The atmosphere/othresphere interface layer will form some violent jet-streams if the othresphere is not moving very strongly in the direction of planetary travel, but a gas cushion like this is the most effective way to remove friction. Also, your planet doesn't have to orbit its star as fast, because habitable planners would be farther away from their star do to an increased heat retention (infinitly thick atmosphere) so they don't have to go as fast, reducing planetery turbulence in the othresphere.

Stars: Light transmission will be the main problem here. Your solution will probably be to say that light passes through othresphere with no interaction. Since othresphere is something we made up, we can simply say that as well as being and anti-gravity particle it doesn't interact with light, except maybe at a certain wavelength in case you want to use some special radar on your space-zeplins to map othresphere density. Otherwise, just leave it as entirely non-interactive.

Note that many of the equilibriums we see in regular orbital mechanics will change, but they will still come to equilibrium somewhere, and that results in a stable, breathable universe.

"Let them breath space!"

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There is no change to the laws of physics that would allow people to be able to breathe in space. Any attempt to explain such a thing through physics is going to create a bunch of plot holes that will greatly reduce the quality of your world.

You're going to be better off avoiding discussing physics at all if you want to have humans to be able to breathe unprotected in outer space. The easiest way to accomplish this is to avoid the explanation all together and treat this fact as entirely unremarkable to the inhabitants of your universe.

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    $\begingroup$ This isn't about breathing vacuum, it's about space not being vacuum. $\endgroup$ – Piomicron Jan 23 '18 at 16:30
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    $\begingroup$ @Piomicron My point still remains. $\endgroup$ – sphennings Jan 23 '18 at 16:31
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    $\begingroup$ The fact that others have provided interesting, potentially workable answers (like a fifth fundamental force) seems to directly contradict your conclusion that it is impossible. As a denier of possibility, the burden of proof falls upon you. Unless you can demonstrate that all other possible answers are wrong based on some inviolable logical principle, this answer is simply unproductive. $\endgroup$ – Blapor Jan 25 '18 at 21:43
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This might be possible with two changes:

  • Add particles with negative mass
  • Add a fifth fundamental force which dominates gravity over large distances

Think of it this way: strong and weak nuclear force hold clumps of subatomic particles together (we call these nuclei). Because both of these forces are dominated by electromagnetic forces at large distances, electromagnetic force holds atoms and groups of atoms (we call these molecules and crystals) together. Since electromagnetic force has both positive and negatively charged particles, and these tend to "cancel" each other out, gravity dominate electromagnetic force over yet larger distances. Gravity holds clumps of molecules and crystals together (we call these planets).

Continuing this chain, we would need to add negative mass, so that a fifth fundamental force, weaker than gravity, could dominate gravity over astronomic distances. Both positive and negative mass planets would exist in this universe, and they would be arranged throughout space in a gigantic crystal lattice. Some molecules of gas that contain both positive and negative mass particles (so that they effectively have zero mass) could diffuse through interplanetary space.

Since this new force is so much weaker than gravity, it wouldn't really be noticeable at a human scale. Also, since planets are clumps of particles with primarily the same sign mass, the negative mass particles wouldn't have a strong effect on any given planet.

As a bonus, planets in this universe would probably be much closer together, and you get the interesting effect that it's difficult to land on planets with he opposite sign mass as your ship.

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This is a major plot point in the Grand Central Arena series by Ryk E. Spoor, the eponymous first of which, in its first edition, is available free from Baen. There's a degree of handwaving about how it all came to be but the scope is massive, set within atmosphere. It's effectively a bounded universe.

Before them was a vast skyscape, a twining, roiling sea of air and cloud, brown and black and white and green, extending beyond the reach of sight in all directions. Through this atmosphere swam tiny shapes, some dimmed by haze of distance, that seemed no more than a meter long, finned or sailed things like strange fish. Then one of them suddenly appeared to the left, emerging from a cloud in majesty, trailing streamers of mist from spars and masts, a titanic ship a kilometer long, lights blinking on its extremities, a distorted image of the massive, impossibly huge Nexus Arena reflected on the polished bronze-colored hull. As it passed, Ariane could see a bridge or forward observation deck, through which tiny figures were visible moving about. In the deepest distance, scarcely visible through the murk and gloom, another spark of light was seen, near to some monstrous shape, a shadow against shadow, of a Sphere that could envelop a world. “Behold the Arena,” Orphan repeated, more quietly, almost reverently. “The endless skies, the worlds that drift in cloud and light and shadow, a place where storms a million million kilometers wide clash above and around embattled Spheres, where trading ships and pirates and mercenaries travel beside, prey upon, and defend explorers, decadent tourists, lost souls searching for a home or a cause, armadas finding new worlds to conquer, and all, all of them looking, watching, asking for news . . . news of First Emergents, of ancient ancient ruins atop a lost Sphere, of rumors of Voidbuilder knowledge or Shadeweaver powers . . . and all of them returning here to hear that news, to behold the newcomers—and perhaps to Challenge them, or be themselves Challenged, and gain or lose all in a single contest. It is my home. Now it is yours.”

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It would be far more plausible to make your characters intelligent beings who don't need to breath. Make them aliens, make them androids (robots built to look like people), make them humans modified into cyborgs for space travel, with built in atomic generators and closed recycling systems for air, food, and wastes.

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